preparation of a magnetic metal organic framework

ANALYTICAL SCIENCES JUNE 2014, VOL. 30 663

Introduction

Metal organic framework (MOF) is the latest class of ordered porous solids among new functional materials.1 Being of highly porous crystalline material, MOF exhibits a larger panel of pore sizes and shapes. Recent developments regarding MOF have advanced very quickly because of its intriguing structure and potential applications, which include use in gas storage, purification, catalysis, sensing, adsorption and drug delivery.2–7 Composites have combined the performances of MOF and other materials. A hybrid composite of multiwalled carbon nanotubes and MOF-5 was synthesized and the hydrostability and hydrogen storage capacity was enhanced.8 Fe3O4 nanoparticles have attracted much attention for their specific characteristics and have potential applications for ferrofluid, bioprocessing, information storage and separation.9–12 Because of its earth-abundant character and high curie temperature, Fe3O4 stands out as a key partner in composites.13 After Sugimoto’s group reported the preparation of magnetite particles with a narrow size distribution in the early 1980s,14 monodisperse Fe3O4 nanoparticles have been fabricated by various chemistry-based synthetic methods.15–17 In this paper, we synthesized a Fe3O4/MOF composite and applied it in electrochemical research. MOF has been used as material for specific electrochemical sensing because of its electrocatalytic activity and large surface area.18 It was reported that the magnetic MOF composite had some specific characteristics and

found applications in drugs delivery.19 In our research, we synthesized a new magnetic composite based on MOF Co3(BTC) (BTC = 1,3,5-benzenetricarboxlic acid) and applied it to detect pesticides.

Organophosphorous pesticides (OPs) are a group of important chemical pollutants that need to be more heavily monitored due to their toxicity, persistence, accumulation in the environment and their effects on the environment and health.20 As a model of OPs, methyl parathion (MP) may be converted in vivo into an active metabolite. It causes accumulation of acetylcholine at nerve endings in the peripheral or central nervous system. Many efforts have been made to develop sensitive, convenient and effective methods for pesticide residue analysis in environmental samples, such as gas chromatography (GC),21 high performance liquid chromatography (HPLC), mass spectrometry (MS) and enzyme-linked immune sorbent assay (ELISA).22–24 However, these methods suffer from expensive and extensive pre-treatment steps, the use of toxic organic solvents and also long analysis time. Recently, a great amount of attention has been paid to electrochemical activity.25 Electrochemical methods have the advantage of low cost, short analysis time and being easily miniaturized. Likewise, MP exhibits good redox activities at the electrode surface.26 A square wave voltammetric detection for MP with ZrO2-nanoparticles modified on a carbon paste electrode and an electrochemical sensor using an acetylene black-chitosan composite modified glassy carbon electrode has been developed.27,28 The enhanced redox current of the MP was due to the nano structure of the composite and the enlarged effective electrode area. But in these electrochemical methods, the materials had to be modified on the electrode surface in advance.

2014 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: [email protected]

Preparation of a Magnetic Metal Organic Framework Composite and Its Application for the Detection of Methyl Parathion

LiPing HU,* Nan WU,* Jing ZHENG,*† JingLi XU,* Min ZHANG,* and PinGang HE**

* College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, P. R. China

** Department of Chemistry, East China Normal University, Shanghai 200062, P. R. China

A magnetic metal organic framework (MOF) composite was prepared. The composite was fabricated by incorporation of Fe3O4 nanoparticals with MOF. It was characterized and expected to offer a promising template for molecular immobilization and sensor fabrication because of its ordered structure and satisfying large specific surface area. The resulting composite was used to detect methyl parathion. Electrochemical measurements showed that the multifunctional composite of MOF provided an excellent matrix for the co-adsorption of methyl parathion. Owing to the ordered structure, the large surface area, excellent compatibility and magnetic property of the material, methyl parathion could be separated, accumulated and directly detected in the solution with high sensitivity. The differential pulse voltammetry (DPV) response was proportional to the concentration range from 5.00 × 10–6 to 5.00 × 10–3 g L–1 with the detection limit of 3.02 × 10–6 g L–1. The experimental results can lead to a widespread use of electrochemical sensors to detect organophosphorous pesticides contaminates and other substances.

Keywords Magnetic, MOF, methyl parathion, detection

(Received February 28, 2014; Accepted April 25, 2014; Published June 10, 2014)

664 ANALYTICAL SCIENCES JUNE 2014, VOL. 30

In this strategy, a new magnetic MOF composite containing Fe3O4 was prepared. The as-synthesized material exhibited both magnetic characteristics and porosity, making it an excellent candidate for the accumulation and detection of MP. The electrochemical behavior of MP was investigated in a phosphate buffer solution. We were able to achieve accumulation of MP directly in the solution due to magnetism of the material, instead of separating in advance via other means. This method can greatly improve the sensitivity of measurement. Under the experimental conditions, the differential pulse voltammetry (DPV) response was proportional to the concentration of MP in a range from 5.00 × 10–6 to 5.00 × 10–3 g L–1, with a correlation coefficient of 0.996. The detection limit was 3.02 × 10–6 g L–1. And in contrast with other electrochemical measurements of MP, the detection limit was much lower.29–31 Owing to the porosity and magnetism of the material, which represents signal amplification through the accumulation of the material, the sensitivity of the system could be enhanced significantly.

Experimental

Reagents and chemicalsThe analytical reagents ferric trichloride hexahydrate

(FeCl3·6H2O), ethylene glycol (HOCH2CH2OH), sodium acetate (NaAc), polyethylene glycol (HO(CH2CH2O)nH), cobalt acetate tetrahydrate (CoAc2·4H2O), 1,3,5-benzenetricarboxylic acid (H3BTC), ethanol, methyl parathion (2.0 mg/mL), sodium phosphate dibasic dodecahydrate (Na2HPO4·12H2O), and dipotassium hydrogen phosphate (KH2PO4) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Ultrapure water was prepared using an Aquapro system (specific resistance is 13 MΩ cm). A phosphate buffer solution (PBS) was prepared by mixing 2.1838 g Na2HPO4·12H2O and 0.5307 g KH2PO4 into 100 mL H2O to form PBS at the concentration of 0.1 M.

ApparatusX-ray diffraction (XRD) (PANalytical X’Pert PRO,

Netherlands), scanning electron microscopy (Hitachi S-3400N, Japan), fourier transform infrared spectroscopy (Thermo Fisher Nicolet AVATAR 380, USA), thermo gravimetric analyzer (Linseis STA PT-1000, Germany), vibrating sample magnetometer (LakeShore 7407, USA) were employed to characterize the material. Cyclic voltammetry (CV) and DPV measurements were carried out using a CHI instruments Model 420 A Electrochemical Analyzer (CHI Instrument Inc., USA).

Preparation of MOFMOF was prepared according to the literature:32 An aqueous

mixture (15 mL) of CoAc2·4H2O (0.4110 g, 1.65 mmol) and H3BTC (0.2037 g, 0.95 mmol) was placed in a stainless steel vessel, which was sealed and placed in a programmable furnace. The mixture was heated to 140°C at 5°C/min and kept at that temperature for 24 h, then cooled at 0.1°C/min to 120°C and kept for 5 h. This was followed by further cooling at the same rate to 100°C, and the mixture was kept for another 5 h before finally cooling to room temperature. The resulting large red crystals were filtered, then washed with ethanol.

Preparation of monodisperse Fe3O4

Fe3O4 nanoparticals were prepared according to the literature:33 FeCl3·6H2O (0.6826 g, 2.5 mmol) was dissolved in HOCH2CH2OH (20 mL) to form a clear solution, followed by the addition of NaAc (1.800 g) and HO(CH2CH2O)nH (0.5000 g). The mixture was stirred vigorously for 30 min and then sealed in a teflonlined stainless-steel autoclave (35 mL capacity). The autoclave was heated to and maintained at 200°C for 8 – 72 h, and allowed to cool to room temperature. The black products were collected with the magnet and were washed several times with ethanol and dried at 80°C for 6 h.

Preparation of the composite Fe3O4/MOFAn aqueous mixture (15 mL) of CoAc2·4H2O (0.4110 g,

1.65 mmol), H3BTC (0.2037 g, 0.95 mmol) and as-synthesized Fe3O4 (0.0232 g, 0.1 mmol) was placed in a stainless steel vessel, which was sealed and placed in a programmable furnace. The mixture was heated to 140°C at 5°C/min and kept at that temperature for 24 h, then cooled at 0.1°C/min to 120°C and kept for 5 h. This was followed by further cooling at the same rate to 100°C, and the mixture was then kept for another 5 h before finally cooling to room temperature. The resulting large crystals were filtered, collected with the magnet, then washed with ethanol.

Accumulation of MP around the working electrodeThe procedure of enriching MP was as follows: 4 mL of PBS

(pH = 7.00) and appropriate amounts of standard MP solution (final concentration of MP ranged between 5.00 × 10–6 and 5.00 × 10–3 g L–1) were added into a 25-mL volumetric flask. Then 5.0 mg magnetic composite was added to the flask. As shown in Fig. 1, the electrochemical system was comprised of a working electrode (d = 5 mm), a reference and an auxiliary electrode. They were dipped into the test solution. The glassy carbon working electrode, Ag/AgCl reference electrode and

Fig. 1　Schematic representation of procedure.(a) Preparation of composite Fe3O4/MOF, (b) Accumulation of MP, (c) Detection of MP.


platinum wire counter electrode were purchased from Gaoss Union Co., Ltd. The magnetic glassy carbon electrode was fabricated by technicians at Gaoss Union Co., Ltd. by adding a magnet upon the glass carbon according to our special requirements. The working electrode was exposed to the test solution for 5 min. MP was accumulated and detected directly in the solution due to magnetism of the material. Prior to use, the working electrode was polished carefully to achieve a mirrorlike surface with 0.3 μm alumina slurry and sequentially infused for 5 min in 1 M NaOH, 6 M HCl and acetone.

Electrochemical detection of MPA magnetic glassy carbon electrode served as a working

electrode, a Ag/AgCl/saturated KCl solution as the reference electrode and a platinum wire as the auxiliary electrode, respectively. Electrochemical experiments were carried out in the same electrochemical cell at room temperature. CV measurements were performed at a potential scan over the range of +0.20 to –0.90 V at a scan rate of 100 mV/s. DPV measurements were performed between +0.20 and –0.30 V, with a pulse height potential of 50 mV and a frequency of 50 Hz. The DPV peak height at a potential of –0.064 V of the reduction of MP was used in all of the measurements.

Results and Discussion

Characterization of Fe3O4/MOFXRD patterns. The XRD patterns are shown in Fig. 2. The peaks of MOF were consistent with an earlier report.32 The crystal structure analysis performed on the Co3(BTC)2·12H2O compound shows that the structure is composed of zigzag chains constructed from two symmetry-inequivalent tetra-aqua cobalt(II) units and BTC ligands. And the pattern of Fe3O4 can be easily indexed to Fe3O4 (JCPDS 75-1609). Its characteristic peaks are (311), (400), (440), the diffraction peaks of the composite corresponding to the lattice of MOF and Fe3O4.

SEM images of Fe3O4/MOFThe size and shape of the products were examined by scanning

electron microscopy (SEM). Figure 3 shows the SEM images

of the Fe3O4/MOF composite. The image of the cross-section of the composite beads shows the presence of Fe3O4 sprinkling on the MOF. The sizes of Fe3O4 were about 200 nm, and the nanoparticals were clearly observed and scattered uniformly on the MOF.

FTIR of Fe3O4/MOFThe FTIR spectra of composite were measured in the range of

400 – 4000 cm–1 (Fig. 4). The broad peak seen at 3458 cm–1 was assigned to the characteristic absorption of the O–H bond of the inorganic chains, indicating the formation of a hydrogen bond between MOF and H2O. The peaks at 3117 and 717 cm–1 corresponded to the characteristic symmetric and bending vibration of unsaturated C–H bond of the phenyl group on the ligand of MOF. The aromatic ring stretching frequency of the pristine 1,3,5-benzenetricarboxylic acid results in the FTIR characterization bands around 1611 – 1433 cm–1. The peak of 1106 cm–1 was ascribed to the vibration absorption of the C=O bond of the carboxylic group of 1,3,5-benzenetricarboxylic acid. The FTIR spectra showed that the characteristic absorption peaks of organic functional groups of MOF were not changed, that is, the structure of MOF in the composite was not damaged. And the peak at 580 cm–1 was the characteristic absorption of

Fig. 2　XRD patterns (a) MOF, (b) Fe3O4, (c) the Fe3O4/MOF composite.

Fig. 3　SEM image of the composite.

Fig. 4　FTIR spectra of the composite.


Fe3O4.34

TGA analysisIn order to elucidate the mechanism of MOF and the composite

thermal cracking in an argon atmosphere, the degradation of the prepared material was studied by thermogravimetric analysis (TGA) (Fig. 5). MOF and the Fe3O4/MOF composite degrade in successive steps with initial loss of water at around 100°C. The materials show further degradation at 415°C in a process identical to the breakdown of BTC. MOF stabilizes at a temperature of 630°C when 31.1% by mass of the material remains (Fig. 5a). However the composite stabilizes at a temperature of 580°C when 37.7% by mass of the material remains (Fig. 5b). The difference might be attributed to the addition of Fe3O4 in the composite.

Magnetic property of Fe3O4/MOFThe magnetic property of the composite system was also

studied (Fig. 6). The results for the composite showed a typical hysteresis behavior characteristic of ferromagnetism at room temperature. The saturation magnetization of the composite

was found to be around 1.212 emu/g, the coercive force 198.5 G, and residual magnetization 4.725 × 10–2 emu/g. In addition, magnetization decreased from the plateau value and reached zero when the magnetic field intensity decreased. The nonlinear curves with nonzero remnant magnetization and coercivity showed a well-pronounced soft magnetic property. So it is likely that the composite was magnetized and demagnetized.

Electrochemical behavior of MPCyclic voltammograms were used to examine the

electrochemical behavior of MP with the composite in PBS (pH = 7.00) at the magnetic glassy carbon electrode (MGCE).

Fig. 5　TGA of MOF (a) and the composite (b).

Fig. 6　Magnetization curves of the composite.

Fig. 7　CV (A) and DPV (B) of (a) in the absence of MP, (b) in the presence of MP, (c) in the presence of MP/Fe3O4, (d) in the presence of MP/MOF, (e) in the presence of MP/Fe3O4/MOF with glassy carbon electrode and (f) in the presence of MP/Fe3O4/MOF with magnetic electrode in the PBS; the inset shows the DPV responseConditions: concentration of MP: 5.00 × 10–4 g L–1; pH: 7.00; accumulation time: 5 min; conditioning potential: –1.00 V; scan rate: 0.10 V s–1.


The potential range from +0.20 to –0.90 V was applied for cyclic voltammetric analysis. The potential range from +0.20 to –0.30 V was applied for differential pulse voltammetry analysis. Figure 7 shows the CV (A) and DPV (B) of (a) in the absence of MP, (b) in the presence of MP, (c) in the presence of MP/Fe3O4, (d) in the presence of MP/MOF, (e) in the presence of MP/Fe3O4/MOF with glassy carbon electrode and (f) in the presence of MP/Fe3O4/MOF with magnetic glassy carbon electrode in the PBS. No redox peak was observed in the absence of MP in PBS (Fig. 7 A(a)). As seen from Fig. 7A(b), the peak current of the bare MGCE to MP was very poor. The voltammogram shows low sensitivity. When Fe3O4 and MOF were added to the solution respectively (Figs. 7A(c) and 7A(d)), the sensitivity was also low. However, when the composite was added to the solution, MP was adsorbed by the composite and accumulated around the magnetic electrode, and the electrochemical signal was amplified (Fig. 7A(f)). While using a normal glassy carbon electrode, MP was adsorbed by the composite but could not collect around the electrode and the sensitivity was lower (Fig. 7A(e)). An irreversible reduction peak (Epc) at –0.691 V corresponded to the reduction of the nitro group to the hydroxylamine group, reaction (I), and a pair of rather well-defined redox peaks (Epa and Epc) at –0.015 and –0.115 V were attributed to two electron-transfer processes, reactions (II) and (III), as shown below. The mechanism for this process has been widely reported.35,36 Owing to the porosity of the material, it offered a benign microenvironment for MP. Figure 7B shows the DPV response at the potential range from +0.20 to –0.30 V in different cases. These results also indicated that MP exhibited high current response when containing the Fe3O4/MOF composite in the PBS. Several factors may have contributed to these results. The high surface area of the composite and the magnetism were certainly favorable for the adsorption and reaction of MP. Moreover, the excellent conductivity of material also played an important role for the fast electron transfer.19 Due to the magnetic property of the composite, MP could be directly detected in solution instead of preparing materials modified onto the electrode.30,37

The effect of amount of the compositeThe amount of the composite could also influence the

determination of MP. Figure 8 shows the effect of the amount of the composite on the reduction peak current of MP. As can be seen, the current of MP increased with the mass of the material until reaching a maximum at 5.0 mg. When the mass

of the material exceeded 5.0 mg, the current hardly increased. Therefore, 5.0 mg of the composite was chosen as the optimal mass to obtain the ideal amount.

Analytical performance for the detection of MPSensitive DPV was chosen to determine trace quantities of

MP at different concentrations. Figure 9(A) displayed the DPV response of the magnetic electrode incubated with increasing concentrations of MP solution under the experimental conditions. Figure 9(B) displays the regression equation for the line in the range of 5.00 × 10–6 to 5.00 × 10–3 g L–1. The equation was Ip = 0.893log C – 0.228 with a correlation coefficient of 0.996, and a detection limit of 3.02 × 10–6 g L–1 based on signal/noise of 3. Compared with other platforms, it is noteworthy that the composite exhibited enhanced performance. The detection limit

Fig. 8　The effect of the amount of the composite on MP adsorption.

Fig. 9 (A) DPV of different concentrations of MP under experimental conditions. (a) 5.00 × 10–6, (b) 1.00 × 10–5, (c) 2.00 × 10–5, (d) 3.00 × 10–5, (e) 5.00 × 10–5, (f) 1.00 × 10–4, (g) 5.00 × 10–4, (h) 1.00 × 10–3, (i) 2.00 × 10–3, (j) 5.00 × 10–3 g L–1. (B) The linear plot of the regression equation in the concentration range of 5.00 × 10–6 to 5.00 × 10–3 g L–1. Error bars = ± relative standard deviation and n = 5.


is much lower than the value of 5.00 × 10–5 g L–1 as reported using Pd/MWCNTs-modified electrode and the value of 5.00 × 10–6 g L–1 as reported using multiwalled carbon nanotubes. This suggested that after signal amplification through the accumulation of the material, the sensitivity of the system could be enhanced significantly.29,30 The linear range is wider than that obtained based on a Pd/MWCNTs nanocomposite-modified glassy carbon electrode, zirconia nanoparticles-modified gold electrode as well as poly-NiTSPc and Nafion® films-modified carbon fiber electrode.30,31,36

The reproducibility of MP was assessed by analyzing samples of 5.00 × 10–4 g L–1 and the relative standard deviation (RSD) of five replicate determinations was 8.07%. The results indicated acceptable precision and reproducibility.

Conclusions

An electrochemical method based on a magnetic MOF composite was developed. MOF showed rich and vivid coordination characteristics and bridging modes to transition metal centers, which offered a pathway for the electronic transfer. The magnetism of the composite facilitated the accumulation of MP around the magnetic electrode surface, allowing for direct detection of MP in the solution. Due to the ordered structure, large surface area and excellent biocompatibility, the composite presented a sensitive response towards MP. With the sensing system, electrochemical signals were detected when the concentration of MP was in a range from 5.00 × 10–6 to 5.00 × 10–3 g L–1 and the detection limit obtained was 3.02 × 10–6 g L–1. The proposed method has been demonstrated to offer a convenient, specific, sensitive way of detecting MP. The strategy and the new material are expected to have wide applications in many other domains.

Acknowledgements

We would like to express our gratitude to the Shanghai Natural Science Foundation (No. 13ZR1418300) and the National Natural Science Foundation of China (No. 21305086) for financial support for this work.

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preparation of a magnetic metal organic framework

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