Synthesis of Fe3O4@MIL-101(Fe) for a Novel Electrochemistry Detection of Citric Acid
Authors: Zhou Shuqi , Liang Tao , Ma Chiyu , Wang Xinyi , Kong Liubing , Lin Wencheng , Kirsanov Dmitry , Legin Andrey , Wan Hao , Wang Ping 
Published: July 2021

Introduction

Urolithiasis is one of the most common diseases in the urinary system with very high incidence worldwide. In recent studies, citric acid (CA) has been considered as a biomarker of urolithiasis due to its vital suppression role in urinary stone formation. The detection of CA in urine is of great significance for early screening and prognostic monitoring of urolithiasis. However, most analytical methods for detecting CA are complicated and require expensive equipment.
In the present study[1, 2], MIL-101(Fe) and Fe3O4 were used for electrochemical detection of CA, based on the reaction of Fe(Ⅲ) and CA. Following these ideas, the electrodes modified with Fe3O4@MIL-101(Fe) nanoparticles were fabricated and investigated for whether they were able to detect CA and improve the sensitivity.

Method

Synthesis of Fe3O4@MIL-101(Fe) nanoparticles(Figure1. A): The Fe3O4@MIL-101(Fe) core-shell nanoparticles were synthesized according to previous literatures[3, 4]. Specifically, 3.6 g NaAC and 0.87 g FeCl3 were dissolved in 75 mL ethylene glycol under ultrasound for 30 minutes. The mixture was transferred to a Teflon-lined autoclave chamber and heated at 200 °C for 16 h, then naturally cooled to room temperature. The obtained magnetic nanoparticles were collected with a magnet and washed with ethanol for three times. 350 mg above nanoparticles was dissolved to 75mL ethanol with 0.58mM Mercapto acetic acid (MAA), and gently stirred for 24h under nitrogen protection. The steps of self-assembly cycle were as follows. Firstly, 100 mg Fe3O4-MAA was dispersed in 10mM FeCl3, shaked for 1 minute and left for 15 minutes, then washed with ethanol for three times. Next, the nanoparticles was re-dispersed in 10 mM benzene-1,3,5-tricarboxylic acid, kept for 30 min at 70 °C with stripping, and then washed with ethanol for three times. After repeating this cycle 31 times, the synthesized core-shell nanoparticles were dried at 75 °C.

Fabrication of the
Fe3O4@MIL-101(Fe)
/GCE: The GCE working electrode was polished with 1.0, 0.3, and 0.05 μm Al2O3 substance and sonicated with an ethyl alcohol and water solution for 5 min respectively then dried under a N2 stream. For the modification, the synthesized Fe3O4@MIL-101(Fe) core-shell nanoparticles was dissolved in ultrapure water and the final concentration was 1.0 mg/mL. After sonicating for 1 hour, 7 µL of the suspension was dripped onto the surface of the GCE and dried at room temperature.

Detection of CA: Electrochemical analyses were carried out using differential pulse voltammetry (DPV) measurements in 0.01M KCl solution with different concentration of CA at pH 7.0. All experiments were performed at room temperature with a three-electrode system: Ag/AgCl (3 M KCl) electrode and platinum wire were used as a reference electrode and counter electrode, respectively, and applying in a potential range of 0 to 0.60 V, with an amplitude of 0.025 V and pulse period of 0.5 s.

Results and Conclusions

Characterization of Fe3O4@MIL-101(Fe): TEM images of as-prepared nanoparticles is shown in Figure 1. B. The Fe3O4@MIL-101(Fe) had spherical morphology and the core–shell structures which indicated the formation of MIL-101(Fe) layer on the Fe3O4 nanoparticles.. The dark core was the Fe3O4 with a diameter of about 348 nm, and the relatively bright outer layer was the MIL-101(Fe) with the thickness of 246 nm.

The detection of CA: The DPVs of Fe3O4@MIL-101(Fe) /GCE in the 0.1M KCl with or without 100 mg/L CA are shown in Figure 1. C. In the presence of CA, a current peak was obtained in 0.324V. This could be explained due to the reaction of Fe3+ ions in Fe3O4@MIL-101(Fe) with CA. The performance of the Fe3O4@MIL-101(Fe) /GCE in different concentrations of CA solutions were monitored by DPV, and the corresponding ΔCurrent( ) are shown in Figure 1.D. The resulting calibration is presented in the Figure 1. E, which was linear over the concentration range of 40 to 100 mg/L and 100-500 mg/L CA, with the linear regression equations of Y = 0.0004*X +0.004 (R2 = 0.9956) and Y = 0.00016*X + 0.03773 (R2 = 0.9766), respectively. Based on three times the background noise, the modified electrode provided a limit of detection of 31 mg/L.

References

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Guivar, J.A.R., et al., Preparation and characterization of cetyltrimethylammonium bromide (CTAB)-stabilized Fe3O4 nanoparticles for electrochemistry detection of citric acid. 2015. 755: p. 158-166.

Chen, Y., et al., Facile preparation of core–shell magnetic metal–organic framework nanoparticles for the selective capture of phosphopeptides. 2015. 7(30): p. 16338-16347.

Salman, F., A. Zengin, and H.Ç.J.I. Kazici, Synthesis and characterization of Fe 3 O 4-supported metal–organic framework MIL-101 (Fe) for a highly selective and sensitive hydrogen peroxide electrochemical sensor. 2020. 26(10): p. 5221-5232.

Figure 1