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A Water-Stable 3D Luminescent Metal−Organic Framework Based on Heterometallic [EuIII6ZnII] Clusters Showing Highly Sensitive, Selective, and Reversible Detection of Ronidazole Qingfu Zhang,*,† Mingyuan Lei,† Hui Yan,† Jinyun Wang,‡ and Yang Shi† †

College of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng 252059, China Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China

‡

S Supporting Information *

MOF materials but also improve the luminescent properties of the lanthanide centers. Up to now, some luminescent MOFs based on Ln-TM heterometallic clusters have been reported.7−15 For example, Yang’s group introduced Ln-Cd (Ln = Tb, Eu, Dy, Gd, Er, Yb, Y) heterometallic clusters into the 3D luminescent MOFs for the first time.8 Subsequently, Sun et al. successfully prepared several luminescent MOFs based on heterometallic Ln6Cd6O12 (Ln = Sm, Eu) wheel clusters9 and tetranuclear [Ln2Cd2(μ2-O)4] (Ln = Sm, Eu) and [Ln2Cd2(μ2-O)6] (Ln = Eu) clusters.10,11 Deng et al. synthesized a 2D-layered luminescent MOF based on tetranuclear Eu2Ag2 clusters.12 Zhang et al. reported a series of isostructural 3D-pillar-layered luminescent MOFs based on planar hexanuclear heterometallic [Ln2M4(PyIDC)2] (Ln = Eu, Gd, Tb; M = Co, Cd, Zn; H3PyIDC = 2-(pyridine-3-yl)-1H-4,5-imidazoledicarboxylic acid) clusters.13 Bo et al. obtained a series of luminescent MOFs based on heterometallic (LnZnO)2(COO)8 (Ln = Sm, Eu) or Zn5Ln(OH)(COO)12 (Ln = Eu, Tb) clusters.14 Liu et al. also reported a family of isostructural luminescent MOFs based on cuboidal heterometallic Zn12Ln8(pdc)6 (Ln = Sm, Eu, Tb; pdc = 3,5-pyrazoledicarboxylate) clusters.15 Nevertheless, to our knowledge, none of the luminescent MOFs based on Ln-TM heterometallic clusters have been applied to detect antibiotics in aqueous solution hitherto, although Li et al. have already employed ZrIV-MOFs for luminescent sensing of antibiotic molecules in water.16 To explore this area, we elaborately selected electron-rich π-conjugated 1,4-naphthalenedicarboxylic acid (H2NDC) to react with zinc nitrate and europium nitrate under solvothermal conditions and successfully obtained a new water-stable 3D luminescent MOF, [Eu6Zn(μ3OH) 8 (NDC) 6 (H 2 O) 6 ] n (1), based on heterometallic [EuIII6ZnII] clusters. Complex 1 exhibits highly sensitive, selective, and reversible luminescent sensing of ronidazole (RNZ), which represents the first example of water-stable luminescent MOFs based on Ln-TM heterometallic clusters for the detection of antibiotics in aqueous solution. Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the trigonal space group R3c and contains two EuIII ions, one-third ZnII ion, two NDC2− ligands, two and twothirds OH− groups, and two coordination water molecules in the asymmetric unit (Figure 1a). In 1, the central ZnII ion that locates at an inversion center displays a hexacoordinated [ZnO6]

ABSTRACT: A water-stable 3D luminescent metal− organic framework (MOF), [Eu6Zn(μ3OH)8(NDC)6(H2O)6]n (1), constructed from heterometallic [EuIII6ZnII] clusters and electron-rich π-conjugated 1,4-naphthalenedicarboxylic acid (H2NDC) ligands exhibits highly sensitive, selective, and reversible detection of ronidazole, which represents the first example of luminescent MOFs based on Ln-TM heterometallic clusters for the detection of antibiotics in aqueous solution.

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ince the landmark discovery of penicillin by Alexander Fleming in 1929,1 antibiotics have developed into a very important group of pharmaceuticals that are widely used for the treatment of bacterial infections in humans and animals. However, the abuse of antibiotics in the past decades has led to high levels of antibiotic residues in various aquatic environments such as wastewaters, surface water, groundwater, and even drinking water, especially in China.2 Current antibiotic detection methods are usually limited because of timeconsuming, high-cost, sophisticated devices and complicated procedures, including liquid chromatography tandem mass spectrometry, capillary electrophoresis, surface enhanced Raman spectra, etc.3 Therefore, there is an urgent need to develop fast, cost-effective, and convenient antibiotic detection methods in water. An optical sensing technique utilizing changes of the luminescent signals induced by sensor−analyte interactions has proven to be a powerful detection method.4 The choice of sensor material is central to achieving an effective detection of the targeted analyte. Lanthanide metal−organic frameworks (LnMOFs) are considered to be one class of promising optical sensing materials because of their unique optical properties, such as high color purity, large Stokes shifts, and visible color with the naked eye, which originally come from f−f transitions via an “antenna effect”.5 Compared with single LnIII ions, the luminescent Ln-MOFs based on LnIII clusters have attracted more attention, owing to their enhanced water stability that could exploit the practical application in an aqueous environment.6 Recently, the use of lanthanide transition-metal (Ln-TM) heterometallic clusters as building blocks to construct luminescent MOFs has become a new field because the coexistence of lanthanide and transition-metal ions in the metallic clusters can not only enrich the structure diversities of © 2017 American Chemical Society

Received: May 6, 2017 Published: June 30, 2017 7610

DOI: 10.1021/acs.inorgchem.7b01156 Inorg. Chem. 2017, 56, 7610−7614

Communication

Inorganic Chemistry

molecules per formula unit (calcd 4.31%). No further weight loss was observed below 360 °C, indicating good thermal stability of the coordination framework (Figure S6). The powder X-ray diffraction (PXRD) demonstrates that there is no structure transformation after treatment in water, such as immersion in water at room temperature for 3 weeks or refluxing in boiling water for 72 h (Figure 2a). Moreover, it should be also pointed

Figure 1. (a) Coordination environment of EuIII and ZnII ions in 1. (b) Trigonal-antiprismatic heterometallic cluster in 1. (c) View of the 3D framework of 1 along the c axis. (d) View of pcu topology of 1.

Figure 2. (a) PXRD patterns of 1: simulated, as-synthesized, upon treatment with water, boiling water, and exposure to RNZ. (b) Fluorescence quenching of 1 by different antibiotics. (c) Fluorescence titration of 1 with the gradual addition of RNZ. (d) Selective detection for RNZ of 1 in the presence of various antibiotics in aqueous solution.

octahedral geometry surrounding by six oxygen atoms from six μ3 -OH groups, while both independent Eu III ions are octacoordinated with [EuO8] bicapped-trigonal-prismatic geometry by three oxygen atoms from three μ3-OH groups, four oxygen atoms from carboxyl groups of three different NDC2− ligands, and one oxygen atom from a coordinated water molecule (Figure S1). All of the bond lengths of Zn−O and Eu−O in 1 are in the normal range (Table S2). As shown in Figure 1b, three EuIII ions are linked by one μ3-OH group to give an equilateraltriangular trinuclear [Eu3(μ3-OH)]8+ unit (Eu···Eu 3.808 and 3.860 Å; Figure S2), and two trinuclear [Eu3(μ3-OH)]8+ units are further joined by the central ZnII ion via the other six μ3-OH groups to form a sandglass-like heptanuclear heterometallic [Eu6Zn(μ3-OH)8]12+ cluster (Zn···Eu 3.471−3.472 Å). In an alternative view, the heptanuclear heterometallic [Eu6Zn(μ3OH)8]12+ cluster can also be described as two vertex-sharing heterometallic tetrahedra fused to each other through the central ZnII ion, and the metallic skeleton of this heptanuclear cluster can be simplified to a trigonal antiprism with a depth of 5.347 Å (Figure S2). Furthermore, these heptanuclear heterometallic clusters are linked by NDC2− ligands to form a 3D framework (Figure 1c), which gives the cavity only of ∼3.6 Å diameter (Figure S3). The effective free volume of this framework is 26.2%, as calculated by PLATON.17 From the topological viewpoint, the heptanuclear heterometallic [EuIII6ZnII] cluster can be viewed as a six-connected node and the NDC2− ligand can be viewed as a connector; thus, 1 exhibits a 3D uninodal sixconnected pcu topology (Figures 1d and S4). Additionally, the face-to-face π−π-stacking interactions between adjacent naphthalene rings are observed in complex 1 (Figure S5 and Table S3), which should enhance the stability of the whole structure. For practical applications, the robustness and water stability of 1 were investigated. Thermogravimetric analysis (TGA) reveals that 1 exhibits a weight loss (obsd 4.17%) between 100 and 230 °C, corresponding to the release of six coordinated water

out that the framework of 1 remains intact, even after exposure to different antibiotics in aqueous solution (Figures 2a and S7). The high thermal and water stability of 1 should be attributed to the combination of [EuIII6ZnII] cluster units and strong coordination interactions between EuIII ions and carboxylate groups.18 The luminescence spectra of 1 dispersed in water were investigated. Upon excitation at 306 nm, the emission spectrum of 1 exhibits the characteristic peaks of EuIII at 592, 614, and 697 nm (Figure S8), which could be attributed to the 5D0 → 7F1, 5D0 → 7F2, and 5D0 → 7F4 transitions, respectively. The intensity ratio (about 2.5) for I(5D0→7F2, electric-dipole transition)/ I(5D0→7F1, magnetic-dipole transition) indicates that the EuIII ions in 1 occupy low-symmetry coordination sites with no inversion centers,19 which is well consistent with the results of Xray structural analysis. As shown in Figure S9, the ligand-based emission band around about 427 nm was not observed in 1, suggesting an efficient energy transfer from the ligand-centered excited states to the Eu f excited states. Interestingly, the luminescence intensities of 1 dispersed in water exhibit different degrees of quenching upon contact with different analytes. Antibiotics such as cycloheximide (CHX), piperacillin (PIP), thiamphenicol (TAP), roxithromycin (RXM), azithromycin (AZM), spiramycin (SPM), amoxicillin (AMX), streptovitacin A (STV), oxacillin (OXA), and common organic chemicals including catechol, phenol, m-dichlorobenzene (mdiCB), o-dichlorobenzene (o-diCB), iodobenzene (IB), pdichlorobenzene (p-diCB), bromobenzene (BB), benzene, chlorobenzene (CB), and aniline basically do not affect the luminescence intensity, whereas nitroimidazoles have a significant quenching effect on the luminescence intensity of 1 in water (Figures 2b and S10). The maximum luminescence intensity at 614 nm was reduced by 91.6, 72.7, and 51.4% upon exposure to 167.0 μM concentration of RNZ, dimetridazole (DMZ), and 7611

DOI: 10.1021/acs.inorgchem.7b01156 Inorg. Chem. 2017, 56, 7610−7614

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Inorganic Chemistry metronidazole (MNZ), respectively. Such a high quenching effect toward RNZ indicates that 1 could serve as a promising sensor for detecting RNZ. Therefore, the fluorescence titration experiment of 1 for RNZ was further performed. As shown in Figure 2c, the luminescence intensity of 1 dispersed in water progressively decreased with the gradual addition of RNZ. The strong red emission of the suspension of 1 can be observed by the naked eye under UV light, and its brightness dramatically vanishes with higher concentrations of RNZ (Figure S11). The fluorescence quenching efficiency was analyzed using the Stern− Volmer (SV) equation F0/F = KSV[A] + 1, where KSV is the quenching constant (M−1), [A] is the molar concentration of the analyte, and F0 and F are the fluorescence intensities before and after the addition of analyte, respectively. The SV plot for RNZ was nearly linear below 125.0 μM but subsequently deviated from linearity and bent upward at higher concentrations (Figure S12). The nonlinear nature of the SV plot of RNZ may be ascribed to self-absorption or an energy-transfer process.20 The calculated KSV value for RNZ, 4.4 × 104 M−1, is close to the reported values of Zr-MOFs for detecting NZF and NFT antibiotics.16 The limit of detection (LOD) of 1 for RNZ was estimated to be 65 ppb (Table S4 and Figure S15). Remarkably, the emission intensity of 1 almost does not change in the presence of excess non-nitroimidazole antibiotics, such as CHX, TAP, AZM, and AMX, but it was significantly quenched upon the introduction of RNZ into the mixture of MOF and CHX, TAP, AZM, or AMX (Figures 2d and S16−S19), which clearly revealed that the interference from non-nitroimidazole antibiotics can be neglected and confirmed the high quenching selectivity of 1 toward RNZ. In addition, complex 1 could be regenerated and reused by centrifuging the dispersed solution after sensing and washing several times with water, and almost regaining the initial luminescence intensity over seven cycles implied a high photostability and reversibility of 1 for its long-time in-field RNZ detection application (Figures S20 and S21). We also made a comparison of our method to those of others (Table S5), and although the LOD of the proposed one is not the lowest, its high sensitivity, selectivity, reversibility, and simplicity made it a new and important approach in the determination of RNZ. In order to better understand the excellent fluorescence quenching effect of 1 toward RNZ, the quenching mechanism was investigated. As shown in Figure 3a, the absorption maximum of 1 red shifts after the addition of RNZ, which clearly suggests the formation of a ground-state complex and confirms the static quenching mechanism because dynamic quenching does not perturb the absorption spectra.21 The values of KSV at higher temperature were found to be smaller (Figure 3b), also indicating that the quenching process follows a static mechanism.22 In addition, the fluorescence lifetimes of 1 were almost unchanged in the absence and presence of RNZ (Figure 3c), further proving the static nature of fluorescence quenching,23 which also indicates that there are no interactions between RNZ and the Eu3+ ions and changes in the emission intensity are related to interactions between RNZ and NDC2− ligands.24 Therefore, the competition of absorption of the light source energy between the analytes and ligands in 1 may be the key factor for high selective detection of RNZ. The absorption spectra of nitroimidazoles (RNZ, DMZ, and MNZ) in water showed that RNZ exhibits the highest absorbance at 306 nm (Figure 3d); thus, RNZ can filter the most light adsorbed by NDC2− moieties, and the total energy transferred from NDC2− ligands to EuIII ions is the largest reduced, which leads to the strongest luminescence quenching of 1 for RNZ. Considering

Figure 3. (a) Absorption spectra of 1 before and after the addition of RNZ. (b) Temperature-dependent SV plots of 1 for RNZ. (c) Timeresolved fluorescence decay of 1 before and after the addition of RNZ. (d) Absorption spectra of nitroimidazoles in water.

that the size of the cavity in 1 is only 3.6 Å (see the above structural analysis), the RNZ molecule (9.136 × 4.488 × 2.642 Å)25 could not enter the cavity except on the surface; thus, the observed fluorescence quenching of 1 with RNZ should mainly be ascribed to surface interaction.26 In summary, a water-stable 3D luminescent MOF has been successfully constructed from heterometallic [EuIII6ZnII] clusters and electron-rich π-conjugated H2NDC ligands, which exhibits highly sensitive, selective, and reversible sensing for RNZ in water. The results indicate that the luminescent MOFs have great potential for the development of new sensory materials for the detection of antibiotics in various aquatic environments.

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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.7b01156. Experimental details, crystallographic data, tables, structures, TGA, PXRD, and luminescent spectra (PDF) Accession Codes

CCDC 1543414 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qingfu Zhang: 0000-0002-5788-7023 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (Grant 21001061) and the Natural Science Foundation of Shandong Province (Grant ZR2014BQ034). 7612

DOI: 10.1021/acs.inorgchem.7b01156 Inorg. Chem. 2017, 56, 7610−7614

Communication

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DOI: 10.1021/acs.inorgchem.7b01156 Inorg. Chem. 2017, 56, 7610−7614