Solvothermal synthesis of Fe3O4 nanospheres for high-performance electrochemical non-enzymatic glucose sensor

Ferroferric oxide (Fe3O4) nanospheres have been synthesized via a facile solvothermal procedure to serve as an electrode material for high performance non-enzymatic glucose sensor. The as-synthesized Fe3O4 nanospheres with a uniform size from 16 to 18 nm, which can increase the reaction contact area and the active sites in the process of glucose detection. Benefiting from the particular nanoscale structure, the Fe3O4 nanospheres obviously enhanced the activity of electrocatalytic oxidation towards glucose. When the Fe3O4 nanospheres material was used for non-enzymatic glucose sensor, several electrochemical properties including the high sensitivity 6560 μA mM−1 cm−2 (0.1–1.1 mM), limit of detection 33 μM (S/N = 3) and good long-term stability were well demonstrated. Furthermore, Fe3O4 nanospheres electrode confirmed the excellent performance of selectivity in glucose detection with the interfering substances existed such as urea, citric acid, ascorbic acid, and NaCl. Due to the excellent electrocatalytic activity in alkaline solution, the Fe3O4 nanospheres material can be considered as a promising candidate in blood glucose monitoring.

and catalytic activity, Fe 3 O 4 nanomaterials have received considerable interest [30][31][32] . Aside from the inherent properties, the structure and morphology of the Fe 3 O 4 nanomaterials are also the key factors affecting the electrochemical sensing performance in glucose detecting 33 . It has been reported that the Fe 3 O 4 nanoparticles in various shapes synthesized via different methods possess different properties 34,35 . With the kinetic capacity in glucose oxidation reaction and the large surface area, nano-spherical structures can offer more active sites and enhance the electrochemical performance [36][37][38] . Therefore, a facile process to synthesize Fe 3 O 4 nanomaterials as the material of electrochemical non-enzymatic glucose sensor has become essential.
In this paper, a high-performance electrochemical non-enzymatic glucose sensor has been presented. The Fe 3 O 4 nanospheres have been synthesized in a facile solvothermal procedure, which are pasted on the Ni foam. Several electron microscopic analysis and diffraction techniques were used to characterize the properties of microstructure and morphology of Fe 3 O 4 nanospheres. Satisfactorily, the Fe 3 O 4 nanospheres displayed superior nanoscale size and electrocatalytic properties for detecting glucose (from 0.1 to 1.1 mM) and a limit of detection is 33 μM.

Results and discussion
The representative synthesis and electrochemical glucose detection process of the Fe 3 O 4 nanospheres electrode is exhibited in Fig. 1. The synthesis of Fe 3 O 4 nanospheres was carried out in a solvothermal procedure and the as-synthesized Fe 3 O 4 nanospheres electrode was acted as a working electrode in three-electrode system for glucose detection. Firstly, FeCl 3 ·6H 2 O, CTAB (hexadecyl trimethyl ammonium bromide), CH 3 COONa and EDA (ethylenediamine) were dissolved in EG (ethylene glycol) under magnetic stirring for 30 min at 50 °C. Then the mixed reagents were transferred to the autoclave to synthesize the target samples. The as-synthesized Fe 3 O 4 nanospheres samples were pasted on Ni foam for electrochemical detecting. In the process of electrochemical sensing towards glucose, the glucose was oxidized into gluconolactone by the strongly oxidized Fe 3+ which was formed rapidly in the NaOH electrolyte. At the same time, the electron transport reactions also occurred. Subsequently, the electrons were transferred to the Ni foam. The electrocatalytic oxidation towards glucose was finally realized under the interaction in the three-electrode system.
Crystallographic data and chemical composition of Fe 3 O 4 nanospheres were identified by the powder X-ray diffraction analysis (XRD, in the 2θ range from 25° to 70°). All characteristic peaks are well consistent with the standard PDF card (JCPDS No. 03-0863) in Fig. 2, which indicate the products are pure In order to observe surface of the samples, the morphologies of Fe 3 O 4 nanospheres has been presented by scanning electron microscope (SEM). The low magnification SEM microstructure images (in Fig. 3a,b) present an overall morphology of Fe 3 O 4 nanospheres. The products with extremely small size exhibit a fine exterior surface and gathered together.    Fig. 5a. It is obvious from these CV curves that the cathodic peak potential shows a slight positive shift when glucose concentration is increased gradually. The observation reveals that the Fe 3 O 4 nanospheres electrode possesses a great advantage in electrocatalytic activity. In the glucose detecting process, some values such as applied working potential has played an important role. Figure 5b shows the current-time curves of Fe 3 O 4 nanospheres electrode when adding glucose successively for every 100 s at various applied potential in 0.5 M NaOH. By comparison, Fe 3 O 4 nanospheres electrode displays a weak sensitivity at the highest applied potential 0.60 V and the noise interference is large. Hence, the appropriate potential value option for Fe 3 O 4 nanospheres electrode is 0.55 V. The ability of the Fe 3 O 4 nanospheres electrode towards glucose electrooxidation was further investigated at the selected potential of 0.55 V. As described in Fig. 5c, amperometric response increased significantly with the increase of glucose concentration in the NaOH. This result reveals an excellent electrocatalytic ability of Fe 3 O 4 nanospheres electrode for detecting glucose. The calibration curve of the current density and glucose concentration are displayed in Fig. 5d, which determined the relevant measurement range of 0.1-1.1 mM (R 2 = 0.9828). The sensitivity of Fe 3 O 4 nanospheres electrode is 6560 μA mM −1 cm −2 by observing corresponding slope parameter and a corresponding detection limit is 33 μM (S/N = 3). We have compared sensitivity of Fe 3 O 4 nanospheres electrode in electrochemical sensing towards glucose with the previously reported data on iron materials and other types of electrochemical sensors. Corresponding comparison of Fe 3 O 4 nanospheres electrode with other electrode materials was summarized in Table 1 39 . Electrochemical impedance spectroscopy (EIS) technique is also important to evaluate the electrochemical glucose sensor, which has been also carried out in other papers 40 . Figure 5e exhibits the Nyquist plot of Fe 3 O 4 nanospheres electrode in the three-electrode system. Inset is a part of the Nyquist plot in the high frequency region which shows a semicircle shape with small diameter. It possesses a certain relationship with the controlled process of the charge transfer 41 . The diameter of the semicircle in Nyquist plot is equal to the charge transfer resistance (Rct) of the active surface area of the Fe 3 O 4 nanospheres electrode, which was calculated to be  www.nature.com/scientificreports/ 1.09 Ω. The low electrochemical impedance indicates a fast glucose oxidation kinetics in the process of glucose detection. The above series of results are sufficient to prove that the non-enzymatic glucose sensor based on the Fe 3 O 4 nanospheres electrode possesses the excellent performance of providing the effective electron transport pathway for glucose detection. Some interfering substances, such as citric acid (CA), urea (UA) and Cl − from NaCl present in the human blood may have an effect on the Fe 3 O 4 nanospheres electrode. Here, we study the antiinterference performance of the Fe 3 O 4 nanospheres electrode through the current-time curve. Figure 5f shows the current-time curve of Fe 3 O 4 nanospheres electrode in NaOH solution with the presence of 1 mM glucose, 0.1 mM CA, UA, ascorbic acid (AA) and NaCl. There was a significant current response when 1 mM of glucose was in NaOH solution. On the contrary, current response had little change with 0.1 mM other interfering substances in NaOH solution, revealing that Fe 3 O 4 nanospheres electrode has great selectivity for glucose detection. Figure 6 shows the CV curves of Fe 3 O 4 nanospheres electrode after 1 day and 30 days in 0.5 M NaOH. As can be seen from the figure, there is no obvious change in the shape of CV curve after 30 days. Furthermore, the value of the current response after 30 days did not change much, which confirmed the satisfactory stability of the Fe 3 O 4 nanospheres electrode.

conclusions
In summary, Fe 3 O 4 nanospheres with nanoscale size have been successfully synthesized in a facile solvothermal procedure. The electrochemical sensing performance of glucose in the three-electrode system has been investigated. The as-synthesized Fe 3 O 4 nanospheres electrode demonstrated a high efficiency and excellent selectivity in electrochemical sensing of glucose. Inspiringly, in relevant measurement range (from 0.1 to 1.1 mM), Fe 3 O 4 nanospheres electrode possesses a satisfactory sensitivity for 6560 μA mM −1 cm −2 . The value of detection limit is 33 μM (S/N = 3). Thus, Fe 3 O 4 nanospheres material has the potential to become a functional electrode material for detecting glucose in the research field of electrochemistry.

experimental details
Synthesis of the fe 3 o 4 nanospheres electrode. All chemicals were of analytical grade. In a conventional procedure, 1.0 g ferric chloride hexahydrate (FeCl 3 ·6H 2 O), 1.0 g hexadecyl trimethyl ammonium bromide (CTAB) and 3.0 g sodium acetate trihydrate (CH 3 COONa) were first added into 20 mL ethylene glycol (EG) and magnetically stirred for 20 min at 50 °C to form a uniform yellow solution. Subsequently, adding 10 mL ethylenediamine (EDA) to above-mentioned yellow mixture with magnetically stirring for 10 min. Then, the as-synthesized sample was added to a Teflon-lined autoclave (50 mL) and kept it for 10 h at 200 °C. Fe 3 O 4 nanospheres were collected by a magnet after cooling down the room temperature, using ethanol and deionized water to rinse them repeatedly. The synthesized Fe 3 O 4 nanospheres were mixed with acetylene black, poly (vinylidene fluoride) (PVDF) and N-methyl-2-pyrrolidone (NMP) in a good percentage. Finally, pasting above mixture on Ni foam (length × width = 20 mm × 10 mm) as the working electrode.