A luminescent Cd(II) coordination polymer as a multi-responsive fluorescent sensor for Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ in water with fluorescence enhancement or quenching

Abstract

A luminescent Cd(II) coordination polymer, namely {[Cd(btic)(phen)]·0.5H₂O}_n (CP-1) (H₂btic = 5-(2-benzothiazolyl)isophthalic acid, phen = 1,10-phenanthroline), was constructed through the mixed-ligand method under solvothermal conditions. CP-1 manifests a chain structure decorated with uncoordinated Lewis basic N and S donors. CP-1 exhibits high sensing towards Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ ions with fluorescence enhancement or quenching. CP-1 exhibited a fluorescence enhancement for Zn²⁺ ions through weak binding to S and N atoms, and a fluorescence quenching for Fe³⁺ and Cr₂O₇²⁻ ions by an energy transfer process. The binding constants were calculated as 1.812 × 10⁴ mol⁻¹ for Zn²⁺, 4.959 × 10⁴ mol⁻¹ for Fe³⁺ and 1.793 × 10⁴ mol⁻¹ for Cr₂O₇²⁻. This study shows CP-1 as a rare multi-responsive sensor material for the efficient detection of Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ ions.

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Introduction

Zn²⁺ and Fe³⁺ are essential metal ions, which are involved in numerous biological processes in human body.^1–6 The abnormal levels of Zn²⁺ and Fe³⁺ can cause numerous adverse health effects. Hence, efficient methods for the detection of Zn²⁺ and Fe³⁺ are highly desirable. On the other hand, Cr₂O₇²⁻ is a type of environmentally non-biodegradable pollutant, which can cause accumulation in living organisms and lead to the visceral damage and water-borne diseases because of its potent mutagenesis and carcinogenesis.^7–9 Therefore, methods for the efficient detection of Cr₂O₇²⁻ are urgent to be explored. Currently, several traditional methods have been developed for the determination of Zn²⁺, Fe³⁺ and Cr₂O₇²⁻, such as atomic absorption spectrophotometry,^10–12 electrochemical methods,^13–15 and inductively coupled plasma mass spectrometry.^16–18 However, these methods are usually expensive, time-consuming, or require complicated sample preparation processes. Therefore, simple and efficient detection methods for such analytes are urgently required.

Coordination polymers (CPs) are coordination compounds with infinite structures (1, 2 or 3 dimensions), which are constructed from metal ions and ligands via coordination bonds.¹⁹ CPs as a type of promising functional materials have received considerable attention because of their intriguing structural features as well as applications in luminescence sensing, magnetic property, gas storage, drug delivery, and so on.^20–28 Particularly, CPs as fluorescent sensors have drew more concern owing to their superiority in long-term stability, efficiency, and operability. Simultaneously, much effort has been made to synthesize fluorescent CP materials for detecting different types of target analytes, such as metal ions,^29,30 anions,^31,32 and small molecules.^33,34 However, many of these specific target analytes are detected in organic solvents, such as ethanol,^35,36 DMF,^37–39 CH₃CN,⁴⁰ DMA,⁴¹ which are not beneficial for the practical applications of fluorescent CPs. Therefore, detecting analytes in water is still an important challenge in biological and environmental sciences.

Organic ligands are crucial in the syntheses of fluorescent CPs. Aromatic or conjugated π moieties within organic ligands can endow the CPs with excellent optical properties, which can improve the efficient recognition for the target analytes.^42–44 Moreover, the Lewis basic sites in fluorescent CPs can interact with certain metal cations, which can enhance the selective sensing capacity towards metal cations.⁴⁵ Based on the above considerations, we adopted a multidentate ligand 5-(2-benzothiazolyl)isophthalic acid (H₂btic) as a main ligand, which contains two aromatic rings. Such ligand was accompanied by an auxiliary ligand with excellent optical properties 1,10-phenanthroline (phen) to fabricate a luminescent CP, namely {[Cd(btic)(phen)]·0.5H₂O}_n (CP-1). CP-1 magnified a chain structure decorated with uncoordinated Lewis basic N and S donors. The luminescence sensing behavior of CP-1 was studied in water solution. The as-synthesized CP-1 showed a striking sensing capability towards Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ ions with fluorescence enhancement or quenching. In addition, the mechanism of luminescence enhancement or quenching has also been discussed.

Experimental section

Materials and instruments

All commercially available chemicals were used as received. Elemental analyses were conducted using a Flash EA 1112 elemental analyzer. IR data were obtained using a BRUKER TENSOR 27 spectrophotometer. PXRD patterns were obtained on a Bruker AXS D8Advance. Thermogravimetric analyses were measured on a NetzschSTA449C thermal analyzer under air atmosphere at a ramp rate of 10 °C min⁻¹. The luminescence properties were studied using a Hitachi F-7000 fluorescence spectrophotometer. XPS data were obtained on a Thermo ESCALAB 250 X-ray photoelectron spectrometer.

Synthesis of the ligand H₂btic

Ligand H₂btic was synthesized according to the reported procedure.⁴⁶ The detailed synthetic method is described in the ESI (Section 1†).

Synthesis of {[Cd(btic)(phen)]·0.5H₂O}_n (CP-1)

A mixture of Cd(NO₃)₂·4H₂O (0.1 mmol), H₂btic (0.1 mmol), and phen (0.05 mmol) in 5 mL of the component solvent (DEF : H₂O = 4 : 1) (DEF = N,N′-diethylformamide) was put into a glass vessel, which was heated at 80 °C until the colorless crystals appeared. Yield: 68% (based on Cd). Anal. calcd for C₅₄H₃₂N₆O₉S₂Cd₂ (%): C, 54.15; H, 2.69; N, 7.02; S, 5.35. Found: C, 53.68; H, 2.51; N, 7.10; S, 5.19. IR (cm⁻¹, KBr): 3422(s), 3131(w), 3060(w), 2970(w), 2926(w), 1610(s), 1551(s), 1513(m), 1433(s), 1364(s), 1310(w), 1249(w), 1139(w), 1104(m), 1031(w), 934(w), 887(w), 777(s), 722(s), 686(w), 638(m).

Fluorescence sensing experiments

1 mg of CP-1 was placed separately into 2 mL aqueous solutions containing numerous analytes (10⁻³ M). After an ultrasonic treatment for 15 min, the photoluminescence responses were recorded with excitation at 278 nm. The fluorescence titrations were conducted through adding analytes into 2 mL suspension of CP-1.

X-ray crystallography

X-ray diffraction data were collected on a Bruker SMART APEX-II CCD diffractometer. The crystal structure was solved and refined using the SHELX-2014 software.⁴⁷ Hydrogen atoms were added geometrically. Crystallographic parameters for CP-1 are shown in Table 1. Table 2 shows the main bond lengths and bond angles.

Table 1 Crystal data and structure refinement details for CP-1

a R1 = ∑‖Fo| − |Fc‖/∑|Fo|.b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Compound	1
Formula	C54H32N6O9S2Cd2
fw	1195.76
Crystal system	Monoclinic
Space group	P2/c
a/Å	10.1136(10)
b/Å	9.7173(10)
c/Å	24.005(3)
α/deg	90
β/deg	92.928(4)
γ/deg	90
V/Å^3	2356.1(4)
Z	2
2θmax (deg)	55.082
Dc/g cm^−3	1.686
Reflns collected/unique	48 969/5434
R(int)	0.0699
Abs coeff/mm^−1	1.059
F(000)	1192
GOF	1.054
R1 [I > 2σ(I)]^a	0.0415
wR2 (all data)^b	0.0791
Largest diff. Peak and hole	0.64 and −0.70 e Å^−3

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Table 2 Selected bond lengths (Å) and bond angles (deg) for CP-1^a

a Symmetry transformations used to generate equivalent atoms in CP (1): #1 −1 + x, y, z; #2 1 − x, y, 3/2 − z.
CP-1
Cd(1)–O(3)#1	2.236(2)	Cd(1)–N(2)	2.314(3)
Cd(1)–O(1)#2	2.353(2)	Cd(1)–N(1)	2.371(3)
Cd(1)–O(2)#2	2.402(3)	Cd(1)–O(1)	2.522(2)
Cd(1)–O(4)#1	2.528(3)	O(3)#1–Cd(1)–N(2)	135.13(9)
O(3)#1–Cd(1)–O(1)#2	101.27(10)	N(2)–Cd(1)–O(1)#2	86.18(9)
O(3)#1–Cd(1)–N(1)	101.03(11)	N(2)–Cd(1)–N(1)	71.08(10)
O(1)#2–Cd(1)–N(1)	155.64(9)	O(3)#1–Cd(1)–O(2)	82.98(9)
N(2)–Cd(1)–O(2)	137.08(9)	O(1)#2–Cd(1)–O(2)	107.77(10)
N(1)–Cd(1)–O(2)	84.58(11)	O(3)#1–Cd(1)–O(1)	129.17(8)
N(2)–Cd(1)–O(1)	95.62(8)	O(1)#2–Cd(1)–O(1)	75.24(9)
N(1)–Cd(1)–O(1)	97.87(9)	O(2)–Cd(1)–O(1)	52.41(8)
O(3)#1–Cd(1)–O(4)#1	54.00(8)	N(2)–Cd(1)–O(4)#1	81.42(8)
O(1)#2–Cd(1)–O(4)#1	95.36(9)	N(1)–Cd(1)–O(4)#1	89.88(10)
O(2)–Cd(1)–O(4)#1	134.66(8)	O(1)–Cd(1)–O(4)#1	170.36(9)

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Results and discussion

Crystal structure of CP-1

CP-1 crystallizes in the monoclinic space group P2/c. As depicted in Fig. 1a, the asymmetric unit contained one Cd(II) ion, one btic²⁻ and one phen, and half lattice H₂O. Each Cd(II) ion adopted a pentagonal-bipyramidal environment finished by five O atoms of three btic²⁻ and two N atoms of one phen. The Cd–O/N bond lengths varied from 2.236(2)–2.528(3) Å, and O/N–Cd–O/N bond angles were 52.41(8)–170.36(9)°, which coincided with those of the reported Cd(II) CPs.^48–50 Different coordination modes were shown by the two carboxylate groups of btic²⁻, namely μ₂-η¹:η² and μ₁-η¹:η¹ modes. Then, Cd(II) ions were connected by carboxylate groups from btic²⁻ ligands to produce a Cd(btic) chain (Fig. 1b). The Cd(btic) chain was made up of dinuclear units Cd₂(COO)₂. The phen ligands took bidentate chelating fashion coordinating to Cd(II) ions to satisfy the coordination demands of Cd(II) ions in the assembly process. As shown in Fig. 1c, these chains in an offset way were stacked into a 3D supramolecule via weak van der Waals interactions. Further, the π–π interactions in each chain between pyridine rings (N2–C16–C17–C18–C19–C20) of phen ligands stabilized the structure. In CP-1, the N and S donors in ligand btic²⁻ were not involved in the coordination to Cd(II) ions. Therefore, the uncoordinated N and S could function as Lewis bases to recognize numerous analytes.

Fig. 1 (a) Coordination environment around the Cd(II) center in CP-1. Hydrogen atoms and solvent molecules are omitted for clarity. Symmetry codes: A = 1 − x, y, 1.5 − z; B = −1 + x, y, z; C = 2 − x, y, 1.5 − z. (b) The chain structure of CP-1. (c) The 3D supramolecular structure of CP-1.

PXRD and thermogravimetric analysis

To check the phase purity of CP-1, its PXRD was performed (Fig. S1†). The peak positions of the as-synthesized sample were consistent with those of the simulated ones. The crystals of CP-1 were stable in air. Moreover, it did not dissolve in water or common organic solvents. To check the chemical stability of CP-1, each finely ground powder of CP-1 was immersed in methanol, ethanol, DMF, H₂O and THF solvents for 24 h. Then, the PXRD of each sample was analyzed (Fig. S2†). The unchanged PXRD patterns revealed that the crystallinity of CP-1 retained after the solvent treatment, indicating the high chemical stability of CP-1. Thermogravimetric analysis was carried out to understand the thermal stability of CP-1 (Fig. S3†). The TGA curve of CP-1 exhibited an initial weight loss of 2.21% from 108 to 193 °C because of the release of H₂O molecules (calcd: 1.51%). The further weight loss from 349–640 °C was ascribed to the disintegration of the structure, leaving CdO as the residue (found, 22.13%; calcd, 21.73%).

Photoluminescence properties

At ambient temperature, the solid-state luminescences of H₂btic, phen, and CP-1 were measured (Fig. S4†). When excited at 278 nm, H₂btic had an emission maximum at 433 nm as well as phen showed a main peak at 382 nm with two shoulder peaks at 365 nm and 403 nm. The excitation of CP-1 at 278 nm led to a dominant peak at 388 nm with two shoulder peaks at 375 nm and 409 nm, which probably originated from intraligand transitions because similar emission was observed for ligand phen. CP-1 showed a small redshift compared to that of phen, which was probably because of the coordination effect of ligands to Cd(II) ions. Further, a stronger emission band in CP-1 was probably assigned to the enhanced rigidity of the ligand that diminished a radiationless decay through the coordination to metal centers.^51–53

Sensing of metal ions

The Lewis basic N and S active sites, good chemical stability and strong luminescence for CP-1 made it a potential candidate as a fluorescent sensor. Thus, we explored the application of CP-1 in detecting metal ions. 1 mg powder of CP-1 was placed separately in 2 mL 10⁻³ M M^x+ aqueous solutions (M^x+ = Na⁺, K⁺, Mg²⁺, Ca²⁺, Cr³⁺, Ag⁺, Zn²⁺, Hg²⁺, Pb²⁺, Ni²⁺, Co²⁺, Cd²⁺, Ba²⁺, Al³⁺, Cu²⁺, Mn²⁺, and Fe³⁺). Then, luminescence responses toward different metal ions were examined. As shown in Fig. 2, these metal ions exhibited different impacts on the fluorescence intensities of CP-1. Notably, Zn²⁺ ions enhanced the emission intensity by 2.17-fold, Fe³⁺ ions almost completely quenched the luminescence, while other metal ions had little or moderate quenching effects on the emission, indicating that CP-1 could act as a sensor material with a great response for Zn²⁺ and Fe³⁺ ions.

Fig. 2 Luminescence intensity histograms of CP-1 (1 mg) dispersed in aqueous solutions of numerous metal ions (2 mL, 10⁻³ M) excited at 278 nm (1) blank, (2) Zn²⁺, (3) Ca²⁺, (4) Ba²⁺, (5) Mg²⁺, (6) K⁺, (7) Na⁺, (8) Cd²⁺, (9) Mn²⁺, (10) Al³⁺, (11) Ag⁺, (12) Cr³⁺, (13) Ni²⁺, (14) Co²⁺, (15) Pb²⁺, (16) Hg²⁺, (17) Cu²⁺, (18) Fe³⁺.

Moreover, to check the sensing sensitivity of CP-1 towards Zn²⁺, the following experiment was carried out. The sample of CP-1 was dispersed in water (0.5 mg mL⁻¹), and executed with an ultrasonic treatment to obtain a suspension. Then, different volumes of Zn²⁺ ions (0.1 M) were added to the above suspension, and the emission spectra were determined (Fig. 3a). With incremental addition of Zn²⁺, the emission intensities gradually increased. The association constant was calculated to be 1.812 × 10⁴ mol⁻¹ based on the fitted linear equation I/I₀ = 1.289 + 1.812 × 10⁴ [M], where I₀ and I are fluorescence intensities before and after analyte incorporation, respectively, and [M] represents the concentration of the analyte (Fig. 3b). The limit of detection (LOD) for Zn²⁺ was 4.172 × 10⁻⁴ M. To the best of our knowledge, the reported sensors for Zn²⁺ are mostly based on organic molecules or composite materials,^54–56 and CP-based fluorescent sensors for detecting Zn²⁺ ions are very rare.

Fig. 3 (a) Fluorescence spectra of CP-1 dispersed in an aqueous suspension upon the incremental addition of Zn²⁺ ions. (b) The linear correlation for the plot of I/I₀ vs. the concentration of Zn²⁺.

The sensing sensitivities of CP-1 for Fe³⁺ ions were also checked with the same method as in case of Zn²⁺. The increasing concentration of Fe³⁺ resulted in a gradual decrease in the fluorescent intensity of CP-1 (Fig. 4a). The quenching efficiency was estimated through the equation: I₀/I = 1.159 + 4.959 × 10⁴ [M] (Fig. 4b). The quenching coefficient was calculated as 4.959 × 10⁴ mol⁻¹ for Fe³⁺. The LOD was 1.524 × 10⁻⁴ M for Fe³⁺.

Fig. 4 (a) Fluorescence spectra of CP-1 dispersed in an aqueous suspension upon the incremental addition of Fe³⁺ ions. (b) The linear correlation for the plot of I₀/I vs. the concentration of Fe³⁺.

Sensing of anions

Simultaneously, to check the effects of anions on the luminescence intensity of CP-1, numerous anions (F⁻, Cl⁻, Br⁻, I⁻, NO₃⁻, CH₃COO⁻, SCN⁻, ClO₄⁻, H₂PO₄⁻, SO₃²⁻, SO₄²⁻, and Cr₂O₇²⁻) were selected. 1 mg of CP-1 was dispersed in aqueous solutions containing above anions (2 mL, 10⁻³ M). Obviously, Cr₂O₇²⁻ ions quenched the emission of CP-1, while other anions almost led to a little change in the fluorescence intensity of CP-1, indicating the potential detection of CP-1 towards Cr₂O₇²⁻ (Fig. 5a). Then, the sensing selectivity towards Cr₂O₇²⁻ was explored through competition experiments. Upon adding Cr₂O₇²⁻ into the mixture of CP-1 and each of other anions, the emission for each case was quenched greatly, showing the high selectivity of CP-1 towards Cr₂O₇²⁻ even in the presence of other interfering anions (Fig. 5b).

Fig. 5 Luminescence intensity histograms of CP-1 (1 mg) dispersed into various anion aqueous solutions (2 mL, 10⁻³ M) excited at 278 nm (a) and subsequent addition of Cr₂O₇²⁻ (b). (1) Blank, (2) F⁻, (3) Cl⁻, (4) Br⁻, (5) I⁻, (6) CH₃COO⁻, (7) SO₄²⁻, (8) H₂PO₄⁻, (9) ClO₄⁻, (10) SO₃²⁻, (11) SCN⁻, (12) Cr₂O₇²⁻ (b).

To further investigate the sensing capability of CP-1 towards Cr₂O₇²⁻, fluorescence titrations were conducted through the addition of different volumes of the Cr₂O₇²⁻ (0.1 M) aqueous solution to the suspension of CP-1. The emission intensity of CP-1 was gradually quenched with incrementally adding Cr₂O₇²⁻ (Fig. 6a). The quenching equation could be written as I₀/I = 0.879 + 1.775 × 10⁴ [M] (Fig. 6b). The quenching coefficient was calculated as 1.793 × 10⁴ M⁻¹. The LOD was calculated to be 4.21 × 10⁻⁴ M.

Fig. 6 (a) Fluorescence spectra of CP-1 dispersed in an aqueous suspension upon the incremental addition of Cr₂O₇²⁻ ions. (b) The linear correlation for the plot of I₀/I vs. the concentration of Cr₂O₇²⁻.

The mechanism of luminescence sensing

To elucidate the reasons for the enhancement caused by Zn²⁺, and the quenching caused by Fe³⁺ and Cr₂O₇²⁻, the sensing mechanisms were also investigated. First, the samples were immersed in the aqueous solutions with numerous analytes for 16 h. Then, the PXRD patterns were recorded to examine the stability of the structure. As confirmed by the PXRD patterns (Fig. S5†), the whole skeleton of CP-1 remained unchanged after immersed in the Zn²⁺ aqueous solution. In the structure of CP-1, there were uncoordinated Lewis basic N and S atoms from ligand H₂btic. Therefore, the hypothesis was that the uncoordinated N and S atoms possibly coordinated to Zn²⁺ ions, which increased the delocalization of CP-1 and improved the energy-transfer efficiency from ligand to metal ions, and finally enhanced the whole fluorescence intensity. To further prove the hypothesis, the XPS of N 1s and S 2p were carried out on CP-1 and Zn²⁺@CP-1 (Fig. S6†). The N 1s peak at 398.65 eV in CP-1 was shifted to 398.75 eV, while the S 2p peak at 163.45 eV in CP-1 shifted to 163.3 eV after Zn²⁺ incorporation, indicating the formation of weak bonds between S/N and Zn²⁺. The formation of weak bonds caused the efficient energy transfer between ligand and metal ions, which further led to the fluorescence enhancement.

After immersed in Fe³⁺ and Cr₂O₇²⁻ aqueous solution, the PXRD patterns showed that the skeleton of CP-1 also did not change, indicating that the fluorescence quenching was unrelated to the structure of CP-1 (Fig. S7 and S8†). Then, the energy transfer mechanism was checked through the measurement of UV-vis spectra of Fe³⁺ and Cr₂O₇²⁻, and the emission spectrum of CP-1, which depended upon the degree of overlap of the two kinds of spectra. As shown in Fig. S9 and S10,† the spectrum of CP-1 partly overlapped with absorption spectra of Fe³⁺ and Cr₂O₇²⁻, while other metal ions or anions displayed no spectral overlap. Therefore, the energy transfer mechanism could account for the luminescence quenching effects induced by Fe³⁺ and Cr₂O₇²⁻ ions.

Conclusions

In summary, we synthesized a Cd(II) coordination polymer with a chain structure (CP-1) based on the mixed-ligand method under solvothermal conditions. The excellent characteristics of high chemical stability, intense fluorescence, and the uncoordinated Lewis basic N and S atoms in the structure of CP-1 made it a rare multi-responsive fluorescence sensor to detect Zn²⁺, Fe³⁺ and Cr₂O₇²⁻ with fluorescence enhancement or quenching. This study also demonstrates that the introduction of functional groups or atoms in the ligands can endow the CPs with expected properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the Natural Science Foundation of Hebei Province (No. B2018201226), the Science and Technology Project of Hebei Education Department (No. ZD2018027), the Youth Top Talent Project of Hebei Province and Key Laboratory of Public Health Safety of Hebei Province.

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic parameters for CP-1, powder XRD, TGA, photoluminescences and related spectra. CCDC reference number 2045504 for CP-1. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ra10203b

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