Study on the performance of MoS₂/Au composites for non-enzymatic electrochemical glucose detection

Thiourea (CH₄N₂S) and sodium molybdate (Na₂MoO₄₎ were purchased from Sigma Aldrich, Cetyltrimethylammonium bromide (CTAB), chloroauric acid (HAuCl₄·3H₂O), graphite powder, and N-Methylpyrrolidone (NMP) were obtained from Aladdin. Sodium hydroxide (NaOH), hydrochloric acid (HCl), potassium dihydrogen phosphate (KH₂PO₄), dipotassium hydrogen phosphate (K₂HPO₄), and glucose (C₆H₁₂O₆·H2O), dipotassium hydrogen phosphate (K₂HPO₄·3H₂O), potassium ferrocyanide (K₄[Fe(CN)₆]·3H₂O) were purchased from Beijing Chemical Factory, while acetylene black (ACET) and polyvinylidene difluoride (PVDF) were purchased from Macklin Reagent. All reagents used were of analytical grade. Highly purified water was used throughout the entire experiment.

The microstructures of the samples analyzed using a field emission scanning electron microscope (SEM, Regulus 8100) and transmission electron microscopy (TEM, FEI Tecnai), with working voltages of 5 kV and 20 kV for SEM and TEM, respectively. The structural analysis of the samples was was analyzed by x-ray diffraction (XRD, Rigaku-D-max2550) equipped with Cu-Kα radiation (λ = 0.154 nm), operating at 40 kV and 30 mA, with a scanning rate of 2°/min. Raman spectroscopy characterization was performed using a 532 nm He-Ne laser (LabRAM HR E). An electrochemical workstation (CS350) was employed for glucose electrochemical detection.

MoS₂ and MoS₂/Au were synthesized using hydrothermal and thermal reduction methods, scheme as shown in figure 1. Initially, 1.0 g of Na₂MoO₄ and 1.4 g of CH₄N₂S were added to 45 ml of deionized water, followed by the addition of 0.5 g of CTAB after stirring. After the solution is fully mixed, the pH was adjusted to 8. The mixed solution was then transferred into a reactor and heated at 240 °C in a forced air oven for 48 h. Upon completion of the reaction, the product was washed with deionized water and absolute ethanol, centrifuged to collect the precipitate, and dried in an oven at 40 °C to obtain MoS₂ powder. Subsequently, 6.5 × 10⁻⁴ M of MoS₂ was added to a boiling solution of HAuCl₄·3H₂O. After 10 min, heating was stopped, and the solution was allowed to cool naturally. The precipitate was collected by centrifugation, washed with deionized water and absolute ethanol, and dried to obtain MoS₂/Au.

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MoS₂ and MoS₂/Au powders were mixed with PVDF, ACET, and graphite powder in a mass ratio of 74:6:10:10 in NMP solvent. The mixture was thoroughly ground into a paste and coated onto nickel foam (NF) to prepare MoS₂ and MoS₂/Au electrodes. Cyclic voltammetry (CV) tests were conducted on the electrochemical workstation. The electrolyte used was a phosphate buffer solution containing 30 mmol/L K₄[Fe(CN)₆]·3H₂O and 30 mmol/L K₃[Fe(CN)₆]. An Ag/AgCl electrode was used as the reference electrode, Pt as the counter electrode, and MoS₂ or MoS₂/Au as the working electrode. The prepared MoS₂ and MoS₂/Au electrodes were immersed in the electrolyte, and different concentrations of glucose solution were added for CV testing.

To investigate the properties of the prepared electrode materials, the morphology of the samples was first observed using SEM and TEM. Figure 2(a) shows the SEM image of MoS₂, revealing numerous interlaced petal-like nanosheets forming a three-dimensional flower-like structure, typically around 2 μm in size. The inset displays the EDS spectrum of MoS₂, indicating the presence of only Mo and S elements, confirming the absence of impurities. The further TEM image (figure 2(b)) illustrates the layered structure of MoS₂, with wrinkles due to the flexibility of the atomic layers, forming a flower-like structure. The HRTEM image in figure 2(c) shows that the average lattice fringe spacing of MoS₂ is approximately 0.64 nm, which is consistent with the theoretical spacing of the (002) plane of hexagonal 2H-MoS₂. Figure 2(d) shows the SEM image of MoS₂/Au, where Au NPs are observed attached to the MoS₂ structure, confirming the formation of the MoS₂/Au composite while maintaining the morphology of MoS₂. The inset displays the EDS spectrum of MoS₂/Au, indicating the presence of only Au, Mo, and S elements, with no other impurities. In the TEM image (figure 2(e)), Au NPs can be clearly seen decorating the MoS₂ nanosheets, with individual Au NPs around 5 nm in size. The HRTEM image (figure 2(f)) shows that the lattice fringe spacing of MoS₂ is approximately 0.64 nm, while the lattice fringe spacing of Au NPs is about 0.23 nm, indicating that the Au NPs do not alter the crystal structure of MoS₂ upon composite formation.

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The crystal structure and purity of the samples were characterized using x-ray diffraction. The XRD patterns of MoS₂ and MoS₂/Au are shown in figure 3. As displayed in figure 3(a), the diffraction peaks of MoS₂ appear at 2θ = 13.88°, 33.06°, 39.3°, and 58.72°, corresponding to the (002), (101), (103), and (110) planes of MoS₂, respectively. The diffraction peaks of 2H-MoS₂ in the hexagonal system are consistent with the standard card. The sharp and prominent peak at the (002) plane indicates a preferred growth orientation along this plane. The absence of strong diffraction peaks corresponding to the (105) plane suggests suppressed growth of MoS₂ on the (105) plane. Furthermore, no impurity peaks from other elements were observed, indicating the high crystallinity of the prepared MoS₂ nanoflowers. Figure 3(b) shows that the diffraction peaks of MoS₂/Au at the (002), (111), and (110) planes are consistent with the standard card (PDF#75–1539) of MoS₂. The other four diffraction peaks correspond to the (111), (200), (220), and (311) planes of Au. No impurity peaks from other elements were detected, indicating the high purity of the synthesized MoS₂/Au composite.

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To further investigate the lattice vibration modes of MoS₂ and MoS₂/Au, Raman characterization was performed. Figure 3(c) shows the Raman spectra of MoS₂ and MoS₂/Au. The ${{\rm{E}}}_{2}^{1}$ g mode represents in-plane vibrations, while the A₁g mode represents out-of-plane vibrations. The ${{\rm{E}}}_{2}^{1}$ g and A₁g peaks of MoS₂ appear at 376.43 cm⁻¹ and 403.35 cm⁻¹, respectively, with a peak difference of 25.92 cm⁻¹, close to the reported value for bulk MoS₂. For the MoS₂/Au composite, the ${{\rm{E}}}_{2}^{1}$ g and A₁g peaks are located at 377.76 cm⁻¹ and 403.79 cm⁻¹, with no additional peaks, indicating that the presence of Au NPs did not affect the MoS₂ nanoflowers during the preparation process. Additionally, the enhanced Raman signal is attributed to plasmon resonance induced by the Au NPs under laser excitation, enhancing the Raman intensity of MoS₂.

Figure 4 shows the CV curves for NF, MoS₂, and MoS₂/Au electrodes. As can be seen, both MoS₂ and MoS₂/Au electrodes exhibit a pair of symmetric redox peaks, while the NF electrode shows almost no redox peaks and a relatively small integrated area. In contrast, the redox peaks for the MoS₂ and MoS₂/Au electrodes are more pronounced, with larger integrated areas. It is evident that the MoS₂/Au electrode has a larger integrated area and higher peak current value compared to the MoS₂ electrode. This indicates that the modification with Au nanoparticles not only increases the specific surface area of the electrode but also enhances the electron transfer rate, making electron transfer at the electrode surface more facile and thereby improving the electrochemical performance.

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To demonstrate the application of MoS₂/Au in non-enzymatic glucose sensing, CV curves of MoS₂ and MoS₂/Au electrodes at different glucose concentrations were obtained, along with the corresponding linear relationship between peak oxidation current and glucose concentration. As observed in figures 5(a) and (c), both MoS₂ and MoS₂/Au display symmetric oxidation and reduction peaks at different glucose concentrations, indicating a reversible redox process at the electrode interface. The peak current increases with the concentration, demonstrating successful electron transfer between the electrode and glucose, leading to electrocatalytic oxidation. The peak current of the MoS₂/Au electrode is significantly higher than that of the MoS₂ electrode, indicating that the modification with Au nanoparticles enhances the catalytic activity and specific surface area of the electrode, resulting in more electrocatalytic active sites, better conductivity, and improved electron transfer capability, thus enhancing the electrocatalytic oxidation of glucose. As shown in figures 5(b) and (d), the peak oxidation currents of both MoS₂ and MoS₂/Au electrodes exhibit a good linear relationship with glucose concentration, with linear detection ranges of 1–25 mM and sensitivities of 373.226 μA mM⁻¹ cm⁻² and 417.556 μA mM⁻¹ cm⁻², respectively. The LOD for both electrodes is 1 mM. The linear equations are as follows:

$$\begin{eqnarray*}&&{{\rm{MoS}}}_{2}:{{\rm{i}}}_{{\rm{pa}}}=0.03631+3{{\rm{.73226E}}-{\rm{4C}}}_{{\rm{mM}}},\,{{\rm{R}}}^{2}=0.98872\end{eqnarray*}$$

$$\begin{eqnarray*}&&{{\rm{MoS}}}_{2}/{\rm{Au}}:{{\rm{i}}}_{{\rm{pa}}}=0.0443+4{{\rm{.17556E}}-{\rm{4C}}}_{{\rm{mM}}},\,{{\rm{R}}}^{2}=0.99579\end{eqnarray*}$$

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Compared to the MoS₂ electrode, the MoS₂/Au electrode exhibits higher linear correlation coefficient and sensitivity.

Table 1 compares the performance of MoS₂ and its composite materials in the field of electrochemical glucose biosensing. It can be seen that electrochemical non-enzymatic glucose sensors based on MoS₂/Cu, AuNPs/CuO/MoS₂, and MoS₂/Ag exhibit higher sensitivity but narrower linear ranges. Our prepared MoS₂/Au sensor has a wide linear range of 1–25 mM. Sensors based on MoS₂/Au-Pd and MoS₂/CuS have lower detection limits and wider linear ranges, but the MoS₂/Au sensor shows higher sensitivity, indicating its potential for accurate glucose concentration detection, providing a new monitoring option for diabetes patients.

The reusability of the sensor is crucial for its practicality and economic feasibility. To investigate the reusability of the glucose sensor, the previously tested MoS₂/Au electrodes were washed with NMP and air-dried at room temperature for subsequent tests. Measurements were performed at a scan rate of 15 mV s⁻¹ with a glucose concentration of 5 mM. The electrodes were tested multiple times over 2, 4, and 6 days. Figures 6(a) and (b) depict the CV curves of the MoS₂/Au electrode at different test periods and the relationship between current intensity ratio and test period, respectively. I₀ represents the initial current response of the MoS₂/Au electrode, and I represents the current response obtained at a specific test period. As shown, the oxidation peak current exhibits minimal fluctuation over different test periods. After 6 days of storage, the electrode still retains 91.22% of its initial current response, indicating good reusability of the sensor.

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The stability of the sensor is critical for the reliability of continuous glucose detection. Figures 7(a) and (b) show the CV curves of the MoS₂/Au electrode and the relationship between oxidation peak current and scan number during eight consecutive tests at a glucose concentration of 5 mM. As observed, the peak current remains relatively constant after eight consecutive CV tests under the same test conditions, demonstrating the sensor's good stability.

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