Facile fabrication of Eu-1,4-NDC-fcu-MOF particles for sensing of benzidine

Abstract

The controllable synthesis of MOF materials is of great significance for its application. At present, the regulation of MOF morphology is mainly limited to those composed of transition metal elements, such as IRMOF, ZIF-8, HKUST-1, UiO-66, and MIL-MOFs etc, and few reports about the fabrication of micro/nano sized lanthanide MOFs could be found in literature. In this paper, micron sized Eu-1,4-NDC-fcu-MOF octahedral particles were obtained via a modified solvothermal method by using methylamine as a modulator. The luminous efficiency of Eu³⁺ was greatly improved due to the efficient energy transfer from the 1,4-NDC ligand to rare earth ion. The resultant Eu-1,4-NDC-fcu-MOFs particles could be used to detect benzidine in ethanol solution effectively, and as the concentration of benzidine increased, the fluorescence intensity of Eu-1,4-NDC-fcu-MOFs gradually decreased. Due to the good stability, high luminescent intensity and excellent sensitivity, this work further demonstrates that Ln-MOF are ideal fluorescent probes to identify specific molecules.

Keywords

Rare earth metal-organic frameworks morphology control photoluminescent property sensing benzidine

1 Introduction

Nano or micron sized metal-organic frameworks (MOFs) have shown prospective futures in the applications of heterogeneous catalysis, controlled drug release, and biomedical fields etc. [1 –3] As an essential part of MOF family, Eu-NMOFs have many prestigious and unique properties attributed to its abundant electrons and 4f orbitals, such as photoluminescent, magnetic and catalytic properties etc. [4 –19] Although great efforts have been made in synthesis of all kinds of novel and functional Eu-MOF single crystals, the report on synthesis of highly uniform nano or micron sized Eu-MOF particles are still scarce [5 –7]. Note that only rod or plate-like products were obtained for the nano or micron sized Eu-MOFs in most cases [5 –7]. Therefore, investigations on preparation of nano or micron sized Eu-MOFs are greatly needed.

Due to the exceptional thermal and chemical stability, herein, we chose [(CH₃)₂NH₂]₂[Eu₆(μ₃-OH)₈(1,4-NDC)₆(H₂O)₆]·(solv)_x (abbreviated as Eu-1,4-NDC-fcu-MOF) as a platform to present the controlled morphogenesis of micron sized Ln-MOF crystals [18]. Eu-1,4-NDC-fcu-MOF crystallizes in the cubic space group of Fm3m. In the crystal structure, each Eu³⁺ ion is surrounded by four μ₃-OH groups and four oxygen atoms from statistically disordered carboxylate groups of four independent 1,4-NDC^2– ligands, leaving the ninth coordination site occupied by a water molecule. The adjacent Eu³⁺ ions are bridged via μ₃-OH and deprotonated carboxylate groups in a bis-monodentate fashion to give a 12-connected [Eu₆(μ₃-OH)₈(O₂C-)₁₂]. Each hexanuclear cluster is bridged through 1,4-NDC^2– to produce a 3D periodic MOF [18].

Among the developed synthetic protocols, the modified solvothermal method is regarded as one of the most effective approach to prepare highly uniform nano or micron sized MOFs [20 –23]. In this study, micron sized Eu-1,4-NDC-fcu-MOF was produced via the modified solvothermal method in the presence of different amounts of modulator, and the PL properties as well as sensing of benzidine in ethanol solution were also investigated.

2 Experimental section

2.1 Reagents and instruments

All of the chemicals and solvents were used as received without further purification. All of the following rare earth metal salts were purchased from Beijing HWRK Chem: Europium (III) nitrate hexahydrate (Eu(NO₃)₃·6H₂O, 99.99%), 1,4-Naphthalenedicarboxylic acid (1,4-NDC, 95.0%, TCI), 2-Fluorobenzoic acid(96%, TCI), (2-FBA, 98%, J&K Scientific), methylamine (40% in water, TCI), dimethylamine (40% in water, aladdin), N,N’-dimethylformamide (DMF, 99.8%, Vetec), Acetonitrile (99.8%, Vetec), Methanol (98%,AR, Beijing Chemical Works) and Ethanol (99%,AR, Beijing Chemical Works). All of the chemicals and solvents were used as received without further purification.

2.2 Characterization

Power X-ray diffraction (PXRD) measurements were performed on a D8 focus diffractometer at a scanning rate of 5°/min in the 2θ range from 5 to 40°, with graphite monochromatized Cu Kα radiation (λ= 0.15405 nm). Thermogravimetric analysis (TGA) data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with a heating rate of 10°/min in a nitrogen flow of 100 mL/min. The morphology of the samples was characterized by using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi) equipped with an energy-dispersive X-ray (EDX) spectrometer. Gas sorption isotherm measurements were carried out on Quantachrome Instrument Autosorb IQ. The samples were fully exchanged with ethanol, and degassed at 150°C for 8 h before the gas sorption measurement. For the PL measurement, the as-synthesized Ln-1,4-NDC-fcu-MOF was first fully exchanged with water for 48 h to remove all of the solvents such as DMF, methanol and ethanol etc., and then air dried for about 3 days before test. The Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as the excitation source was used to conduct PL measurement.

2.3 Synthesis

A precursor solution was prepared by mixing Eu(NO₃)₃·6H₂O (20 mg, 0.0441 mmol), 1,4-NDC (5 mg, 0.0231 mmol), and 2-FBA (15 mg, 0.108 mmol) in a mixture of 1 mL DMF, 0.5 mL methanol and 0.5 mL ethanol, and then methylamine modulator was added into the reaction system. The mixture was heated to 150°C and held for 2 hours on a hot plate with continuous stirring. The products were harvested by centrifugation and washed with anhydrous ethanol twice.

3 Results and discussion

3.1. Analysis of XRD and N₂ absorption isotherm

Figure 1 showed the PXRD of the as-synthesized samples, the octahedral and sphere-like products obtained in the presence of different amounts of methylamine showed similar PXRD patterns, and the diffraction peaks matched well with that of the calculated pattern for Eu-1,4-NDC-fcu-MOF [18]. It is indicated that Eu-1,4-NDC-fcu-MOF micron particles with different shapes were successfully prepared by this modified solvothermal method, and the octahedral products obtained in the presence of 0.02 mL methylamine showed good crystallinity. Figure 2 showed the N₂ absorption-desorption isotherm of Eu-1,4-NDC-fcu-MOF, the gas sorption properties were also tested with BET surface area of 375.7 cm²/g and pore volume of 0.2457 cc/g, and the analysis of pore size distribution was performed at NLDFT model by assuming a carbon surface. These results of Eu-1,4-NDC-fcu-MOF particles were consistent with the single crystalline counterparts.

Fig.1

PXRD patterns of the as-synthesized Eu-1,4-NDC-fcu-MOF particles produced by adding different amounts of methylamine (0.71 M).

Fig.2

N₂ absorption-desorption isotherm of Eu-1,4-NDC-fcu-MOF.

3.2 Analysis of morphology

In order to prepare monodispersed Eu-1,4-NDC-fcu-MOF, alkaline modulators were introduced. Indeed, monodispersed micron sized product of Eu-1,4-NDC-fcu-MOF was obtained in the presence of these modulators. Figure 3 shows scanning electron microscope (SEM) images of the products prepared by adding 0–0.06 mL of methylamine (0.71 M in DI water). As shown in Fig. 3a, non-uniform octahedral particles with random size distributions from hundreds nanometers to several micronmeters were obtained in the absence of methylamine. By adding 0.02 mL of methylamine solution, highly monodispersed octahedral particles with sizes around 1-2 μm were produced. Further increasing the amount of methylamine to 0.04 mL, sphere-like products with rough surface were formed. Finally, by adding 0.06 mL of methylamine, sphere-like products with more smoothed surface were obtained. In this study, 0.02 mL of methylamine was chosen as optimized amount of modulator added into the system to get highly monodispersed octahedral particles. We use different amines, solvents and modulators, the morphology and dispersion of Eu-1,4-NDC-fcu-MOF has good dispersion and uniformity when methylamine was used as solvent.

Fig.3

SEM images of the as-synthesized products prepared by adding: (a) 0, (b) 0.02, (c) 0.04 and (d) 0.06 mL of methylamine (0.71 M in DI water). The scale bar in Fig. 3a-d is 5 μm, and that in the inset of the figure is 0.5 μm.

3.3 Fluorescence properties of Eu-1,4-NDC-fcu-MOF

Due to the forbidden f-f transitions, lanthanide ions suffer from weak light absorption, which makes the direct excitation of lanthanide ions inefficiently unless high-power laser source is utilized. The problem could be overcome by introduction of coupling species to improve the energy transfer efficiency, such phenomenon was known as “luminescence sensitization” or “antenna effect” [2 , 24]. For Eu-MOFs, the ligand is commonly used as antenna, and the luminescence sensitization progress is mainly comprised of the following steps: light is absorbed by the organic ligands around lanthanide ions, then energy is transferred from the organic ligands to lanthanide ions, finally luminescence is generated from lanthanide ions [2 , 24]. Figure 4 shows the PL spectra of the as-synthesized Eu-1,4- NDC-fcu-MOF particles. The excitation spectrum was obtained by monitoring the ⁵D₀-⁷F₂ transition of Eu³⁺ at 617 nm. It could be found from Fig. 4 that the excitation spectrum consists of a broad intense band from 250 to 400 nm, with a peak centered at around 330 nm, which should be assigned to the absorption band from ligand. Due to the existence of absorption band from ligand in the excitation spectrum, it means that LMCT (ligand to metal charge transfer) process was involved in this study. Excitation into samples with the wavelength of 330 nm yields the emissions corresponding to f-f transitions of Eu³⁺: ⁵D₀-⁷F_J (J = 0, 1, 2, 3, 4), which were dominated by the hypersensitive red emission of ⁵D₀-⁷F₂ transition at 617 nm and also confirmed that energy efficiently transferred from ligand to Eu³⁺. The life time of Eu-1,4-NDC-fcu-MOF was shown in Fig. 5.

Fig.4

PL spectra of the as-synthesized Eu-1,4-NDC-fcu-MOF.

Fig.5

Life time for Eu-1,4-NDC-fcu-MOF.

The fluorescence performance of rare earth MOFs can be used to effectively detect small organic molecules, metal ions and explosives, etc. In addition, it can also be used to detect toxic substances, which are harmful to the environment and human health [25 –27]. Herein, the resultant Eu-1,4-NDC-fcu-MOF particles were used to detect benzidine (BD, class I carcinogen), a dye intermediate widely used in the textile dye industry. During the test, the benzidine ethanol solutions with different concentrations (2000, 1000, 500, 250, 125, 62.5, 31.2, 15.6, 7.8, 3.9 μg/mL) were prepared, and then Eu-1,4-NDC-fcu-MOF particles with a concentration of 100 μg/mL were dispersed in the above solutions and stirred evenly. As shown in Fig. 6, with increasing the concentration of BD in ethanol solution, the luminescence intensity of Eu³⁺ gradually decreased. The possible reason is that the amino groups in benzidine can be easily oxidized to generate amine oxides, and the OH group in the amine oxides will coordinate with Eu³⁺ and block the energy transfer from 1,4-NDC to Eu³⁺, thus leading to the luminescence quenching of Eu³⁺. The above results showed that Eu-1,4-NDC-fcu-MOF particles could be used to detect benzidine in ethanol solution effectively.

Fig.6

UV-vis spectra of Eu-1,4-NDC-fcu-MOF particles under different concentrations of benzidine.

4 Conclusions

In conclusion, Eu-1,4-NDC-fcu-MOF micron particles with octahedral morphologies were prepared by the solvothermal method, and the PL properties of Eu-1,4-NDC-fcu-MOF was also investigated. Furthermore, owing to the fast responsive, the micron sized Eu-1,4-NDC-fcu-MOF particles could be used as a potential sensing material for benzidine.

Footnotes

Acknowledgments

This project is financially supported by the National Natural Science Foundation of China (NSFC 21471145), “Hundred Talents Program” of Chinese Academy of Science, the Youth Backbone Teacher Training Project of Henan Higher Education Institutions in 2017 (2017GGJS282), Postdoctoral Science Foundation of Henan province in 2018 (001803005), the Postdoctoral Foundation(BHJF001), Startup Project of Doctor Scientific Research (BSJH001) and the Youth Fund (2020-Qnjj-002) of Shangqiu Medical College, the Higher School Key Scientific Research Project of Henan Province (20B150018).

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Facile fabrication of Eu-1,4-NDC-fcu-MOF particles for sensing of benzidine

Abstract

Keywords

1 Introduction

2 Experimental section

2.1 Reagents and instruments

2.2 Characterization

2.3 Synthesis

3 Results and discussion

3.1. Analysis of XRD and N2 absorption isotherm

Footnotes

Acknowledgments

References

3.1. Analysis of XRD and N₂ absorption isotherm