Rapid and Large-Scale Synthesis of IRMOF-3 by Electrochemistry

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Rapid and Large-Scale Synthesis of IRMOF‑3 by Electrochemistry Method with Enhanced Fluorescence Detection Performance for TNP Jin-Zhi Wei,* Xue-Liang Wang, Xiao-Jun Sun, Yan Hou, Xin Zhang, Dou-Dou Yang, Hong Dong, and Feng-Ming Zhang* Key Laboratory of Green Chemical Engineering and Technology of College of Heilongjiang Province, College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150040, People’s Republic of China S Supporting Information *

ABSTRACT: Rapid and large-scale synthesis of metal− organic frameworks (MOFs) materials is of great significance for their practical applications. For the first time, we have electrochemically synthesized IRMOF-3 at room temperature by applying a voltage to a zinc electrode immersed in electrolyte containing 2-aminoterephthalic acid (NH 2 H2BDC). The reaction conditions, including the ratio of solvent (electrolyte), the applied voltage, and different reaction times, were investigated and optimized. The degree of crystallinity and nanomorphology of the synthesized IRMOF-3 can be controlled by changing the reaction conditions. More importantly, we demonstrated that the electrochemical synthesis strategy can rapidly obtain nanoscale IRMOF-3 with high crystallinity on a gram scale. In addition, in comparison with the product of solvothermal synthesis, the electrochemically synthesized nanoscale IRMOF-3 exhibits improved fluorescent detection ability to 2,4,6-trinitrophenol (TNP) with a detection limit of about 0.1 ppm.

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INTRODUCTION Metal−organic frameworks, as a class of crystalline porous materials formed by metal centers (metal ions or clusters of metal) and bridged organic ligands through self-assembly processes, have been extensively studied in various fields, including gas storage and separation,1−5 catalysis,6−9 drug loading,10,11 and sensing,12−18 due to their inherent high internal surface area, tunable structure, and diverse functionality. The synthesis of MOFs reported in the literature has mainly focused on hydrothermal and solvothermal methods.19,20 This may be attributed to the requirement of determining the structure, to obtain proper single crystals and confirm the single-crystal structure by X-ray irradiation. Obviously, the synthesis method under extreme conditions is energy and time consuming, has low reproducibility, and is too complicated to give large-scale amounts of product to satisfy the demands of practical applications of MOFs.21,22 Thus, the development of synthesis methods of MOFs that can rapidly prepare the existing MOFs with high reproducibility under mild conditions is quite important. Until now, reports on the room-temperature synthesis of MOFs have been very limited. Only some microporous MOFs, such as ZIF-8, MOF-5, and MOF-74, have been successfully synthesized by mixing and stirring their precursors in solution at room temperature.23,24 This is because additional energy to improve the self-assembly reaction is necessary for the syntheses of most MOFs. Over the past decade, electrochemical synthesis, as an emerging new type of synthesis, has received tremendous attention from researchers, owing to the © XXXX American Chemical Society

advantages of mild reaction conditions, simple operation, short reaction time, and high conversion efficiency.25−27 The practicality of electrochemical synthesis of MOFs has also been proved by some research groups.28,29 Electrochemical synthesis of MOFs can be realized in two different manners. One is anodic dissolution, resulting in the metal ions required for the MOFs to be released by anodic oxidation,30,31 and the other is reductive deprotonation, which relies on the HO− to cause a rise in pH and subsequent deprotonation of the linker.32,33 Since anodic dissolution does not require the addition of metal salts, this method is more favored by researchers. Among the various MOFs, IRMOF-3, a Zn-based isoreticular MOF comprised of Zn4O secondary building units (SBUs) and NH2-BDC linkers, has the characteristics of stability in natural atmosphere, large pores (around 15 Å), and high surface area and has exhibited excellent catalysis34 and fluorescent properties.35−37 Previously, IRMOF-3 has been mainly synthesized under solvothermal conditions for at least 18 h,38−40 while mild and rapid synthesis routes at room temperature have not been reported until now. Different from the case for ZIF-8, IRMOF3 cannot be prepared by simply mixing metal ions and ligand together at room temperature in the same solution used in its solvothermal reaction. Electrochemical synthesis of MOFs such as IRMOF-3 has not been reported. Received: December 18, 2017

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DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry In this report, we successfully synthesized IRMOF-3 by an electrochemical synthesis strategy at room temperature. The electrochemical reaction system contains a zinc plate as the anode, a copper plate as the cathode, tetrabutylammonium bromide (TATB) as supporting electrolyte, and DMF−ethanol as solvent (Figure 1). The reaction conditions, including the

Figure 1. Schematic diagram for synthesis of IRMOF-3 by electrochemical method.

ratio of solvent, the applied voltage, and different reaction times, were investigated and optimized. It was found that the degree of crystallinity and nanomorphology of the synthesized IRMOF-3 can be controlled by changing the reaction conditions. The electrochemical synthesis strategy can prepare nanoscale IRMOF-3 on a gram scale within 3 h. In addition, in comparison with the product of solvothermal synthesis, the electrochemically synthesized nanoscale IRMOF-3 exhibits improved fluorescent detection properties to TNP with a detection limit up to 0.1 ppm.

Figure 2. (a) XRD patterns of samples electrochemically synthesized at different ratios of DMF to ethanol in comparison to the simulated pattern of IRMOF-3, where a−e represent 50:0, 40:10, 30:20, 20:30 and 10:40 ratios of DMF to ethanol, respectively. (b−e) SEM of samples electrochemically synthesized at different ratios of DMF to ethanol, where (b)−(e) represent 40:10, 30:20, 20:30, and 10:40 ratios of DMF to ethanol, respectively.

RESULTS AND DISCUSSION The theory of the electrochemical method is that metal ions are released from the anode due to anodic oxidation. At the same time, ligand anions are moved to the anode driven by the electric field. The coordination self-assembly of metal ions and ligand anions subsequently occurs, forming MOFs in the electrolytic cell. According to the literature on the hydrothermal method,41 a certain proportion of Zn(NO3)2 was added to the electrochemical synthesis system, in the case of no voltage, with stirring at room temperature for 12 h. As a result, the solution remained clear (Figure S1), showing that IRMOF3 cannot be synthesized at room temperature in the absence of voltage. During the process of electrochemical synthesis, several important conditions such as solvent composition, voltage, and reaction time have to be considered. The nucleation rate of MOFs tends to be affected by the composition of the solvent.42 Therefore, different solvent volume ratios were first investigated. The products of the electrolytic cell with different volume ratios of DMF to ethanol were analyzed by XRD. The results are shown in Figure 2a, keeping the time, the applied voltage, and the amount of conductive salt at 3 h, 5 V and 0.6 g, respectively. The results reveal that the synthesized material exists in an amorphous structure when the electrolyte is only DMF. As the ratio of ethanol is increased, the peak position of the synthesized material tends to be consistent with the peak position of simulated IRMOF-3. This phenomenon is ascribed to the effect of ethanol on the nucleation rate of IRMOF-3.29 However, an increased ratio of ethanol is not always better. When the ratio of DMF to ethanol is 10:40, the peak has a weakening trend. A reasonable explanation is that a higher ratio of ethanol will lead to a lower solubility of the NH2-H2BDC.

When the ratio of DMF to ethanol is 20:30, the peak position of the synthesized material matches well with that of simulated IRMOF-3, indicating that the synthesized material is the expected IRMOF-3. As shown in Figure 2b−e, we also observed the morphology of the synthesized material according to the above ratio of DMF to ethanol. When there is no ethanol in the electrolyte, the morphology of the synthesized material is petal-like (Figure S2). As the ratio of ethanol is increased, the morphology of the petal-like material gradually changes to granular particles. When the ratio of DMF to ethanol is 20:30, the morphology is uniformly granular. However, the petal-like morphology appears again when the ratio of ethanol is increased. In other words, the morphology of synthesized samples is petal-like when their structure is mainly amorphous. Only at the appropriate voltage can the metal ions be released from the anode.43 The effect of applied voltage on the IRMOF-3 has also been explored. The results are shown in Figure 3a, keeping the ratio of DMF to ethanol, the time, and the amount of conductive salt at 20:30, 3 h, and 0.6 g, respectively. When the voltage is 4 V, the synthesized material has a low degree of crystallinity. With an increase in voltage, the peak position of the synthesized material is more and more consistent with the peak position of simulated IRMOF-3. Since the voltage affects the release rate of Zn2+, the amount of Zn2+ is insufficient and is not enough to be combined with the NH2BDC2− when the voltage is low, resulting in a synthesized material with a low degree of crystallinity. However, when the voltage is too high, the Zn2+ is more likely to form ZnO on the surface of the anode, resulting in an insufficient number of Zn2+ ions in the electrolytic cell.44 Here, the ratio of Zn2+ to NH2BDC2− is appropriate for a relatively high crystallization when the voltage is 6 V. As shown in Figure 3b−e, the morphology of

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DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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In order to further confirm the structure of IRMOF-3, FTIR measurement was performed on the synthesized IRMOF-3 (Figure S5). In the FTIR spectrum of NH2-H2BDC, the peak around 1692 cm−1 is ascribed the CO stretching vibration in free carboxylic acids. However, a peak occurs at 1658 cm−1 shifted from the original 1692 cm−1 in the FTIR spectrum of IRMOF-3, showing that a coordination reaction occurs between the Zn2+ and deprotonation of −COOH groups in NH2-H2BDC. In addition, the peaks around 3504 and 3384 cm−1 signify the presence of the amine groups of NH2-H2BDC. The nature of the thermal stability was studied by TGA (Figure S6). The thermogram shows three major weight loss stages: viz., (1) the first decline appeared from 30 to 251 °C, and the 15.5% mass loss may be due to the evaporation of physically adsorbed solvent molecules (such as ethanol and DMF); (2) the second stage ranges from 251 to 400 °C, and the 4.8% mass loss was ascribed to the further release of solvent and unreacted ligand physically absorbed within pores or on the surface; (3) the third decline appeared from 400 to 510 °C, and the 29.7% mass loss corresponds to the decomposition of the framework. The acid−base stability of the synthesized IRMOF-3 after immersion in different pH solution mixtures of DMF and H2O (9/1, v/v) for 12 h was determined by XRD patterns (Figure S7), and the results show that the synthesized IRMOF-3 can retain its structural integrality in a pH 6−8 solution mixture. The fluorescence emission spectrum of NH2-H2BDC shows a emission band at 435 nm upon excitation at 340 nm (Figure S8), while the synthesized IRMOF-3 exhibits a emission peak around 450 nm. The red shift of the emission peak of IRMOF3 in comparison to the peak for the ligand should be attributed to the coordination of the ligand to the metal centers.47 The quantum yield of the synthesized IRMOF-3 was measured to be 0.12, which is comparable with that of relevant zinc complexes.48 IRMOF-3 with fluorescence synthesized by the electrochemistry method is expected to detect nitro explosives in water, such as 2,4,6-trinitrophenol (TNP), nitromethane (DMNB), p-nitrotoluene (4-NT), nitrobenzene (NB), and 2,4,6-trinitrotoluene (TNT). According to the results of pH stability, the following detection experiments were carried out by adding 1 mL of a water solution of a nitro explosive to 9 mL of a DMF soution contianing 0.9 mg of IRMOF-3. Then the fluorescence intensity at different times was investigated when the concentration of TNP was 10 ppm (Figure S9). After only 10 s, the fluorescence intensity of the IRMOF-3 is significantly quenched and the degree of quenching remains stable. Therefore, 10 s was chosen as the test time in the following experiment. The quenching effect of different concentrations of nitro explosives on IRMOF-3 was studied at pH 7 (Figure 4 and Figures S10−S13). When the nitro explosive concentration is zero (only water was added), the fluorescence intensity of IRMOF-3 was slightly improved, which is due to the different polarities between water and DMF (Figure S14). With an increase in concentration of nitro explosives, the fluorescence intensity of IRMOF-3 is quenched to varying degrees. The quenching effect of nonaliphatic DMNB on the fluorescence of IRMOF-3 is lower than that of other aromatic nitro explosives. (Figure S15). The quenching mechanism for aromatic nitro explosives may be due to π−π interactions with IRMOF-3, which effectively trap the nitro explosives on the surface of IRMOF-3 and is favorable for electron transport from the conduction band of IRMOF-3 to the lowest unoccupied molecule orbital (LUMO) energy of the nitro explosives.

Figure 3. (a) XRD patterns of samples electrochemically synthesized at different voltages in comparison to the simulated pattern of IRMOF-3, where a−d represent the voltages at 4, 5, 6, and 7 V, respectively. (b)−(e) SEM of samples electrochemically synthesized at different voltages, where (b)−(e) represent the voltages at 4, 5, 6, and 7 V, respectively.

the synthesized material at different voltages was observed. The morphology of the low crystallinity at 4 V is petal-like. With an increase in voltage, the petal-like morphology is gradually replaced by granular particles. When the voltage is 6 V, the morphology of the synthesized IRMOF-3 is nanoscale spheres. In comparison with other methods, one of the advantages of the electrochemical method is the shorter synthesis time.45 Thus, the effect of time on the structure was studied (Figure S3), with the ratio of DMF to ethanol,the voltage, and the amount of conductive salt kept constant at 20:30, 6 V, and 0.6 g, respectively. After 3 h, the degree of crystallization is relatively high. Yields of synthesized IRMOF-3 at different times were also recorded (Figure S4). The result shows that the yield increases slightly with time. After 3 h, the yield of IRMOF-3 synthesized is about 0.31 g. In combination with a yield−time graph, it was surmised that the crystallinity of IRMOF-3 increases slowly over a period of time (about 1 h) by XRD patterns. In addition, in comparison with other literature on synthesized IRMOF-3, 41,46 the yield of IRMOF-3 synthesized by the electrochemical method is compelling and the time is relatively short, showing that the electrochemical method has the potential to realize industrialization. The morphology of the synthesized material at different times was also studied (Figure S3). The morphology is petal-like when the crystal has a low degree of crystallinity, which is consistent with previous results. In summary, the optimized conditions for electrochemical synthesis of IRMOF-3 with high crystallinity are 20:30 as the ratio of DMF to ethanol, 6 V as the applied voltage, and 3 h as the reaction time. IRMOF-3 is obtained with a relatively high crystallization and nanoscale spherical morphology. The IRMOF-3 synthesized under optimized conditions is chosen as the material in the following detection experiment of nitro explosives. C

DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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IRMOF-3, which means that IRMOF-3 has the potential to detect TNP in water by this method.58 The S-V plots of 4-NT, NB, DMNB, and TNT are linear at any concentration (0−100 ppm) (Figure S17). Interestingly, the S-V plot of TNP is linear when its concentration is less than 50 ppm. When the concentration is higher than 50 ppm, the S-V plot is an upward curve. To the best of our knowledge, the superquenching ability of TNP is due to an energy transfer between TNP and IRMOF3. The fluorescence inner filter effect, a special energy transfer, refers to the phenomenon that the fluorescence of a fluorophor is weakened due to the absorption of the emitted light when the fluorophor coexists with other light-absorbing substances.59 As shown in Figure 6, the overlap between fluorescence spectra

Figure 4. Fluorescence emission spectra of IRMOF-3 upon incremental increases in the concentration of TNP.

Although the same is true for aromatic nitro explosives, the quenching effect of TNP on the fluorescence of IRMOF-3 is outstanding (Figure S15), which is due to the different LUMO energy (Figure S16).49 In general, the lower the LUMO level, the stronger its ability to accept electrons.50 It is remarkable that the fluorescence quenching of IRMOF-3 can be clearly observed when the concentration of TNP is as low as 0.1 ppm, which is comparable to the case for MOF-based sensors reported previously.51−54 Nevertheless, in comparison with TNP, the detection limit of IRMOF-3 for other aromatic nitro explosives is inferior, which is attributed to the hydrogen bonds formed between the pendant primary amine group of linkers and the −OH of TNP leading to intense intermolecular π−π stacking, which is more conducive to the electronic transmission.55,56 Therefore, TNP has the most obvious quenching effect on the fluorescence of IRMOF-3. In addition, the fluorescence spectra show a significant red shift (∼480 nm) with an increase in concentration of TNP, which does not appear in detection of 4-NT, NB, and TNT, indicating that fluorescence quenching of IRMOF-3 may be caused by a certain effect between IRMOF-3 and TNP. The quenching efficiencies of the analytes and quenching constants were analyzed and calculated using the Stern− Volmer (S-V) equation I0/I = KSV[A] + 1, where I0 and I are the fluorescence intensities before and after the addition of the analytes, [A] is the molar concentration of the analytes, and KSV is the quenching constant (M−1).57 The S-V plots of the nitro explosives are shown in Figure 5. The KSV value of TNP is 2.99 × 104. However, the KSV values of 4-NT, NB, DMNB and TNT are 685, 243, 113 and 754, respectively. This indicates that TNP has an obvious quenching effect on the fluorescence of

Figure 6. Spectral overlap between absorbance spectra of nitro explosives and the emission spectrum of IRMOF-3.

of IRMOF-3 and ultraviolet absorption spectra of various nitro explosives was investigated. The overlap between the fluorescence spectrum of IRMOF-3 and absorption spectrum of TNP is the largest. However, the overlap between the emission spectrum of IRMOF-3 and the ultraviolet absorption spectra of 4-NT, NB, DMNB, and TNT is negligible. Therefore, we believe that the internal filtration effect of TNP is an important mechanism for fluorescence quenching of IRMOF-3.60 In other words, the mechanism of fluorescence quenching of IRMOF-3 by TNP is the result of the combined effect of electron transfer and fluorescence inner filter. In comparison with TNP, the fluorescence quenching efficiency of 4-NT, NB, DMNB, and TNT for IRMOF-3 is not as sensitive. Encouraged by these results, we investigated the selectivity of IRMOF-3 toward TNP in the presence of other nitro explosives (Figures S18−S21). The fluorescence quenching efficiency and trends of IRMOF-3 between the coexistence of other nitro explosives with TNP and TNP alone are basically similar (Figure S22). In other words, IRMOF-3 has a certain anti-interference ability for these nitro explosives in the detection of TNP. Finally, the effects of 2,4-dinitrophenol (2,4-DNP) and 4nitrophenol (NP), which are also nitrophenols, on the emission spectrum of IRMOF-3 have been studied (Figures S23 and S24). Similar to the case for TNP, there is a clear fluorescence quenching when the concentration of 2,4-DNP or NP is higher. The fluorescence quenching degree was found to be TNP > 2,4-DNP > NP, which is in complete agreement with the number of nitro groups of these nitro explosives (Figure S25). In addition, the detection limits of IRMOF-3 for 2,4-DNP and NP are 1 and 2 ppm, respectively. These results are significantly lower than that of TNP, which further testifies to the

Figure 5. Stern−Volmer (S-V) plots for various nitro explosives (histogram). D

DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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mechanism having a combined effect of electron transfer and fluorescence inner filter.61 The fluorescent detection ability of IRMOF-3 for solutions at different pH was investigated (Figures S26 and S27), and the results demonstrated that the detection limit of the synthesized IRMOF-3 also reached the ppm level under pH 6 and 8 conditions, but the limit was not as good as that under neutral conditions, which may be attributed to weaker hydrogen bond interactions between TNP and the framework of IRMOF-3 in acid or base solution.62 The recyclability of IRMOF-3 to TNP was tested (Figure S28), and the results showed an equal detection limit during five consecutive cycles, indicating an excellent recyclability of the synthesized IRMOF-3 to TNP. Since the size of IRMOF-3 synthesized by electrochemistry is approximately 200 nm, which may be less than that synthesized by the solvothermal method, we speculate that IRMOF-3 synthesized by electrochemistry has a great advantage in detecting TNP. IRMOF-3 was synthesized by previously reported procedures.41 The XRD patterns and the optical microscope photograph of IRMOF-3 synthesized by the solvothermal method were also investigated (Figure S29). The yield of IRMOF-3 synthesized by the solvothermal method is 64.52% of that synthesized by the electrochemical method in just 3 h (based on 0.1 g of NH2-H2BDC) (Figure S30). The IRMOF-3 synthesized by the solvothermal method was also used to detect TNP in water by this method (Figure S31). However, the detection limit of IRMOF-3 synthesized by solvothermal method is 5 ppm for TNP, which is lower than that of IRMOF-3 synthesized by the electrochemical method (0.1 ppm). A possible explanation is that IRMOF-3 with a large size cannot be in good contact with TNP, affecting electron transfer. Therefore, IRMOF-3 synthesized by the electrochemical method not only improves the synthesis conditions but also enhances the detection performance for TNP.

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.-Z.W.: [email protected]. *E-mail for F.-M.Z.: [email protected]. ORCID

Xin Zhang: 0000-0003-4305-145X Hong Dong: 0000-0001-5839-7846 Feng-Ming Zhang: 0000-0002-2738-306X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 21501036 and 21676066) and the Science Foundation of Heilongjiang Province, People’s Republic of China (Grant No. E2016042).

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CONCLUSIONS In conclusion, we successfully developed an electrochemically synthetic strategy for large-scale preparation of IRMOF-3 at room temperature. The influence of reaction conditions on the crystallinity and morphology of the synthesized IRMOF-3 was systematically investigated. It can be demonstrated that the electrochemical synthesis strategy in our work can rapidly produce nanoscale IRMOF-3 with a high degree of crystallinity on a gram scale within 3 h. In addition, due to the nano scale and special morphology, the synthesized IRMOF-3 exhibits improved fluorescent detection ability to TNP with a detection limit of 0.1 ppm, in comparison to the product of solvothermal synthesis. We believe that such a synthetic strategy will open up new avenues for the mild and rapid synthesis of MOFs at room temperature.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03174. Materials and methods, synthesis of IRMOF-3, SEM, XRD, FTIR, and TG studies, and the fluorescence detection performances of IRMOF-3 for nitro explosives (PDF) E

DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03174 Inorg. Chem. XXXX, XXX, XXX−XXX