Pd2+ Cations in Aqueous ... - ACS Publications

Aug 29, 2017 - Department of Chemistry, RCU Government Post Graduate College, Uttarkashi-249193, Uttarakhand, India. •S Supporting Information. ABST...
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Mixed-Ligand LMOF Fluorosensors for Detection of Cr(VI) Oxyanions and Fe3+/Pd2+ Cations in Aqueous Media Bhavesh Parmar,†,‡ Yadagiri Rachuri,†,‡ Kamal Kumar Bisht,§ and Eringathodi Suresh*,†,‡ †

Academy of Scientific and Innovative Research (AcSIR), CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364 002, Gujarat, India ‡ Analytical Division and Centralized Instrument Facility, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364 002, Gujarat, India § Department of Chemistry, RCU Government Post Graduate College, Uttarkashi-249193, Uttarakhand, India S Supporting Information *

ABSTRACT: Zn(II)/Cd(II)-based dual ligand Luminescent Metal−Organic Frameworks (LMOFs) {[M(ATA)(L)]}n· xH2O (1) and (2) were synthesized by versatile synthetic routes, viz., diffusion of precursor solutions, conventional reflux, and green mechanochemical (grinding) reactions from bipyridyl-based Schiff base, (E)-N′-(pyridin-4-ylmethylene)isonicotinohydrazide (L) and amino functionalized 2-aminoterephthalic acid (H2ATA) as linkers. Chemical and thermal stability, phase purity, and characterization of both LMOFs were established by various analytical methods. SXRD analysis revealed the 3D framework is composed of two-dimensional [M(ATA)]n nets doubly pillared by L through the terminal nitrogen atom. Selective and sensitive detection of chromate anions (CrO42−/Cr2O72−) and Fe3+/Pd2+ cations in the aqueous phase by fluorescent quenching of the LMOFs 1 and 2 has been established. Competitive experiments in the presence of interfering anions/cations with 1 and 2 revealed no major change in the quenching efficiency. The observed limits of detection (LOD) values by 1 for CrO42−/Cr2O72− were 0.25 μM (48 ppb)/0.43 μM (126 ppb) and for Fe3+/Pd2+ were 3.76 μM (0.61 ppm)/0.20 μM (35 ppb), whereas LOD values by 2 were 0.18 μM (35 ppb)/0.19 μM (55 ppb) and 1.77 μM (0.29 ppm)/0.10 μM (18 ppb), respectively. Simple fluorescent-based test paper strips have been developed for reliable and visual detection of the mentioned analytes in practical applications. The present investigation clearly demonstrates selective detection of CrO42−/ Cr2O72− and Fe3+/Pd2+ in aqueous media, and the probable mechanism for the quenching phenomena based on structural aspects has also been discussed.

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INTRODUCTION Metal−Organic Frameworks (MOFs), crystalline materials offering tunable properties by the sensible choice of precursors, have rapidly emerged in the past two decades for their versatile applications in gas storage, gas separation, chemical sensing, heterogeneous catalysis, magnetism, and optoelectronics as well as for their tempting structural aesthetics.1−6 Studies have established that these applications depend not only on constitution of MOFs but also on the structural aspects like framework topology and pore geometries.7−11 It is also documented that small changes in experimental parameters such as time, temperature, solvent, and concentration may result in entirely different end products.12−14 Therefore, devising efficient nonconventional protocols for bulk preparation of stable and practically useful new MOFs is of considerable significance.15−19 On the other hand, recognition of toxic cations, anions, or other noxious chemicals has become a very important issue because growing industrialization has contributed to increasing levels of pollutants in the environment. For instance, © 2017 American Chemical Society

chromium(VI) oxyanions, responsible for many human ailments from allergy to gene mutation, are being used and released to the environment as nonbiodegradable effluents by leather tanning, paint, steel, agrochemicals, and a few other industries.20−24 Similarly, minute concentrations of an industrially useful catalyst, palladium(II) cation, not only causes allergies and cytotoxic effects but can bind with DNA and proteins.25−29 Another example is ferric cation, a precise quantity of which is essential for a few biochemical processes in humans. Deficiency of Fe3+ causes anemia, whereas its excess may induce damages to liver and kidneys.30−32 These concerns have driven research efforts to develop new chemical sensors for detecting the minute concentrations of toxic ions in soils and water bodies.33−42 In recent years, luminescent MOFs (LMOFs) have emerged as excellent chemical sensors to detect ppm levels and in some cases even ppb levels of pollutants or analytes.43−57 NeverReceived: May 4, 2017 Published: August 29, 2017 10939

DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

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Inorganic Chemistry

Synthesis and General Characterizations. Synthesis of {[Zn(ATA)(L)]·H2O}n (1). Zn(ClO4)2·6H2O (37.2 mg, 0.1 mmol) was dissolved in 3 mL of H2O. Separately, 2-aminoterephthalic acid (H2ATA) (18.11 mg, 0.1 mmol), KOH (12 mg, 0.2 mmol), and (E)N′-(pyridin-4-ylmethylene)isonicotinohydrazide (L) (22.62 mg, 0.1 mmol) were dissolved in 3 mL of a solution of H2O:EtOH (1:1/v:v). In a narrow test tube, the aqueous metal solution was placed, followed by 8 mL of a solution of H2O:MeOH (1:1/v:v). The ligand solution was then carefully layered over the H2O:MeOH layer, and the test tube was sealed and left for the crystallization. Crystals of X-ray diffraction quality appeared within 1 week. (Yield ∼ 75%) Elemental analysis (%) Calcd. for {[Zn(ATA)(L)]·H2O}n: C, 49.15; H, 3.51; N, 14.33; found: C, 49.18; H, 3.63; N, 14.47; IR cm−1 (KBr): 3463 (m), 3359 (m), 3207 (m), 3040 (m), 1693 (m), 1569 (s), 1426 (s), 1380 (s), 1286 (s), 1146 (m), 1068 (w), 1015 (w), 947 (w), 828 (w), 772 (m), 697 (m), 538 (w). Synthesis of {[Cd(ATA)(L)]·2H2O}n (2). The same synthetic procedure for 1 was adopted for growing crystals for 2 also. However, Cd(ClO4)2·xH2O (31.2 mg, 0.1 mmol) was used in place of Zn(ClO4)2.6H2O. (Yield ∼ 78%) Elemental analysis (%) Calcd. for {[Cd(ATA)(L)]·2H2O}n: C, 43.38; H, 3.46; N, 12.65; found: C, 44.43; H, 3.29; N, 13.11; IR cm−1 (KBr): 3475 (m), 3361 (m), 3209 (m), 3046 (m), 1695 (m), 1552 (s), 1422 (s), 1375 (s), 1285 (s), 1146 (m), 1069 (w), 1014 (w), 941 (w), 848 (w), 773 (m), 697 (m), 513 (w). Conventional (Reflux) Synthesis. Bulk powders of 1 or 2 were synthesized via a reflux method. 1 mmol of Zn(NO3)2·6H2O (for 1) or Cd(NO3)2·4H2O (for 2), 1 mmol of H2ATA, 2 mmol of KOH, and 1 mmol L in 40 mL of H2O:MeOH (1:1/v:v) solvent were refluxed in a 100 mL round-bottom flask at ca. 110 °C for 6 h. The resulting precipitates were filtered and washed with H2O:MeOH (1:1/v:v), followed by acetone, then dried at 100 °C in an oven. For 1: Elemental analysis (%) Calcd.: C, 49.15; H, 3.51; N, 14.33; found: C, 49.00; H, 3.72; N, 14.82; IR cm−1 (KBr): 3464 (m), 3332 (m), 3206 (m), 3062 (m), 1689 (m), 1566 (s), 1422 (s), 1378 (s), 1287 (m), 1144 (m), 1065 (w), 1016 (w), 957 (w), 845 (w), 773 (m), 695 (m), 535 (w). For 2: Elemental analysis (%) Calcd.: C, 43.38; H, 3.46; N, 12.65; found: C, 44.33; H, 3.38; N, 13.24; IR cm−1 (KBr): 3410 (br), 3361 (m), 3212 (m), 3046 (m), 1687 (m), 1552 (s), 1416 (s), 1370 (s), 1287 (m), 1145 (m), 1067 (w), 1013 (w), 944 (w), 843 (w), 770 (w), 693 (m), 510 (w). Mechanochemical (Grinding) Synthesis. Alternatively, 1 or 2 was synthesized via manually grinding a mixture of 1 mmol of Zn(OAc)2· 2H2O (for 1G) or Cd(OAc)2·2H2O (for 2G), 1 mmol of H2ATA, and 1 mmol of L in a mortar and pestle for 25 min. The resulting solids were washed with H2O:MeOH (1:1/v:v, 5 mL), followed by acetone (5 mL) to remove any unreacted starting material, and then dried at 100 °C for 24 h in an oven and designated as 1G and 2G. For 1G: Elemental analysis (%) Calcd.: C, 49.15; H, 3.51; N, 14.33; found: C, 48.26; H, 3.65; N, 14.07; IR cm−1 (KBr): 3457 (m), 3358 (m), 3211 (m), 3067 (m), 1690 (m), 1570 (s), 1425 (s), 1376 (s), 1287 (m), 1147 (w), 1068 (w), 1016 (w), 945 (w), 829 (w), 772 (m), 697 (m), 539 (w). For 2G: Elemental analysis (%) Calcd.: C, 43.38; H, 3.46; N, 12.65; found: C, 42.68; H, 3.58; N, 12.44; IR cm−1 (KBr): 3409 (br), 3358 (m), 3211 (m), 3046 (m), 1690 (m), 1556 (s), 1420 (s), 1374 (s), 1286 (s), 1147 (m), 1067 (w), 1014 (w), 941 (w), 846 (w), 773 (w), 695 (m), 536 (w). Fluorescence Study. For anion/cation sensing experiment, 3 mg of 1 or 2 is weighed and added to the cuvette of path length of 1 cm containing 3 mL of aqueous solutions of anions/cations under stirring. The fluorescence spectra in the 360−600 nm range upon excitation at 340 nm were recorded, and fluorescence intensity was monitored at 428 nm (1), 431 nm (2). To maintain homogeneity, the suspension was constantly stirred with the help of the stirring attachment present in the fluorescence instrument. Standard aqueous solutions of potassium salts of F−, Cl−, Br−, I−, SCN−, NO2−, NO3−, SO42−, ClO4−, IO3−, MoO42−, AsO2−, CrO42−, Cr2O72− and standard aqueous solutions of chloride salts of Na+, K+, Mg2+, Ba2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Hg2+, Cr3+, Fe3+, Pd2+, each of 10−2 M

theless, there are not many reports on LMOF sensors for detection of analytes in aqueous medium.39,45,58−64 Two reports on detection of Pd(II) ions in organic phase and a few studies on aqueous phase recognition of Cr(VI) oxyanions and Fe(III) cation by LMOFs are worth mentioning here.59−76 These studies accentuate that stability of the LMOF framework in water or moist conditions is most crucial for developing a chemical sensor for recognizing pollutants in aqueous phase toward practical applications. Additionally, the concomitant effect of metal ions and linkers employed to construct LMOFs, free functional groups on the pore surfaces, and framework topology can regulate the host−guest interactions in specific manners which in turn can result in sensitive and selective sensing.77−79 In continuation to our previous investigations,60,80−82 herein we present two 3D isostructural LMOFs, {[M(ATA)(L)]· xH2O}n comprising Zn2+ (1) or Cd2+ (2) metal nodes and a bipyridyl-based Schiff base ligand, 4-pyridyl carboxaldehyde isonicotinoylhydrazone (L) in combination with 2-aminoterephthalic acid (H2ATA). LMOFs 1 and 2 are crystallized by diffusion of precursor solutions, and bulk amounts are synthesized by conventional reflux and green mechanochemical (grinding) methods. Syntheses, characterization, chemical as well as thermal stability of both LMOFs and their application for the selective and recyclable detection of Cr(VI) oxyanions, Fe3+ and Pd2+ cations in aqueous media by fluorescence quenching method have been demonstrated. Practical applicability of these fluorosensors is also established by developing LMOF coated paper strips displaying high fluorescence under UV (365 nm) irradiation and their complete quenching in the presence of the aforementioned analytes. To the best of our knowledge, this is the first report for aqueous phase detection of Pd2+ by LMOFs as a fluorosensor.

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EXPERIMENTAL SECTION

Materials and General Methods. All reagents and solvents were purchased from commercial sources and were used without further purification. The N-donor, (E)-N′-(pyridin-4-ylmethylene)isonicotinohydrazide (L), was synthesized according to our previous report.80 Distilled water was used for synthetic manipulations. CHNS analyses were done using an elementar vario MICRO CUBE analyzer. IR spectra were recorded using the KBr pellet method on a PerkinElmer GX FTIR spectrometer. For each IR spectra, 10 scans were recorded at 4 cm−1 resolution. 1H NMR spectra for the ligand L were recorded on a Bruker AX 500 spectrometer (500 MHz) at a temperature 25 °C and was calibrated with respect to the internal reference TMS. TGA analysis was carried out using a Mettler Toledo Star SW 8.10. TG analysis was performed in a nitrogen environment while the heating rate was ramped from room temperature to 700 °C at 10 °C/min. Powder X-ray diffraction (PXRD) and variable temperature XRD (VTXRD) data were collected using a PANalytical Empyrean (PIXcel 3D detector) system with CuKα radiation. Single crystal structures were determined using a BRUKER SMART APEX (CCD) diffractometer. BET surface area was measured on a Micromeritics, ASAP 2010, instrument. Solid state UV−vis spectra were recorded using a Shimadzu UV-3101PC spectrometer and BaSO4 as a reference. Field emission-scanning electron microscopy (FE-SEM) micrographs were recorded using a JEOL JSM-7100F instrument employing an 18 kV accelerating voltage. Luminescence spectra and fluorescence decay lifetime spectra were recorded at room temperature utilizing a Fluorolog Horiba Jobin Yvon spectrophotometer. ICP analysis was performed using a PerkinElmer, Optima 2000 DV, instrument. Crystallographic data, characterization, and stability study data for both LMOFs are included in the Supporting Information (Tables S1 and S2). 10940

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Figure 1. Depiction of (a) coordination environment of Zn2+ in 1; (b) sql net [Zn2(ATA)2]n; (c) double-pillaring of sql net [Zn2(ATA)2]n by L resulting in pcu net of 1; (d) 2-fold interpenetration of pcu nets; (e) overall 3D network of LMOF 1, {[Zn(ATA)(L)]n. concentration, were prepared for sensing experiments. All titrations were performed in triplicate to establish the consistency of the results.

It is noteworthy that the structural features of LMOF 1 are closely related to a 3D MOF, {[Zn2(terephthalate)2(L)2]· solvents}n, reported by Matoga et al. and {[M(terephthalate)(L)]·solvent}n (M = Zn and Cd) reported by us.80,83 These recently reported MOFs also feature [M2(dicarboxylate)2]n type sql nets which are doubly pillared by ligand L and realize 2-fold interpenetrated pcu type 3D networks. Comparison of structural details between LMOF 1 and {[Zn2(terephthalate)2(L)2]·solvents}n MOF reported by Matoga et al. clearly revealed the conformational variation of ligand L. The ligand L assumes a nearly planar conformation (dihedral angle between planes of pyridine rings, ϕpy−py = 5.38°) in LMOF 1, whereas the pyridine rings of L are twisted (ϕpy−py = 34.10°) with respect to each other in the case of {[Zn2(terephthalate)2(L)2]·solvents}n to make effective coordination with the metal. Good luminescence property of LMOF 1 can be partly attributed to the planar conformation of L which has better ability to communicate electrons. In the case of 2, the coordination geometry around the Cd2+ and the mode of coordination by the ATA ligands are quite different, retaining the overall pcu topology as observed in the case of 1. Thus, Cd2+ is involved in N2O4 coordination with a distorted octahedral geometry provided by two symmetrically disposed ATA ligands as well as nitrogen atoms of different bipyridyl-based Schiff base L (Figure S1a). Symmetrically disposed ATA ligands are involved in a dimeric [Cd2(COO)2] type secondary building unit (Cd···Cd separation 3.945 Å) via μ4-η1,η1; η1,η1 coordination by one of the ATA ligands involving O1 and O2. [Cd2(COO)2] dimeric chains are cross-linked by another ATA moiety with a carboxylate group in a chelated μ2-η1,η1; η1,η1 mode of coordination (O3,O4), generating the sql sheets [Cd2(ATA)2]n with ca. 10.3 × 11.3 Å rectangular channels (Figure S1b). Adjacent square lattice sheets of [Cd2(ATA)2]n doubly pillared by N-donor ligands with a Cd···Cd bridging distance 16.02 Å generate a robust 2-

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RESULTS AND DISCUSSION Crystal and Molecular Structure of 1 and 2. Crystallographic studies revealed that 1 and 2 crystallized in the same space group P1̅ and feature 3D doubly interpenetrated nets composed of Zn2+/Cd2+ nodes connected through ATA and pillared by the bipyridine-based Schiff base ligand L. The asymmetric unit of both LMOFs consists of a crystallographically independent M2+, two half ATA moieties with C2 symmetry (passing through the 1,4-carbon atom of the ATA moiety in which C2 symmetry is generated by disordered amino nitrogen atoms occupying at 2 and 5 positions), and one molecule of Schiff base ligand L. In the case of 1, N2O3 coordination around Zn2+ with distorted trigonal bipyramidal geometry was provided by carboxylate oxygen atoms (O1, O2, and O3) from three different ATA ligands and axially pillared by terminal pyridyl nitrogen atoms (N3 and N6) from different Schiff base ligands (Figure 1a). Symmetrically disposed ATA ligands are involved in a dimeric [Zn2(COO)2] secondary building unit (Zn···Zn separation 3.131(3) Å) by means of one of the ATA ligands in syn−syn coordination mode involving O1 and O2, which is further coupled by the carboxylate oxygen O3 from the second symmetric ATA moiety via a μ2-coordination mode creating sql sheets (17.468 Å × 12.284 Å) with the general formula [Zn2(ATA)2]n oriented diagonal to the acplane Figure 1b. Thus, [Zn2(COO)2] dimeric chains are crosslinked by another ATA moiety via a μ2-η1,η0; η1,η0 mode of coordination, which resulted in basic parallelogram units in the structure of 1 (Figure 1b). sql sheets [Zn2(ATA)2]n are further double pillared through the terminal nitrogen atoms (N3, N6) of ligand L, generating a robust three-dimensional distorted pcu net with a Zn···Zn distance of 15.735 Å between two adjacent sql nets (Figure 1c−e). 10941

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Figure 2. (a, b) The fluorescence quenching by suspension of LMOFs 1 and 2 (3.0 mg/3 mL) in different aqueous anion solutions. (c−f) Luminescence responses of 1 and 2 (2 mg dispersed in 2 mL of water) toward different concentrations of CrO42−/Cr2O72− (0−2.5 mM) in water.

fluorescence intensities of 1 and 2 excited at 340 nm (Figure 2a,b), whereas other tested anions showed negligible effect (Figure S13). Therefore, concentration-based studies were performed by recording the fall in PL intensities of the 1 and 2 suspensions (2 mg in 2 mL) upon incremental concentrations of CrO42−/Cr2O72− ions in the range 0−2.5 mM (Figure 2c−f). The quenching efficiency of CrO42−/Cr2O72− was rationalized and calculated based on Stern−Volmer plots presented in Figure 3a,b. Stern−Volmer plots drawn for respective PL intensities (I0/I) of 1 or 2 against concentration of CrO42−/

fold interpenetrated 3D framework with pcu topology as observed in 1. The packing diagram disclosed that interpenetrated 3D nets in both LMOFs 1 and 2 are involved in supramolecular interactions via hydrogen bonding. Thus, disordered amino hydrogen atoms attached to N1 and N2 at 2 and 5 positons act as donors and are involved in intermolecular N-H···N and N-H···O contacts. Amide hydrogen is making strong intermolecular N-H···O contact with the carboxylate oxygen atoms in both LMOFs. In addition to intramolecular contacts, weak C-H···O contacts are also observed in both LMOFs in stabilizing the 3D net in the crystal lattice. Details of pertinent hydrogen bonding interaction and symmetry code are provided in Table S3. Photoluminescence Properties. Luminescent MOFs constructed from the organic ligands containing aromatic or conjugated π moieties and a d10 metal center can promote optical emission or photoluminescence. PL spectra of 1 and 2 were recorded in both solid state and in aqueous media (Figures S11 and S12). PL spectra in aqueous suspensions were acquired by dispersing 3 mg of LMOF in 3 mL of water with constant stirring in a cuvette equipped with a stirring attachement. A water suspension of 1 and 2 showed good emission intensities at 428 and 431 nm, respectively, upon excitation at 340 nm which prompted us to evaluate the suitability of both LMOFs for aqueous phase sensing applications for different analytes. The blue shift of emission intensity in water (∼11−18 nm) compared to solid state can be attributed to the solvent effect. Fluorescence Studies for Sensing of Chromium(VI) Oxyanions. Standard potassium salt solutions (10−2 M) of anions, F−, Cl−, Br−, I−, SCN−, NO2−, NO3−, SO42−, ClO4−, IO32−, MoO42−, AsO2−, CrO42−, and Cr2O72−, in water were prepared, and emission intensities of both LMOFs dispersed in standard anionic solutions were explored. The luminescence sensing experiments were performed on uniform dispersions of 1 or 2 (3 mg in 3 mL of aqueous anion solution (10−2 M)) under constant stirring. Strikingly, the CrO42− and Cr2O72− ions afforded the most significant turnoff quenching effect on

Figure 3. (a, b) Nonlinear Stern−Volmer (SV) plots for CrO42−/ Cr2O72− in the presence of a water suspension of 1 and 2 (2.0 mg/2 mL). 10942

DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

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Figure 4. (a, b) Fluorescence lifetime decay profiles of 1 and 2 in the presence and absence of CrO42− and Cr2O72−. (c) Spectral overlap between normalized absorption spectra of Cr(IV) anions solution and normalized emission spectra of LMOFs 1 and 2 in water. (d) Possible chelating sites for metal cation interaction in ATA and Schiff base L ligand unit in the LMOFs 1 and 2.

Cr2O72− behave nonlinearly with upward bending which fit well with an exponential quenching model, I0/I = Aek[Q] + B (where I0 and I are the fluorescence intensities before and after analytes addition, respectively; [Q] is the molar concentration of analytes; and A, B, and k are constants).84−86 Quenching constants (Ksv) for CrO42−/Cr2O72− specified by the product of constants A and k,87 are calculated to be 1.485 × 10−3 M−1/ 2.623 × 10−3 M−1 for 1 and 0.97 × 10−3 M−1/3.119 × 10−3 M−1 for 2. The calculated limit of detection (LOD) values for CrO42−/Cr2O72− were 0.25 μM (48 ppb)/0.43 μM (126 ppb) for 1 and 0.18 μM (35 ppb)/0.19 μM (55 ppb) for 2, respectively (section S1 and Figure S14). On the other hand, interference by other anions in the detection phenomenon was studied by performing quenching experiments with a mixture of anionic solutions which revealed that the presence of other anions does not affect the sensing ability of LMOFs 1 and 2 toward CrO42−/Cr2O72− anions (Figure S15). Previously reported MOF sensors for detection of chromates and pertinent quenching data are provided in Table S4 for the sake of comparison. Concomitantly, the excellent quenching efficiencies, LOD values, and interference studies establish the potential of 1 and 2 as suitable complete turn-off fluorosensors for quick detection of noxious CrO42−/ Cr2O72− anions in aqueous phase detection (Figure S16). The mechanism for selective detection of CrO42−/Cr2O72− by 1 and 2 is investigated by corroborating evidence from Stern−Volmer plots, lifetime measurements, and spectral overlap of emission profiles of LMOFs and absorption spectra of analytes. PXRD, FTIR, and FE-SEM analyses of LMOFs after soaking in aqueous solutions of chromate ions for several hours show that structural and textural properties remain intact, ruling out the possibility of luminescence quenching by collapse of the material framework. The deviation from linearity in the Stern−Volmer plot (Figure 3a,b) indicates the quenching behavior of LMOFs is either static or a combination of static and dynamic processes. To further understand the nature of the quenching phenomenon, fluorescence lifetime decay profiles of LMOFs 1 and 2 were recorded in the presence and absence of

quenchers. As shown in Figure 4a,b, the lifetime decay of LMOFs in the presence and absence of quenchers remains almost the same, indicating the static quenching mechanism is operational. As depicted in Figure 4c, absorption spectra of an aqueous solution of K2CrO4 and K2Cr2O7 displayed two broad bands in the wavelength range of 230−420 nm (λmax at 274, 372 nm for K2CrO4 and at 257, 351 nm for K2Cr2O7). Spectral overlap between these absorption peaks with emission peaks of the LMOFs signifies the probability of electron/energy transfer from LMOF to analytes due to weak interactions between the two, thus leading to the selective quenching phenomenon.85−89 No such spectral overlap was observed for other anions as depicted in Figure S17, which explains the selectivity toward chromate anions. Further, amide and amino functionalities present on linkers can uphold supramolecular interaction such as hydrogen bonding with chromate oxyanions favoring the electron/energy transfer processes (Figure 4d). Conversely, absorbance bands of none of the anions but K2CrO4 (λmax at 274, 372 nm) and K2Cr2O7 (λmax at 257, 351 nm) are in the same wavelength range (Figure S18) as that for LOMFs 1 (261 nm, 340 nm) and 2 (261, 343 nm). Therefore, chromate anions in water are also expected to compete with LMOFs for absorption wavelength that in turn would diminish the emission of the LMOFs in the presence of chromate anions. Therefore, the static quenching process involving electron/ energy transfer due to suitable spectral overlap with analytes can be asserted as the plausible quenching mechanism operating in the present case. Fluorescence Studies for Sensing of Fe 3+ /Pd 2+ Cations. Multifunctional probing ability of LMOFs 1 and 2 was investigated by employing them for aqueous phase detection of metal cations. This study was in line with that carried out for chromate anions. Thus, 10−2 M aqueous solutions of chloride salts of Na+, K+, Mg2+, Ba2+, Ca2+, Zn2+, Cd2+, Mn2+, Ni2+, Hg2+, Co2+, Cr3+, Cu2+, Pb2+, Fe3+, and Pd2+ were tested when fluorescence emission of LMOFs 1 (λem = 428 nm) and 2 (λem = 431 nm) observed upon excitation at 10943

DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

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Figure 5. (a, b) The fluorescence quenching by suspension of LMOFs 1 and 2 (3.0 mg/3 mL) in different aqueous cation solutions. (c−f) Luminescence responses of 1 and 2 (2 mg dispersed in 2 mL of water) toward different concentrations of Fe3+/Pd2+ (0−2.0 mM/0−25 μM) in water.

Figure 6. (a, b) Nonlinear Stern−Volmer (SV) plots for Fe3+/Pd2+ in the presence of water suspension of 1 and 2 (2.0 mg/2 mL). (c, d) Fluorescence lifetime decay profile of 1 and 2 in the presence and absence of Fe3+ and Pd2+.

3.838 × 10−3 M−1/7.872 × 10−4 M−1 in the case of LMOF 2 (Figure 6a,b). Calculated LOD values for Fe3+/Pd2+ were 3.76 μM (0.61 ppm)/0.20 μM (35 ppb) for LMOF 1 and 1.77 μM (0.29 ppm)/0.10 μM (18 ppb) for LMOF 2 (section S2 and Figure S20). Interference studies showed that other added metal cations barely affect the selective sensing ability of 1 or 2 for Fe3+ or Pd2+ (Figure S21). Interestingly, the complete fluorescent turn-off in the presence of Fe3+/Pd2+ can be visualized by the naked eye under UV illumination (365 nm) as shown in Figure S22. On the other hand, aqueous phase detection of Fe3+ by LMOFs is well documented in the

340 nm was almost completely quenched by Fe3+ and Pd2+ cations (Figures 5a,b and S19). Ni2+, Hg2+, Co2+, Cr3+, Cu2+, and Pb2+ cations also showed some degree of quenching, whereas other cations did not quench the emission intensity. Concentration-based studies were performed to record the fall in PL intensities of the 1 and 2 suspensions (2 mg in 2 mL) upon increasing concentration of Fe3+ (0−2.0 mM) and Pd2+ (0−25 μM) ions (Figure 5c−f). Stern−Volmer fitting of relative emission versus analyte concentration revealed quenching constants (Ksv) for Fe3+/Pd2+ as 0.557 × 10−3 M−1/4.182 × 10−4 M−1 in the case of 1 and 10944

DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

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Inorganic Chemistry

spectra of host materials confirms incorporation of these analytes in the frameworks (Figure S23). FTIR analysis also suggests that Fe3+ and Pd2+ cations interact with the amino functionality present on linker ATA. Coordination of metal cations with the amino group of the ATA linker reflects in the shifting of amino ν(N-H) stretching, δ(H-N-H) scissoring, and ν(ϕ-N) stretching frequencies toward the lower energies. As depicted in Figure S24, ν(N-H) stretching frequencies appearing at 3398 and 3359 cm−1 in the case of pristine LMOF shifts to ca. 3359 and 3319 cm−1 after interacting with Fe3+ and Pd2+ ions. The amino scissoring band, δ(H-N-H), and C-N stretching band, ν(ϕ-N), appear at 1693 and 1258 cm−1, respectively, in the case of pristine materials. These two bands lower to 1681, 1251 cm−1 and 1673, 1240 cm−1 after interaction with Fe3+ and Pd2+ ions, respectively. Similar lowering of amino stretching and bending energies was observed for Cd LMOF, 2, as well (Figure S25). Therefore, it is evident from ICP and FTIR analysis that the chromates, Fe3+, and Pd2+ ions selectively incorporate in the frameworks and interact with the functional groups of the ligand moieties in both LMOFs. Test Paper Strip for Practical Application. Detection of CrO42−/Cr2O72− anions and Fe3+/Pd2+ cations was also performed by developing LMOF coated paper strips. Test paper strips were prepared by coating the water suspension of LMOFs into a filter paper strip and drying it in an oven at 100 °C for 2 h. LMOFs coated paper strips showed strong emission visible by the naked eye upon excitation at 365 nm in the UV cabinet. After partially dipping the LMOF coated paper strips in aqueous solutions containing CrO42−/Cr2O72− (10−2 M) and Fe3+/Pd2+ (10−2 M), quenching of the emission intensity was observed upon UV irradiation. The digital photographs of LMOF coated paper strips after partial treatment with analyte solutions is captured under UV irradiation. As shown in Figure 7a,b, both LMOF 1 and 2 coated test strips showed good response for CrO42−/Cr2O72− anions and Fe3+/Pd2+ cations. As is evident from the figure, emission from the area of the LMOF

literature as compiled in Table S5, but the aqueous phase sensing of Pd2+ by LMOFs is scantly reported. As far as we know, there are only two reports available in the literature for the detection of Pd2+ by LMOFs. Xu et al. reported detection and encapsulation of Pd2+ into MOFs constituted by 2,5dithioalloxyterephthalic acid for colorimetric determination of Pd2+ concentrations up to the 0.5 ppm level.75 Another report by Konar et al. involves the detection of Pd2+ (0.03 ppm) below the permissible limits set by the WHO using a Zn(II)-based LMOF.76 Hence, the newly synthesized LMOFs are promising sensors for the selective detection of hazardous Fe3+/Pd2+ cations in aqueous solutions which may also contain interfering cations. To understand the nonlinear behavior of Stern−Volmer plots, fluorescence lifetime decay profiles of LMOFs 1 and 2 in the presence and absence of quenchers (Fe3+/Pd2+) were recorded. As shown in Figure 6c,d, lifetime decay of LMOFs in the presence and absence of quenchers remains almost the same, indicating the static quenching mechanism was applicable. As depicted in Figure 4d, both the ligands, ATA and L, used for construction of frameworks 1 and 2 possess unsaturated coordination sites, viz., Lewis basic site, −NH2 and amide functionality. These unsaturated coordination sites have susceptibility toward analyte metal ions, Pd2+ and Fe3+.76,90 It is reasoned that metal cations associated with available coordination sites do accept electrons in their vacant d orbitals, facilitating the donor−acceptor electron-transfer quenching mechanism.91 The selectivity of 1 and 2 for Pd2+ and Fe3+ can be attributed to their versatile coordination geometries supported by delicate supramolecular interactions such as hydrogen bonding and van der Waal forces realizing in the framework interiors. Even though both LMOFs are sensitive for Fe3+ and Pd2+, the promising LOD values toward Pd2+ of these smart materials have significance as a fluorosensor for Pd2+ detection in aqueous phase for which no relevant reports are available in the literature. Evidence of Interaction between LMOFs and Analytes. ICP analysis verified the physical incorporation of analytes in the host frameworks, whereas FTIR analysis provided the crucial information about the nature of interaction between host frameworks and anaytes. After sensing experiments, the materials 1 and 2 were washed with water to remove analyte solution adhered on the surface of MOF particles. Washed LMOF samples were digested using a standard protocol (S3) and subjected to ICP analysis which confirmed the presence of Cr, Fe, or Pd in addition to the metal employed for framework construction. LMOF 1 was found to contain 0.472 mg of Cr per 100 mg of Zn after sensing of chromate anion. Similarly, 0.296 mg of Cr, 17.866 mg of Fe, and 15.780 mg of Pd per 100 mg of Zn metal in the bulk of LMOF 1 was estimated after the sensing of dichromate, Fe3+, and Pd2+ ions, respectively. Corresponding values after the respective sensing experiments in the case of Cd LMOF, 2, were found to be 0.120 mg of Cr (for chromate), 0.239 mg of Cr (for dichromate), 6.644 mg of Fe (for Fe3+), and 4.495 mg Pd (for Pd2+) per 100 mg of Cd. These results apparently confirm that analytes interact with the frameworks 1 and 2. FTIR analysis of the LMOF samples recovered after sensing experiments by washing with water and then drying in ambient atmosphere offered valuable information about the nature of interactions between host frameworks and analytes (Figures S23−S25). In the case of chromate and dichromate anions, the appearance of bending vibrational modes, δ(O-Cr-O), at ca. 1315, 735, and 624 cm−1 in the FTIR

Figure 7. (a, b) Digital images of blank paper strip, LMOF 1 or 2 coated test paper strip and after dipped in aqueous solution of CrO42−, Cr2O72−, Fe3+, and Pd2+ (10−2 M). 10945

DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

Inorganic Chemistry

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coated paper strips that is in contact with CrO42−/Cr2O72− and Fe3+/Pd2+ was quenched significantly (became dark). It shows that LMOF 1 and 2 coated strips are a good probe for sensitive and selective detection of CrO42−/Cr2O72 anions and Fe3+/Pd2+ cations from different pools of cationic/anionic species which has relevance in monitoring the toxic industrial waste toward practical application. For examining the recyclable sensing ability of LMOFs 1 and 2, fluorescence experiments were repeatedly conducted using the materials recovered after the first set of experiments, followed by washing and drying. The recovered LMOFs showed no significant variation of the luminescence intensity over three recovery cycles in either case, indicating the good recyclability of 1 and 2 (Figure S26).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], sureshe123@rediffmail.com. ORCID

Bhavesh Parmar: 0000-0003-4263-7635 Yadagiri Rachuri: 0000-0003-2979-2876 Kamal Kumar Bisht: 0000-0003-0987-6765 Eringathodi Suresh: 0000-0002-1934-6832 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The registration number of this publication is PRIS 050/2017. Financial support from CSIR (BP), UGC (YR) and analytical support by AD&CIF of CSIR-CSMCRI is gratefully acknowledged. We thank Ms. Ridhdhi Laiya for PXRD, Mr. Parthrajsinh Sodha for TGA data, Ms. Megha Yadav for FTIR data, Mr. Viral Vakani for CHN analysis, Mr. Jayesh Chaudhari for FE-SEM images, and Mr. Rajesh Patidar for surface area, porosity and ICP analysis.

CONCLUSIONS In summary, thermal, chemical, and water-stable Zn(II)/ Cd(II)-based 3D LMOFs (1 and 2) were successfully synthesized by versatile synthetic approaches, viz., diffusion of precursor solutions, conventional reflux method, and green mechanochemical (grinding) reactions. Both robust LMOFs are characterized by various analytical methods including SXRD, and good chemical, thermal, stability has been established. Notably, aqueous dispersions of 1 and 2 exhibit a highly sensitive and selective fluorescence quenching effect in the presence of Cr(VI) oxyanions and Fe3+/Pd2+ cations. Competitive experiments clearly demonstrate that fluorescence quenching remains largely unaffected in the presence of other anions/cations. These selective quenching phenomena toward chromium oxyanions are attributed to the competitive absorption of excitation wavelength energy between CrO42−/ Cr2O72− ions and LMOFs. Selective detection of Fe3+/Pd2+ cations may be due to the binding (or weak) interaction between the functional groups such as −NH2 from ATA or the amide group from L of the LMOFs, which may probably diminish the energy-transfer efficiency. The present study provides insight for the design of MOF-based water stable fluorosensors for the selective and sensitive detection of hazardous anions as well as cations. LMOF-based paper strips have been developed for practical applications for lethal environmental pollutants that could offer sensitive and repetitive sensing function in aqueous medium which has relevance in environmental concerns and human health.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01130. Crystallography, crystallographic figures, characterization and stability study, FE-SEM images, FTIR, TGA, PXRD, adsorption isotherm, fluorescence spectra, UV−vis spectra, LOD calculations, ICP analysis, bond length, bond angle and H-bonding table, comparison table for Cr(VI) anions and Fe3+ detection (PDF) Accession Codes

CCDC 1538770 and 1538771 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. 10946

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DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949

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DOI: 10.1021/acs.inorgchem.7b01130 Inorg. Chem. 2017, 56, 10939−10949