An Ultrastable Luminescent MetalâOrganic Framework for Selective

Article Cite This: Cryst. Growth Des. 2018, 18, 431−440

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An Ultrastable Luminescent Metal−Organic Framework for Selective Sensing of Nitroaromatic Compounds and Nitroimidazole-Based Drug Molecules Bao-Xia Dong,* Yong-Mei Pan, Wen-Long Liu,* and Yun-Lei Teng School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, P. R. China S Supporting Information *

ABSTRACT: A chemically stable and thermally stable luminescent Cd(II)-based metal−organic framework (MOF), [Cd3(DBPT)2(H2O)4]·5H2O (1), featuring an open Lewis basic triazolyl active site in the host, was successfully assembled by using the multifunctional ligand of 3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1-H-1,2,4-triazole (H3DBPT) to bridge hexanuclear {Cd6} clusters. The host material exhibits ligand-based photoluminescence and solvent-dependent fluorescent intensities, which could selectively detect nitroaromatic compounds (NACs) of 2,4,6-trinitrophenol (TNP) and 4-nitroaniline (4NA). The most striking property of 1 is the remarkable sensitivity and selectivity toward TNP and 4-NA even in the presence of other NACs of nitrobenzene (NB), which can be attributed to a photoinduced electron transfer mechanism and resonance energy transfer mechanism. Their photoluminescence quenching could be detected at very low concentrations of 1.14 and 0.70 ppm, respectively, and the maximum quenching efficiencies were found to be 98% and 95%, respectively, at 0.08 mM. Significantly, compound 1 also exhibits excellent sensing performance toward nitroimidazole-based drug molecules of ornidazole (ONZ), metronidazoles (MNZ), dimetridazole (DMZ), and 2-methyl-5-nitroimidazole (2-M-5-MZ), which represents one of the rare MOFs-based fluorescent sensors for simultaneous selective detecting drug molecules and NACs. Their photoluminescence quenching efficiencies increase drastically with the analyte amount even in the low concentration range (95%). The remarkable chemical stability and the unusual sensing performance make this Cd-based MOF a promising multiresponsive sensory material for chemical sensing of NACs and nitroimidazole-based drugs.

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INTRODUCTION Metronidazoles (MNZ), ornidazole (ONZ), dimetridazole (DMZ), and 2-methyl-5-nitroimidazole (2-M-5-MZ) are all nitroimidazole-based drugs. MNZ is used to treat diseases caused by bacterium and anaerobic protozoan infections.1,2 ONZ is active against protozoa and anaerobic bacteria, and is used like MNZ in the prevention and treatment of susceptible protozoal infections and anaerobic bacterial infections.3 Long-term use of MNZ or ONZ leads to drug resistance, which poses health hazards to consumers. When MNZ accumulates in human body, some toxic effects including seizures and peripheral neuropathy can occur, which is harmful to the health of both humans and wildlife.2 DMZ is used in veterinary medicine to treat histomoniasis and coccidiosis in poultry and game birds.4 The use of DMZ in veterinary practice is strictly regulated in many © 2017 American Chemical Society

countries because of DMZ and its metabolites are suspected of being human carcinogens and mutagens. The residue of DMZ is often found in food producing animals or in products intended for human consumption. Therefore, the quantitative determination and sensing of these antibiotics are important for pharmacokinetic studies and therapeutic drug monitoring. Recently, several methods and techniques have been reported for determination of MNZ, which include polarography,5 gas chromatography,6 supercritical fluid chromatography,7 highperformance liquid chromatography (HPLC),8 thin-layer chromatography (TLC),9 electrochemical sensing,10−12 specReceived: October 12, 2017 Revised: November 26, 2017 Published: November 27, 2017 431

DOI: 10.1021/acs.cgd.7b01430 Cryst. Growth Des. 2018, 18, 431−440

Crystal Growth & Design

Article

trophotometry,13−16 etc. Among them, much attention has been paid to electrochemical sensors and spectrophotometry, due to their excellent recognition capabilities, high sensitivity and selectivity, fast response, and real-time detection nature. However, for the electrochemical approach, people are always obsessed by the drawbacks of poor reproducibility, stability, and vulnerability, which are attributed to outside condition interfering (such as the pH value and scan rate) or surface poisoning (such as the absorbed intermediates). The fluorescence sensor techniques can overcome these problems and emerged as the most suitable one for biochemical analysis. Many conventional organic fluorophores, for example, the dye molecules of pyrene, pyrenebutyric acid, 3-amino-9-ethylcarbazole (AEC), etc., have been developed for using as fluorescence probe.16 A fluorescent membrane for MNZ sensing were prepared by covalent immobilization of a pyrenebutyric acid derivative of 2-(methacryloyloxy)ethyl-4-(1-pyrenyl)butanote (MPB) with 2-hydroxylenthlmethacrylate (HEMA) on the activated glass surface by thermal initiation, which responds linearly in the concentration range of 1.23−35.48 mg L−1 in aqueous solution with a detection limit of 0.36 mg L−1.13 Recently, graphene quantum dots embedded silica molecular imprinted polymer was developed, which exhibits a linear response in the range of 0.2−15 μM, and the detection limit of 0.15 μM.14 BSA-protected gold nanoclusters (AuNCs@BSA) with near-infrared (NIR) fluorescence were used in the determination of MTZ and other nitroimidazole derivatives, which exhibits a much lower detection limit of 0.01 μM, and wider range of 0.1−10000 μM for MTZ detection.15 Metal−organic frameworks (MOFs), as an outstanding subclass of organic−inorganic hybrid solids with infinite, uniform, and porous framework structures, have emerged as promising materials for molecular recognition,7−18 gas storage,19−23 heterogeneous catalysis,24 electrocatalysis,25−28 photoluminescence,29−31 adsorption of volatile organic compounds,32−34 and drug delivery35 owing to their designable and adjustable structures and properties. Among the various potential applications of MOFs, the fluorescence sensing of chemicals is a recently emerging research area, which is of great promise due to their operability, rapid response, high selectivity, sensitivity, and long-term stability.36−38 Highly sensitive and efficient sensory materials for the sensing of trace amounts of nitroaromatic compounds (NACs), especially for TNP (2,4,6-trinitrophenol) and 4-NA (4-nitroaniline), are in high demand because of their explosive nature and high toxicity.39−41 In this subject, MOF materials are of considerable interest and have been widely explored by researchers. However, to the best of our knowledge, the relevant studies on MOFs-based fluorescent probes for antibiotics assay are still scarce up to now.42−45 According to the analysis of recent work, the fluorescence sensing ability of MOFs toward NACs is based on the interaction between their backbones and the analyte molecules.46 In the sensing process, the analyte molecule is first adsorbed by the MOF, and then the excited state charge or energy transfer between the analyte and the MOF backbone causes the quenching (turn off) or enhancement (turn on) of the emission. Therefore, the fluorescent behavior of a MOF-based sensor is highly dependent on the organic ligands and metal ion/cluster. The organic ligands in MOFs often contain aromatic or conjugated π moieties, which not only endow the MOFs with photoluminescence properties, but also make the frontier orbitals of the compounds susceptible to the interference of analytes. Especially, the MOF-based sensor constructed with π-

electron-rich ligands usually show a sensitive response to electron deficient analytes, such as NACs, due to the redox quenching mechanism.47−59 The accessible Lewis-base sites in the host of MOFs also display excellent monofunctional sensing of single organic molecules or metal cations.56,60,61 For this reason, we are especially interested in ligand containing multiple aromatic rings and open Lewis basic triazolyl active site, i.e., H3DBPT (3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1-H1,2,4-triazole, Figure S1), as such a π-electron rich ligand would achieve high sensitivity and high selectivity for sensing electron deficient analytes or detecting metal ions. In this work, the synthesis and structure of a luminescent Cd(II)-based MOF, [Cd3(DBPT)2(H2O)4]·5H2O (1) are described, which exhibits ultrahigh chemical stability and thermal stability. It features ligand-based photoluminescence (PL) and solvent-dependent fluorescent intensities. Its fluorescence in suspension could be sensitively and selectively quenched by metal ions of Zr4+ and Fe3+, and NACs of TNP and 4-NA. Significantly, compound 1 also exhibits remarkable sensing performance toward nitroimidazole-based drug molecules of ONZ, MNZ, DMZ, and 2-M5-MZ, which represents one of the rare MOFs-based fluorescent sensors for simultaneous selective detecting antibiotics and NACs. Moreover, aiming for TNP, 4-NA, and drug molecules, the PL quenching titrations were performed with the incremental addition of analytes to suspensions of 1. The PL quenching efficiencies increase drastically with the analyte amount even in the low concentration range (95%). When the concentration of the analyte reaches 0.08 mM, it was found to quench 98%, 95%, 95%, 85%, 76%, and 66%, respectively, of the emission of 1 by TNP, ONZ, 4-NA, MNZ, DMZ, and 2-M-5-MZ, respectively. These results indicate that compound 1 is a promising multiresponsive sensory material for chemical sensing of NACs and nitroimidazole-based drugs.

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EXPERIMENTAL SECTION

Materials and General Methods. All chemicals purchased were of reagent grade and were used as received. FT-IR spectrum (KBr pellets) was recorded in the range of 4000−400 cm−1 on a BRUKER TENSOR 27 Fourier-transform infrared spectrometer. TGA/DSC measurement (TGA, thermal gravimetric analysis; DSC, differential scanning calorimetry) was performed on a TG/DSC model STA 449 F3 Netzsch instrument at a ramp rate of 10 °C min−1, by heating the sample under nitrogen and air atmosphere. Powder XRD data were collected with CuKα (λ = 1.5406 Å) radiation on a Bruker-AXS D8 Advance X-ray diffractometer in the angular range 2θ = 5−50° at 296 K. Each pattern was recorded with a 2 s per step scan. The fluorescence spectra were recorded using a PerkinElmer LS50B luminescence spectrophotometer. Both the excitation and emission pass width were 5.0 nm. The UV absorption characteristics were measured on a Shimadzu UV-2550 UV− vis spectrophotometer. Synthesis of [Cd3(DBPT)2(H2O)4]·5H2O (1). A mixture of Cd(NO3)2· 4H2O (0.077 g, 0.25 mmol) and H3DBPT (0.0177 g, 0.05 mmol) was dissolved in 5.0 mL of H2O/ethanol/NaOH (2/2/1, v:v, and then the solution was stirred at room temperature for 0.5 h. The mixture was sealed in the Teflon-lined autoclave and heated at 130 °C for 3 days. After slow cooling to room temperature, colorless block-like crystals 1 were collected (yield: 85% based on Cd). Elemental analysis (%) calculated for C34H34Cd3N6O21: C, 78.41; H, 2.83; N, 7.00; Cd, 28.10; Found: C, 78.22; H, 2.82; N, 7.05; Cd, 27.95. Prominent FT-IR peaks for 1 (KBr Pellet, cm−1): 3200(m), 1623(m), 1541(s), 1367(s), 1284(m), 977(w), 865(w), 751(m). X-ray Crystallographic Study. Single-crystal X-ray diffraction analysis data were collected on a Bruker Smart Apex II CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 293 K. The absorption correction was performed by using the SADABS program. 432



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The structure was solved by direct methods and refined on F2 by fullmatrix least-squares methods using the SHELXTL package.62 Anisotropic thermal parameters were used to refine all Cd, C, N, and O atoms. The hydrogen atoms attached to carbon positions were placed in geometrically calculated positions. The crystal data and structure refinement result of compound 1 is summarized in Table 1. Selected

emission spectrum for each suspension was recorded immediately upon excitation at 283 nm. Sensing of Metal Ions. Finely ground samples of 1 (3.0 mg) were first dispersed in methanol, forming a suspension solution at a concentration of 0.05 mM by an ultrasound method for 2 h. Then 5.0 μL (0.10 M) of aqueous solution of M(NO3)x (M = Na+, Cs+, K+, Cu2+, Zn2+, Nd2+, Cd2+, Ni2+, Mn2+, Pb2+, Mg2+, Co2+, Ba2+, Fe2+, Gd3+, Ce3+, Cr3+, Sm3+, Sr2+, Ca2+, In3+, Al3+, Ag+, Fe3+, or Zr4+) was added into the above emulsion (2.0 mL) and sonicated for about 2 min. The photoluminescence response for the obtained emulsion containing different metal ions (0.25 mM) was recorded immediately upon excitation at 283 nm. Sensing of Aromatic Compounds and Drug Molecules. The same procedures as those for sensing the metal ions were followed, except that the aqueous solutions of metal ions were replaced by those of methanol solutions of analyte (0.25 mM).

Table 1. Crystal Data and Structure Refinement for Compound 1 compound

1

formula formula weight temperature (K) crystal system space group a (Å) b (Å) c (Å) a (deg) ß (deg) γ (deg) V (Å3) Z Dcalc (g/cm3) μ (mm−1) F(000) reflns collected/unique R(int) θ range (deg) R1/wR2 (I > 2σ(I)) R1/wR2 (all data)

C34H34Cd3N6O21 1199.87 293(2) triclinic P1̅ 9.027(5) 14.262(5) 15.548(5) 92.420(5) 101.491(5) 96.540(5) 1944.5(14) 2 2.049 1.723 1184 20695/6573 0.0162 1.34−25.00 R1 = 0.0240, wR2 = 0.0707 R1 = 0.0423, wR2 = 0.0894

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RESULTS AND DISCUSSION The Structural Features. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the triclinic P1̅ space group. The asymmetric unit contains three crystallographically independent CdII centers, two DBPT3− ligands (DBPT-1 and DBPT-2), four coordinated water molecules (O13, O14, O15, and O16), and five lattice water molecules, as shown in Figure S2. The CdII centers display three different coordination surroundings in which Cd2 is coordinated with five oxygen atoms (O2, O4#3, O7#1, O10#4, O11) from five DBPT3− ligands, exhibiting a distorted trigonal bipyramid coordination geometry. Cd1 and Cd3 are six-coordinated with a distorted octahedral coordination geometry (Figure 1a). Cd1 is coordinated by three oxygen atoms (O1#1, O8, O10#2) from three DBPT3− ligands and three oxygen atoms from three terminal water molecules (O14, O15, O16). Cd3 is coordinated by five oxygen atoms (O3#5, O3#6, O4#5, O6, O8#7) from four DBPT3− ligands and one oxygen atom from terminal water molecule of O13. The Cd−O distances are in the range of 2.189(2)−2.498(2) Å. The O−Cd−O bond angles are in the range of 54.75(8)−174.30(10)°. Both DBPT-1 and DBPT-2 bridge to six CdII centers, exhibiting two kinds of coordination modes of mode I and mode II, respectively (Scheme 1, Figure S3). In DBPT-1, three carboxylic groups act as μ1-η1: η0, μ2-η1: η1 and μ3-η2:η2 mode, respectively. In DBPT-2, three carboxylic groups act as μ1-η0: η1, μ3-η1: η2 and μ2-η0: η2

bond lengths and angles are listed in Table S1. Crystallographic data for the structure reported in this paper has been deposited in the Cambridge Crystallographic Data Center with CCDC number of 1561890. Fluorescence Measurements. Sensing of Solvents. Finely ground samples of 1 (3.0 mg) were first dispersed in 50.0 mL of different solvents: H2O, methanol, ethanol, acetone, acetonitrile, cyclohexane, toluene, isopropyl alcohol, N,N-dimethylacetamide (DMA), and N,Ndimethylformamide (DMF) (0.05 mM), and then were treated by ultrasonication for 2 h to form stable suspensions. The fluorescence

Figure 1. Ball-and-stick (a) and polyhedral (b) presentations of three types of CdII centers; presentations of the 3D framework of 1 constructed from the {Cd6} cluster (c) as viewed from the a axis (d) and c axis (e). 433



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linked by 12 DBPT ligands to form a three-dimensional (3D) open framework (Figure 1c−d). The framework of 1 exhibits porosity in the ab plane with ca. 12 Å × 4 Å dimension, which are occupied by lattice water molecules. The solvent-accessible void factions calculated using PLATON63 is ∼14.4% (280.7 Å3), of the total crystal volume of 1944.5(14) Å3. The accessible special surface area, which is calculated by Material Studio 5.5 software, can reach 824.3 m2 g−1 for 1. However, the activation for 1 failed, and the N2 sorption experiment for 1 gave no result for the Brunauer−Emmett−Teller surface area, which is probably due to the partial loss of the long-range orderly structure under activation, and the weak affinity of the framework toward N2. PXRD and Thermal Stability. PXRD patterns for the assynthesized materials and samples after treatment at different conditions are shown in Figure 2a. The diffraction peaks of the as-synthesized 1 match well with the simulated pattern on the basis of the single-crystal structure. The crystalline of 1 is stable in air and is insoluble in water and common organic solvents. A prominent structural feature is the ultrahigh chemical stability it possesses, as confirmed from the PXRD tests. Each ground sample was immersed into water and organic solvents (such as DMF, DMA, methanol, ethanol, isopropyl alcohol, acetone, acetonitrile, methylbenzene, and cyclohexane) for 48 h, then separated by simple centrifugation, and dried at 50 °C for 12 h. These results revealed that the characteristic peaks maintain nearly the same positions and intensities as those of assynthesized one, suggesting that compound 1 is chemically

Scheme 1. Diverse Coordination Modes of H3DBPT in Compound 1

mode, respectively. Four octahedra and two trigonal bipyramids share edges and corner through eight μ3-O and four μ2-O, respectively, forming a hexanuclear {Cd6} cluster by the virtue of eight carboxylate groups from eight DBPT ligands (Figure 1b). Each {Cd6} cluster acts as basic building block, which is further

Figure 2. (a) PXRD patterns of the as-synthesized 1, the simulated one from single-crystal X-ray data and the samples after immersion in different solvents; (b) TGA/DSC curves of compound 1 recorded in N2 and air atmospheres; (c) the temperature-variable PXRD patterns of 1 in air atmosphere. 434



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Figure 3. (a) The fluorescence spectra of 1 dispersed in various solvents; (b) fluorescence spectra of the emulsions of 1 by introducing different metal ions with the same concentration of 0.25 mM in the final mixture; λex = 283 nm.

Because of the establishment of the chemical stability of 1, we also examined the luminescence properties of 1 in common organic solvents. The ground powders of as-synthesized 1 of known amount were first dispersed in water and common organic solvents, and then were treated by ultrasonication for 2 h to form stable emulsions prior to PL measurements. As shown in the Figure 3a, the emission intensities of 1 are highly dependent on the solvents. Acetone has the most significant quenching effect on the emission of the emulsion. This phenomenon is to a great extent ascribed to the interactions between the framework of 1 and the “CO” of the acetone.64,65 It is revealed that acetone has a wide absorption range from 250 to 325 nm, which has a maximum overlap with the absorbing band of H3DBPT ligand and the excitation spectrum of compound 1 (Figure S6). For the ligand-based emission system, organic ligands absorb energy upon excitation and the corresponding fluorescence was observed. However, in the presence of acetone molecules, the energy absorbed by H3DBPT ligands is transferred to acetone molecules, and as a result, the fluorescence intensity decreased. Such solvent-dependent fluorescent properties are of significant interest in the sensing of acetone, which is dangerous to human health and the environment. In addition, compound 1 shows the strongest emission in methanol, and the emission peak is close to that of solid state sample. Accordingly, 1 was dispersed in methanol (λem = 380 nm) in the following measurements, in order to achieve optimal analytic effect and make the results easier to distinguish. The PXRD patterns for the sample before and after ultrasonic dispersion are shown in Figure S7. The results further confirm the stability of 1 in methanol after ultrasonic treatment. Sensing of Metal Ions. The existence of open Lewis basic imidazolyl active sites in compound 1 promoted us to investigate its ability for sensing common metal ions. To examine the PL response toward different metal ions, 5.0 μL (0.10 M) M(NO3)x og aqueous solution (Mx+ = Na+, Cs+, K+, Cu2+, Zn2+, Nd2+, Cd2+, Ni2+, Mn2+, Pb2+, Mg2+, Co2+, Ba2+, Fe2+, Gd3+, Ce3+, Cr3+, Sm3+, Sr2+, Ca2+, In3+, Al3+, Ag+, Fe3+, or Zr4+) was added separately into the emulsion of 1. As shown in Figure 3b (Figure S8), most of the metal ions, such as Na+, K+, and Mn2+, have little effect on the fluorescence intensity of it. Interestingly, the emission intensity of 1 decreases obviously in the presence of Fe3+ and Zr4+, whereas the QP (quenching percentage) are 33% and 38%, respectively.

stable in common organic solvents, which is prerequisite for practical application such as fluorescence sensing. Thermogravimetric analyses (TGA) were conducted to determine the thermal stability of 1 at N2 and air atmospheres, respectively. As shown in Figure 2b, the results indicate that 1 may completely release its coordinated and free solvent molecules until to 300 °C (ca. 13.0%, cal.; ca. 13.5%, exp.) in N2 atmosphere. The temperature-variable PXRD studies were performed for the as-synthesized 1 and the samples which have been pretreated at 250, 300, 330, 360, 400, 430, and 460 °C, respectively, in air atmosphere (Figure 2c). The narrow diffraction peaks obtained at 300 °C indicate the good crystallinity of the desolvated sample (1a). The desolvation process does not cause the changes in peak position but causes minor changes in peak intensities. As revealed from the TGA curves, the desolvated 1a is thermally stable up to ca. 430 °C at N2 and ca. 400 °C at air atmosphere. During the heating process from 300 to 330 °C, the diffraction peak at 2θ = 6.2° disappeared, and the diffraction peaks at 2θ = 9.6, 10.9, and 11.2° appeared, which indicates a new phase happened. The new generated phase maintains the structural integrity as high as 400 °C. For the pattern obtained at 430 °C, two diffraction peaks at 2θ = 33.0, and 38.2°, corresponding to the (111) and (200), respectively, of the CdO phase, appeared, which indicates the collapse of the framework. The temperature-variable PXRD analyses in combination with the TGA results confirmed that 1 also has ultrahigh thermal stability. Fluorescent Properties. Sensing of Solvent Molecules. The solid-state PL spectra of compound 1 and free H3DBPT ligand were investigated at ambient temperature. As shown in Figure S4, compound 1 shows strong PL with the emission maximum at ∼402 nm (λex = 283 nm). The free ligand of H3DBPT (Figure S5) displays a similar emission band peaking at ∼406 nm (λex= 283 nm). On the basis of the similarity in the luminescence spectra between the free ligand and the constructed compound, we assigned the PL of 1 to ligandcentered electronic excitations. Moreover, blue shift of the emission peak was observed in compound 1, which is probably due to the increase of the conformational rigidity of the ligand, and reduction of nonradioactive decay because of the coordination between CdII and DBPT3−. 435



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Here, QP = 1 − I/I0, I0 is the initial fluorescence intensity without the analyte, and I is the fluorescence intensity after adding the analyte. To further assess the sensing sensitivity toward Fe3+ and Zr4+, a batch of suspensions of 1 with gradually increasing Fe3+ or Zr4+ from 5.0 × 10−5 to 1.9 × 10−3 M were prepared to test the emissive response (Figure S9). The emission intensity of 1 is gradually quenched with the addition of increasing concentrations of Fe3+ or Zr4+. The quenching efficiency could reach 85% or 87%, respectively, while the concentration reaches 1.9 × 10−4 M. These results indicate that 1 is sensitive toward Zr4+ and Fe3+. The PXRD were recorded after the sensing of metal ions. As confirmed by PXRD patterns (Figure S10), the crystal structure of 1 remains unchanged, showing that this quenching phenomenon has no relation with the framework of the crystal. The UV/vis absorption data of Fe3+ shows a wide absorption band from 260 to 400 nm, which has a majority of overlap with the excitation spectrum of 1 (Figure S11). Moreover, the absorption intensity of Fe3+ ion is much stronger than those of other metal ions (for example, Ba2+ and Sr2+). This fact indicates that the UV/vis absorption of Fe3+ upon excitation may prevent the absorption of 1 and result in the decrease or quenching of the luminescence.44,66 However, since no absorbance was observed for Zr4+ ion, the mechanism of fluorescence quenching by Zr4+ is not clear at the current stage. Sensing of Aromatic Compounds and Drug Molecules. The sensing experiments were carried out using suspensions of 1 in methanol (2.0 mL). Identical volumes (5.0 μL) of a series of nitroaromatics (NACs), aromatic compounds (ACs), and drug molecules were added to the emulsion of 1, including 5 NACs of TNP, 4-NA, nitrobenzene (NB), 4-nitrotoluene (4-NT), and 1,3-dinitrobenzene (1,3-DNB); 11 ACs of chlorobenzene (CB), bromobenzene (BrB), toluene (TO), 1,2-dimethylbenzene (1,2TO), 1,3-dimethylbenzene (1,3-TO), isophthalonitrile (ISN), 4chloroaniline (4-ClA), 4-bromoaniline (4-BrA), 1,2-diaminobenzene (1,2-DA), o-aminophenol (2-MA), and p-methoxyaniline (4-MOA), and four drug molecules of MNZ, ONZ, DMZ, and 2-M-5-MZ. As shown in Figure 4 (Table S2), the results

aromatics such as BrB, ISN, 2-MA, TO, 1,3-TO and ClB had a negligible effect. To further examine the sensitivity of detecting NACs and drug molecules, the PL quenching titrations were performed with the incremental addition of analytes to suspensions of 1. The quenching effect of TNP, 4-NA, MNZ, DMZ, ONZ, and 2-M-5MZ toward 1 is shown in Figures 5−6. The PL quenching efficiencies increase drastically with the analyte amount even in the low concentration range (95%), as shown in Figure 7a. Fast and high PL quenching was observed upon incremental addition of TNP and drug molecule of ONZ, which show maximum quenching efficiencies of 98% and 95%, respectively, at 0.08 mM. The QP plots are nearly linear before reaching equilibrium. In contrast, the QP plots for 4-NA, MNZ, and DMZ are almost linear at low concentrations (up to 0.05 mM for 4-NA; 0.08 mM for MNZ and DMZ) and subsequently begin to bend downward. However, they exhibit extraordinary quenching efficiencies of >99% at 0.10 mM for 4-NA, 0.13 mM for MNZ, and 0.15 mM for DMZ, respectively. A linear increase of PL quenching efficiency was observed for 2-M-5-MZ below 0.12 mM, and it reaches maximum quenching efficiency of 98% at 0.15 mM. When the concentration of the analyte reaches 0.08 mM, it was found to quench 98%, 95%, 95%, 85%, 76%, and 66%, respectively, of the emission of 1 by TNP, ONZ, 4-NA, MNZ, DMZ, and 2-M-5MZ, respectively. On the basis of the titration experiments, the order of the quenching efficiency was established to be TNP > ONZ ≈ 4-NA > MNZ > DMZ > 2-M-5-MZ. Moreover, the fluorescence quenching by drug molecules could also be detected at a very low concentration (ONZ: 5.0 μM, 1.10 ppm; MNZ: 10.0 μM, 1.71 ppm; DMZ: 10.0 μM, 1.41 ppm; 2-M-5-MZ: 10.0 μM, 1.27 ppm). The luminescence quenching efficiency was analyzed by fitting the experimental data to the Stern−Volmer (SV) equation, (I0/I) = KSV[Q]+1, in which [Q] is the molar concentration of the analyte, and KSV is the quenching constant (Figure 7b). The quenching constants of 1 for ONZ, MNZ, DMZ, and 2-M-5-MZ were found to be 2.4 × 104, 2.0 × 104, 1.7 × 104, and 1.1 × 104 M−1, respectively. High sensitivity and high selectivity for NACs are the most important criteria by which new MOF-based sensors for improving the sensing performance will be judged.51 As shown from Figure 5, the PL quenching by NACs could be detected at a very low concentration (TNP: 5.0 μM, 1.14 ppm; 4-NA: 5.0 μM, 0.70 ppm), further indicating their extremely high sensitivities toward TNP and 4-NA detections. The quenching constants of 1 for TNP and 4-NA are 2.8 × 104 M−1 and 2.5 × 104 M−1, respectively, in the low-concentration range, which are higher than those found for supramolecular-polymer-based sensors67−69 and most MOF-based sensors (Table 2), for example, the Eu-based MOF of [Eu 3 (bpydb) 3 (HCOO)(μ 3 OH)2(DMF)](DMF)3(H2O)2,56 the Cd-based MOF of [Cd(NDC)(H2O)]n,57 the Zn-based MOF of [Zn2(NDC)2(bpy)]· Gx (byp = 4,4′-bipyridine, G = guest solvent molecules),58 and the magnetic MOF of Fe3O4@Tb-BTC (H3BTC = benzene1,3,5-tricarboxylic acid).59 With QP of 98% at the same concentration of 0.08 mM, compound 1 is also comparable to the Eu-doped Y-based MOF (QP, 95−96%),54 which exhibits the highest KSV (15.9 × 104 M−1) among those luminescencebased MOF sensors reported to date, although the KSV of 1 toward TNP is much lower. Motivated by the above results, we also investigated the selectivity for TNP/4-NA in the presence of another nitro-based compound of NB. In the above-mentioned sensing experiments

Figure 4. Percentage of fluorescence quenching obtained by introducing different NACs, ACs, and drug molecules (0.25 mM) into the emulsion of 1.

display that all organic compounds (0.25 mM) can weaken the fluorescence intensity to different degrees. Significant emission intensities above 90% were quenched by drug molecules of ONZ, MNZ, DMZ, 2-M-5-MZ and NACs of TNP, 4-NA. The addition of NB and 4-NT caused a 30−62% quenching of fluorescence intensity of 1. In contrast, the addition of non-nitro-containing 436



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Figure 5. Effect on the photoluminescence spectra of 1 upon incremental addition of TNP (a) or 4-NA (b) solutions.

Figure 6. Effect on the photoluminescence spectra of 1 upon incremental addition of ONZ (a); MNZ (b); DMZ (c); or 2-M-5-MZ (d) solutions.

Mechanisms for the Photoluminescent Sensing of NACs and Nitroimidazole-Based Drug Molecules. Among these test, compound 1 is sensitive to nitro-substituted NACs and nitroimidazole-based drug molecules. Since the nitro group is a typical electron-withdrawing substituent, we tentatively explain the fluorescence response with a photoinduced electron transfer (PET) mechanism; that is, the electron presented in the excited state of compound 1 is transferred to the LUMO of the electron-deficient nitro-analytes leading to the disappearance or quenching of PL. In a gigantic 3D MOFs like compound 1, the valence and conduction band may be treated similarly to molecular orbitals. Normally the conduction bands of the MOFs are located at higher energy than LUMOs of NACs, which helps

of different NACs, the observed QP for sensing NB was 62% at 0.25 mM concentration. In a specially designed experiment, the blank fluorescence spectrum for 1, which has been dispersed in methanol, was recorded. To the emulsions of 1, the methanol solutions of (NB + TNP)/(NB + 4-NA) were added step by step, and the corresponding emissions were monitored, as shown in Figure 8. In both cases, the fluorescence intensity changed little upon the addition of NB (QP, ca. 0.6% for NB + TNP system and ca. 1.8% for NB + 4-NA system for the first addition), while obvious fluorescence quenching was observed upon the addition of TNP (QP, ca. 16.1%) or 4-NA (QP, ca. 19.6%), indicating great selectivity toward them. 437



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Figure 7. Plots of quenching efficiency vs concentrations of TNP, 4-NA, MNZ, ONZ, DMZ, and 2-M-5-MZ for compound 1 (a) and the corresponding Stern−Volmer plots of the analytes (b).

compounds (TNP > ONZ > 2-M-5-MZ > DMZ > MNZ > 4NA), especially for that of 4-NA, which lies in the unfavorable energy state of −1.93 eV for electron transfer but exhibits high quenching efficiency. These results indicate that the PET is not the only mechanism for quenching. In addition to the PET mechanism, the resonance energy transfer (RET) mechanism can also drastically enhance the PL quenching efficiency and improve sensitivity, which depend on the extent of overlap of the emission spectrum of the donor (compound 1) and the absorption spectrum of the acceptor (each analyte). As shown from Figure 10, the absorption spectrum of 4-NA (∼370 nm) has a maximum overlap with the emission spectrum of 1. This fact reveals that the RET is undoubtedly the right way to perfectly explain the high sensitivity and selectivity of 1 toward 4-NA. Moreover, the suitable spectral overlap of TNP (∼353 nm) with the spectrum of 1 also indicates that there might exist an energy transfer process. As for the drug molecules, they show small spectral overlap with the following order: ONZ (∼310 nm) > MNZ (∼310 nm) > DMZ (∼308 nm) > 2-M-5-MZ (∼299 nm). Almost zero spectral overlap was observed for NB. As a result, the combination of PET and RET mechanisms could reasonably explain the PL quenching by the nitro-based compounds in this work.

Table 2. List of Quenching Constant Values and Detect Limit Towards TNP/4-NA in 1 and the Corresponding Reference Sensory Materials KSV/(L/mol) analyte

1

TNP

2.8 × 10 (0−0.08 mM), (5.0 μM, 1.14 ppm) 4

references 3.5 × 10 (0−0.07 mM), (4.0 μM, 0.91 ppm) 3.2 × 104 (0−0.10 mM), (5.0 μM, 1.14 ppm) 6.56 × 104 (0−0.05 mM) 4

7.09 × 104 (0−0.10 mM), (0.8 μM, 0.23 ppm) 15.9 × 104 (0−0.10 mM), (0.1 μM, 23 ppb) 2.1 × 104 (0−0.12 mM), (5.0 μM, 1.14 ppm) 2.4 × 104 (0−0.10 mM), (4.0 μM, 0.92 ppm) 0.42 × 104 (0−8.0 mM) 1.84 × 104 (0−0.09 mM) 4-NA

2.5 × 104 (0−0.11 mM), (5.0 μM, 0.70 ppm)

9.8 × 104 (0−0.1 mM), (3.75 μM, 0.52 ppm)

Cd-based MOF51 Zn-based MOF52 Cd-based MOF53 Tb-doped Ybased MOF54 Eu-doped Ybased MOF55 Eu-based MOF56 Cd-based MOF57 Zn-based MOF58 Fe3O4@TbBTC59 Cd-based MOF41

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CONCLUSION

In summary, a chemical-stable and thermal-stable luminescent Cd(II)-based MOF material of 1 was successfully isolated by hydrothermal synthesis. It exhibits ligand-based photoluminescence with solvent-dependent fluorescent intensities. The host material may serve as a multiresponsive luminescent sensor, capable of detecting the metal ions of Zr4+ and Fe3+, NACs of TNP and 4-NA. The most striking property of 1 is the remarkable sensitivity and selectivity toward TNP and 4-NA even in the presence of another nitro compound of NB, which can be attributed to the combination of photoinduced electron transfer mechanism and resonance energy transfer mechanism. The PL quenching by them could be detected at very low concentrations of 1.14 and 0.70 ppm, respectively. The maximum quenching efficiencies were found to be 98% and 95%, respectively, at 0.08 mM. Significantly, compound 1 also exhibits excellent sensing performance toward nitroimidazole-

to improve electron transfer from electron-rich MOFs to electron-deficient NACs. Thus, the lower the energy of the LUMO of the analyte, the higher will be the probability of electron transfer from the fluorophore and in turn the effective quenching.47−59 The HOMO and LUMO orbital energy plots of selected electron-deficient NACs and nitroimidazole-based drug molecules are shown in Figure 9. The DFT calculations were performed with Gaussian 09 package, and subsequent geometry optimization was carried out at B3LYP level of DFT. The LUMOs of TNP (−3.92 eV) and ONZ (−2.77 eV) lie in a favorable energy state for maximum electron transfer from the conduction band of 1. However, the order of observed quenching efficiency (TNP > ONZ ≈ 4-NA > MNZ > DMZ > 2-M-5-MZ) is not fully in accordance with the LUMO energies of other nitro 438



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Figure 8. Emission spectra of 1 upon addition of methanol solution of NB followed by TNP (a) or 4-NA (b) (10.0 mM, 2.0 μL addition for each time).

unusual sensing performance make this Cd-based MOF a promising sensory material for versatile chemical sensing of NACs and antibiotics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01430. Tables with crystal data, bond lengths and angles, additional structures, figures, IR spectra, etc. (PDF) Accession Codes

CCDC 1561890 contains 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 data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.

Figure 9. HOMO and LUMO energies for selected NACs, nitroimidazole-based drug molecules and the free H3DBPT ligand.

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AUTHOR INFORMATION

Corresponding Authors

*(B.-X.D.) E-mail: [email protected]. *(W.-L.L.) E-mail: [email protected]. ORCID

Bao-Xia Dong: 0000-0002-3114-9188 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (Nos. 21371150, 21671169, 21573192), Six Talent Peaks Project in Jiangsu Province (No. 2017-XNY-043), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions, and the Foundation from the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Figure 10. Spectral overlap between the adsorption spectra of analytes and the emission spectrum of 1 in methanol.

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