Schiff Base-functionalized Metal–organic Frameworks for Selective Sensing of Chromate and Dichromate in Water

In this research, Zn- or Cd-based metal–organic frameworks (coded ZnMOF-1 and CdMOF-1) containing benzene-1,4-dicarboxylic acid (H2bdc) and pyridyl-based Schiff base (4-pyridylcarboxaldehydeisonicotinoylhydrazone (L)) dual ligands were successfully assembled via a conventional solvothermal method. The photoluminescence quenching response of ZnMOF-1 and CdMOF-1 and their sensing sensitivity and selectivity towards various inorganic anions were evaluated in aqueous media. Crystallographic and thermogravimetric studies confirm the formation of both MOFs with good crystallinity and thermal stability. Photoluminescence studies also verify the selectivity of ZnMOF-1 and CdMOF-1 for efficient sensing of inorganic oxyanions (like chromate/dichromate: CrO42− and Cr2O72−). Further, it was noted that only chromate/dichromate (CrO42−/Cr2O72−) anions showed a significant turn-off quenching effect while other anions (like F−, Br−, I−, Cl−, ClO4−, SCN−, SO42−, NO3−, and NO2−) have a low/negligible effect on the photoluminescence intensity of both MOFs. The limit of detection (LOD) of chromate/dichromate by ZnMOF-1 and CdMOF-1 was 9.79/10.94 µM and 2.68/1.48 µM, respectively. A probable mechanism for turn-off quenching response towards chromate and dichromate anions could be attributed to the spectral overlap of both excitation and emission spectra of ZnMOF-1/CdMOF-1 with the absorption spectra of chromate/dichromate anions. As a result, the energy transfer from ZnMOF-1 or CdMOF-1 to the target chromate and dichromate anions decreased fluorescence intensity (i.e., fluorescence quenching effect).


Introduction
With the growing industrialization and modern farming practices, environmental contaminants have become a seriously increasing issue. A variety of pollutants (such as toxic cations (Hg 2+ , As 3+ , and Cd 2+ ), anions (F − , CN − , CrO 4 2− , and Cr 2 O 7 2− ), explosives (2,4,6-trinitrophenol, nitrobenzene, and 2,4-dinitrophenol), and pesticides (methyl parathion, glyphosate, and atrazine)) have been discharged into the environment from different sources such as dye, leather, chemical, plastic, and pharmaceutical industries [1,2]. These pollutants have a serious impact on human health and the ecosystem. Currently, a plethora of methods are used to detect these toxic cations, anions, and explosives, such as high-performance liquid chromatography (HPLC), atomic absorption spectroscopy (AAS), electrochemical method, voltammetry, mass spectrometry, flame atomic absorption spectroscopy, X-ray fluorescence spectrometry, inductively coupled plasma mass spectrometry, etc. [3][4][5]. However, most of these methods are time-consuming (especially during sample preparation), costlier, and exhibit low sensitivity. To overcome these limitations, attention has been devoted to the user-friendly, cost-effective fluorescence-based sensing methods, which provide rapid response, excellent selectivity/sensitivity, portability, and compatibility in liquid and solid media [6].
Chromate and dichromate (CrO 4 2− /Cr 2 O 7 2− ) are hazardous anions in aqueous solutions, causing skin allergy, cancer, and gene mutations in humans [7]. These anions are being commonly utilized and discharged to the environment by agrochemicals, steel, paint, leather, tanning, and various other industries. Thus, precise, selective, and efficient detection of these anions in variety of samples like industrial wastewater, soil, and groundwater is a challenge of prime importance. In recent years, metal-organic frameworks (MOFs) have emerged as excellent fluorescent sensors for detecting trace amount of CrO 4 2− /Cr 2 O 7 2− anions owing to their good emission properties, porosities, and feasibility of viable supramolecular interactions between the host frameworks and target analytes. Typically, MOFs are crystalline materials composed of metal clusters bridged by organic linkers [8]. To date, huge number of luminescent MOF sensors for detection of toxic cations and explosives have been reported [9][10][11][12]. Nevertheless, only a few studies were reported on developing luminescent MOF-based sensors for inorganic anions detection, especially oxoanions like chromate species [13]. Thus, more selective and sensitive sensors for detection of hazardous anions are needed.
In the present work, a pyridyl-based Schiff base ligand (4-pyridylcarboxaldehyde isonicotinoylhydrazone (L)) was developed and used to prepare a dual-ligand Zn(II)/Cd(II) MOFs (coded as ZnMOF-1 and CdMOF-1) by a conventional solvothermal method. The prepared materials were characterized by various techniques to confirm their successful synthesis methods. The effect of pyridyl-based Schiff base ligand and metal centers (i.e., Zn(II) and Cd(II)) on the fluorescence sensing behavior (selectivity and sensitivity) of MOFs was studied towards the detection of chromate and dichromate oxoanions. The photoluminescence selectivity and sensitivity of both MOFs for chromate and dichromate anions and their limit of detection (LOD) were also evaluated in the presence of other inorganic anions. Besides, the luminescence sensing mechanism was explained based on the excitation and emission energy transfer diagram from both MOFs to the target analyte and their luminescence quenching effects.

Materials
All the procured reagents/chemicals were commercial products and purchased from Avra synthesis Pvt. Ltd. ( On the other hand, benzene-1,4-dicarboxylic acid (98%) was procured from Sigma-Aldrich. All materials mentioned above were of analytical reagent grade and used without further purification. Triply distilled water was used for synthetic manipulations and stock solution preparations.

Synthesis of 4-pyridylcarboxaldehyde Isonicotinoylhydrazone (L)
A mixture of isonicotinic acid hydrazide (1.37 gm, 10 mmo l) and pyridine-4-carboxaldehyde (0.94 mL, 10 mmol) was dissolved in 50 mL ethanol, followed by the addition of a few drops of glacial acetic acid in 100 mL round bottom flask [14]. The contents of the flask were refluxed with continuous stirring for 4 h. On completion of the reaction, the reaction mixture was cooled to room temperature, yielding a white precipitate. The obtained precipitate was filtered, washed with methanol, and finally recrystallized using ethanol to afford the pure product (Yield = 85%). Note that the Schiff base condensation reaction pathway was monitored by Thin-Layer Chromatography (TLC) using silica gel G (Sigma Aldrich). The spots on the TLC plates were cautiously visualized by their exposure to the I 2 fumes in the iodine chamber. 1

Measurements
Fourier transform-infrared (FTIR) spectrum of the ligand L and Zn(II)/Cd(II) MOFs were recorded in the range of 450-4000 cm −1 by using a Perkin Elmer FTIR Spectrophotometer (RXIFT). 1 H and 13 C NMR spectra for the developed pyridyl-based Schiff base ligand (L) were performed in DMSO-d 6 solvent on a Bruker Avance NEO 500 MHz NMR spectrometer calibrated with respect to the internal reference tetramethylsilane (TMS). Powder X-ray diffraction (PXRD) diffractograms were obtained using XPERT PRO Powder X-ray Diffractometer [CuK α X-ray (λ = 1.5406 Å), 1800 W (45 kV, 40 mA)] in 2θ range 5° to 45° keeping step size of 0.026° for crystal structure determination. Field-Emission Scanning electron microscope (FE-SEM) micrographs were obtained with the HITACHI, JAPAN instrument MODEL: SU8010 SERIES using gold-coated sample at accelerating voltage of 5.0 kV at a working distance of 7.9 and 7.7 mm for ZnMOF-1 and CdMOF-1, respectively. Thermogravimetric analysis (TGA) was carried out using a STA7300 (Hitachi) instrument under a pure nitrogen atmosphere in the temperature range from 35 to 700 • C at a heating rate of 10 • C/ min. Shimadzu spectrophotometer (UV 1800 model) and Shimadzu RF-5301PC spectrofluorophotometer were used for absorption and photoluminescence studies, respectively.

Photoluminescence Study
To perform the anion sensing experiments, standard aqueous solutions of anions at similar concentrations (10 mM

Recyclability Test
The recyclability of the ZnMOF-1 and CdMOF-1 sensors was studied by conducting a simple fluoroscnce sensing experiment by dispersing each MOF (3 mg) in 3 mL of standard aqueous solutions of CrO 4 2− /Cr 2 O 7 2− of 2.0 mM concentration separately via ultrasonication for 30 min. Subsequently, fluorescence spectra of these aqueous suspensions were recorded in the 300 − 600 nm spectral range under the same excitation wavelength (at 282 nm), while the emission intensity was examined at 427 nm for ZnMOF-1 and 418 nm for CdMOF-1. After every fluroroscence sensing experiment, each MOF was isolated by centrifugation and then washed many times with water, methanol, and acetone before using in next sensing recovery cycle.

H and 13 C NMR Analysis
The 1 H NMR analysis of the synthesized Schiff base ligand L is shown in Fig. 1. As it can be seen, the sharp peaks located at 2.50 ppm and 3.40 ppm are ascribed to the DMSO and H 2 O used during analysis and synthesis, respectively. The sharp peak at 8.46 ppm can be assigned to the -CH = Nproton, while the peak at 12.38 ppm can be attributed to the N-H proton. The four doublets seen in the range of 7.69 to 8.81 ppm can be attributed to the four pairs of equivalents protons of pyridine rings in the Schiff base ligand. From the 13 C NMR analysis in Fig. 2, the septet at 39.74 ppm can be assigned to the carbon of DMSO-d 6 solvent used during analysis. The sharp peak at 161.88 ppm is assigned to the carbonyl carbon of the hydrazone group of Schiff base ligand. The carbon atom of the pyridine ring directly connected to the imine carbon and carbonyl carbon of the hydrazone group also appears at 140.02 and 141.07 ppm, Fig. 4 A, B The PXRD spectra of ZnMOF-1 and CdMOF-1 (a) experimentally synthesized, (b) 7 days in 10 mM chromate solution, (c) 7 days in 10 mM dichromate solution, and (d) after 3 rd sensing cycle; C, D The PXRD spectra of ZnMOF-1 and CdMOF-1 syn-thesized via diverse synthesis techniques reported in the previous literature [14]. Note. Here Fig. 4C and 4D are added for the PXRD spectral comparison in order to provide the proof of formation of high purity MOFs in this research work 1 3 respectively. Further, the imine carbon of the hydrazone group appears as a singlet peak at 146.50 ppm. The two sharp peaks located at 150.21-150.28 can be assigned to the two pairs of equivalent carbons directly bonded to the nitrogen of two pyridine rings of Schiff base ligand. While the two peaks located at 121.01 and 121.46 can be attributed to the rest of two pairs of equivalent carbons of two pyridine rings of Schiff base ligand, proving the successful formation of pyridyl Schiff base ligand L.

FTIR Analysis
The FTIR analysis was conducted to confirm the synthesis of Schiff base ligand L and ZnMOF-1 and CdMOF-1 using H 2 bdc and Schiff base ligand L. The sharp peak located in the imine or Schiff (-CH = N-) group region at 1587 cm −1 corresponding to C = N stretching in the FTIR spectra of L indicates the successful formation of L (Fig. 3). In addition, the absence of a peak at 3300-3500 cm −1 indicates the nonexistence of an amine group, which should be present in the starting material, i.e., isonicotinic acid hydrazide. The peaks appearing 1685 and 3188 cm −1 can be related to the C = O and N-H vibration of the hydrazone moiety of L, respectively. However, a slight decrease in their intensity without any significant shift in position was observed in the ZnMOF-1 and CdMOF-1 (Fig. 3). As such, the N-H and C = O groups of the hydrazone moiety of L remain free within the pore cages of Zn-and CdMOF-1 without participating in the MOF formation. The appearance of a weak absorption peak in the fingerprint area at 482 cm −1 , corresponding to Zn-N or Cd-N vibration, confirmed that ligand L coordinates with Zn or Cd through the nitrogen of the pyridine ring in the Znand CdMOF-1. The FTIR spectra of ZnMOF-1 was found to match with that recorded for a single crystal in the Zn-and CdMOF-1. This observation confirms the successful synthesis of in the Zn-and CdMOF-1 from L and H 2 bdc ligands [10].
As observed in Fig. 3, the bands at 1390/1569 cm −1 and 1383/1568 cm −1 in the FTIR spectrum of both Zn-and CdMOF-1, respectively can be assigned to the symmetric and antisymmetric carbonyl stretching vibrations, respectively. The differences in these stretching vibrations in both Zn-and CdMOF-1 were about 179 and 185 cm −1 , respectively. As such, these differences should back up the existence of bidentate binding mode of chelated carboxylate groups [10]. This indicates that carboxylic groups of the H 2 bdc ligand should be involved in the coordination with Zn 2+ /Cd 2+ ions during Zn-and CdMOF-1 preparation. The broad peaks at 3445 cm −1 (ZnMOF-1) and 3440 cm −1 (CdMOF-1) were allotted to O-H stretching vibrations to reflect the presence of lattice water molecules. The weak absorption peak seen in the fingerprint area at 575 and 590 cm −1 are also related to Zn-O and Cd-O vibration modes in the Zn-and CdMOF-1, respectively. Thus, it can be concluded that FTIR studies were supportive to validate the formation of high quality crystalline Zn-and CdMOF-1 from H 2 bdc and L.

Powder XRD Analysis
To examine MOFs' crystallinity and phase purity, Fig. 4A and B shows the recorded powder XRD (PXRD) diffractograms for the synthesized ZnMOF-1 and CdMOF-1. The observed PXRD diffraction patterns of ZnMOF-1 and CdMOF-1 on 2ϴ scale match the standard pattern recorded for their reported crystal structures in literature and the corresponding patterns of these MOFs are shown in Fig. 4C and D for comparison purpose [14], indicating their successful synthesis. The phase purity of these synthesized MOFs was established by the absence of any impurity peaks in Fig. 4A and B. Based on crystallographic PXRD studies, the structure and coordination environment around Zn(II) or Cd(II) metal nodes of ZnMOF-1 and CdMOF-1 are presented in Figs. 5 and 6, respectively.

FE-SEM Analsysis
The topographical and morphological characteristics of the synthesized ZnMOF-1 and CdMOF-1 were studied by FE-SEM analysis, as shown in Fig. 7. The FE-SEM micrographs of ZnMOF-1 displayed the rod-like morphology (Fig. 7a,  b). On the other hand, CdMOF-1 displayed a mixed morphology of both flower-like (Fig. 7c) and rod-like (Fig. 7d) structures. Figure 8 reflects the thermal stability of ZnMOF-1 and CdMOF-1 as a function of temperature (35 to 700 °C) under nitrogen atmosphere based on TGA analysis. The TGA curves of both ZnMOF-1 and CdMOF-1 showed two steps of weight losses steps. Notably, ZnMOF-1 exhibited higher thermal stability than CdMOF-1, with a total weight loss of 62% at 700 °C (relative to 73% for CdMOF-1). The first weight loss in both MOFs may occur due to the desolvation of the axially or weekly coordinated solvents with metal sites or trapped solvent molecules in the porous structures. The significant weight loss in both MOFs was observed upon increasing temperature range above 300 °C, resulting from the decomposition of the organic linkers. Thus, these results establish that both MOFs are thermally stable up to 300 °C after which thermal degradation of the framework starts.

Photoluminescence Sensing Properties
It is well-known that MOFs constructed from the transition metal ions having d 10 configuration and conjugated organic ligands showed excellent photoluminescent (PL) properties. Figure 9 shows the PL spectra of ZnMOF-1 and CdMOF-1 in water (3 mg/3 mL) upon excitation at 282 nm wavelength and room temperature. As seen, the water suspension of ZnMOF-1 and CdMOF-1 (3 mg/3 mL) showed good emission intensities at 427 and 418 nm, respectively, indicating their suitability for photoluminescence sensing applications for analytes in the aqueous phase (Fig. 9).  Fig. 10. Noticeably, results of fluorescence quenching reveal that only the presence of CrO 4 2− and Cr 2 O 7 2− anions showed a significant turn-off quenching effect on the PL intensities of ZnMOF-1 and CdMOF-1 (Fig. 10a, b), while the presence of other anions had a low/ negligible effect on the PL intensity (Fig. S1 in the Supporting Information, SI). Therefore, the effect of incremental addition of To calculate the limits of detection (LOD) for CrO 4 2− /Cr 2 O 7 2− by ZnMOF-1 and CdMOF-1, the fluorescence quenching titrations were performed with the incremental addition of 100 µM aqueous CrO 4 2− /Cr 2 O 7 2− solution. Detailed procedure for calculating LOD (LOD = 3σ/ m) is presented in Section S1 (SI), where σ = standard  . 9 The photoluminescence spectra of water suspensions of ZnMOF-1 and CdMOF-1 (3 mg/ 3 mL) upon excitation at 282 nm at room temperature deviation from five blank measurements for each MOF and m = slope of the linear curve plotted at the lowest concentration for LOD calculations. The respective LOD values for CrO 4 2− /Cr 2 O 7 2− in aqueous media were 1.90 ppm (9.79 μM)/ 3.2 ppm (10.94 μM) by ZnMOF-1 and 0.52 ppm (2.68 μM)/ 0.43 ppm (1.48 μM) by CdMOF-1, respectively. These observations demonstrate the excellent potential of the as-prepared ZnMOF-1 and CdMOF-1 materials for sensitive sensing of CrO 4 2− /Cr 2 O 7 2− in aqueous media (section S1 and Fig. S4, SI). However, it should be noted that the sensitivity of CdMOF-1 for CrO 4 2− /Cr 2 O 7 2− detection was higher than ZnMOF-1. Compared with some of the recently reported fluorescent sensors (Table S1, SI), the LOD values of CrO 4 2− and Cr 2 O 7 2− ions by the prepared dual ligand Zn/Cd-MOFs were comparable to that of the previously reported MOF sensors (Table S1).

Chromium Oxyanions Detection (CrO
To demonstrate the potential of ZnMOF-1 and CdMOF-1 to be used as turn-off fluorosensors for fast detection of hazardous hexavalent chromate anions in the aqueous phase. To find out the recyclability of ZnMOF-1 and CdMOF-1, each MOF was isolated by centrifugation after every fluorescence sensing experiment and then washed numerous times with water, methanol, and acetone. It was found that the recovered ZnMOF-1 and CdMOF-1 showed no substantial changes of the fluroscence intensity over three sensing-recovery cycles, in either case, indicating the good recyclability of ZnMOF-1 and CdMOF-1 (Fig. 13a, b). The PXRD spectra of both MOFs also reinforced this fact after recycling and soaking in 10 mM CrO 4 2− /Cr 2 O 7 2− solutions up to 7 days (Fig. 4A(a-d) and B(a-d)), confirming the formation of chemical, thermal, and water-stable luminescent ZnMOF-1/CdMOF-1 for photoluminescence sensing of CrO 4 2− /Cr 2 O 7 2− anions in aqueous solutions. Generally speaking, the detection of anions by luminescent MOFs can occur via three mechanisms (1) Collapse of framework structure and (2) Competitive energy absorption between MOFs and anions (3) Fluorescence resonance energy transfer (FRET). On this basis, the possible sensing mechanisms of ZnMOF-1 and CdMOF-1 for CrO 4 2− /Cr 2 O 7 2− detection were explored. The possibility of quenching of luminescence by framework collapse is ruled out by the consistent PXRD spectra of ZnMOF-1 and CdMOF-1 before and after soaking in aqueous solutions of chromate/ dichromate ions for 7 days. The PXRD of soaked MOFs showed no change in the diffraction patterns, indicating their good crystalline stability (i.e., a common way to quench the luminescence) ( Fig. 4A(a,b,c) and B(a,b,c)). Further, the PL intensities of ZnMOF-1 and CdMOF-1 could be changed by the possible competition for the excitation energy between the anions themselves and ZnMOF-1/ CdMOF-1.
As depicted in Fig. S5, two broad absorption bands were observed in the wavelength range of 200 − 450 nm in the UV-vis spectra of aqueous solutions of K 2 CrO 4 (λ max at 270, 382 nm) and K 2 Cr 2 O 7 (λ max at 260, 350 nm). The absorption range of the CrO 4 2− /Cr 2 O 7 2− covers the absorption bands in the range of 250 − 400 nm, including the excitation wavelength (282 nm) of ZnMOF-1 and CdMOF-1 (Fig. S6). Further, upon excitation of ZnMOF-1/CdMOF-1 at 282 nm, CrO 4 2− /Cr 2 O 7 2− in the solution can significantly absorb the energy of the excitation, which in turn discourages the UV-vis absorption of the target ZnMOF-1/CdMOF-1, resulting in a quenching of their PL intensities. Only chromate and dichromate anions absorption spectra overlap with the emission spectra of both ZnMOF-1 and CdMOF-1, causing fluorescence resonance energy transfer from ZnMOF-1 and CdMOF-1 to chromate and dichromate anions (Figs. 14a and S7). This energy transfer led to the quenching of PL intensities of both ZnMOF-1 and CdMOF-1. However, none of the anions have UV-vis absorption peaks in the range of absorption bands of ZnMOF-1 and CdMOF-1 as well as the excitation wavelength, and hence no turn-off quenching effect was observed. These observations demonstrate the selectivity of ZnMOF-1/CdMOF-1 toward chromate and dichromate anions detection (Fig. S7). Further, amide functionality present on Schiff base linkers L can support supramolecular interaction like hydrogen bonding with chromate oxyanions preferring the electron/energy transfer processes (Fig. 14b). Therefore, in the present case, the plausible quenching mechanism can be declared as the electron/ energy transfer due to suitable spectral overlap with analytes. Reports on similar mechanisms for detection of CrO 4 2− /Cr 2 O 7 2− are available in the literature [7,15].

Conclusion
In this work, chemical, thermal, and water-stable Zn(II)/ Cd(II)-based MOFs were successfully synthesized by conventional reflux technique and characterized by various analytical methods. Notably, aqueous dispersions of both Zn(II)/ Cd(II)-based MOFs showed a highly sensitive and selective fluorescence turn-off response only towards CrO 4 2− / Cr 2 O 7 2− ions. This selective turn-off effect is attributed to the competitive absorption of excitation wavelength energy and FRET between CrO 4 2− / Cr 2 O 7 2− ions and Cd-/Zn-MOF-1. Competitive experiments also demonstrate that fluorescence quenching remains largely unaffected in the presence of other competing anions, with LOD values for CrO 4 2− /Cr 2 O 7 2− at 9.79 µM/ 10.94 µM and 2.68 µM/ 1.48 µM by the ZnMOF-1 and CdMOF-1, respectively. The utility of both ZnMOF-1 and CdMOF-1 as sensing materials showed good recyclability up to three recycles without tedious work-up. Our present study opens avenues for the design and synthesis of robust MOFs with chemical stability by judicious selection of ligand moiety for the desired functional properties, including selective detection of hazardous anions in the real-field analysis.