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Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: Multifunction detection and photocatalysis property
Journal Pre-proof Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: Multifunction detection and photocatalysis property Jiapeng Cheng, Tao Hu, Wenjun Li, Zhidong Chang, Changyan Sun PII:
S0022-4596(19)30630-9
DOI:
https://doi.org/10.1016/j.jssc.2019.121125
Reference:
YJSSC 121125
To appear in:
Journal of Solid State Chemistry
Received Date: 28 October 2019 Revised Date:
6 December 2019
Accepted Date: 10 December 2019
Please cite this article as: J. Cheng, T. Hu, W. Li, Z. Chang, C. Sun, Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: Multifunction detection and photocatalysis property, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/ j.jssc.2019.121125. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Jiapeng Cheng synthesized the Zn-MOF and studied the fluorescent probe property. Tao Hu studied the photocatalytic property of Zn-MOF. Wenjun Li supervised the study of the photocatalytic property. Zhidong Chang supervised the synthesis and characterization of MOFs. Changyan Sun supervised the whole work and accomplished the manuscript.
Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: multifunction detection and photocatalysis property Jiapeng Cheng, Tao Hu, Wenjun Li, Zhidong Chang, Changyan Sun* School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083, China Abstract: In this paper, we synthesized a zinc metal-organic framework material (Zn-MOF) based on fluorenone carboxylate ligand and studied the application of Zn-MOF in the fields of fluorescent detection and photocatalysis degradation. The experiment results show that it is a fluorescent probe that can quickly recognize organic solvent nitrobenzene (NB) from other solvents. The detection limit is 2.9 µM, and Ksv (quenching efficiency) is 3.547× 103 M-1. In particular, Zn-MOF can identify nitrobenzene derivatives, such as nitrophenol and nitrotoluene, and the quenching efficiency can reach 16.54% and 92.64%, respectively. Moreover, Zn-MOF can be used to detect metal ions and has high selectivity for Al3+ and Fe3+ ions. Competitive experiments show that other metal ions will not interfere with the detection. In addition, Zn-MOF can also be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours. Keywords: metal-organic framework; fluorescent probe; nitrobenzene; photocatalytic degradation; Rhodamine B 1. Introduction As one kind of new functional materials, metal-organic frameworks (MOFs) have been widely used in various fields [1], such as gas separation and storage [2], fluorescent chemosensors [3], multi-phase catalysis [4] and drug transport [5] due to tunable porosity, high specific surface area, and luminescent property. Especially in the field of fluorescent chemosensors, metal-organic frameworks are receiving more and more attentions because of their good selectivity, high sensitivity, low cost and simple operation [6]. People are focused on designing new MOFs as fluorescent chemosensors by introducing large poroties [7], or empty coordination sites via
pre-design of the structure or post-synthetic modification [8, 9]. A great number of MOFs materials have been synthesized through various methods [10-13], and used as fluorescent chemosensors to detect small organic solvents, nitro aromatics explosives and metal ions [14-18]. With the development of industry, large amounts of chemical pollutants, such as toxic organic small molecules and nitro aromatics explosives are released into the environment, which have caused a lot of adverse influences on human health [19]. Therefore, the rapid detection of these harmful substances is very important. Nowadays, fluorescent probes based on MOFs materials are becoming a hotspot in the detection of organic pollutants. Pan et al. reported a new Zn(II)-based coordination polymer which displayed a highly selectivity for nitrobenzene (NB) based on the photo-induced electron transfer mechanism, and the detection limit could reach 9.6 µM [20]. Zhao et al. reported a zinc carboxylate MOF which could be used to detect traces of nitrobenzene (NB) rapidly and the detection limit was 7.2µM [21], It was reported that the electron transfer and electrostatic interactions may play an intrinsic role in the detection process. Aluminum and iron are two common elements in daily life. It has been found that excessive deposition of aluminum in brain tissue can lead to memory loss and aging. Deficiency of Iron can lead to anemia, while excessive iron may cause human tissue damage and other diseases [22, 23]. Therefore, the detection of Al3+ and Fe3+ ions are particularly important for human life. Recently, there are a lot of MOFs reported for metal-ions detection. Cao et al. has synthesized a water-stable luminescent terbium-based metal-organic framework which could quickly and specifically recognize Fe3+ and Al3+ ions in aqueous solution [24]. Liu et al. reported a ZIF-8 which could be used to detect Cu2+ and Cd2+ ions [25]. As a common industrial raw material, dye molecules are widely used in fluorescent dyes, colored glass, and so on. However, due to its poor biodegradability, dye molecules will cause harm to human beings and environment. Therefore, it is necessary to effectively degrade dye molecules. TiO2 was the first photocatalysis reported [26]. After that, a lot of nano catalyses including metal oxides [27], metal
sulfides [28] and other nanoparticles [29-31] have been investigated, and they could effectively degrade dye molecules. But the fast electron-hole pair recombination speed of these nano catalyses becomes a problem that should be solved. Recently, porous MOFs materials become research hotsports in this field [32-34]. The introduction of organic groups made electron-hole recombination difficult and the charge separation efficiency of MOFs is high [35, 36]. Alvaro et al. reported the first example MOF-5 which could be used as photocatalysts [37]. Experiments showed that MOF-5 exhibited comparable activity to commercial TiO2 (Degussa P-25) in the degradation of phenol in aqueous solution. Considering their tunable porosity and high specific surface area, researches on MOFs materials with high photocatalysts efficiency are getting lots of attention. In this paper, a metal-organic framework material with open pores was selected, which employed 9-fluorenone-2,7-dicarboxylicacid (H2FODC) and ethylene glycol as organic ligands and Zn2+ as central metal ions to form Zn-MOF [38]. The characteristics of this material are as followings [38, 39]: (i) The existence of the conjugation system results in fluorescent property of Zn-MOF; (ii) The resulting framework contains a 3D interconnected pore system with small channels; (iii) The carbonyl group possesses high chemical reactivity and may act as the basic site in base-catalyzed reactions. The incorporation of this group onto the pore surface would constitute a new interaction site for selective sorption and/or catalysis inside the pores. Based on the above characteristics, this Zn-MOF might be potential materials as a fluorescent probe or catalysis. But up to now, there is only one literature reporting that this Zn-MOF could rapidly and selectively adsorb cationic dyes [39]. So in this work, this Zn-MOF was used as a fluorescent probe to detect organic solvents, nitro aromatics explosives and metal ions, and the photocatalytic property of Zn-MOF was studied.
2. Experimental Section 2.1 Materials and Measurements All chemicals purchased were of reagent grade or better and were used without
further purification. Fluorescence spectra were measured on a F-4500 fluorescence spectrophotometer (Hitachi, China) with a xenon lamp as the excitation source and quartz cuvette (path length=1cm). The infrared data (IR) were collected on a Digilab FTIR-8400S spectrophotometer with KBr pellets in the range 4000-400 cm-1.The UV-vis absorption spectra were determined on a T9S. Powder X-ray diffraction (PXRD) patterns were recorded on an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation in the 2θ range of 5-40º.
2.2 Synthesis of Zn-MOF. Zn-MOF was synthesized according to the previous literature with a small modification [38, 39]. Zn (OAc) 2·2H2O (0.044 g, 0.2 mmol) and H2FODC (0.027 g, 0.1 mmol) were dissolved in N,N-dimethylformamide (DMA, 6 mL) and 6mL glycol was added. Then the mixture was transferred to a 25mL Teflon-lined stainless steel and heated at 120 °C for 72 h. Upon cooling, orange crystals were obtained by filtration, washed with DMA and H2O, and dried in the air. Fig.S1 showed the infrared spectrum of Zn-MOF. The peaks at 1595 and 1387 cm-1 are the characteristic absorption of coordinated carboxylate groups. While the peak at 1715 cm-1 could be attributed to the stretching vibration of noncoordinated carbonyl groups. The PXRD spectrum of Zn-MOF was carried out (Fig. S2), which was the same as that in the literature [39], indicating that the target product [Zn5(FODC)2(OCH2CH2O)3 (H2O)]·4DMA·5.5H2O was successfully synthesized.
2.3 Spectroscopic measurements. The 0.1 M stock solutions of metal ions (Na+, Al3+, Mg2+, Li+, Ca2+, K+, Ba2+, Zn2+, Co2+, Cu2+, Mn2+, Ni2+, Hg2+, Fe3+ and Cd2+) were prepared in aqueous solution using their chlorides. The 1.0 × 10-3 M stock solutions of nitro aromatics (nitrobenzene (NB), p-nitrophenol (PNP), o-nitrophenol (ONP), m-nitrotluene (MNT), o-nitrotoluene (ONT)) were prepared in dimethyl sulfoxide (DMSO). The excitation wavelength was 315 nm. And both of the excitation and emission
slit widths were 5.0 nm.
3. Results and discussion 3.1 Structure of Zn-MOF According to the literature [38], the Zn-MOF crystallizes in hexagonal P63/mcm space group with the formula [Zn5(FODC)2(OCH2CH2O)3(H2O)]·4DMA·5.5H2O. It contains a 3D interconnected pore system with large chambers and small channels, and the pore surface is decorated by both uncoordinated carbonyl groups and potential unsaturated metal centers.
3.2 Fluorescent property of Zn-MOF Fig. 1 shows the fluorescent spectra of the Zn-MOF and H2FODC ligand at room temperature. It can be observed that upon excitation of 315 nm, both Zn-MOF and H2FODC ligand exhibit an emission at 515 nm. The difference is that the formation of the Zn-MOF enhances fluorescence.
Fig 1. Solid-state fluorescent spectrum of the Zn-MOF and H2FODC ligand.
3.2 Fluorescent Sensing Applications Sensing of small organic solvents. 3 Mg fully ground Zn-MOF samples were dispersed in 3 mL different small organic solvents including chloroform(CHCl3), ethanol(EtOH), methanol(MeOH), N,N′-dimethylacetamide (DMA), diethyl ether, N,N′-dimethylformamide (DMF), acetone, nitrobenzene(NB), acetonitrile, ethylene glycol, dimethyl sulfoxide (DMSO), benzene, toluene, CCl4, and ethyl acetate, and ultrasonicated for 5 min, which led to stable suspensions. Under UV lamp, all
suspensions were bright except that Zn-MOF in NB was black, as shown in Fig. 2a. The fluorescent spectra of Zn-MOF suspensions were shown in Fig. 2b. It could be seen that NB had the largest quenching effect on the emission of Zn-MOF (Fig. 2c). It showed that Zn-MOF could effectively detect NB as a fluorescent probe. Owing to the highly electron deficient property of NB and the electron rich nature of Zn-MOF, the quenching mechanism might be ascribed to the excited electron transfer between Zn-MOF and NB.
Fig. 2. (a) Zn-MOF in different organic solvents under UV lamp; (b) Fluorescent spectra of Zn-MOF (1mg/mL) in different organic solvents; (c) Comparison of fluorescent intensity of Zn-MOF in different organic solvents.
To explore the detection limit of Zn-MOF for detecting NB, titration experiments were carried out. The fluorescent intensity of Zn-MOF gradually decreased with the increasing concentration of NB. As expected, as the NB concentration increased from 0 to 2.4 mM, the fluorescence of Zn-MOF suspensions showed significant quenching (Figs. 3a, 3b). The detection limit was calculated to be 2.9 µM based on 3σ/K, which was superior to most of reported values [20, 21, 40]. The fluorescence quenching efficiency was analyzed using the Stern-Volmer equation I0/I = KSV [M] + 1, where I0 and I are the fluorescent intensities before and after the addition of NB, respectively, [M] is the molar concentrations of NB, and KSV is the quenching efficiency (M-1). As shown in the inset of Fig. 3c, the Stern-Volmer plots of Zn-MOF exhibited a good
linear correlation with the concentration of NB. And the Ksv value was calculated 3.547× 103 M-1. (a)
(b)
(c)
Fig. 3. (a) Fluorescent spectra of Zn-MOF with different concentrations of NB in DMSO; (b)The fluorescent intensity of Zn-MOF with different concentrations of NB in DMSO at 530 nm. (c)The quenching efficiency Ksv.
In practical applications, the repeatability of fluorescent probes is essential. The four-cycle recovery experiments were performed to study the repeatability of Zn-MOF in the detection of NB. After each fluorescence test, Zn-MOF was collected by centrifugation and then washed with DMSO several times. As illustrated in Fig. 4, it is noted that Zn-MOF showed the similar relative fluorescent intensity after each cycle, and the PXRD of Zn-MOF after four-cycle recovery was matched well with that of the as-synthesized samples (Fig. S3), which revealed that Zn-MOF had good repeatability.
Fig. 4. The repeatability of Zn-MOF (The red column is relative intensity of washed Zn-MOF, the black column is relative intensity of washed Zn-MOF with NB)
Sensing for nitrobenzene derivatives. Based on the above experiments, it can be concluded that this Zn-MOF could be used to detect NB from common organic solvents. Then, the detection of nitrobenzene derivatives by Zn-MOF was tested. The fluorescent spectra of Zn-MOF were recorded in a series of nitrobenzene derivatives such as p-nitrophenol (PNP), o-nitrophenol (ONP), m-nitrotluene (MNT), and o-nitrotoluene (ONT), as shown in Fig. 5. It could be seen that Zn-MOF had different response to different nitro aromatics. The quenching efficiencies of MNT and ONT were 92.64% and 87.96%, while the quenching efficiencies of PNP and ONP were 16.54% and 24.94%. The reason for this difference might be that -OH is a strong electron donating group, which weakens the excited electron transfer between the Zn-MOF and nitro aromatics [40]. (a)
(b)
Fig. 5. (a) Fluorescent spectra of Zn-MOF (1 mg/mL) in DMSO with nitrobenzene derivatives; (b) Quenching efficiency of nitrobenzene derivatives.
Sensing for metal ions. 3.0 Mg fully ground Zn-MOF sample was dispersed in 3.0 mL H2O/DMF (v/v=1:1) and ultrasonicated for 30 min to make a suspension of
Zn-MOF. Then, 15 kinds of metals ions including Na+, Al3+, Mg2+, Li+, Ca2+, K+, Ba2+, Zn2+, Co2+, Cu2+, Mn2+, Ni2+, Hg2+, Fe3+, and Cd2+ were added respectively. The fluorescent intensities of these suspensions were recorded at room temperature. As shown in Fig. 6, upon the addition of metal ions except Al3+ and Fe3+ ions, there were no significant changes in the fluorescence spectra of Zn-MOF. But when Al3+ ions were added, the fluorescence of Zn-MOF had a 30 nm blue shift. The reason for this phenomenon may be that the uncoordinated carbonyl groups of the Zn-MOF coordinate with Al3+ ions, resulting in the change of the HOMO-LUMO energy level of the Zn-MOF. When Fe3+ ions were added, the fluorescence of Zn-MOF was completely quenched. In order to explain this phenomenon, the UV-vis absorption spectra of metal ions and the fluorescence excitation spectrum of the Zn-MOF were tested. As shown in Fig.S4, the UV-vis absorption peaks of Fe3+ ions and the fluorescence excitation spectrum of the Zn-MOF were almost completely overlapped. This means that there may be competitive absorption between Fe3+ ions and the Zn-MOF, resulting in the fluorescence quenching. This showed that Zn-MOF could be used to detect Al3+ and Fe3+ ions with fluorescent spectra.
Fig. 6. Fluorescent spectra of Zn-MOF with different metal ions in H2O/DMF (v/ v = 1:1)
Competitive experiments. To test the effect of other metal ions in the detection of Al3+ and Fe3+ ions, competitive experiments were performed. As shown in Fig. 7(a), it can be observed that when other metal ions except Fe3+ ions were added to the Zn-MOF-Al system, the emission peak position of Zn-MOF-Al did not move. This indicated that the existence of other metal ions did not affect the detection of Al3+ ions. However, the addition of Fe3+ ions completely quenched the fluorescence of Zn-MOF-Al system. But if ascorbic acid was added first, this interference could be eliminated. The same competitive experiment was also conducted in the detection of Fe3+ ions. The result (Fig. 7(b)) showed that the presence of other metal ions did not affect the detection of Fe3+ ions. Therefore, Zn-MOF could be effectively used to detect Al3+ and Fe3+ ions with high selectivity. (a)
(b)
Fig. 7. (a) Fluorescence emission wavelength of Zn-MOF (Black bars: with Al3+ ions; Red bars: with Al3+ ions and other metal ions); (b) Florescent intensity of Zn-MOF (Red bars: with other metal ions; Black bars: with Fe3+ ions and other metal ions).
3. Photocatalysis Rhodamine B (RhB) is chosen as a representative dye to evaluate the capability of Zn-MOF for degradation of organic pollutions. Control experiments were firstly discussed to illustrate the photocatalytic nature of the reaction. As shown in Fig. 8(a), no significant variation in the concentration of RhB is observed under UV-vis light irradiation, indicating that RhB is resistant to UV-vis light irradiation. When 30 mg Zn-MOF solid sample was dispersed into RhB solution (30 mL) in the dark, in the first one hour, there is a slight decrease in the concentration of RhB, indicating that a small fraction of RhB is adsorbed into the pores of the Zn-MOF. In the next four
hours, the concentration of RhB remains unchanged, suggesting that the resultant mixture have reached adsorption-desorption equilibrium. Then the photocatalytic activities of the Zn-MOF was evaluated by the degradation of RhB under UV irradiation from 200 to 800 nm on a 400 W Xe lamp. 3.0 ML samples were taken out for analysis every hour using a 7230G visible spectrophotometer which measured the absorbance of the solution to estimate the photocatalytic performance of Zn-MOF to RhB. The calculation results showed that Zn-MOF could degrade 85% of RhB within 3 hours, which was comparable to the literature [34, 40]. Emam and co-workers explored a Ln-MOF which could be used as photocatalysts to degrade 85% of RhB in 120 min [34]. Chen and co-workers synthesized a doubly interpenetrated porous MOF which could degrade methyl orange (MO) completely in 120 min under UV irradiation [40]. These results show that the Zn-MOF can be used as an effective photocatalyst. The possible degradation mechanism was shown in Fig. 8(b) [29]. According to the energy band theory, the empty outer orbitals of metal centers in MOF are the conduction band (CB), and the outer orbitals of organic ligands are the valence band (VB). Under the light, the electrons (e-) are excited from the valence band to the conduction band, leaving holes (h+) in the valence band, thus produce electron-hole pairs. The reaction of valence band holes (h+) and photogenerated electrons (e-) with water and oxygen can produce ·OH radicals and superoxideions (·O2-· ), which are known to degrade organic dyes.
Fig. 8. (a) Control experiments for the photocatalytic degradation of RhB. (b) The deduced mechanism of the degradation of RhB by Zn-MOF.
Next, the recycling of Zn-MOF was tested. After each photodegradation test, Zn-MOF was washed with H2O several times and dried, then was added into a new RhB solution to test. After 3 cycles, the results were show in Fig. 9. It can be found that the degradation effect of Zn-MOF on RhB was still obvious. And the PXRD of Zn-MOF after 4 cycles was matched well with that of the as-synthesized samples (Fig. S5), indicating that the Zn-MOF had good recycling.
Fig. 9. Four cycle experiments of Zn-MOF
4. Conclusions In this work, a Zn-MOF with open channels has been successfully synthesized. The application in the fields of fluorescent detection and photocatalysis degradation were studied. The experiments showed that this Zn-MOF could rapidly and selectively detect NB from other organic solvents, and the detection limit was 2.92
µM. Moreover, Zn-MOF could selectively detect Al3+ and Fe3+ ions from 15 kinds of metal ions. Competitive experiments indicated that other metal ions did not interfere with the detection. In addition, Zn-MOF showed good photocatalytic activities. It could effectively degrade RhB under UV-vis, and the degradation efficiency reached 85% within 3 hours.
Acknowledgements This project is supported by National Nature Science Foundation of China (21101013) and Special Science Project of University of Science and Technology Beijing (FRF-BR-17-002B).
Conflict of interest The authors declare that they have no conflict of interest.
Supplementary information: The IR spectra of Zn-MOF are shown in Fig.S1. The PXRD curves for Zn-MOF as-synthesized and after 4 times repeatability experiments, after 4 times photodegradation experiments are shown in Figs.S2, S3 and S5. The UV-vis absorption spectra of metal ions and the fluorescence excitation spectrum of Zn-MOF are shown in Fig. 4.
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The Zn-MOF not only has high selectivity and sensitivity for nitrobenzene, Al3+ and Fe3+ ions, but also can be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours.
Highlights 1. A zinc metal-organic framework material with small channels was synthesized. 2. The Zn-MOF could quickly recognize organic solvent nitrobenzene (NB) from other solvents, and the detection limit is 2.9 µM. 3. The Zn-MOF can be used to detect metal ions and has high selectivity for Al3+ and Fe3+ ions. 4. The Zn-MOF can also be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours.
The authors declare no competing financial interest.
S0022-4596(19)30630-9
DOI:
https://doi.org/10.1016/j.jssc.2019.121125
Reference:
YJSSC 121125
To appear in:
Journal of Solid State Chemistry
Received Date: 28 October 2019 Revised Date:
6 December 2019
Accepted Date: 10 December 2019
Please cite this article as: J. Cheng, T. Hu, W. Li, Z. Chang, C. Sun, Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: Multifunction detection and photocatalysis property, Journal of Solid State Chemistry (2020), doi: https://doi.org/10.1016/ j.jssc.2019.121125. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
Jiapeng Cheng synthesized the Zn-MOF and studied the fluorescent probe property. Tao Hu studied the photocatalytic property of Zn-MOF. Wenjun Li supervised the study of the photocatalytic property. Zhidong Chang supervised the synthesis and characterization of MOFs. Changyan Sun supervised the whole work and accomplished the manuscript.
Stable zinc metal-organic framework materials constructed by fluorenone carboxylate ligand: multifunction detection and photocatalysis property Jiapeng Cheng, Tao Hu, Wenjun Li, Zhidong Chang, Changyan Sun* School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing, 100083, China Abstract: In this paper, we synthesized a zinc metal-organic framework material (Zn-MOF) based on fluorenone carboxylate ligand and studied the application of Zn-MOF in the fields of fluorescent detection and photocatalysis degradation. The experiment results show that it is a fluorescent probe that can quickly recognize organic solvent nitrobenzene (NB) from other solvents. The detection limit is 2.9 µM, and Ksv (quenching efficiency) is 3.547× 103 M-1. In particular, Zn-MOF can identify nitrobenzene derivatives, such as nitrophenol and nitrotoluene, and the quenching efficiency can reach 16.54% and 92.64%, respectively. Moreover, Zn-MOF can be used to detect metal ions and has high selectivity for Al3+ and Fe3+ ions. Competitive experiments show that other metal ions will not interfere with the detection. In addition, Zn-MOF can also be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours. Keywords: metal-organic framework; fluorescent probe; nitrobenzene; photocatalytic degradation; Rhodamine B 1. Introduction As one kind of new functional materials, metal-organic frameworks (MOFs) have been widely used in various fields [1], such as gas separation and storage [2], fluorescent chemosensors [3], multi-phase catalysis [4] and drug transport [5] due to tunable porosity, high specific surface area, and luminescent property. Especially in the field of fluorescent chemosensors, metal-organic frameworks are receiving more and more attentions because of their good selectivity, high sensitivity, low cost and simple operation [6]. People are focused on designing new MOFs as fluorescent chemosensors by introducing large poroties [7], or empty coordination sites via
pre-design of the structure or post-synthetic modification [8, 9]. A great number of MOFs materials have been synthesized through various methods [10-13], and used as fluorescent chemosensors to detect small organic solvents, nitro aromatics explosives and metal ions [14-18]. With the development of industry, large amounts of chemical pollutants, such as toxic organic small molecules and nitro aromatics explosives are released into the environment, which have caused a lot of adverse influences on human health [19]. Therefore, the rapid detection of these harmful substances is very important. Nowadays, fluorescent probes based on MOFs materials are becoming a hotspot in the detection of organic pollutants. Pan et al. reported a new Zn(II)-based coordination polymer which displayed a highly selectivity for nitrobenzene (NB) based on the photo-induced electron transfer mechanism, and the detection limit could reach 9.6 µM [20]. Zhao et al. reported a zinc carboxylate MOF which could be used to detect traces of nitrobenzene (NB) rapidly and the detection limit was 7.2µM [21], It was reported that the electron transfer and electrostatic interactions may play an intrinsic role in the detection process. Aluminum and iron are two common elements in daily life. It has been found that excessive deposition of aluminum in brain tissue can lead to memory loss and aging. Deficiency of Iron can lead to anemia, while excessive iron may cause human tissue damage and other diseases [22, 23]. Therefore, the detection of Al3+ and Fe3+ ions are particularly important for human life. Recently, there are a lot of MOFs reported for metal-ions detection. Cao et al. has synthesized a water-stable luminescent terbium-based metal-organic framework which could quickly and specifically recognize Fe3+ and Al3+ ions in aqueous solution [24]. Liu et al. reported a ZIF-8 which could be used to detect Cu2+ and Cd2+ ions [25]. As a common industrial raw material, dye molecules are widely used in fluorescent dyes, colored glass, and so on. However, due to its poor biodegradability, dye molecules will cause harm to human beings and environment. Therefore, it is necessary to effectively degrade dye molecules. TiO2 was the first photocatalysis reported [26]. After that, a lot of nano catalyses including metal oxides [27], metal
sulfides [28] and other nanoparticles [29-31] have been investigated, and they could effectively degrade dye molecules. But the fast electron-hole pair recombination speed of these nano catalyses becomes a problem that should be solved. Recently, porous MOFs materials become research hotsports in this field [32-34]. The introduction of organic groups made electron-hole recombination difficult and the charge separation efficiency of MOFs is high [35, 36]. Alvaro et al. reported the first example MOF-5 which could be used as photocatalysts [37]. Experiments showed that MOF-5 exhibited comparable activity to commercial TiO2 (Degussa P-25) in the degradation of phenol in aqueous solution. Considering their tunable porosity and high specific surface area, researches on MOFs materials with high photocatalysts efficiency are getting lots of attention. In this paper, a metal-organic framework material with open pores was selected, which employed 9-fluorenone-2,7-dicarboxylicacid (H2FODC) and ethylene glycol as organic ligands and Zn2+ as central metal ions to form Zn-MOF [38]. The characteristics of this material are as followings [38, 39]: (i) The existence of the conjugation system results in fluorescent property of Zn-MOF; (ii) The resulting framework contains a 3D interconnected pore system with small channels; (iii) The carbonyl group possesses high chemical reactivity and may act as the basic site in base-catalyzed reactions. The incorporation of this group onto the pore surface would constitute a new interaction site for selective sorption and/or catalysis inside the pores. Based on the above characteristics, this Zn-MOF might be potential materials as a fluorescent probe or catalysis. But up to now, there is only one literature reporting that this Zn-MOF could rapidly and selectively adsorb cationic dyes [39]. So in this work, this Zn-MOF was used as a fluorescent probe to detect organic solvents, nitro aromatics explosives and metal ions, and the photocatalytic property of Zn-MOF was studied.
2. Experimental Section 2.1 Materials and Measurements All chemicals purchased were of reagent grade or better and were used without
further purification. Fluorescence spectra were measured on a F-4500 fluorescence spectrophotometer (Hitachi, China) with a xenon lamp as the excitation source and quartz cuvette (path length=1cm). The infrared data (IR) were collected on a Digilab FTIR-8400S spectrophotometer with KBr pellets in the range 4000-400 cm-1.The UV-vis absorption spectra were determined on a T9S. Powder X-ray diffraction (PXRD) patterns were recorded on an Ultima IV with D/teX Ultra diffractometer at 40 kV, 40 mA with Cu Kα (λ = 1.5406 Å) radiation in the 2θ range of 5-40º.
2.2 Synthesis of Zn-MOF. Zn-MOF was synthesized according to the previous literature with a small modification [38, 39]. Zn (OAc) 2·2H2O (0.044 g, 0.2 mmol) and H2FODC (0.027 g, 0.1 mmol) were dissolved in N,N-dimethylformamide (DMA, 6 mL) and 6mL glycol was added. Then the mixture was transferred to a 25mL Teflon-lined stainless steel and heated at 120 °C for 72 h. Upon cooling, orange crystals were obtained by filtration, washed with DMA and H2O, and dried in the air. Fig.S1 showed the infrared spectrum of Zn-MOF. The peaks at 1595 and 1387 cm-1 are the characteristic absorption of coordinated carboxylate groups. While the peak at 1715 cm-1 could be attributed to the stretching vibration of noncoordinated carbonyl groups. The PXRD spectrum of Zn-MOF was carried out (Fig. S2), which was the same as that in the literature [39], indicating that the target product [Zn5(FODC)2(OCH2CH2O)3 (H2O)]·4DMA·5.5H2O was successfully synthesized.
2.3 Spectroscopic measurements. The 0.1 M stock solutions of metal ions (Na+, Al3+, Mg2+, Li+, Ca2+, K+, Ba2+, Zn2+, Co2+, Cu2+, Mn2+, Ni2+, Hg2+, Fe3+ and Cd2+) were prepared in aqueous solution using their chlorides. The 1.0 × 10-3 M stock solutions of nitro aromatics (nitrobenzene (NB), p-nitrophenol (PNP), o-nitrophenol (ONP), m-nitrotluene (MNT), o-nitrotoluene (ONT)) were prepared in dimethyl sulfoxide (DMSO). The excitation wavelength was 315 nm. And both of the excitation and emission
slit widths were 5.0 nm.
3. Results and discussion 3.1 Structure of Zn-MOF According to the literature [38], the Zn-MOF crystallizes in hexagonal P63/mcm space group with the formula [Zn5(FODC)2(OCH2CH2O)3(H2O)]·4DMA·5.5H2O. It contains a 3D interconnected pore system with large chambers and small channels, and the pore surface is decorated by both uncoordinated carbonyl groups and potential unsaturated metal centers.
3.2 Fluorescent property of Zn-MOF Fig. 1 shows the fluorescent spectra of the Zn-MOF and H2FODC ligand at room temperature. It can be observed that upon excitation of 315 nm, both Zn-MOF and H2FODC ligand exhibit an emission at 515 nm. The difference is that the formation of the Zn-MOF enhances fluorescence.
Fig 1. Solid-state fluorescent spectrum of the Zn-MOF and H2FODC ligand.
3.2 Fluorescent Sensing Applications Sensing of small organic solvents. 3 Mg fully ground Zn-MOF samples were dispersed in 3 mL different small organic solvents including chloroform(CHCl3), ethanol(EtOH), methanol(MeOH), N,N′-dimethylacetamide (DMA), diethyl ether, N,N′-dimethylformamide (DMF), acetone, nitrobenzene(NB), acetonitrile, ethylene glycol, dimethyl sulfoxide (DMSO), benzene, toluene, CCl4, and ethyl acetate, and ultrasonicated for 5 min, which led to stable suspensions. Under UV lamp, all
suspensions were bright except that Zn-MOF in NB was black, as shown in Fig. 2a. The fluorescent spectra of Zn-MOF suspensions were shown in Fig. 2b. It could be seen that NB had the largest quenching effect on the emission of Zn-MOF (Fig. 2c). It showed that Zn-MOF could effectively detect NB as a fluorescent probe. Owing to the highly electron deficient property of NB and the electron rich nature of Zn-MOF, the quenching mechanism might be ascribed to the excited electron transfer between Zn-MOF and NB.
Fig. 2. (a) Zn-MOF in different organic solvents under UV lamp; (b) Fluorescent spectra of Zn-MOF (1mg/mL) in different organic solvents; (c) Comparison of fluorescent intensity of Zn-MOF in different organic solvents.
To explore the detection limit of Zn-MOF for detecting NB, titration experiments were carried out. The fluorescent intensity of Zn-MOF gradually decreased with the increasing concentration of NB. As expected, as the NB concentration increased from 0 to 2.4 mM, the fluorescence of Zn-MOF suspensions showed significant quenching (Figs. 3a, 3b). The detection limit was calculated to be 2.9 µM based on 3σ/K, which was superior to most of reported values [20, 21, 40]. The fluorescence quenching efficiency was analyzed using the Stern-Volmer equation I0/I = KSV [M] + 1, where I0 and I are the fluorescent intensities before and after the addition of NB, respectively, [M] is the molar concentrations of NB, and KSV is the quenching efficiency (M-1). As shown in the inset of Fig. 3c, the Stern-Volmer plots of Zn-MOF exhibited a good
linear correlation with the concentration of NB. And the Ksv value was calculated 3.547× 103 M-1. (a)
(b)
(c)
Fig. 3. (a) Fluorescent spectra of Zn-MOF with different concentrations of NB in DMSO; (b)The fluorescent intensity of Zn-MOF with different concentrations of NB in DMSO at 530 nm. (c)The quenching efficiency Ksv.
In practical applications, the repeatability of fluorescent probes is essential. The four-cycle recovery experiments were performed to study the repeatability of Zn-MOF in the detection of NB. After each fluorescence test, Zn-MOF was collected by centrifugation and then washed with DMSO several times. As illustrated in Fig. 4, it is noted that Zn-MOF showed the similar relative fluorescent intensity after each cycle, and the PXRD of Zn-MOF after four-cycle recovery was matched well with that of the as-synthesized samples (Fig. S3), which revealed that Zn-MOF had good repeatability.
Fig. 4. The repeatability of Zn-MOF (The red column is relative intensity of washed Zn-MOF, the black column is relative intensity of washed Zn-MOF with NB)
Sensing for nitrobenzene derivatives. Based on the above experiments, it can be concluded that this Zn-MOF could be used to detect NB from common organic solvents. Then, the detection of nitrobenzene derivatives by Zn-MOF was tested. The fluorescent spectra of Zn-MOF were recorded in a series of nitrobenzene derivatives such as p-nitrophenol (PNP), o-nitrophenol (ONP), m-nitrotluene (MNT), and o-nitrotoluene (ONT), as shown in Fig. 5. It could be seen that Zn-MOF had different response to different nitro aromatics. The quenching efficiencies of MNT and ONT were 92.64% and 87.96%, while the quenching efficiencies of PNP and ONP were 16.54% and 24.94%. The reason for this difference might be that -OH is a strong electron donating group, which weakens the excited electron transfer between the Zn-MOF and nitro aromatics [40]. (a)
(b)
Fig. 5. (a) Fluorescent spectra of Zn-MOF (1 mg/mL) in DMSO with nitrobenzene derivatives; (b) Quenching efficiency of nitrobenzene derivatives.
Sensing for metal ions. 3.0 Mg fully ground Zn-MOF sample was dispersed in 3.0 mL H2O/DMF (v/v=1:1) and ultrasonicated for 30 min to make a suspension of
Zn-MOF. Then, 15 kinds of metals ions including Na+, Al3+, Mg2+, Li+, Ca2+, K+, Ba2+, Zn2+, Co2+, Cu2+, Mn2+, Ni2+, Hg2+, Fe3+, and Cd2+ were added respectively. The fluorescent intensities of these suspensions were recorded at room temperature. As shown in Fig. 6, upon the addition of metal ions except Al3+ and Fe3+ ions, there were no significant changes in the fluorescence spectra of Zn-MOF. But when Al3+ ions were added, the fluorescence of Zn-MOF had a 30 nm blue shift. The reason for this phenomenon may be that the uncoordinated carbonyl groups of the Zn-MOF coordinate with Al3+ ions, resulting in the change of the HOMO-LUMO energy level of the Zn-MOF. When Fe3+ ions were added, the fluorescence of Zn-MOF was completely quenched. In order to explain this phenomenon, the UV-vis absorption spectra of metal ions and the fluorescence excitation spectrum of the Zn-MOF were tested. As shown in Fig.S4, the UV-vis absorption peaks of Fe3+ ions and the fluorescence excitation spectrum of the Zn-MOF were almost completely overlapped. This means that there may be competitive absorption between Fe3+ ions and the Zn-MOF, resulting in the fluorescence quenching. This showed that Zn-MOF could be used to detect Al3+ and Fe3+ ions with fluorescent spectra.
Fig. 6. Fluorescent spectra of Zn-MOF with different metal ions in H2O/DMF (v/ v = 1:1)
Competitive experiments. To test the effect of other metal ions in the detection of Al3+ and Fe3+ ions, competitive experiments were performed. As shown in Fig. 7(a), it can be observed that when other metal ions except Fe3+ ions were added to the Zn-MOF-Al system, the emission peak position of Zn-MOF-Al did not move. This indicated that the existence of other metal ions did not affect the detection of Al3+ ions. However, the addition of Fe3+ ions completely quenched the fluorescence of Zn-MOF-Al system. But if ascorbic acid was added first, this interference could be eliminated. The same competitive experiment was also conducted in the detection of Fe3+ ions. The result (Fig. 7(b)) showed that the presence of other metal ions did not affect the detection of Fe3+ ions. Therefore, Zn-MOF could be effectively used to detect Al3+ and Fe3+ ions with high selectivity. (a)
(b)
Fig. 7. (a) Fluorescence emission wavelength of Zn-MOF (Black bars: with Al3+ ions; Red bars: with Al3+ ions and other metal ions); (b) Florescent intensity of Zn-MOF (Red bars: with other metal ions; Black bars: with Fe3+ ions and other metal ions).
3. Photocatalysis Rhodamine B (RhB) is chosen as a representative dye to evaluate the capability of Zn-MOF for degradation of organic pollutions. Control experiments were firstly discussed to illustrate the photocatalytic nature of the reaction. As shown in Fig. 8(a), no significant variation in the concentration of RhB is observed under UV-vis light irradiation, indicating that RhB is resistant to UV-vis light irradiation. When 30 mg Zn-MOF solid sample was dispersed into RhB solution (30 mL) in the dark, in the first one hour, there is a slight decrease in the concentration of RhB, indicating that a small fraction of RhB is adsorbed into the pores of the Zn-MOF. In the next four
hours, the concentration of RhB remains unchanged, suggesting that the resultant mixture have reached adsorption-desorption equilibrium. Then the photocatalytic activities of the Zn-MOF was evaluated by the degradation of RhB under UV irradiation from 200 to 800 nm on a 400 W Xe lamp. 3.0 ML samples were taken out for analysis every hour using a 7230G visible spectrophotometer which measured the absorbance of the solution to estimate the photocatalytic performance of Zn-MOF to RhB. The calculation results showed that Zn-MOF could degrade 85% of RhB within 3 hours, which was comparable to the literature [34, 40]. Emam and co-workers explored a Ln-MOF which could be used as photocatalysts to degrade 85% of RhB in 120 min [34]. Chen and co-workers synthesized a doubly interpenetrated porous MOF which could degrade methyl orange (MO) completely in 120 min under UV irradiation [40]. These results show that the Zn-MOF can be used as an effective photocatalyst. The possible degradation mechanism was shown in Fig. 8(b) [29]. According to the energy band theory, the empty outer orbitals of metal centers in MOF are the conduction band (CB), and the outer orbitals of organic ligands are the valence band (VB). Under the light, the electrons (e-) are excited from the valence band to the conduction band, leaving holes (h+) in the valence band, thus produce electron-hole pairs. The reaction of valence band holes (h+) and photogenerated electrons (e-) with water and oxygen can produce ·OH radicals and superoxideions (·O2-· ), which are known to degrade organic dyes.
Fig. 8. (a) Control experiments for the photocatalytic degradation of RhB. (b) The deduced mechanism of the degradation of RhB by Zn-MOF.
Next, the recycling of Zn-MOF was tested. After each photodegradation test, Zn-MOF was washed with H2O several times and dried, then was added into a new RhB solution to test. After 3 cycles, the results were show in Fig. 9. It can be found that the degradation effect of Zn-MOF on RhB was still obvious. And the PXRD of Zn-MOF after 4 cycles was matched well with that of the as-synthesized samples (Fig. S5), indicating that the Zn-MOF had good recycling.
Fig. 9. Four cycle experiments of Zn-MOF
4. Conclusions In this work, a Zn-MOF with open channels has been successfully synthesized. The application in the fields of fluorescent detection and photocatalysis degradation were studied. The experiments showed that this Zn-MOF could rapidly and selectively detect NB from other organic solvents, and the detection limit was 2.92
µM. Moreover, Zn-MOF could selectively detect Al3+ and Fe3+ ions from 15 kinds of metal ions. Competitive experiments indicated that other metal ions did not interfere with the detection. In addition, Zn-MOF showed good photocatalytic activities. It could effectively degrade RhB under UV-vis, and the degradation efficiency reached 85% within 3 hours.
Acknowledgements This project is supported by National Nature Science Foundation of China (21101013) and Special Science Project of University of Science and Technology Beijing (FRF-BR-17-002B).
Conflict of interest The authors declare that they have no conflict of interest.
Supplementary information: The IR spectra of Zn-MOF are shown in Fig.S1. The PXRD curves for Zn-MOF as-synthesized and after 4 times repeatability experiments, after 4 times photodegradation experiments are shown in Figs.S2, S3 and S5. The UV-vis absorption spectra of metal ions and the fluorescence excitation spectrum of Zn-MOF are shown in Fig. 4.
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The Zn-MOF not only has high selectivity and sensitivity for nitrobenzene, Al3+ and Fe3+ ions, but also can be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours.
Highlights 1. A zinc metal-organic framework material with small channels was synthesized. 2. The Zn-MOF could quickly recognize organic solvent nitrobenzene (NB) from other solvents, and the detection limit is 2.9 µM. 3. The Zn-MOF can be used to detect metal ions and has high selectivity for Al3+ and Fe3+ ions. 4. The Zn-MOF can also be used as photocatalytic degradation of Rhodamine B (RhB), and the photodegradation efficiency of RhB can reach 85% within 3 hours.
The authors declare no competing financial interest.