A quinazolin-based Schiff-base chemosensor for colorimetric detection of Ni²⁺ and Zn²⁺ ions and ‘turn-on’ fluorometric detection of Zn²⁺ ion

Abstract

Herein, we have reported a novel quinazolin-based Schiff base chemosensor (E)-2-benzamido-N′-(1-(pyridin-2-yl)ethylidene)benzohydrazide (L). L has been designed, synthesised and characterised by ¹H-NMR, IR spectroscopy, ESI-MS spectrometry and theoretical studies. The receptor showed appreciable colorimetric λ_max shift for both Ni²⁺ and Zn²⁺ ions and fluorometric “turn on” response in presence of only Zn²⁺ ion. The Jobs plot analysis revealed that receptor forms 2 : 1 complex with both the ions Ni²⁺ and Zn²⁺, further confirmed by ESI-MS analysis. The single crystal structure of L–Ni²⁺ complex (1) has also been determined. The colorimetric detection limits were calculated to 7.9 nM and 7.5 nM respectively for Ni²⁺ and Zn²⁺ in methanol–Tris–HCl buffer medium (10 mM, pH 7.2, 1 : 1 v/v). The chemosensor L can be applied for the recovery of contaminated water samples.

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Introduction

Metal ion detection continues to be an important area of research both in the environment and in biological systems.¹ Nickel has long been thought of as a potentially toxic metal. Decomposition or degradation of rocks and soil, biological cycles, particularly industrial processes, and water disposal are the main sources of nickel in aquatic systems.² The tolerable concentration of Ni(II) ions in drinking water is 2.5 mg mL⁻¹.³ Among the transition metal series, Ni²⁺ plays a significant part in biological activities like metabolism, respiration, and biosynthesis. It is a necessary component of various metallo-enzymes such hydrogenases, carbon monoxide dehydrogenases, and acireductone dioxygenases.^4–6 In the chemical industry, it is widely used for the manufacture of practical electronics devices including Ni–Cd batteries, electroplating, and electroforming.^7,8 One of the nickel alloys utilised in the manufacture of tools, machinery, and weapons is stainless steel.⁹ The adverse effects of excessive Ni²⁺ accumulation over the recommended limits on human physical health include respiratory issues, lung cancer, pneumonitis, allergies, and central nervous system disorders.^10,11 Thus, one of the difficult issues in current research is the specific detection of Ni²⁺ ion at the biological and environmental level.

Zinc ion is the second-most prevalent heavy metal ion and it is well known that zinc plays a significant role in cellular processes.¹² Because of its structural characteristics, the zinc, a biologically significant metal ion, is essential for many pathophysiologic processes, including gene expression, cell apoptosis, enzymatic regulation, and neurotransmission.^13–15 The breakdown of Zn²⁺ ion homeostasis, which is toxic in excess or absence may in the pathology of intellectual development as well as other neurological issues including Alzheimer's and Parkinson's diseases.^16,17 Due to this, the detection of Zn²⁺ has been studied intensively. Many selective colorimetric and/or fluorescent probes targeting Zn²⁺ have been reported, that are mostly based on quinoline, fluorescein, benzazole, anthracene, coumarin, BODIPY, Schiff bases, indole, thiazole, terpyridin, naphthalimide, nanoparticles and so on.^18–40

From the above reports most of them are fluorescent based and few are colorimetric fluorescent chemosensors. Xu et al. (2009) has reported an NBD-based colorimetric and fluorescent chemosensor for Zn²⁺ and its use for detection of intracellular zinc ions.⁴¹ S. Lee et al. (2016) has reported a Zn²⁺ colorimetric sensor using label-free silver nanoparticles.⁴² On the other hand, a fluorescent chemosensor that is “switching on” and selective to Zn²⁺ was described by Li et al.⁴³ and applied in MCF-7 cells. Two new ratiometric chemosensors were also created and tested by Lee et al.⁴⁴ for the detection of potentially harmful free Zn²⁺ ions in aqueous media. A novel fluorescent chemosensor was recently created by Chen and Li et al.⁴⁵ using a cyclen moiety as the ionophore and a helical imide as the fluorophore. Moreover, Ding et al.⁴⁶ developed selective and responsive “turn-on” fluorescentric Zn²⁺ sensors based on di- and tri-pyridines with easily modulatable emission wavelengths.

Inspired by the above research and our search on fluorescent colorimetric chemosensor,^47,48 in this work, we have reported an optical sensor for colorimetric detection of Ni²⁺ and Zn²⁺ ions and selective fluorometric detection of Zn²⁺ ion. A quinazolinone derived Schiff base chemosensor (E)-2-benzamido-N′-(1-(pyridin-2-yl)ethylidene)benzohydrazide (L) has been designed, synthesised and characterised by ¹H-NMR, IR spectroscopy, ESI-MS spectrometry and single crystal XRD analysis. The receptor showed colorimetric λ_max shift for Ni²⁺ and Zn²⁺ (detection limit was 7.9 nM and 7.5 nM respectively) and fluorometric “turn-on” response for Zn²⁺ ion with detection limit 1.7 nM in methanol–Tris–HCl buffer medium (10 mM, pH 7.2, 1 : 1 v/v). The Jobs plot analysis revealed that receptor forms 2 : 1 complex with both the ions Ni²⁺ and Zn²⁺, further confirmed by ESI-MS analysis. The single crystal structure of L–Ni²⁺ complex, 3 has also been isolated. The chemosensor L can be applied to the recovery of contaminated water samples.

Experimental

Materials and general information

The necessary components were purchased from Sigma-Aldrich and employed for synthesis without further purification. For the majority of the experiments, analytical-grade solvents were utilised, and double-deionized water was used for dilution and the preparation of Tris buffer (pH = 7.2) solution. The metal ion solutions were made from their nitrate salts. ¹H-NMR and ¹³C-NMR spectra were recorded on a Bruker DRX spectrometer operating at 400 MHz in CDCl₃ and chemical shifts were recorded in ppm relative to TMS. On a Shimadzu UV 1800 spectrophotometer, absorption spectra were captured using a quartz cuvette with a 10 mm path length and a wavelength of between 200 and 800 nm. Using a Waters mass spectrometer with mixed solvent HPLC methanol and triple-distilled water, high resolution mass (HRMs) spectra were captured. The pH measurements were done using a digital pH meter (Merck) by adjusting dilute hydrochloric acid and sodium hydroxide in buffer solution. Solutions of the receptor L (1 × 10⁻⁵ M) and metal salts (1 × 10⁻⁴ M) were prepared in CH₃OH–H₂O (1 : 1, v/v) and H₂O respectively.

X-ray data collection and structural determination

X-ray single crystal data were collected using MoKα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement were carried out using the software package of SMART APEX.⁴⁹ The structures were solved by direct methods (SHELXS-97) and standard Fourier techniques, and refined on F² using full matrix least squares procedures (SHELXL-97) using the SHELX-97 package⁵⁰ incorporated in WinGX.⁵¹ In most of the cases, non-hydrogen atoms were treated anisotropically. Hydrogen atoms were fixed geometrically at their calculated positions following riding atom model. Structural information of 3 have been deposited at the Cambridge Crystallographic Data Center (CCDC number 2205292†).

Synthesis of 2-phenyl-4(3H)-3,1-benzoxazinone (1)

The 2-phenyl-4H-benzoxalin-4-one (1) has been synthesized by following the reported procedure⁵² with some modification. To a stirred solution of anthranilic acid (2.74 g, 0.020 mol) in pyridine (12 mL), benzoyl chloride (2.81 g, 0.020 mol) was added dropwise, maintaining the temperature about 0–5 °C for 1 h. The reaction mixture was stirred for another 3 h at room temperature until a solid product was formed. The reaction mixture was neutralized with saturated sodium bicarbonate solution and the pale yellow solid which separated was filtered, washed with water and re-crystallized from ethanol. Yield, 3.8 g, 85%; mp 112–115 °C. ¹³C NMR (400 MHz, CDCl₃, TMS): δ 100.2 (1C, C=N), 130.4–137.0 (12C, aromatic carbons), 172.1 (1C, C=O). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.31 (d, 2H), 8.24 (d, 1H), 7.82 (t, 1H), 7.69 (d, 1H), 7.58 (t, 1H), 7.51 (m, 3H) (Fig. S1†).

Synthesis of 3-amino-2-phenyl-4(3H)-quinazolinone (2)

To a stirred solution of 1 (2.23 g, 0.01 mol) in ethanol (20 mL), 80% N₂H₄·H₂O (0.5 g, 0.01 mol) and a catalytic amount of pyridine (2–4 drop) was added. The reaction mixture was stirred and refluxed for 2 h at 117 °C. After cooling, the crude product was obtained by filtration and the crude product was recrystallized from ethanol to afford 2 as a white product.⁵³ Yield, 2.15 g, 90%; mp 178–180 °C. ¹³C NMR (400 MHz, CDCl₃, TMS): δ 110.5 (1C, C=N), 126.6–140.3 (12C, aromatic carbons), 167.4 (1C, C=O). ¹H NMR (400 MHz, CDCl₃, TMS): δ 8.45 (d, 2H), 7.92 (s, 1H), 7.68 (m, 4H), 7.51 (d, 2H), 6.85 (s, 2H) (Fig. S2†).

Synthesis of N-(E)-N′-(4-(diethylamino)-2-hydroxybenzylidene)-2-(benzamido) benzohydrazide (L)

2.37 g (0.01 mol) of 2 was dissolved in 20 mL of absolute ethanol and to this solution 1.21 g (0.01 mol) 1-(pyridin-2-yl)ethanone was added dropwise with constant stirring. Then the reaction mixture was refluxed for 12 h under dry condition. The resulting mixture was kept in air for 24 h. Needle shaped crystals separated out was filtered, washed with methanol and dried in air. Yield, 3.15 g, 87%; mp = 235–237 °C. Single crystals suitable for X-ray analysis were obtained from the slow evaporation of dichloromethane solution of L. Anal. calc. for C₂₁H₁₈N₄O₂: C, 70.38; H, 5.06; N, 15.63. Found C, 70.68; H, 5.05; N, 15.07%. EI-MS: m/z 359.14 (M + Na⁺, 100%); (Fig. S3†). FTIR/cm⁻¹ (KBr): 3216 (-NH), 3040 (m, aromatic CH– str), 1676 (vs., C=O), 1631 (vs., C=N), 1525 (s), 1454 (s), 1370 (m), 1316 (s), 1260 (s), 1167 (s), 1092 (s), 1037 (m), 972 (s), 934 (s) (S4). ¹H NMR (400 MHz, CDCl₃, TMS): δ 11.57 (s, NH), 11.32 (s, 1H, NH), 8.87 (s, 1H, azomethine), 8.1–6.9 (m, 12H, aromatic), 1.19 (s, 3H, methyl) (Fig. S5†). ¹³C NMR (CDCl₃, δ ppm, TMS): 165.81, 158.93, 151.27, 139.01, 134.09, 132.67, 131.31, 130.38, 129.39, 129.22, 128.35, 128.09, 127.61, 127.02, 126.76, 123.63, 122.34, 119.60, 117.72, 117.52, 19.45 (Fig. S6†).

Synthesis of Ni²⁺ complex of L (NiL₂, 3)

A methanolic solution (10 mL) of Ni(NO₃)₂·3H₂O (0.120 g, 0.5 mmol) was added dropwise with stirring to the saturated solution of L (0.181 g, 0.5 mmol) in methanol (20 mL). The resulting mixture was continuously stirred for 1 h at room temperature and then filtered. Yield, 69%. Anal. calc. for C₄₂H₃₆N₈NiO₄: C, 65.05; H, 4.68; N, 14.45%. Found C, 64.96; H, 4.59; N, 14.53%. EI-MS: m/z 772.2 (2L–2H⁺ + Ni²⁺) (Fig. S7†). FTIR/cm⁻¹ (KBr):3440 (–NH), 3024 (m, aromatic CH– str), 1660 (vs., C=O), 1606 (vs., C=N), 1504 (s), 1443 (m), 1385 (s), 1313 (s), 1216 (s), 1167 (s), 1037 (m), 708 (s) (Fig. S8†). Slow evaporation of the methanol solution of 3 yielded dark red crystalline solid (suitable for X-ray analysis).

Synthesis of Zn²⁺ complex of L (ZnL₂, 4)

A methanolic solution (10 mL) of Zn(NO₃)₂·6H₂O (0.150 g, 0.5 mmol) was added dropwise with stirring to the saturated solution of L (0.181 g, 0.5 mmol) in methanol (20 mL). The resulting mixture was continuously stirred for 1 h at room temperature and then filtered. Yield, 72%. Anal. calc. for C₄₂H₃₆N₈ZnO₄: C, 64.49; H, 4.64; N, 14.33%. Found C, 64.56; H, 4.55; N, 14.47%. EI-MS: m/z 803.2 (2L + Zn²⁺ + Na⁺) (Fig. S9†). FTIR/cm⁻¹ (KBr): 3068 (w), 2928 (m), 1665 (vs., C=O), 1643 (vs., C=N), 1595 (s), 1519 (s), 1446 (m), 1342 (m), 1242 (s), 1137 (m), 762 (m), (Fig. S10†).

Photo physical measurements

Chemosensor L (3.58 mg) was dissolved separately in 10 mL of homogeneous solvent CH₃OH–H₂O (1 : 1, v/v) to make a solution of 1 × 10⁻³ M and 30 μL of each of this solution were diluted with 2.97 mL of CH₃OH–Tris buffer (1 : 1 v/v) mixture to make a final concentration of 10 μM. The analyte solutions have been prepared using nitrate salt of metals in the order of 10⁻⁴ M in double-distilled water. After mixing L with each of the metal ions for a few seconds, absorption and fluorescence spectra were obtained at room temperature.

Job's plot measurements

A methanol–Tris buffer (1 : 1) solutions containing L (10 μM) and aqueous solution of Ni(NO₃)₂ and Zn(NO₃)₂ (10 μM each) were prepared separately. Then changing the mole ratio of the L from 0.1 to 0.9 in such a manner that the sum of the total metal ion and L volume remained constant (2 mL). All the solutions were diluted to 3 mL. After shaking them for a minute, UV-vis spectra were obtained at room temperature.

Computational details

The program package GAUSSIAN-09 Revision C.01 was employed for all calculations.⁵⁴ The gas phase geometries of the compound were fully optimized symmetry restrictions in singlet ground state with the gradient-corrected DFT level coupled with B3LYP.⁵⁵ Basis set LanL2DZ was used for the whole molecules L and NiL₂ (3). The HOMOs and LUMOs of molecular ions were calculated with the same basis set and functional.

Result and discussion

Synthesis and structure of L

The coupling reaction between aldehyde and amine generate the imine functionalised ligand L (Scheme 1) with high yield (over 85%) and it was characterised by elemental analysis, FTIR, ¹H NMR and ESI-mass spectra.

Scheme 1 Synthesis procedure of ligand L.

UV-vis spectroscopic studies of L towards different metal ions

The receptor contains imine linker as well as several amide functional moieties thus interaction with cations and anions were expected. Thus absorption studies of L in presence of common cations and anions were performed at room temperature in methanol–Tris–HCl buffer (1 : 1 v/v, 10 mM, pH 7.2) solution. The receptor L shows main absorption band at 280 nm due to π–π* transition. This absorption band of L significantly red shifted to around 375 nm only in presence of 2 equiv. of Ni²⁺ and Zn²⁺ ions with the change of colour of the solution from colourless to yellow. Addition of same amount of several metal like Cu²⁺, Co²⁺, Cd²⁺, Cr³⁺, Mn⁷⁺, Sm³⁺, Al³⁺ to the solution of receptor L, also absorption band shift observed, but the color and absorbance intensity produced by these ions is not very prominent when compare with Ni²⁺ and Zn²⁺ ions. The remaining cations and anions e.g. Fe³⁺, Hg²⁺, Pb²⁺, Cr³⁺, Ag⁺, Mn²⁺, Fe²⁺ etc. cannot produced any significant change of receptor solution at that condition (Fig. S11†).

After that, using same UV-vis spectrophotometer, receptor L was titrated separately in presence of Ni²⁺ and Zn²⁺ ions in methanol–Tris–HCl buffer (1 : 1 v/v, 10 mM, pH 7.2) solution. Here successive amount addition of analyte ions to the ligand L, absorption band gradually enhanced at ∼375 nm for Ni²⁺ whereas 333 nm and 375 nm for Zn²⁺ ion. As a results the absorption band at 280 nm goes on decreasing simultaneously and well-defined isosbestic point developed at 332 nm and 331 nm for Ni²⁺ and Zn²⁺ ions respectively. This change in absorption behaviour of L with addition of Ni²⁺ and Zn²⁺ ions remained consistent up to 0.5 equiv. for both ions and then remained well saturated (Fig. 1).

Fig. 1 UV-vis titration of L with (a) Ni²⁺ and (b) Zn²⁺ in methanol–Tris–HCl buffer. (Inset) Plot of intensity vs. analyte concentration.

From the titration experiments the calibration curves were plotted where the absorbance intensity around 375 nm for both the metal ions varies almost linearly with the concentration. Thus detection limit was calculated to 7.9 × 10⁻⁹ M for Ni²⁺ and 7.9 × 10⁻⁹ M for Zn²⁺ using convenient formula DL = 3σ/m, here ‘σ’ is standard deviation of the blank measurement and ‘m’ is the slope of the linear calibration curve (Fig. 2). This low level DL values are far below the guideline provided by WHO in drinking water and several reported Ni²⁺ and Zn²⁺ sensors. As per WHO, the permissible limits of Ni²⁺ and Zn²⁺ ions in drinking waters are 0.0008 and 5.0 mg L⁻¹ respectively.⁵⁶

Fig. 2 Colorimetric detection limit of (a) Ni²⁺ (b) Zn²⁺.

The precise detection of analyte like Zn²⁺ and Ni²⁺ ion in real water sample can be hampered in presence of other interfering ions due to their cross responses. Thus analyte signalling of receptor L was performed in presence of common competiting ions as a background. The presence of same equivalent of other interfering cations with analyte ion, the absorption spectra of host–guest complex cannot be disturbed (Fig. 3). Even 3 fold excessive addition of competitive metal ions than analyte, the original peak intensity of L + M²⁺ remained almost negligible change. Moreover, presence of equal amount of Zn²⁺ ion to the L–Ni²⁺ solution can replace Ni²⁺, thus corresponding absorbance band intensity to L–Zn²⁺ complex was observed.

Fig. 3 Interference test of L with different metal ions.

The Job's plot indicates that here 2 : 1 binding stoichiometric ratio obtained between receptor L and both the analyte metal ions Ni²⁺ and Zn²⁺ (Fig. S12†), which was further confirmed by ESI-MS analysis (Fig. S7 and S9†) and single crystal structure of the L–Ni²⁺ (3) was isolated complex (vide infra). In the ESI-MS spectra peaks at m/z 772.2 (Fig. S7†) and m/z at m/z 803.2 (Fig. S9†) indicated 2 : 1 binding stoichiometric ratio between the ligand L and the metal ions Ni²⁺ and Zn²⁺ respectively. Assuming 2 : 1 host–guest ratio, the association constants were calculated from Benesi–Hildebrand (B–H) equation by the linear fitting of graph 1/(A − A₀) vs. 1/[M²⁺]^1/2 (Fig. 4). The values obtained as 2.37 × 10³ M^−1/2 and 2.31 × 10³ M^−1/2 for Ni²⁺ and Zn²⁺ ion respectively, indicates very strong affinity between receptor and analytes.

Fig. 4 Association constant for (a) Ni²⁺ (b) Zn²⁺.

As we are interested on analyte sensing in aqueous or semi aqueous medium, thus pH of this medium plays a very crucial role as receptor contain pH sensitive amide (–CONH–) and imine (>C=N) moieties. For that purpose, receptor solutions with variable pH range 2 to 12 were adjusted by dilute HCl and NaOH and then analyte ions were added independently. The sequential UV-vis studies of each set of solution indicates that absorbance intensity at 375 nm for L and its metal complexes are mostly increases with pH. At pH = 2–4, there is less absorbance intensity difference observed between L and L + M²⁺ (M = Zn and Ni), thus not very suitable to sense at this pH range. Thus in a wide range of pH = 5–12 the proposed chemosensor can successfully sense both Ni²⁺ and Zn²⁺ ion including physiological environment (Fig. 5).

Fig. 5 pH effect test.

Generally, metal ions like Ni²⁺ and Zn²⁺ ion form stable chelate complex with multidentate ligand Na₂EDTA. Thus reversibility was checked by absorption studies L + M²⁺ (M = Zn and Ni) solution with Na₂EDTA in working medium. The characteristic absorption bands of host–guest complexes around 375 nm, instantly disappeared when L + M²⁺ (M = Zn and Ni) complex reacted with 1 equiv. Na₂EDTA (Fig. 6). Thus absorbance intensity at 280 nm restored with solution turns yellow to colourless due to generation of L. Further addition of analytes, the absorbance intensity at 375 nm was almost recovered. The alternative addition of chelator and analytes, absorption switching between L and L + M²⁺ were almost consistent up to 4 cycles with negligible loss of absorbance intensity which indicate that sensing process is reversible than any irreversible metal catalysed reaction.

Fig. 6 Absorbance spectra of L in the absence and presence of (a) Ni²⁺ and Na₂EDTA, and (b) Zn²⁺ and Na₂EDTA.

Fluorescence studies

The emission properties of the receptor L in presence of selective metal ions was also examined in methanol–Tris–HCl buffer (10 mM, pH 7.2) mixture (1 : 1 v/v). Among the selected metal ions like Fe³⁺, Cu²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Cr³⁺, Ag⁺, Al³⁺, Mn²⁺ and Fe²⁺ only addition of 2 equiv. Zn²⁺ ion to the ligand solution emission intensity at 463 nm significantly enhanced whereas the same amount of other metal ions including Ni²⁺ induced very significant emission change of receptor L (Fig. 7). To get the details aspect of the emission spectra, titration experiment was conducted. When excited at 290 nm free receptor L showed weak emission at 455 nm. Upon increasing the Zn²⁺ ion concentration in the receptor solution, emission intensity gradually enhanced with slightly red shifted to 463 nm that got saturated up to 5 equiv. of Zn²⁺ ion (Fig. 8a). The metal ion coordination with receptor L via imine side probably restricted both the non-radiative PET and >C=N isomerisation process, thus emission intensity increase due to chelation enhancement fluorescence effect (CHEF). At the same time, there is no change in emission intensity of L in case of Ni²⁺ ion indicate that in L–Ni²⁺ complex both the opposing process i.e. paramagnetic fluorescence quenching and chelation enhancement fluorescence effect mutually compensate to each other. The emission enhancement of L in presence of Zn²⁺ ion, cannot be disturbed in addition of 2-fold higher concentration of other metal ions. The fluorometric detection limit was found to 1.7 × 10⁻⁹ M calculated using the same formula 3σ/m (Fig. 8b).

Fig. 7 Fluorescence selectivity of the receptor L in methanol–Tris–HCl buffer.

Fig. 8 (a) Fluorescence titration of L with Zn²⁺ (b) fluorometric limit of detection for Zn²⁺.

DFT calculation

The geometry optimizations staring from the Gauss-view structure of L lead to a global minimum as a stationary level. The optimized structures of the L and its Ni²⁺ complex (3) and schematic representation of the energy of MOs and contours of selected HOMO and LUMO orbitals of L and 3 are presented in Fig. 9. The HOMO to LUMO energy gap for L is 3.25 eV, whereas for 3, the gap is 2.92 eV.

Fig. 9 (a) Energy minimized structure of the L and NiL₂ (3); (b) a schematic representation of the energy of MOs and contours of selected HOMO and LUMO orbitals of L and NiL₂.

X-ray single crystal structure of NiL₂ (3)

The ligand L reacts with Ni(NO₃)₂ (2 : 1 molar ratio) in methanol at room temperature to form the complex 3. It crystallizes in the monoclinic space group C2/c (Table 1). The asymmetric unit consists of one L ligand and one Ni²⁺ ion (Fig. 10a) with occupancy half. The overall complex contains one Ni²⁺ ion and two L ligands (Fig. 10b). The metal ion shows distorted octahedral coordination with N₄O₂ donor sets from two pyridyl N, two amide O and two hydrazide N (Fig. 10c). The Ni–N and Ni–O bond distances and bond angles (Tables S1 and S2†) were found to be within normal ranges. The charges on Ni(II) ion were internally compensated by two deprotonated hydrazide NH originating from two L. Carefully looking into the structure of 1 revealed that the complex is associated through intermolecular π–π stacking and CH–π interactions (Fig. 10d).

Table 1 Crystal data and structure refinement for 3

Empirical formula	C42H34N8NiO4
Formula weight	773.48
Temperature/K	221.68
Crystal system	Monoclinic
Space group	C2/c
a/Å	24.902(7)
b/Å	11.539(3)
c/Å	16.250(6)
α/°	90
β/°	127.330(17)
γ/°	90
Volume/Å^3	3713(2)
Z	4
ρcalc g cm^−3	1.384
μ/mm^−1	0.578
F(000)	1608.0
Crystal size/mm^3	0.25 × 0.22 × 0.18
Radiation	MoKα (λ = 0.71073)
2θ range for data collection/°	5.022 to 50.208
Index ranges	−29 ≤ h ≤ 29, −13 ≤ k ≤ 13, −19 ≤ l ≤ 19
Reflections collected	17 858
Independent reflections	2761 [Rint = 0.2075, Rsigma = 0.1661]
Data/restraints/parameters	2761/0/250
Goodness-of-fit on F^2	1.073
Final R indexes [I ≧ 2σ (I)]	R1 = 0.0978, wR2 = 0.1692
Final R indexes [all data]	R1 = 0.1542, wR2 = 0.1924
Largest diff. peak/hole/e Å^−3	0.43/−1.11

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Fig. 10 (a) Asymmetric unit of 3, (b) overall structure of the complex 3, (c) coordination environment around the Ni²⁺ ion, (d) view of 3 illustrating the intramolecular π–π stacking and CH–π interactions (dotted line).

Comparison of the performance of L with other reported fluorescent and colorimetric chemosensors

Colorimetric detection of Ni²⁺ and Zn²⁺ ions and ‘turn-on’ fluorometric detection of Zn²⁺ ion using a single probe is not very common in the literature. Compared to the other systems, our system has several attractive analytical features, such as high sensitivity, wide linear range, high selectivity, lower detection limit, simple operation technology, good solubility, sensitive visualization and good practical applicability (Table 2). Moreover, the synthesis of our proposed chemosensor L requires only two steps and less hazardous reagents, and no hazardous by-product is formed.

Table 2 Comparison of the performance of L with other reported chemosensors

Type of sensors	Ion sensing	Limit of detection (M)		Reference
		Colorimetrically	Fluorometrically
Aminopyridine Schiff bases based	Ni^2+, Zn^2+	6.96 × 10^−7 and 4.17 × 10^−7	—	57
	Zn^2+	—	1.13 × 10^−7, 2.95 × 10^−7	57
Aminoantipyrine based	Zn^2+	—	1.4 × 10^−8	58
Octopamine-based	Zn^2+	—	9.9 × 10^−7	59
Benzothiazole-based	Ni^2+, Zn^2+	Not provided	2.50 × 10^−7 and 3.0 × 10^−7	60
Naphthalimide-based	Ni^2+, Zn^2+	—	3.3 × 10^−7 and 1.8 × 10^−7	61
Quinazolin-based	Ni^2+, Zn^2+	7.9 × 10^−9 and 7.5 × 10^−9	—	This work
	Zn^2+	—	1.7 × 10^−9 M	This work

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Application

To explore the Ni²⁺ and Zn²⁺ sensing ability of the present chemosensor L in real water samples, we have tested the probe L in waste water and we got some result. Artificial M²⁺ (M = Ni, Zn) polluted tap water samples have been made by adding known amounts of standard M²⁺ solutions in order to test its sensing in various samples. The prepared samples were analyzed by the same UV-vis spectrometer and the recovery amounts were calculated using the calibration curve (intensity vs. conc.) with the help of Lambert–Beer law. As shown in Table 3, the M²⁺ concentration recovered for each sample are close in agreement with spiked amount with good precision. Thus, the receptor L can be suitable to detect M²⁺ ion quantitatively in real water samples, indicating that it could be applied in real environments (Table 3).

Table 3 Analysis of M²⁺ (nM) in water sample by the developed method (mean ± standard deviation, n = 3)

Sample	M^2+ added	M^2+ found	Recovery (%)	Relative error (%)
Ni^2+	5	5.02 ± 1.5	100.5	0.4
	10	10.15 ± 1.54	101.5	1.5
	20	19.70 ± 0.87	98.5	2.0
Zn^2+	5	5.13 ± 0.97	102.6	2.6
	10	9.87 ± 1.1	98.7	1.3
	20	20.06 ± 1.2	100.3	0.6

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Conclusion

In summary, we have successfully designed and synthesized a quinazolinone derived Schiff base chemosensor L for colorimetric nanomolar detection of Ni²⁺ and colorimetric fluorescent detection of Zn²⁺ with high selectivity and sensitivity over other competitive ions in aqueous medium. A 2 : 1 stoichiometry between L and Ni²⁺ were supported by Job-plot, ESI-mass spectral analysis and DFT studies. The single crystal of the L + Ni²⁺ was also isolated. The colorimetric detection limits reached up to 7.9 nM for Ni²⁺ and 7.5 nM for Zn²⁺ respectively. Fluorescent measurement of Zn²⁺ were obtained with the detection limit of 1.70 nM, which were far lower than those recommended by the WHO guidelines for drinking water. The reversible response to Ni²⁺ and Zn²⁺ ion was established by the Na₂EDTA experiments. Moreover, L could operate in a wide range of pH and can be successfully applied for detection and quantification of Ni²⁺ in environmental samples.

Conflicts of interest

Authors declare no conflicts of interest.

Acknowledgements

G. K. P. would like to thank the Department of Science and Technology (SR/FST/CSI-264/2014 and EMR/2017/001789) New Delhi for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Contains the supplementary crystallographic data for the Ni²⁺ complex 3. Supplementary data containing Fig. S1–S12 and Tables S1 and S2 of this article. CCDC 2205292. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ra05564c

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