Schiff base-functionalized Metal-Organic Frameworks for Selective Sensing of Chromate and Dichromate in Water

Schiff base-functionalized Metal-Organic Frameworks for Selective Sensing of Chromate and Dichromate in Water | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Schiff base-functionalized Metal-Organic Frameworks for Selective Sensing of Chromate and Dichromate in Water Manpreet Kaur, Mohamad Yusuf, Ashok Kumar Malik This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-2042384/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 7 You are reading this latest preprint version Abstract In this research, Zn- or Cd-based metal-organic frameworks (coded ZnMOF-1 and CdMOF-1) containing benzene-1,4-dicarboxylic acid (H 2 bdc) 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: CrO 4 2− and Cr 2 O 7 2− ). Further, it was noted that only chromate/dichromate (CrO 4 2− /Cr 2 O 7 2− ) anions showed a significant turn-off quenching effect while other anions (like F − , Br − , I − , Cl − , ClO 4 − , SCN − , SO 4 2− , NO 3 − , and NO 2 − ) 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 chromate/dichromateanions. 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). Metal-organic Frameworks (MOFs) Pyridyl-based Schiff base ligand Fluorescent sensor Chromate anions turn-off quenching effect Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Highlights Two metal-organic frameworks (ZnMOF-1 and CdMOF-1) associated with pyridyl Schiff-base ligand were successfully synthesized. Both MOFs displayed a good photoluminescent recognition ability for oxyanions (like chromate) in the aqueous phase. Energy transfer, FRET, and H-bonding interactions explained the fluorescence quenching effect of both MOFs. The prepared Zn or Cd-MOFs exhibited excellent sensing recyclability for at least three cycles. 1. 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 – 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 – 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 their limit of detection (LOD) were also evaluated in the presence of other inorganic anions and cations. Besides, the luminescence sensing mechanism was explained based on the excitation and emission energy transfer diagram from both MOFs to the target analyteand their luminescence quenching effects. 2. Experimental Methods 2.1. Materials All the procured reagents/chemicals were commercial products and purchased from Avra synthesis Pvt. Ltd. (Bangalore, India), TCI Chemicals (Tamil Nadu, India), LobaChemie (Maharashtra, India), and Sigma-Aldrich. The purity of the purchased materials from Avra synthesis Pvt. Ltd are as follows: 4-pyridine carboxaldehyde (98%), zinc nitrate hexahydrate (Zn(NO 3 ) 2 .6H 2 O, 98%), cadmium nitrate tetrahydrate (Cd(NO 3 ) 2 .4H 2 O, 98%), potassium hydroxide (KOH, 85%), potassium nitrite (KNO 2 , 97%), potassium bromide (KBr, 98%), potassium iodide (KI, 98%), potassium chromate (KCrO 4 , 98%), and potassium nitrate (KNO 3 , 99%). Further, those purchased from TCI Chemicals (Tamil Nadu, India) was: potassium chloride (KCl, > 99.5%). The purity of materials purchased from LobaChemie (Maharashtra, India) were: isonicotinic acid hydrazide (99%), potassium fluoride (KF, 99%), potassium thiocyanate (KSCN, 98%), potassium sulphate (K 2 SO 4 , 98.5%), potassium dichromate (K 2 Cr 2 O 7 , 99.5%). potassium perchlorate (KClO 4 , 99.99%). 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. 2.2. Synthesis procedures 2.2.1. Synthesis of 4-pyridylcarboxaldehyde isonicotinoylhydrazone (L) A mixture of isonicotinic acid hydrazide (1.37 gm, 10 mmol) 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. FTIR cm -1 : 3188 (N-H stretching), 3005 (aromatic C-H stretching), 1685 (C = O stretching), 1567 (C = N stretching), 1415 (N-H bending), 1284, 1148 (C-O stretching) (Figure S1); 1 H NMR (500 MHz, DMSO‐ d 6 ): δ ppm: 7.69 (2 H, d, J o = 5.05 Hz, H-13, 17), 7.85 (2H, d, J o = 5.15 Hz, H-3, 5), 8.46 (1H, s, -CH = N-), 8.67 (2H, d, J o = 4.6 Hz, H-14, 16), 8.81 (2H, d, J o = 5.0 Hz, H-2, 6), 12.38 (1H, s, -N-H) (Fig. 1); 13 C NMR (500 MHz, DMSO‐ d 6 ): δ ppm: 121.01(2 C, s, C-13, 17), 121.46 (2 C, s, C-3, 5), 140.02 (1 C, s, C-12), 141.07 (1 C, s, C-4), 146.50 (1 C, s, C-11), 150.21 (2 C, s, C-14, 16), 150.28 (2 C, s, C-2, 6), 161.88 (1 C, s, C-7) (Fig. 2). 2.2.2. Synthesis of ZnMOF-1 and CdMOF-1 The crystalline ZnMOF-1 and CdMOF-1 materials were synthesized by the reaction of the Zn(II)/Cd(II) salts d with benzene-1,4-dicarboxylic acid (H 2 bdc) ligand and pyridyl Schiff base ligand (4-pyridylcarboxaldehydeisonicotinoylhydrazone (L)), as shown in Scheme 1 [ 14 ]. Typically, a stock ligand solution containing L (226 mg, 1.0 mmol), H 2 bdc (166 mg, 1.0 mmol) and KOH (112 mg, 2.0 mmol) in 20 ml water-methanol (1:1 v/v) was prepared and added to the 10 ml aqueous M(NO 3 ) 2 .xH 2 O [M = Zn and x = 6 for ZnMOF-1, M = Cd and x = 4 for CdMOF-1] (1.0 mmol) in 100 mL of round bottom flask. The resulting mixture was refluxed with continuous stirring at 110 o C for 6 h. The obtained yellow-colored crystalline material was separated by simple filtration, washed several times with methanol, then acetone, and finally dried in a preheated oven at 80°C. 2.3. Measurements Fourier transform-infrared (FTIR) spectrum of the ligand L and Zn(II)/Cd(II) MOFs were recorded in the range of 400–4000 cm − 1 by using a Perkin Elmer FTIR Spectrophotometer (RXIFT). 1 H and 13 C NMR spectra for the developed pyridyl 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 = 1.5406Å), 1800W (45 kV, 40 mA)] in 2θ range 5⁰ to 45⁰ keeping step size of 0.026⁰ for crystal structure determination. Field-Emission Scanning electron microscope (FE-SEM) micrographs were obtained with the HITACHI, JAPAN instrumentMODEL: 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. 2.4. Photoluminescence Study To perform the anion sensing experiments, standard aqueous solutions of anions at similar concentrations (10 mM) were prepared using potassium salts of each anion, F − , Cl − , Br − , I − , SCN − , NO 2 − , NO 3 − , SO 4 2− , ClO 4 − , CrO 4 2− , and Cr 2 O 7 2− . For selectivity study, 3 mg of ZnMOF-1/CdMOF-1 was dispersed into 3 mL of above prepared aqueous solutions of anions via ultrasonication for 30 min. Subsequently, fluorescence spectra of 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. The emission quenching efficiency of ZnMOF-1/CdMOF-1 was calculated by [(I o -I)/I o ] ×100%, where I o represents the emission intensity of ZnMOF-1/CdMOF-1 without analyte and I is the emission intensity after analyte addition. 3. Results And Discussion 3.1. Characterization data 3.1.1. 1 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 = N- proton, 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 140.07 ppm, 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 121.06 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. 3.1.2. FTIR analysis Figure 3 shows the FTIR spectra of ZnMOF-1 and CdMOF-1 formed by the coordination of H 2 bdc and Schiff base ligand L with metal centers (Zn or Cd). The FTIR spectra of both MOFs showed symmetric and antisymmetric v C = O bands at 1390 − 1383 cm − 1 and 1569–1568 cm − 1 , respectively (Fig. 3 ). The difference in antisymmetric and symmetric carbonyl stretching frequencies for both ZnMOF-1 and CdMOF-1 was about 179 and 185, respectively, indicating the chelating bidentate coordination mode of carboxylate moieties in these LMOFs. The broad absorption bands centering near 3445 cm − 1 (ZnMOF-1) and 3440 cm − 1 (CdMOF-1) are assigned to v O-H vibrations indicating the presence of the lattice water molecules. 3.1.3. Powder XRD analysis To examine MOFs' crystallinity and phase purity, Fig. 4 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[ 14 ], indicating their successful synthesis. The phase purity of these synthesized MOFs was established by the absence of any impurity peaks in Fig. 4 . 3.1.4. 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. 5 . The FE-SEM micrographs of ZnMOF-1 displayed the rod-like morphology (Fig. 5 a,b). On the other hand, CdMOF-1 displayed a mixed morphology of both flower-like (Fig. 5 c) and rod-like (Fig. 5 d) structures. 3.1.5. Thermogravimetric analysis (TGA) Figure 6 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 ramp 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. 3.2. 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 7 shows the PL spectra of ZnMOF-1 and CdMOF-1 in water (1 mg/ mL) upon excitation at 282 nm wavelength and room temperature. As seen, the water suspension of ZnMOF-1 and CdMOF-1 (1 mg/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. 7 ). 3.3. Chromium oxyanions detection ( CrO 4 2− /Cr 2 O 7 2− ) To explore the aqueous phase sensing capabilities of both ZnMOF-1 and CdMOF-1, the PL emission profiles of individually suspended ZnMOF-1/CdMOF-1 in standard anionic solutions of F − , Br − , I − , Cl − , ClO 4 − , SCN − , SO 4 2− , NO 3 − , NO 2 − , CrO 4 2− , and Cr 2 O 7 2− are evaluated under an excitation wavelength of 282 nm in Fig. 8 . 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. 8 a,b), while the presence of other anions had a low/ negligible effect on the PL intensity (Figure S2 in the Supporting Information, SI). Therefore, the effect of incremental addition of CrO 4 2− /Cr 2 O 7 2− concentrations (in the range 0 − 2.0 mM) on the PL emission profiles of ZnMOF-1/CdMOF-1 water suspensions was evaluated in Fig. 8 c − f. As it can be seen that the luminescence of the aqueous dispersions of ZnMOF-1 and CdMOF-1 falls sharply in the presence of incremental concentrations of CrO 4 2− /Cr 2 O 7 2− ions in the range 0 − 2.0 mM. The quenching effects on ZnMOF-1/CdMOF-1 PL intensity by incremental addition of CrO 4 2− /Cr 2 O 7 2− concentrations (0 − 2.0 mM) and the concentration versus quenching percentage are delivered in Figures S3 and S4. The quenching response of chromate and dichromate anions can be simplified and calculated based on the Stern − Volmer equation, I 0 /I = 1 + Ksv[A], where I 0 represents the PL intensity of ZnMOF-1/CdMOF-1 without CrO 4 2− /Cr 2 O 7 2− solutions, I is the PL intensity with added CrO 4 2− /Cr 2 O 7 2− solutions of molar concentration [A], and Ksv is the Stern − Volmer constant/quenching constant. As shown in Fig. 9 , the Stern − Volmer plots for CrO 4 2− /Cr 2 O 7 2− exhibits a good linear correlation at lower concentrations (0 − 1.0 mM) and showed linear correlation coefficients (R) 0.994/0.996 for ZnMOF-1 and 0.995/0.998 for CdMOF-1, respectively. However, the Stern − Volmer plots became nonlinear at higher concentrations of CrO 4 2− /Cr 2 O 7 2− > 1.0 mM (Figure S3). The quenching constants (Ksv) of ZnMOF-1 and CdMOF-1 were 1.19 × 10 3 M − 1 and1.38 × 10 3 M − 1 for CrO 4 2− and 1.48 × 10 3 M − 1 and 3.63 × 10 3 M − 1 for Cr 2 O 7 2− , respectively. These results reveal the significant quenching effect of CrO 4 2− /Cr 2 O 7 2− on the PL intensity of ZnMOF-1/ CdMOF-1. 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 10 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 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 Figure S5, 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). To explore the impact of interfering anions on the detection of CrO 4 2− /Cr 2 O 7 2− anions, the PL emission spectra of ZnMOF-1 and CdMOF-1 were measured in mixed anions solutions containing either CrO 4 2− or Cr 2 O 7 2− plus other (F − , Br − , I − , Cl − , ClO 4 − , SCN − , SO 4 2− , NO 3 − , NO 2 − , CrO 4 2− , and Cr 2 O 7 2− ) anions. As shown in Fig. 10 , the interfering anions have no apparent influence on the fluorescence detection of CrO 4 2− /Cr 2 O 7 2− anions by ZnMOF-1/CdMOF-1. In other words, the quenching efficiencies of ZnMOF-1/CdMOF-1 emissions by CrO 4 2− /Cr 2 O 7 2− anions are almost the same in the presence or absence of interfering anions. These obtained data (Ksv, LOD, and interference studies) 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. 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-1for 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. 4 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 Figure S6, 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 (Figure S7). 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 (Fig. 11 a and S8). 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 (Figure S8). 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. 11 b). 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 ]. 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. The recovered ZnMOF-1 and CdMOF-1 showed no substantial changes of the PL intensity over three sensing-recovery cycles, in either case, indicating the good recyclability of ZnMOF-1 and CdMOF-1 (Fig. 12 a, 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. 4 b-d), confirming the formation of chemical, thermal, and water-stable luminescence ZnMOF-1/CdMOF-1 for photoluminescence sensing of CrO 4 2− /Cr 2 O 7 2− anions in aqueous solutions. 4. 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. Declarations Acknowledgements The authors are thankful to the Chemistry Department, Punjabi University, Patiala, for providing lab and instrument facilities. One of the author Manpreetkaur is highly obliged to the UGC, New Delhi, India, for UGC-SRF fellowship. Author Contributions Manpreet Kaur has performed the experiment and written the paper. AKM and MY has written and edited the paper. All authors reviewed the manuscript. Competing Interests “The authors have no relevant financial or non-financial interests to disclose.” Funding No Funding was available supporting this work. Data Availability All relevant data will be provided on request. Ethics approval There is no ethical approval required. Consent to participate All authors give their consent to participate in the publication of this paper. Consent to publish All authors give consent to publish the paper. Supporting Information FT-IR, PL spectra, SV plots, quenching percentage graph, UV−vis spectra, LOD calculations, comparison table for chromate and dichromate anions. References K. Vikrant, D.C. Tsang, N. Raza, B.S. Giri, D. Kukkar, K.-H. Kim, Potential utility of metal–organic framework-based platform for sensing pesticides, ACS applied materials & interfaces 10(10) (2018) 8797-8817. Y. Zhang, S. Yuan, G. Day, X. Wang, X. Yang, H.-C. Zhou, Luminescent sensors based on metal-organic frameworks, Coordination Chemistry Reviews 354 (2018) 28-45. Y.D. Farahani, V. Safarifard, A luminescent metal-organic framework with pre-designed functionalized ligands as an efficient fluorescence sensing for Fe3+ ions, Journal of Solid State Chemistry 270 (2019) 428-435. C. Liu, B. Yan, A novel photofunctional hybrid material of pyrene functionalized metal-organic framework with conformation change for fluorescence sensing of Cu2+, Sensors and Actuators B: Chemical 235 (2016) 541-546. W. Zhang, Y. Tang, A. Shi, L. Bao, Y. Shen, R. Shen, Y. Ye, Recent developments in spectroscopic techniques for the detection of explosives, Materials 11(8) (2018) 1364. Y.D. Farahani, V. Safarifard, Highly selective detection of Fe3+, Cd2+ and CH2Cl2 based on a fluorescent Zn-MOF with azine-decorated pores, Journal of Solid State Chemistry 275 (2019) 131-140. B. Parmar, Y. Rachuri, K.K. Bisht, E. Suresh, Mixed-ligand LMOF fluorosensors for detection of Cr (VI) oxyanions and Fe3+/Pd2+ cations in aqueous media, Inorganic chemistry 56(18) (2017) 10939-10949. M. Kaur, S. Kumar, S.A. Younis, M. Yusuf, J. Lee, S. Weon, K.-H. Kim, A.K. Malik, Post-Synthesis Modification of Metal-Organic Frameworks Using Schiff Base Complexes for Various Catalytic Applications, Chemical Engineering Journal (2021) 130230. M. Pamei, A. Puzari, Luminescent transition metal–organic frameworks: An emerging sensor for detecting biologically essential metal ions, Nano-Structures & Nano-Objects 19 (2019) 100364. S.A.A. Razavi, A. Morsali, Metal ion detection using luminescent-MOFs: Principles, strategies and roadmap, Coordination Chemistry Reviews 415 (2020) 213299. B. Parmar, K.K. Bisht, Y. Rachuri, E. Suresh, Zn (II)/Cd (II) based mixed ligand coordination polymers as fluorosensors for aqueous phase detection of hazardous pollutants, Inorganic Chemistry Frontiers 7(5) (2020) 1082-1107. M. Kaur, S. Kumar, M. Yusuf, J. Lee, R.J. Brown, K.-H. Kim, A.K. Malik, Post-synthetic modification of luminescent metal-organic frameworks using schiff base complexes for biological and chemical sensing, Coordination Chemistry Reviews 449 (2021) 214214. L. Fan, D. Zhao, B. Li, F. Wang, Y. Deng, Y. Peng, X. Wang, X. Zhang, Luminescent Binuclear Zinc (II) Organic Framework as Bifunctional Water-stable Chemosensor for Efficient Detection of Antibiotics and Cr (VI) Anions in Water, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2021) 120232. B. Parmar, Y. Rachuri, K.K. Bisht, E. Suresh, Syntheses and structural analyses of new 3D isostructural Zn (II) and Cd (II) luminescent MOFs and their application towards detection of nitroaromatics in aqueous media, ChemistrySelect 1(19) (2016) 6308-6315. B. Parmar, Y. Rachuri, K.K. Bisht, R. Laiya, E. Suresh, Mechanochemical and conventional synthesis of Zn (II)/Cd (II) luminescent coordination polymers: dual sensing probe for selective detection of chromate anions and TNP in aqueous phase, Inorganic chemistry 56(5) (2017) 2627-2638. Scheme 1 Scheme 1 is available in Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Scheme1.png Scheme 1. Synthetic details of the ZnMOF-1 and CdMOF-1 obtained with the use of a zinc/cadmium salt in combination with L and H2bdc linkers. SupplementaryInformation.docx Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major revision 20 Sep, 2022 Reviews received at journal 18 Sep, 2022 Reviewers agreed at journal 12 Sep, 2022 Reviewers invited by journal 12 Sep, 2022 Editor assigned by journal 12 Sep, 2022 Submission checks completed at journal 12 Sep, 2022 First submitted to journal 07 Sep, 2022 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-2042384","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":135855399,"identity":"1b34e5a3-8606-46ab-9e44-87981c0806a5","order_by":0,"name":"Manpreet Kaur","email":"","orcid":"","institution":"Punjabi University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Manpreet","middleName":"","lastName":"Kaur","suffix":""},{"id":135855401,"identity":"133d91ee-bc8b-406f-9c81-84932da4bba8","order_by":1,"name":"Mohamad Yusuf","email":"","orcid":"","institution":"Punjabi University","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Mohamad","middleName":"","lastName":"Yusuf","suffix":""},{"id":135855403,"identity":"09ed6e21-dafd-4774-89e7-21921a4cdb76","order_by":2,"name":"Ashok Kumar Malik","email":"data:image/png;base64,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","orcid":"","institution":"Punjabi University","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Ashok","middleName":"Kumar","lastName":"Malik","suffix":""}],"badges":[],"createdAt":"2022-09-07 16:29:27","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-2042384/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-2042384/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":26418577,"identity":"859d0953-f4c4-482e-9035-c3e9317b96b1","added_by":"auto","created_at":"2022-09-13 20:39:58","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":592777,"visible":true,"origin":"","legend":"\u003cp\u003e\u003csup\u003e1\u003c/sup\u003eH-NMR spectrum of Schiff base ligand L.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/50ec092e332e429cf58553e5.png"},{"id":26418578,"identity":"2393cd1e-c3a0-4f72-9500-7af7f21d04df","added_by":"auto","created_at":"2022-09-13 20:39:58","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":460938,"visible":true,"origin":"","legend":"\u003cp\u003e13C-NMR spectrum of Schiff base ligand L.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/1ef50f341d91615279048b2a.png"},{"id":26419105,"identity":"eed88e32-bcaf-4dce-9ed5-309473362f84","added_by":"auto","created_at":"2022-09-13 20:44:58","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":28190,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectrum of ZnMOF-1 and CdMOF-1.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/168e8b5a4e7112da9b4b9774.png"},{"id":26419797,"identity":"64c67031-a5e0-4526-a984-f702ccedeee2","added_by":"auto","created_at":"2022-09-13 20:49:58","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":137566,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eThe \u003c/strong\u003ePXRD 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 3rdsensing cycle.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/5177a3d18dea7dd91041afef.png"},{"id":26419113,"identity":"31aac91c-216b-4c41-bf2f-4e033178ac56","added_by":"auto","created_at":"2022-09-13 20:44:58","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1469245,"visible":true,"origin":"","legend":"\u003cp\u003eFE-SEM images of ZnMOF-1 (a, b) and CdMOF-1 (c, d) at different magnifications (a) ×15,000; (b) ×20,000; (c) ×18,000; (a) ×70,000.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/83d32108b896808f2301acf4.png"},{"id":26420136,"identity":"90c99e5b-ec82-43a9-960e-8d6a99849e98","added_by":"auto","created_at":"2022-09-13 20:54:58","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":16090,"visible":true,"origin":"","legend":"\u003cp\u003eTGA plot of ZnMOF-1 and CdMOF-1\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/3f2f259002e184cea621c732.png"},{"id":26418580,"identity":"27191ff6-5e04-49bf-82ca-25322a9020b9","added_by":"auto","created_at":"2022-09-13 20:39:58","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":17034,"visible":true,"origin":"","legend":"\u003cp\u003eThe photoluminescence spectra of water suspensions of ZnMOF-1 and CdMOF-1 (3 mg/ 3mL) upon excitation at 282 nm at room temperature.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/07dbae7714ba82e7e05babcb.png"},{"id":26420421,"identity":"8feb1040-f4c3-42db-92bd-c562c460a566","added_by":"auto","created_at":"2022-09-13 20:59:58","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1232280,"visible":true,"origin":"","legend":"\u003cp\u003e(a, b) The fluorescence quenching by suspension of ZnMOF-1 and CdMOF-1 (3.0 mg/3 mL) in different aqueous anion solutions.(c−f) Luminescence responses of ZnMOF-1 and CdMOF-1 (3 mg dispersed in 3 mL of water) toward different concentrations of CrO42−/Cr2O72−(0−2.0 mM) in water.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/2413fce101fddaa78653ddda.png"},{"id":26419798,"identity":"60dac91a-59bb-4d0f-9e37-39bf01eba21c","added_by":"auto","created_at":"2022-09-13 20:49:58","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":248247,"visible":true,"origin":"","legend":"\u003cp\u003eStern−Volmer (SV) plots for CrO42−/Cr2O72−in the presence of water suspensions of ZnMOF-1 and CdMOF-1 (3.0 mg/3 mL).\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/aa30eb792fdd4749139cf3ce.png"},{"id":26418589,"identity":"6715c303-fc95-45d6-a0f0-c23aa3f17d1c","added_by":"auto","created_at":"2022-09-13 20:39:58","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":296591,"visible":true,"origin":"","legend":"\u003cp\u003eInterference study of ZnMOF-1 and CdMOF-1 for CrO42−/Cr2O72−in the presence of different anions.\u003c/p\u003e","description":"","filename":"floatimage10.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/e605fbd9d59ffa9ab562ac4d.png"},{"id":26419117,"identity":"1693c03c-f7ec-43d7-8152-186b8b017214","added_by":"auto","created_at":"2022-09-13 20:44:58","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":39338,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Spectral overlap between normalized absorption spectra of Cr(IV) anions solution and normalized emission spectra of LMOFs ZnMOF-1 and CdMOF-1 in water. (b) Possible chelating sites for Cr(IV) anions interaction in Schiff base L ligand unit in the ZnMOF-1 and CdMOF-1.\u003c/p\u003e","description":"","filename":"floatimage11.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/c32d3e29be87488b46924caa.png"},{"id":26419116,"identity":"1fc59479-887b-4a34-8ee3-9a4ecb99de6a","added_by":"auto","created_at":"2022-09-13 20:44:58","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":482399,"visible":true,"origin":"","legend":"\u003cp\u003e(a, b)\u003cstrong\u003e.\u003c/strong\u003e Bar diagram showing the recyclability of ZnMOF-1 and CdMOF-1 for fluorescence quenching experiment with chromium oxyanions up to 3 cycles.\u003c/p\u003e","description":"","filename":"floatimage12.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/4e6cb37c2f39131af9af6deb.png"},{"id":26420850,"identity":"82c55aa7-0391-4ece-829d-4cc7a9ab941f","added_by":"auto","created_at":"2022-09-13 21:05:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3534874,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/fd208a5a-7b2d-4bea-9561-4b1f19578afa.pdf"},{"id":26418576,"identity":"f55255cd-89ed-4488-a809-50d7af0e6848","added_by":"auto","created_at":"2022-09-13 20:39:57","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11100,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1. \u003c/strong\u003eSynthetic details of the ZnMOF-1 and CdMOF-1 obtained with the use of a zinc/cadmium salt in combination with L and H2bdc linkers.\u003c/p\u003e","description":"","filename":"Scheme1.png","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/11b57d85df9920c0a82d9cab.png"},{"id":26418588,"identity":"a6090f4e-efae-431e-82c3-fc677034b9e9","added_by":"auto","created_at":"2022-09-13 20:39:58","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":637282,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-2042384/v1/a837672e55ab180f5a4b2303.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Schiff base-functionalized Metal-Organic Frameworks for Selective Sensing of Chromate and Dichromate in Water","fulltext":[{"header":"Highlights","content":"\u003cul\u003e\n \u003cli\u003eTwo metal-organic frameworks (ZnMOF-1 and CdMOF-1) associated with pyridyl Schiff-base ligand were successfully synthesized.\u003c/li\u003e\n \u003cli\u003eBoth MOFs displayed a good photoluminescent recognition ability for oxyanions (like chromate) in the aqueous phase.\u003c/li\u003e\n \u003cli\u003eEnergy transfer, FRET, and H-bonding interactions explained the fluorescence quenching effect of both MOFs.\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eThe prepared Zn or Cd-MOFs exhibited excellent sensing recyclability for at least three cycles.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eWith the growing industrialization and modern farming practices, environmental contaminants have become a seriously increasing issue. A variety of pollutants (such as toxic cations (Hg\u003csup\u003e2+\u003c/sup\u003e, As\u003csup\u003e3+\u003c/sup\u003e, and Cd\u003csup\u003e2+\u003c/sup\u003e), anions (F\u003csup\u003e\u0026minus;\u003c/sup\u003e, CN\u003csup\u003e\u0026minus;\u003c/sup\u003e, CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e), 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 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. 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, \u003cem\u003eetc.\u003c/em\u003e[\u003cspan additionalcitationids=\"CR4\" citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. 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 [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eChromate and dichromate (CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) are hazardous anions in aqueous solutions, causing skin allergy, cancer, and gene mutations in humans [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. 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\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e 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 [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. To date, huge number of luminescent MOF sensors for detection of toxic cations and explosives have been reported [\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Nevertheless, only a few studies were reported on developing luminescent MOF-based sensors for inorganic anions detection, especially oxoanions like chromate species [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Thus, more selective and sensitive sensors for detection of hazardous anions are needed.\u003c/p\u003e \u003cp\u003eIn 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 their limit of detection (LOD) were also evaluated in the presence of other inorganic anions and cations. Besides, the luminescence sensing mechanism was explained based on the excitation and emission energy transfer diagram from both MOFs to the target analyteand their luminescence quenching effects.\u003c/p\u003e"},{"header":"2. Experimental Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003eAll the procured reagents/chemicals were commercial products and purchased from Avra synthesis Pvt. Ltd. (Bangalore, India), TCI Chemicals (Tamil Nadu, India), LobaChemie (Maharashtra, India), and Sigma-Aldrich. The purity of the purchased materials from Avra synthesis Pvt. Ltd are as follows: 4-pyridine carboxaldehyde (98%), zinc nitrate hexahydrate (Zn(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.6H\u003csub\u003e2\u003c/sub\u003eO, 98%), cadmium nitrate tetrahydrate (Cd(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.4H\u003csub\u003e2\u003c/sub\u003eO, 98%), potassium hydroxide (KOH, 85%), potassium nitrite (KNO\u003csub\u003e2\u003c/sub\u003e, 97%), potassium bromide (KBr, 98%), potassium iodide (KI, 98%), potassium chromate (KCrO\u003csub\u003e4\u003c/sub\u003e, 98%), and potassium nitrate (KNO\u003csub\u003e3\u003c/sub\u003e, 99%). Further, those purchased from TCI Chemicals (Tamil Nadu, India) was: potassium chloride (KCl, \u0026gt; 99.5%). The purity of materials purchased from LobaChemie (Maharashtra, India) were: isonicotinic acid hydrazide (99%), potassium fluoride (KF, 99%), potassium thiocyanate (KSCN, 98%), potassium sulphate (K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e, 98.5%), potassium dichromate (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, 99.5%). potassium perchlorate (KClO\u003csub\u003e4\u003c/sub\u003e, 99.99%). 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.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Synthesis procedures\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1. Synthesis of 4-pyridylcarboxaldehyde isonicotinoylhydrazone (L)\u003c/h2\u003e \u003cp\u003eA mixture of isonicotinic acid hydrazide (1.37 gm, 10 mmol) 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 [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. 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\u0026thinsp;=\u0026thinsp;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\u003csub\u003e2\u003c/sub\u003e fumes in the iodine chamber. FTIR cm\u003csup\u003e-1\u003c/sup\u003e: 3188 (N-H stretching), 3005 (aromatic C-H stretching), 1685 (C\u0026thinsp;=\u0026thinsp;O stretching), 1567 (C\u0026thinsp;=\u0026thinsp;N stretching), 1415 (N-H bending), 1284, 1148 (C-O stretching) (Figure S1); \u003csup\u003e1\u003c/sup\u003eH NMR (500 MHz, DMSO‐\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sub\u003e): δ ppm: 7.69 (2 H, d, J\u003csub\u003eo\u003c/sub\u003e = 5.05 Hz, H-13, 17), 7.85 (2H, d, J\u003csub\u003eo\u003c/sub\u003e = 5.15 Hz, H-3, 5), 8.46 (1H, s, -CH\u0026thinsp;=\u0026thinsp;N-), 8.67 (2H, d, J\u003csub\u003eo\u003c/sub\u003e = 4.6 Hz, H-14, 16), 8.81 (2H, d, J\u003csub\u003eo\u003c/sub\u003e = 5.0 Hz, H-2, 6), 12.38 (1H, s, -N-H) (Fig.\u0026nbsp;1); \u003csup\u003e13\u003c/sup\u003eC NMR (500 MHz, DMSO‐\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sub\u003e): δ ppm: 121.01(2 C, s, C-13, 17), 121.46 (2 C, s, C-3, 5), 140.02 (1 C, s, C-12), 141.07 (1 C, s, C-4), 146.50 (1 C, s, C-11), 150.21 (2 C, s, C-14, 16), 150.28 (2 C, s, C-2, 6), 161.88 (1 C, s, C-7) (Fig.\u0026nbsp;2).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2. Synthesis of ZnMOF-1 and CdMOF-1\u003c/h2\u003e \u003cp\u003eThe crystalline ZnMOF-1 and CdMOF-1 materials were synthesized by the reaction of the Zn(II)/Cd(II) salts d with benzene-1,4-dicarboxylic acid (H\u003csub\u003e2\u003c/sub\u003ebdc) ligand and pyridyl Schiff base ligand (4-pyridylcarboxaldehydeisonicotinoylhydrazone (L)), as shown in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Typically, a stock ligand solution containing L (226 mg, 1.0 mmol), H\u003csub\u003e2\u003c/sub\u003ebdc (166 mg, 1.0 mmol) and KOH (112 mg, 2.0 mmol) in 20 ml water-methanol (1:1 v/v) was prepared and added to the 10 ml aqueous M(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e.xH\u003csub\u003e2\u003c/sub\u003eO [M\u0026thinsp;=\u0026thinsp;Zn and x\u0026thinsp;=\u0026thinsp;6 for ZnMOF-1, M\u0026thinsp;=\u0026thinsp;Cd and x\u0026thinsp;=\u0026thinsp;4 for CdMOF-1] (1.0 mmol) in 100 mL of round bottom flask. The resulting mixture was refluxed with continuous stirring at 110 \u003csup\u003eo\u003c/sup\u003eC for 6 h. The obtained yellow-colored crystalline material was separated by simple filtration, washed several times with methanol, then acetone, and finally dried in a preheated oven at 80\u0026deg;C.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Measurements\u003c/h2\u003e \u003cp\u003eFourier transform-infrared (FTIR) spectrum of the ligand L and Zn(II)/Cd(II) MOFs were recorded in the range of 400\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e by using a Perkin Elmer FTIR Spectrophotometer (RXIFT). \u003csup\u003e1\u003c/sup\u003eH and \u003csup\u003e13\u003c/sup\u003eC NMR spectra for the developed pyridyl Schiff base ligand (L) were performed in DMSO-\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sub\u003e 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\u003csub\u003eα\u003c/sub\u003e X-ray (\u003cb\u003eλ\u003c/b\u003e\u0026thinsp;=\u0026thinsp;1.5406\u0026Aring;), 1800W (45 kV, 40 mA)] in 2θ range 5⁰ to 45⁰ keeping step size of 0.026⁰ for crystal structure determination. Field-Emission Scanning electron microscope (FE-SEM) micrographs were obtained with the HITACHI, JAPAN instrumentMODEL: 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\u003csup\u003e◦\u003c/sup\u003eC at a heating rate of 10\u003csup\u003e◦\u003c/sup\u003eC/min. Shimadzu spectrophotometer (UV 1800 model) and Shimadzu RF-5301PC spectrofluorophotometer were used for absorption and photoluminescence studies, respectively.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e2.4. Photoluminescence Study\u003c/h2\u003e \u003cp\u003eTo perform the anion sensing experiments, standard aqueous solutions of anions at similar concentrations (10 mM) were prepared using potassium salts of each anion, F\u003csup\u003e\u0026minus;\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, I\u003csup\u003e\u0026minus;\u003c/sup\u003e, SCN\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e. For selectivity study, 3 mg of ZnMOF-1/CdMOF-1 was dispersed into 3 mL of above prepared aqueous solutions of anions \u003cem\u003evia\u003c/em\u003eultrasonication for 30 min. Subsequently, fluorescence spectra of aqueous suspensions were recorded in the 300\u0026thinsp;\u0026minus;\u0026thinsp;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. The emission quenching efficiency of ZnMOF-1/CdMOF-1 was calculated by [(I\u003csub\u003eo\u003c/sub\u003e-I)/I\u003csub\u003eo\u003c/sub\u003e] \u0026times;100%, where I\u003csub\u003eo\u003c/sub\u003e represents the emission intensity of ZnMOF-1/CdMOF-1 without analyte and I is the emission intensity after analyte addition.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results And Discussion","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Characterization data\u003c/h2\u003e \u003cdiv id=\"Sec11\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1. \u003csup\u003e1\u003c/sup\u003eH and \u003csup\u003e13\u003c/sup\u003eC NMR analysis\u003c/h2\u003e \u003cp\u003eThe \u003csup\u003e1\u003c/sup\u003eH NMR analysis of the synthesized Schiff base ligand L is shown in Fig.\u0026nbsp;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\u003csub\u003e2\u003c/sub\u003eO used during analysis and synthesis, respectively. The sharp peak at 8.46 ppm can be assigned to the \u0026ndash;CH\u0026thinsp;=\u0026thinsp;N- proton, 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 \u003csup\u003e13\u003c/sup\u003eC NMR analysis in Fig.\u0026nbsp;2, the septet at 39.74 ppm can be assigned to the carbon of DMSO-\u003cem\u003ed\u003c/em\u003e\u003csub\u003e\u003cem\u003e6\u003c/em\u003e\u003c/sub\u003e 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 140.07 ppm, 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 121.06 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.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2. FTIR analysis\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the FTIR spectra of ZnMOF-1 and CdMOF-1 formed by the coordination of H\u003csub\u003e2\u003c/sub\u003ebdc and Schiff base ligand L with metal centers (Zn or Cd). The FTIR spectra of both MOFs showed symmetric and antisymmetric \u003cem\u003ev\u003c/em\u003e C\u0026thinsp;=\u0026thinsp;O bands at 1390\u0026thinsp;\u0026minus;\u0026thinsp;1383 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1569\u0026ndash;1568 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The difference in antisymmetric and symmetric carbonyl stretching frequencies for both ZnMOF-1 and CdMOF-1 was about 179 and 185, respectively, indicating the chelating bidentate coordination mode of carboxylate moieties in these LMOFs. The broad absorption bands centering near 3445 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (ZnMOF-1) and 3440 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (CdMOF-1) are assigned to \u003cem\u003ev\u003c/em\u003e O-H vibrations indicating the presence of the lattice water molecules.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3. Powder XRD analysis\u003c/h2\u003e \u003cp\u003eTo examine MOFs' crystallinity and phase purity, Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e 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[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], indicating their successful synthesis. The phase purity of these synthesized MOFs was established by the absence of any impurity peaks in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.1.4. FE-SEM analsysis\u003c/h2\u003e \u003cp\u003eThe topographical and morphological characteristics of the synthesized ZnMOF-1 and CdMOF-1 were studied by FE-SEM analysis, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The FE-SEM micrographs of ZnMOF-1 displayed the rod-like morphology (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea,b). On the other hand, CdMOF-1 displayed a mixed morphology of both flower-like (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) and rod-like (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) structures.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.1.5. Thermogravimetric analysis (TGA)\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e reflects the thermal stability of ZnMOF-1 and CdMOF-1 as a function of temperature (35 to 700\u003csup\u003e◦\u003c/sup\u003eC) 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\u003csup\u003e◦\u003c/sup\u003eC (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 ramp above 300\u0026deg;C, resulting from the decomposition of the organic linkers. Thus, these results establish that both MOFs are thermally stable up to 300\u0026deg;C after which thermal degradation of the framework starts.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Photoluminescence sensing properties\u003c/h2\u003e \u003cp\u003eIt is well-known that MOFs constructed from the transition metal ions having d\u003csup\u003e10\u003c/sup\u003e configuration and conjugated organic ligands showed excellent photoluminescent (PL) properties. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the PL spectra of ZnMOF-1 and CdMOF-1 in water (1 mg/ mL) upon excitation at 282 nm wavelength and room temperature. As seen, the water suspension of ZnMOF-1 and CdMOF-1 (1 mg/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.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec17\" class=\"Section2\"\u003e \u003ch2\u003e\u003cem\u003e3.3. Chromium oxyanions detection (\u003c/em\u003eCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e)\u003c/h2\u003e \u003cp\u003eTo explore the aqueous phase sensing capabilities of both ZnMOF-1 and CdMOF-1, the PL emission profiles of individually suspended ZnMOF-1/CdMOF-1 in standard anionic solutions of F\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, I\u003csup\u003e\u0026minus;\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, SCN\u003csup\u003e\u0026minus;\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e are evaluated under an excitation wavelength of 282 nm in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. Noticeably, results of fluorescence quenching reveal that only the presence of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions showed a significant turn-off quenching effect on the PL intensities of ZnMOF-1 and CdMOF-1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea,b), while the presence of other anions had a low/ negligible effect on the PL intensity (Figure S2 in the Supporting Information, SI). Therefore, the effect of incremental addition of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentrations (in the range 0\u0026thinsp;\u0026minus;\u0026thinsp;2.0 mM) on the PL emission profiles of ZnMOF-1/CdMOF-1 water suspensions was evaluated in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ec\u0026thinsp;\u0026minus;\u0026thinsp;f. As it can be seen that the luminescence of the aqueous dispersions of ZnMOF-1 and CdMOF-1 falls sharply in the presence of incremental concentrations of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ions in the range 0\u0026thinsp;\u0026minus;\u0026thinsp;2.0 mM. The quenching effects on ZnMOF-1/CdMOF-1 PL intensity by incremental addition of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentrations (0\u0026thinsp;\u0026minus;\u0026thinsp;2.0 mM) and the concentration versus quenching percentage are delivered in Figures S3 and S4.\u003c/p\u003e \u003cp\u003eThe quenching response of chromate and dichromate anions can be simplified and calculated based on the Stern\u0026thinsp;\u0026minus;\u0026thinsp;Volmer equation, I\u003csub\u003e0\u003c/sub\u003e/I\u0026thinsp;=\u0026thinsp;1\u0026thinsp;+\u0026thinsp;Ksv[A], where I\u003csub\u003e0\u003c/sub\u003e represents the PL intensity of ZnMOF-1/CdMOF-1 without CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e solutions, I is the PL intensity with added CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e solutions of molar concentration [A], and Ksv is the Stern\u0026thinsp;\u0026minus;\u0026thinsp;Volmer constant/quenching constant. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, the Stern\u0026thinsp;\u0026minus;\u0026thinsp;Volmer plots for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e exhibits a good linear correlation at lower concentrations (0\u0026thinsp;\u0026minus;\u0026thinsp;1.0 mM) and showed linear correlation coefficients (R) 0.994/0.996 for ZnMOF-1 and 0.995/0.998 for CdMOF-1, respectively. However, the Stern\u0026thinsp;\u0026minus;\u0026thinsp;Volmer plots became nonlinear at higher concentrations of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e\u0026gt; 1.0 mM (Figure S3). The quenching constants (Ksv) of ZnMOF-1 and CdMOF-1 were 1.19 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and1.38 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003eand 1.48 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 3.63 \u0026times; 10\u003csup\u003e3\u003c/sup\u003e M\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, respectively. These results reveal the significant quenching effect of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e on the PL intensity of ZnMOF-1/ CdMOF-1.\u003c/p\u003e \u003cp\u003eTo calculate the limits of detection (LOD) for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e by ZnMOF-1 and CdMOF-1, the fluorescence quenching titrations were performed with the incremental addition of 10 \u0026micro;M aqueous CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e solution. Detailed procedure for calculating LOD (LOD\u0026thinsp;=\u0026thinsp;3σ/ m) is presented in Section S1 (SI), where σ\u0026thinsp;=\u0026thinsp;standard deviation from five blank measurements for each MOF and m\u0026thinsp;=\u0026thinsp;slope of the linear curve plotted at the lowest concentration for LOD calculations. The respective LOD values for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003ein aqueous media were 1.90 ppm (9.79 \u0026micro;M)/ 3.2 ppm (10.94 \u0026micro;M) by ZnMOF-1 and 0.52 ppm (2.68 \u0026micro;M)/ 0.43 ppm (1.48 \u0026micro;M) 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\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e in aqueous media (section S1 and Figure S5, SI). However, it should be noted that the sensitivity of CdMOF-1 for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003edetection was higher than ZnMOF-1. Compared with some of the recently reported fluorescent sensors (Table S1, SI), the LOD values of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ions by the prepared dual ligand Zn/Cd-MOFs were comparable to that of the previously reported MOF sensors (Table S1).\u003c/p\u003e \u003cp\u003eTo explore the impact of interfering anions on the detection of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions, the PL emission spectra of ZnMOF-1 and CdMOF-1 were measured in mixed anions solutions containing either CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e or Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e plus other (F\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, I\u003csup\u003e\u0026minus;\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, SCN\u003csup\u003e\u0026minus;\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) anions. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e, the interfering anions have no apparent influence on the fluorescence detection of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions by ZnMOF-1/CdMOF-1. In other words, the quenching efficiencies of ZnMOF-1/CdMOF-1 emissions by CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions are almost the same in the presence or absence of interfering anions. These obtained data (Ksv, LOD, and interference studies) 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.\u003c/p\u003e \u003cp\u003eGenerally 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-1for CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e 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.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb,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.\u003c/p\u003e \u003cp\u003eAs depicted in Figure S6, two broad absorption bands were observed in the wavelength range of 200\u0026thinsp;\u0026minus;\u0026thinsp;450 nm in the UV-vis spectra of aqueous solutions of K\u003csub\u003e2\u003c/sub\u003eCrO\u003csub\u003e4\u003c/sub\u003e (λ\u003csub\u003emax\u003c/sub\u003e at 270, 382 nm) and K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e (λ\u003csub\u003emax\u003c/sub\u003e at 260, 350 nm). The absorption range of the CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e covers the absorption bands in the range of 250\u0026thinsp;\u0026minus;\u0026thinsp;400 nm, including the excitation wavelength (282 nm) of ZnMOF-1 and CdMOF-1 (Figure S7). Further, upon excitation of ZnMOF-1/CdMOF-1 at 282 nm, CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e 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 (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea and S8). 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 (Figure S8). 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.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb). 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\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e are available in the literature [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo 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. The recovered ZnMOF-1 and CdMOF-1 showed no substantial changes of the PL intensity over three sensing-recovery cycles, in either case, indicating the good recyclability of ZnMOF-1 and CdMOF-1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea, b). The PXRD spectra of both MOFs also reinforced this fact after recycling and soaking in 10 mM CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e solutions up to 7 days (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb-d), confirming the formation of chemical, thermal, and water-stable luminescence ZnMOF-1/CdMOF-1 for photoluminescence sensing of CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e anions in aqueous solutions.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn 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\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/ Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e ions. This selective turn-off effect is attributed to the competitive absorption of excitation wavelength energy and FRET between CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/ Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e 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\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e at 9.79 \u0026micro;M/ 10.94 \u0026micro;M and 2.68 \u0026micro;M/ 1.48 \u0026micro;M 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.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors are thankful to the Chemistry Department, Punjabi University, Patiala, for providing lab and instrument facilities. One of the author Manpreetkaur is highly obliged to the UGC, New Delhi, India, for UGC-SRF fellowship.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eManpreet Kaur has performed the experiment and written the paper. AKM and MY has \u0026nbsp;written and edited the paper. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u0026ldquo;The authors have no relevant financial or non-financial interests to disclose.\u0026rdquo;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNo Funding was available supporting this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll relevant data will be provided on request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThere is no ethical approval required.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors give their consent to participate in the publication of this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to publish\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors give consent to publish the paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFT-IR, PL spectra, SV plots, quenching percentage graph, UV\u0026minus;vis spectra, LOD calculations, comparison table for chromate and dichromate anions.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eK. Vikrant, D.C. Tsang, N. Raza, B.S. Giri, D. Kukkar, K.-H. Kim, Potential utility of metal\u0026ndash;organic framework-based platform for sensing pesticides, ACS applied materials \u0026amp; interfaces 10(10) (2018) 8797-8817.\u003c/li\u003e\n\u003cli\u003eY. Zhang, S. Yuan, G. Day, X. Wang, X. Yang, H.-C. Zhou, Luminescent sensors based on metal-organic frameworks, Coordination Chemistry Reviews 354 (2018) 28-45.\u003c/li\u003e\n\u003cli\u003eY.D. Farahani, V. Safarifard, A luminescent metal-organic framework with pre-designed functionalized ligands as an efficient fluorescence sensing for Fe3+ ions, Journal of Solid State Chemistry 270 (2019) 428-435.\u003c/li\u003e\n\u003cli\u003eC. Liu, B. Yan, A novel photofunctional hybrid material of pyrene functionalized metal-organic framework with conformation change for fluorescence sensing of Cu2+, Sensors and Actuators B: Chemical 235 (2016) 541-546.\u003c/li\u003e\n\u003cli\u003eW. Zhang, Y. Tang, A. Shi, L. Bao, Y. Shen, R. Shen, Y. Ye, Recent developments in spectroscopic techniques for the detection of explosives, Materials 11(8) (2018) 1364.\u003c/li\u003e\n\u003cli\u003eY.D. Farahani, V. Safarifard, Highly selective detection of Fe3+, Cd2+ and CH2Cl2 based on a fluorescent Zn-MOF with azine-decorated pores, Journal of Solid State Chemistry 275 (2019) 131-140.\u003c/li\u003e\n\u003cli\u003eB. Parmar, Y. Rachuri, K.K. Bisht, E. Suresh, Mixed-ligand LMOF fluorosensors for detection of Cr (VI) oxyanions and Fe3+/Pd2+ cations in aqueous media, Inorganic chemistry 56(18) (2017) 10939-10949.\u003c/li\u003e\n\u003cli\u003eM. Kaur, S. Kumar, S.A. Younis, M. Yusuf, J. Lee, S. Weon, K.-H. Kim, A.K. Malik, Post-Synthesis Modification of Metal-Organic Frameworks Using Schiff Base Complexes for Various Catalytic Applications, Chemical Engineering Journal (2021) 130230.\u003c/li\u003e\n\u003cli\u003eM. Pamei, A. Puzari, Luminescent transition metal\u0026ndash;organic frameworks: An emerging sensor for detecting biologically essential metal ions, Nano-Structures \u0026amp; Nano-Objects 19 (2019) 100364.\u003c/li\u003e\n\u003cli\u003eS.A.A. Razavi, A. Morsali, Metal ion detection using luminescent-MOFs: Principles, strategies and roadmap, Coordination Chemistry Reviews 415 (2020) 213299.\u003c/li\u003e\n\u003cli\u003eB. Parmar, K.K. Bisht, Y. Rachuri, E. Suresh, Zn (II)/Cd (II) based mixed ligand coordination polymers as fluorosensors for aqueous phase detection of hazardous pollutants, Inorganic Chemistry Frontiers 7(5) (2020) 1082-1107.\u003c/li\u003e\n\u003cli\u003eM. Kaur, S. Kumar, M. Yusuf, J. Lee, R.J. Brown, K.-H. Kim, A.K. Malik, Post-synthetic modification of luminescent metal-organic frameworks using schiff base complexes for biological and chemical sensing, Coordination Chemistry Reviews 449 (2021) 214214.\u003c/li\u003e\n\u003cli\u003eL. Fan, D. Zhao, B. Li, F. Wang, Y. Deng, Y. Peng, X. Wang, X. Zhang, Luminescent Binuclear Zinc (II) Organic Framework as Bifunctional Water-stable Chemosensor for Efficient Detection of Antibiotics and Cr (VI) Anions in Water, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2021) 120232.\u003c/li\u003e\n\u003cli\u003eB. Parmar, Y. Rachuri, K.K. Bisht, E. Suresh, Syntheses and structural analyses of new 3D isostructural Zn (II) and Cd (II) luminescent MOFs and their application towards detection of nitroaromatics in aqueous media, ChemistrySelect 1(19) (2016) 6308-6315.\u003c/li\u003e\n\u003cli\u003eB. Parmar, Y. Rachuri, K.K. Bisht, R. Laiya, E. Suresh, Mechanochemical and conventional synthesis of Zn (II)/Cd (II) luminescent coordination polymers: dual sensing probe for selective detection of chromate anions and TNP in aqueous phase, Inorganic chemistry 56(5) (2017) 2627-2638.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Metal-organic Frameworks (MOFs), Pyridyl-based Schiff base ligand, Fluorescent sensor, Chromate anions, turn-off quenching effect","lastPublishedDoi":"10.21203/rs.3.rs-2042384/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-2042384/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this research, Zn- or Cd-based metal-organic frameworks (coded ZnMOF-1 and CdMOF-1) containing benzene-1,4-dicarboxylic acid (H\u003csub\u003e2\u003c/sub\u003ebdc) and pyridyl-based Schiff base (4-pyridylcarboxaldehydeisonicotinoylhydrazone (L)) dual ligands were successfully assembled \u003cem\u003evia\u003c/em\u003e 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: CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e). Further, it was noted that only chromate/dichromate (CrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e/Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e) anions showed a significant turn-off quenching effect while other anions (like F\u003csup\u003e\u0026minus;\u003c/sup\u003e, Br\u003csup\u003e\u0026minus;\u003c/sup\u003e, I\u003csup\u003e\u0026minus;\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, ClO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, SCN\u003csup\u003e\u0026minus;\u003c/sup\u003e, SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e, NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, and NO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) 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 \u0026micro;M and 2.68/1.48 \u0026micro;M, 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 chromate/dichromateanions. 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).\u003c/p\u003e","manuscriptTitle":"Schiff base-functionalized Metal-Organic Frameworks for Selective Sensing of Chromate and Dichromate in Water","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2022-09-13 20:39:55","doi":"10.21203/rs.3.rs-2042384/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revision","date":"2022-09-20T13:58:04+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2022-09-18T11:18:32+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"ba3b79f9-5e0d-432b-b54c-9ab314318fb7","date":"2022-09-12T18:43:21+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2022-09-12T16:37:32+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2022-09-12T04:45:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2022-09-12T04:45:03+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Fluorescence","date":"2022-09-07T16:23:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-fluorescence","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jofl","sideBox":"Learn more about [Journal of Fluorescence](https://www.springer.com/journal/10895)","snPcode":"10895","submissionUrl":"https://submission.nature.com/new-submission/10895/3","title":"Journal of Fluorescence","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"2e7c94f8-67ee-4ebb-850f-31dfd90fdbc7","owner":[],"postedDate":"September 13th, 2022","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2022-09-27T11:14:33+00:00","versionOfRecord":[],"versionCreatedAt":"2022-09-13 20:39:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-2042384","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-2042384","identity":"rs-2042384","version":["v1"]},"buildId":"cBFmMYwuxLRRLfASyISRj","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}