A novel cadmium metal–organic framework-based multiresponsive fluorescent sensor demonstrating outstanding sensitivities and selectivities for detecting NB, Fe3+ ions and Cr2O72− anions

Xiuli Wang *, Yu Liu , Hongyan Lin , Na Xu , Guocheng Liu , Xiang Wang , Zhihan Chang and Jianrong Li *
College of Chemistry and Materials Engineering, Bohai University, Jinzhou, 121013, P. R. China. E-mail: wangxiuli@bhu.edu.cn; jrli6491@163.com; Fax: +86 416 3400158; Tel: +86 416 3400158

Received 5th August 2020 , Accepted 9th September 2020

First published on 10th September 2020


A Cd(II) metal–organic framework (MOF) [Cd3(L)(NTB)2(DMA)2]·2DMA (Cd-MOF, DMA = N,N-dimethylacetamide) was solvothermal prepared in the presence of N-donor ligand L (L = (E)-4,4′-(ethene-1,2-diyl)bis[(N-pyridin-3-yl)benzamide]) and O-donor ligand H3NTB (H3NTB = 4,4′,4′′-nitrilotribenzoic acid). The Cd-MOF is a 3D framework based on 2D [Cd3(NTB)2]n bilayers and semi-rigid bidentate L ligands, which exhibit a novel 2,3,6-connected network with {82·108·12·184}{8}6 topology. Moreover, the Cd-MOF represents the first metal–organic complex assembled by the H3NTB and bis(amide)-bis(pyridyl) co-ligands and displays remarkable fluorescence and stability as well as fluorescence quenching phenomenon towards Al3+, Cu2+, Cr3+, Fe3+ cations, Cr2O72− anion and nitrobenzene (NB). In particular, Cd-MOF can sensitively and selectively detect Fe3+, Cr2O72− and NB, and the limit of detection (LOD) values of Fe3+, Cr2O72− ions and NB are 5.1 × 10−5 M, 5.3 × 10−5 M and 4.2 × 10−5 M, respectively.


Introduction

For the last couple of decades, some recent progresses on the applications of metal–organic frameworks (MOFs),1 such as in catalysis,2 gas storage,3,4 separation,5,6 fluorescent sensors,7,8 luminescence property,9,10 electroluminescent devices,11,12 batteries,13,14 drug-delivery,15,16 and proton conduction,17,18 have been attracting considerable attention. Recently, considering the human health and environmental protection, the selective and sensitive detection of toxic metal cations, anions and organic solvents has become significant.19,20 Some metal cations and anions are essential for various physiological processes; however, their contents should be carefully controlled. Nitrobenzene (NB), as a persistent pollutant, is usually used to prepare aniline, and the excessive drain of untreated nitrobenzene can seriously pollute ecosystems such as rivers, groundwater and soil. Since Al3+, Cu2+, Cr3+ and Fe3+ metal cations, Cr2O72− ions and NB as toxic and harmful pollutants pose high risks to human health and environments, it is essential to develop a method for the fast and sensitive detection of Al3+, Cu2+, Cr3+, Fe3+, Cr2O72− ions and NB in environment.21–26 The traditional detection methods are expensive, complicated and time-consuming.27,28 In recent years, some MOFs as a type of fluorescence sensing materials have been investigated.29–31 In particular, Cd-based MOFs can be used as fluorescence sensing materials that are simple, environmental-friendly and low cost, and they possess a low detection limit and can selectively identify targeted molecules.32–34

It is well known that polycarboxylic ligands with intrinsic characteristics of a spacer and substituent groups play an important role in determining the final structures and chemical properties of target complexes.35,36 4,4′,4′′-Nitrilotribenzoic acid (H3NTB) as a π electron-rich tricarboxylic acid has been used to synthesize MOFs. For example, Gu et al. reported a 2D complex [Cd3(NTB)2(DMA)3]·2DMA, which was used as a fluorescence sensor for detecting nitrobenzene (NB) and 2,4,6-trinitrophenol (TNP) with high sensitivity, selectivity, and recyclability.37 However, reports on MOFs assembled from H3NTB are still limited;38–40 particularly, as there are no MOFs reported on the H3NTB and bis(amide)-bis(pyridyl) co-ligands. On the other hand, a series of amide-pyridyl-based ligands have been introduced into metal-polycarboxylates systems, and numerous novel MOFs have been obtained in the past decades.41–47 Our group has synthesized the first 3D Cu-MOF [Cu2(L)(BTC)(μ3-OH)] based on the semi-rigid bis(amide)-bis(pyridyl) ligand (E)-4,4′-(ethene-1,2-diyl)bis[(N-pyridin-3-yl)benzamide] (L) and 1,3,5-benzenetricarboxylic acid (H3BTC) under a hydrothermal condition. This Cu-MOF possessed significant electrocatalytic activities towards hydrogen peroxide (Scheme 1).


image file: d0ce01139h-s1.tif
Scheme 1 Structures of bis(pyridyl-amide) ligand and tricarboxylic acid used in this work.

In this study, we selected the semi-rigid bis(amide)-bis(pyridyl) L and H3NTB as co-ligands to construct the target MOFs based on the following two points: first, both ligand L and H3NTB have large conjugate structures, which is essential for MOF-based complexes with potential fluorescence properties. Second, the ligands L and H3NTB have abundant coordination sites and superior flexibilities in favor of meeting the coordination environment of metal ions to generate stable polynuclear MOF. Finally, we successfully obtained a novel Cd-MOF [Cd3(L)(NTB)2(DMA)2]·2DMA under solvothermal conditions. To the best of our knowledge, this Cd-MOF is the first metal–organic complex based on the tricarboxylate NTB3− and bis(amide)-bis(pyridyl) co-ligand.

Results and discussion

Structural description

The single-crystal X-ray diffraction indicates that Cd-MOF crystallizes in the triclinic space group P[1 with combining macron], which was constructed from a trinuclear {Cd3} cluster, NTB3− anions and L ligands. It can be seen from Fig. 1a that the asymmetric unit consists of three Cd(II) ions, one L ligand, two NTB3− anions, two coordinated DMA ligands and two free DMA molecules. The Cd1 ion was coordinated by six O atoms (O1, O1#1, O3#4, O3#5, O5#2 and O5#3) from six NTB3− anions, showing an octahedral geometry (Cd–O distances: 2.2120(2)–2.309(2) Å). The Cd2 ion also showed a six-coordinated disordered octahedral geometry defined by four O atoms (O2#7, O4, O5#6 and O6#6) from three NTB3− anions, one O atom (O7) from one coordinated DMA (Cd–O = 2.198(5)–2.572(5) Å) and one N atom (Cd–N = 2.267(3) Å) from one L ligand (Fig. 1a). Each NTB3− anion adopts a μ6121111 mode to connect six Cd(II) ions, yielding a 2D [Cd3(NTB)2]n bilayer structure (Fig. S1b). As illustrated in Fig. 1b, three Cd(II) ions are connected by twelve carboxyl oxygen atoms of six NTB3− anions to form a trinuclear {Cd3} cluster secondary building unit (SBU), and then each {Cd3} SBU is linked with adjacent six {Cd3} SBUs by NTB3− anions to generate a “flower-shaped” structure. Fig. S1c indicates that the adjoining 2D [Cd3(NTB)2]n bilayers are further linked by L ligands to construct a 3D framework. The solvent-accessible volume in fully desolvated Cd-MOF was calculated to be 33.6% using the PLATON program. It can seen from Fig. 1b that {Cd3} SBUs and NTB3− anions can be considered as 6-connected and 3-connected nodes, respectively, and L ligands are regarded as linkers, so the overall framework was considered as a 2,3,6-connected network with the {82·108·12·184}{8}6 topology.
image file: d0ce01139h-f1.tif
Fig. 1 (a) The coordination environment of the Cd(II) ions in Cd-MOF; (b) the 3D topology structure of Cd-MOF.

Compared with the reported 2D bilayer coordination network [Cd3(NTB)2(DMA)3]·2DMA,37 Cd-MOF in this work is a 3D coordination framework, in which the 2D “flower-shaped” [Cd3(NTB)2]n bilayers are connected by a semi-rigid N-ligand L. This may be the reason why the 2D bilayer structure could be extended into a 3D framework.

Fluorescent property of Cd-MOF

It is well known that Cd-MOFs constructed by Cd(II) metal centres and electron-rich π-conjugated organic ligands may exhibit broad applications as fluorescent materials in fluorescence sensors, optical information storage and others.48,49 However, the fluorescence properties and fluorescence sensing activities of Cd-MOF based on the semi-rigid bis(amide)-bis(pyridyl) and the electron-rich H3NTB have not been found up to now. Therefore, we have systematically studied the fluorescence sensing properties of the title Cd-MOF.

As shown in Fig. 2, the solid-state fluorescence emission of the powder samples of L, H3NTB ligands and Cd-MOF as well as the Cd-MOF dispersed in water and DMA were investigated. The free L ligand and Cd-MOF display a strong emission peak at 465 nm and 503 nm (λex = 350 nm), respectively. The H3NTB ligand shows an emission peak at 460 nm (λex = 400 nm). Compared to those of ligands, the emission bands of Cd-MOF are apparently red-shifted. The result showed that the emission band of Cd-MOF might have been attributed to the intraligand π* → π electronic transition due to the organic ligand.50 Furthermore, the liquid fluorescence properties of Cd-MOF dispersed in water and DMA were investigated. The Cd-MOF dispersed in H2O showed a strong emission peak at 460 nm (λex = 370 nm), and when dispersed in DMA, Cd-MOF showed a strong emission peak at 410 nm (λex = 350 nm). It was observed that the suspensions of Cd-MOF in water and DMA solution exhibited somewhat blue-shift compared with that of solid-state. It suggests that the interactions between the ligand and solvent molecules may have resulted in the changes in the Cd-MOF emission.51


image file: d0ce01139h-f2.tif
Fig. 2 Fluorescence spectra of ligand L, H3NTB and Cd-MOF.

Sensing metal cations and anions

20 mg ground Cd-MOF was dispersed in 100 mL water or DMA solution, which then was aged for one day after sonicating for an hour. The upper suspension of Cd-MOF was collected to perform fluorescence sensing experiments. The PXRD patterns of ground Cd-MOF, soaked in water and DMA solvent, was tested. Besides, the stability of Cd-MOF in water was studied further via powder X-ray diffraction analysis. The related PXRD patterns of Cd-MOF, after soaking in an aqueous solution of pH 2–10 for 24 h were obtained (Fig. S3), which indicated that Cd-MOF remained stable in the pH range of 2–10. The results indicate that Cd-MOF maintains its crystallinity, which lays the foundations for fluorescence sensing experiments of metal cations, anions and organic solvents (Fig. S2a). We selected ten metal cations, ten anions and ten organic solvents to perform the fluorescence sensing experiment. Metal cations and anions were detected in the aqueous solution, and organic solvents were detected in the DMA solution.

We choose ten metal cations and ten anions, including Al3+, Ca2+, Co2+, Cu2+, Ni2+, Ba2+, Ag+, Sr2+, Cr3+, and Fe3+ cations, and Cl, NO2, MnO4, F, CrO42−, SO42−, SO32−, HCO3, CO32−, and Cr2O72− anions. 0.21 mL of a metal ion or anion (1.0 × 10−2 M) aqueous solution was added to 3 mL of the dispersion solution, respectively. The fluorescence intensity of the suspension was measured on a fluorescence spectrophotometer. As illustrated in Fig. 3, metal cations Al3+, Cu2+, Cr3+, Fe3+ and anion Cr2O72− showed fluorescence quenching for Cd-MOF at different degrees, in which the quenching efficiencies of Fe3+ and Cr2O72− ions can reach as much as 97.2% and 97.4% (Q = (I0I)/I0 × 100%, in which I0 and I are the luminescence intensities of Cd-MOF in the absence and presence of Fe3+ or Cr2O72−). The titration experiments of Al3+, Cu2+, Cr3+, and Fe3+ ions showed that the fluorescence intensity of Cd-MOF decreases with increasing concentration. A 1.0 × 10−3 M aqueous solution of Fe(NO3)3 was gradually added (20 μL for each addition) to 3 mL DMA solution of Cd-MOF. It was noted that the titration concentration of other metal cations was 1.0 × 10−2 M. Also, it was found that Fe3+ was more sensitive than Al3+, Cu2+ and Cr3+. Therefore, only the Ksv and limit of detection (LOD) value of Fe3+ ions were calculated. Fig. 4b indicates that the fluorescence intensities of Cd-MOF decreased gradually as the concentration of Fe3+ solution increased. The fluorescence intensity did not further decrease until the concentration of Fe3+ ions increased to 0.9 mM. As shown in Fig. 4a, the quenching ability of Fe3+ ions was quantitatively rationalized by the Stern–Volmer equation, I0/I = 1 + Ksv [Fe3+], which was calculated as 3.1 × 104 M−1 (I0 and I are the fluorescence intensities of Cd-MOF in the absence and presence of Fe3+, respectively, [Fe3+] is the molar concentration of Fe3+ and Ksv is the Stern–Volmer quenching coefficient of Cd-MOF for Fe3+). The LOD value calculated by 3σ/Ksv presented a low reading of 5.1 × 10−5 M, where σ is the standard deviation for ten cycles of fluorescence tests using the blank solution at room temperature.


image file: d0ce01139h-f3.tif
Fig. 3 The fluorescence spectra of Cd-MOF after the addition of various ten cations (a), ten anions (b) and ten organic solvents (c).

image file: d0ce01139h-f4.tif
Fig. 4 (a) The effect of adding other anions on the fluorescence intensity of Fe3+; (b) fluorescence spectra of Cd-MOF with Fe3+ ions at different concentrations, inset: Ksv plot for Fe3+ ions.

We also investigated the fluorescence recognition for Cr2O72− anions under the same conditions as for Fe3+ ions. The data of the titration experiment indicated that the fluorescence intensities of Cd-MOF decreased with the increase in the Cr2O72− anion concentration from 0 to 0.9 mM (Fig. 5b). The Ksv value of the Cr2O72− anion was 3.0 × 104 M−1, and the LOD value of Cr2O72− anion was 5.3 × 10−5 M. Then, anti-interference experiments were performed towards Al3+, Cu2+, Cr3+, Fe3+ ions, and Cr2O72− anions. Fig. 4, 5 and S6–S8 demonstrate the excellent anti-interference of Al3+, Cu2+, Cr3+, Fe3+ ions, and Cr2O72− anions.


image file: d0ce01139h-f5.tif
Fig. 5 (a) The effect of adding other anions on the fluorescence intensity of Cr2O72−; (b) fluorescence spectra of Cd-MOF with Cr2O72− anions at different concentrations, inset: Ksv plot for Cr2O72− anions.

Sensing organic solvents

Meanwhile, ten common organic solvents, including cyclohexane (CyH), benzene (PhH), acetonitrile (ACN), tetrachloromethane (CTC), dichloromethane (DCM), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), ether (DEE), acetylacetone (ACAC), and nitrobenzene (NB), were selected to investigate the fluorescence sensing properties of Cd-MOF in the DMA solution. Compared with other solvents, the quenching effect of NB was the most obvious. Because the other organic solvent molecules are often doped in industrial wastewater and domestic sewage, the anti-interference experiments were performed towards NB (Fig. 6a). The experimental results showed high anti-jamming capability for NB (as shown in Fig. 6b). In order to evaluate the sensitivity of Cd-MOF towards NB, titration experiments were performed in the DMA solution. When the volumes of NB increased from 0 to 0.2 mL, the fluorescence intensity gradually weakened. The Ksv and LOD values of NB was 3.8 × 104 M−1 and 4.2 × 10−5 M, respectively.
image file: d0ce01139h-f6.tif
Fig. 6 (a) The effect of adding other anions on the fluorescence intensity of NB; (b) fluorescence spectra of Cd-MOF with NB at different concentrations, inset: Ksv plot for NB.

Sensing mechanism

A series of fluorescence sensing experiments demonstrated that Cd-MOF could sensitively and selectively detect Fe3+ions, Cr2O72− anions and NB. Therefore, the mechanism of the quenching effect was further studied via UV-vis absorption spectroscopy and PXRD. The PXRD patterns (Fig. S2b) of Cd-MOF after soaking in a solution of Fe3+ ions, Cr2O72− anions and NB matched well with their simulated patterns, demonstrating that the 3D network of Cd-MOF remained intact, so that the fluorescence quenching effect should not be attributed to the destruction of the structure. The UV-vis absorption spectra of Fe3+ ions and Cr2O72− anions, and the emission spectra of Cd-MOF overlapped with different degrees (Fig. 7). Thus, the quenching effect of Fe3+ ions and Cr2O72− anions could be a result of competition for the absorption of UV light.52,53 Since NB has an electron-withdrawing substituent –NO2 group, NB is an excellent electron acceptor with a low molecular orbital (LUMO) energy, which might drive the electrons from the ligand to the guest species, resulting in the fluorescence quenching phenomenon.54–56
image file: d0ce01139h-f7.tif
Fig. 7 Absorbance spectra of Fe3+ ions, Cr2O72− anions, NB and emission spectra of Cd-MOF showing different overlap degrees.

Conclusions

In summary, a novel 3D Cd-MOF was designed and constructed by introducing a long semi-rigid bis(amide)-bis(pyridyl) co-ligand into the Cd-NTB system. Notably, Cd-MOF exhibited remarkable fluorescence sensing behaviours towards numerous metal cations, anions and organic solvents with excellent sensitivity and selectivity, indicating that it can be considered as multiple probes for Al3+, Cu2+, Cr3+, and Fe3+ cations, Cr2O72− anion and nitrobenzene (NB). The LOD value of Fe3+ ions, Cr2O72− anions and NB are 5.1 × 10−5 M, 5.3 × 10−5 M and 4.2 × 10−5 M, respectively. Our studies further suggested that the introduction of a bis(amide)-bis(pyridyl) ligand within MOFs may represent a prospective strategy for designing the desired fluorescent MOF materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The support of the National Natural Science Foundation of China (no. 21671025, 21501013, 21471021) and the Liao Ning Revitalization Talents Program (XLYC1902011) is gratefully acknowledged.

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Footnote

Electronic supplementary information (ESI) available: IR spectra, PXRD, and additional figures. CCDC 1992610. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0ce01139h

This journal is © The Royal Society of Chemistry 2020