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

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.


Introduction
Zn 2+ and Fe 3+ are essential metal ions, which are involved in numerous biological processes in human body. 1- 6 The abnormal levels of Zn 2+ and Fe 3+ can cause numerous adverse health effects. Hence, efficient methods for the detection of Zn 2+ and Fe 3+ are highly desirable. On the other hand, Cr 2 O 7 2À 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][8][9] Therefore, methods for the efficient detection of Cr 2 O 7 2À are urgent to be explored. Currently, several traditional methods have been developed for the determination of Zn 2+ , Fe 3+ and Cr 2 O 7 2À , such as atomic absorption spectrophotometry, [10][11][12] electrochemical methods, [13][14][15] and inductively coupled plasma mass spectrometry. [16][17][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 innite 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][21][22][23][24][25][26][27][28] Particularly, CPs as uorescent sensors have drew more concern owing to their superiority in long-term stability, efficiency, and operability. Simultaneously, much effort has been made to synthesize uorescent 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 specic target analytes are detected in organic solvents, such as ethanol, 35,36 DMF, 37-39 CH 3 CN, 40 DMA, 41 which are not benecial for the practical applications of uorescent CPs. Therefore, detecting analytes in water is still an important challenge in biological and environmental sciences.
Organic ligands are crucial in the syntheses of uorescent CPs. Aromatic or conjugated p moieties within organic ligands can endow the CPs with excellent optical properties, which can improve the efficient recognition for the target analytes. [42][43][44] Moreover, the Lewis basic sites in uorescent 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 (H 2 btic) 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)]$ 0.5H 2 O} n (CP-1). CP-1 magnied 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 Zn 2+ , Fe 3+ and Cr 2 O 7 2À ions with uorescence enhancement or quenching. In addition, the mechanism of luminescence enhancement or quenching has also been discussed.

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 À1 . The luminescence properties were studied using a Hitachi F-7000 uorescence spectrophotometer. XPS data were obtained on a Thermo ESCALAB 250 X-ray photoelectron spectrometer.

Synthesis of the ligand H 2 btic
Ligand H 2 btic was synthesized according to the reported procedure. 46 The detailed synthetic method is described in the ESI (Section 1 †). Fluorescence sensing experiments 1 mg of CP-1 was placed separately into 2 mL aqueous solutions containing numerous analytes (10 À3 M). Aer an ultrasonic treatment for 15 min, the photoluminescence responses were recorded with excitation at 278 nm. The uorescence 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 rened using the SHELX-2014 soware. 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.

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 btic 2À and one phen, and half lattice H 2 O. Each Cd(II) ion  (9) , which coincided with those of the reported Cd(II) CPs. [48][49][50] Different coordination modes were shown by the two carboxylate groups of btic 2À , namely m 2 -h 1 :h 2 and m 1 -h 1 :h 1 modes. Then, Cd(II) ions were connected by carboxylate groups from btic 2À ligands to produce a Cd(btic) chain (Fig. 1b). The Cd(btic) chain was made up of dinuclear units Cd 2 (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.
155.64 Further, the p-p 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 btic 2À 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.

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 Fig. 1 (a) Coordination environment around the Cd(II) center in CP-1. Hydrogen atoms and solvent molecules are omitted for clarity. Symmetry codes: The chain structure of CP-1. (c) The 3D supramolecular structure of CP-1.
common organic solvents. To check the chemical stability of CP-1, each nely ground powder of CP-1 was immersed in methanol, ethanol, DMF, H 2 O 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 aer 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 H 2 O 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 H 2 btic, phen, and CP-1 were measured (Fig. S4 †). When excited at 278 nm, H 2 btic 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 redshi 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][52][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 uorescent sensor. Thus, we explored the application of CP-1 in detecting metal ions.   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 Zn 2+ and Fe 3+ ions. Moreover, to check the sensing sensitivity of CP-1 towards Zn 2+ , the following experiment was carried out. The sample of CP-1 was dispersed in water (0.5 mg mL À1 ), and executed with an ultrasonic treatment to obtain a suspension. Then, different volumes of Zn 2+ ions (0.1 M) were added to the above suspension, and the emission spectra were determined (Fig. 3a). With incremental addition of Zn 2+ , the emission intensities gradually increased. The association constant was calculated to be 1.812 Â 10 4 mol À1 based on the tted linear equation I/I 0 ¼ 1.289 + 1.812 Â 10 4 [M], where I 0 and I are uorescence intensities before and aer analyte incorporation, respectively, and [M] represents the concentration of the analyte (Fig. 3b). The limit of detection (LOD) for Zn 2+ was 4.172 Â 10 À4 M. To the best of our knowledge, the reported sensors for Zn 2+ are mostly based on organic molecules or composite materials, 54-56 and CP-based uorescent sensors for detecting Zn 2+ ions are very rare.
The sensing sensitivities of CP-1 for Fe 3+ ions were also checked with the same method as in case of Zn 2+ . The increasing concentration of Fe 3+ resulted in a gradual decrease in the uorescent intensity of CP-1 (Fig. 4a). The quenching efficiency was estimated through the equation: I 0 /I ¼ 1.159 + 4.959 Â 10 4 [M] (Fig. 4b). The quenching coefficient was calculated as 4.959 Â 10 4 mol À1 for Fe 3+ . The LOD was 1.524 Â 10 À4 M for Fe 3+ .

The mechanism of luminescence sensing
To elucidate the reasons for the enhancement caused by Zn 2+ , and the quenching caused by Fe 3+ and Cr 2 O 7

2À
, 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 conrmed by the PXRD patterns (Fig. S5 †), the whole skeleton of CP-1 remained unchanged aer immersed in the Zn 2+ aqueous solution. In the structure of CP-1, there were uncoordinated Lewis basic N and S atoms from ligand H 2 btic. Therefore, the hypothesis was that the uncoordinated N and S atoms possibly coordinated to Zn 2+ ions, which increased the delocalization of CP-1 and improved the energy-transfer efficiency from ligand to metal ions, and nally enhanced the whole uorescence intensity. To further prove the hypothesis, the XPS of N 1s and S 2p were carried out on CP-1 and Zn 2+ @CP-1 (Fig. S6 †). The N 1s peak at 398.65 eV in CP-1 was shied to 398.75 eV, while the S 2p peak at 163.45 eV in CP-1 shied to 163.3 eV aer Zn 2+ incorporation, indicating the formation of weak bonds between S/N and Zn 2+ . The formation of weak bonds caused the efficient energy transfer between ligand and metal ions, which further led to the uorescence enhancement.
Aer immersed in Fe 3+ and Cr 2 O 7 2À aqueous solution, the PXRD patterns showed that the skeleton of CP-1 also did not change, indicating that the uorescence 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 Fe 3+ and Cr 2 O 7 2À , 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 Fe 3+ and Cr 2 O 7 2À , while other metal ions or anions displayed no spectral overlap. Therefore, the energy transfer mechanism could account for the luminescence quenching effects induced by Fe 3+ and Cr 2 O 7 2À 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 uorescence, and the uncoordinated Lewis basic N and S atoms in the structure of CP-1 made it a rare multi-responsive uorescence sensor to detect Zn 2+ , Fe 3+ and Cr 2 O 7 2À with uorescence 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 conicts to declare.