Selective fluorescence sensors for detection of nitroaniline and metal Ions based on ligand-based luminescent metal-organic frameworks

Journal of Solid State Chemistry 232 (2015) 96–101

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Selective fluorescence sensors for detection of nitroaniline and metal Ions based on ligand-based luminescent metal-organic frameworks Zongchao Yu a, Fengqin Wang a, Xiangyi Lin b, Chengmiao Wang a, Yiyuan Fu a, Xiaojun Wang a, Yongnan Zhao c, Guodong Li d a College of Environment and Chemical Engineering & Key Lab of Hollow Fiber Membrane Materials & Membrane Process, Tianjin Polytechnic University, Tianjin 300387, China b Suzhou Huihe Pharmaceutical Limited Company, Suzhou 215200, China c College of Materials and Engineering & Key Lab of Hollow Fiber Membrane Materials & Membrane Process, Tianjin Polytechnic University, Tianjin 300387, China d The State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, China

art ic l e i nf o

a b s t r a c t

Article history: Received 5 June 2015 Received in revised form 6 September 2015 Accepted 8 September 2015 Available online 14 September 2015

Metal-organic frameworks (MOFs) are porous crystalline materials with high potential for applications in fluorescence sensors. In this work, two solvent-induced Zn(II)–based metal-organic frameworks, Zn3L3(DMF)2 (1) and Zn3L3(DMA)2(H2O)3 (2) (L ¼ 4,4′-stilbenedicarboxylic acid), were investigated as selective sensing materials for detection of nitroaromatic compounds and metal ions. The sensing experiments show that 1 and 2 both exhibit selective fluorescence quenching toward nitroaniline with a low detection limit. In addition, 1 exhibits high selectivity for detection of Fe3 þ and Al3 þ by significant fluorescence quenching or enhancement effect. While for 2, it only exhibits significant fluorescence quenching effect for Fe3 þ . The results indicate that 1 and 2 are both promising fluorescence sensors for detecting and recognizing nitroaniline and metal ions with high sensitivity and selectivity. & 2015 Published by Elsevier Inc.

Keywords: Metal-organic frameworks (MOFs) Sensing Nitroaniline Metal ions

1. Introduction Now, it is well known that the environmental problems are becoming more and more serious in our daily life and even have a lethal influence on the living organisms. Therefore, governments and scientists have both been investing money and focusing attention towards environmental issues, such as the presence of toxic organic substances and metal ions [1–3]. Among these poisonous substances, there is a great demand for nitroaromatic compounds, which are the broadly raw materials in the chemical synthesis of the pesticides and explosives [4–6]. Moreover, the excessive ingestion of these toxic pollutants may cause some diseases, such as the vomiting and coma etc. Similarly, the metal ions, particularly for Fe3 þ and Al3 þ ions are also essential to the smelting industries and humans [7–9]. As well, the excessive intake of them might lead to the health problems, such as hepatitis, cancer, neurodegenerative Alzheimer's disease and Parkinson’s disease etc. Therefore, developing highly sensitive and selective sensors for rapid and effective detection of nitroaromatic compounds and metal ions is an extremely urgent issue concerning homeland security, environmental protection and humanitarian E-mail address: [email protected] (F. Wang). http://dx.doi.org/10.1016/j.jssc.2015.09.010 0022-4596/& 2015 Published by Elsevier Inc.

concerns [10,11]. To date, many kinds of modern instruments, such as ion mobility spectrometry, X-ray dispersion and Raman spectroscopy, are being employed for the sensitive and selective sensing of nitroaromatic compounds. However, these traditional detection methods are limited for low portability, complex pretreatments and high operational cost etc [12]. Thus, it is very necessary to develop some methods that can be easily applied to effectively detect pollutants. The recent studies show that MOF-based photoluminescence chemosensors have great promise because of their excellent sensitivity, short response time, reusability, and operability [13]. Especially, the diverse structures of MOFs can usually be fine-tuned by incorporation of functional groups and/or manipulation of reaction conditions to improve their emission properties. An effective strategy to synthesize MOF sensors is to use πconjugated organic molecules, which endows the MOFs with good fluorescence properties [14]. Although much efforts has been focused on the construction of fluorescence MOFs for detection of organic small molecule pollutants and metal ions [15–23]. Reports on the multifunctionality for detection nitroaniline and metal ions are scare. Considering the above mentioned, we selected 4,4′-stilbenedicarboxylic acid (L) as the organic linker to construct MOF-based fluorescent materials for these reasons: (1) it is a kind of flexible

Z. Yu et al. / Journal of Solid State Chemistry 232 (2015) 96–101

aromatic dicarboxylate ligand with highly conjugated π system; (2) as a emissive linker it can be used to construct novel topologies MOFs with good photophysical properties; (3) despite some MOFs based on L ligand reported in the literature [24–29], however, reports on luminescent MOFs are scarce, especially those that display fluorescence sensing properties. Therefore, in this paper, we synthesized two luminescent MOFs materials: Zn3L3(DMF)2 (1) and Zn3L3(DMA)2(H2O)3 (2) (L ¼4,4′-stilbenedicarboxylic acid), based on L ligand in different solvent systems according to the literature reported by Bauer et al. [30]. They exhibit ligand-based emission with the increased emission properties compared to free trans-stilbene ligand for the rigidity of the stilbene linker increasing upon coordination to the inorganic units through inhibition of torsion about the central ethylene bond. Just because of the good optical properties of 1 and 2, we selected them as the fluorescence sensing materials to study the applications for selective and sensitive detection of nitroaniline and metal ions. The results indicate that the two MOFs are promising fluorescence probes for detecting nitroaniline and metal ions via fluorescence quenching or enhancement effect, which can be potentially used for pollutants detection or environmental monitoring.

2. Experimental 2.1. Chemicals and characterizations All reagents were obtained from commercial sources and used as received. Infrared spectra were recorded on a FTIR-650 system using KBr pellets in the range 4000–400 cm  1. Elemental analyses (C, H and N) were measured on a Perkin-Elmer auto-analyzer. Thermogravimetric (TG) analysis was carried out on a Netzch STA 449c analyzer at a heating rate of 5 °C/min from ambient temperature to 800 °C. Powder X-ray diffraction (PXRD) data were recorded on a D/MAX-2500 automated diffractmeter. The simulated PXRD patterns were derived from the single crystal data through the diffraction-crystal module of the Mercury program version 3.0. The photoluminescence spectra of the studied complexes in solid state and their samples in acetonitrile were measured on an F–380 Spectrophotometer. 2.2. Synthesis of Zn3L3(DMF)2 (1) A mixture of L (0.04 g, 0.15 mmol), Zn(NO3)2  6H2O (0.045 g, 0.15 mmol), 10 mL DMF were homogeneously mixed under stirring, sealed in a 25 mL stainless steel vessel and statically heated at 85 °C for 3 days under autogenous pressure, and then cooled to room temperature. Elemental analysis (%), Calculated for C54H44N2O14Zn3: C, 56.84; H, 3.89; N 2.45; found: C, 55.92; H, 3.89; N, 2.66. IR spectra (KBr pellet cm  1): 3434 br, 2923 w, 1658 s, 1592 s, 1544 w, 1398 s, 1181 w, 1106 w,1078 w, 1014 w, 979 w, 958 s, 856 w, 790 s, 711 m, 684 m, 644 s, 547 m, 522 m, 472 w, 431 s.

2.4. X-ray crystallography Single crystal X-ray diffraction measurement for 2 was carried out on computer-controlled Bruker SMART 1000 CCD diffractometer equipped with graphite-monochromatized Mo-Kα with a radiation wavelength of 0.71073 Å using the ω-scan technique. The structures were solved by direct methods and refined by full-matrix least squares on F2 using the SHELXS 97 and SHELXL 97 programs [31,32]. Semiempirical absorption corrections were applied using the SADABS program [33]. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were generated geometrically and treated by a mixture of independent or constrained refinement. The crystallographic data for 2 are listed in Table S1. 2.5. Fluorescence titrations in dispersed medium To examine the potential of 1 and 2 for sensing nitroaromatic compounds and metal ions, the MOFs (1.5 mg) were immersed in acetonitrile (4.5 mL), respectively, which were treated by ultrasonication for 1.5 h. All titrations were carried out by gradually adding analytes in an incremental fashion. The corresponding fluorescence emission spectra were recorded at 298 K. Each titration was repeated several times to get concordant values. For all measurements, the suspensions of 1 were excited at λex ¼355 nm and the corresponding emission wavelengths were monitored from 365 to 600 nm. The suspensions of 2 were excited at λex ¼365 nm and the corresponding emission wavelengths were monitored from 375 to 600 nm. The fluorescence efficiency was calculated using the formula [(I0  I)/I0]  100% (I0 is the initial fluorescence intensity). Fluorescence quenching titration was further evaluated using the Stern–Volmer equation I0/I ¼1 þKsv[M], where the values I0 and I are the fluorescence intensity of the MOFs suspension without and with addition of analytes, respectively, Ksv is the quenching constant, [M] is the analytes concentration.

3. Results and discussion Compound 1 and 2 were prepared according to the literature under different solvent systems. The structure of 1 was confirmed by powder X-ray diffraction analysis. The PXRD pattern of assynthesized sample of 1 is very close to the simulated pattern from single crystal structure of 1 (Fig. S1), which suggest that the structure of 1 is identical to the literature reported [29]. In compound 1, each L ligand uses its four oxygen atoms to link four Zn (II) ions into a 2D layer metal-organic framework containing trinuclear Zn3(RCO2)6 SBUs, as shown in Fig. 1. The SBU contains a linear array of three zinc atoms lying on a 3-fold axis, the central zinc atom rests on a crystallographic inversion center. The central zinc atom has octahedral coordination geometry, while the terminal zincs are tetrahedral, their apical sites are occupied by O

2.3. Synthesis of Zn3L3(DMA)2(H2O)3 (2) A mixture of L (0.0134 g, 0.05 mmol), Zn(NO3)2  6H2O (0.015 g, 0.05 mmol), 8 mL DMA and 3 mL H2O were homogeneously mixed under stirring, sealed in a 25 mL stainless steel vessel and statically heated at 85 °C for 3 days under autogenous pressure, and then cooled to room temperature. Colorless block crystals were obtained by filtration. Elemental analysis (%), Calculated for C56H54N2O17Zn3: C, 54.99; H 4.45; N 2.29; found: C 54.90; H 4.03; N 2.05. IR spectra (KBr pellet cm  1): 3432 br, 2923 w, 1589 s, 1546 s, 1398 s, 1166 w, 1072 w, 975 w, 956 w, 858 w, 792 s, 792 s, 713 m, 642 s, 617 m, 549 m, 524 m, 474 w, 428 s.

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Fig. 1. The 2D layer structure of 1.

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3.1. The photophysical properties The luminescent MOFs with d10 metal ions have been the potential candidates for their excellent photoactive materials. Thereby, the solid state luminescent properties of 1 and 2 were investigated at room temperature, as shown in Fig. S6. The maximum emission bands of 1 and 2 both located at 447 nm (λex ¼355 nm for 1 and λex ¼365 nm for 2). The emission spectra of 1 and 2 display the obvious blue shift compared to that of L ligand (λem ¼ 467 nm), probably due to substantial electronic coupling between the neighbor ligands through the Zn (II) metal ions. The similar emission bands of 1 and 2 comparing with L ligand in the solid state demonstrate that the fluorescence emissions of 1 and 2 are ligand-based. And also, they both exhibit strong visible blue light when excited by the ultraviolet light. We also examined the fluorescence properties of 1 and 2 in common organic solvents. The emission spectra of 1 and 2 (Fig. S7 and S8) are both similar to those of solid state samples. The strong emissions of 1 and 2 in solid state and organic suspensions confirm that they have potential applications in liquid phase fluorescence detection. 3.2. Detection of nitro aromatic compounds Based on the fluorescence properties of 1 and 2, fluorescence detection experiments were carried out with the acetonitrile suspension of 1 and 2 by gradually adding the nitroaromatic compounds. The addition of nitroaromatic compounds, such as nitrobenzene, p-chloronitrobenzene, p-nitrotoluene, m-nitrotoluene, o-nitroaniline, m-nitroaniline and p-nitroaniline promoted the fluorescence quenching effect in the fluorescence intensities of the two MOFs to various extents. The percentage of

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6 I /I =0.51+5.99×10 [M]

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R =0.9701

4

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atoms of monodentate DMF molecules. The disordered DMF molecules filled the space above and below the layer units. Furthermore, individual layers stack together with a cubic ABCABC motif, resulting in a dense structure without significant overall porosity. Compound 2 has been successfully synthesized using the same initial reactants but in different solvent system. Single crystal X-ray diffraction analysis shows that compound 2 also has a 2D layer skeleton structure with disordered DMA molecules filled the space above and below the layer units (Fig. S2), which is identical to that of 1. Powder X-ray diffraction and IR spectrum of the assynthesized sample of 2 (Figs. S3 and S4) further confirmed their consistency. All these demonstrate that the framework structure of 2 is identical to that of 1 except for the differences of DMA and water molecules. The thermal stabilities of 1 and 2 were studied using thermogravimetric analysis (TGA). Similar behaviors occurred between them. As shown in Fig. S5 the framework of 1 and 2 both collapsed following by the release of solvent molecules. The results show that the solvent molecules play an important role in stabilizing the structures of the two MOFs.

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Wavelength(nm) Fig. 3. Fluorescence titration of 1 dispersed in acetonitrile with the addition of different volumes of 10  3 M acetonitrile solution of p-nitroaniline. Excitation wavelength was 355 nm and fluorescence emission was monitored from 365 nm to 600 nm. The slit widths for excitation and emission were both 5 nm. The insert is Stern–Volmer plot of 1 quenched by p-nitroaniline acetonitrile solution.

fluorescence quenching of 1 and 2 obtained for different analytes were given in Fig. 2. For selected analytes, 1 and 2 show similar sensing behaviors (Fig. S9–S14). They both exhibit significant responds to nitroanilines. In particular, p-nitroaniline gives the best quenching performance under the same experimental conditions. The fluorescence quenching efficiency increased drastically in the presence of p-nitroaniline even in the low concentration range and reached above 83.1% for 1 (Fig. 3) and 74.4% for 2 (Fig. 4) with the addition of 0.2 mmol p-nitroaniline. For the other selected analytes, the quenching efficiency is obviously decreased and reached about lower 20%. It might be because that the –NH2 group as hydrogen bonds acceptor and donor can form hydrogen bonds with MOFs. We also studied the relationships between the fluorescence intensities of the two MOFs and the concentrations of p-nitroaniline. As shown in Figs. 3 and 4 (the insert curve), the linear correlation coefficient (R) in the Ksv curve suggest that the quenching effect of p-nitroaniline on the fluorescence of 1 and 2 fit the Stern–Volmer mode well. Meanwhile, the different concentration of p-nitroaniline molecules was added slowly to the suspension of 1 and 2 to study the quantitative quenching effect of them (Figs. S15 and S16). The results show that the concentrations of p-nitroaniline have a remarkable influence on the fluorescence intensity of 1 and 2. When the concentration of p-nitroaniline was 10  2 mol L  1, the quenched efficiency of 1 is about 98.0% and 2 is about 99.1% with the addition amount of p-nitroaniline only 60 mL. However, with the concentration of p-nitroaniline decreasing from 10  3 to 10  5 mol L  1, the fluorescence quenching efficiencies of 1 and 2 were decreased. The fluorescence quenching efficiencies of 1 were about 83.1% (10  3), 23.1% (10  4) and 12.5% (10  5) with the addition amount of p-nitroaniline 200 mL, respectively. For 2, the fluorescence quenching efficiencies were about 74.8% (10  3), 30.6% (10  4) and 8.4% (10  5) under the same conditions, respectively. This further confirmed that the concentrations of p-

Fig. 2. Percentage of fluorescence quenching of 1(a) and 2(b) obtained for different analytes in acetonitrile solutions at room temperature.

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Fig. 4. Fluorescence titration of 2 dispersed in acetonitrile with the addition of different volumes of 10  3 M acetonitrile solution of p-nitroaniline. Excitation wavelength was 365 nm and fluorescence emission was monitored from 375 nm to 600 nm. The slit widths for excitation and emission were both 5 nm. The insert is Stern–Volmer plot of 2 quenched by p-nitroaniline acetonitrile solution.

nitroaniline obviously affect the fluorescence intensities of the two MOFs. For comparison, triethylamine and nitromethane were also selected as the probes to investigate the influence of aliphatic compounds on the fluorescence of MOFs. As shown in Fig. 2, no obvious fluorescence intensity change occurred after adding triethylamine or nitromethane into the acetonitrile suspension of MOFs (Figs. S17 and S18). As a sensor, the recyclable usability is very important for evaluating the sensing performance. Herein, we studied the repeated usability for sensing of p-nitroaniline of 1 by simply filtering and washing with fresh solvent. As shown in Fig. 5, compound 1 exhibits excellent reversible sensing ability. The initial fluorescence intensities and the quenching efficiencies did not

change much over five repeated cycles. The structure was still stable after five cycles of p-nitroaniline detection, as confirmed by powder X-ray diffraction (Fig. S19). For the two MOFs, the mechanism of fluorescence quenching for nitroaromatic compounds may be attributed to the effective interactions between the analytes and MOFs, which lead to electron transfer between the MOFs and the analytes adsorbed on the surfaces of the MOFs. When the electrons transfer from the framework of MOFs to analyte molecules will result in the detectable fluorescence quenching effect. The fact that nitroanilines exhibits the significant fluorescence quenching effect may be attributed to the effective interactions containing hydrogen bonds and π–π stacking between the MOFs and analytes. The -NH2 groups as hydrogen bonds acceptor and donor can take part in forming hydrogen bonds with MOFs and reinforce the π–π stacking interactions between the framework of MOFs and benzene ring of nitroanilines. The cooperation of the two interactions induces facile intermolecular electron transfer and consequently leads to significant fluorescence quenching of the two MOFs in the presence of nitroanilines [34,35]. 3.3. Detection of metal ions To show the potential application of 1 and 2 for detecting metal ions, the responses of the fluorescence of 1 and 2 toward metal ions in solvent media, including M(NO3)x (M ¼Ag þ , Li þ , Na þ , Ni2 þ , Co2 þ , Pb2 þ , Zn2 þ , Ba2 þ , Ca2 þ , Mg2 þ , Al3 þ , Cu2 þ and Fe3 þ ) were studied. As shown in Fig. 6, the fluorescence responses of 1 and 2 are both heavily dependent on the species of metal ions. The 1400 1200

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6 I /I =0.62+5.85×10 [M]

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Wavelength(nm) Fig. 5. Recyclability test on 1. Fluorescence was recovered by simply filtering and washing with fresh solvent. p-nitroaniline was used as a model analyte. Red bar: emission of sample before sensing of p-nitroaniline. Blue bar: emission of sample after sensing of p-nitroaniline. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. Fluorescence titration of 1 dispersed in acetonitrile with the addition of different volume of 10  3 M acetonitrile solution of Fe3 þ . Excitation wavelength was 355 nm and fluorescence emission was monitored from 365 nm to 600 nm. The slit widths for excitation and emission were both 5 nm. The insert is Stern– Volmer plot of 1 quenched by Fe3 þ acetonitrile solution.

Fig. 6. (a) Percentage of fluorescence quenching of 1 interacting with different metal ions in 10  3 M acetonitrile solution of M(NO3)x, excited at 355 nm and monitored from 365 to 600 nm; for clarity, we only gave the enhancing efficiency of Al3 þ with the addition amount of 0.1 mmol in the bar diagram, the others are all 0.2 mmol). (b) Percentage of fluorescence quenching of 2 interacting with different metal ions in 10  3 M acetonitrile solution of M(NO3)x (excited at 365 nm and monitored from 375 to 600 nm). The slit width for both excitation and emission was 5 nm.

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detailed fluorescence quenching efficiency of different metal ions of 1 and 2 is shown in Fig. S20–S31. For the given metal ions, the Fe3 þ and Al3 þ ions have a significant effect on the fluorescence intensity of 1 (Fig. 6a), whereas others have a negligible effect on the fluorescence intensity. After Fe3 þ is incorporated, the fluorescence intensity of 1 was quenched by about 89.7% after the addition of 0.2 mmol Fe3 þ (Fig. 7). The relationships between the fluorescence intensity of 1 and the concentration of Fe3 þ ions is in good agreement with the first-order exponential equation (insert of Fig. 7). However, with the addition of Al3 þ , 1 exhibits a new emission at 393 nm in addition to the original peak at 447 nm. Furthermore, the fluorescence intensity at 393 nm considerably

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Fig. 8. Fluorescence titration of 1 dispersed in acetonitrile with the addition of different volume of 10  3 M acetonitrile solution of Al3 þ . Excitation wavelength was 355 nm and fluorescence emission was monitored from 365 nm to 600 nm. The slit width for both excitation and emission was 5 nm.

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2.4 I /I =1.1+9.05×10 [M]

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Wavelength(nm) Fig. 9. Fluorescence titration of 2 dispersed in acetonitrile with the addition of different volume of 10  3 M acetonitrile solution of Fe3 þ . Excitation wavelength was 365 nm and fluorescence emission was monitored from 375 nm to 600 nm. The slit width for both excitation and emission was 5 nm. The insert is Stern– Volmer plot of 2 quenched by Fe3 þ acetonitrile solution.

intensified by 325.5% with the addition of only 0.1 μmol Al3 þ , along with a small blue-shift of the emission wavelength. With the continuous adding the Al3 þ , the fluorescence intensity increased by 777.7% with the addition amount of 0.2 μmol. However, the original emission peak at 447 nm slightly quenched by 23.5% (Fig. 8). The titration experiments indicate that the fluorescence intensity of 1 is gradually weakened or increased by increasing the amount of Fe3 þ or Al3 þ ions. To further study the effect of metal ions concentrations on the fluorescence intensity of 1, we took Fe3 þ as an example and measured the fluorescence spectra of 1 in acetonitrile solutions containing Fe(NO3)3 of different concentration. It is notable that the concentrations of metal ions also have a remarkable influence on the fluorescence intensity of 1. The fluorescence intensity of 1 gradually quenched as the Fe3 þ concentration increased from 10  5 to 10  2 mol L  1 (Fig. S32). When the concentration is 10  2, the fluorescence intensity of Fe3 þ -incorporated 1 is quenched by 97% with the addition amount of Fe3 þ only 60 uL. With the concentration of Fe3 þ decreasing from 10  3 to 10  5, the quenching effect is up to 89.8%, 30.5% and 0.44%, although the addition amount of Fe3 þ increased to 200 mL, respectively. The interesting quenching effect of Fe3 þ and the enhancing effect of Al3 þ to 1 indicate that it is a good candidate as a fluorescence sensor for highly selective sensing of Fe3 þ and Al3 þ ions. Likewise, the detection experiment of 2 for metal ions was also conducted. As shown in Fig. 6b, the fluorescence intensity of 2 shows the quenching effect for all metal ions, but Fe3 þ gives the most significant quenching effect. The fluorescence efficiency quenched by 54.3% after the addition of 0.2 mmol Fe3 þ (Fig. 9), indicating the high selectivity for detection and specific recognition of Fe3 þ . The relationship between the fluorescence intensity of 2 and the concentration of Fe3 þ ions is also in good agreement with the first-order exponential equation (insert of Fig. 9). Similarly, the concentration control experiment also demonstrates that the concentration of Fe3 þ have a significant effect on the fluorescence efficiency of 2 (Fig. S33). The fluorescence intensity of 2 gradually quenched as the Fe3 þ concentration increased. For the two MOFs, the different fluorescence sensing behaviors for Al3 þ may be attributed to the difference of solvent molecules, which results in the different interactions between the Al3 þ and MOFs. Furthermore, we took sensitive Fe3 þ as an example and conducted the completing experiments to clarify the selectivity and anti-interference sensing abilities of 1 and 2 in the presence of other metal ions. For simplicity, we selected some representative metal ions to study the completing experiments (Figs. S34–S41). As shown in Fig. 10, the selected metal ions have no obvious influence on the sensing performance of Fe3 þ . Therefore, the qualitative detection of Fe3 þ did not interfere. These results demonstrate that 1 and 2 are reliable and efficient fluorescence sensors for detection of Fe3 þ with high sensitivity and selectivity.

Fig. 10. Competitive binding studies of the different metal ions: The blue bars represent the fluorescence intensity of MOFs in acetonitrile solution and 0.1 mmol (10  3 M, 100 mL) other metal ions. The red bars represent the fluorescence intensity of MOFs in acetonitrile solution, 0.1 mmol (10  3 M, 100 mL) other metal ions and 0.2 mmol (10  3 M, 200 mL) Fe3 þ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Z. Yu et al. / Journal of Solid State Chemistry 232 (2015) 96–101

4. Conclusions In summary, we have synthesized two MOFs and explored the applications as fluorescence sensing materials. The results demonstrate that the two MOFs both possess sensitive selectivity for sensing nitroanilines. The control experiment on concentration of p-nitroaniline has a remarkable influence on the fluorescence intensity of the two MOFs. And also, 1 and 2 can be conveniently reused by simply washing with fresh solvent. In addition, the two MOFs both exhibit obviously fluorescence sensing effect on metal ions. Of which, complex 1 exhibits sensitive fluorescence quenching or enhancement effect for Fe3 þ and Al3 þ ions. However, complex 2 only shows an obvious fluorescence quenching effect for Fe3 þ ions. The high selectivity make the two MOF materials promising candidates for the development of low-cost and recyclable luminescence sensors for selectively sensing nitroaniline and Fe3 þ or Al3 þ in liquid phase. Further investigations are underway, including the study of the mechanism and the extension of the construction strategy to other MOF systems.

Acknowledgments This project is supported by NSFC (Grant no. 21271138) and the NSF of Tianjin (No.14JCYBJC17500).

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.jssc.2015.09.010.

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