Cd(II

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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 Bhavesh Parmar,†,‡ Yadagiri Rachuri,†,‡ Kamal Kumar Bisht,§ Ridhdhi Laiya,‡ 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: Isostructural Zn(II)/Cd(II) mixed ligand coordination polymers (CPs) {[M(IPA)(L)]}n (CP1 and CP2) built from isophthalic acid (H2IPA) and 3pyridylcarboxaldehyde nicotinoylhydrazone (L) were prepared using versatile synthetic routes: viz., diffusion of precursor solutions, conventional reflux methods, and green mechanochemical (grinding) reactions. Both robust CPs synthesized by different routes were characterized by various analytical methods, and their thermal and chemical stability as well as the phase purity was established. Crystallographic studies revealed that CP1 and CP2 are isostructural frameworks and feature a double-lined two-dimensional network composed of Zn2+/Cd2+ nodes connected through IPA and pillared by the Schiff base ligand L with a double-walled edge. The photoluminescent (PL) properties of CP1 and CP2 have been exploited as dual detection fluorosensors for hexavalent chromate anions (CrO42−/Cr2O72−) and 2,4,6-trinitrophenol (TNP) because it was observed that the emission intensity of aqueous suspensions of CPs selectively quenches by chromate anions or TNP among large pools of different anions or nitro compounds, respectively. Competitive experiments in the presence of interfering anions/other nitro compounds also revealed no major effect in the quenching efficiency, suggesting the selective detection of hexavalent chromate anions as well as TNP by the LCPs. The limits of detection by CP1 for CrO42−/Cr2O72− and TNP are 4 ppm/4 ppm and 28 ppb, respectively, whereas the limits of detection by CP2 for the same analytes are 1 ppm/1 ppm and 14 ppb, respectively. A probable mechanism for the quenching phenomena is also discussed.

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has been well documented in the literature.17−34 Luminescent ability can be improved by the introduction of electron-rich aromatic ligands, because sensing by quenching is generally realized by the electron transfer of the photoexcited electrons from the MOFs to the electron-deficient analytes. Development of user-friendly and cost-effective detection methods for toxic chemicals, cations, anions, explosives, etc. by deploying new materials including LCPs is important in view of environmental and safety considerations. Due to industrialization and modern living, water contamination and environmental problems have become major issues and large volumes of wastewater with high concentrations of heavy metal ions/anions and various organic pollutants have been discharged. Because of its increasing utilization in various industries such as leather tanning, paint making, and steel manufacturing, CrO42−/Cr2O72− is one of the simplest environmental nonbiodegradable pollutants. Hexava-

INTRODUCTION Significant attention has been given to coordination polymers (CPs) or metal−organic frameworks (MOFs) with different dimensionalities in recent years due to their application in the area of gas storage/separation, catalysis, sensing, and so forth.1−8 Extensive efforts have been devoted to MOFs toward chemical-sensing applications, owing to their porosity and functionality, which allows sustenance of supramolecular interactions between the selective guest analytes and the MOF host, translating into detectable luminescent responses. The luminescent properties of CPs mainly depend on the structure and other factors such as choice of the metal centers/ ligands, porosity and features of the pore surfaces, supramolecular interactions such as hydrogen bonding, π−π stacking interactions, and coordination bonds between the guest species and the CP.9−16 Vapor- and solution-phase detection of hazardous chemicals as well as sensing of cations, anions, and small molecules by a variety of luminescent coordination polymers (LCPs) involving lanthanide cations/d10 metal ions © XXXX American Chemical Society

Received: November 21, 2016

A

DOI: 10.1021/acs.inorgchem.6b02810 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

reference TMS. TGA analysis was carried out using a Mettler Toledo Star SW 8.10 instrument. TG analysis was performed in a nitrogen environment while the heating rate was ramped from room temperature to 600 °C at 10 °C/min. Powder X-ray diffraction (PXRD) and variable-temperature XRD (VT-XRD) data were collected using a PANalytical Empyrean (PIXcel 3D detector) system with Cu Kα radiation. Single-crystal structures were determined using a Bruker SMART APEX (CCD) diffractometer. The 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 were recorded at room temperature utilizing a Fluorolog Horiba Jobin Yvon spectrophotometer. The PL images were captured using an Olympus BX53 fluorescence microscope. Synthesis and General Characterizations. Preparation of Stock Ligand Solution used for CP Synthesis. H2IPA (isophthalic acid; 415 mg, 2.5 mmol) was added to KOH (280 mg, 5 mmol) in 20 mL of water with constant stirring to make a clear solution. L (282 mg, 1.25 mmol) was dissolved in 20 mL of methanol separately. Both solutions were mixed together by constant stirring for 15 min and diluted to 70 mL using a methanol/water (1/1) mixture, filtered, and used as a stock ligand solution. Synthesis of {[Zn(IPA)(L)]}n (CP1). A 3 mL portion of the ligand solution was carefully layered above 3 mL of a Zn(ClO4)2·6H2O (372 mg in 15 mL of water) solution with 8 mL of buffer (methanol/water 1/1). The narrow test tube was covered and allowed to stand for slow diffusion of reactants at room temperature, which afforded colorless crystals within 1 week (yield ∼75%). Anal. Calcd for {[Zn(IPA)(L)]}n: C, 52.71; H, 3.10; N, 12.29. Found: C, 52.23; H, 3.23; N, 13.36. IR (cm−1, KBr): 3432 (br), 3207 (m), 3032 (s), 1693 (s), 1611 (s), 1544 (s), 1394 (s), 1278 (s), 1126 (m), 1030 (m), 963 (w), 919 (w), 832 (w), 696 (s), 533 (w), 435 (w). Synthesis of {[Cd(IPA)(L)]}n (CP2). The same procedure as for CP1 was followed but the metal solution was replaced with 3 mL of a Cd(ClO4)2·6H2O solution (312 mg in 15 mL of water). The narrow test tube was covered and allowed to stand for slow diffusion of reactants at room temperature, which afforded yellowish crystals within 1 week (yield ∼77%). Anal. Calcd for {[Cd(IPA)(L)]}n: C, 47.78; H, 2.81; N, 11.14. Found: C, 47.16; H, 3.07; N, 9.24. IR (cm−1, KBr): 3434 (br), 3205 (m), 3032 (s), 1694 (s), 1603 (s), 1555 (s), 1385 (s), 1278 (s), 1196 (w), 1126 (m), 1029 (w), 963 (w), 918 (w), 807 (w), 722 (s), 513 (w), 427 (w). Conventional (Reflux) Synthesis. Bulk powders of CP1 or CP2 were synthesized via refluxing at ca. 110 °C 1 mmol of Zn(NO3)2· 6H2O (for CP1) or Cd(NO3)2·4H2O (for CP2), 1 mmol of H2IPA, 2 mmol of KOH, and 1 mmol of L in 40 mL of methanol/water (1/1) solvent in a 100 mL round-bottom flask for 6 h. The resulting precipitates were filtered, washed with methanol/water (1/1) followed by acetone, and then dried at 100 °C in an oven (yield ∼85% for CP1 and ∼88% for CP2). Data for CP1 are as follows. Anal. Calcd: C, 52.71; H, 3.10; N, 12.29. Found: C, 51.18; H, 3.32; N, 13.09. IR (cm−1, KBr): 3438 (br), 3210 (m), 3036 (s), 1690 (s), 1616 (s), 1548 (s), 1391 (s), 1273 (s), 1123 (m), 1032 (m), 965 (w), 922 (w), 836 (w), 695 (s), 531 (w), 432 (w). Data for CP2 are as follows. Anal. Calcd: C, 47.78; H, 2.81; N, 11.14. Found: C, 47.23; H, 3.15; N, 11.29. IR (cm−1, KBr): 3436 (br), 3209 (m), 3029 (s), 1697 (s), 1610 (s), 1549 (s), 1388 (s), 1273 (s), 1195 (w), 1124 (m), 1032 (w), 966 (w), 920 (w), 810 (w), 720 (s), 511 (w), 424 (w). Mechanochemical (Grinding) Synthesis. CP1 and CP2 were synthesized in bulk via manually grinding a mixture of 1 mmol of Zn(OAc)2·2H2O (for CP1G) or Cd(OAc)2·2H2O (for CP2G), 1 mmol of H2IPA, and 1 mmol of L in a mortar and pestle for 25 min. The resulting solids were washed with methanol/water (1/1, 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 (yield ∼82% for CP1G and ∼85% for CP2G). Data for CP1G are as follows. Anal. Calcd: C, 52.71; H, 3.10; N, 12.29. Found: C, 51.65; H, 3.39; N, 12.33. IR (cm−1, KBr): 3428 (br), 3202 (w), 3037 (m), 1694 (m), 1612 (s),

lent chromium is classified as a human carcinogen, and hence detection and removal of CrO42−/Cr2O72− in wastewater discharge and drinking water distribution is indispensable.35−41 It has been demonstrated that MOFs are effective adsorbents for the removal of hazardous pollutants including chromate anions in the aqueous phase.42−48 Explosive components in landmines and commercial explosives are nitroaromatics (NACs) such as, TNT, TNB, DNB, DNT, TNP, etc. Both aromatic and aliphatic nitro compounds are hazardous and/or explosive in nature. TNP is commonly used in leather, dyes, fireworks, and the glass industry, and exposure to TNP vapor can lead to adverse health effects. Improper disposal of these organic compounds can cause environmental contamination in soils as well as in aquatic systems.49−57 Precise, efficient, selective detection and sensing of hazardous pollutants such as hexavalent chromium (CrO42−/Cr2O72−) and TNP in industrial waste, soil, and groundwater are major challenges. Fluorescence-based sensing methods have attracted great attention in recent times due to the good sensitivity/selectivity, quick response time, portability, and compatibility in both solid and liquid phases. LCPs are promising candidates for the detection of trace amounts of hazardous ions and molecules due to their good emission properties, porosities, and viable supramolecular interactions between the host frameworks and the target analytes. Some pioneering work in the area of LCP-based sensing materials involved detection of hexavalent chromium (CrO42−/Cr2O72−) and NACs in the vapor phase and nonaqueous phase.58−74 For selective detection of hexavalent chromium or nitro explosives in soil and groundwater, it is desirable for the LCP probes to work in aqueous media. LCPbased probes for aqueous-phase detection of anions and explosives are scantly reported in the literature, and this may be due to their poor water stability, which limits their applications to organic solvents only.75−92 Herein, we report two new water-stable Cd(II)/Zn(II) LCPs, {[Zn(IPA)(L)]}n (CP1) and {[Cd(IPA)(L)]}n (CP2), based on the less investigated N-donor ligand 3-pyridylcarboxaldehyde nicotinoylhydrazone (L) and isophthalic acid (H2IPA). These LCPs are synthesized by diffusion of precursor solutions, reflux, and mechanochemical routes and possess isostructural two-dimensional structures, as established by single-crystal analysis. Remarkably, photoluminescence (PL) properties of both of these d10 LCPs have been exploited as a dual detection tool for the selective sensing of hexavalent chromate anions (CrO42−/Cr2O72−) and TNP in the aqueous phase containing a variety of competing anions or nitro organics, respectively. This work demonstrates versatile syntheses of highly water stable mixed ligand 2D LCPs and their application to the selective, recyclable detection of lethal environmental pollutants: namely, CrO42−/Cr2O72− and TNP in aqueous media.

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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 detailed synthetic procedure of ligand L and crystallographic data are included in the Supporting Information. 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 Perkin−Elmer GX FTIR spectrometer. For each IR spectrum 10 scans were recorded at 4 cm−1 resolution. The 1H NMR spectrum for the ligand L was recorded on a Bruker AX 500 spectrometer (500 MHz) at a temperature of 25 °C and was calibrated with respect to the internal B


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Inorganic Chemistry 1546 (s), 1395 (s), 1279 (m), 1148 (m), 1036 (w), 961 (w), 922 (w), 832 (w), 751 (m), 696 (m), 532 (w). Data for CP2G are as follows. Anal. Calcd: C, 47.78; H, 2.81; N, 11.14. Found: C, 47.13; H, 3.08; N, 11.27. IR (cm−1, KBr): 3430 (br), 3206 (w), 3040 (w), 1694 (m), 1602 (s), 1548 (s), 1386 (s), 1280 (m), 1127 (w), 1085 (w), 1044 (w), 960 (w), 834 (w), 723 (w), 696 (w), 510 (w). Fluorescence Study. For anion sensing experiments, 3 mg of CP1/ CP2 was weighed and placed in a cuvette with a path length of 1 cm containing 3 mL of aqueous solutions of anions with stirring. For nitroaromatic sensing experiments, 2 mg of CP1 or CP2 was weighed and placed in a cuvette of path length of 1 cm containing 2 mL of water with stirring. The fluorescence spectra in the 330−540 nm range upon excitation at 295 nm were measured in situ after gradual addition of freshly prepared aqueous nitroanalyte solutions (2 mM), and the fluorescence intensity was monitored at ca. 417 nm (CP1) and ca. 430 nm (CP2). The solution was stirred at a constant rate in the fluorescence instrument with a stirring attachment during the whole experiment to maintain the homogeneity of the solution. For sensing experiments, standard aqueous solutions containing potassium salts of F−, Cl−, Br−, I−, SCN−, NO2−, NO3−, SO42−, ClO4−, IO3−, MoO42−, CrO42−, and Cr2O72− at the same concentration (10 mM) were prepared. In the case of nitro analytes standard solutions (2 mM) of 2,4,6-trinitrophenol (TNP), 2,4-dinitrophenol (2,4-DNP), 4-nitrophenol (4-NP), 2-nitrophenol (2-NP), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 4-nitrotoluene (4-NT), 3-nitrotoluene (3-NT), 2-nitrotoluene (2-NT), nitrobenzene (NB), nitromethane (NM), and 2,3-dimethyl-2,3-dinitrobutane (DMDNB) were used. All titrations were carried out in triplicate to establish the consistency of the results. The fluorescence efficiency was calculated by [(I0 − I)/I0] × 100%, where I0 and I are the fluorescence intensities before and after addition of the analyte.

Figure 1. (a) 1D chain of [Cd2(IPA)2]n comprised of metal nodes and isophthalate units. (b) Double pillaring of [Cd2(IPA)2]n chains with Schiff base linker (L) resulting in the formation of the 2D network of CP2. (c) μ3 and μ2 coordination modes of IPA and L, respectively, and simplified topology for 2D nets.

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RESULTS AND DISCUSSION Crystal Structure. Crystallographic studies revealed that CP1 and CP2 are isostructural and feature 2D frameworks composed of Zn2+/Cd2+ nodes connected through IPA and pillared by the Schiff base ligand L. The asymmetric unit of both LCPs consists of a crystallographically independent M2+ and one molecule each of IPA and L, respectively (Figure S1 in the Supporting Information). The distorted-octahedral geometry around M2+ with an O4N2 binding set is provided by four O atoms from carboxylates of two IPA moieties, generating a double-chain motif (Figure 1a) which is axially double pillared by terminal pyridyl nitrogen atoms from different Schiff base ligands (Figure 1b). Thus, IPA and M2+ forms a planar double chain which is constituted by [M2(IPA)2] secondary building units (SBUs). M2+ centers within the SBU are separated by 4.21 and 4.16 Å in CP1 and CP2, respectively. As depicted in Figure 1a, M2+ nodes are bridged by symmetrically disposed IPA moieties which are involved in a μ3-η1,η1,η1,η1 mode of coordination within the double chain. Consequently, carboxylate oxygen atoms O1 and O2 are involved in a syn-anti relationship and O3 and O4 have chelated coordination with the metal center (Figure 1c). Further, [M2(IPA)2] SBUs within the double chains are connected axially by the terminal nitrogen from the doubly lined Schiff base ligands. Accordingly, within the double-lined two-dimensional network, each edge length is double-walled and constituted by an N-donor ligand or IPA (Figure 1b). [M2(IPA)2]n double chains axially linked by the N-donor ligand L are separated by distances of 14.54 and 14.80 Å in CP1 and CP2, respectively. Terminal pyridyl rings of the N-donor Schiff base ligand within the double wall of the 2D net overlap each other and are involved in good π−π interactions with centroid···centroid (Cg1···Cg2) separations of 3.89 Å in CP1

and 3.82 Å in CP2 (Figure 2a and Figure S2a in the Supporting Information). The topology of the isostructural LCPs can be designated as a double-walled rectangular net (Figure 1c). The packing diagram disclosed that the 2D nets are oriented almost diagonal to the bc/ac plane (in CP1 and CP2) and the alternate

Figure 2. (a) π−π stacking interactions of the terminal pyridyl rings of the doubly pillared N-donor Schiff base ligand (L) in CP2 (hydrogen atoms are omitted for clarity). (b) Intermolecular H-bond interaction between the adjacent 2D sheets observed in CP2 (only H atoms involved in intermolecular H bonding are shown for clarity). C


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Inorganic Chemistry

Figure 3. (a, b) Comparison of PXRD data of CP1 and CP2 synthesized by different routes with simulated SXRD. (c, d) VT-PXRD for CP1 and CP2 in the temperature range 30−400 °C. (e, f) PXRD data of CP1 and CP2 dispersed for 1 week in water and in aqueous solutions of TNP, chromate, and dichromate.

powder XRD (VT-PXRD) data (Figure 3c,d and Figure S5 in the Supporting Information). The chemical stability of the LCPs in water, aqueous solutions of TNP and CrO42−/Cr2O72−, and different organic solvents was established by soaking the CPs in the respective media for 7 days and subsequent PXRD analysis (Figure 3e,f and Figure S6 in the Supporting Information). The water stability of CP1 and CP2 can be attributed to the robust double-pillared structure and various hydrogen-bonding interections between the layers. Gas adsorption studies show negligible surface area and porosity for CP1 and CP2 (Figure S7 in the Supporting Information), which may be due to the offset stacking of the adjacent 2D layers to cause effective hydrogen bonding and prevent the formation of through channels. The nonporous nature and tight coordination around the metal center of the LCPs inhibiting the accessibility of water molecules toward the metal nodes probably impart the hydrolytic stability of CP1 and CP2.93−95 Photoluminescence Properties. CPs with d10 transitionmetal ions are well-known for their photoluminescence (PL) properties. PL spectra of CP1 and CP2 were recorded in the solid state and in aqueous media. PL spectra in aqueous suspensions were acquired by dispersing 2 mg of the LCP in 2 mL of water with stirring in a fluorescence cuvette equipped with stirring mechanisms. The solid-state PL spectra of CP1 and CP2 exhibit emission at 460 and 458 nm, respectively, upon excitation by 295 nm radiation at room temperature (Figure S8 in the Supporting Information). In particular, water suspensions of CP1 and CP2 showed good emission at ca. 417 and 430 nm, respectively, upon excitation at 295 nm (Figure S9 in the Supporting Information). This prompted us to evaluate the suitability of both LCPs for aqueous-phase sensing applications. The blue shift of emission intensity in water (∼33−38 nm) in comparison to that in the solid state can be attributed to solvent effects.

2D nets are stacked offset to make effective and identical hydrogen-bonding interactions in the supramolecular assembly. Subsequently, the carboxylate oxygen O2/O4 in the respective compounds (CP1/CP2) is involved in bifurcated N−H···O/ C−H···O contacts with amide and methylidene hydrogen atoms H2C/H15 and, in the case of CP2, O3 has an added C− H···O contact with phenyl hydrogen H4 in connecting the offset 2D nets (Figure 2b and Figure S2b). Additional weak intramolecular C−H···O contacts are also observed in both LCPs. Details of all hydrogen-bonding interactions with their symmetry codes are provided in Table S3 in the Supporting Information. Characterization and Stability Study. LCPs CP1 and CP2 obtained via different routes were characterized by various analytical methods and found to have identical respective phases. Both LCPs have been synthesized in bulk quantities by neat grinding of precursors in the suitable stoichiometry. The phase purity of CP1 and CP2 samples synthesized by different methods has been confirmed by comparing the experimental PXRD with patterns simulated from single-crystal X-ray (SXRD) data (Figure 3a,b). FE-SEM images of CP1 and CP2 synthesized via diffusion, conventional, and mechanochemical routes revealed rectangular block-shaped and platelike tiny crystals (Figure S3 in the Supporting Information). CHN analysis of synthesized LCP samples suggests an elemental composition corroborating the empirical formula {[M(IPA)(L)]}n, where M = Zn(II), Cd(II). FTIR of CP1 and CP2 showed symmetric and antisymmetric νCO bands at ca. 1394, 1544 cm−1 and 1385, 1555 cm−1, respectively. The difference in antisymmetric and symmetric carbonyl stretching frequencies (Δν) for all samples of CP1 and CP2 was ca. 170, indicating the chelating bidentate coordination mode of the carboxylate moieties (Figure S4 in the Supporting Information). LCPs CP1 and CP2 showed thermal stability up to ∼325 °C by TGA analysis, which is in agreement with the variable-temperature D


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Inorganic Chemistry

Figure 4. (a, b) Fluorescence quenching for different anions by water suspensions of CP1 and CP2 (3.0 mg/3 mL). (c−f) Luminescence responses of CP1 and CP2 (2 mg dispersed in 2 mL of water) toward different concentrations of CrO42−/Cr2O72− (0−2.0 mM) in water. Conditions: λem ca. 417 nm for CP1 and ca. 430 nm for CP2; slit widths 14 mm.

Figure 5. (a, b) Stern−Volmer (SV) plots for CrO42−/Cr2O72− in the presence of water suspensions of CP1 and CP2 (2.0 mg/2 mL). (c, d) Interference study of CP1 and CP2 for CrO42−/Cr2O72− in the presence of different anions.

Chromate Anion Sensing. Ten millimolar aqueous solutions containing potassium salts of each of the anions F−, Cl−, Br−, I−, SCN−, NO2−, NO3−, SO42−, ClO4−, IO32−, MoO42−, CrO42−, and Cr2O72− were prepared, and dispersions of 3 mg of CP1/CP2 in 3 mL of each of the salt solutions were prepared and subjected to fluorescence emission recording with constant stirring. Interestingly, the presence of anions such as F−, Cl−, Br−, I−, SCN−, NO2−, NO3−, SO42−, ClO4−, IO32−, and MoO42− has a negligible effect on the PL intensity, while hexavalent chromate anions (CrO42− and Cr2O72−) significantly quenched the emission intensity of CP1/CP2 dispersions on excitation at 295 nm (Figure 4a,b and Figure S10 in the Supporting Information).).

To understand luminescence quenching in the presence of CrO42−/Cr2O72−, PL spectra of both CPs in the presence of incremental concentrations of chromate anions were studied. It was observed that the luminescence of CP1 and CP2 is gradually quenched with an increase in the concentration of CrO42−/Cr2O72− anions. As depicted in Figure 4c−f, the intensity of the aqueous dispersions of CP1 and CP2 declines sharply with an increase in CrO42−/Cr2O72− concentration from 0 to 2.0 mM. Furthermore, the quenching ability of CrO42−/Cr2O72− can be rationalized and calculated by the Stern−Volmer equation, I0/I = 1 + Ksv[A], where I0 and I are the fluorescence intensities before and after addition of CrO42−/Cr2O72− solutions, respectively, [A] is the molar E


Article

Inorganic Chemistry Table 1. Comparison among Various LCP/LMOF Sensors for Aqueous-Phase Cr(VI) Detection analyte (CrO42−/ Cr2O72−)

entry

material (LCP/LMOF)

1

{[Ln(μ3-ddpp)]·4H2O}n (Ln = Ce, Nd, Sm, Eu, Tb, Er) {[Zn2(TPOM) (NH2-BDC)2]·4H2O}n {[Zn2.5(cpbda) (OH)2]·solvent2}n {[Eu(Hpzbc)2(NO3)]·H2O}n [Y(BTC)(H2O)6]n:0.1Eu [Zn(2-NH2bdc) (bibp)]n {Cd3(L)(bipy)2·4DMA}n [Cd6(L)2(bib)2(DMA)4] [Cd3(L)(tib) (DMF)2] [Zn(btz)]n [Zn(ttz)H2O]n Ln3+@MIL-121 [Zn(IPA)(L)]n (CP1)

CrO42−

[Cd(IPA)(L)]n (CP2)

2 3 4 5 6 7

8 9 10

quenching constant Ksv (M−1)

CrO42−/Cr2O72− CrO42−/Cr2O72− Cr2O72− CrO42−/Cr2O72− Cr2O72− CrO42−/Cr2O72−

4.45 × 103/7.59 × 103

CrO42−/Cr2O72− CrO42−/Cr2O72− Cr2O72− CrO42−/Cr2O72−

3.19 2.35 4.34 1.00

CrO42−/Cr2O72−

1.30 × 103/2.91 × 103

1.18 × 103/4.52 × 103 6555070

concentration of CrO42−/Cr2O72−, and Ksv is the Stern−Volmer constant/quenching constant.58,83 On the basis of the experimental data, Stern−Volmer plots for CrO42−/Cr2O72− at lower concentrations (0−1 mM) showed linear correlation coefficients (R) 0.992/0.990 for CP1 and 0.997/0.990 for CP2 (Figure 5a,b). However, at higher concentrations of CrO42−/Cr2O72− the Stern−Volmer plots became nonlinear (Figure S11 in the Supporting Information). The quenching constant value Ksv was calculated to be Ksv = 1.00 × 103 M−1 for CrO42− and Ksv = 1.37 × 103 M−1 for Cr2O72− by CP1 and Ksv = 1.30 × 103 M−1 for CrO42− and Ksv = 2.91 × 103 M−1 for Cr2O72− by CP2, respectively, disclosing the quenching effect on the CP luminescence. The progress of quenching effects on LCPs at higher concentration by CrO42−/Cr2O72− solutions (0−400 μL; 10 mM) by incremental addition and the quenching percentage vs concentration are provided in Figures S11 and S12 in the Supporting Information. The limit of detection (LOD) for CrO42−/Cr2O72− was calculated using the equation LOD = 3σ/ m, where σ = standard deviation from five blank measurements for each LCP and m = slope of the linear curve plotted at lower concentration for LOD measurements.58,83 The LOD values for CrO42−/Cr2O72− were found to be 18.33 μM (3.56−4 ppm)/ 12.02 μM (3.53−4 ppm) for CP1 and 2.52 μM (0.48−1 ppm)/ 2.26 μM (0.66−1 ppm) for CP2, respectively (section S1 and Figure S13 in the Supporting Information). The Ksv and LOD values demonstrate both CP1 and CP2 are good fluorosensors for aqueous-phase detection of CrO42−/Cr2O72−. The sensing ability of CP1 and CP2 in the present investigation is comparable to that of previously reported excellent MOF sensors for detection of hexavalent chromate (Table 1). Interference by other anions in the detection of CrO42− and Cr2O72− ions was investigated by competitive experiments: i.e., luminescence measurement of mixed solutions containing CrO42−/Cr2O72− and another anion. The results indicated that the presence of other anions did not make any significant change in the sensing of CrO42− and Cr2O72− (Figure 5c,d). Since wastewater contains a variety of interfering ions, the present investigation of the newly developed LCPs is greatly promising and has significance in the selective detection of hazardous hexavalent chromium from industrial waste. The mechanism involved in the luminescent quenching of CPs by anions can be assigned to either collapse of the framework

× × × ×

103/4.23 × 103 103/2.19 × 103 103 103/1.37 × 103

LOD (μM)

4.8/3.9 22 0.03/0.04

media (aqueous/organic) NH3·H2O/NH4Cl buffered aq soln/DMF DMF DMF/DMA:H2O ethanol H2O H2O H2O

10/2 20/2 0.054 18.33/12.02

H2O H2O H2O H2O

2.52/2.26

H2O

ref 44 58 59 60 75 76 77

78 79 this work

structure or energy loss by collision between anions and the framework. The present investigation revealed that the concentrations of CrO42− and Cr2O72− ions cause a remarkable decrease in the luminescence intensity in comparison to other anions. As mentioned earlier, the stability of both LCPs was established by PXRD patterns, which remained well consistent with the simulated patterns after dispersing the LCPs in the respective analyte for 1 week. Hence, a quenching mechanism by collapse of the CP framework can be ruled out. The possibility of weak interactions between the analyte and the LCPs as well as competition for the excitation energy between the anions themselves and LCPs could change the luminescent signals. UV−vis spectra of aqueous solutions of K2CrO4 and K2Cr2O7 showed two broad absorption bands that range between 230 and 420 nm (274, 372 nm (K2CrO4); 257, 351 nm (K2Cr2O7)) (Figure S14 in the Supporting Information). The absorption range of the CrO42−/Cr2O72− covers the absorption bands of CP1 and CP2 (230−350 nm) as well as the excitation wavelength of CP1 and CP2 (295 nm) (Figure S15 in the Supporting Information). Further, upon excitation of CPs at 295 nm, CrO42−/Cr2O72− in the solution can significantly absorb the energy of the excitation, which in turn deters the UV/vis absorption of the target LCPs, resulting in a decrease or even full quenching of the luminescence intensities. However, in the case of the rest of the anions the UV/vis absorption peaks are far below the absorption bands of CP1 and CP2 as well as the excitation wavelength and hence no significant effect on fluorescence quenching is detected. It is also observed that there is an overlap between the absorption peaks of CrO42−/ Cr2O72− as well as emission peaks of the LCPs, indicating an electron transfer mechanism may also be operative, probably due to weak interactions between the LCP framework and hexavalent chromium analytes (Figure S16 in the Supporting Information). Reports on similar mechanisms for detection of CrO42−/Cr2O72− are available in the literature.66,76,77,96,97 TNP Detection. To explore the multifunctional applications of isostructural CP1 and CP2 as luminescent probes, we further studied the ability of both LCPs for the selective detection of TNP from a large pool of nitro compounds. In order to investigate their sensing ability, emission profiles of known concentrations of both aromatic and aliphatic and nitro compounds were studied upon gradual addition to water F


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Figure 6. (a, b) PL spectra of CP1 and CP2 (2 mg) dispersed in water (2 mL) upon addition of a TNP (2,4,6-trinitrophenol) solution in water (2 mM). (c, d) Fluorescence quenching efficiency of water suspensions of CP1 and CP2 (2.0 mg/2 mL) for different analytes. (e, f) Stern−Volmer (SV) plots for various nitro analytes in water suspensions of CP1 and CP2 (2.0 mg/2 mL). Conditions: λem ca. 417 nm for CP1 and ca. 430 nm for CP2; slit widths 14 mm.

Figure 7. (a, b) Interference study of CP1 and CP2 for TNP with different nitroanalytes. (c, d) Bar diagrams depicting the recyclability of CP1 and CP2 for luminescence quenching experiment with TNP, CrO42−, and Cr2O72− up to three cycles.

suspensions of both LCPs. In fact, fluorescence-quenching titrations were performed with incremental addition of different nitro analytes into a suspension of LCPs in aqueous media (2 mg/2 mL) with constant stirring. Thus, systematic fluorescence quenching titrations were studied relating the aromatic nitro compounds 2,4,6-trinitrophenol (TNP), 2,4-dinitrotoluene (2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), 4-nitrotoluene (4NT), 3-nitrotoluene (3-NT), 2-nitrotoluene (2-NT), and nitrobenzene (NB) as well as the aliphatic nitro compounds nitromethane (NM) and 2,3-dimethyl-2,3-dinitrobutane

(DMDNB) as analytes for the sensing experiments. Emission profiles of CP1 and CP2 excited at 295 nm in the fluorescence titration showed selective and significant quenching in the presence of TNP and relatively low/negligible quenching was observed for other nitro analytes, as depicted in Figure 6a,b and Figures S17 and S18 in the Supporting Information. Although the order of the quenching efficiency of analytes differs on the basis of CP1 or CP2, TNP showed the highest quenching in both cases. The order of the quenching efficiency was found to be TNP > 2,4-DNP > NP in the case of nitrophenols, which is G


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Figure 8. Digital microscopic images under white light/UV filter for both CPs and under UV filter after addition of CrO42−/Cr2O72−/TNP.

not sensitive to steric factors. However, the relative orientation of the donor and acceptor dipoles is crucial for RET to be operative.99−101 We propose the same has been achieved probably due to the strong hydrogen-bonding interactions between the phenolic group of TNP and carbonyl hydrazone moiety of L on the crystal surface. Interference by other nitro anlytes in the detection of TNP was also studied by competitive experiments, i.e. luminescence measurement of mixed solutions, containing TNP in the presence of another nitro analyte. As depicted in Figure 7a,b, the results indicated that the presence of other nitro analytes did not cause any significant change in the sensing efficiency of TNP by both CP1 and CP2. For a check of the recyclability of CP1 and CP2, after every fluorescence titration experiment the material was recovered by centrifugation, followed by washing several times with water, methanol, and acetone. The recovered LCP showed no significant variation of the luminescence intensity over three sensing-recovery cycles in either case, indicating the recyclability of CP1 and CP2 (Figure 7c,d). This was also supported by stability PXRD measurements of LCPs which conserved the crystallinity and structural integrity after soaking in water, 2 mM TNP, and 10 mM CrO42−/Cr2O72− solutions up to 7 days (Figure 3e,f). The digital images of CP1 and CP2 under white light and under a UV filter in fluorescence microscopy were captured for blank aqueous solutions of CPs and solutions after addition of CrO42−/Cr2O72−/2,4,6-trinitrophenol (TNP). These fluorescence microscopic images clearly show the quenching of emission intensity after addition of CrO42−/Cr2O72−/TNP (Figure 8). Moreover, an obvious luminescence quenching of CP1 and CP2 as a dual detection probe for the selective detection of CrO42−/Cr2O72− from different groups of anions as well as toward TNP from different pools of nitro aromatic compounds has been established. The green mechanochemical synthetic approach and high thermal as well as chemical stability in water, aqueous solutions, and organic solvents provide a promising footing for both LCPs CP1 and CP2 in future applications as dual detection fluorosensors for the pollutant anions CrO42−/ Cr2O72− as well as the hazardous nitro aromatic compound trinitrophenol (TNP) from industrial wastewater.

in agreement with the order of acidity of these analytes (Figure S19 in the Supporting Information). The best quenching efficiencies observed for TNP were calculated to be 42, 62, 71, 77, and 81% for CP1 and 30, 49, 62, 69, and 75% for CP2 upon incremental addition (Figure 6a, b). Thus, addition of 200 μL (2 mM) of TNP solution resulted in fast and selective fluorescence quenching as high as 75 and 81% for CP1 and CP2, respectively, and addition of equivalent amounts of the other mentioned nitro analytes resulted in relatively low or negligible quenching, as depicted in Figure 6c,d and Table S2 in the Supporting Information. The fluorescence quenching efficiency for TNP was further analyzed using the Stern−Volmer (SV) equation:I0/I = Ksv[A] + 1, where I0 is the initial fluorescence intensity without analyte, I represents the fluorescence intensity with added analyte of the molar concentration [A], and Ksv is the quenching constant (M−1). The nonlinear nature of the SV plot of TNP may be attributed to many reasons such as self-absorption and energy transfer processes between TNP and the LMOFs (Figure 6e,f and Figure S20 in the Supporting Information). The quenching constants (Ksv) of CP1 and CP2 for TNP were calculated to be 2.16 × 104 and 1.52 × 104 M−1, respectively, which are comparable to the highest values reported by Ghosh et al. for selective aqueous-phase detection of TNP using the LMOF [Zn4(DMF)(urotropine)2(L4)4].84 Moreover, the LOD in the case of the present LCPs is superior to that observed for the LMOF [Zn4(DMF)(urotropine)2(L4)4]. We have also tabulated some of the recent fluorescent sensors for detection of TNP in the aqueous phase (Table S5 in the Supporting Information). The respective limits of detection (LOD) for TNP by CP1/CP2 dispersed in water were found to be 28 ppb (0.12 μM) and 14 ppb (0.06 μM), which clearly demonstrate the excellent potential of these materials as highly sensitive sensors for TNP in aqueous media (section S2 and Figure S21 in the Supporting Information). To explore the quenching mechanism, normalized absorption spectra of various analytes and emission spectra of water suspensions of LCPs were analyzed, which showed good spectral overlap between the absorption band of TNP and emission profiles of CP1 and CP2, while almost no overlap was observed for other nitro analytes (Figure S22 in the Supporting Information). The spectral overlap advocates for a resonance energy transfer (RET) mechanism, which is a long-range process usually yielding considerably high fluorescence-quenching efficiencies and improved sensitivity.98 Quenching results obtained with the nonporous LCPs CP1 and CP2 can be substantiated on the basis of RET being a long-range energy transfer process that is

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CONCLUSIONS In summary, isostructural Zn(II)/Cd(II)-based water-stable two-dimensional LCPs (CP1 and CP2) were successfully synthesized by versatile synthetic approaches: viz., diffusion of precursor solutions, conventional reflux methods, and green H


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mechanochemical (grinding) reactions. Both robust LCPs have been characterized by various analytical methods and showed high chemical and thermal stability. Importantly, aqueous dispersions of CP1 and CP2 exhibit a highly sensitive and selective fluorescence quenching effect in the presence of CrO42−/Cr2O72− or TNP; however, their fluorescence remains largely unaffected in the presence of numerous competing anions or nitro organics. It is significant that these LCPs offer sensitive sensing results even after several times of recycling. These selective quenching phenomena are attributed to the competitive absorption of excitation wavelength energy between CrO42−/Cr2O72− ions and LCPs and resonance energy transfer (RET) between TNP and LCPs. The present study provides insight into the design of CP-based water-stable fluorosensors for the selective and sensitive detection of hazardous anions as well as explosive molecules. This study also opens avenues for designing LCP-based sensors for lethal environmental pollutants that could offer sensitive and repetitive sensing functions under realistic conditions such as aqueous medium and the presence of large number of competing analytes.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

information files. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.6b02810. Synthesis of ligands, crystallography, ORTEP diagrams, crystallographic figures, FE-SEM images, FTIR, TGA, PXRD, adsorption isotherm, fluorescence spectra, UV− vis spectra, LOD calculations, bond lengths, bond angles, and H-bonding data, and a comparison table for TNP detection (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for E.S.: [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 Ridhdhi Laiya: 0000-0001-9468-6485 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 CSIR-CSMCRI159/2016. Financial support from the CSIR (B.P.), the UGC (Y.R.), and the CSIR-Network project (CSC-0134) and analytical support by the AD&CIF of CSIR-CSMCRI is gratefully acknowledged. We thank Mrs. Monika Gupta for TGA data, Ms. Megha Yadav for FTIR data, Mr. Viral Vakani for CHN analysis, Mr. Jayesh Chaudhari for FE-SEM images, Mr. Senthil Kumar for surface area and porosity analysis, and Mr. Ishan H. Raval for fluorescence microscopic images. I


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Inorganic Chemistry

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