One-step in situ Controllable Synthesis of MnFe2O4/rGO Nanocomposite and Its Application to Electrochemical Sensing of Hydrogen Peroxide

In this work, we prepared a reduced graphene oxide (rGO)-supported manganese ferrite (MnFe2O4) hybrid material by a simple one-pot solvothermal synthesis method, using graphite oxide (GO) and metal ions (Fe3+, Mn2+) as raw materials. The reduction of GO and the in situ formation of MnFe2O4 nanoparticles were simultaneously achieved during the synthesis. The properties of MnFe2O4/rGO were characterized by scanning electron microscopy, powder X-ray diffraction, Fourier-transform infrared spectroscopy, and energy-dispersive X-ray spectrometry. The electrochemical characterizations of the resulting sensor were carried out by cyclic voltammetry and chronoamperometry. The results of electrochemical experiments show that the composite has improved hydrogen peroxide (H2O2) reduction performance. The linear range of the as-prepared sensor for H2O2 detection is 0.025 to 1.5 mM, with a detection limit of 0.796 μM (S/N = 3) and a response time of less than 4 s. In this paper, an effective, economical, and green experimental method for the synthesis of metal-oxide/graphene nanocomposites is proposed.


Introduction
Hydrogen peroxide (H 2 O 2 ) plays an important role in life activities and pharmaceutical, environmental, and industrial processes. (1)(2)(3) It is also an important by-product of many oxidation reactions (active oxygen) and is a major pollutant in the environment. (4)(5)(6) In living organisms, living cells secrete H 2 O 2 in the mitochondria to regulate the normal function of the cells. (7) H 2 O 2 is also closely related to aging and nervous system diseases. Therefore, the construction of a controllable, low-cost, and rapid H 2 O 2 -response recognition system has attracted widespread attention. (8) Since Gao et al. (9) reported, for the first time, that ferromagnetic nanoparticles have mimetic enzymes similar to common peroxidases, many research groups have begun to study the catalysis of magnetic materials on H 2 O 2 by spectroscopy (10) and electrochemical methods. (11) MFe 2 O 4 (M = Co, Mn, Zn, Ni, Cu, etc.), which has a cubic spinel structure, is an important ferromagnetic nanomaterial. (12) It has shown a great application value in the fields of information storage, pollutant removal, medical diagnosis, and drug delivery. (13) However, unmodified spinel-type ferromagnetic particles are easily agglomerated and extremely unstable, so their applications are also facing enormous challenges. In addition, sensors based on MFe 2 O 4 usually have problems such as low sensitivity and narrow linear range. (14,15) Therefore, it is necessary to modify the surfaces of magnetic nanoparticles by an appropriate method to overcome these limitations. (16,17) A two-dimensional layered nanomaterial, graphene, has received extensive attention owing to its unique electrical, thermal, mechanical, and chemical properties, and has good application prospects in environmental detection and repair, energy conversion, energy storage, and biosensing. (18)(19)(20) Therefore, the combination of graphene and MFe 2 O 4 magnetic nanoparticles can not only enhance the electrocatalytic activity and electron transfer ability of the material, but also prevent the agglomeration of magnetic nanoparticles, which makes it an ideal sensing platform for electrochemical sensors/biosensors. (21)(22)(23) Unfortunately, the controlled synthesis of MFe 2 O 4 nanoparticles on reduced graphene oxide (rGO) nanosheets remains a challenge.
Herein, we prepared MnFe 2 O 4 /rGO by a one-pot solvothermal process using GO and metal chlorides as raw materials. The morphology and structure of the nanocomposites were investigated. In addition, the catalytic properties, including the reduction of H 2 O 2 , were systematically investigated.

Preparation of MnFe 2 O 4 /rGO
The synthesis method for MnFe 2 O 4 /rGO nanocomposites is as follows. (24) Certain amounts of NaAc and PEG are added to the EG dispersion of 2 mg/mL GO, 0.04 M FeCl 3 ·6H 2 O, and 0.02 M MnCl 2 ·4H 2 O. After stirring for a certain period of time, the mixture is transferred to a hydrothermal reaction kettle and heated at 200 °C for 10 h. The final product is centrifuged and washed with ethanol several times and dried in a vacuum oven at 60 °C. Additionally, MnFe 2 O 4 bare spheres and pure rGO are prepared under the same experimental conditions.

Preparation of the modified electrodes
Our laboratory-made screen-printed carbon electrodes (SPCEs) were used as substrates for immobilized catalysts. The printing and processing methods for SPCEs were described in our previous work. (25) The modified electrodes were dried and stored in a desiccator for later use. The MnFe 2 O 4 , rGO, and MnFe 2 O 4 /rGO nanostructure-modified electrodes are specified as MnFe 2 O 4 /SPCE, rGO/SPCE, and MnFe 2 O 4 /rGO/SPCE, respectively.

Electrochemical measurements
Cyclic voltammetry (CV), amperometry measurements, and electrochemical impedance spectroscopy (EIS) were carried out on a CHI-760E electrochemical workstation (CH Instruments, Shanghai, China). The three-electrode system consisted of a screen-printed electrode, a platinum wire, and an SCE as the working, auxiliary, and reference electrodes, respectively. All electrochemical tests were performed in 0.1 M PBS (pH 7.4). Additionally, prior to testing, all solutions were bubbled with high-purity nitrogen for 15 min to completely remove oxygen.

Results and Discussion
In this work, a simple in situ synthesis method was adopted to prepare MnFe 2 O 4 /rGO. The whole process is as follows: GO is first exfoliated by an ultrasonic method, and then the MnFe 2 O 4 microspheres simultaneously grow on the rGO nanosheets during the solvothermal process. The mechanism associated with structural transformation is that, during the dispersion of the raw materials, Fe 3+ and Mn 2+ cations are adsorbed on the negatively charged GO surface as nucleation sites by electrostatic interaction. Subsequently, in the hydrothermal process, EG and PEG are used as both solvents and reducing agents. NaAc can change the alkalinity of the solution, which causes Fe 3+ and Mn 2+ to grow into MnFe 2 O 4 nanocrystallites under alkaline conditions and further aggregate into microspheres.

Characterization of samples
The morphology of MnFe 2 O 4 /rGO nanocomposites was characterized by SEM. It can be seen from Fig. 1 Fig. 1(b). These microspheres have good monodispersity and their diameter is larger than that of the MnFe 2 O 4 microspheres in the MnFe 2 O 4 /rGO nanocomposite.
The XRD pattern of the synthetic MnFe 2 O 4 /rGO nanocomposite is shown in Fig. 2(a). The disappearance of the peak at 2θ = 10.3° indicates the removal of the oxygen-containing group, and GO is reduced to rGO. This conclusion can also be confirmed by FTIR spectroscopy. The diffraction peaks at 18.

Electrochemical sensing of H 2 O 2
The electrocatalytic activity of the developed MnFe 2 O 4 /rGO nanocomposites for H 2 O 2 detection was evaluated by a CV technique within a potential range from −0.9 to +1.0 V in 0.1 M PBS with a scan rate of 50 mV/s. Figure 3( Fig. 3(b), MnFe 2 O 4 / rGO/SPCE shows a larger reduction peak than rGO/SPCE and MnFe 2 O 4 /SPCE. The excellent electrocatalytic activity of MnFe 2 O 4 /rGO/SPCE can be derived from the good synergistic effect between MnFe 2 O 4 microspheres and rGO nanosheets for the following reasons. On the one hand, MnFe 2 O 4 microspheres are attached to or distributed over the rGO nanosheets, which can inhibit the agglomeration of the magnetic nanoparticles and the curling of the sheet structure, thereby increasing the effective area of the composite. On the other hand, the rGO nanosheets are formed as the core of MnFe 2 O 4 magnetic microspheres. The nuclear matrix can induce the nucleation and growth distribution of the nanocrystals, form fine nanostructures, and uniformly disperse and control the morphology on the surface of the lamellar structure through chemical functions. Thus, a perfect overall structure can be formed between the MnFe 2 O 4 magnetic microspheres and the rGO nanosheets. This structure is a conductive network that can shorten the path of electron and ion transport.  Table 1, it is clear that the electrochemical sensing performance characteristics of as-fabricated MnFe 2 O 4 /rGO/SPCE are analogous and even superior to those in the recent reports. (26)(27)(28)(29)(30)

Conclusions
We successfully prepared MnFe 2 O 4 /rGO nanocomposites by a simple one-step solvothermal method. The reduction of GO and the growth of MnFe 2 O 4 were simultaneously achieved during the synthesis. In addition, by changing the metal ion concentration in the raw material, the size of the MnFe 2 O 4 microspheres and the dispersion density on the rGO nanosheets are well controlled. Because of the excellent biocompatibility and good synergy between MnFe 2 O 4 and rGO, the prepared nanocomposites of MnFe 2 O 4 /rGO have high selectivity and sensitivity (0.113 mA/mM), a large linear range (0.025 to 1.5 mM), and a lower detection limit (0.796 μM). This sensor can be used as an effective tool for the qualitative and quantitative monitoring of dynamic changes in H 2 O 2 concentration in different systems.