Fluorescent Schiff base sensors as a versatile tool for metal ion detection: strategies, mechanistic insights, and applications

Abstract

Metal ions exist globally. Therefore, the study of the design strategies, mechanistic insights, and applications of fluorescent Schiff base sensors for metal ion detection constitutes an active topic of research. Further, theoretical studies on these systems using various spectroscopic techniques add new flavor in terms of providing a better understanding of the photo-electronic properties of Schiff base sensors and their complexes with metals. The presented review article starts with the general introduction of fluorescent Schiff base sensors for metal ion detection, and consists of two main sections. The first section illustrates the strategies and different mechanistic insights into metal ion detection using Schiff base sensors. The second section demonstrates the application of fluorescent Schiff base sensors in metal ion sensing, biomedical imaging, environmental monitoring, and optoelectronic systems. Additionally, a summary of the reviewed fluorescent Schiff base sensors is presented in a table format. Finally, the conclusion and future scope of fluorescent Schiff base sensors for metal ion detections are discussed.

---

Manoj Kumar Goshisht

Dr Manoj Kumar Goshisht is an Assistant Professor of Chemistry at Govt. Naveen College Tokapal, Bastar, Chhattisgarh, India. He obtained his master's degree from Guru Jambheshwar University of Science & Technology, Hisar, Haryana, and PhD degree from Dr B. R. Ambedkar National Institute of Technology, Jalandhar, Punjab, India. He has cleared the National Eligibility Test (CSIR-UGC NET) for Assistant Professor in the subject of Chemical Sciences. He is a winner of the Editors Pick Award of the “9th DST & ACS Workshop” held on 11th August 2021. His research interests include organic chemistry, materials chemistry, supramolecular chemistry, and nanomaterials. He has published various research papers in reputed international peer-reviewed journals such as The American Chemical Society, The Royal Society of Chemistry, and Springer Nature. He also has a book to his credit published by CRC Press (an imprint of the Taylor & Francis group).

Goutam Kumar Patra

Prof. Goutam Kumar Patra received his PhD from Jadavpur University, under the supervision of Prof. Dipankar Datta at Indian Association for the Cultivation of Science, Kolkata. Then he joined Tel Aviv University, Israel, as a postdoctoral research fellow with Prof. Israel Goldberg (2000–2002). Subsequently he moved to Carnegie Mellon University, USA, where he worked with Prof. Catalina Achim. Then he joined as Asst. Prof. in Vijoygarh Jyotish Ray College, Kolkata, in December 2003. He visited Max Planck Institute of Bioinorganic Chemistry, Mülheim, Germany, as a BOYSCAST fellow during 2006–2007 and worked with the then Director, Prof. Karl Wieghardt. He has been a Professor in Guru Ghasidas Central University, Bilaspur, since 2012. His research interests include chemosensors, redox activity, azamacrocyclic chemistry, crystal engineering, porphyrin and supramolecular chemistry, peptide nucleic acids (PNAs) and free radical chemistry. So far, he has published more than hundred research papers in journals of national and international repute.

Neetu Tripathi

Dr Neetu Tripathi obtained her PhD from Guru Nanak Dev University, Amritsar, India. She completed her master's with 1st rank (Gold medal) in MSc Chemistry (Instrumental Analysis). She has been a recipient of the prestigious INSPIRE fellowship of the Government of India during her PhD. She has also cleared National Eligibility Test (CSIR-UGC NET) for Assistant Professor and Graduate Aptitude test (GATE) in the subject of chemical sciences. Her research interest includes organic chemistry, materials chemistry, supramolecular chemistry, and nanomaterials. She has published various research papers in international peer-reviewed journals (Royal Society of Chemistry, Elsevier journals and Springer journal). She is also the co-author of a book published by CRC Press (an imprint of the Taylor & Francis group).

---

1. Introduction

The metal ions constitute an active topic of research owing to their global existence. In the human body, metal ions enjoy special status because of their participation in numerous biological processes. In our surroundings, metal ions are present in food, soil, and water.¹ However, the presence of metal ions at concentrations above or below the permissible limit (Table 1) may cause hazardous effects on the environment and human beings.^2–5 In the past, various methods such as inductively coupled plasma atomic emission spectroscopy, inductively coupled plasma mass spectroscopy, electrochemical detection, and ion chromatography have been reported for metal ion detection.^6–10

Table 1 Permissible concentration levels of metals in drinking water by WHO (World Health Organization) and recommended daily amount (RDA) of metal ions in the human body

Metal ion	In drinking water (mg l^−1)	In human body	Ref.
Mg^2+	50	∼300–400 mg day^−1	3 and 5
Ca^2+	75	—	5
Al^3+	0.03 mg l^−1	3–10 mg day^−1	3 and 4
Sn^2+	1–2 μg l^−1	—	2
Pb^2+	0.05	93.5 μg day^−1	5
Cr^3+	0.05	50–200 mg day^−1	4
Mn^2+	0.1 mg/l	2.3 mg day^−1	2–4
Fe^2+/Fe^3+	0.3	8 mg day^−1	3–5
Co^2+	0.001	5–40 μg day^−1	3–5
Ni^2+	0.0008	80–130 μg day^−1	3–5
Cu^2+	1.0	2 mg day^−1	3–5
Zn^2+	5.0	8 mg day^−1	3 and 4
Hg^2+	0.001–65	—	4 and 5
Cd^2+	0.005	—	4 and 5

---

However, these methods suffer from limitations such as inadequate practical applications, expensive instruments, time-consuming processes, complicated procedures, etc. Recently, significant attention has been paid to developing fluorescent chemosensors for metal ion detection owing to their excellent selectivity, sensitivity as well as rapid response time.^4,11,12

In recent years, the design and synthesis of fluorescent chemosensors are of central importance because of their straightforward synthesis, tailor-made properties, and rapid, selective, and sensitive detection of the analyte on-site.^13–19 Here, positive metal ion detection may result in visible color change, fluorescence color change, a shift in emission wavelength or intensity (blue-shift or red-shift) or may result in the appearance of new emission maxima.^20–25 Fluorescent chemosensors for metal ion detection generally contain Schiff base,^26–28 pyridine,^29,30 pyrene,^31,32 anthracene,^33,34 quinoline,^35,36 naphthalene,³⁷ urea,^38,39 coumarin,^40,41 phenolphthalein,^42,43 and rhodamine^44,45 groups as the binding site. Among various fluorescent chemosensors, Schiff base-based sensors have shown outstanding results owing to their simple synthesis, and provide a suitable electronic and geometrical environment for coordination with single metal ions as well as with multiple metal ions simultaneously. Schiff bases commonly include hydrazone, acyl hydrazone, salicylimine, azine, and other groups, and provide nitrogen and oxygen atoms for coordination with metal ions.^46–50 Schiff base-based chemosensors may show the aggregation-induced emission (AIE) phenomenon because of the free rotation around the C=N bond. Additionally, the excited-state intramolecular proton transfer (ESIPT) process was exhibited by chemosensors containing the salicylimine group.^51–54

Nowadays, Schiff base sensors and their complexes with metal ions have become a hot topic of research owing to their practical application in numerous scientific areas such as in environmental monitoring and in biological cell imaging of target ions within the cells, tissues, and organelles.^4,55–59 Furthermore, the tailor-made and multi-stimuli responsive properties of Schiff bases enable their application in optoelectronic systems.^60–62 The molecular switching properties of Schiff base sensors have been applied in the construction of molecular keypads and logic gates.^63,64

In the past, significant research work has been done in developing fluorescent Schiff base sensors for metal ion detection.^63,65–71 However, to the best of our knowledge, no review has been reported on metal ion detection using fluorescent Schiff base sensors. In this review article, we start with the general introduction of fluorescent Schiff base sensors for metal ion detection and then divide the article into main sections. The first section illustrates the strategies and different mechanistic insights into metal ion detection using Schiff base sensors. The second section demonstrates the practical applications of fluorescent Schiff base sensors in metal ion sensing, biomedical imaging, environmental monitoring, and optoelectronic systems (Fig. 1). Additionally, a summary of the reviewed fluorescent Schiff base sensors is presented in a table format (Table 2). Finally, the conclusion and future opportunities of fluorescent Schiff base sensors for metal ion detection are discussed.

Fig. 1 Practical applications of fluorescent Schiff base sensors.

Table 2 Summary of the reported fluorescent Schiff base sensors, fluorescence response, excitation wavelength (λ_ex), emission wavelength (λ_em), metal ion, limit of detection (LOD), Recovery rates (%) ± RSD (%) (relative standard deviation) in the spiked samples, mechanism, application, and references

Sensor	Fluorescence response	Solvent	λ ex (nm)	λ em (nm)	Metal ion	LOD	Recovery rates (%) ± RSD (%) in the spiked samples	Mechanism	Application	Ref.
1	Turn-on	H2O–DMF (1 : 1, v/v)	370	455	Cu^2+	35 nM	—	Cu^2+ induced self-assembly	Metal ion sensor, fluorescence imaging of intracellular Cu^2+	72
2	Turn-off	99% HEPES buffer–CH3CN	390	526	Cu^2+	0.261 ppb	95 to 120 ± <0.5 in tap water, pond water, and industrial waste water	Disassembly of self-assembled aggregates	Metal ion sensor, cell imaging of HeLa cells, logic circuit, TLC strips	73
3	Blue shift of emission, turn-on	CH3CN	393	454.5	Cu^2+	2.5 μM	—	Coordination of sensor 3 with Cu^2+ blocks the PET process	Metal ion sensor	32
4	Turn-off	H2O/DMSO (v/v, 1 : 1)	400	585	Cu^2+	32.8 nM	—	Coordination of Cu^2+ with sensor 4 caused activation of the PET process between coumarin and Cu^2+	Metal ion sensor, fluorescence imaging of intracellular Cu^2+	41
5	Blue shift of emission, turn-on	Tris–HCl buffer (CH3OH/H2O = 9 : 1, v/v)	430	516	Zn^2+	0.144 μM	—	PET and CHEF effect	Metal ion sensor	74
6	Turn-on, red-shift in the case of Cu^2+	CH3OH–tris buffer (1 : 1)	340	440	Cu^2+ and Pb^2+	1.2 μM and 0.9 μM for Cu^2+ and Pb^2+, respectively	98.3 to 101.6 ± <1.5 for Cu^2+, and 100.7 to 109.8 ± <2 for Pb^2+ in water samples	PET and ICT process	Metal ion sensor, logic gate, cell imaging	75
7	Turn-off	THF–H2O (9 : 1, v/v)	365	484	Cu^2+	Less than 1 μM	—	Metal to ligand charge transfer, CHEQ	Metal ion sensor	76
8	Turn-off	Aqueous medium	405	517	Cu^2+	12.3 nM	—	Ligand to metal charge transfer	Metal ion sensor, secondary sensor for CN^− (turn-on), logic gate, cellular imaging	77
9	Turn-off	DMF/water mixtures	400	525 and 630	Cu^2+	9.12 nM	—	FRET	Metal ion sensor	78
10	Turn-on	PBS buffer at pH 7.4	360	415 and 548	Al^3+	10 nM	—	ESIPT, metal-template aggregation	Metal ion sensor	79
11	Turn-off	Aqueous medium	350	438	Cr(VI)	0.175 μM	99 to 104 ± <2 in ground and tap water, 102.5 ± <2 in human serum, and 101.5 to 105 ± <2 in vitamin C tablets	Inner filter effect	Metal ion sensor, secondary sensor for ascorbic acid (turn-on), tap water, ground water	80
12	Turn-on	CH3CN–H2O (7 : 3, v/v)	305	389	Al^3+ and Cr^3+	88.7 nM and 63.1 nM for Al^3+ and Cr^3+, respectively	97.8 to 103.8 ± <1 for Al^3+ and 96.5 to 104.8 ± <1.5 for Cr^3+ in tap and deionized water	Metal induced chemical change, C=N isomerization	Metal ion sensor, tap water	81
13	Turn-on	Ethanol	491	547	Mg^2+	0.516 μM	—	PET process, CHEF effect	Metal ion sensor	82
14	Ratiometric fluorescence change	HEPES buffer solution (pH = 7.2)	370	545 and 483	Ca^2+	0.3 μM	—	ICT process	Metal ion sensor	83
15	Turn-on	HEPES buffer solution	365	488	Al^3+	15.0 nM	94.4 ± 2.2 in tap water	PET process, CHEF effect	Metal ion sensor, test paper, tap water	84
16	Turn-on, blue-shift	Methanol	316	550	Al^3+	14 nM	—	CHEF effect	Metal ion sensor	85
17	Turn-on	DMF–H2O (v/v, 9 : 1)	390	490	Al^3+	6.7 μM	—	ICT process, CHEF effect and C=N isomerization	Metal ion sensor	86
18	Turn-on	Ethanol	428	502	Al^3+	38.7 nM	—	PET phenomenon, CHEF effect	Metal ion sensor	87
19	Turn-on	Ethanol	350	391	Al^3+	0.1 μM	—	PET process, CHEF effect	Metal ion sensor	88
20	Turn-on	Ethanol	410	480	Al^3+	31.9 nM	—	PET process, CHEF effect	Metal ion sensor	89
21	Turn-on	Ethanol–H2O (v/v, 3 : 1)	420	475 and 508	Al^3+	18.2 nM	—	PET process	Metal ion sensor, solid state sensor	90
22	Turn-on, blue-shift	Aqueous medium	389	460	Al^3+	0.6.9 μM	—	ICT phenomenon	Metal ion sensor	91
23	Turn-on	Ethanol	363	420	Al^3+	0.2 μM	—	PET phenomenon, CHEF effect	Metal ion sensor	92
24 and 25	Turn-on	DMSO–H2O		493 and 476 for sensors 24 and 25, respectively	Al^3+	14.9 nM and 15.1 nM for sensors 24 and 25, respectively	—	PET process, CHEF effect	Metal ion sensor	93
26 and 27	Turn-on	Methanol	410	506 and 489 for sensors 26 and 27, respectively	Al^3+	47.9 nM and 82.8 nM for sensors 26 and 27, respectively	—	CHEF effect	Metal ion sensor	94
28	Turn-on	Ethanol	394	478	Al^3+	4.2 ppm	—	CHEF effect	Metal ion sensor	95
29	Turn-on	DMF	378	481	Al^3+	1.2 μM	—	ICT process, CHEF effect	Metal ion sensor	96
30	Turn-on	Methanol–H2O (9 : 1, v/v)	334	521	Al^3+	0.1587 μM	—	PET process, CHEF effect	Metal ion sensor	97
31	Turn-on	DMSO–H2O (1 : 1, v/v)	320	460	Sn^2+	0.314 μM	—	PET process, ICT process, C=N isomerization and CHEF effect	Metal ion sensor	98
32	Turn-on	CH3CN–H2O (95/5%)	273	537	Cr^3+	0.22 μM	—	PET process, CHEF effect	Metal ion sensor	99
33	Turn-on	CH3CN–H2O (95/5%)	360	663	Cr^3+	0.13 μM	—	PET process, CHEF effect	Metal ion sensor	100
34	Turn-off	DMF	280	356 and 372	Fe^3+	37.5 nM	103 ± 1.9 and 97.5 ± 2.1 in distilled water and tap water, respectively	LMCT	Metal ion sensor	101
35	Turn-on	Aqueous medium	256	505	Fe^3+	48.0 nM	—	PET process, CHEF effect	Metal ion sensor	102
36	Turn-on, red-shift	CH3CN–H2O (1 : 1, v/v)	300	370	Fe^2+	0.15 μM	104.8 ± 0.04, 100.8 ± 0.03, 97.0 ± 0.03, and 93.3 ± 0.05 in tap water, tablets, dark chocolate and tomato juice, respectively	ICT process, CHEF effect	Metal ion sensor, tap water, tablets, dark chocolate and tomato juice	103
37 and 38	Turn-off	DMF/HEPES buffer (7 : 3, v/v)	318 and 304 for sensors 37 and 38, respectively	446 and 426 for sensors 37 and 38, respectively	Fe^3+	4.28 μM and 0.83 μM for sensors 37 and 38, respectively	99.7 to 101.9 ± <1.0 in bottled water, and 100.1 to 100.3 ± <0.8 in tap water	Electron/energy transfer	Metal ion sensor, water samples	104
39	Turn-on	Ethanol	298	350	Co^2+	0.782 μM	97.2 to 98.8 ± <3.5 in lake water, and 99.6 to 102.5 ± <2 in tap water	AIE phenomenon	Metal ion sensor, lake water, tap water	105
40	Turn-off	CH3CN–H2O (9/1, v/v)	384	429	Cu^2+	0.503 μM	102 ± 0.49 in tap water, and 100.5 ± 0.76 in drinking water	LMCT	Metal ion sensor, tap water, drinking water	106
41	Turn-off	DMSO	278	307	Cu^2+	0.037 μM	—	Electron transfer	Metal ion sensor	107
42	Turn-off	CH3CN	296	542	Cu^2+	2.4 μM	—	—	Metal ion sensor	108
43	Turn-off, blue-shift	CH3CN/H2O (1 : 1, v/v)	310	435	Cu^2+	10 μM	—	Nitrogen coordination and cation–π interaction	Metal ion sensor	109
44	Turn-on	CH3CN	340	369	Cu^2+	1.54 nM	—	ICT process, C=N isomerization and CHEF effect	Metal ion sensor	110
45	Turn-off	Aqueous medium	386	554	Cu^2+	0.35 μM	—	Electron transfer	Metal ion sensor	111
46 and 47	Turn-on	DMF	364 and 365 for sensors 46 and 47, respectively	411 and 416 for sensors 46 and 47, respectively	Cu^2+	1 nM for sensor 47	—	CHEF effect	Metal ion sensor	112
48	Turn-on	CH3CN	298	335 and 381	Cu^2+	27.4 nM	—	ICT process and C=N isomerization	Metal ion sensor	113
49	Turn-off	CH3CN/PBS (v/v =9/1)	365	624	Cu^2+	0.67 μM	—	Ligand to metal charge transfer (LMCT)	Metal ion sensor, fluorescence bio-imaging	114
50	Turn-on	Ethanol/HEPES buffer (95/5, v/v)	242	450	Zn^2+	0.5034 μM	—	(i) Restriction in rotation of the C=N group, (ii) blockage of the PET process, (iii) Zn^2+ induced disaggregation of sensor 50	Metal ion sensor, fluorescence bio-imaging	115
51	Turn-on	DMF	430	509	Zn^2+	8.6 nM	—	CHEF effect and C=N isomerization	Metal ion sensor	116
52	Turn-on	DMF–H2O (9 : 1, v/v)	380	500	Zn^2+	2.59 μM	—	PET process and C=N isomerization	Metal ion sensor	117
53	Turn-on	Ethanol	430	498	Zn^2+	0.173 μM	—	PET process, C=N isomerization and CHEF effect	Metal ion sensor	118
54	Turn-on	Water– DMSO (3 : 1 v/v)	340	427	Ag^+	3.15 μM	—	PET process, and CHEF effect	Metal ion sensor, real samples	119
55	Turn-off, red-shift	DMSO-buffered H2O (9/1, v/v)	365	500	Hg^2+	0.907 μM	—	Intramolecular charge transfer	Metal ion sensor, lake water, tap water, distilled water	120
56	Turn-on	Methanol/H2O (8 : 2, v/v)	365	503	Hg^2+	20 μM	—	CHEF effect	Metal ion sensor	121
57	Turn-on	CH3CN–H2O (9/1, v/v)	349	415 and 473	Hg^2+	22.68 nM	—	ESIPT process and C=N isomerization	Metal ion sensor	122
58	Turn-on	Aqueous medium	355	417	Hg^2+	0.270 μM	99.5 to 101.0 ± <3 in drinking water and tap water	Metal induced chemical change	Metal ion sensor, drinking water, and tap water	123
61	Turn-on	10% methanol solution (v/v, 0.2 M PBS, pH 7.0)	246	425	Cd^2+	2.7 nM	—	Interaction between Cd^2+ and nitrogen containing moieties in sensor 61	Metal ion sensor	124
62	Turn-on	CH3CN–H2O (8 : 2, v/v)	380	510	Cd^2+	14.8 nM	99.5 to 100.5 ± <1.5 in double distilled water, and 99.2 to 101.0 ± <1.5 in tap water	CHEF effect and C=N isomerization	Metal ion sensor, double distilled water, and tap water	125
63	Turn-off	Ethanol	341	385	Ag^+, Cu^2+ and Fe^3+	0.132 μM, 0.088 μM, and 0.037 μM, for Ag^+, Cu^2+ and Fe^3+, respectively	99.43 ± 0.14, 100.65 ± 0.38, and 101.4 ± 0.39 for Ag^+, Cu^2+ and Fe^3+, respectively.	PET process	Multi-metal ion sensor, water samples	126
64	Turn-off	THF/water (6 : 4, v/v)	259	622	Co^2+, Cu^2+ and Hg^2+	3.26 nM, 89.1 pM and 0.387 nM for Co^2+, Cu^2+ and Hg^2+, respectively	—	The heavy metal ion effect and paramagnetic behavior triggered non-radiative decay	Multi-metal ion sensor, distilled water, tap water	127
65	Turn-on with Hg^2+ and turn-off with Co^2+ and Cu^2+	DMSO–H2O (9 : 1, v/v)	460	540	Co^2+, Cu^2+ and Hg^2+	0.007 ppb, 0.29 ppb, and 0.20 ppb, respectively, for Co^2+, Cu^2+ and Hg^2+	—	The coordination and desulfurization mechanism for Hg^2+ and disruption of the intramolecular H-bond in the presence of Co^2+ or Cu^2+	Multi-metal ion sensor, tap, lake, drink, ditch and ground water	128
66	Turn-on, red-shift	CH3CN	355	500	Al^3+, Cr^3+ and Fe^3+	0.117 μM, 0.111 μM and 0.106 μM for Al^3+, Cr^3+ and Fe^3+	—	Excimer formation and PET process	Multi-metal ion sensor, multi-metal ion sensor, living cell imaging	129
67	Turn-on	Ethanol	510	572 and 580 for Zn^2+ and Fe^3+, respectively	Zn^2+, Fe^3+	0.664 μM and 0.212 μM for Zn^2+ and Fe^3+, respectively	—	Opening of the lactam ring	Multi-metal ion sensor, distilled water, lake water, running water, well water, and Yangtze river	130
68	Turn-off, red-shift	DMSO–H2O (1 : 1, v/v)	310	550	Cu^2+, Hg^2+, Ag^+	64.8 μM, 52.7 μM and 63.7 μM for Cu^2+, Hg^2+ and Ag^+	—	Coordination of Cu^2+, Hg^2+, and Ag^+ with phenolic hydroxyl O, imine N atoms and thiol S, hydrogen bonding	Multi-metal ion sensor	131
69	Turn-off	DMF	341	393	Fe^2+, Fe^3+ and Cu^2+	2.06 μM, 2.17 μM, and 2.48 μM for Fe^2+, Fe^3+ and Cu^2+, respectively	—	CHEQ	Multi-metal ion sensor	132
70	Turn-on	Ethanol–HEPES buffer (9 : 1, v/v)	420 and 520 for Zn^2+ and Fe^3+, respectively	490 and 583 for Zn^2+ and Fe^3+, respectively	Zn^2+ and Fe^3+	0.134 μM and 0.139 μM for Zn^2+ and Fe^3+	—	PET process, CHEF effect for Zn^2+ and opening of the spirolactam ring for Fe^3+	Multi-metal ion sensor, solid state sensor	133
71	Turn-on, red-shift	CH3CN	350	539 and 565 for Zn^2+ and Hg^2+, respectively	Zn^2+ and Hg^2+	0.040 μM and 0.011 μM for Hg^2+ and Zn^2+, respectively	—	ICT process and CHEF effect	Multi-metal ion sensor	134
72	Turn-on	DMSO–H2O (1 : 99, v/v)	300	460 and 434 for Zn^2+ and Ga^3+, respectively	Zn^2+ and Ga^3+	0.32 μM and 0.01 μM for Zn^2+ and Ga^3+, respectively	—	PET process	Multi-metal ion sensor	135
73	Turn-on	CH3CN	333	536	Al^3+ and Cr^3+	0.31 μM and 0.25 μM for Al^3+ and Cr^3+	—	ESIPT process and C=N isomerization	Multi-metal ion sensor	136
74	Turn-off response with Cu^2+ and turn-on response with Hg^2+	THF–H2O (7 : 3, v/v)	385	537	Cu^2+ and Zn^2+	70.6 nM and 0.116 μM for Cu^2+ and Zn^2+, respectively	—	ICT process, CHEQ for Cu^2+ and PET process, CHEF for Hg^2+	Multi-metal ion sensor	137
75	Turn-on	CH3CN	380	495	Fe^3+ and Cr^3+	4.57 μM and 2.75 μM for Fe^3+ and Cr^3+, respectively	—	ESIPT process and C=N isomerization	Multi-metal ion sensor	138
77 and 78	Turn-on	DMSO–H2O (1 : 2, v/v)	405 for sensors 77 and 455 for sensor 78	505 for sensors 77 and 509 for sensor 78	Al^3+	14.8 nM and 42.3 nM for sensors 77 and 78, respectively	—	ESIPT process, C=N isomerization and CHEF effect	Metal ion sensor, secondary sensor for F^− (turn-off), filter paper, cell imaging	139
79	Turn-on	DMSO–H2O (1 : 9, v/v, HEPES buffered)	450	525	Zn^2+	0.52 μM	—	ICT	Metal ion sensor, secondary sensor for PPi (turn-off), test paper strips and bioimaging of intracellular Zn^2+ in HeLa cells	140
80	Turn-off with Co^2+ and Cu^2+ and turn-on with Hg^2+	THF–H2O (1 : 99)	360	620	Co^2+-Hg^2+-Cu^2+	0.0937 μM, 0.0221 μM, and 0.0988 μM for Co^2+, Hg^2+ and Cu^2+, respectively		Metal ion binding in the cavity of the sensor through oxygen and nitrogen atoms	Multi-metal ion sensor, sequential sensor for Co^2+-Hg^2+-Cu^2+, test strip, living cell imaging	141
81	Turn-on	Ethanol–H2O buffered (9 : 1, v/v)	330	511	Zn^2+	1.2 nM		PET process, C=N isomerization and CHEF effect	Metal ion sensor, secondary sensor for PPi (turn-off), INHIBIT type logic gate	142
82	Turn-off	H2O–CH3CN (1 : 1, v/v) PBS buffered	450	521	Cu^2+	24.0 nM	—	Sensor 82 carbonyl and imino group binding with Cu^2+	Metal ion sensor, secondary sensor for glutathione (turn-on), bio-imaging of Cu^2+ in living cells and zebrafish	143
83	Turn-on, blue-shift	Methanol–water (3 : 1, v/v)	315	512	Zn^2+	0.078 μM	—	ESIPT and PET process and CHEF effect	Metal ion sensor, secondary sensor for ATP (turn-off), live cell imaging	144
84	Turn-on	Aqueous medium	350	588	Al^3+	1.68 μM	—	Al^3+ induced hydrolysis of sensor 84, forming the free –CHO group	Metal ion sensor, secondary sensor for CN^− (turn-off), Al^3+ detection in A549 cells through live cell imaging	145
85	Turn-off	DMSO–HEPES buffer (4 : 6, v/v)	338	419	Cu^2+	2.11 ppb	—	Cu^2+ induced hydrolysis of the imine functionality of sensor 85	Metal ion sensor, secondary sensor for PO4^3− (turn-on), paper strips	146
86	Turn-on	90% aqueous methanol	410	520	Al^3+	1.62 μM	—	CHEF effect and C=N isomerization	Metal ion sensor, secondary sensor for nucleotides and phosphate ions (turn-off), live cell imaging	147
87	Turn-on, red-shift	Ethanol	400	500	Al^3+	63.0 nM	—	Schiff base hydrolysis and CHEF effect	Metal ion sensor, live cell imaging in living HeLa cells	148
88	Turn-on	Methanol/H2O (9 : 1, v/v)	310	343	Al^3+	0.64 μM	—	ICT process, C=N isomerization and CHEF effect	Metal ion sensor, live cell imaging	149
89	Turn-on, red-shift	DMSO–H2O (1 : 1, v/v) solution	393	467	Cu^2+	0.26 μM	—	Strong coordination of sensor 89 with Cu^2+	Metal ion sensor, live cell imaging	150
90	Turn-on	DMSO–H2O (3/7, v/v) solution	346	472	Zn^2+	0.018 μM	—	ESIPT, CHEF effect	Metal ion sensor, living cell (African Monkey Vero cells) imaging	151
91	Turn-on	Ethanol–H2O (1/1, v/v) solution	545	584	Fe^3+	28.1 nM	—	Opening of the spirolactam ring	Metal ion sensor, living cell (SMMC-7721 cells) imaging	152
94	Turn-off	CH3CN/PBS (1 : 1, v/v)	450	540	Cu^2+	—	92.90 to 109.3 ± <2 in water samples	—	Metal ion sensor, live cell (MCF-7 and A549 cells) imaging	153
95	Turn-off	HEPES buffer	418	516	Cu^2+	0.16 μM	—	MLCT, LLCT	Metal ion sensor, two-photon fluorescence imaging of Cu^2+ in cellular mitochondria and in the liver and intestine of larval zebrafish	154
96	Turn-on	Ethanol	400	458	Al^3+	1.11 nM	—	ESIPT	Metal ion sensor, cell (MCF-7) imaging	155
97	Turn-on	CH3CN–water (8 : 2) mixture	440	530	Hg^2+	1 nM	—	PET and hydrolysis of the C=N group	Metal ion sensor, live cell (MCF-7 cells) imaging	156
98	Turn-on	Buffer solution (ethanol/Tris–HCl, v/v, 4/1)	380	458	Zn^2+	1.37 μM	—	PET process, C=N isomerization and CHEF effect	Metal ion sensor, live cell (A549) imaging	157
101	Turn-on	CH3OH/HEPES buffer (3/7, v/v)	350	435	Al^3+	0.05 μM	—	CHEF effect and C=N isomerization	Metal ion sensor, fluorescence imaging in living cells (SW480 cells)	158
102	Turn-on	Ethanol (0.01%)–HEPES buffer	321	450	Al^3+	0.1 μM	—	ESIPT process, C=N isomerization and CHEF effect	Metal ion sensor, fluorescence imaging in living cells (HepG2 cells)	159
103	Turn-on	DMSO/H2O (HEPES buffer, v/v = 3/7)	346	555	Zn^2+	60 nM	—	PET process, ESIPT process and CHEF effect	Metal ion sensor, living cell (Vero cells) imaging	160
104	Turn-on, red-shift	DMSO–H2O (7 : 3, v/v)	347	445	Hg^2+	2.82 μM	—	Excimer formation and CHEF effect	Metal ion sensor, live cell (HeLa cells) imaging	161
105	Turn-on	CH3CN	350	430	Cu^2+	86.3 nM	—	PET process and CHEF effect	Metal ion sensor, live cell (Raw264.7 cells) imaging	162
106	Turn-on	Methanol	373	517	Al^3+	0.12 μM	—	PET process	Metal ion sensor, live cell (HeLa cells) imaging	163
107	Turn-on	Methanol	307	430	Al^3+	0.137 μM	—	ESIPT process	Metal ion sensor, live cell (HeLa cells) imaging, in vivo bioimaging of Al^3+ in zebrafish larvae	164
108	Turn-on	Phosphate buffer–ethanol (7 : 3, v/v) solution	480	516	Au^3+	60 nM	—	Metal induced chemical change	Metal ion sensor, PC12 cell and zebrafish imaging	165
109	Turn-on	Buffered CH3CN–water (1 : 1)	370	450	Zn^2+	0.50 μM	—	Complexation of Zn^2+ with oxygen, nitrogen and sulfur atoms of sensor 109	Metal ion sensor, live cell (MCF-7 cells) imaging	166
110	Turn-off	Buffered ethanol–water (1 : 1, v/v)	400	566	Cu^2+	15.4 nM	—	Excited state energy/electron transfer	Metal ion sensor, live cell (HeLa cells) imaging	167
111	Turn-on	DMSO–H2O (9 : 1, v/v)	377	479	Al^3+	9.79 nM	—	PET process and CHEF effect	Metal ion sensor, live cell (HeLa cells) imaging	168
112	Turn-on, red-shift	Aqueous medium	350	460	Zn^2+	10.4 nM	—	C=N isomerization	Metal ion sensor, bacterial cells, viz. Staphylococcus aureus and E. coli	169
113	Turn-on	Methanol–H2O (1/9, v/v)	392	448	Al^3+ and Zn^2+	0.804 μM	For Al^3+, 96.33 and 96.24 in water and suspension, respectively	ESIPT, PET process, C=N isomerization and CHEF effect	Metal ion sensor, drug samples such as Gelucil and Zincovit, live cell (HeLa cells) imaging	170
							For Zn^2+, 98.60 and 95.05 in water and suspension, respectively
114	Turn-on	Methanol–water (2 : 1, v/v)	340	442	Pb^2+	8 nM	105 ± 1.7, 117 ±1.6, and 93 ±1.3 in tap water, river water and urine samples, respectively	PET process and CHEF effect	Metal ion sensor, BSA medium, environmental samples such as tap water, river water and urine samples	171
115	Turn-on	CH3CN–H2O (1 : 1, v/v)	360	430	Al^3+	50 μM	—	PET process, CHEF effect	Metal ion sensor, BSA medium, live cell (human embryonic kidney (HEK)) imaging	172
116	Turn-on	Methanol–water (2 : 1, v/v)	310	373	Al^3+	0.903 μM	98.5% to 101.91% in food samples	PET process, C=N isomerization and CHEF effect	Metal ion sensor, BSA medium	173
117 and 118	Turn-off	Aqueous medium	370	540	Cu^2+, Fe^3+	—	—	Host–guest interaction	Anticancer	174
119-124	-	DMSO–H2O (2 : 8, v/v)	—	422 nm for 119-121, and 474 nm for 122-124	—	—	—	Energy dependent, antitumor mechanism of oxidation	Anticancer	175
125	Turn-on	Ethanol–H2O (4 : 6, v/v)	397	550 nm	Fe^3+	0.8 ppb	—	ESIPT	Cancer cells	176
126	Turn-off	DMSO	360	475	Cu^2+	—	—	PET process and CHEF effect or due to structural change in the ligand upon complexation	Optical sensor, catalytic oxidation, and Lewis acid–base colour indicator	177
127	Turn-off	DMSO–H2O (1 : 1, v/v)	414	549	Hg^2+	0.284 nM	118.5 to 154.0, 108.0 to 114.0, and 97.7 to 112.0 in lake water, tap water, and mineral water, respectively	Hg^2+ interaction with the nitrogen atom of the imine group and with the oxygen atom	Adsorption, real water samples, and logic gate	178
128 and 129	Turn-on	DMF–H2O (v:v = 1 : 9)	360 and 300 for sensors 128 and 129, respectively	483 and 467 for sensors 128 and 129, respectively	Al^3+	3.2 nM and 29.0 nM with sensors 128 and 129, respectively	—	PET and ESIPT process, CHEF effect	Metal ion sensor, lake water and tap water	179
130	Turn-off, hypso-chromic shift	DMF/H2O (2/3, v/v)	450	560	Cu^2+	28.1 nM	94 to 105 ± <2 in tap water, 106 to 117 ± <2.7 in river, and 87 to 93 ± <2.6 in simulated urine samples	Paramagnetic nature of Cu^2+ and CHQF mechanism	Metal ion sensor, real samples such as water and food	180
131	Turn-on	Methanol	401	524	Al^3+	7.4 nM	100.96 to 101.84 ± <0.5 in ultrapure water and 100.0 to 100.10 ± <1.0 in tap water	ICT process and CHEF effect	Metal ion sensor, ultrapure water, tap water	181
132	Turn-on	CH3CN	353	487 and 532	Mg^2+	19.1 ppb	—	C=N isomerization and CHEF effect	Metal ion sensor, tap, ground and lake water	182
133	Turn-off	THF–H2O (4/1, v/v)	428	565	Cu^2+	16.4 nM	99.10 to 102.90 ± <2.01 in lake water and 98.49 to 102.37 ± <2.07 in tap water	ESIPT process, LMCT	Metal ion sensor, running water, artificial lake	183
134	Turn-on	DMF–HEPES (1 : 1, v/v)	350	531	Al^3+	86.5 nM	99.7 to 101.7 ± <0.5 in yellow river and 99.0 to 103.4 ± <0.5 in tap water	PET process and CHEF effect	Metal ion sensor, yellow river and tap water	184
135	Turn-on	Tris–HCl buffer (1 : 1, v/v) solution	350	526	Al^3+	0.34 μM	100.5 to 102.3 ± <0.3 in yellow river and 99.5 to 102.8 ± <0.3 in tap water	PET process and CHEF effect	Metal ion sensor, yellow river and tap water	185
136	Turn-on	Ethanol–HEPES buffer (95 : 5, v/v)	273	473 and 499	Al^3+	0.108 μM	—	PET process	Metal ion sensor, live cell (HeLa cells) imaging, lake, river and distilled water	186
137	Turn-on, blue-shift	THF–H2O (1 : 1, v/v) mixture	490	550	Zn^2+	38.9 nM	—	ESIPT, PET, non-radiative process and CHEF effect	Metal ion sensor, inkless rewritable paper, self-erasing paper, live cell (SiHa cells) imaging	187
138	Turn-on	—	—	Strong emission band between 550 and 700		—	—	Enol to keto form photoisomerization	Rewritable paper and UV sensor	188
141	Turn-on	CH3CN–H2O mixture	350	463	Cu^2+	0.23 μM		CHEF effect	Metal ion sensor, mechanochromic fluorescence	189
143	Turn-off	HEPES buffered methanol–H2O (4/1, v/v)	432	508	Cu^2+ and Zn^2+	0.212 μM and 71.9 nM for Cu^2+ and Zn^2+, respectively	—	PET process	Multi-metal ion sensor, solid state piezochromic luminescence	190
145	Turn-on	THF–H2O	367	408 and 481	—	—	—	AIE, ICT	Anti-counterfeiting and printed electronics	191
146 and 147	Turn-on	DMSO	372	504 and 444 for sensors 146 and 147, respectively	Zn^2+	—	—	ICT process	Color tunable	192
148–152	Turn-on	DMSO	365	447, 440, 436, 433, and 430 for complexes 148, 149, 150, 151, and 152, respectively	Zn^2+	—	—	Relaxation due to intra-ligand transition	Organic white light emitting devices	193
153	Blue fluorescent	CHCl3, THF, ethanol, CH3CN, and solid state	345–370	430–460	Zn^2+	—	—	Combination of monomer–excimer emission	Blue fluorescent OLED	194
154	Turn-on	HEPES buffered ethanol–H2O (9 : 1, v/v)	375	520	Zn^2+	1.59 μM	—	PET process, C=N isomerization and CHEF effect	Metal ion sensor, logic gate, TLC supported paper strips, and bioimaging of intracellular Zn^2+ in Vero cells (African green monkey kidney cells)	195
155	Turn-on	Methanol–water (1 : 1, v/v)	370	464	Al^3+	10 μM	—	PET process	Metal ion sensor, secondary sensor for PO4^3−, logic gate, BSA medium, live cell (rat L6 myoblast cells) imaging	196
156 and 157	Turn-on	Methanol	375	465 and 464 for sensors 156 and 157, respectively	Al^3+	0.525 μM and 2.38 μM for sensors 156 and 157, respectively	—	CHEF effect	Metal ion sensor, logic gate	197
158	Turn-on	Aqueous medium	359	454	Zn^2+	0.477 μM	—	ESIPT process, C=N isomerization, and CHEF effect	Metal ion sensor, logic gate, keypad lock, and fluorescence imaging of Zn^2+ in rice seedlings	198
159	Turn-on	DMSO–water (9 : 1, v/v)	482	With Zn^2+ (545) and Cd^2+ (560)	Zn^2+ and Cd^2+	2.7 nM (for Zn^2+) and 6.6 nM (for Cd^2+)	For Zn^2+, 83 to 91 in drinking water, and for Cd^2+, 103 to 104 in drinking water	CHEF effect	Multi-metal ion sensor, logic gate, TLC plates, and drinking water	199
160	Turn-on	CH3CN	400	542	Ni^2+	8.67 nM	89 to 104.7 ± <0.2 in tap water, 94 to 98.7 ± <0.2 in river water, and 97.3 to 109.5 ± <0.2 in bottled water	CHEF effect	Metal ion sensor, logic gate, live cell imaging (HeLa cells), real samples	200
161	Turn-off	CH3CN–H2O (7 : 3)	344	531	Fe^3+	0.81 μM	98.56 ± 0.19, 98.81 ± 0.35, 97.91 ± 0.21, 98.81 ± 1.03, 98.46 ± 0.28, 97.96 ± 0.23, and 94.51 ± 0.26 in tap water, lake, well, mineral water, distilled water, purified water and human serum albumin, respectively	ICT process and C=N isomerization	Metal ion sensor, logic gate, test paper, water samples and fluorescent bioimaging	201

---

2. Strategies and mechanistic insights into fluorescence-based metal ion detection

The general approach for metal ion detection involves sensor–metal ion coordination interactions. A Schiff base sensor contains a binding functional unit that can coordinate with a metal ion, causing enhancement/quenching of emission intensity, ratiometric fluorescence change, or enhancement/quenching of fluorescence color of the solution, leading to chelation-enhanced fluorescence (CHEF) or chelation-enhanced quenching (CHEQ) effect. The commonly used mechanisms for metal ion detection are formation/disruption of self-assembly/disassembly, aggregation/disaggregation, photo-induced electron transfer (PET), intra/intermolecular charge transfer (ICT), metal to ligand charge transfer (MLCT), ligand to metal charge transfer (LMCT), Förster resonance energy transfer (FRET), excited state intramolecular proton transfer (ESIPT), inner filter effect (IFE), metal-induced chemical change and C=N isomerization.

2.1 Self-assembly/disassembly

Self-assembled molecules are versatile materials for a variety of applications, including analyte detection and fluorescence imaging. In the sensing system, the fluorescence of the sensor can be modulated by analyte interaction and the self-assembly process. In the literature, various strategies such as self-assembly-disassembly driven detection of metal ions, metal ion-induced disassembly, self-assembly coupled with the AIE phenomenon, etc. have been used for fluorescence-based detection of metal ions.^202,203 Herein, we will discuss the self-assembly-based mechanism for metal ion detection using fluorescent Schiff base molecules.

Wu et al. developed pyrene-based Schiff base 1 with AEE (Aggregation emission enhancement) properties for fluorescence-based detection of Cu²⁺.⁷² In H₂O–DMF (1 : 1, v/v) solution, sensor 1 was weakly fluorescent (Fig. 2 and Table 2). In the presence of Cu²⁺, sensor 1 displayed strong fluorescence enhancement (4.5-fold) at 455 nm (λ_ex = 370 nm). The LOD was 35 nM for Cu²⁺. The recognition mechanism was attributed to Cu²⁺ induced self-assembly. TEM and SEM results confirmed the spherical particle morphology and aggregation of sensor 1 in the presence of Cu²⁺.

Fig. 2 The sensing mechanism for Cu²⁺ detection using sensor 1. Reproduced with permission from ref. 72 Copyright 2018, Elsevier.

2.2 Aggregation/disaggregation

Recently, Schiff base sensors with AIE properties have attracted tremendous attention for metal ion detection. Schiff base-based sensors are excellent AIE active molecules in which free rotation around the C=N band is restricted upon coordination with metal ions, leading to emission change. These sensors are weakly emissive in dilute solution but emit strongly upon aggregation assisted by probe–metal ion interaction in solution or in solid-state.

Singh et al. developed probe 2 with AIE and ESIPT properties for Cu²⁺ detection in an aqueous medium (Fig. 3A and Table 2).⁷³ On excitation at 390 nm, the fluorescence spectrum of probe 2 displayed green emission at 526 nm, attributed to the AIE + ESIPT process. Upon gradual addition of an increasing concentration of Cu²⁺, the emission spectrum of probe 2 revealed quenching of emission intensity and saturation was achieved after the addition of 1 μM Cu²⁺. The limit of detection was 0.261 ppb for Cu²⁺. The coordination of probe 2 with Cu²⁺ resulted in the disassembly of self-assembled aggregates of probe 2 from self-assembled nanorods into smaller spherical nanostructures (Fig. 3B).

Fig. 3 (A) The chemical structure of sensor 2. (B) SEM images of (a) sensor 2; (b and c) sensor 2 + Cu²⁺; TEM images of (d) sensor 2; (e and f) sensor 2 + Cu²⁺; (inset) SAED of aggregates. Reproduced with permission from ref. 73 Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

2.3 Photoinduced electron transfer (PET)

PET is a process where electron transfer occurs from the donor to the acceptor in the excited state. The coordination of metal ions with the Schiff base sensor may favor or disrupt the PET process, resulting in emission change.

Chakraborty et al. developed pyrene-based Schiff base 3 with AIE properties for fluorescence-based detection of Cu²⁺ (Fig. 4 and Table 2).³² Sensor 3 was found to be selective for Cu²⁺ with an LOD of 2.5 μM. Sensor 3 was weakly fluorescent because of the PET process between the imine bond and the pyrene ring. The complexation of sensor 3 with Cu²⁺ blocked the PET process and resulted in fluorescence enhancement.

Fig. 4 PET-based mechanism for Cu²⁺ detection with sensor 3. Reproduced with permission from ref. 32 Copyright 2018, Elsevier.

Wang et al. developed coumarin-based Schiff base 4 with AIRE (Aggregation induced ratiometric emission) properties and employed it as a turn-off sensor for Cu²⁺ detection (Fig. 5 and Table 2).⁴¹ The complexation of Cu²⁺ with sensor 4 caused activation of the PET process between coumarin and the Cu²⁺ center, resulting in fluorescence quenching.

Fig. 5 PET-based mechanism for Cu²⁺ detection with sensor 4. Reproduced with permission from ref. 41 Copyright 2018, Elsevier.

Dong et al. synthesized Schiff base 5 as a turn-on sensor for Zn²⁺ detection.⁷⁴ Sensor 5 showed excellent sensitivity for Zn²⁺ over other tested metal ions. In the off-state, the PET process occurs from the HOMO of the azomethine group to the HOMO of the conjugated aromatic ring, whereas in the on-state the PET process is inhibited due to coordination of Zn²⁺ with the nitrogen atom and oxygen atom of the azomethine group, and electron transfer occurs from the LUMO to the HOMO of the conjugated aromatic ring (Fig. 6A, B and Table 2).

Fig. 6 (A) Sensing mechanism of sensor 5 for Zn²⁺ detection. (B) Orbital energy diagrams showing (a) fluorescence quenching by the PET process; (b) fluorescence enhancement due to the CHEF effect. Reproduced with permission from ref. 74 Copyright 2017, Elsevier.

2.4 Intra/intermolecular charge transfer (ICT)

In the ICT process, energy relaxation of the excited molecule occurs via the charge transfer phenomenon. Charge transfer can be intramolecular where redistribution of charge occurs within the excited molecule or intermolecular in which charge is transferred from the excited donor molecule to the neighboring acceptor molecule.

Rout et al. designed triazole-based Schiff base 6 as a turn-on fluorescent sensor for Cu²⁺ and Pb²⁺ detection (Fig. 7 and Table 2).⁷⁵ In methanol–tris buffer (1 : 1) medium, the free sensor displayed weak emission at 440 nm (λ_ex = 340 nm), attributed to the PET process from the triazole and imine moiety to the fluorophore. In the presence of Cu²⁺/Pb²⁺, the fluorescence spectrum of sensor 6 displayed an increase in emission intensity attributed to blockage of the PET process and strengthening of the ICT process. The limit of detection was 1.2 μM and 0.9 μM for Cu²⁺ and Pb²⁺, respectively.

Fig. 7 ICT- and PET-based sensing mechanism for Pb²⁺ and Cu²⁺ detection with sensor 6. Reproduced with permission from ref. 75 Copyright 2019, Royal Society of Chemistry.

2.5 Metal to ligand charge transfer (MLCT)

Pannipara et al. developed an AIE-active Schiff base 7 as a turn-off sensor for Cu²⁺ detection (Fig. 8 and Table 2).⁷⁶ In the THF–H₂O (9 : 1, v/v) mixture, sensor 7 showed intense emission at 484 nm. Upon addition of Cu²⁺ to the solution of sensor 7, a significant decrease in fluorescence intensity was observed, attributed to metal to ligand charge transfer between Cu²⁺ and sensor 7, resulting in the CHEQ effect.

Fig. 8 Chemical structure of sensors 7 and 8. Reproduced with permission from ref. 76 Copyright 2017, Elsevier.

2.6 Ligand to metal charge transfer (LMCT)

Bhardwaj et al. developed chromone-based fluorescent organic nanoparticles (FON-8) for Cu²⁺ and CN⁻ detection in an aqueous medium (Fig. 8 and Table 2).⁷⁷ The fluorescence spectrum of FON-8 showed a gradual decrease in emission intensity at 517 nm (λ_ex = 405 nm) upon the addition of an increasing concentration of Cu²⁺, attributed to ligand to metal charge transfer. The limit of detection was 12.3 nM for Cu²⁺. From the DFT method, for sensor 8, the HOMO was located on the naphthalene moiety and the LUMO was located on the whole sensor 8. However, in the case of the 8-Cu²⁺ complex, the LUMO was shifted toward the metal center, demonstrating ligand to metal charge transfer.

2.7 Förster resonance energy transfer (FRET)

FRET is a non-radiative process where energy transfer occurs from excited donors to acceptor molecules. The metal–Schiff base complexation may inhibit or trigger the FRET process resulting in emission change.

Yang et al. developed a novel method for Cu²⁺ detection through the AIE and FRET strategy in an aqueous medium (Fig. 9 and Table 2).⁷⁸ The Schiff base sensor 9 exhibited AIE properties with green emission at 525 nm. In the presence of Cu²⁺, sensor 9 underwent fluorescence quenching and the limit of detection was 0.132 μM. However, through the sensor 9–NiR FRET system, the LOD for Cu²⁺ was reduced to 9.12 nM upon monitoring emission intensity at 620 nm.

Fig. 9 FRET-based sensing mechanism for Cu²⁺ detection with sensor 9. Reproduced with permission from ref. 78 Copyright 2019, Elsevier.

2.8 Excited-state intramolecular proton transfer (ESIPT)

In the ESIPT process, generally, the transfer of protons occurs from the donor (NH, OH, etc.) to the acceptor (carbonyl, nitrogen, etc.). ESIPT based sensors generally show two emission bands, one due to the enol-form, and the other due to the tautomeric keto-form. Additionally, ESIPT based sensors show a large Stokes shift, high quantum yield, a low energy gap, etc. Usually, salicylimine type Schiff bases exhibit the ESIPT phenomenon because of keto–enol tautomerization.^48,49

Puri et al. developed Schiff base sensor 10 with AIE properties for fluorescence-based detection of Al³⁺ in methanol solution.⁷⁹ On excitation at 360 nm, sensor 10 displayed emission at 548 nm accompanied by a large Stokes shift (188 nm), attributed to the ESIPT phenomena. Upon addition of Al³⁺ to the solution of sensor 10, further enhancement in emission intensity was observed with emission maxima at 476 nm and a Stokes shift of 116 nm, attributed to metal–template aggregation (Fig. 10 and Table 2).

Fig. 10 ESIPT based sensing mechanism for Al³⁺ detection with sensor 10. Reproduced with permission from ref. 79 Copyright 2018, Royal Society of Chemistry.

2.9 Inner filter effect (IFE)

The inner filter effect is a radiative process where the excitation and/or emission energy of the probe is absorbed by the absorber in the sensing system. Earlier, it was considered as a source of error in fluorescence measurements. Nowadays, researchers have shown more interest in developing IFE-based sensors because (i) this approach is simple and straightforward, i.e. it does not require sensor–analyte interaction, and (ii) absorber's absorbance change transforms exponentially into emission change, resulting in increased sensitivity. The basic requirement for IFE to act is spectral overlap between the absorber's absorbance band and the excitation and/or emission band of the sensor.²⁰⁴

Abbasi and Shakir et al. developed IFE-based Schiff base sensor 11 for Cr (VI) detection in an aqueous medium (Fig. 11A and Table 2).⁸⁰ On excitation at 350 nm, sensor 11 showed emission maxima at 438 nm. Upon addition of Cr(VI), the fluorescence of sensor 11 was adequately quenched. The sensing mechanism was attributed to IFE, based on the good overlap between the absorption band of Cr(VI) (centered at 260 nm, 350 nm, and 440 nm) and the excitation band (at 350 nm) and/or emission band (at 438 nm) of sensor 11 (Fig. 11B). The limit of detection for Cr(VI) was 0.175 μM.

Fig. 11 (A) Chemical structure of sensor 11. (B) (a) Absorbance spectrum of Cr(IV); normalized fluorescence (b) excitation and (c) emission spectrum of sensor 11. Reproduced with permission from ref. 80 Copyright 2018, Royal Society of Chemistry.

2.10 Metal-induced chemical change and C=N isomerization

Sometimes, the interaction of metal ions with the sensor results in some chemical reaction that changes the chemical structure of the ligand, resulting in emission change.

Hu et al. developed fluorene-naphthalene-based Schiff base sensor 12 for Al³⁺ and Cr³⁺ detection.⁸¹ In CH₃CN–H₂O (7 : 3, v/v) solution, free sensor 12 was weakly emissive (λ_ex = 305 nm). In the presence of Al³⁺ or Cr³⁺, sensor 12 underwent fluorescence enhancement 722 times and 923 times, respectively. The limit of detection was 0.887 nM and 0.631 nM for Al³⁺ and Cr³⁺, respectively. The recognition mechanism was a reaction-based process, wherein first oxidation of the C=N group of sensor 12 C=N to O = C-NH occurred in the presence of Al³⁺ or Cr³⁺, and then coordination, which resulted in inhibition of C=N isomerization and in strong emission (Fig. 12 and Table 2).

Fig. 12 Sensing mechanism of sensor 12 with Al³⁺. Reproduced with permission from ref. 81 Copyright 2021, Elsevier.

3. Applications

3.1 Metal ion sensors

Some metal ions are very crucial for life such as Fe³⁺, Zn²⁺, K⁺, etc., whereas others such as Hg²⁺, Pb²⁺, Cd²⁺, etc., may cause serious environmental and health issues.¹ Accordingly, detection of metal ions present in trace amounts is important. Recently, fluorescent Schiff base sensors for metal ion detection have grabbed significant attention because of their simplicity, selectivity, sensitivity, real-time detection, fast response time, and non-destructive properties. In addition, Schiff bases are eminent hard bases and have electron-rich oxygen and nitrogen donor atoms for coordination with metal ions.

3.1.1 Mg²⁺ sensor. Magnesium is an essential mineral for human beings, playing several important functions such as blood pressure regulation, bone development, muscle contraction, immune system improvement, catalysis of many enzyme-driven biochemical reactions, acting as a cofactor for many enzymes, etc.²⁰⁵ However, both excess intake or deficiency of magnesium ions results in serious diseases such as Alzheimer's disease, hypertension (due to excess intake of magnesium), and hypocalcemia, diabetes, migraine (due to deficiency of magnesium).^206,207 Thus, the detection of magnesium using fluorescent chemosensors is an interesting area of research.

Wang et al. developed Schiff base 13 as a turn-on sensor for fluorescence-based detection of Mg²⁺ (Fig. 13 and Table 2).⁸² In ethanol solution, sensor 13 displayed weak emission at wavelength 547 nm (λ_ex = 491 nm). Upon addition of Mg²⁺, sensor 13 displayed a 70-fold increase in fluorescence intensity at 547 nm. The detection limit was 0.516 μM for Mg²⁺. Job's plot showed 1 : 1 (13/Mg²⁺) stoichiometry and the association constant was 3.33 × 10⁴ M⁻¹ as calculated using the Benesi–Hildebrand equation. The chelation of sensor 13 with Mg²⁺ through oxygen and nitrogen atoms resulted in blockage of the PET process, causing fluorescence enhancement through the CHEF effect.

Fig. 13 Chemical structure of sensors 13–28.

3.1.2 Ca²⁺ sensor. Calcium ions play key roles in human biology, for example, signal transduction, muscle contraction, bone formation, etc. However, the abnormal concentration of calcium ions in the body is an indicator of serious health problems.²⁰⁸ Excess calcium ion concentration (hypocalcaemia) in the body can cause hyperparathyroidism, sarcoidosis, and cardiac arrhythmias,²⁰⁹ whereas its deficiency (hypocalcaemia) may lead to osteoporosis, vitamin D deficiency, and hypoparathyroidism.²¹⁰ Therefore, the use of fluorescent sensors for Ca²⁺ detection is a topic of immense interest for health monitoring and diagnostic purpose.

Saleh et al. reported Schiff base sensor 14 as a ratiometric fluorescent sensor for Ca²⁺ (Fig. 13 and Table 2).⁸³ In HEPES buffer solution (pH = 7.2), the free sensor showed a broad band with emission maxima at 545 nm (λ_ex = 370 nm). The fluorescence titration with Ca²⁺ revealed ratiometric fluorescence change with the simultaneous increase in emission intensity at 483 nm and decrease in emission intensity at 545 nm. The limit of detection was 0.3 μM Ca²⁺ and the stoichiometry of the complex was 1 : 1 (14 : Ca²⁺). The binding of Ca²⁺ with sensor 14 at the coordination sites, i.e., OH, C=N, SH, decreases the ICT process present in the sensor and results in ratiometric fluorescence change.

3.1.3 Al³⁺ sensor^{96,97,211–216}. Aluminum is the third most abundant element (8.3%) in the earth crust after oxygen and silicon. While aluminum has no biological role, over-exposure to aluminum may lead to neurotoxicity and Alzheimer's disease.^217,218 Al³⁺ is a hard acid and coordinates well with hard bases such as oxygen and nitrogen atoms of the Schiff base ligand.

Wang et al. synthesized an imidazo[2,1-b]thiazole-based Schiff base (15) as a turn-on sensor for Al³⁺ detection (Fig. 13 and Table 2).⁸⁴ In HEPES buffer solution, sensor 15 was almost non-fluorescent. Upon addition of Al³⁺, the fluorescence spectrum of sensor 15 displayed emission enhancement at 488 nm (λ_ex = 365 nm) along with a color change of the solution from colorless to bluish-green (Fig. 14). The complexation stoichiometry was 1 : 1 (15/Al³⁺). The limit of detection and the association constant for Al³⁺ were 15.0 nM and 7.42 × 10⁴ M⁻¹, respectively. The sensing mechanism could be attributed to inhibition of the PET process and the CHEF effect.

Fig. 14 Fluorescence spectrum of sensor 15 in the presence of different metal ions. Inset: photographs of 15 (L) and 15 + Al³⁺ solution under UV lamp illumination. Reproduced with permission from ref. 84 Copyright 2021, Wiley-VCH GmbH.

Chang et al. synthesized dansyl-based sensor 16 for fluorescence-based detection of Al³⁺ (Fig. 13 and Table 2).⁸⁵ In methanol solution, sensor 16 exhibited weak emission at 550 nm (λ_ex = 316 nm). Fluorescence titration with Al³⁺ revealed a prominent increase in emission intensity accompanied by a blue shift of the emission maxima from 550 nm to 448 nm (Δλ_shift = 102 nm). Job's plot revealed the 1 : 1 (16 : Al³⁺) stoichiometry of the complex (Fig. 15A). The association constant was 9.40 × 10⁶ M⁻¹ and the limit of detection was 14 nM for Al³⁺. In the ¹H NMR titration experiment, the presence of a broad peak at 9.9 ppm and two singlets of the imine protons suggested Al³⁺ coordination at the imine site, thus inhibiting C=N isomerization, causing the CHEF effect (Fig. 15B).

Fig. 15 (A) Fluorescence titration of sensor 16 with the increasing concentration of Al³⁺ in CH₃OH. (B) ¹H NMR titration of sensor 16 with the increasing concentration of Al³⁺ in CD₃OH. Reproduced with permission from ref. 85 Copyright 2015, Elsevier.

Hossain et al. developed coumarin-based Schiff base chemosensor 17 for Al³⁺ detection (Fig. 13 and Table 2).⁸⁶ Sensor 17 exhibited high selectivity for Al³⁺ among various other metal ions in DMF–H₂O (v/v, 9 : 1). Upon addition of Al³⁺, sensor 17 underwent a ∼34-fold increase in fluorescence intensity at 490 nm (λ_ex = 390 nm). The stoichiometry of the complex was 1 : 1 (17 : Al³⁺) and the detection limit was 6.7 μM for Al³⁺. The turn-on fluorescence response of sensor 17 with Al³⁺ was associated with the combined effect of ICT, CHEF, and C=N isomerization.

Li et al. designed and synthesized chromone-based sensor 18 as a turn-on sensor for Al³⁺ (Fig. 13 and Table 2).⁸⁷ In ethanol solution, sensor 18 showed weak emission in the range of 440–690 nm. Among various metal ions, sensor 18 underwent 171-fold fluorescence enhancement with Al³⁺ at 502 nm (λ_ex = 428 nm) and to a lesser extent with Mg²⁺ and Zn²⁺, whereas other metal ions had an insignificant effect on the fluorescence spectrum of sensor 18. The limit of detection was 38.7 nM for Al³⁺. The binding constant was 1.74 × 10⁷ M⁻¹ and the stoichiometry of the complex was 1 : 1 (18 : Al³⁺). The turn-on mechanism could be ascribed to the deactivation of the PET phenomenon upon chelation of Al³⁺ with sensor 18, forming a rigid system, and causing the CHEF effect.

Li et al. prepared pyrazine-based sensor 19 for fluorescence-based detection of Al³⁺ (Fig. 13 and Table 2).⁸⁸ Sensor 19 was weakly fluorescent with emission at 391 nm (λ_ex = 350 nm) in ethanol solution. Upon addition of various metal ions, viz. Al³⁺, Ca²⁺, Ba²⁺, Mg²⁺, Fe³⁺, Cr³⁺, Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺, Hg²⁺, Cd²⁺ and Zn²⁺, sensor 19 showed a drastic increase in fluorescence intensity at 488 nm with Al³⁺ and to a lesser extent with Fe³⁺ and Cr³⁺ ions, whereas other metal ions had an insignificant effect. The binding constant was 1.35 × 10⁴ M⁻¹ as calculated using the Benesi–Hildebrand equation. The detection limit was 0.1 μM. Job's plot revealed the 1 : 1 (19 : Al³⁺) stoichiometry of the complex. The complexation of Al³⁺ with sensor 19 caused the deactivation of the PET process and resulted in turn-on emission through the CHEF effect.

Liu et al. synthesized chromone-based Schiff base sensor 20 and evaluated it for Al³⁺ detection (Fig. 13 and Table 2).⁸⁹ In ethanol solution, sensor 20 (λ_ex = 410 nm) was non-fluorescent. On addition of various metal ions to the solution of sensor 20, only Al³⁺ caused significant (∼500-fold) fluorescence enhancement at 480 nm. The association constant of the complex was 1.21 × 10⁴ M⁻¹ as calculated using the Benesi–Hildebrand equation. The detection limit was 31.9 nM for Al³⁺. Job's plot and ESI mass spectra revealed the 1 : 1 (20 : Al³⁺) stoichiometry of the complex. The coordination of Al³⁺ caused inhibition of the PET phenomenon and resulted in turn-on fluorescence through the CHEF effect.

Liu and Yang further synthesized chromone-based Schiff base sensor 21 for Al³⁺ detection (Fig. 13 and Table 2).⁹⁰ Sensor 21 exhibited high selectivity and sensitivity for Al³⁺ detection among various other metal ions. In ethanol-H₂O (v/v, 3 : 1), upon addition of Al³⁺, sensor 21 underwent a drastic increase in fluorescence intensity at 475 nm and 508 nm (λ_ex = 420 nm). The detection limit was 0.182 μM for Al³⁺. Job's plot revealed 2 : 1 (21 : Al³⁺) stoichiometry and the association constant was 9.24 × 10³ M⁻¹. Sensor 21 coordinates with Al³⁺ through two nitrogen atoms of azomethine groups forming a 2 : 1 (21:Al³⁺) complex, which inhibits the PET process and results in a turn-on fluorescence response. Sensor 21 also found potential application in the detection of Al³⁺ in solid-state.

Qin and Yang prepared bis-Schiff base sensor 22 for Al³⁺ detection (Fig. 13 and Table 2).⁹¹ In an aqueous solution, free sensor 22 was weakly fluorescent with emission maxima at 506 nm (λ_ex = 389 nm). Among various metal ions, sensor 22 displayed strong fluorescence enhancement only with Al³⁺ and to a lesser extent with Zn²⁺. Fluorescence titration with Al³⁺ showed an increase in fluorescence intensity with a blue shift of the emission maxima from 506 nm to 460 nm (Δλ_shift = 40 nm). The detection limit was 0.69 μM for Al³⁺. Job's plot revealed 1 : 1 (22 : Al³⁺) stoichiometry and the binding constant was 1.67 × 10⁴ M⁻¹. The turn-on fluorescence response could be attributed to the inhibition of the ICT phenomenon due to the chelation of sensor 22 with Al³⁺ through imine nitrogen, the oxygen atom of the carboxylate group, and the oxygen atom of the phenolic hydroxyl group.

Qin et al. designed Schiff base sensor 23 for fluorescence-based detection of Al³⁺ (Fig. 13 and Table 2).⁹² In ethanol solution, the fluorometric behavior of sensor 23 toward various metal ions revealed high selectivity only for Al³⁺. The detection limit was 0.2 μM. The complexation stoichiometry was 1 : 1 (23 : Al³⁺) and the binding constant was 7.54 M⁻¹. In the presence of Al³⁺, sensor 23 underwent a drastic increase in emission intensity at 420 nm (λ_ex = 363 nm). The sensing mechanism could be attributed to the inhibition of the PET phenomenon due to the chelation of the –C=N– group nitrogen atom with Al³⁺, causing the CHEF effect.

Ruo et al. synthesized 5,5′-methylenebis(salicylaldehyde)-based Schiff base sensors 24 and 25 for Al³⁺ detection (Fig. 13 and Table 2).⁹³ In DMSO-H₂O solution, both sensors 24 and 25 are weakly fluorescent with emission at 491 nm and 473 nm, respectively. Among various metal ions, sensors 24 and 25 displayed a 70-fold and 40-fold increase in fluorescence intensity at 493 nm and 476 nm, respectively, only with Al³⁺. The binding constants of sensors 24 and 25 with Al³⁺ were 2.01 × 10⁴ M⁻¹ and 5.46 × 10⁵ M⁻¹, respectively, and the stoichiometry of the complex was 1 : 1 (24/25 : Al³⁺). The detection limit of sensors 24 and 25 for Al³⁺ was 14.9 nM and 15.1 nM, respectively. The ¹H NMR titration experiment revealed that sensors 24 and 25 chelated with Al³⁺ through the nitrogen atom of the imine moiety and the oxygen atom of the hydroxyl group, which resulted in deactivation of the PET process and fluorescence enhancement through the CHEF effect.

Shoora et al. synthesized Schiff base sensors 26 and 27 for fluorescence-based detection of Al³⁺ (Fig. 13 and Table 2).⁹⁴ Both sensors 26 and 27 are weakly fluorescent in methanol solution. Among various metal ions, both sensors 26 and 27 showed strong fluorescence enhancement with Al³⁺ and to a lesser extent with Zn²⁺ and Cr³⁺, whereas other metal ions had an insignificant effect. Fluorescence titration of sensors 26 and 27 with Al³⁺ revealed an increase in emission intensity at 506 nm (λ_ex = 410 nm) and 489 nm (λ_ex = 410 nm), respectively. The increase in fluorescence intensity was associated with a change in the color of the solution from colorless to blue, which can be detected by the naked eye under UV lamp illumination. The detection limit of sensors 26 and 27 for Al³⁺ was 47.9 nM and 82.8 nM, respectively. Job's plot indicated 1 : 1 (26/27 : Al³⁺) complex stoichiometry and the stability constant was 1.41 × 10⁴ M⁻¹ and 1.59 × 10⁴ M⁻¹ for sensors 26 and 27, respectively. The mechanism of sensing could be ascribed to the CHEF process.

Zhang et al. synthesized Schiff base sensor 28 as a fluorescence turn-on sensor for Al³⁺ (Fig. 13 and Table 2).⁹⁵ In ethanol solution, free sensor 28 was weakly fluorescent with emission at 478 nm (λ_ex = 394 nm). Among various metal ions, sensor 28 showed 200-fold fluorescence enhancement at 478 nm only with Al³⁺, whereas other metal ions had an insignificant effect. The fluorescence change was associated with a change in the color of the solution from colorless to blue, which can be detected by the naked eye under UV lamp illumination. The limit of detection was 4.2 ppm for Al³⁺. The association constant was 1.4 × 10⁷ M⁻¹ and the complex stoichiometry was 1 : 1 (28 : Al³⁺). The coordination of Al³⁺ with sensor 28 caused the CHEF effect.

Kumar et al. prepared 4-(N,N-diethylamino)salicylaldehyde-based Schiff base sensor 29 for Al³⁺ detection in DMF solution.⁹⁶ Upon the addition of Al³⁺, sensor 29 displayed a turn-on fluorescence response (cyan color, λ_em = 481 nm) toward Al³⁺ over other tested metal ions. Sensor 29 binds with Al³⁺ in a 2 : 1 manner, forming a six coordinated Al³⁺-complex, and resulting in inhibition of the ICT process and leading to turn-on emission through the CHEF effect (Fig. 16, 17 and Table 2).

Fig. 16 Sensing mechanism for Al³⁺ detection with sensor 29. Reproduced with permission from ref.96 Copyright 2022, Elsevier.

Fig. 17 Photographs of sensor 29 solution in the presence of different metal ions under 364 nm UV light illumination. Reproduced with permission from ref. 96 Copyright 2022, Elsevier.

Wang et al. synthesized tetrastyrene based sensor 30 (AIE-active) for fluorescence-based detection of Al³⁺ in methanol–H₂O (9 : 1, v/v) solution (Fig. 18, Table 2).⁹⁷ The free sensor 30 was weakly fluorescent. With the addition of Al³⁺, the fluorescence spectrum of sensor 30 displayed an increase in emission intensity at 521 nm (λ_ex = 334 nm) and a good linear relationship (0–1.8 × 10⁻⁵ M). The binding ratio and the binding constant were 1 : 1 (30 : Al³⁺) and 2.24 × 10⁴ M⁻¹, respectively. The sensing mechanism was attributed to inhibition of the PET process and the CHEF effect. From the ¹H NMR titration experiment, with the addition of Al³⁺, the disappearance of proton signals at 10.97 ppm (–OH), a slight shift of proton signals at 12.17 ppm (–CH=N–), and splitting of proton signals at 9.10 ppm (–CH=N–) indicated chelation of Al³⁺ with the nitrogen atom of the imine group, the oxygen atom of the phenol hydroxyl group and the oxygen atom of aldehyde in sensor 30.

Fig. 18 Sensing mechanism for Al³⁺ detection with sensor 30. Reproduced with permission from ref. 97 Copyright 2021, Elsevier.

3.1.4 Sn²⁺ sensor. Sn(II) is an essential trace mineral for the human body.²¹⁹ In our daily life, stannous chloride (SnCl₂) is widely used in food manufacturing and processing, and to conserve soft drinks. According to the World Health Organization, the maximum limit of tin in canned food is 250 μg per kg. However, tin consumption above 200 μg per kg can damage living cells and can lead to serious gastrointestinal problems.^2,220

Kolcu et al. designed and synthesized sensor 31 as a turn-on sensor for Sn(II) (Fig. 19 and Table 2).⁹⁸ In DMSO–H₂O (1 : 1, v/v) solution, sensor 31 was non-fluorescent due to C=N isomerization and the PET process. However, in the presence of Sn²⁺, the fluorescence spectrum of sensor 31 displayed a strong emission band at 460 nm (λ_ex = 320 nm) along with a color change of the solution from yellow to deep blue under a UV lamp. The chelation of Sn²⁺ with sensor 31 resulted in inhibition of the PET process and C=N isomerization, resulting in fluorescence enhancement through the CHEF effect. The complexation stoichiometry was 1 : 2 (31/Sn²⁺) and the binding constant was 6.8 × 10⁹ M⁻². The limit of detection was 0.314 μM for Sn²⁺.

Fig. 19 Chemical structure of sensors 31–45.

3.1.5 Cr³⁺ sensor. Cr³⁺ is an essential micronutrient of our balanced diet. In the human body, Cr³⁺ is involved in various processes such as maintaining insulin balance, metabolism of carbohydrates, proteins and fats, etc. However, excess concentration of Cr³⁺ leads to genotoxic diseases, whereas deficiency of Cr³⁺ causes cardiovascular diseases and diabetes.²²¹ Therefore, the development of a fluorescent sensor for Cr³⁺ detection is an interesting field of research.

Chalmardi et al. utilized Schiff base sensor 32 for Cr³⁺ detection (Fig. 19 and Table 2).⁹⁹ In acetonitrile–water (95/5%), sensor 32 displayed high selectivity for Cr³⁺ over other metal ions. In the presence of Cr³⁺, sensor 32 underwent fluorescence enhancement at 537 nm (λ_ex = 273 nm). The complex stoichiometry was 1 : 1 (32 : Cr³⁺). The binding constant was 8.77 × 10⁴ M⁻¹ as calculated using the Benesi–Hildebrand equation. The limit of detection was 0.22 μM for Cr³⁺. The sensing mechanism could be ascribed to the blockage of the PET process due to coordination of Cr³⁺ with the imine moiety and the phenolic hydroxyl group of sensor 32, causing fluorescence enhancement through the CHEF effect.

The same group further reported another Schiff base sensor 33 for Cr³⁺ detection (Fig. 19 and Table 2).¹⁰⁰ In acetonitrile–water (95/5%), free sensor 33 was weakly fluorescent. In the presence of Cr³⁺, sensor 33 showed a rapid turn-on fluorescence response at 663 nm (λ_ex = 360 nm). The stoichiometry of the complex was 1 : 1 (33 : Cr³⁺) and the association constant was 2.28 × 10⁵ M⁻¹. The detection limit was 0.13 μM for Cr³⁺. The mechanism of detection was inhibition of the PET phenomenon because of Cr³⁺ coordination with sensor 33, causing turn-on fluorescence through the CHEF effect.

3.1.6 Fe²⁺/Fe³⁺ sensor^222–224. Iron is one of the essential elements and it plays a very significant role in living systems such as in oxygen uptake, oxygen metabolism, the electron transfer process, and as a cofactor in various enzymatic reactions. However, an unbalanced concentration of iron in living systems is connected with serious diseases such as anaemia, heart failure, cell damage, liver damage, etc. Therefore, detection of iron is important for monitoring health problems and illness diagnosis. Though iron exists in Fe²⁺ and Fe³⁺ forms, comparatively Fe³⁺ is more stable and highly studied as an analyte based on the fluorescent sensor.²²⁵

He et al. designed carbazole-based Schiff base 34 as an on–off sensor for Fe³⁺ detection (Fig. 19 and Table 2).¹⁰¹ Upon excitation at 280 nm, the fluorescence spectrum of 34 displayed emission in the range of 330–520 nm in DMF solution. Sensor 34 exhibited high selectivity for Fe³⁺ among various other metal ions. 60% quenching in fluorescence intensity of sensor 34 was observed with 10 equivalents of Fe³⁺. Job's plot revealed 1 : 1 (34/Fe³⁺) complex stoichiometry. The binding constant was 7.98 × 10⁻⁶ M⁻¹ and the limit of detection was 37.5 nM. The mechanism of interaction could be attributed to the charge transfer process (LMCT) from sensor 34 to Fe³⁺.

Li et al. developed quinoline-based sensor 35 for fluorescence-based detection of Fe³⁺ (Fig. 19 and Table 2).¹⁰² In an aqueous solution, free sensor 35 was non-fluorescent. Upon addition of Fe³⁺, sensor 35 underwent a 44-fold increase in emission intensity at 505 nm (λ_ex = 256 nm). Various other tested metal ions had insignificant effects on the fluorescence spectrum of sensor 35. Job's plot indicated 1 : 1 (35/Fe³⁺) binding stoichiometry. The binding constant was 7.22 × 10⁴ M⁻¹ and the detection limit was 4.8 × 10⁻⁸ M for Fe³⁺. The coordination of Fe³⁺ with sensor 35 resulted in blockage of the PET process and turn-on emission through the CHEF effect.

Santhoshkumar et al. reported naphthalene pyridine-based Schiff base 36 as a fluorescent sensor for Fe²⁺ detection (Fig. 19 and Table 2).¹⁰³ Sensor 36 was weakly fluorescent in acetonitrile–H₂O (1 : 1, v/v) solution. In the presence of Fe²⁺ (λ_ex = 300 nm), sensor 36 underwent fluorescence enhancement along with red-shift in emission wavelength from 345 nm to 370 nm. In comparison with other metal ions, sensor 36 exhibited selective fluorescence response with Fe²⁺ only. Job's plot indicated 2 : 1 (36/Fe²⁺) binding stoichiometry and the binding constant was 5.02 × 10⁴ M⁻¹. The limit of detection was 0.15 μM for Fe²⁺. The coordination of Fe²⁺ with sensor 36 through the imine moiety inhibited the ICT process present in free sensor 36 and caused fluorescence enhancement through the CHEF effect. Sensor 36 found potential application in Fe²⁺ detection in real samples.

Zhao et al. designed phenanthro[9,10-d]imidazole-coumarin-based Schiff bases 37 and 38 as fluorescent sensors for Fe³⁺ detection (Fig. 19 and Table 2).¹⁰⁴ In DMF/HEPES buffer (7 : 3, v/v) solution, the fluorescence spectrum of free sensors 37 and 38 displayed emission maxima at 446 nm (λ_ex = 318 nm) and 426 nm (λ_ex = 304 nm), respectively. Among various metal ions, both sensors 37 and 38 underwent fluorescence quenching only with Fe³⁺. The detection limit of sensors 37 and 38 was 4.28 μM and 0.83 μM, respectively, for Fe³⁺. The binding constant of sensors 37 and 38 with Fe³⁺ was 1.52 × 10⁵ M⁻¹ and 7.69 × 10⁴ M⁻¹, respectively. The binding stoichiometry was 1 : 1 (37/38 : Fe³⁺). The observed fluorescence quenching could be attributed to electron/energy transfer from Fe³⁺ to sensors 37 and 38 because of the paramagnetic nature of Fe³⁺. Both sensors 37 and 38 found potential application in Fe³⁺ detection in real water samples.

3.1.7 Co²⁺ sensor. Co²⁺ is an essential trace element in the human body. It is an active center of vitamin B₁₂ and enzymes. It is also used in lithium-ion batteries, alloys, etc. Co-60 is an important radioisotope used for gamma ray production and in radioactive tracer studies.^226,227 However, an increase/decrease in the concentration of cobalt ions from normal in humans and animals causes adverse effects on the respiratory system and central nervous system, kidney failure, anaemia, etc.²²⁸ Therefore research on the development of fluorescent sensors which can selectively and sensitively coordinate with Co²⁺ is of immense interest.²²⁹

Zhang et al. synthesized salicylal derived Schiff base sensor 39 for fluorescence-based detection of Co²⁺ (Fig. 19 and Table 2).¹⁰⁵ Among various metal ions, sensor 39 showed high selectivity only for Co²⁺ in ethanol solution (Fig. 20). Fluorescence titration of sensor 39 with Co²⁺ revealed an increase in emission intensity at 350 nm (λ_ex = 298 nm). The plot of fluorescence intensity vs the concentration of Co²⁺ was linear in the range of 8.0 × 10⁻⁷ to 2.0 × 10⁻⁶ M. The limit of detection was 0.782 μM for Co²⁺. Job's plot revealed the 1 : 4 (39 : Co²⁺) stoichiometry of the complex. ¹H NMR studies in the absence and presence of Co²⁺ suggested the involvement of the nitrogen atom of the N=C–H group and the phenolic O–H group in coordination with Co²⁺. Sensor 39 could be used for Co²⁺ monitoring in environmental systems.

Fig. 20 Fluorescence response of sensor 39 with different metal ions in ethanol solution. Reproduced with permission from ref. 105 Copyright 2017, Elsevier.

3.1.8 Cu²⁺ sensor^{108,110,114,230–236}. Cu²⁺ is an essential trace dietary element in the human body and participates in various physiological processes, for example in normal functioning of the immune system, nervous system and cardiovascular system.^237,238 The unbalanced concentration of Cu²⁺ is associated with diseases such as Wilson's diseases, hypoglycemia, liver damage, etc. Furthermore, due to excessive use of copper in industry and agriculture, copper is a major pollutant of the environment.²³⁹ Thus, development of selective and sensitive methods for Cu²⁺ detection is a challenging area of research.

Dalbera et al. developed pyrene-based Schiff base sensor 40 for Cu²⁺ detection (Fig. 19 and Table 2).¹⁰⁶ In CH₃CN–H₂O (9/1, v/v), the fluorescence experiment revealed the high selectivity of sensor 40 towards Cu²⁺ over other metal ions, viz. K⁺, Na⁺, Ni²⁺, Pb²⁺, Hg²⁺, Zn²⁺, Al³⁺, Co²⁺, and Fe³⁺. Upon gradual addition of Cu²⁺, sensor 40 underwent a 93% decrease in emission intensity at 429 nm (λ_ex = 384 nm). The fluorescence quenching could be ascribed to the reduction of π-conjugation in sensor 40 upon coordination with Cu²⁺, because of sensor 40 to metal charge transfer (LMCT). Job's plot indicated 2 : 2 (40 : Cu²⁺) stoichiometry and the detection limit was 0.503 μM for Cu²⁺. The quenching constant as calculated using the Stern–Volmer equation was 1.38 × 10⁵ M⁻¹. The binding constant as obtained using the Benesi–Hildebrand equation was 5.65 × 10⁵ M⁻¹. Sensor 40 also found potential application in Cu²⁺ detection in real water samples such as tap water and drinking water.

Carlos et al. prepared fluorene-based Schiff base sensor 41 and utilized it for metal ion detection in DMSO solutions (Fig. 19 and Table 2).¹⁰⁷ Among various metal ions, sensor 41 exhibited significant fluorescence quenching at 307 nm (λ_ex = 278 nm) with Cu²⁺. The stoichiometry of the complex was 2 : 1 (Cu²⁺/41). The Stern–Volmer constant (K_sv) and the binding constant of sensor 41 with Cu²⁺ were 2.26 × 10¹² M⁻¹ and 1.74 × 10¹² M⁻¹, respectively. The detection limit was 0.037 μM for Cu²⁺. The coordination of one Cu²⁺ through the imine group as well as the nitrogen atom of the amide group and the second Cu²⁺ through the pyridine ring nitrogen atom as well as oxygen atoms from solvent molecules facilitated electron transfer from Cu²⁺ to sensor 41 and resulted in fluorescence quenching.

Fu et al. synthesized Schiff base sensor 42 using diarylethene and the 1,8-naphthalimide unit (Fig. 19 and Table 2).¹⁰⁸ In acetonitrile solution, sensor 42 showed significant fluorescence and color change with Cu²⁺ in comparison with other metal ions. Sensor 42 underwent fluorescence quenching at 542 nm (λ_ex = 296 nm) with Cu²⁺ and the color of the solution changed from bright green to dark. Job's plot and the mass spectrum revealed 1 : 1 (42/Cu²⁺) stoichiometry. The binding constant was 3.13 × 10⁴ M⁻¹ as calculated using the modified Benesi–Hildebrand equation. The limit of detection was 2.4 μM.

Kumar et al. reported naphthylamine and benzyl-derived sensor 43 for Cu²⁺ detection (Fig. 19 and Table 2).¹⁰⁹ In CH₃CN/H₂O (1 : 1, v/v) solution, the fluorescence spectrum of sensor 43 displayed emission at 435 nm (λ_ex = 310 nm). Upon addition of Cu²⁺, sensor 43 underwent a decrease in fluorescence intensity associated with the blue shift of the emission maxima from 435 nm to 410 nm. The plot of fluorescence intensity ratio vs the concentration of Cu²⁺ was linear in the range of 5 × 10⁻⁵ to 3 × 10⁻⁴ M. The detection limit was 10⁻⁵ M. The Stern–Volmer constant (K_sv) was 1.1 × 10⁴ M⁻¹. The binding constant was log β = 6.4. The stoichiometry of the complex was 1 : 2 (43/Cu²⁺). One Cu²⁺ coordinates with the nitrogen atom of the imine moiety and the second Cu²⁺ binds to benzyl rings through cation–π interaction.

Moghadam et al. designed fluorene-based Schiff base sensor 44 and utilized it for fluorescence-based detection of Cu²⁺ (Fig. 19 and Table 2).¹¹⁰ Sensor 44 was non-fluorescent in acetonitrile solution. In the presence of Cu²⁺, sensor 44 showed strong fluorescence enhancement at 369 nm (λ_ex = 340 nm). The binding constant was 2.84 × 10⁵ M⁻¹ as calculated using the Benesi–Hildebrand equation. The limit of detection was 1.54 nM for Cu²⁺. Job's plot revealed 1 : 1 (44/Cu²⁺) stoichiometry. The chelation of sensor 44 with Cu²⁺ resulted in the formation of a more rigid system because of blocking of the ICT process and restriction in C=N isomerization, causing turn-on emission through the CHEF effect. Sensor 44 found practical application in the detection of copper in electroplating wastewater.

Torawane et al. prepared Schiff base sensor 45 as a turn-off sensor for Cu²⁺ detection (Fig. 19 and Table 2).¹¹¹ Among various tested metal ions, sensor 45 underwent instant fluorescence quenching only with Cu²⁺ at emission wavelength 554 nm (λ_ex = 386 nm) in an aqueous solution. The detection limit of sensor 45 was 0.35 μM for Cu²⁺. The association constant was 2.67 × 10⁵ M⁻¹ as determined using the non-linear curve fitting method and the binding stoichiometry was 1 : 1 (45/Cu²⁺). The mechanism of sensing could be ascribed to the excited state electron transfer from sensor 45 to Cu²⁺.

Wang et al. synthesized imidazole- and benzimidazole-based Schiff base sensors 46 and 47, respectively, for Cu²⁺ detection (Fig. 22 and Table 2).¹¹² In DMF solution, sensors 46 and 47 displayed emission at wavelength 411 nm and 416 nm, respectively. Both sensors 46 and 47 exhibited specificity for Cu²⁺ over other tested metal ions and underwent fluorescence enhancement with Cu²⁺. The detection limit of Sensor 47 was 1 nM for Cu²⁺. The coordination of Cu²⁺ with sensors 46 and 47 through the –C=N– group and the imidazole ring resulted in the formation of a rigid structure and caused fluorescence enhancement through the CHEF effect. On comparison, sensor 47 was found to be more selective than sensor 46 for Cu²⁺ because of its large conjugated rigid structure.

Yin et al. designed carbazole-based Schiff base sensor 48 for fluorescence-based detection of Cu²⁺ in acetonitrile solution (Fig. 22 and Table 2).¹¹³ On excitation at 298 nm, sensor 48 displayed weak emission at wavelength 335 nm and 381 nm. Among various tested metal ions, sensor 48 showed strong fluorescence enhancement only with Cu²⁺. On gradual addition of an increasing concentration of Cu²⁺, sensor 48 displayed an increase in fluorescence intensity at 450 nm and reached a plateau after the addition of 6 equivalents of Cu²⁺. The binding constant (K_a) was 1.26 × 10⁻⁶ M⁻¹ and the binding stoichiometry was 1 : 1 (48/Cu²⁺). The detection limit was 27.4 nM for Cu²⁺. The sensing mechanism could be ascribed to the coordination of Cu²⁺ with sensor 48 through the diaminomalenonitrile unit, causing inhibition of the ICT process and C=N isomerization, resulting in fluorescence enhancement.

Gao et al. incorporated Schiff base-based sensor 49 into the pore of periodic mesoporous organosilica (49-PMO) for fluorescence-based detection of Cu²⁺ (Fig. 21 and Table 2).¹¹⁴ Upon the addition of Cu²⁺, the fluorescence spectrum of 49-PMO revealed fluorescence quenching, and the detection limit was 0.67 μM for Cu²⁺. The sensing mechanism was attributed to ligand to metal charge transfer (LMCT).

Fig. 21 Chemical structure of Schiff base 49 and the sensing mechanism of Cu²⁺ with 49-PMO. Reproduced with permission from ref.114 Copyright 2021, Wiley.

3.1.9 Zn²⁺ sensor^240–244. Zinc is an essential trace element, involved in several important functions in the human body such as metabolism of RNA and DNA, cell apoptosis, reproduction, immune system function, protein metabolism, etc.²⁴⁵ Deficiency of zinc results in serious diseases such as growth retardation, a weak immune system, Parkinson's disease, Alzheimer's disease, etc. On the other hand, excess intake of zinc may lead to health problems.^246–248 In addition, zinc is an important pollutant of the environment, having toxic effects on aquatic microorganisms, plants, and invertebrates. Therefore, environmental monitoring and detection of zinc in a biological system constitute an interesting and challenging field of research.

Zhu et al. developed 2-hydroxy-1-naphthaldehyde and 2-benzylthio-ethanamine based Schiff base sensor 50 for Zn²⁺ detection (Fig. 22 and Table 2).¹¹⁵ In ethanol/HEPES buffer (95/5, v/v) solution, sensor 50 hardly displayed detectable fluorescence (λ_em = 450 nm, λ_ex = 242 nm). Among various tested metal ions, sensor 50 displayed superb fluorescence enhancement (19-fold) only with Zn²⁺. The binding stoichiometric ratio was 2 : 1 (50 : Zn²⁺) and the binding constant was 85.7 M⁻². The detection limit of sensor 50 was 0.5034 μM. The sensing mechanism could be attributed to (i) restriction in rotation of the C=N group, (ii) blockage of the PET process, and (iii) Zn²⁺ induced disaggregation of sensor 50.

Fig. 22 Chemical structure of sensors 46–62.

Kumar et al. synthesized diaminobenzidine-based Schiff base sensor 51 for Zn²⁺ detection (Fig. 22 and Table 2).¹¹⁶ In DMF solution, sensor 51 displayed selective fluorescence enhancement only with Zn²⁺ over various other competing metal ions such as Cd²⁺, Ni²⁺, Co²⁺, Cu²⁺, Mn²⁺, Fe³⁺, and Al³⁺. Fluorescence titration of sensor 51 with Zn²⁺ showed a progressive increase in emission intensity at 509 nm (λ_ex = 430 nm). The detection limit was 8.6 nM for Zn²⁺. Job's plot revealed 1 : 1 (51/Zn²⁺) binding stoichiometry and the binding constant was 7.8 × 10⁴ M⁻¹. The chelation of sensor 51 with Zn²⁺ and inhibition of –C=N– isomerization upon coordination with Zn²⁺ resulted in a turn-on fluorescence response through the CHEF effect.

Qin et al. synthesized coumarin-based Schiff base 52 as a two-photon fluorescent sensor for Zn²⁺ detection (Fig. 22 and Table 2).¹¹⁷ On excitation at 380 nm, sensor 52 displayed weak emission at 500 nm in DMF–H₂O (9 : 1, v/v) solution. Fluorescence titration of sensor 52 with Zn²⁺ showed a gradual increase in fluorescence intensity at 500 nm which reached a plateau after the addition of 0.5 equivalent of Zn²⁺. The binding stoichiometry was 2 : 1 (52/Zn²⁺) and the binding constant (log K) was 6.04. The limit of detection was 2.59 μM for Zn²⁺. The sensing mechanism could be ascribed to the inhibition of the PET process and restriction in –C=N– isomerization upon coordination of Zn²⁺ with sensor 52.

Yan et al. developed Schiff base sensor 53 for Zn²⁺ detection in ethanol solution (Fig. 22 and Table 2).¹¹⁸ Among various metal ions under investigation, sensor 53 underwent strong fluorescence enhancement at 498 nm (λ_ex = 430 nm) only with Zn²⁺. The stoichiometry of the complex was 1 : 1 (53/Zn²⁺) and the detection limit was 0.173 μM for Zn²⁺. The association constant (K_a) was 2.0 × 10⁴ M⁻¹ as calculated using the Benesi–Hildebrand equation. The complexation of Zn²⁺ with sensor 53 makes the system more rigid by inhibiting the PET process and C=N isomerization, resulting in fluorescence enhancement through the CHEF effect.

3.1.10 Ag⁺ sensor. Ag⁺ is a heavy metal ion and has been widely utilized in various industrial processes such as photography, batteries, and electrical industries. However, intake of Ag⁺ directly/indirectly from industrial waste has a toxic impact on living beings. Thus, detection of Ag⁺ is important, which is challenging. Recently, thiophene appended cyclotriphosphagene-based sensor 54 has been developed for Ag⁺ detection (Fig. 23 and Table 2).¹¹⁹ Sensor 54 showed a turn-on fluorescence response toward Ag⁺ over other competitive metal ions. The coordination of sensor 54 with Ag⁺ in a 1 : 1 manner resulted in inhibition of the PET process and the CHEF effect. The LOD was 3.15 μM for Ag⁺. Interestingly, sensor 54 found practical application in the determination of Ag⁺ in real samples.

Fig. 23 Sensing mechanism of Ag⁺ with sensor 54. Reproduced with permission from ref.119 Copyright 2021, Wiley-VCH GmbH, Weinheim.

3.1.11 Hg²⁺ sensor^249,250. Among heavy metal ions, mercury has an extremely toxic impact on living beings and the environment. Exposure to Hg²⁺ can lead to liver, kidney, and brain damage, neurological problems, etc. The main sources of mercury contamination are industry and mining activities. Therefore, the detection of mercury in biological systems and the environment is important. Because of the high affinity of mercury for sulfur, chelating agents such as 2,3-dimercapto-1-propanesulfonic acid (DMPS) and dimercaptosuccinic acid (DMSA) have been used for treating acute mercury poisoning. ²⁵¹ Generally, most of the sensors for mercury ion detection contain sulfur, pyridine, thymine, and quinoline groups as binding sites.

Lei et al. synthesized thiosemicarbazone-based Schiff base 55 as an on–off sensor for Hg²⁺ detection in DMSO-buffered water (9/1, v/v) (Fig. 22 and Table 2).¹²⁰ Among various metal ions, the fluorescence spectrum of sensor 55 displayed dramatic fluorescence quenching only with Hg²⁺ and to a lesser extent with Cu²⁺, whereas other metal ions had an insignificant effect on the fluorescence spectrum of sensor 55. Upon addition of Hg²⁺, the color of the solution of sensor 55 changed from colorless to light yellow and from yellow-green to light red under natural light and UV lamp illumination, respectively. Fluorescence titration of sensor 55 with Hg²⁺ revealed a decrease in emission intensity at 500 nm (λ_ex = 365 nm) along with a red-shift of the emission wavelength by 50 nm. The detection limit was 0.907 μM for Hg²⁺. Job's plot revealed 1 : 2 (55/Hg²⁺) complex stoichiometry. The complexation constant was 2.51 × 10⁵ M⁻¹ as calculated using the Benesi–Hildebrand equation. The downfield shift of protons of the thiosemicarbazone moiety and the low field shift of protons of the benzoquinoxaline moiety in the ¹H NMR titration experiment suggested the involvement of the thiosemicarbazide group and the imine group in coordination with Hg²⁺.

Singhal et al. designed thiophene-based Schiff base 56 as a turn-on sensor for Hg²⁺ detection (Fig. 22 and Table 2).¹²¹ Sensor 56 was weakly fluorescent in methanol/H₂O (8 : 2, v/v) solution. On excitation at 365 nm, sensor 56 showed a turn-on fluorescence response at 503 nm with Hg²⁺, whereas other tested metal ions had an insignificant effect. The complexation of Hg²⁺ with sensor 56 caused fluorescence enhancement through the CHEF effect. The binding constant as calculated using Hill's method was log β = 13.36 and the limit of detection was 20 μM for Hg²⁺. The job's plot and ¹H NMR titration experiment revealed 2 : 1 (56/Hg²⁺) binding stoichiometry.

Su et al. reported benzophenone and aminobenzenethiol based Schiff base sensor 57 for Hg²⁺ detection (Fig. 22 and Table 2).¹²² In the CH₃CN-H₂O (9/1, v/v) mixture, sensor 57 displayed weak fluorescence at 415 nm (λ_ex = 349 nm). In the presence of Hg²⁺, sensor 57 showed an increase in fluorescence intensity at 415 nm along with the formation of a new emission band at 473 nm. The sensing mechanism could be attributed to the inhibition of the ESIPT process and C=N isomerization due to the coordination of Hg²⁺ with sensor 57 through the nitrogen atom and the sulfur atom of the imine group and the thiophenol moiety, respectively, forming a five-membered ring structure. The complex stoichiometry was 2 : 1 (57/Hg²⁺) and the association constant was 4.48 × 10⁵ M⁻¹ as obtained from the Benesi–Hildebrand method. The limit of detection for Hg²⁺ was 22.68 nM.

Tekuri et al. synthesized pyrene-based Schiff base chemodosimeters 58, 59, and 60 for Hg²⁺ detection in aqueous medium (Fig. 22 and Table 2).¹²³ Among sensors 58, 59, and 60, only sensor 58 displayed high selectivity for Hg²⁺ over other tested metal ions. Fluorescence titration of sensor 58 with Hg²⁺ exhibited an increase in fluorescence intensity at 417 nm (λ_ex = 355 nm). The binding constant was 5.8 × 10⁻² M⁻¹ and the complex stoichiometry was 2 : 1 (58/Hg²⁺). The limit of detection was 0.270 μM for Hg²⁺. The sensing mechanism could be ascribed to the Hg²⁺ induced hydrolysis of sensor 58 resulting in the formation of more fluorescent pyrene carboxyaldehyde molecules.

3.1.12 Cd²⁺ sensor^{35,124,252,253}. Cadmium is an extremely hazardous metal ion that is widely utilized in industrial processes, for example in electroplating, fabrication of batteries, etc. Human exposure to Cd²⁺ may occur through industrial discharge and excessive use of fertilizers. Inside the human body, Cd²⁺ may damage the respiratory system, liver, kidney, etc.¹¹ Therefore, detection of cadmium ions in biological fluids (such as blood samples and urine samples) and in real samples (such as water and food) is important for early identification of cadmium poisoning.

Wan et al. utilized quinoline-based Schiff base sensor 61 for Cd²⁺ detection (Fig. 22 and Table 2).¹²⁴ In 10% methanol solutions, sensor 61 displayed high selectivity only for Cd²⁺ among various other metal ions. In the presence of Cd²⁺, sensor 61 showed a 40 times increase in fluorescence intensity at 425 nm (λ_ex = 246 nm). The limit of detection was 2.7 nM for Cd²⁺. Job's plot revealed 1 : 1 (61 : Cd²⁺) stoichiometry. ¹H NMR analysis of sensor 61 in the presence and absence of Cd²⁺ demonstrated the participation of –C=N–, quinoline, the nitrogen atom of ethylenediamine, and the oxygen atom of ester in complexation with Cd²⁺.

Mohanasundaram et al. synthesized quinoline-based Schiff base sensor 62 for Cd²⁺ detection (Fig. 22 and Table 2).¹²⁵ In CH₃CN–H₂O (8 : 2, v/v) solution, free sensor 62 showed weak emission at 510 nm (λ_ex = 380 nm). Among various tested metal ions, sensor 62 underwent fluorescence enhancement only with Cd²⁺ (37-fold) and Zn²⁺ (14-fold), whereas other metal ions had an insignificant effect. Sensor 62 also exhibited color change from colorless to yellow under UV light illumination. The binding stoichiometry was 2 : 1 (62/Cd²⁺) and the association constant was 1.77 × 10⁵ M⁻¹. The limit of detection was 14.8 nM for Cd²⁺. The coordination mechanism of sensor 62 with Cd²⁺ could be the combined effect of restriction in C=N isomerization and the CHEF effect. Furthermore, sensor 62 demonstrated potential application in the quantification of Cd²⁺ in different water samples.

3.2 Multi-metal ion sensors^57,254–264

Nowadays, the development of sensors with multiple ion recognition sites is a challenging and emerging area of interest, because of the advantages of such systems such as cost reduction and faster analytical processing.

Hammud et al. synthesized Schiff base sensor 63 for fluorescence-based detection of Ag⁺, Cu²⁺, and Fe³⁺ in ethanol solution (Fig. 24 and Table 2).¹²⁶ On excitation at 341 nm, sensor 63 exhibited strong fluorescence at emission wavelength 385 nm. Fluorescence titration of sensor 63 with Ag⁺, Cu²⁺, and Fe³⁺ revealed a linear decrease in fluorescence intensity with the addition of an increasing concentration of these ions. The detection limit was 0.132 μM, 0.088 μM, and 0.037 μM, respectively, for Ag⁺, Cu²⁺, and Fe³⁺. The Stern–Volmer constant (K_sv) value was 9.575 × 10² M⁻¹, 8.273 × 10² M⁻¹ and 23.2 × 10² M⁻¹ for Ag⁺, Cu²⁺, and Fe³⁺, respectively. The stoichiometry of the complex was 2 : 1 (63/Ag⁺ or Cu²⁺) and 1 : 2 (63/Fe³⁺). The Benesi–Hildebrand plot revealed a binding constant value of 1.4062 × 10³ M⁻¹, 1.111 × 10³ M⁻¹ and 1.2909 × 10⁴ M⁻¹ with Ag⁺, Cu²⁺, and Fe³⁺, respectively. The mechanism of fluorescence quenching could be attributed to the initiation of the PET process upon binding with metal ions. Sensor 63 found analytical application in the detection of Ag⁺, Cu²⁺, and Fe³⁺ in real water samples.

Fig. 24 Chemical structure of sensors 63–76.

Saleem et al. developed triazole-based Schiff base sensor 64 for fluorescence-based detection of Co²⁺, Cu²⁺, and Hg²⁺ (Fig. 24 and Table 2).¹²⁷ In THF/water (6 : 4, v/v) solution, sensor 64 displayed emission at 622 nm (λ_ex = 259 nm). Among various metal ions, sensor 64 exhibited strong fluorescence quenching only with Hg²⁺ and to a lesser extent with Co²⁺ and Cu²⁺, whereas other metal ions had an insignificant effect. The quenching constant (K_sv) value was 9.25 × 10⁷, 1.829 × 10⁷ and 1.14 × 10⁷ M⁻¹ for Co²⁺, Cu²⁺, and Hg²⁺, respectively. The association constant value was 1.52 × 10⁸ M⁻¹, 5.47 × 10¹¹ M⁻¹, and 1.16 × 10⁸ M⁻¹ for Co²⁺, Cu²⁺, and Hg²⁺, respectively. The limit of detection was 3.26 nM, 89.1 pM, and 0.387 nM for Co²⁺, Cu²⁺, and Hg²⁺, respectively. The stoichiometry of the complex was 1 : 1 (64 : Co²⁺/Cu²⁺/Hg²⁺). The heavy metal ion effect and paramagnetic behavior triggered non-radiative decay could be responsible for the observed fluorescence quenching.

Sie et al. synthesized Schiff base sensor 65 for fluorometric and colorimetric detection of Co²⁺, Cu²⁺, and Hg²⁺ (Fig. 24, 25 and Table 2).¹²⁸ In DMSO–H₂O (9 : 1, v/v) solution, sensor 65 exhibited emission at wavelength 540 nm (λ_ex = 460 nm). The fluorescence titration of sensor 65 with Hg²⁺ showed first a decrease (4–12 μM), then an increase (16–40 μM), and again a gradual decrease (60–800 μM) in emission intensity with the addition of an increasing concentration of Hg²⁺. Coordination and desulfurization could be the possible mechanism of the observed fluorescence change of sensor 65 with Hg²⁺. Upon addition of an increasing concentration of Co²⁺ or Cu²⁺, sensor 65 showed a decrease in emission intensity at 540 nm. The disruption of the intramolecular H-bond in the presence of Co²⁺ or Cu²⁺ could be responsible for the observed fluorescence quenching. The association constant was 3.52 × 10⁶ M⁻¹, 5.69 × 10⁵ M⁻¹, and 1.85 × 10⁴ M⁻¹, respectively, for Co²⁺, Cu²⁺, and Hg²⁺. The stoichiometry of the complex was 1 : 1 (65 : Co²⁺/Cu²⁺/Hg²⁺). The limit of detection was 0.007 ppb, 0.29 ppb, and 0.20 ppb, respectively, for Co²⁺, Cu²⁺, and Hg²⁺. Sensor 65 showed potential for Co²⁺, Cu²⁺, and Hg²⁺ detection in real water samples.

Fig. 25 Color change of sensor 65 (host) upon addition of Hg²⁺, Co²⁺, and Cu²⁺ (5 equiv.): (A) naked eye detectable color change; (B) under UV light illumination. Reproduced with permission from ref. 128 Copyright 2017, Royal Society of Chemistry.

Simon et al. developed pyrene-based Schiff base 66 as an off–on sensor for Al³⁺, Cr³⁺, and Fe³⁺ in acetonitrile solution (Fig. 24 and Table 2).¹²⁹ Among various metal ions, sensor 66 showed turn-on fluorescence response only with trivalent metal ions such as Al³⁺, Cr³⁺, and Fe³⁺ along with red-shift of the emission maxima from 417 nm to 500 nm (λ_ex = 355 nm). The complexation stoichiometry was 2 : 1 (66 : Al³⁺/Cr³⁺/Fe³⁺). The association constant value was 2.26 × 10⁶ M⁻², 2.24 × 10⁶ M⁻² and 2.25 × 10⁶ M⁻² for Al³⁺, Cr³⁺, and Fe³⁺ ions, respectively. The detection limits were 0.117 μM, 0.111 μM, and 0.106 μM for Al³⁺, Cr³⁺, and Fe³⁺, respectively. The turn-on sensing mechanism was explained by means of excimer formation and the PET process. Sensor 66 found application in Al³⁺, Cr³⁺, and Fe³⁺ detection in living cells.

Tang et al. synthesized rhodamine B derivative-based Schiff base sensor 67 for Zn²⁺, Fe³⁺, and Cu²⁺ (Fig. 24 and Table 2).¹³⁰ In ethanol solution, sensor 67 displayed fluorescence enhancement (λ_ex = 510 nm) at 572 nm (125-fold) and at 580 nm (150-fold) along with a change in the fluorescence color of the solution from colorless to orange with Zn²⁺ and from colorless to red with Fe³⁺, respectively, whereas other metal ions had an insignificant effect (Fig. 26A and B). In ethanol–H₂O (1 : 1, v/v) solution, sensor 67 behaved as a colorimetric sensor for Cu²⁺ with a naked-eye detectable color change of the solution from colorless to pink. Job's plot revealed a complexation ratio of 1 : 1 (67/Zn²⁺), 2 : 1 (67/Fe³⁺) and 1 : 1 (67/Cu²⁺). The binding constant was 1.53 × 10³ M⁻¹, 4.57 × 10⁴ M⁻¹ and 2.62 × 10³ M⁻¹ for Zn²⁺, Fe³⁺, and Cu²⁺, respectively. The detection limit was 0.664 μM, 0.212 μM and 82.7 nM for Zn²⁺, Fe³⁺, and Cu²⁺, respectively. The turn-on response could be attributed to the opening of the lactam ring of sensor 67 upon binding with Zn²⁺ and Fe³⁺, leading to the formation of a more conjugated system. However, in the case of Cu²⁺, the naked-eye detectable color change could be due to electron transfer from Cu²⁺ to sensor 67. Sensor 67 found application in Zn²⁺, Fe³⁺, and Cu²⁺ detection in real water samples and in the fluorescence imaging of Fe³⁺ in biological cells.

Fig. 26 Fluorescence change of sensor 67 in the presence of various tested metal ions: (A) emission spectrum; (B) visible color change under 365 nm UV lamp irradiation. Reproduced with permission from ref. 130 Copyright 2017, Elsevier.

Zhang et al. developed Schiff base sensor 68 for colorimetric and fluorometric detection of Cu²⁺, Hg²⁺, and Ag⁺ (Fig. 24 and Table 2).¹³¹ In DMSO–H₂O (1 : 1, v/v) solution, sensor 68 showed emission maxima at 527 nm (λ_ex = 310 nm). In the presence of Cu²⁺, Hg²⁺, and Ag⁺, sensor 68 displayed fluorescence quenching along with a red-shift of the emission maxima from 527 to 550 nm. The binding ratio was 1 : 2 (68 : Cu²⁺/Hg²⁺/Ag⁺). The plot of fluorescence intensity ratio vs the concentration of Cu²⁺, Hg²⁺, and Ag⁺ revealed a good linear range between 0 and 20 μM. The detection limit was 64.8 μM, 52.7 μM, and 63.7 μM for Cu²⁺, Hg²⁺, and Ag⁺, respectively. The association constant value was 1.64 × 10⁵ M⁻¹, 9.27 × 10⁴ M⁻¹, and 1.37 × 10⁵ M⁻¹ for Cu²⁺, Hg²⁺, and Ag⁺, respectively, as calculated using the Benesi–Hildebrand equation. From the FT-IR spectra of complexes [68–Cu²⁺], [68–Hg²⁺], and [68–Ag⁺], a new band at 3462, 3427, and 3437 cm⁻¹, respectively, and the low-frequency shift of ν_C=N and ν_C–S, the high-frequency shift of the ν_C–O stretching band, and disappearance of the δ_O–H band indicated coordination of Cu²⁺, Hg²⁺, and Ag⁺ with sensor 68 through the nitrogen atom of the imine group, the sulfur atom of thiol and the oxygen atom of the phenolic hydroxyl group.

Zhu et al. developed Schiff base sensor 69 for Fe²⁺, Fe³⁺ and Cu²⁺ detection in DMF solution (Fig. 24 and Table 2).¹³² Among various metal ions, sensor 69 showed selectivity only for Fe²⁺, Fe³⁺, and Cu²⁺ and naked eye detectable color change from colorless to yellow with Cu²⁺ and colorless to light brown with Fe²⁺ and Fe³⁺, and fluorescence color change from purple to colorless with Fe²⁺, Fe³⁺, and Cu²⁺ under UV light. On excitation at 341 nm, sensor 69 displayed emission at wavelength 393 nm. In the presence of Fe²⁺, Fe³⁺, and Cu²⁺, sensor 69 underwent fluorescence quenching (∼98% with Fe²⁺ and Fe³⁺ and ∼92% with Cu²⁺). The fluorescence quenching mechanism could be attributed to the CHEQ effect of paramagnetic Fe²⁺, Fe³⁺, and Cu²⁺. The detection limit was 2.06 μM, 2.17 μM, and 2.48 μM for Fe²⁺, Fe³⁺, and Cu²⁺, respectively. Job's plot revealed 1 : 1 (69 : Fe²⁺/Fe³⁺/Cu²⁺) stoichiometry. The association constant value was 3.61 × 10⁵ M⁻¹, 3.82 × 10⁵ M⁻¹, and 3.93 × 10⁵ M⁻¹ for Fe²⁺, Fe³⁺, and Cu²⁺, respectively.

Xue et al. developed rhodamine-chromone-based Schiff base sensor 70 for Zn²⁺ and Fe³⁺ detection (Fig. 24 and Table 2).¹³³ In ethanol-HEPES buffer (9 : 1, v/v) solution, sensor 70 showed 66-fold fluorescence enhancement at 490 nm (λ_ex = 420 nm) with Zn²⁺ over other tested metal ions. In contrast, in DMSO–ethanol (2/3, v/v) solution, sensor 70 exhibited 51-fold fluorescence enhancement at 583 nm (λ_ex = 520 nm) with Fe³⁺ over other metal ions. The fluorescence responses of sensor 70 with Zn²⁺ could be due to inhibition of the PET process and the CHEF effect. The fluorescence enhancement at 583 nm could be due to the opening of the spirolactam ring of rhodamine upon coordination with Fe³⁺. The complexation stoichiometry was 1 : 1 (70 : Zn²⁺/Fe³⁺). The limit of detection was 0.134 μM and 0.139 μM for Zn²⁺ and Fe³⁺, respectively. The binding constant was 5.82 × 10⁴ M⁻¹ and 5.85 × 10³ M⁻¹ for Zn²⁺ and Fe³⁺, respectively. A test strip coated with sensor 70 displayed yellow-green color with Zn²⁺ and pink color with Al³⁺, demonstrating the application of sensor 70 as a solid-state sensor (Fig. 27).

Fig. 27 Photographs of test strips coated with sensor 70 and after addition of different metal ions under 365 nm UV lamp irradiation. Reproduced with permission from ref. 133 Copyright 2019, Elsevier.

Lee et al. developed salicylaldehyde- and fluorene-based Schiff base sensor 71 for fluorescence-based detection of Zn²⁺ and Ga³⁺ (Fig. 24 and Table 2).¹³⁴ In acetonitrile solution, free sensor 71 was weakly fluorescent. Compared with other tested metal ions, sensor 71 (λ_ex = 300 nm) exhibited fluorescence enhancement with Zn²⁺ and Ga³⁺ at 460 nm and 434 nm, respectively. The binding ratio was 1 : 1 (71 : Zn²⁺/Ga³⁺). The binding constant was 7.0 × 10⁴ M⁻¹ and 1.0 × 10⁴ M⁻¹ for Zn²⁺ and Ga³⁺ ions, respectively. The limit of detection was 0.32 μM and 0.01 μM for Zn²⁺ and Ga³⁺, respectively. The turn-on fluorescence response of sensor 71 with Zn²⁺ and Ga³⁺ could be due to the suppression of the PET process.

Dong et al. developed quinoline-based Schiff base 72 as a turn-on sensor for Zn²⁺ and Hg²⁺ (Fig. 24 and Table 2).¹³⁵ In the DMSO–H₂O (1 : 99, v/v) mixture, sensor 72 showed weak emission at 473 nm (λ_ex = 350 nm). Among various metal ions, sensor 72 exhibited turn-on response with red-shift of the emission maxima from 473 to 539 (Δλ_shift = 57 nm) with Hg²⁺, and from 473 to 565 nm (Δλ_shift = 92 nm) with Zn²⁺, whereas other metal ions had an insignificant effect. Under UV light, sensor 72 displayed color change from colorless to yellowish-green with Hg²⁺, and from colorless to intense yellow with Zn²⁺ (Fig. 28A and B). Job's plot indicated 1 : 1 (72 : Hg²⁺/Zn²⁺) stoichiometry. The limit of detection was 0.040 μM and 0.011 μM for Hg²⁺ and Zn²⁺, respectively. The association constant was 1.41 × 10⁵ M⁻¹ and 1.52 × 10⁵ M⁻¹ for Hg²⁺ and Zn²⁺, respectively. The sensing mechanism could be attributed to the lowering of the energy gap between HOMO and LUMO of sensor 72 as well as increased structural rigidity upon complexation with Hg²⁺ and Zn²⁺.

Fig. 28 Fluorescence change of sensor 72 in the absence and presence of different metal ions: (A) emission spectrum; (B) plot of fluorescence intensity change. Inset: Photographs of sensor 72, 72 + Zn²⁺ and 72 + Hg²⁺ under 365 nm UV lamp irradiation. Reproduced with permission from ref. 135 Copyright 2017, Royal Society of Chemistry.

Tajbakhsh et al. developed fluorene-based Schiff base 73 as a turn-on sensor for Al³⁺ and Cr³⁺ (Fig. 24 and Table 2).¹³⁶ In acetonitrile solution, sensor 73 exhibited emission at 536 nm (λ_ex = 333 nm). In the presence of Al³⁺ or Cr³⁺, sensor 73 displayed fluorescence enhancement at 536 nm. The binding ratio was 2 : 1 (73 : Al³⁺/Cr³⁺) and the binding constant was 5.44 × 10⁴ M⁻¹ and 8.33 × 10⁴ M⁻¹ for Al³⁺ and Cr³⁺ ions, respectively. The detection limit was 0.31 μM and 0.25 μM for Al³⁺ and Cr³⁺, respectively. The chelation of sensor 73 with Al³⁺ and Cr³⁺ resulted in inhibition of C=N isomerization and the ESIPT process, causing fluorescence enhancement (Fig. 29).

Fig. 29 Sensing mechanism of sensor 73 with Al³⁺. Reproduced with permission from ref. 136 Copyright 2018, Elsevier.

Zhang et al. developed phenylamine-oligothiophene-based Schiff base sensor 74 for fluorescence-based detection of Cu²⁺ and Zn²⁺ (Fig. 24 and Table 2).¹³⁷ In THF–H₂O (7 : 3, v/v) solution, sensor 74 displayed emission at 537 nm (λ_ex = 385 nm). Compared with other metal ions, sensor 74 exhibited a turn-off response with Cu²⁺ and a turn-on response with Hg²⁺. Under 365 nm UV light, the fluorescence color of sensor 74 changed from light green to dark green (with Cu²⁺), and to bright yellow-green (with Hg²⁺). The binding stoichiometry was 1 : 1 (74 : Cu²⁺/Hg²⁺). The association constant was 9.40 × 10⁴ M⁻¹ and 8.69 × 10⁴ M⁻¹ for Cu²⁺ and Zn²⁺, respectively. The detection limit was 70.6 nM and 0.116 μM for Cu²⁺ and Zn²⁺, respectively. The turn-on fluorescence response of sensor 74 with Hg²⁺ could be due to inhibition of the PET process upon coordination and the CHEF effect. The turn-off fluorescence response of sensor 74 with Cu²⁺ could be ascribed to paramagnetic characteristics of Cu²⁺ and the ICT process, resulting in the CHEQ effect (Fig. 30).

Fig. 30 Sensing mechanism of sensor 74 for Cu²⁺ and Hg²⁺. Reproduced with permission from ref. 137 Copyright 2017, Elsevier.

Zhu et al. synthesized carbazole-based Schiff base sensors 75 and 76 for Fe³⁺ and Cr³⁺ detection in acetonitrile solution (Fig. 24 and Table 2).¹³⁸ Among various metal ions, sensor 75 displayed significant fluorescence enhancement selectively with Fe³⁺ and Cr³⁺, credited to inhibition of the ESIPT process and C=N isomerization. Sensor 75, without the o-OH group, lacks selectivity towards tested metal ions. Fluorescence titration of sensor 75 with Fe³⁺ and Cr³⁺ showed a gradual increase in fluorescence intensity with the increasing concentration of Fe³⁺/Cr³⁺ at 495 nm (λ_ex = 380 nm) along with red-shift from 495 nm to 502 nm. The detection limit was 4.57 μM and 2.75 μM for Fe³⁺ and Cr³⁺, respectively. The binding stoichiometry was 2 : 1 (75 : Fe³⁺/Cr³⁺) and the association constant was 4.67 × 10⁴ M⁻¹ and 5.97 × 10⁴ M⁻¹ for Fe³⁺ and Cr³⁺, respectively.

3.3 Schiff-base–metal ion ensembles as a sequential sensor for ions^265,266

Nowadays, researchers are working on the design and synthesis of Schiff-base–metal ion ensembles as a sequential sensor for the detection of various analytes, viz. anions, cations, and metal ions. Generally, the interaction of the ensemble with metal ions results in recovery of the original fluorescence of the Schiff base, illustrating the reusability of the Schiff base sensor.

Fu et al. developed 8-hydroxyquinoline based Schiff base sensors 77 and 78 for Al³⁺ detection (Fig. 31 and Table 2).¹³⁹ In DMSO-H₂O (1 : 2, v/v) solution, both sensors 77 (λ_ex = 405 nm) and 78 (λ_ex = 455 nm) are non-fluorescent. In the presence of Al³⁺, sensors 77 and 78 exhibited a 270-fold and 460-fold increase in emission intensity at 505 nm and 509 nm, respectively. The complexation of sensor 77 or 78 with Al³⁺ inhibited the ESIPT process and C=N isomerization and resulted in fluorescence enhancement through the CHEF effect. The binding constant of sensors 77 and 78 with Al³⁺ was 9.40 × 10⁴ M⁻¹ and the limit of detection was 14.8 nM and 42.3 nM, respectively. The complexation stoichiometry was 1 : 1 (77/78 : Al³⁺). Further, the 77–Al³⁺ and 78–Al³⁺ complexes displayed fluorescence quenching selectively with F⁻. The limit of detection of the 77–Al³⁺ and 78–Al³⁺ complexes was 0.164 μM and 35.8 nM for F⁻, respectively. Both sensors 77 and 78 found practical application in Al³⁺ and F⁻ detection using filter paper and in the bioimaging of Al³⁺ and F⁻ in living cells.

Fig. 31 Chemical structure of chemosensors 77–83.

Gomathi et al. synthesized benzoxazole appended Schiff base 79 for fluorescence-based detection of Zn²⁺ and PPi (Fig. 31 and Table 2).¹⁴⁰ In the DMSO–H₂O (1 : 9, v/v, HEPES buffered) solvent system, sensor 79 showed weak emission maxima at 525 nm (λ_ex = 450 nm). On addition of different metal ions to the solution of sensor 79, no significant change in the fluorescence spectrum was observed, except for Zn²⁺. Fluorescence titration of sensor 79 with Zn²⁺ displayed a gradual increase in fluorescence intensity with the addition of an increasing concentration of Zn²⁺ and saturation was reached after the addition of 50 μM Zn²⁺. The binding stoichiometry was 1 : 1 (79/Zn²⁺) and the binding constant was 4.53 × 10⁴ M⁻¹. The detection limit was 0.52 μM for Zn²⁺. Further the sensor 79–Zn²⁺ ensemble showed significant fluorescence quenching selectively with PPi over other ions. The detection limit of the sensor 79-Zn²⁺ ensemble was 0.58 μM for PPi and the binding ratio was 1 : 1. Test strips coated with sensor 79 exhibited a greenish-yellow color with Zn²⁺ under UV lamp illumination. Sensor 79 was also applied for monitoring intracellular Zn²⁺ in HeLa cells.

Qiu et al. synthesized diphenylacrylonitrile- and pyridine-based Schiff base 80 for fluorescence-based detection of Co²⁺–Hg²⁺–Cu²⁺ sequentially (Fig. 31, 32 and Table 2).¹⁴¹ In the THF–H₂O (1 : 99) mixture, sensor 80 displayed AIE property and red fluorescence at emission wavelength 620 nm (λ_ex = 360 nm). Compared with various other tested metal ions, sensor 80 exhibited fluorescence quenching with Co²⁺ and Cu²⁺, and a slight blue-shift with Ag⁺ and Hg²⁺, whereas other metal ions had an insignificant effect. From the interference experiment, the quenched fluorescence of sensor 80 + Co²⁺ could be recovered with Hg²⁺ addition and quenched again by the addition of Cu²⁺. The limit of detection was 0.0937 μM, 0.0221 μM, and 0.0988 μM for Co²⁺, Hg²⁺ and Cu²⁺, respectively. The binding stoichiometry ratio was 2 : 1 (80 : Co²⁺/Hg²⁺/Cu²⁺). The binding constant value was 4.06 × 10¹¹ M⁻¹, 2.23 × 10¹³ M⁻¹, and 2.87 × 10¹³ M⁻¹ for Co²⁺, Hg²⁺ and Cu²⁺, respectively. In the ¹H NMR spectra of sensor 80 + Cu²⁺, the low field shift of CH=N protons and the disappearance of OH group protons suggested the complexation of Cu²⁺ with sensor 80 through nitrogen and oxygen atoms. Sensor 80 found application in test paper detection and bio-imaging of living cells.

Fig. 32 Photographs of sensor 80 solutions showing sequential detection of Co²⁺–Hg²⁺–Cu²⁺ under 365 nm UV light irradiation. Reproduced with permission from ref. 141 Copyright 2019, Elsevier.

Xu et al. developed imidazo[2,1-b]thiazole-based Schiff base sensor 81 for Zn²⁺ and PPi detection (Fig. 31 and Table 2).¹⁴² In ethanol–H₂O buffered (9 : 1, v/v) solution, sensor 81 showed turn-on fluorescence only with Zn²⁺ as well as a color change of the solution from colorless to yellow-green under a UV lamp, whereas other metal ions displayed insignificant change. Upon gradual addition of an increasing concentration of Zn²⁺ to the solution of sensor 81, the fluorescence intensity increased gradually with emission maxima at 511 nm (λ_ex = 330 nm) and reached a plateau after the addition of 6 × 10⁻⁵ M concentration of Zn²⁺. The binding ratio was 1 : 1 (81 : Zn²⁺) and the association constant value was 2.2 × 10⁵ M⁻¹. The lowest detection limit was 1.2 nM for Zn²⁺. The sensing mechanism could be credited to snapping of the PET process and C=N isomerization upon coordination of sensor 81 with Zn²⁺, leading to turn-on fluorescence through the CHEF process. The sensor 81–Zn²⁺ ensemble exhibited fluorescence quenching selectively with PPi over other tested ions. The detection limit of the ensemble was 1.9 nM for PPi. Additionally, INHIBIT type logic gates were constructed using the fluorescence signal of sensor 81 at the molecular level.

Wang et al. synthesized coumarin-based Schiff base 82 and utilized it as a fluorometric/colorimetric sensor for Cu²⁺ and glutathione detection (Fig. 31 and Table 2).¹⁴³ In H₂O–CH₃CN (1 : 1, v/v) PBS buffered solution, sensor 82 exhibited emission maxima at 521 nm (λ_ex = 450 nm). Compared with different metal ions, sensor 82 underwent selective fluorescence quenching (89.8% quenching) with Cu²⁺, credited to the paramagnetic nature of Cu²⁺ (Fig. 33A). Further, the 82–Cu²⁺ ensemble showed 9.2-fold fluorescence enhancement with glutathione through the Cu²⁺ displacement approach (Fig. 33B). The lowest detection limit of sensor 82 for Cu²⁺ and the 82–Cu²⁺ ensemble for glutathione was 24.0 nM and 0.129 μM, respectively. The binding constant was 2.63 × 10⁵ M⁻¹ for the 2 : 1 (82/Cu²⁺) complex. Sensor 82 and the 82–Cu²⁺ ensemble found application in the bio-imaging of Cu²⁺ and glutathione, respectively, in living cells and zebrafish.

Fig. 33 Fluorescence titration spectrum of (A) sensor 82 with Cu²⁺; (B) 82 + Cu²⁺ system with GSH. Reproduced with permission from ref. 143 Copyright 2020, Elsevier.

Patra et al. synthesized hydroxycoumarin-pyridylthioether-based Schiff base 83 for fluorescence-based detection of Zn²⁺ and ATP (Fig. 31 and Table 2).¹⁴⁴ In methanol–water (3 : 1, v/v) solution, sensor 83 exhibited poor yellow emission at 555 nm (λ_ex = 315 nm), assigned to activation of the ESIPT and PET processes. In the presence of Zn²⁺, sensor 83 displayed ∼240 times enhancement in fluorescence along with the blue shift of the emission maxima from 555 nm to 512 nm, credited to inhibition of the ESIPT and PET processes and the CHEF effect. The binding constant was 6.49 × 10⁴ M⁻¹ for 1 : 1 (83/Zn²⁺) stoichiometry. Additionally, the 83–Zn²⁺ ensemble displayed fluorescence quenching selectively with ATP, compared with other anions. The detection limit of sensor 83 and the 83–Zn²⁺ ensemble for Zn²⁺ and ATP was 0.078 μM and 6.6 μM, respectively. Sensor 83 demonstrated application in bio-imaging and intracellular Zn²⁺ detection in RAW 264.7 cells.

Mehta et al. synthesized rhodamine-based Schiff base 84 as a fluorometric sensor for Al³⁺ detection in an aqueous medium (Fig. 34 and Table 2).¹⁴⁵ Upon addition of various metal ions to the solution of sensor 84, turn-on emission with emission maxima at 588 nm (λ_ex = 350 nm) was observed only with Al³⁺ along with the color change of the solution from colorless to bright pink under UV light. Further, the addition of cyanide ions into the solution of the 84–Al³⁺ ensemble resulted in fluorescence quenching at 588 nm. The limit of detection of sensor 84 and the 84–Al³⁺ ensemble for Al³⁺ and CN⁻ was 1.68 μM and 0.815 μM, respectively. The binding constant was 3.98 mM⁻¹ for a 1 : 1 (84/Al³⁺) binding ratio. The sensing mechanism could be attributed to Al³⁺ induced hydrolysis of sensor 84, forming the free –CHO group, and the further nucleophilic attack of CN⁻ on the free –CHO group of the ensemble resulting in the removal of Al³⁺ and closing of the spirolactam ring by the charge transfer process. Sensor 84 found application in Al³⁺ detection in A549 cells.

Fig. 34 The binding mechanism of Al³⁺ and CN⁻ to sensor 84 and the 84–Al³⁺ complex, respectively. Reproduced with permission from ref. 145 Copyright 2019, Elsevier.

Kaur et al. synthesized Schiff base sensor 85 for fluorescence-based detection of Cu²⁺ and PO₄³⁻via the complexation/decomplexation approach (Fig. 35 and Table 2).¹⁴⁶ In DMSO–HEPES buffer (4 : 6, v/v) solution, sensor 85 showed fluorescence quenching only with Cu²⁺ over other tested metal ions. Fluorescence titration of sensor 85 with Cu²⁺ (λ_ex = 338 nm) revealed a continuous decrease in fluorescence intensity with the addition of an increasing concentration of Cu²⁺, which reached stabilization after the addition of 73.0 μM Cu²⁺. The sensing mechanism could be attributed to Cu²⁺ ion-induced hydrolysis of the imine functionality, forming the sensor 85–Cu²⁺ complex. Furthermore, the addition of PO₄³⁻ to the 85–Cu²⁺ ensemble solution resulted in turn-on emission at 422 nm via the decomplexation process. The detection limit of sensor 85 and the 85–Cu²⁺ ensemble for Cu²⁺ and PO₄³⁻ was 2.11 ppb and 31.6 ppb, respectively. Sensor 85 demonstrated practical application in Cu²⁺ detection and subsequently PO₄³⁻ detection using paper strips (Fig. 36).

Fig. 35 The sensing mechanism of sensor 85 for the sequential detection of Cu²⁺ and PO₄³⁻. Reproduced with permission from ref. 146 Copyright 2018, Elsevier.

Fig. 36 Photographs of paper strips showing fluorescence change of sensor 85 for the sequential detection of Cu²⁺ and PO₄³⁻ under 365 nm UV light illumination. Reproduced with permission from ref. 146 Copyright 2018, Elsevier.

Sheet et al. synthesized coumarin-based Schiff base 86 as a light-up probe for Al³⁺ detection (Fig. 37 and Table 2).¹⁴⁷ In 90% aqueous methanol solution, sensor 86 exhibited weak emission at 520 nm (λ_ex = 410 nm). Among various metal ions, sensor 86 showed ∼7-fold fluorescence enhancement only with Al³⁺ along with the color change from colorless to bright green under a UV lamp. Fluorescence titration of sensor 86 with Al³⁺ revealed a gradual increase in fluorescence intensity along with the blue shift of the emission maxima from 520 nm to 510 nm (Fig. 38). The binding constant was (9.9 ± 0.04) × 10³ M⁻¹ for the 1 : 1 (86/Al³⁺) complex. The lowest detection limit was 1.62 μM for Al³⁺. The complexation of Al³⁺ with sensor 86 makes the system more rigid by inhibiting C=N isomerization, leading to fluorescence enhancement through the CHEF effect. Additionally, the 86–Al³⁺ ensemble acted like an on–off type fluorescent sensor for biologically important nucleotides and phosphate ions. Sensor 86 also found application in exogenous Al³⁺ detection in living cells.

Fig. 37 The binding mechanism of Al³⁺ and H₂PO₄⁻/PPi to sensor 86 and 86–Al³⁺, respectively. Reproduced with permission from ref. 147 Copyright 2017, Elsevier.

Fig. 38 (A) Fluorescence spectrum of sensor 86 in the absence and presence of different metal ions. Inset: Visible color change of sensor 86 in the absence and presence of Al³⁺. (B) Fluorescence spectrum of the sensor 86–Al³⁺ ensemble in the absence (left) and presence (right) of different anions. Inset: visible color change of the sensor 86–Al³⁺ ensemble in the absence (left) and presence of PPi ions (right). Reproduced with permission from ref. 147 Copyright 2017, Elsevier.

3.4 Biomedical applications

3.4.1 Fluorescence imaging^267,268. Fluorescence imaging permits monitoring of various processes in cells, tissues, and organelles in real-time via fluorescence microscopy. In the literature, the various instrumental methods for metal ion detection such as atomic absorption spectrometry, inductively coupled plasma mass spectrometry, electrochemical voltammetry, etc. when applied in live-cell imaging have resulted in cell damage.^269,270 However, fluorescent sensors proved to be an effective tool for live-cell imaging because of their selective, sensitive, and rapid operation and ability to provide non-damaging images of living cells.

Mu et al. developed Schiff base sensor 87 based on carbazole and the triphenylamine unit for Al³⁺ detection (Fig. 39 and Table 2).¹⁴⁸ In ethanol solution, sensor 87 displayed weak fluorescence at 455 nm (λ_ex = 400 nm). Upon gradual addition of an increasing concentration of Al³⁺, sensor 87 underwent an increase in fluorescence intensity along with a red-shift of the emission maxima from 455 nm to 500 nm. The limit of detection was 63.0 nM for Al³⁺. The stoichiometry of the complex was 2 : 1 (87/Al³⁺). In ¹H NMR titration, the appearance of a new peak at 9.83 ppm upon addition of Al³⁺ to the solution of sensor 87 suggested Schiff base hydrolysis and fluorescence enhancement through the CHEF effect. Sensor 87 found potential application in live-cell imaging in living HeLa cells.

Fig. 39 Chemical structure of sensors 87–109.

Salarvand et al. designed and synthesized Schiff base sensor 88 based on 2-hydroxynaphthalene as an off–on sensor for Al³⁺ detection (Fig. 39 and Table 2).¹⁴⁹ In methanol/water (9 : 1, v/v) solution, sensor 88 displayed weak emission at 343 nm (λ_ex = 310 nm). Fluorescence titration of sensor 88 with Al³⁺ showed a gradual increase in fluorescence intensity which reached a plateau after the addition of one equivalent of Al³⁺. The binding stoichiometry was 1 : 1 (88/Al³⁺) and the binding constant was 1.546 × 10⁵ M⁻¹ as calculated using the Benesi–Hildebrand equation. The detection limit was 0.64 μM for Al³⁺. The chelation of sensor 88 with Al³⁺ inhibited the ICT process and C=N isomerization, resulting in fluorescence enhancement through the CHEF effect. Sensor 88 showed no cytotoxicity to living cells as evaluated using MTT assay on MCF-7 cell lines and found potential application in imaging and tracing Al³⁺ in living cells.

Saravanan et al. synthesized pyrene-based sensor 89 and utilized it for fluorescence-based detection of Cu²⁺ inside living cells (Fig. 39 and Table 2).¹⁵⁰ In DMSO–water (1 : 1, v/v) solution, sensor 89 displayed weak emission at 463 nm (λ_ex = 393 nm). Upon addition of an increasing concentration of Cu²⁺, sensor 89 underwent fluorescence enhancement along with red-shift of the emission maxima from 463 nm to 467 nm. Job's plot revealed 1 : 1 (89/Cu²⁺) stoichiometry and the association constant was 1.16 × 10⁴ M⁻¹. The detection limit was 0.26 μM for Cu²⁺. From the DFT calculation, the huge energy difference (−44637.828 eV) between sensor 89 and the sensor 89–Cu²⁺ (1 : 1) complex suggested strong coordination of sensor 89 with Cu²⁺. The bioimaging study of sensor 89 in living RAW 264.7 cells showed the good biocompatibility and cell permeability of sensor 89 and proved it to be an excellent biosensor for Cu²⁺ detection inside living RAW 264.7 cells.

Bhanja et al. developed vanillinyl-based Schiff base 90 and utilized it for fluorescence-based detection of Zn²⁺ (Fig. 39 and Table 2).¹⁵¹ In DMSO–water (3/7, v/v) solution, sensor 90 exhibited turn-on fluorescence response only with Zn²⁺ among various other metal ions studied. The fluorescence titration of sensor 90 with Zn²⁺ displayed a significant increase in fluorescence intensity at 472 nm (λ_ex = 346 nm), which became saturated after the addition of ∼1.1 equivalent of Zn²⁺. The binding stoichiometry was 1 : 1 (90/Zn²⁺) and the binding constant was 13.22 × 10⁴ M⁻¹. The detection limit was 0.018 μM for Zn²⁺. The sensing mechanism could be attributed to Zn²⁺ induced deprotonation and the CHEF effect. Sensor 90 found potential application in the examination of Zn²⁺ in living cells (African Monkey Vero cells) by a fluorescence-based cell imaging procedure.

Chen et al. synthesized rhodamine-based Schiff base sensors 91, 92, and 93 for fluorescence-based detection of Fe³⁺ (Fig. 39 and Table 2).¹⁵² In ethanol–water (1/1, v/v) solution, all the three sensors 91, 92, and 93 underwent fluorescence enhancement selectively with Fe³⁺ over various tested metal ions. Among sensors 91, 92, and 93, sensor 91 displayed 1.5-fold higher fluorescence enhancement with Fe³⁺ in comparison with sensors 92 and 93. The fluorescence titration of sensor 91 with Fe³⁺ displayed a gradual increase in fluorescence intensity at 584 nm (λ_ex = 545 nm). The complexation stoichiometry was 1 : 1 (91/Fe³⁺) and the binding constant was 7.66 × 10⁴ M⁻¹. The limit of detection was 28.1 nM for Fe³⁺. The sensing mechanism could be attributed to the opening of the spirolactam ring upon coordination of sensor 91 with Fe³⁺ through the oxygen and nitrogen atom of benzoylhydrazine and the oxygen atom of the oxole unit. Sensor 91 found potential application in Fe³⁺ detection in living cells. SMMC-7721 cells incubated with sensor 91 displayed bright red fluorescence (λ_ex = 545 nm) upon the addition of Fe³⁺ (Fig. 40a and b).

Fig. 40 Confocal fluorescence images of SMMC-7721 cells incubated with sensor 91 in the (a) absence and (b) presence of Fe³⁺. Reproduced with permission from ref. 152 Copyright 2018, Elsevier.

Feng et al. synthesized coumarin based sensor 94 for Cu²⁺ detection in CH₃CN/PBS (1 : 1, v/v) medium (Fig. 39 and Table 2).¹⁵³ On excitation at 450 nm, sensor 94 displayed strong emission at wavelength 540 nm. On gradual addition of an increasing concentration of Cu²⁺, sensor 94 underwent fluorescence quenching and reached a plateau after the addition of 5 equivalents of Cu²⁺. In the mass spectrum, the intense peak at m/z = 454.05 due to [94 + Cu²⁺–H]⁺ suggested 1 : 1 binding stoichiometry. Sensor 94 showed good biocompatibility and low cytotoxicity to MCF cells and A549 cells as determined by the CCK8 assay. Sensor 94 found potential application in Cu²⁺ detection in real water samples, and in living cells through confocal microscopy. The A549 cells and MCF-7 cells incubated with sensor 94 displayed strong fluorescence, and when treated with Cu²⁺ showed diminished fluorescence (Fig. 41).

Fig. 41 Confocal fluorescence images of A549 cells incubated with sensor 94 (probe) in the (a1) absence and (a2–a4) presence of Cu²⁺. Confocal fluorescence images of MCF-7 cells incubated with sensor 94 in the (b1) absence and (b2–b4) presence of Cu²⁺. Reproduced with permission from ref. 153 Copyright 2019, Elsevier.

Hu et al. reported Schiff base 95 as a two-photon fluorescent sensor for Cu²⁺ detection (Fig. 39 and Table 2).¹⁵⁴ In HEPES buffer solution, the one-photon fluorescence spectrum of sensor 95 displayed emission at wavelength 516 nm (λ_ex = 418 nm). Fluorescence titration of sensor 95 with Cu²⁺ revealed a decrease in emission intensity upon the addition of an increasing concentration of Cu²⁺. The detection limit was 0.16 μM for Cu²⁺. The binding stoichiometry was 2 : 1 (95/Cu²⁺) and the binding constant was 1.02 × 10⁻⁵ M⁻¹. The coordination of sensor 95 with Cu²⁺ through the nitrogen atom of the imine group and the oxygen atom of the phenolic hydroxyl moiety in a 2 : 1 (95 : Cu²⁺) manner resulted in fluorescence quenching as confirmed from ¹H NMR titration, mass spectra, and DFT calculations. Sensor 95 found practical application in the two-photon fluorescence imaging of Cu²⁺ in cellular mitochondria and in the liver and intestine of larval zebrafish (Fig. 42).

Fig. 42 Practical application of sensor 95 in two-photon fluorescence imaging of Cu²⁺ in cellular mitochondria and in the liver and intestine of larval zebrafish. Reproduced with permission from ref. 154 Copyright 2017, Elsevier.

Huang et al. developed isoquinoline-based Schiff base 96 as a turn-on sensor for Al³⁺ detection (Fig. 39 and Table 2).¹⁵⁵ In ethanol solution, sensor 96 displayed a gradual increase in fluorescence intensity at 458 nm (λ_ex = 400 nm) with the addition of an increasing concentration of Al³⁺. The limit of detection was 1.11 nM for Al³⁺. The binding constant was 2.78 × 10⁶ M⁻¹ and the binding stoichiometry was 2 : 1 (96/Al³⁺). The sensing mechanism could be attributed to inhibition of the ESIPT process upon coordination of Al³⁺ with sensor 96. Sensor 96 found potential application in the examination of intracellular Al³⁺ concentration in cells.

Jiao et al. synthesized Schiff base sensor 97 and utilized it as a dual emissive ratiometric fluorescent sensor for Hg²⁺ detection (Fig. 39 and Table 2).¹⁵⁶ In the acetonitrile–water (8 : 2) mixture, sensor 97 was weakly emissive with emission at 530 nm (λ_ex = 440 nm) because of the PET process. The fluorescence titration of sensor 97 with Hg²⁺ showed a 20-fold increase in fluorescence intensity (530 nm to 490 nm) upon excitation at 440 nm, and 60-fold fluorescence enhancement (530 nm to 415 nm) upon excitation at 315 nm. The emission maxima at 490 nm and 415 nm could be attributed to inhibition of the PET process and hydrolysis of the C=N group following the addition of Hg²⁺. The detection limit was 1 nM for Hg²⁺. Sensor 97 displayed potential for Hg²⁺ determination in living cells at nano-molar concentration through confocal microscopy. As shown in Fig. 43, MCF-cells stained with sensor 97 when exposed to Hg²⁺ demonstrated green fluorescence (410–470 nm) and red fluorescence (490–590 nm).

Fig. 43 Confocal microscope images of MCF-cells incubated with sensor 97 (a) under bright field; (b) green channel; (c) red channel; and further incubated with Hg²⁺ (d) under bright field; (e) green channel; (f) red channel. Reproduced with permission from ref. 156 Copyright 2017, Elsevier.

Li et al. reported Schiff base 98 as a turn-on sensor for Zn²⁺ (Fig. 39 and Table 2).¹⁵⁷ In buffer solution (ethanol/Tris–HCl, v/v, 4/1), sensor 98 underwent fluorescence enhancement at 458 nm (λ_ex = 380 nm) with the addition of an increasing concentration of Zn²⁺ and reached a plateau after the addition of 4 equivalents of Zn²⁺. The complex stoichiometry was 1 : 1 (98/Zn²⁺) and the binding constant was 4.457 × 10⁵ M⁻¹ as calculated using the Benesi–Hildebrand equation. The limit of detection was 1.37 μM for Zn²⁺. The coordination of Zn²⁺ with sensor 98 through imine nitrogen and the oxygen atom of the phenolic group makes the system more rigid by inhibiting C=N isomerization and the PET process, causing turn-on fluorescence through the CHEF effect. Sensor 98 found potential application in Zn²⁺ detection in living cells. The lung cancer A549 cells treated with sensor 98 exhibited a significant fluorescence response to Zn²⁺.

Liu et al. synthesized naphthol-based sensors 99, 100, and 101 for Al³⁺ detection (Fig. 39 and Table 2).¹⁵⁸ Among sensors 99, 100, and 101, sensor 101 proved to be the best for Al³⁺ detection over other tested metal ions in CH₃OH/HEPES buffer (3/7, v/v). Fluorescence titration of sensor 101 with Al³⁺ revealed 140-fold fluorescence enhancement at emission wavelength 435 nm (λ_ex = 350 nm) upon addition of 1.0 equivalent of Al³⁺. The stoichiometry of the complex was 1 : 1 (101/Al³⁺) and the binding constant was 1.01 × 10^6.5 M⁻¹. The detection limit was 0.05 μM for Al³⁺. The sensing mechanism could be attributed to inhibition of C=N isomerization upon coordination of Al³⁺ with sensor 101 through the nitrogen atom of the imine group and the oxygen atom of the –OH group, forming two saturated 5-membered rings and two unsaturated 6-membered rings, resulting in fluorescence enhancement through the CHEF effect. Sensor 101 showed potential for imaging intracellular Al³⁺ in living cells. The colon cancer SW480 cells stained with sensor 101 exhibited bright blue fluorescence with intracellular Al³⁺.

Naskar et al. developed benzophenone-derived Schiff base sensor 102 for Al³⁺ detection by a fluorometric method (Fig. 39 and Table 2).¹⁵⁹ In ethanol (0.01%)–HEPES buffer (50 mM) solution, sensor 102 (Φ_F = 0.067) exhibited weak fluorescence at 450 nm (λ_ex = 321 nm). In the presence of Al³⁺, sensor 102 underwent fluorescence enhancement (Φ_F = 0.237) at 450 nm. Job's plot showed 1 : 2 (Al³⁺/102) stoichiometry and the binding constant was 10.188 × 10³ M⁻¹. The limit of detection was 10⁻⁷ M for Al³⁺. The complexation of sensor 102 with Al³⁺ resulted in the formation of sensor 102–Al³⁺ aggregates, which in turn caused inhibition of C=N isomerization and the ESIPT process, leading to the CHEF effect. Sensor 102 displayed no cytotoxicity up to 40 μM as analyzed by MTT assay. The non-fluorescent HepG2 cells stained with sensor 102 exhibited bright blue fluorescence upon binding with intracellular Al³⁺.

Patra et al. reported vanillinyl thioether-based Schiff base 103 as a turn-on sensor for Zn²⁺ detection (Fig. 39 and Table 2).¹⁶⁰ On excitation at 346 nm, sensor 103 (Φ_F = 0.0051) displayed weak emission at 411 nm in DMSO/H₂O (HEPES buffer, v/v = 3/7) solution. In the presence of Zn²⁺, sensor 103 exhibited new emission at 555 nm and its intensity gradually increased with the increasing concentration of Zn²⁺ from 0 to ∼1 mol equiv. (Φ_F = 0.0399). The complexation of sensor 103 with Zn²⁺ caused blockage of the PET process, inhibition of the ESIPT process through deprotonation of the phenolic hydroxyl group and turn-on emission through the CHEF effect. The complexation stoichiometry was 1 : 1 (103/Zn²⁺) and the binding constant was 3.9 × 10⁴ M⁻¹. The detection limit was 60 nM for Zn²⁺. Vero cells treated with sensor 103 exhibited strong fluorescence when exposed to Zn²⁺, demonstrating the potential of sensor 103 for imaging intracellular Zn²⁺ in living cells (Fig. 44).

Fig. 44 Confocal microscope images of (a) Vero cells; (b) cells stained with sensor 103; (c) cells stained with Zn²⁺ and then exposed to sensor 103 for 10 minutes. Reproduced with permission from ref. 160 Copyright 2016, Elsevier.

Shellaiah et al. synthesized pyrene-based Schiff base 104 and utilized it as a turn-on sensor for Hg²⁺ detection by a fluorometric method (Fig. 39 and Table 2).¹⁶¹ In DMSO–H₂O (7 : 3, v/v) solution, sensor 104 showed selective turn-on fluorescence response with Hg²⁺ over other metal ions. Fluorescence titration of sensor 104 with Hg²⁺ displayed a 31-fold increase in fluorescence intensity along with a red-shift of the emission maxima from 427 nm to 445 nm (λ_ex = 347 nm). The stoichiometry of the complex was 2 : 1 (104/Hg²⁺) and the association constant was 7.36 × 10⁴ M⁻¹. The limit of detection was 2.82 μM for Hg²⁺. The turn-on sensing mechanism could be attributed to excimer formation (104–104*) and the CHEF effect (Fig. 45A). Sensor 104 displayed potential for imaging Hg²⁺ in living cells using a confocal microscope. The non-fluorescent HeLa cells incubated with sensor 104 exhibited bright blue fluorescence upon treatment with Hg²⁺ (Fig. 45B).

Fig. 45 (A) HOMOs, LUMOs and the energy gap of sensor 104 and 104 + Hg²⁺ (104–104*) complexes. (B) Confocal microscope images of HeLa cells treated with (a) sensor 104 and (b) sensor 104 + Hg²⁺. Reproduced with permission from ref. 161 Copyright 2015, Royal Society of Chemistry.

Simon et al. developed anthracene- and pyridine-derived Schiff base 105 as an off–on sensor for Cu²⁺ detection (Fig. 39 and Table 2).¹⁶² On excitation at 350 nm, sensor 105 displayed 23-fold fluorescence enhancement at 430 nm selectively with Cu²⁺ over other tested metal ions in acetonitrile solution. The stoichiometry of the complex was 1 : 1 (105/Cu²⁺) and the association constant was 2.12 × 10⁶ M⁻². The detection limit was 86.3 nM for Cu²⁺. The sensing mechanism could be ascribed to suppression of the PET process upon Cu²⁺ coordination with sensor 105, leading to the formation of the 5-membered stable chelate ring and turn-on emission through the CHEF effect. Sensor 105 showed excellent cell permeability, no cytotoxicity up to 40 μM concentration, and potential for fluorescence imaging of sub-cellular Cu²⁺. The non-fluorescent Raw264.7 cells incubated with sensor 105 exhibited bright blue fluorescence upon treatment with Cu²⁺ under a confocal microscope.

Tian et al. synthesized quinoline-based Schiff base sensor 106 and utilized it as a turn-on sensor for Al³⁺ detection (Fig. 39 and Table 2).¹⁶³ In methanol solution, the complexation behavior of sensor 106 towards various metal ions revealed selective 38-fold fluorescence enhancement at 517 nm (λ_ex = 373 nm) only with Al³⁺. Job's plot revealed a 1 : 1 (106/Al³⁺) complexation and the association constant as determined using the Benesi–Hildebrand equation was 2.4 × 10⁴ M⁻¹. The detection limit was 0.12 μM for Al³⁺. The turn-on sensing mechanism could be credited to suppression of the PET process upon Al³⁺ coordination with sensor 106. HeLa cells incubated with different concentrations of Al³⁺ displayed fluorescence emission when exposed to sensor 106, demonstrating the potential of sensor 106 for imaging Al³⁺ in living cells.

Tian et al. developed 2-hydroxynaphthalene-based Schiff base sensor 107 for Al³⁺ detection (Fig. 39 and Table 2).¹⁶⁴ In methanol medium, free sensor 107 was non-fluorescent. Upon addition of Al³⁺, sensor 107 underwent a significant increase in fluorescence intensity (150-fold) at 430 nm (λ_ex = 307 nm). The binding constant was 3.01 × 10⁴ M⁻¹ and the limit of detection was 0.137 μM for Al³⁺. ¹H NMR and Job's plot analysis indicated inhibition of the ESIPT process upon coordination of Al³⁺ with sensor 107 through the nitrogen atom of imine, secondary amine, and the naphthol hydroxyl group in 1 : 1 stoichiometry. HeLa cells incubated with Al³⁺ showed blue fluorescence when exposed to sensor 107 under a confocal microscope, demonstrating the potential of sensor 107 for fluorescence imaging of Al³⁺ in living cells. Sensor 107 also found application in the in vivo bioimaging of Al³⁺ in zebrafish larvae (Fig. 46A and B).

Fig. 46 Confocal microscopic images of HeLa cells incubated with (i) sensor 107 and (ii) sensor 107 and further with Al³⁺. (b) Confocal microscopic images of zebrafish larvae treated with (i) sensor 107 and (ii, iii) sensor 107 and further with Al³⁺. Reproduced with permission from ref. 164 Copyright 2019, Elsevier.

Wang et al. developed boron-dipyrromethene-based Schiff base 108 for fluorescence-based detection of Au³⁺ (Fig. 39 and Table 2).¹⁶⁵ In phosphate buffer–ethanol (7 : 3, v/v) solution, sensor 108 was weakly fluorescent. Among various metal ions, significant enhancement in fluorescence intensity was observed only with Au³⁺ (Fig. 47A). Upon gradual addition of Au³⁺ to the solution of sensor 108, the fluorescence intensity increased linearly with concentration from 0 to 50 μM at 516 nm (λ_ex = 480 nm) and reached a plateau after the addition of 6 equvi. of Au³⁺. The limit of detection was 60 nM for Au³⁺. The sensing mechanism was a reaction-based process, wherein the reaction of sensor 108 with Au³⁺ resulted in the conversion of the C=N bond into aldehyde and formation of a new boron-dipyrromethene derivative (bright green fluorescent). Sensor 108 found potential application in monitoring Au³⁺ in living cells and in zebrafish. As shown in Fig. 47C, PC12 cells incubated with sensor 108 displayed bright fluorescence when exposed to Au³⁺. Fig. 47B shows an increase in fluorescence intensity with the increasing concentration of Au³⁺ ions in zebrafish.

Fig. 47 (A) Fluorescence spectrum of sensor 108 in the presence of different metal ions. (Inset:) Visible color change of sensor 108 in the presence of Au³⁺. (B) Confocal microscopic images of (a) zebrafish incubated with sensor 108; (b–f) zebrafish incubated with different concentrations of Au³⁺ followed by the addition of sensor 108. (C) Confocal microscopic images of PC12 cells (a₁, b₁, c₁) incubated with sensor 108; (a₂, b₂, c₂) incubated with sensor 108 and further incubated with Au³⁺. Reproduced with permission from ref. 165 Copyright 2016, Elsevier.

Rahman et al. developed methionine-based Schiff base 109 as a turn-on sensor for Zn²⁺ detection (Fig. 39 and Table 2).¹⁶⁶ In buffered acetonitrile–water (1 : 1) solution, sensor 109 displayed significant fluorescence enhancement at 450 nm (λ_ex = 370 nm) only with Zn²⁺ over other metal ions. The binding constant was 5.3 × 10⁶ M⁻¹ and the detection limit was 0.5 μM for Zn²⁺. The sensing mechanism could be attributed to the complexation of Zn²⁺ with the oxygen, nitrogen, and sulfur atom of sensor 109 in a 1 : 1 ratio as confirmed from ¹H NMR titration. Breast cancer cells incubated with sensor 109 and then with Zn²⁺ displayed strong fluorescence under a confocal microscope, demonstrating the potential of sensor 109 for monitoring intracellular Zn²⁺.

Yang et al. developed biphenyl- and benzohydrazide-based Schiff base 110 for Cu²⁺ detection (Fig. 48 and Table 2).¹⁶⁷ In buffered ethanol–water (1 : 1, v/v) solution, sensor 110 exhibited fluorescence at 566 nm (λ_ex = 400 nm). Fluorescence titration of sensor 110 with Cu²⁺ revealed a gradual decrease in fluorescence intensity, which reached saturation after the addition of 4.0 equivalents of Cu²⁺. The association constant was 13.87 × 10⁴ M⁻¹ and the binding stoichiometry was 1 : 1 (110/Cu²⁺). The limit of detection was 15.4 nM for Cu²⁺. The coordination of Cu²⁺ with sensor 110 through two oxygen atoms of the hydroxyl group and two nitrogen atoms of the imine group resulted in excited state energy/electron transfer from sensor 110 to Cu²⁺, causing fluorescence quenching. The bright fluorescence of HeLa cells incubated with sensor 110 diminished after treatment with Cu²⁺, demonstrating the potential application of sensor 110 in biological systems.

Fig. 48 Chemical structure of sensors 110–136.

Zhu et al. developed Schiff base 111 as an on–off sensor for Al³⁺ detection in DMSO–H₂O (9 : 1, v/v) solution (Fig. 48 and Table 2).¹⁶⁸ On excitation at 377 nm, sensor 111 exhibited a weak emission band at 479 nm. In the presence of Al³⁺, sensor 111 displayed a more than 700-fold increase in fluorescence intensity at 479 nm. The detection limit was 9.79 nM for Al³⁺. The complexation stoichiometry was 2 : 1 (111/Al³⁺). The chelation of sensor 111 with Al³⁺ inhibited the PET process and resulted in fluorescence enhancement through the CHEF effect. The Benesi–Hildebrand equation revealed an association constant value of 7.106 × 10¹⁰ M⁻². HeLa cells incubated with sensor 111 displayed insignificant fluorescence. Upon further incubation with Al³⁺, remarkable bright blue fluorescence was observed under a confocal microscope, demonstrating the application potential of sensor 111 in biological systems.

3.4.2 Bacterial detection. Pathogenic bacteria cause serious diseases such as typhoid fever, tetanus, tuberculosis, pneumonia, etc., globally. Therefore, for safety purposes, detection of bacteria is important. In the literature, various methods such as PCR-based assays, immunological methods, colony counting and culturing, etc. have been used for the detection of bacteria. However, these methods suffer from complicated procedures, time-consuming operations, and costly apparatus. Fluorescent biosensors offer the advantages of high selectivity, sensitivity, and fast response time in bacterial detection.

Subha et al. developed Schiff base 112 as a turn-on sensor for Zn²⁺ (Fig. 48 and Table 2).¹⁶⁹ In an aqueous medium, sensor 112 exhibited weak emission at 458 nm (λ_ex = 350 nm). Sensor 112 showed selective fluorescence enhancement only with Zn²⁺ over other metal ions. Fluorescence titration of sensor 112 with Zn²⁺ revealed ∼40-fold fluorescence enhancement at 460 nm along with a slight red-shift, attributed to inhibition of C=N isomerization. The stoichiometry of the complex was 1 : 1 (112/Zn²⁺) and the binding constant was 1.57 × 10⁵ M⁻¹. The limit of detection was 10.4 nM for Zn²⁺. Sensor 112 exhibited potential application in sensing bacterial cells, viz. Staphylococcus aureus and E. coli. The turn-on fluorescence enhancement with bacterial cells could be credited to the formation of an amide or ester bond with the cell wall of bacteria (Fig. 49).

Fig. 49 Fluorescence photographs of (a) Streptococcus cells; (b) E. coli cells; (c) Streptococcus cells incubated with sensor 112; (d) E. coli cells incubated with sensor 112. Reproduced with permission from ref. 169 Copyright 2016, Elsevier.

3.4.3 Drug sample analysis. The improvement in the field of analytical chemistry is supported by the use of fluorescent materials,¹⁷⁰ biomaterials,^271,272 and nanomaterials^273–275 in the analysis of samples. Fluorescent materials offer various advantages such as fast analysis time, high sensitivity and selectivity, low detection limit, and in vivo and in vitro investigation facility.

Kundu et al. reported Schiff base sensor 113 for Al³⁺ and Zn²⁺ detection in methanol–H₂O (1/9, v/v) solution (Fig. 48 and Table 2).¹⁷⁰ On excitation at 254 nm, sensor 113 exhibited weak emission with maxima at 355 nm. In the presence of Al³⁺ or Zn²⁺, sensor 113 underwent fluorescence enhancement with emission maxima centred at 495 nm (λ_ex = 392 nm) for Al³⁺ and with emission maxima at 448 nm (λ_ex = 384 nm) for Zn²⁺ (Fig. 50a and b). Job's plot proved a 1 : 1 (113 : Al³⁺/Zn²⁺) complexation. The binding constant value was 1.66 × 10⁴ M⁻¹ and 0.6 × 10⁴ M⁻¹ for Al³⁺ and Zn²⁺, respectively. The detection limit was 0.804 μM and 0.795 μM for Al³⁺ and Zn²⁺, respectively. The complexation of sensor 113 with Al³⁺/Zn²⁺ provides rigidity to the system by inhibiting the ESIPT process, the PET process, and C=N isomerization, causing fluorescence enhancement through the CHEF effect. Sensor 113 found application in Al³⁺ and Zn²⁺ analysis in drug samples such as Gelucil and Zincovit, as well as in bioimaging of Al³⁺ and Zn²⁺ in HeLa cells.

Fig. 50 The emission spectrum of sensor 113 in the absence and presence of different metal ions: (a) fluorescence response on excitation at 392 nm; (b) fluorescence response on excitation at 384 nm. Reproduced with permission from ref. 170 Copyright 2019, Elsevier.

3.4.4 Biological sample analysis²⁷⁶. Bovine serum albumin is a large globular protein composed of 582 amino acid residues. BSA is most abundant in plasma protein (0.38 mL⁻¹). BSA performs important functions in our circulatory system such as transport of endogenous and exogenous compounds such as drugs, etc.^277,278 Thus, quantification of BSA is important. Recently, researchers are developing fluorescent biosensors for metal ion detection using BSA as a medium, since it provides rapid, highly selective, and sensitive detection under physiological conditions.

Ghorai et al. developed sensor 114 based on azino bis-Schiff base for fluorescence-based detection of Pb²⁺ (Fig. 48 and Table 2).¹⁷¹ In methanol–water (2 : 1, v/v) solution, sensor 114 exhibited weak emission at 429 nm (λ_ex = 340 nm). Among various metal ions, sensor 114 displayed 34-fold fluorescence enhancement at 442 nm (λ_ex = 340 nm) only with Pb²⁺. The limit of detection was 8 nM for Pb²⁺. The binding stoichiometry was 2 : 1 (114 : Pb²⁺). The coordination of Pb²⁺ with sensor 114 resulted in blockage of the PET process along with the formation of a rigid system, causing fluorescence enhancement through the CHEF effect. Fluorescence titration of sensor 114 with Pb²⁺ in BSA medium displayed a ∼32-fold increase in emission intensity, illustrating the potential application of sensor 114 in biological samples. In addition, sensor 114 found application in Pb²⁺ determination in environmental samples such as tap water, river water, and urine samples.

Kumar et al. developed Schiff base sensor 115 for fluorescence-based detection of Al³⁺ by turn-on mode (Fig. 48 and Table 2).¹⁷² In CH₃CN–H₂O (1 : 1, v/v) solution, sensor 115 exhibited emission maxima at 430 nm (λ_ex = 360 nm). Fluorescence titration of sensor 115 with Al³⁺ revealed a 42-fold increase in fluorescence intensity. The binding constant was 0.4 × 10⁶ M⁻¹ and the binding stoichiometry ratio was 3 : 1 (115 : Al³⁺). The limit of detection was 50 μM for Al³⁺. The off–on fluorescence behavior could be attributed to inhibition of the PET process and enhanced electron delocalization upon coordination of Al³⁺ with sensor 115. The fluorescence titration of sensor 115 with Al³⁺ in BSA aqueous medium showed 30 times fluorescence enhancement in comparison to that of the initial one, demonstrating the application of sensor 115 in biological samples. In addition, sensor 115 found application in Al³⁺ detection in living cells (human embryonic kidney (HEK)).

Ghorai et al. developed Schiff base sensor 116 for fluorescence-based detection of Al³⁺ and HSO₃⁻ in methanol–water (2 : 1, v/v) solution (Fig. 48 and Table 2).¹⁷³ Fluorescence titration of sensor 116 with Al³⁺ showed a gradual increase in fluorescence intensity at 373 nm (λ_ex = 310 nm) upon addition of an increasing concentration of Al³⁺ along with the blue-shift of the emission maxima by 28 nm (Fig. 51A). The interaction of Al³⁺ with sensor 116 resulted in inhibition of the PET process and C=N isomerization, causing fluorescence enhancement through the CHEF effect. The stoichiometry of the complex was 1 : 2 (116/Al³⁺) and the association constant was 1.26 × 10⁵ M⁻¹. The limit of detection was 0.903 μM for Al³⁺. Sensor 116 showed remarkable detection over a wide pH range of 4–11 and found application in Al³⁺ detection in the aqueous solution of bovine serum albumin protein (Fig. 51B).

Fig. 51 Fluorescence titration of (A) sensor 116 with Al³⁺ in methanol–water (2 : 1, v/v) solution; (B) sensor 116 with Al³⁺ in bovine serum albumin medium. Reproduced with permission from ref. 173 Copyright 2015, Royal Society of Chemistry.

3.4.5 Anticancer activity^{174–176,279}. Nowadays cancer is a very serious disease, and looking into the present situation of drugs for cancer treatment, WHO predicted that more than 13.1 million people may die by 2030 due to cancer. Currently, platinum-based complexes are used as antitumor drugs in the clinic worldwide. However, these drugs have shown multiple side effects such as drug resistance, tissue damage, etc. This necessitates the development of an efficient drug for cancer treatment.

Liu et al. synthesized Schiff base complexes 117 and 118 for fluorescence-based detection of Cu²⁺ and Fe³⁺ in an aqueous medium (Fig. 52 and Table 2).¹⁷⁴ Among various metal ions, sensor 117 underwent fluorescence quenching selectively with Cu²⁺ and Fe³⁺ only. The sensing mechanism could be attributed to host–guest interaction, metal-ion exchange, or competition between the metal ions and complexes for absorption and excitation energy. Furthermore, complex 118 demonstrated excellent anticancer activity against four human tumor cell lines, namely, MDA-MB-231 (human breast cancer), HT29 (human colon cancer), HeLa (human cervical cancer), and PC-3 (human prostate cancer).

Fig. 52 Chemical structure of complexes 117 and 118.

Considering the important applications of coumarin derivatives such as antifungal, anticoagulant, and anticancer, Liu et al. synthesized Schiff base-Ir(III) complexes (119–124) as an antitumor agent (Fig. 53 and Table 2).¹⁷⁵ All the six complexes exhibited excellent biological stability, and their anticancer activity can be tuned by slight adjustment in the number of phenyl groups and electron donor ability of the Schiff base. In DMSO–H₂O (2 : 8, v/v), the fluorescence spectrum of complexes 119–121 showed maxima at 422 nm and this was red-shifted to 474 nm in complexes 122–124. Further, these complexes successfully targeted lysosomes and mitochondria, could induce apoptosis, and displayed potential application as antitumor agents.

Fig. 53 Chemical structure of complexes 119–124.

Kim et al. provided a naphthalene-based donor–acceptor type platform for fluorescence-based detection of Fe³⁺ (Fig. 54 and Table 2).¹⁷⁶ The fluorescence spectrum of sensor 125 showed enhancement in emission intensity at 550 nm upon the addition of Fe³⁺, credited to inhibition of the ESIPT based quenching effect. Further, the sensor displayed application in the fluorescence imaging of Fe³⁺ in HeLa cells and cancer cell lines. The yellow emission of HeLa cells following incubation with exogenous Fe³⁺ was more intense than that of the control group incubated with sensor 125.

Fig. 54 Sensing mechanism of Fe³⁺ with sensor 125.

3.5 Environmental monitoring

3.5.1 Catalytic applications. Over the years, numerous Schiff base–metal complexes have been developed and efficiently used as catalysts in organic transformation reactions.^280,281 Thus, discussion of the catalytic applications of fluorescent Schiff base–metal complexes constitutes an interesting topic of research.

Salih et al. prepared fluorescent Schiff base ligand 126 using 2-(1-piperazinyl)ethylamine and 2-hydroxy-5-nitrobenzaldehyde, and the corresponding Cu(II)-complexes (Fig. 55 and Table 2).¹⁷⁷ In DMSO solution, the emission spectrum of the Schiff base ligand 126 revealed turn-off response upon complexation with Cu(II) (λ_ex = 360 nm, λ_em = 475 nm), could be attributed to the PET process and the CHEF effect or to structural change in the ligand upon complexation. Further under mild catalytic conditions for oxidation and in the presence of H₂O₂ (green oxidant), the Schiff base–Cu(II) complex successfully catalyzed the benzyl alcohol to benzaldehyde oxidation reaction.

Fig. 55 (A) Chemical structure of Schiff base ligand 126 and the 126–Cu²⁺ complex. (B) Benzyl alcohol oxidation reaction catalysed by 126–Cu²⁺ complex.

3.5.2 Adsorption. Fluorescent Schiff base doped polymeric materials have emerged as new candidates for metal ion detection, owing to several advantages offered by fluorescent polymeric materials such as amplified optical properties, high detection limit, etc.

Xue et al. developed sulfonylhydrazone type Schiff base sensor 127 (AIE-active) for fluorescence-based detection of Hg(II) (Fig. 56 and Table 2).¹⁷⁸ The emission spectrum of sensor 127 showed turn-off response (λ_ex = 414 nm, λ_em = 549 nm) with the addition of an increasing concentration of Hg²⁺, and a good linear range between 0.1 and 0.6 μM concentration of Hg²⁺. The limit of detection was 0.284 nM, and the response time was 15 seconds for Hg²⁺. Interestingly, polyacrylamide (PAM) doped Schiff base 127 (127–PAM polymer) demonstrated high efficiency of Hg²⁺ adsorption, with a 99.9% removal rate. Additionally, sensor 127 displayed potential for Hg²⁺ detection in real water samples and for logic gate construction.

Fig. 56 Sensing mechanism of Hg²⁺ with sensor 127.

3.5.3 Real sample analysis^{185,280,282–285}. Although the fluorescent sensors for metal ion detection have shown advantages such as simplicity, selectivity, sensitivity, and rapid response time over other analytical methods, working with some toxic metal ions has always been problematic due to strong hydration, poor coordination, and lack of spectroscopic characteristics. Therefore, researchers are working on the development of fluorescent materials for direct detection of metal ions in real samples such as water, food, etc.

Peng et al. synthesized pyridine-based Schiff base sensors 128 and 129 for Al³⁺ detection (Fig. 48 and Table 2).¹⁷⁹ In DMF–H₂O (v : v = 1 : 9) solution, both sensors 128 and 129 exhibited weak emission centered at 483 nm (λ_ex = 360 nm) and 467 nm (λ_ex = 300 nm), respectively. However, upon the addition of Al³⁺, sensors 128 and 129 displayed an increase in fluorescence intensity at 483 nm and 467 nm, respectively. The turn-on fluorescence was associated with colorless to aquamarine color change of the solution. The limit of detection for Al³⁺ was 3.2 nM and 0.29 nM with sensors 128 and 129, respectively. Job's plot and HRMS revealed 1 : 1 (128/129 : Al³⁺) stoichiometry of the complex. The mechanism of interaction could be ascribed to the blockage of the PET and ESIPT processes upon addition of Al³⁺ as confirmed from the ¹H NMR titration experiment. Sensors 128 and 129 found potential application in the detection of Al³⁺ in lake water and tap water.

Guo et al. synthesized oligothiophene-based Schiff base 130 as a fluorometric probe for Cu²⁺ detection (Fig. 48 and Table 2).¹⁸⁰ In DMF/H₂O (2/3, v/v) solution, free sensor 130 exhibited strong fluorescence with emission maxima at 580 nm (λ_ex = 450 nm). In the presence of Cu²⁺, sensor 130 underwent fluorescence quenching along with the hypsochromic shift of the emission maxima from 580 nm to 560 nm. The binding constant was 2.52 × 10⁴ M⁻¹ for the 1 : 1 (130/Cu²⁺) complex. The limit of detection was 28.1 nM for Cu²⁺. The decrease in the fluorescence intensity of sensor 130 upon complexation with Cu²⁺ could be ascribed to the paramagnetic nature of Cu²⁺ and the CHEQ mechanism. Furthermore, sensor 130 found application in Cu²⁺ detection in real samples such as water and food with excellent accuracy and precision.

Kang et al. synthesized naphthalimide functionalized Schiff base 131 as a fluorescent turn-on sensor for Al³⁺ detection (Fig. 48 and Table 2).¹⁸¹ In methanol solution, sensor 131 exhibited weak emission maxima at 524 nm (λ_ex = 401 nm). Among various tested metal ions, sensor 131 showed significant fluorescence enhancement with Al³⁺ (39-fold), and fluorescence quenching to some extent with Ni²⁺, Fe²⁺, Co²⁺, and Cu²⁺, whereas other metal ions had an insignificant effect. Fluorescence titration of sensor 131 with Al³⁺ revealed linear enhancement in emission intensity along with the blue shift of the emission maxima from 524 nm to 508 nm, attributed to the combined effect of inhibition of the ICT process and the CHEF effect (Fig. 57A and B). The detection limit was 7.4 nM for Al³⁺. The association constant was 1.62 × 10⁴ M⁻¹ for the 1 : 1 (131/Al³⁺) complex. In addition, the sensor displayed potential application in measuring Al³⁺ concentration in real water samples.

Fig. 57 (A) Fluorescence titration of sensor 131 with Al³⁺. (B) Plot of fluorescence intensity vs. Al³⁺ concentration. Inset: naked eye detectable color change of sensor 131 in the absence (left) and presence of Al³⁺ (right) under 365 nm UV light illumination. Reproduced with permission from ref. 181 Copyright 2017, Elsevier.

Kao et al. synthesized quinoline-derived Schiff base 132 as a fluorescence turn-on sensor for Mg²⁺ detection (Fig. 48 and Table 2).¹⁸² On excitation at 353 nm, free sensor 132 displayed no significant emission in acetonitrile solution. However, in the presence of Mg²⁺, sensor 132 underwent a significant increase in emission intensity at 487 nm and 532 nm. The association constant was 1.91 × 10⁷ M⁻¹ for the 1 : 1 (132/Mg²⁺) complex. The detection limit was 19.1 ppb for Mg²⁺. The coordination of sensor 132 with Mg²⁺ resulted in the formation of a rigid system by inhibiting C=N isomerization, leading to fluorescence enhancement through the CHEF effect. In addition, sensor 132 found practical application in the determination of Mg²⁺ in real water samples such as tap, ground, and lake water (Fig. 58).

Fig. 58 Photographs of Mg²⁺-induced light-up emission of sensor 132 in different media such as CH₃CN, lake water, groundwater, and tap water. Reproduced with permission from ref. 182 Copyright 2016, Elsevier.

Zhang et al. developed Schiff base 133 with AIEE (Aggregation induced emission enhancement) characteristics as a turn-off fluorescent sensor for Cu²⁺ (Fig. 48 and Table 2).¹⁸³ In THF-H₂O (4/1, v/v) solution, sensor 133 exhibited emission maxima at 565 nm (λ_ex = 428 nm). Both UV-Vis and fluorescence spectra of sensor 133 displayed good selectivity only for Cu²⁺ over other tested metal ions. Fluorescence titration of sensor 133 with Cu²⁺ showed a gradual decrease in fluorescence intensity at 565 nm with the addition of an increasing concentration of Cu²⁺. The limit of detection was 16.4 nM for Cu²⁺. The binding stoichiometry was 1 : 2 (133/Cu²⁺) and the binding constant was 1.22 × 10³ M⁻¹. The transfer of lone pairs of electrons from the nitrogen and oxygen atoms to the empty orbitals of Cu²⁺ blocked the initial ESIPT process and resulted in fluorescence quenching (Fig. 59). Furthermore, sensor 133 found application in monitoring Cu²⁺ concentration in environmental water samples. The percentage recovery of sensor 133 was 98.49–102.37% in tap water and 99.10–102.90% in lake water.

Fig. 59 Sensing mechanism of sensor 133 for Cu²⁺ detection. Reproduced with permission from ref.183 Copyright 2021, MDPI, Basel, Switzerland.

Peng et al. developed naphthalimide-based Schiff base 134 as a fluorescent turn-on probe for Al³⁺ (Fig. 48 and Table 2).¹⁸⁴ In DMF-HEPES (1 : 1, v/v) solution, free sensor 134 was weakly fluorescent with emission maxima at 531 nm (λ_ex = 350 nm). Upon addition of Al³⁺ into the solution of sensor 134, a significant increase in fluorescence intensity was observed at 524 nm, attributed to inhibition of the PET process and the CHEF effect. The binding constant value was 4.95 × 10⁴ M⁻¹ for the 1 : 1 (134/Al³⁺) complex. The lowest limit of detection was 86.5 nM for Al³⁺. Furthermore, sensor 134 was found to be useful for Al³⁺ determination in real water samples such as yellow river and tap water.

Further, the same group reported another naphthalimide-based Schiff base 135 for fluorescence-based detection of Al³⁺ (Fig. 48 and Table 2).¹⁸⁵ In Tris–HCl buffer (1 : 1, v/v) solution, free sensor 135 exhibited weak emission at 526 nm (λ_ex = 350 nm). Upon addition of various metal ions to the solution of sensor 135, no significant change in emission intensity was observed except with Al³⁺. In the presence of Al³⁺, sensor 135 showed fluorescence enhancement with emission maxima at 526 nm, attributed to strong coordination between sensor 135 and Al³⁺ resulting in inhibition of the PET process and the CHEF effect. The limit of detection was 0.34 μM for Al³⁺. The binding constant was 2.6 × 10⁴ M⁻¹ for the 1 : 1 (135/Al³⁺) complex. Moreover, sensor 135 demonstrated potential application in monitoring Al³⁺ concentration in real water samples such as yellow river and tap water.

Zhu et al. developed Schiff base 136 as a turn-on fluorescent sensor for Al³⁺ detection (Fig. 48 and Table 2).¹⁸⁶ In ethanol-HEPES buffer (95 : 5, v/v) solution, sensor 136 showed high selectivity for Al³⁺ over other tested metal ions. On excitation at 273 nm, sensor 136 displayed weak emission at 473 nm and 499 nm. However, upon the addition of Al³⁺, a significant enhancement in emission intensity (63-fold) was observed at 473 nm and 499 nm along with the color change of the solution from green to bright blue under 273 nm UV light illumination. The complexation stoichiometry was 1 : 1 (136/Al³⁺). The binding constant of the complex was 6.53 × 10³ M⁻¹ and the detection limit was 0.108 μM for Al³⁺. The sensing mechanism could be attributed to blockage of the PET process upon complexation of sensor 136 with Al³⁺. Furthermore, sensor 136 displayed potential application in detecting Al³⁺ in living cells and environmental samples.

3.5.4 Rewritable paper. Paper is a versatile material with various uses such as writing, reading, printing, cleaning, etc. Although paper is recyclable, huge energy consumption and chemical discharge during the recycling process are unavoidable. In the present world, the massive use of paper is being continuously alleviated by digital storage devices such as computers, mobile, etc. Nowadays, researchers are focusing on the development of organic photochromic materials due to their potential application as rewritable paper, sensors, data storage systems, molecular switches, etc.^286–291

Sun et al. designed tetraphenylethene-based Schiff base 137 as a colorimetric/fluorescent sensor for Zn²⁺ and for inkless rewritable paper (Fig. 60 and Table 2).¹⁸⁷ In the THF–H₂O (1 : 1, v/v) mixture, sensor 137 showed weak emission maxima at 600 nm, which upon addition of Zn²⁺ resulted in significant fluorescence enhancement along with the blue shift of the emission maxima from 600 nm to 550 nm. Moreover, the emission color of the solution changed from weak red to strong yellow (Fig. 61A). The limit of detection was 38.9 nM for Zn²⁺. The sensing mechanism could be attributed to the CHEF effect which inhibits the ESIPT, PET, and non-radiative processes, thus resulting in fluorescence enhancement. Further, an inkless rewritable, self-erasing paper was fabricated by utilizing sensor 137 as the imaging layer and H₂O as ink, which showed potential application in data storage, information security, and anti-counterfeiting (Fig. 61B and C). Additionally, sensor 137 found application in monitoring Zn²⁺ in SiHa cells.

Fig. 60 Chemical structure of chemosensors 137–161.

Fig. 61 (A) Fluorescence spectrum; (B) absorbance spectrum of sensor 137 in the absence and presence of Zn²⁺. Inset: Naked eye detectable color change of sensor 137 solution in the absence and presence of Zn²⁺ under (A) 365 nm UV light and (B) ambient light. (C) Sensor 137 application as inkless rewritable paper in information safety. Reproduced with permission from ref. 187 Copyright 2021, Royal Society of Chemistry.

Sun et al. designed tetraphenylethene-based Schiff base sensor 138 with photochromic properties and employed it as rewritable paper and a UV sensor (Fig. 60 and Table 2).¹⁸⁸ Sensor 138 exhibited weak fluorescence in solid-state, which after grinding with a pestle resulted in the appearance of a strong emission band between 550 nm and 700 nm, longer response time and reduced photochromic efficiency. The photochromic mechanism of sensor 138 resulted from enol to keto form photoisomerization. Further sensor 138 found application as rewritable paper. With violet light at 410 nm, sensor 138 served as a pen, while with white light and 420 nm–590 nm visible light, sensor 138 was used as an eraser. Moreover, sensor 138 also served as a UV sensor for the detection of UV radiation pollution by the naked eye (Fig. 62A–D).

Fig. 62 Application of sensor 138 as rewritable paper: (A) different letter, pattern, and number on rewritable paper; (B) proposed system-based reversible photochromic dragon. (C) Writing and erasing process on rewritable paper. (D) Photographs of sensor 138 under 365 nm UV light and visible light: (1 and 3) pristine sample; (2 and 4) ground sample. Reproduced with permission from ref. 188 Copyright 2019, Elsevier.

3.6 Optoelectronic applications

3.6.1 Mechanoluminescence^191,193,194. The tunable emission properties of mechanochromic materials have grabbed significant attention because of their potential application in devices, optical recordings, security inks, and sensors.

Yang et al. synthesized Schiff base derivatives 139, 140, and 141 for fluorescence-based detection of Cu²⁺ (Fig. 60 and Table 2).¹⁸⁹ All the three sensors (139, 140, and 141) displayed AIE characteristics in the CH₃CN–H₂O mixture with different water fractions. From the fluorescence experiment, sensor 141 was found to be a specific sensor for Cu²⁺, whereas sensors 139 and 140 lacked selectivity and displayed fluorescence enhancement with various metal ions (Fig. 63A and C). The fluorescence titration of sensor 141 with Cu²⁺ in CH₃CN–H₂O (4 : 1, v/v) solution revealed a linear increase in fluorescence intensity at 463 nm (λ_ex = 350 nm) with the addition of an increasing concentration of Cu²⁺ (0–20 μM). The limit of detection was 2.3 × 10⁻⁷ M for Cu²⁺ and the stoichiometry of the complex was 2 : 1 (141 : Cu²⁺). The turn-on fluorescence response of sensor 141 with Cu²⁺ could be attributed to the CHEF effect. Sensor 141 showed reversible mechanochromic fluorescence behavior with the mechanical stress-induced shift of fluorescence spectra by 21 nm, attributed to transformation between crystalline and amorphous states (Fig. 63B and D).

Fig. 63 (A) Fluorescence response of sensor 141 in the presence of different metal ions. (B) Normalized fluorescence spectrum of sensor 141 in different solid-states, viz. initial sample, ground sample, and fumed sample. (C) Photographs of sensor 141 solutions in the presence of different metal ions under 365 nm UV light. (D) Color change of sensor 141 after grinding and fuming under 365 nm UV light illumination. Reproduced with permission from ref. 189 Copyright 2017, Royal Society of Chemistry.

Yang et al. developed anthryl-based Schiff bases 142, 143, and 144 for Cu²⁺ and Zn²⁺ detection (Fig. 60 and Table 2).¹⁹⁰ Among various tested metal ions, only sensor 143 (λ_ex = 432 nm) showed on–off type fluorescence response (λ_em = 508 nm) with Cu²⁺ in HEPES buffered methanol-H₂O (4/1, v/v) solution, and off–on type fluorescence response (λ_em = 510 nm) with Zn²⁺ in pure methanol solution. The binding stoichiometry was 2 : 1 (143 : Cu²⁺/Zn²⁺), and the binding constant was 1.28 × 10⁸ M⁻¹ and 1.05 × 10⁶ M⁻¹ for Cu²⁺ and Zn²⁺, respectively. The limit of detection was 0.212 μM and 71.9 nM for Cu²⁺ and Zn²⁺, respectively. The different fluorescence responses of sensor 143 with Cu²⁺ and Zn²⁺ could be due to the different electronic configurations of metal ions. For Cu²⁺, the PET process could be responsible for fluorescence quenching. And for Zn²⁺, the filled d-orbital and no possibility of d-d transition could be responsible for fluorescence enhancement.

Muthamma et al. used fluorene-based Schiff base 145 (AIE-active) for developing environmentally friendly flexographic ink for printed electronics and for security printing applications (Fig. 60 and Table 2).¹⁹¹ The Schiff base 145 writing and fingerprints on UV dull security paper using flexo ink were invisible under daylight but showed bluish green fluorescence under UV light. Further, the emission of Schiff base 145 writing and fingerprints was sensitive to pH, demonstrating the potential application of the Schiff base in anti-counterfeiting. Further, the mechanical and electrical measurements of flexo-ink printing on UV dull security paper substrates exhibited p-type semiconductivity, thus signifying the potential for application of Schiff base 145 in organic printed transistor devices.

3.6.2 Color tunable. Currently, various fluorescent materials have been fabricated into devices because of their potential application as light-emitting components in display devices. The optical properties of fluorescent materials can be tuned by chemical modification, functional group addition or substitution, or by complexation with ions.^60,292 The diverse application of these fluorescent materials encouraged the researchers to develop materials with improved fluorescence properties.

Gondia and Sharma et al. studied comparative optical properties of sensor 146–Zn²⁺ and sensor 147–Zn²⁺ complexes (Fig. 60 and Table 2).¹⁹² Both sensors 146 and 147 are similar in structure except for the presence of additional phenyl groups in sensor 147. The fluorescence spectrum of sensor 146–Zn²⁺ and sensor 147–Zn²⁺ complexes revealed significant emission intensity peaks at 504 nm and 444 nm (λ_ex = 372 nm), respectively. Further, the sensor 146–Zn²⁺ complex showed nearly two times increased fluorescence as well as blue shift as compared to the emission spectrum of the sensor 147–Zn²⁺ complex, attributed to the presence of an additional phenyl group. The change in emission of sensors 146 and 147 upon complexation with Zn²⁺ through N-azomethine and the oxygen atom of the –OH group could be due to the highest to lowest energy state intraligand transition. The CIE (Commission Internationale de L’Eclairage) chromaticity diagram for 146–Zn²⁺ and 147–Zn²⁺ complexes revealed a bluish-green region for the 146–Zn²⁺ complex and a blue region for the 147–Zn²⁺ complex, suggesting potential application of these complexes in electroluminescent devices (Fig. 64 and Table 3).

Fig. 64 CIE diagram of 146–Zn²⁺ and 147–Zn²⁺ complexes. Reproduced with permission from ref. 192 Copyright 2019, Elsevier.

Table 3 CIE parameter of the complexes and quantum yield. Reproduced with permission from ref. 192 Copyright 2019, Elsevier

Compound	Slope	Quantum yield	Color coordinates
			x	y
146	6.37	0.46	0.187	0.355
147	16.59	1.21	0.168	0.152
Standard	777.95	0.54	—	—

---

Nishal et al. presented a series of zinc-Schiff base complexes 148–152 for organic light-emitting device (OLED) application (Fig. 60 and Table 2).¹⁹³ On excitation at 365 nm, Schiff base complexes 148, 149, 150, 151, and 152 emitted blue with the emission maxima centered at 447, 440, 436, 433, and 430 nm, respectively. The blue luminescence of the Schiff-base complexes was assigned to relaxation from higher to lower energy levels because of intra-ligand transition. Notably, as the number of alkyl groups in bridging diamines increased, the emission maxima were more shifted toward the blue region. The CIE chromaticity diagram and color coordinate calculation showed the potential application of Schiff base-zinc complexes in organic light-emitting devices (OLEDs).

Gusev et al. presented an ethylenediamine Schiff base-Zn²⁺ complex (153) as a promising emissive material for blue fluorescent OLED applications (Fig. 60 and Table 2).¹⁹⁴ The complex 153 emitted strongly both in solution and solid state with maximum emission in the blue region (430–460 nm), assigned to combination of monomer (419 nm)–excimer (433 and 468 nm). Interestingly, complex 153 was fabricated into an organic-light emitting device and demonstrated promising blue fluorescent OLED applications (maximum external quantum efficiency, EQE_max = 5%, and maximum brightness 17 000 cd m⁻²).

3.6.3 Logic gate^175,178,293. Modern science may benefit from computations based on molecular logic functions. The logic operation at the molecular level is performed using two input signals and one output signal. The Schiff base-based logic gates have a lot of promise for optical sensing, which opens up new possibilities for multidirectional memory device innovation in the future.

Das et al. prepared fluorescein-based Schiff base 154 as a fluorometric turn-on sensor for Zn²⁺ (Fig. 60 and Table 2).¹⁹⁵ In HEPES buffered ethanol–H₂O (9 : 1, v/v) solution, sensor 154 displayed fluorescence enhancement only with Zn²⁺ over other metal ions. Fluorescence titration of sensor 154 with Zn²⁺ revealed an increase in emission intensity at 520 nm (λ_ex = 375 nm) along with color change of the solution from colorless to greenish-yellow under UV light. The complexation of sensor 154 with Zn²⁺ through the imine group and –OH groups inhibits the PET process and C=N isomerization, leading to fluorescence enhancement by the CHEF effect. The binding constant was 2.86 × 10⁴ M⁻¹ for the 1 : 1 (154/Zn²⁺) complex. The limit of detection was 1.59 μM for Zn²⁺. Additionally, an INHIBIT type logic gate was designed using Zn²⁺ and EDTA as inputs and emission intensity at 520 nm as output at the molecular level (Fig. 65). Sensor 154 also found application in Zn²⁺ detection using TLC supported paper strips and in the bioimaging of intracellular Zn²⁺ ions in Vero cells (African green monkey kidney cells).

Fig. 65 Truth table and INHIBIT type logic gate representation with Zn²⁺ and EDTA as input and the emission intensity of sensor 154 at 520 nm as output at the molecular level. Reproduced with permission from ref. 195 Copyright 2019, Elsevier.

Das et al. constructed Schiff base sensor 155 using ethylenediamine and 2-hydroxyacetophenone and utilized it for Al³⁺ detection by fluorescence off–on mode (Fig. 60 and Table 2).¹⁹⁶ In methanol–water (1 : 1, v/v) solution, sensor 155 exhibited weak emission in the 380 nm to 700 nm range (λ_ex = 370 nm). Upon the addition of Al³⁺, sensor 155 underwent an increase in emission intensity along with red-shift. The binding constant was log β = 5.14 for 1 : 1 (155 : Al³⁺) stoichiometry. The limit of detection was 10 μM for Al³⁺. The off–on fluorescence response could be attributed to inhibition of the PET process upon complexation of Al³⁺ with sensor 155, resulting in fluorescence enhancement. The 155–Al³⁺ complex further acted as an on–off type fluorescent sensor for PO₄³⁻. An INHIBIT type logic gate was constructed utilizing Al³⁺ and PO₄³⁻ as two inputs. Sensor 155 found application in Al³⁺ determination in BSA and in rat L6 myoblast cells.

Gupta et al. synthesized sensors 156 and 157 for fluorescence-based detection of Al³⁺ (Fig. 60 and Table 2).¹⁹⁷ The fluorescence spectrum of sensors 156 and 157 (λ_ex = 375 nm) in methanol solution displayed fluorescence enhancement selectively with Al³⁺ over other tested metal ions and the emission maxima were centered at 465 nm and 464 nm for sensors 156 and 157, respectively. The detection limit of sensors 156 and 157 was 0.525 μM and 2.38 μM, respectively for Al³⁺ ions. The binding constant was 6.64 × 10³ M⁻¹ and 7.29 × 10³ M⁻¹ for 156 : Al³⁺ (1 : 1) and 157 : Al³⁺ (1 : 1) complexes, respectively. The sensing mechanism could be attributed to the CHEF effect. A combination of AND and NOT gates was developed using Al³⁺ and EDTA as two inputs and emission intensity as output (Fig. 66).

Fig. 66 Truth table and AND and NOT gate representation with Al³⁺ and EDTA as two inputs and emission intensity as output. Reproduced with permission from ref. 197 Copyright 2015, Elsevier.

Naskar et al. synthesized Schiff base sensor 158 using 2-amino-1-ethanol and salicyladehyde for Zn²⁺ detection in an aqueous medium (Fig. 60 and Table 2).¹⁹⁸ On excitation at 359 nm, sensor 158 exhibited emission maxima at 454 nm. In the presence of various metal ions, sensor 158 displayed strong fluorescence enhancement with Zn²⁺ and moderate response with Al³⁺, whereas other metal ions had an insignificant effect. The limit of detection of sensor 158 for Zn²⁺ was 0.477 μM. The binding constant was 24.75 × 10⁴ M⁻¹ for 2 : 1 (158/Zn²⁺) stoichiometry. The coordination of sensor 158 with Zn²⁺ caused inhibition of the ESIPT process and C=N isomerization, leading to fluorescence enhancement through the CHEF effect. An ‘OR’ logic gate and a ‘Keypad lock’ logic network were fabricated using Zn²⁺ and Al³⁺ as input with absorbance at 359 nm and emission at 454 nm as output (Fig. 67A and B). Additionally, sensor 158 found potential application in the fluorescence imaging of Zn²⁺ in rice seedlings.

Fig. 67 (A) Truth table and OR gate representation with Zn²⁺ and Al³⁺ as inputs and absorbance at 359 nm as output. (B) Truth table for fluorescent keypad logic operation. Reproduced with permission from ref. 198 Copyright 2016, Elsevier.

Purkait et al. synthesized Schiff base sensor 159 for fluorescence-based detection of Zn²⁺, Cd²⁺, and I⁻ (Fig. 60 and Table 2).¹⁹⁹ In DMSO–water (9 : 1 v/v) solution, sensor 159 was non-fluorescent (λ_ex = 482 nm). Among various tested metal ions, sensor 159 displayed fluorescence enhancement only with Zn²⁺ (λ_em = 545 nm) and Cd²⁺ (λ_em = 560 nm) along with naked eye detectable color change from colorless to bright yellow (for Zn²⁺) and from colorless to orange color (for Cd²⁺) under UV light illumination. The interaction of Zn²⁺/Cd²⁺ with sensor 159 resulted in fluorescence enhancement due to the CHEF effect. The limit of detection was 2.7 nM (for Zn²⁺) and 6.6 nM (for Cd²⁺). The binding constant was 2.7 × 10⁴ M⁻¹ (for the 159–Zn²⁺ (1 : 1) complex) and 0.96 × 10⁴ M⁻¹ (for the 159–Cd²⁺ (1 : 1) complex). Upon addition of Na₂EDTA to the solution of the sensor 159–Zn²⁺/Cd²⁺ complex, on–off type fluorescence response was observed, signifying reversibility and reusability of sensor 159. An INHIBIT type logic gate was generated using Zn²⁺/Cd²⁺ and EDTA as input and emission intensity as output (Fig. 68). Sensor 159 also displayed practical application in sensing Zn²⁺ and Cd²⁺ in TLC plates and in drinking water.

Fig. 68 Truth table and INHIBIT logic gate representation with the Zn²⁺ and Cd²⁺ complex of sensor 159 and EDTA as inputs. Reproduced with permission from ref. 199 Copyright 2018, Royal Society of Chemistry.

Vinoth Kumar et al. synthesized Schiff base 160 as a turn-on sensor for Ni²⁺ (Fig. 60 and Table 2).²⁰⁰ In acetonitrile solution, sensor 160 showed an off–on type fluorescence response with Ni²⁺, whereas other metal ions had an insignificant effect. Fluorescence titration of sensor 160 with Ni²⁺ revealed a continuous increase in fluorescence intensity at 542 nm (λ_ex = 400 nm) upon addition of an increasing concentration of Ni²⁺ (0–60 μM), attributed to the CHEF effect. The detection limit was 8.67 nM for Ni²⁺ and the association constant was 4.18 × 10⁵ M⁻¹ for the 160–Ni²⁺ (1 : 1) complex. Upon addition of Na₂EDTA to the solution of the sensor 160–Ni²⁺ complex, fluorescence quenching was observed, suggesting reversibility and reusability of sensor 160. Additionally, the molecular logic gate of AND, OR, NOT, and NOR type and truth table were created based on Ni²⁺ and EDTA as input and absorbance and emission intensity at 428 nm and 542 nm as output. Sensor 160 found application in the fluorescence imaging of intracellular Ni²⁺ in HeLa cells.

Jothi et al. synthesized naphthalimide-based Schiff base 161 for detection of Fe³⁺ based on a fluorometric method (Fig. 60 and Table 2).²⁰¹ In the acetonitrile–water (7 : 3) mixture, sensor 161 showed strong emission at 531 nm (λ_ex = 344 nm). The fluorescence titration of sensor 161 with Fe³⁺ revealed a gradual decrease in fluorescence intensity with the addition of an increasing concentration of Fe³⁺. The limit of detection was 0.81 μM for Fe³⁺. The stoichiometry of the complex was 1 : 1 (161/Fe³⁺) and the binding constant was 2.49 × 10⁴ M⁻¹. The sensing mechanism could be attributed to blockage of ICT and C=N isomerization upon complexation of sensor 161 with Fe³⁺. Furthermore, NOT gate, OR gate logic circuit, and truth table were constructed using Fe³⁺ and EDTA as input and emission intensity at 531 nm as output (Fig. 69). Additionally, sensor 161 demonstrated potential application in detecting Fe³⁺ in test paper and water samples, and in fluorescent bioimaging.

Fig. 69 Truth table and logic gate representation with Fe³⁺and EDTA as inputs. Reproduced with permission from ref. 201 Copyright 2021, Royal Society of Chemistry.

4. Conclusion and future opportunities

Metal ions are ubiquitous in nature, and up to certain concentrations as fixed by WHO and EPA they are non-toxic. In fact, some metal ions are essential for the normal functioning of living organisms. Nowadays, researchers are using Schiff base metal ion complexes for productive purposes owing to their outstanding applications in optoelectronic systems. However, with the increase in their concentration due to industrialization and urbanization, metal ions have shown toxic effects on human health and the environment. Therefore, their detection is crucial, which is challenging. This review summarized the recent advances in fluorescent Schiff base sensors for metal ion detection in the past six years.

The commonly used mechanisms for metal ion detection include self-assembly/disassembly, aggregation/disaggregation, photo-induced electron transfer (PET), intra/intermolecular charge transfer (ICT), metal to ligand charge transfer (MLCT), ligand to metal charge transfer (LMCT), Förster resonance energy transfer (FRET), excited state intramolecular proton transfer (ESIPT), inner filter effect (IFE), metal-induced chemical change and C=N isomerization. Metal ions upon coordination with Schiff base sensors generally show fluorescence enhancement or quenching leading to the CHEF or CHEQ effect, respectively. However, for a few systems, ratiometric fluorescence change was also observed. Usually, paramagnetic metal ions such as Cu²⁺ and Fe³⁺ act as a fluorescence quencher and show CHEQ effects when bound with chemosensors. However, there are several reports wherein fluorescence enhancement was also observed with these metal ions. Additionally, the ESIPT process was associated with the sensors containing the salicylimine group. Because of the free rotation around the C=N bond, Schiff base-based sensors may show the AIE phenomenon. Schiff bases commonly include hydrazone, acyl hydrazone, salicylimine, azine, and other groups, and provide nitrogen and oxygen atoms for coordination with metal ions.

Due to the global existence of metal ions, environmental monitoring and biomedical application are the most common practical application of Schiff base sensors. Other applications of fluorescent Schiff base sensors include multi-metal ion sensors, a sequential sensor for other ions, and optoelectronic systems. The molecular switching properties of Schiff base sensors have been applied in the construction of molecular keypads and logic gates.

The development of fluorescent Schiff base sensors and their commercial applications will always be of immense interest to researchers. Although existing results are plentiful, future efforts need to be undertaken in the following aspects. First, the main drawbacks of as-developed fluorescent Schiff base sensors are instability and poor water solubility in an aqueous medium. Except for sensors 2, 8, 11, 22, 35, 45, 58, 84, 112, 117, 118, and 158, the optical properties of Schiff base sensors were studied in either H₂O-DMF/DMSO/THF/CH₃CN/ethanol/methanol buffered binary mixtures or in CH₃CN, ethanol, methanol solutions (Table 2), thus limiting their application in biological and environmental systems. Second, from Table 2 we can conclude that most of the work has been done on the detection of Al³⁺, Cu²⁺, and Zn²⁺, and insufficient literature is available for other important and toxic metal ions. Third, Schiff base sensors can be applied for fluorescence-based speciation of metal ions such as Fe²⁺/Fe³⁺, As³⁺/As⁵⁺, Cr³⁺/Cr⁴⁺, etc. Fourth, the selectivity and sensitivity of the Schiff base sensor need to be improved. Fifth, although there are several practical applications of Schiff base sensors, so far there have been no available well known commercial applications because of the poor stability (as Schiff bases are prone to hydrolysis, the carbonyl and amine groups might be regenerated on hydrolysis), pH sensitivity, and decrease in fluorescence of Schiff base sensors with time. Therefore, the prototype should be made for the commercialization of these techniques. This will be useful for diagnostic purposes and pollutant detection on-site. Sixth, the majority of the developed sensors are based on one-photon excitation and few reports are available for two-photon fluorescent sensors. In comparison, two-photon fluorescent probes offer several advantages over conventional one-photon fluorescent probes such as decreased photo-damage to biological samples, deeper penetration into cells, tissues, and organelles, and minimization of background interference. Therefore, future efforts should be on the development of two-photon fluorescent sensors.

Although fluorescent-based Schiff base sensors encounter many challenges in terms of both exploratory and practical application, by taking into consideration the above-mentioned aspects in the sensor design and application-oriented research work, we believe that fluorescent Schiff base sensors can be utilized as important materials in the upcoming days and their commercial availability will be expected in the future.

Conflicts of interest

There are no conflicts to declare.

References

1. K. S. Egorova and V. P. Ananikov, Organometallics, 2017, 36, 4071–4090.
2. J. K. Fawell, E. Ohanian, M. Giddings, P. Toft, Y. Magara and P. Jackson, Inorganic Tin in Drinking-water WHO, Guidelines for Drinking-water Quality, 2004.
3. F. S. Al-fartusie and S. N. Mohssan, Indian J. Adv. Chem. Sci., 2017, 5, 127–136.
4. A. L. Berhanu, Gaurav, I. Mohiuddin, A. K. Malik, J. S. Aulakh, V. Kumar and K.-H. Kim, Trends Anal. Chem., 2019, 116, 74–91.
5. M. Kumar and A. Puri, Indian J. Occup. Environ. Med., 2012, 16, 40–44.
6. T. Gong, J. Liu, X. Liu, J. Liu, J. Xiang and Y. Wu, Food Chem., 2016, 213, 306–312.
7. J. Dalmieda and P. Kruse, Sensors, 2019, 19, 5134.
8. Y. Lu, X. Liang, C. Niyungeko, J. Zhou, J. Xu and G. Tian, Talanta, 2018, 178, 324–338.
9. L. Cui, J. Wu and H. Ju, Biosens. Bioelectron., 2015, 63, 276–286.
10. S. Sikdar and M. Kundu, ChemBioEng Rev., 2018, 5, 18–29.
11. M. H. Chua, H. Zhou, Q. Zhu, B. Z. Tang and J. W. Xu, Mater. Chem. Front, 2021, 5, 659–708.
12. J. Zhang, L. Xu and W. Wong, Coord. Chem. Rev., 2018, 355, 180–198.
13. S. Sandhu, R. Kumar, N. Tripathi, H. Singh, P. Singh and S. Kumar, Sens. Actuators, B, 2017, 241, 8–18.
14. N. Tripathi, P. Singh and S. Kumar, New J. Chem., 2017, 41, 8739–8747.
15. N. Tripathi and M. K. Goshisht, Aggregation of Luminophores in Supramolecular Systems: From Mechanisms to Applications, CRC Press, Taylor Fr., 2020, pp. 1–220.
16. N. Tripathi, R. Kumar, P. Singh and S. Kumar, Sens. Actuators, B, 2017, 246, 1001–1010.
17. M. K. Goshisht and N. Tripathi, J. Mater. Chem. C, 2021, 9, 9820–9850.
18. N. Tripathi, P. Singh, V. Luxami, D. Mahajan and S. Kumar, Sens. Actuators, B, 2018, 270, 552–561.
19. N. Tripathi, S. Sandhu, P. Singh, A. Mahajan, M. Kaur and S. Kumar, Sens. Actuators, B, 2016, 231, 79–87.
20. P. Sabari and M. Ravikanth, J. Chem. Sci., 2021, 133, 59–67.
21. M. Sahu, A. K. Manna and G. K. Patra, Inorg. Chim. Acta, 2021, 517, 120199.
22. X. Liu, X. Peng, F. Xu, L. Wang and M. Liu, Russ. J. Gen. Chem., 2021, 91, 1093–1098.
23. S. D. Pradeep, D. Sebastian, A. K. Gopalakrishnan, D. K. Manoharan, D. T. Madhusudhanan and P. V. Mohanan, J. Fluoresc., 2021, 31, 1113–1123.
24. Y. Xing, Z. Liu, B. Li, L. Li, X. Yang and G. Zhang, Sens. Actuators, B, 2021, 347, 130497.
25. M. Liu, H. Yang, D. Li, Q. Yao, H. Wang, Z. Zhang and J. Dou, Inorg. Chim. Acta, 2021, 522, 120384.
26. C. Pan, K. Wang, S. Ji, H. Wang, Z. Li, H. He, Y. Huo and Y. Huo, RSC Adv., 2017, 7, 36007–36014.
27. J. Harathi and K. Thenmozhi, Mater. Chem. Front, 2020, 4, 1471–1482.
28. C. Chen, P. Hung, C. Wan and A. Wu, Inorg. Chem. Commun., 2013, 38, 74–77.
29. X. Feng, Y. Li, X. He, H. Liu, Z. Zhao, R. T. K. Kwok, M. R. J. Elsegood, J. W. Y. Lam and B. Z. Tang, Adv. Funct. Mater., 2018, 28, 1802833.
30. A. Mukherjee and M. Chakravarty, New J. Chem., 2020, 44, 6173–6181.
31. V. Kachwal, P. Alam, H. R. Yadav, S. S. Pasha, A. R. Choudhury and I. R. Laskar, New J. Chem., 2018, 42, 1133–1140.
32. N. Chakraborty, A. Chakraborty and S. Das, J. Lumin., 2018, 199, 302–309.
33. J. Xiong, Z. Li, J. Tan, S. Ji, J. Sun, X. Li and Y. Huo, Analyst, 2018, 143, 4870–4886.
34. M. Shyamal, S. Maity, A. Maity, R. Maity, S. Roy and A. Misra, Sens. Actuators, B, 2018, 263, 347–359.
35. W. Chen, H. Xu, L. Ju and H. Lu, Tetrahedron, 2021, 88, 132123.
36. J. Qin, T. Li, B. Wang, Z. Yang and L. Fan, Synth. Met., 2014, 195, 141–146.
37. R. Azadbakht and S. Rashidi, Spectrochim. Acta, Part A, 2014, 127, 329–334.
38. R. Bhaskar and S. Sarveswari, ChemistrySelect, 2020, 5, 4050–4057.
39. A. Kumar, S. Mondal, K. S. Kayshap, S. K. Hira, P. P. Manna, W. Dehaend and S. Dey, New J. Chem., 2018, 42, 10983–10988.
40. Y. Wang, X. Hao, L. Liang, L. Gao, X. Ren, Y. Wu and H. Zhao, RSC Adv., 2020, 10, 6109–6113.
41. Y. Wang, H. Wu, W. Wu, S. Li, Z. Xu, Z. Xu, Y. Fan, X. Zhao and B. Liu, Sens. Actuators, B, 2018, 260, 106–115.
42. D. Aydin, S. Dinckan, S. Nihan, K. Elmas, T. Savran, F. N. Arslan and I. Yilmaz, Food Chem., 2021, 337, 127659.
43. O. Alici and D. Aydin, J. Photochem. Photobiol., A, 2021, 404, 112876.
44. H. Yu, J. Zhi, Z. Chang, T. Shen, W. Ding, X. Zhang and J. Wang, Mater. Chem. Front., 2019, 3, 151–160.
45. Y. Yang, C. Gao, N. Zhang and D. Dong, Sens. Actuators, B, 2016, 222, 741–746.
46. H. Fang, G. Cai, Y. Hu and J. Zhang, Chem. Commun., 2018, 54, 3045–3048.
47. H. Yao, J. Wang, Q. Zhou, X.-W. Guan, Y.-Q. Fan, Y.-M. Zhang, T.-B. Wei and Q. Lin, Soft Matter, 2018, 14, 8390–8394.
48. A. C. Sedgwick, L. Wu, H. H. Han, S. D. Bull, X. P. He, T. D. James, J. L. Sessler, B. Z. Tang, H. Tian and J. Yoon, Chem. Soc. Rev., 2018, 47, 8842–8880.
49. J. Zhao, S. Ji, Y. Chen, H. Guo and P. Yang, Phys. Chem. Chem. Phys., 2012, 14, 8803–8817.
50. S. Gupta and M. D. Milton, New J. Chem., 2018, 42, 2838–2849.
51. V. Kachwal, I. S. V. Krishna, L. Fageria, J. Chaudhary, R. K. Roy, R. Chowdhury and I. R. Laskar, Analyst, 2018, 143, 3741–3748.
52. Q. Yu, X. Zhang, S.-T. Wu, H. Chen, Q.-L. Zhang, H. Xu, Y.-L. Huang, B.-X. Zhu and X.-L. Ni, Chem. Commun., 2020, 56, 2304–2307.
53. S. Sharma, S. Virk, P. Pradeep and A. Dhir, Eur. J. Inorg. Chem., 2017, 2457–2463.
54. A. Sen Gupta, K. Paul and V. Luxami, Sens. Actuators, B, 2017, 246, 653–661.
55. Y. Jia and J. Li, Chem. Rev., 2015, 115, 1597–1621.
56. F. Faridbod, M. R. Ganjali, R. Dinarvand, P. Norouzi and S. Riahi, Sensors, 2008, 8, 1645–1703.
57. A. Q. Alorabi, M. Abdelbaset and S. A. Zabin, Chemosensors, 2020, 8, 1.
58. M. J. Reimann, D. R. Salmon, J. T. Horton, E. C. Gier and L. R. Jefferies, ACS Omega, 2019, 4, 2874–2882.
59. A. Singh and P. Barman, Top. Curr. Chem., 2021, 379, 29.
60. W. Liping, J. Shibo, Z. Weifeng, L. I. U. Yunqi and Y. Gui, Chinese Sci. Bull., 2013, 58, 2733–2740.
61. İ. Sıdır, Y. G. Sıdır, H. Berber and F. Demiray, J. Mol. Struct., 2019, 1176, 31–46.
62. P. H. A. Nayak, H. S. B. Naik, H. B. Teja, B. R. Kirthan, R. Viswanath, P. H. A. Nayak, H. S. B. Naik, H. B. Teja, B. R. Kirthan and R. Viswanath, Mol. Cryst. Liq. Cryst., 2021, 1–7.
63. A. Sakthivel, K. Jeyasubramanian, B. Thangagiri and J. D. Raja, J. Mol. Struct., 2020, 1222, 128885.
64. S. K. Patil and D. Das, ChemistrySelect, 2017, 2, 6178–6186.
65. M. T. Kaczmarek, M. Zabiszak, M. Nowak and R. Jastrzab, Coord. Chem. Rev., 2018, 370, 42–54.
66. D. Udhayakumari and V. Inbaraj, J. Fluoresc., 2020, 30, 1203–1223.
67. S. Shekhar, A. M. Khan, S. Sharma, B. Sharma and A. Sarkar, Emergent Mater., 2021 DOI:10.1007/s42247-021-00234-1.
68. M. Kaur, S. Kumar, M Yusuf, J. Lee, R. J. C. Brown, K.-H. Kim and A. K. Malika, Coord. Chem. Rev., 2021, 449, 214214.
69. H. Wan, Q. Xu, P. Gu, H. Li, D. Chen, N. Li, J. He and J. Lu, J. Hazard. Mater., 2021, 403, 123656.
70. C. C. Vidyasagar, B. M. Muñoz Flores, V. M. Jiménez-Pérez and P. M. Gurubasavaraj, Mater. Today Chem., 2019, 11, 133–155.
71. C. Immanuel David, G. Prabakaran and R. Nandhakumar, Microchem. J., 2021, 169, 106590.
72. W.-N. Wu, P.-D. Mao, Y. Wang, X.-J. Mao, Z.-Q. Xu, Z.-H. Xu, X.-L. Zhao, Y.-C. Fan and X.-F. Hou, Sens. Actuators, B, 2018, 258, 393–401.
73. P. Singh, H. Singh, V. Vanita and R. Sharma, ChemistrySelect, 2016, 1, 6880–6887.
74. W. Dong, S. F. Akogun, Y. Zhang, Y. Sun and X. Dong, Sens. Actuators, B, 2017, 238, 723–734.
75. K. Rout, A. K. Manna, M. Sahu, J. Mondal, S. K. Singh and G. K. Patra, RSC Adv., 2019, 9, 25919–25931.
76. M. Pannipara, A. G. Al-Sehemi, A. Kalam, A. M Asiri and M. N. Arshad, Spectrochim. Acta, Part A, 2017, 183, 84–89.
77. S. Bhardwaj, N. Maurya and A. K. Singh, Sens. Actuators, B, 2018, 260, 753–762.
78. J. Yang, J. Chai, B. Yang and B. Liu, Spectrochim. Acta, Part A, 2019, 211, 272–279.
79. P. Puri, G. Kumar, K. Paul and V. Luxami, New J. Chem., 2018, 42, 18550–18558.
80. A. Abbasi and M. Shakir, New J. Chem., 2018, 42, 293–300.
81. T. Hu, L. Wang, J. Li, Y. Zhao, J. Cheng, W. Li, Z. Chang and C. Sun, Inorg. Chim. Acta, 2021, 524, 120421.
82. G. Wang, J. Qin, L. Fan, C. Li and Z. Yang, J. Photochem. Photobiol., A, 2016, 314, 29–34.
83. S. M. Saleh and R. F. M. Elshaarawy, RSC Adv., 2016, 6, 68709–68718.
84. H. Wang, X. Xu, J. Yin, Z. Zhang and L. Xue, ChemistrySelect, 2021, 6, 1–7.
85. Y.-J. Chang, S.-S. Wu, C.-H. Hu, C. Cho, M. X. Kao and A.-T. Wu, Inorg. Chim. Acta, 2015, 432, 25–31.
86. S. M. Hossain, K. Singh, A. Lakma, R. N. Pradhan and A. K. Singh, Sens. Actuators, B, 2017, 239, 1109–1117.
87. C. Li, J. Qin, B. Wang, L. Fan, J. Yan and Z. Yang, J. Fluoresc., 2016, 26, 345–353.
88. C. Li, J. Qin, G. Wang, B. Wang, A. Fu and Z. Yang, Inorg. Chim. Acta, 2015, 430, 91–95.
89. C. Liu, Z. Yang, L. Fan, X. Jin, J. An, X. Cheng and B. Wang, J. Lumin., 2013, 158, 172–175.
90. L. Liu and Z. Yang, Inorg. Chim. Acta, 2018, 469, 588–592.
91. J. Qin and Z. Yang, J. Photochem. Photobiol., A, 2015, 303–304, 99–104.
92. J.-C. Qin, X. Cheng, K. Yu, R. Fang, M. Wang and Z. Yang, Anal. Methods, 2015, 7, 6799–6803.
93. W. Ruo, J. Guang-qi and L. Xiao-hong, Inorg. Chim. Acta, 2017, 455, 247–253.
94. S. K. Shoora, A. K. Jain and V. K. Gupta, Sens. Actuators, B, 2015, 216, 86–104.
95. S. J. Zhang, H. Li, C. L. Gong, Z. J. Wang, Z. Y. Wu and F. Wang, Synth. Met., 2016, 217, 37–42.
96. M. Kumar, A. Kumar, S. Kishor, S. Kumar, N. Manav, A. K. Bhagi, S. Kumar and R. P. John, J. Mol. Struct., 2022, 1247, 131257.
97. M. Wang, L. Lu, W. Song, X. Wang, T. Sun, J. Zhu and J. Wang, J. Lumin., 2021, 233, 117911.
98. F. Kolcu, D. Erdener and İ. Kaya, Synth. Met., 2021, 272, 116668.
99. G. B. Chalmardi, M. Tajbakhsh, A. Bekhradnia and R. Hosseinzadeh, Inorg. Chim. Acta, 2017, 462, 241–248.
100. G. Babaei, M. Tajbakhsh, N. Hasani and A. Bekhradnia, Tetrahedron, 2018, 74, 2251–2260.
101. Y. He, J. Yin and G. Wang, Chem. Heterocycl. Compd., 2018, 54, 146–152.
102. B. Li, D. Zhang and F. Tian, Luminescence, 2017, 32, 1567–1573.
103. S. Santhoshkumar, K. Velmurugan, J. Prabhu, G. Radhakrishnan and R. Nandhakumar, Inorg. Chim. Acta, 2016, 439, 1–7.
104. B. Zhao, T. Liu, Y. Fang, L. Wang, B. Song and Q. Deng, Tetrahedron Lett., 2016, 57, 4417–4423.
105. J. Zhang, Y. Liu, Q. Fei, H. Shan, F. Chen, Q. Liu, G. Chai, G. Feng and Y. Huan, Sens. Actuators, B, 2017, 239, 203–210.
106. S. Dalbera, S. Kulovi and S. Dalai, ChemistrySelect, 2018, 3, 6561–6569.
107. S. Carlos, M. C. Nunes, L. De Boni, G. S. Machado and F. S. Nunes, J. Photochem. Photobiol., A, 2017, 348, 41–46.
108. Y. Fu, C. Fan, G. Liu and S. Pu, Sens. Actuators, B, 2017, 239, 295–303.
109. J. Kumar, P. K. Bhattacharyya and D. Kumar, Spectrochim. Acta, Part A, 2015, 138, 99–104.
110. F. N. Moghadam, M. Amirnasr, S. Meghdadi, K. Eskandari, A. Buchholz and W. Plass, Spectrochim. Acta, Part A, 2019, 207, 6–15.
111. P. Torawane, S. K. Sahoo, A. Borse and A. Kuwar, Luminescence, 2017, 32, 1426–1430.
112. X. Wang, T. Xu and H. Duan, Sens. Actuators, B, 2015, 214, 138–143.
113. J. Yin, Q. Bing, L. Wang and G. Wang, Spectrochim. Acta, Part A, 2018, 189, 495–501.
114. M. Gao, C. Xing, X. Jiang, L. Xu, P. Li and C. Der Hsiao, Luminescence, 2021, 36, 951–957.
115. J. Zhu, Y. Zhang, Y. Chen, T. Sun, Y. Tang, Y. Huang, Q. Yang, D. Ma, Y. Wang and M. Wang, Tetrahedron Lett., 2017, 58, 365–370.
116. M. Kumar, A. Kumar, M. K. Singh, S. K. Sahu and R. P. John, Sens. Actuators, B, 2017, 241, 1218–1223.
117. C. Liu, Q. Zhou, Y. Li, L. V. Garner, S. P. Watkins, L. J. Carter, J. Smoot, A. C. Gregg, A. D. Daniels, S. Jervey and D. Albaiu, ACS Cent. Sci., 2020, 6, 315–331.
118. J. Yan, L. Fan, J. Qin, C. Li and Z. Yang, Tetrahedron Lett., 2016, 57, 2910–2914.
119. S. O. Tümay, ChemistrySelect, 2021, 6, 10561–10572.
120. F. Lei, W. Shi, J. Ma, Y. Chen, F. Kui, Y. Hui and Z. Xie, Sens. Actuators, B, 2016, 237, 563–569.
121. D. Singhal, N. Gupta and A. K. Singh, RSC Adv., 2015, 5, 65731–65738.
122. Q. Su, Q. Niu, T. Sun and T. Li, Tetrahedron Lett., 2016, 57, 4297–4301.
123. V. Tekuri, S. K. Sahoo and D. R. Trivedi, Spectrochim. Acta, Part A, 2019, 218, 19–26.
124. X. Wan, H. Ke, J. Tang and G. Yang, Talanta, 2019, 199, 8–13.
125. D. Mohanasundaram, R. Bhaskar, G. Gangatharan, V. Kumar, J. Rajesh and G. Rajagopal, Microchem. J., 2021, 164, 106030.
126. H. H. Hammud, S. El, G. Sonji, N. Sonji and K. H. Bouhadir, Spectrochim. Acta, Part A, 2015, 150, 94–103.
127. M. Saleem, C. H. Khang and K. H. Lee, J. Fluoresc., 2016, 26, 11–22.
128. Y. Sie, C. Wan and A. Wu, RSC Adv., 2017, 7, 2460–2465.
129. T. Simon, M. Shellaiah, V. Srinivasadesikan, C.-C. Lin, F.-H. Ko, K. W. Sun and M.-C. Lin, Sens. Actuators, B, 2016, 231, 18–29.
130. X. Tang, J. Han, Y. Wang, L. Ni, X. Bao, L. Wang and W. Zhang, Spectrochim. Acta, Part A, 2017, 173, 721–726.
131. S. Zhang, X. Wu, Q. Niu, Z. Guo and T. Li, J. Fluoresc., 2017, 27, 729–737.
132. X. Zhu, Y. Duan, P. Li, H. Fan, T. Han and X. Huang, Anal. Methods, 2019, 11, 642–647.
133. J. Xue, L. Tian and Z. Yang, J. Photochem. Photobiol., A, 2019, 369, 77–84.
134. S. Y. Lee, K. H. Bok, T. G. Jo, S. Y. Kim and C. Kim, Inorg. Chim. Acta, 2017, 461, 127–135.
135. Y. Dong, R. Fan, W. Chen, P. Wang and Y. Yang, Dalton Trans., 2017, 46, 6769–6775.
136. M. Tajbakhsh, G. B. Chalmardi, A. Bekhradnia, R. Hosseinzadeh, N. Hasani and M. A. Amiri, Spectrochim. Acta, Part A, 2018, 189, 22–31.
137. S. Zhang, Q. Niu, L. Lan and T. Li, Sens. Actuators, B, 2017, 240, 793–800.
138. W. Zhu, L. Yang, M. Fang, Z. Wu, Q. Zhang and F. Yin, J. Lumin., 2015, 158, 38–43.
139. J. Fu, B. Li, H. Mei, Y. Chang and K. Xu, Spectrochim. Acta, Part A, 2020, 227, 117678.
140. A. Gomathi, P. Viswanathamurthi and K. Natarajan, J. Photochem. Photobiol., A, 2019, 370, 75–83.
141. J. Qiu, S. Jiang, B. Lin, H. Guo and F. Yang, Dyes Pigm., 2019, 170, 107590.
142. Y. Xu, H. Wang, J. Zhao, X. Yang, M. Pei, G. Zhang, Y. Zhang and L. Lin, J. Photochem. Photobiol., A, 2019, 383, 112026.
143. Z.-G. Wang, X.-J. Ding, Y.-Y. Huang, X.-J. Yan, B. Ding, Q.-Z. Li, C.-Z. Xie and J.-Y. Xu, Dyes Pigm., 2020, 175, 108156.
144. C. Patra, C. Sen, A. Das Mahapatra, D. Chattopadhyay, A. Mahapatra and C. Sinha, J. Photochem. Photobiol., A, 2017, 341, 97–107.
145. R. Mehta, P. Kaur, D. Choudhury, K. Paul and V. Luxami, J. Photochem. Photobiol., A, 2019, 380, 111851.
146. I. Kaur, P. Kaur and K. Singh, Sens. Actuators, B, 2018, 257, 1083–1092.
147. S. K. Sheet, B. Sen, R. Thounaojam, K. Aguan and S. Khatua, J. Photochem. Photobiol., A, 2017, 332, 101–111.
148. Y. Mu, C. Zhang, Z. Gao, X. Zhang, Q. Lu, J. Yao and S. Xing, Synth. Met., 2020, 262, 116334.
149. Z. Salarvand, M. Amirnasr and S. Meghdadi, J. Lumin., 2019, 207, 78–84.
150. A. Saravanan, G. Subashini, S. Shyamsivappan, T. Suresh, K. Kadirvelu, N. Bhuvanesh, R. Nandhakumar and P. S. Mohan, J. Photochem. Photobiol., A, 2018, 364, 424–432.
151. A. K. Bhanja, C. Patra, S. Mondal, D. Ojha, D. Chattopadhyay and C. Sinha, RSC Adv., 2015, 5, 48997–49005.
152. X. Chen, W. Sun, Y. Bai, F. Zhang, J. Zhao and X. Ding, Spectrochim. Acta, Part A, 2018, 191, 566–572.
153. S. Feng, Q. Gao, X. Gao, J. Yin and Y. Jiao, Inorg. Chem. Commun., 2019, 102, 51–56.
154. L. Hu, H. Wang, B. Fang, Z. Hu, Q. Zhang, X. Tian, H. Zhou, J. Wu and Y. Tian, Sens. Actuators, B, 2017, 251, 993–1000.
155. Y. Huang, F. Wang, S. Mu, X. Sun, Q. Li, C. Xie and H. Liu, Spectrochim. Acta, Part A, 2020, 243, 118754.
156. Y. Jiao, X. Liu, L. Zhou, H. He, P. Zhou and C. Duan, Sens. Actuators, B, 2017, 247, 950–956.
157. Z. Li, H. Su, K. Zhou, B. Yang, T. Xiao, X. Sun, J. Jiang and L. Wang, Dyes Pigm., 2018, 149, 921–926.
158. B. Liu, P. Wang, J. Chai, X. Hu, T. Gao, J. Chao, T. Chen and B. Yang, Spectrochim. Acta, Part A, 2016, 168, 98–103.
159. B. Naskar, R. Modak, Y. Sikdar, D. K. Maiti, A. Bauzá, A. Frontera, A. Katarkar, K. Chaudhuri and S. Goswami, Sens. Actuators, B, 2017, 239, 1194–1204.
160. C. Patra, A. K. Bhanja, C. Sen, D. Ojha, D. Chattopadhyay, A. Mahapatra and C. Sinha, Sens. Actuators, B, 2016, 228, 287–294.
161. M. Shellaiah, Y. C. Rajan, P. Balu and A. Murugan, New J. Chem., 2015, 39, 2523–2531.
162. T. Simon, M. Shellaiah, V. Srinivasadesikan, C.-C. Lin, F.-H. Ko, K. W. Sun and M.-C. Lin, New J. Chem., 2016, 40, 6101–6108.
163. J. Tian, X. Yan, H. Yang and F. Tian, RSC Adv., 2015, 5, 107012–107019.
164. H. Tian, X. Qiao, Z. Zhang, C. Xie, Q. Li and J. Xu, Spectrochim. Acta, Part A, 2019, 207, 31–38.
165. E. Wang, L. Pang, Y. Zhou, J. Zhang, F. Yu, H. Qiao and X. Pang, Biosens. Bioelectron., 2016, 77, 812–817.
166. F.-Ur-Rahman, A. Ali, R. Guo, J. Tian, H. Wang, Z.-T. Li and D.-W. Zhang, Sens. Actuators, B, 2015, 211, 544–550.
167. Y. Yang, S. Ma, Y. Zhang, J. Ru, X. Liu and H. Guo, Spectrochim. Acta, Part A, 2018, 199, 202–208.
168. J. Zhu, L. Lu, M. Wang, T. Sun, Y. Huang, C. Wang, W. Bao, M. Wang, F. Zou and Y. Tang, Tetrahedron Lett., 2020, 61, 151893.
169. L. Subha, C. Balakrishnan, S. Natarajan, M. Theetharappan, B. Subramanian and M. A. Neelakantan, Spectrochim. Acta, Part A, 2016, 153, 249–256.
170. B. K. Kundu, P. Mandal, B. G. Mukhopadhyay, R. Tiwari, D. Nayak, R. Ganguly and S. Mukhopadhyay, Sens. Actuators, B, 2019, 282, 347–358.
171. A. Ghorai, J. Mondal, R. Saha, S. Bhattacharya and G. K. Patra, Anal. Methods, 2016, 8, 2032–2040.
172. J. Kumar, M. J. Sarma, P. Phukan and D. K. Das, Dalton Trans., 2015, 44, 4576–4581.
173. A. Ghorai, J. Mondal, R. Chandra and G. K. Patra, Dalton Trans., 2015, 44, 13261–13271.
174. M. Liu, H. Yang, D. Li, Q. Yao, H. Wang, Z. Zhang and J. Dou, Inorg. Chim. Acta, 2021, 522, 120384.
175. C. Liu, X. Liu, X. Ge, Q. Wang, L. Zhang, W. Shang, Y. Zhang, X. A. Yuan, L. Tian, Z. Liu and J. You, Dalton Trans., 2020, 49, 5988–5998.
176. N. H. Kim, J. Lee, S. Park, J. Jung and D. Kim, Sensors, 2019, 19, 2500.
177. K. S. M. Salih, A. M. Shraim, S. R. Al-Mhini, R. E. Al-Soufi and I. Warad, Emergent Mater., 2021, 4, 423–434.
178. S. Xue, Z. Xie, Y. Wen, J. He, Y. Liu and W. Shi, ChemistrySelect, 2021, 6, 7123–7129.
179. H. Peng, Y. Han, N. Lin and H. Liu, Opt. Mater., 2019, 95, 109210.
180. Z. Guo, Q. Niu, T. Li, T. Sun and H. Chi, Spectrochim. Acta, Part A, 2019, 213, 97–103.
181. L. Kang, Y.-T. Liu, N.-N. Li, Q.-X. Dang, Z.-Y. Xing, J.-L. Li and Y. Zhang, J. Lumin., 2017, 186, 48–52.
182. M. Kao, T. Chen, Y. Cai, C. Hu, Y. Liu, Y. Jhong and A. Wu, J. Lumin., 2016, 169, 156–160.
183. X. Zhang, L. Shen, Q. Zhang, X. Yang, Y. Huang, C. Redshaw and H. Xu, Molecules, 2021, 26, 1233.
184. H. Peng, K. Shen, S. Mao, X. Shi, Y. Xu, S. O. Aderinto and H. Wu, J. Fluoresc., 2017, 27, 1191–1200.
185. K. Shen, S. Mao, X. Shi, F. Wang, Y. Xu, S. O. Aderinto and H. Wu, Luminescence, 2018, 33, 54–63.
186. J. Zhu, Y. Zhang, L. Wang, T. Sun, M. Wang, Y. Wang, D. Ma, Q. Yang and Y. Tang, Tetrahedron Lett., 2016, 57, 3535–3539.
187. H. Sun, Y. Jiang, J. Nie, J. Wei, B. Miao, Y. Zhao, L. Zhang and Z. Ni, Mater. Chem. Front., 2021, 5, 347–354.
188. H. Sun, J. Li, F. Han, R. Zhang, Y. Zhao, B. Miao and Z. Ni, Dyes Pigm., 2019, 167, 143–150.
189. X. Yang, W. Zhang, Z. Yi, H. Xu, J. Wei and L. Hao, New J. Chem., 2017, 41, 11079–11088.
190. M. Yang, Y. Zhang, W. Zhu, H. Wang, J. Huang, L. Cheng, H. Zhou, J. Wu and Y. Tia, J. Mater. Chem. C, 2015, 3, 1994–2002.
191. K. Muthamma, D. Sunil, P. Shetty, S. D. Kulkarni, P. J. Anand and D. Kekuda, Prog. Org. Coat., 2021, 161, 106463.
192. N. K. Gondia and S. K. Sharma, Mater. Chem. Phys., 2019, 224, 314–319.
193. V. Nishal, D. Singh, R. K. Saini, V. Tanwar, S. Kadyan, R. Srivastava and P. S. Kadyan, Cogent Chem., 2015, 1, 1079291.
194. A. N. Gusev, M. A. Kiskin, E. V. Braga, M. Chapran, G. Wiosna-Salyga, G. V. Baryshnikov, V. A. Minaeva, B. F. Minaev, K. Ivaniuk, P. Stakhira, H. Ågren and W. Linert, J. Phys. Chem. C, 2019, 123, 11850–11859.
195. B. Das, A. Jana, A. Das, D. Chattopadhyay, A. Dhara, S. Mabhai and S. Dey, Spectrochim. Acta, Part A, 2019, 212, 222–231.
196. D. K. Das, J. Kumar, A. Bhowmick, P. K. Bhattacharyya and S. Banu, Inorg. Chim. Acta, 2017, 462, 167–173.
197. V. K. Gupta, A. K. Jain and S. K. Shoora, Sens. Actuators, B, 2015, 219, 218–231.
198. B. Naskar, R. Modak, Y. Sikdar, D. K. Maiti, A. Banik, T. K. Dangar, S. Mukhopadhyay, D. Mandal and S. Goswami, J. Photochem. Photobiol., A, 2016, 321, 99–109.
199. R. Purkait, S. Dey and C. Sinhaa, New J. Chem., 2018, 42, 16653–16665.
200. G. G. V. Kumar, M. P. Kesavan, M. Sankarganesh, K. Sakthipandi, J. Rajesh and G. Sivaraman, New J. Chem., 2018, 42, 2865–2873.
201. D. Jothi, S. K. Munusamy, S. Sawminathana and S. K. Iyer, RSC Adv., 2021, 11, 11338–11346.
202. Q. Lin, X. Jiang, X. Ma, J. Liu, H. Yao, Y. Zhang and T. Wei, Sens. Actuators, B, 2018, 272, 139–145.
203. X. Ma, Z. Zhang, H. Xie, Y. Ma, C. Liu, S. Liu and M. Liu, Chem. Commun., 2018, 54, 13674–13677.
204. S. K. Panigrahi and A. K. Mishra, J. Photochem. Photobiol., A, 2019, 41, 100318.
205. H. Rubin, Arch. Biochem. Biophys., 2007, 458, 16–23.
206. R. Y. Walder, B. Yang, J. B. Stokes, P. A. Kirby, X. Cao, P. Shi, C. C. Searby, R. F. Husted and V. C. Sheffield, Hum. Mol. Genet., 2009, 18, 4367–4375.
207. M. Piskacek, L. Zotova, G. Zsurka and R. J. Schweyen, J. Cell. Mol. Med., 2009, 13, 693–700.
208. J. Tfelt-Hansen and E. M. Brown, Crit. Rev. Clin. Lab. Sci., 2005, 42, 35–70.
209. S. Minisola, J. Pepe, S. Piemonte and C. Cipriani, BMJ, 2015, 350, 2723.
210. J. Fong and A. Khan, Can. Fam. Phys., 2012, 58, 158–62.
211. Y. Wang, Z. Y. Ma, D. L. Zhang, J. L. Deng, X. Chen, C. Z. Xie, X. Qiao, Q. Z. Li and J. Y. Xu, Spectrochim. Acta, Part A, 2018, 195, 157–164.
212. H. Wang, X. Xu, J. Yin, Z. Zhang and L. Xue, ChemistrySelect, 2021, 6, 6454–6459.
213. L. Fan, J. C. Qin, C. R. Li and Z. Y. Yang, Spectrochim. Acta, Part A, 2019, 218, 342–347.
214. L. Bai, F. Tao, L. Li, A. Deng, C. Yan, G. Li and L. Wang, Spectrochim. Acta, Part A, 2019, 214, 436–444.
215. W. Anbu Durai, A. Ramu and A. Dhakshinamoorthy, Inorg. Chem. Commun., 2020, 121, 108191.
216. L. Bai, G. Li, L. Li, M. Gao, H. Li, F. Tao, A. Deng, S. Wang and L. Wang, React. Funct. Polym., 2019, 139, 1–8.
217. L. Tomljenovic, J. Alzheimer's Dis., 2011, 23, 567–598.
218. A. Schiff-base, A. Iii, L. Fan, J. Qin, T. Li, Z. Yang, L. Fan, J. Qin, T. Li, B. Wang and Z. Yang, J. Lumin., 2014, 155, 84–88.
219. K. S. Patil, P. G. Mahajan and S. R. Patil, Spectrochim. Acta, Part A, 2017, 170, 131–137.
220. C. R. Silva, M. B. N. Oliveira, S. F. Melo, F. J. S. Dantas, J. C. P. De Mattos, R. J. A. C. Bezerra, A. Caldeira-De-Araujo, A. Duatti and M. Bernardo-Filho, Brain Res. Bull., 2002, 59, 213–216.
221. R. Mcrae, P. Bagchi, S. Sumalekshmy and C. J. Fahrni, Chem. Rev., 2009, 109, 4780–4827.
222. Y. S. Yang, C. Liang, C. Yang, Y. P. Zhang, B. X. Wang and J. Liu, Spectrochim. Acta, Part A, 2020, 237, 118391.
223. G. Singh, J. Sindhu, Manisha, V. Kumar, V. Sharma, S. K. Sharma, S. K. Mehta, M. H. Mahnashi, A. Umar and R. Kataria, J. Mol. Liq., 2019, 296, 111814.
224. R. Kouser, S. Zehra, R. A. Khan, A. Alsalme, F. Arjmand and S. Tabassum, Spectrochim. Acta, Part A, 2021, 247, 119156.
225. L. Dong, X. Zeng, L. Mu, S. Xue, Z. Tao and J. Zhang, Sens. Actuators, B, 2010, 145, 433–437.
226. H. Chen, F. Yuan, J. Xu, Y. Zhang, Y. Wu and L. Wang, Spectrochim. Acta, Part A, 2013, 107, 151–155.
227. L. Wang, L. Yang and D. Cao, J. Fluoresc., 2014, 24, 1347–1355.
228. W. Bian, J. Ma, Q. Liu, Y. Wei, Y. Li, C. Dong and S. Shuang, Luminescence, 2014, 29, 151–157.
229. S. Ramezani, M. Pordel and A. Davoodnia, Inorg. Chim. Acta, 2019, 484, 450–456.
230. P. Aussawaponpaisan, P. Nusuwan, P. Tongraung, P. Jittangprasert, K. Pumsa-Ard and M. Kuno, Mater. Today Proc., 2017, 4, 6022–6030.
231. D. Vashisht, S. Sharma, R. Kumar, V. Saini, V. Saini, A. Ibhadon, S. C. Sahoo, S. Sharma, S. K. Mehta and R. Kataria, Microchem. J., 2020, 155, 104705.
232. X. Zhang, S. T. Wu, X. J. Yang, L. Y. Shen, Y. L. Huang, H. Xu, Q. L. Zhang, T. Sun, C. Redshaw and X. Feng, Inorg. Chem., 2021, 60, 8581–8591.
233. F. Yan, X. Sun, Y. Zhang, Y. Jiang, L. Chen, T. Ma and L. Chen, J. Photochem. Photobiol., A, 2021, 407, 113065.
234. S. Wang, K. Cai, Y. Song and Y. Zhu, ChemistrySelect, 2021, 6, 3788–3794.
235. W. Pan, X. Yang, Y. Wang, L. Wu, N. Liang and L. Zhao, J. Photochem. Photobiol., A, 2021, 420, 113506.
236. A. Goel, N. Tomer, V. D. Ghule and R. Malhotra, Spectrochim. Acta, Part A, 2021, 249, 119221.
237. H. Li, H. Li, Y. Zhu, B. Shi, W. Qu, Y. Zhang, Q. Lin, H. Yao and T. Wei, Supramol. Chem., 2014, 27, 471–477.
238. M. Liu, T. Wei, Q. Lin and Y. Zhang, Spectrochim. Acta, Part A, 2011, 79, 1837–1842.
239. L. M. Gaetke, H. S. Chow and C. K. Chow, Arch. Toxicol., 2014, 88, 1929–1938.
240. X. Wang, D. Shi, Y. Xu, S. Yu, F. Zhao, Y. Wang, L. Hu, J. Tian, X. Yu and L. Pu, Tetrahedron Lett., 2018, 59, 2332–2334.
241. S. Ullmann, R. Schnorr, M. Handke, C. Laube, B. Abel, J. Matysik, M. Findeisen, R. Rüger, T. Heine and B. Kersting, Chem. – Eur. J., 2017, 23, 3824–3827.
242. S. J. Malthus, S. A. Cameron and S. Brooker, Inorg. Chem., 2018, 57, 2480–2488.
243. K. Rout, A. K. Manna, M. Sahu and G. K. Patra, Inorg. Chim. Acta, 2019, 486, 733–741.
244. X. Mu, L. Shi, L. Yan and N. Tang, J. Fluoresc., 2021, 31, 971–979.
245. C. T. Chasapis, A. C. Loutsidou, C. A. Spiliopoulou and M. E. Stefanidou, Arch. Toxicol., 2012, 86, 521–534.
246. C. F. Walker and R. E. Black, Annu. Rev. Nutr., 2004, 24, 255–275.
247. Z. Yang, C. Yan, Y. Chen, C. Zhu, C. Zhang, X. Dong, W. Yang, Z. Guo, Y. Lub and W. He, Dalton Trans., 2011, 40, 2173–2176.
248. M. Hosseini, A. Ghafarlo, M. R. Ganjali, F. Faridbod, P. Norouzi and M. S. Niasari, Sens. Actuators, B, 2014, 198, 411–415.
249. J. Afshani, A. Badiei, M. Karimi, N. Lashgari and G. Mohammadi Ziarani, Appl. Organomet. Chem, 2017, 31, 1–12.
250. W. Fang, G. Zhang, J. Chen, L. Kong, L. Yang, H. Bi and J. Yang, Sens. Actuators, B, 2016, 229, 338–346.
251. G. N. George, R. C. Prince, G. A. Buttigieg, M. B. Denton, H. H. Harris and I. J. Pickering, Chem. Res. Toxicol., 2004, 17, 999–1006.
252. J. Yan, L. Fan, J. C. Qin, C. R. Li and Z. Y. Yang, J. Fluoresc., 2016, 26, 1059–1065.
253. D. Mohanasundaram, R. Bhaskar, G. Gangatharan Vinoth Kumar, J. Rajesh and G. Rajagopal, Microchem. J., 2021, 164, 106030.
254. B. X. Shen and Y. Qian, ChemistrySelect, 2017, 2, 2406–2413.
255. H. Tokunaga, K. Kazama, M. Tsuboi and M. Miyasaka, Luminescence, 2021, 36, 1561–1568.
256. B. Li, Z. Liu, L. Li, Y. Xing, Y. Liu, X. Yang, M. Pei and G. Zhang, New J. Chem., 2021, 45, 6753–6759.
257. P. S. Hariharan and S. P. Anthony, Spectrochim. Acta, Part A, 2015, 136, 1658–1665.
258. Ö. Şahin, Ü. Ö. Özdemir, N. Seferoğlu, Z. K. Genc, K. Kaya, B. Aydıner, S. Tekin and Z. Seferoğlu, J. Photochem. Photobiol., B, 2018, 178, 428–439.
259. B. Kaur and N. Kaur, J. Coord. Chem., 2019, 72, 2189–2199.
260. N. Kaur and B. Kaur, Inorg. Chem. Commun., 2020, 121, 108239.
261. C. Li, L. Xiao, Q. Zhang and X. Cheng, Spectrochim. Acta, Part A, 2020, 243, 118763.
262. E. K. İnal, J. Fluoresc., 2020, 30, 891–900.
263. S. Dey, R. Purkait, D. Mallick and C. Sinha, ChemistrySelect, 2020, 5, 8274–8283.
264. P. Vijayakumar, E. Dhineshkumar, M. Arockia Doss, S. Nargis Negar and R. Renganathan, Mater. Today Proc., 2020, 42, 1050–1064.
265. S. Li, D. Cao, X. Meng, Z. Hu, Z. Li, C. Yuan, T. Zhou, X. Han and W. Ma, J. Photochem. Photobiol., A, 2020, 392, 112427.
266. B. Li, X. Gu, M. Wang, X. Liu and K. Xu, Dyes Pigm., 2021, 194, 109637.
267. S. A. Khan, Q. Ullah, A. S. A. Almalki, S. Kumar, R. J. Obaid, M. A. Alsharif, S. Y. Alfaifi and A. A. Hashmi, J. Mol. Liq., 2021, 328, 115407.
268. H. He, Z. Cheng and L. Zheng, J. Mol. Struct., 2021, 1227, 129522.
269. O. A. Urucu and A. Aydin, Anal. Lett., 2015, 48, 1767–1776.
270. E. S. Almeida, E. M. Richter and R. A. A. Munoz, Anal. Chim. Acta, 2014, 837, 38–43.
271. M. K. Goshisht, L. Moudgil, M. Rani, P. Khullar, G. Singh, H. Kumar, N. Singh, G. Kaur and M. S. Bakshi, J. Phys. Chem. C, 2014, 118, 28207–28219.
272. A. Mahal, M. K. Goshisht, P. Khullar, H. Kumar, N. Singh, G. Kaur and M. S. Bakshi, Phys. Chem. Chem. Phys., 2014, 16, 14257–14270.
273. M. K. Goshisht, Adv. Mater. Proc, 2017, 2, 535–546.
274. P. Khullar, M. K. Goshisht, L. Moudgil, G. Singh, D. Mandial, H. Kumar, G. K. Ahluwalia and M. S. Bakshi, ACS Sustainable Chem. Eng., 2017, 5, 1082–1093.
275. M. K. Goshisht, L. Moudgil, P. Khullar, G. Singh, A. Kaura, H. Kumar, G. Kaur and M. S. Bakshi, ACS Sustainable Chem. Eng., 2015, 3, 3175–3187.
276. H. Dezhampanah, R. Firouzi, Z. Moradi Shoeili and R. Binazir, J. Mol. Struct., 2020, 1205, 127557.
277. M. Cui, Y. Xin, R. Song, Q. Sun, X. Wang and D. Lu, Cellulose, 2020, 27, 1621–1633.
278. X. Zeng, X. Zhang, B. Zhu, H. Jia and J. Xue, Analyst, 2011, 136, 4008–4012.
279. Y. Sun, Y. Lu, M. Bian, Z. Yang, X. Ma and W. Liu, Eur. J. Med. Chem., 2021, 211, 113098.
280. S. Hazra, B. G. M. Rocha, A. K. M. Fátima, C. Guedes da Silva and A. J. L. Pombeiro, Inorganics, 2019, 7, 17.
281. A. Soroceanu, M. Cazacu, S. Shova, C. Turta, J. Kožíšek, M. Gall, M. Breza, P. Rapta, T. C. O. Mac Leod, J. T. Armando, J. L. Pombeiro, A. A. Dobrov and V. B. Arion, Eur. J. Inorg. Chem., 2013, 1458–1474.
282. Y. Wang, Q. Guo, X. Wu, H. Gao, R. Lu and W. Zhou, Spectrochim. Acta, Part A, 2022, 265, 120358.
283. J. J. Celestina, P. Tharmaraj, C. D. Sheela and J. Shakina, J. Lumin., 2021, 239, 118359.
284. S. O. Tümay and S. Yeşilot, J. Photochem. Photobiol., A, 2021, 407, 113093.
285. D. Aydin, S. Dinckan, S. N. Karuk Elmas, T. Savran, F. N. Arslan and I. Yilmaz, Food Chem., 2021, 337, 127659.
286. Z. Li, X. Zhang, S. Wang, Y. Yang, B. Qin, K. Wang, T. Xie, Y. Wei and Y. Ji, Chem. Sci., 2016, 7, 4741–4747.
287. D. Genovese, A. Aliprandi, E. A. Prasetyanto, M. Mauro, M. Hirtz, H. Fuchs, Y. Fujita, H. Uji-i, S. M. K. Lebedkin and L. De Cola, Adv. Funct. Mater., 2016, 26, 5271–5278.
288. C. Ma, B. Xu, G. Xie, J. He, X. Zhou, B. Peng, L. Jiang, B. Xu, W. Tian, Z. Chi, S. Liu, Y. Zhang and J. Xu, Chem. Commun., 2014, 50, 7374–7377.
289. B. Yoon, J. Lee, I. S. Park, S. Jeon, J. Lee and J.-M. Kim, J. Mater. Chem. C, 2013, 1, 2388–2403.
290. L. Peng, Y. Zheng, X. Wang, A. Tong and Y. Xiang, Chem. – Eur. J., 2015, 21, 4326–4332.
291. K. Li, Y. Xiang, X. Wang, J. Li, R. Hu, A. Tong and B. Z. Tang, J. Am. Chem. Soc., 2014, 136, 1643–1649.
292. Y. Jhong, W. H. Hsieh, J. Chir and A. Wu, J. Fluoresc., 2014, 24, 1723–1726.
293. Z. Y. Yin, J. H. Hu, K. Gui, Q. Q. Fu, Y. Yao, F. L. Zhou, L. L. Ma and Z. P. Zhang, J. Photochem. Photobiol., A, 2020, 396, 112542.

---