Preparation of cuprous oxide-supported silver-modified reduced graphene oxide nanocomposites for non-enzymatic electrochemical sensor

2.1 Reagents and materials

Ethanol (>99.9%, analytically pure), polyvinylpyrrolidone (PVP, K30, M _w = 30,000–40,000), hydrogen peroxide (H₂O₂, 30 wt%), silver nitrate, copper nitrate, sodium citrate dihydrate (1 wt%), hydrazine hydrate (40%, analytically pure), concentrated nitric acid (68 wt%), glucose (analytically pure), and ascorbic acid (AA, >99.7%, analytically pure) were purchased from Beijing Chemical Plant. Monolayer graphene oxide (GO) aqueous dispersion (10 mg·g⁻¹) was purchased from Hangzhou Gaosen Technology Co., Ltd. Dopamine hydrochloride (98%, analytically pure) was purchased from Aladdin Reagent Co., Ltd., and uric acid (UA, analytically pure) was purchased from Sigma Aldrich Chemical Reagent Co., Ltd. All reagents were used directly without purification. Ultrapure water was used in the experiment.

2.2 Preparation of Cu₂O/Ag-rGO nanocomposites

2.3 Instruments

The morphology of the obtained nanocomposites was investigated by an S-4800 scanning electron microscope (SEM) and a TECNAI G2 high-resolution transmission electron microscope (TEM). The crystalline nature of the nanostructures was analyzed using a Rigaku D/max 2500 V X-ray diffractometer. X-ray photoelectron spectroscopy (XPS) analyses were carried out on an Esca-lab-MkII energy spectrometer.

2.4 Detection of H₂O₂

H₂O₂ aqueous solutions with different concentrations were prepared by diluting H₂O₂ in a gradient. All electrochemical properties of the samples were tested by a CHI 760e electrochemical analyzer (Shanghai Chenhua Instrument Co., Ltd.) with the conventional three-electrode system. Saturated calomel electrode was used as the reference electrode, platinum wire as the counter electrode, and modified glassy carbon electrode (GCE) as the working electrode. The 3 mm diameter GCE was polished with alumina slurry, and then ultrasonically washed in water and ethanol for 5 min. When preparing the H₂O₂ sensor, 4 μL of Cu₂O/Ag-rGO solution was dropped on the clean GCE surface and dried in the air. In the control experiment, 4 μL of 1 mg·mL⁻¹ rGO solution was dropped on the bare GCE and dried as the working electrode. In the selective test, 5 μL of glucose, AA, UA, and dopamine (DA) were used as control. The current change in the system was investigated under the same experimental conditions as those for H₂O₂. Before the experiment, N₂ was pumped into the 0.1 M phosphate-buffered saline (PBS) solution (pH = 7.0) to remove the oxygen in the buffer and N₂ was used in the whole electrochemical measurements.

3.1 Characterizations of Cu₂O/Ag-rGO nanocomposites

The morphology and crystal structure of Cu₂O/Ag-rGO were characterized by SEM and TEM, as shown in Figure 1. SEM images show that Cu₂O/Ag nanoparticles are spherical in shape with the size of about 50–80 nm and the surface of the particles is rough (Figure 1a and b). To further confirm their micro-configuration and geometrical structure, the morphologies of the as-prepared Cu₂O/Ag nanocomposites were further analyzed by TEM (Figure 1c and d). It can be seen from Figure 1c that most Cu₂O/Ag nanoparticles are supported on rGO nanosheets. The Ag nanoparticles are evenly encapsulated by the rough Cu₂O layer. The evident contrast in the brightness between the dark cores and bright shells indicates the formation of core–shell architecture. Two distinct crystal structures can be observed in Figure 1d. High-resolution transmission electron microscopy shows that the fringe spacing of 0.244 nm is ascribed to (111) facet of Cu₂O nanoparticles, indicating that the exposed crystal facet of Cu₂O nanocrystals is (111) crystal facets. The fringe spacing of 0.238 nm corresponds to the (111) crystal planes of Ag nanoparticles [31].

Figure 1

(a and b) SEM and (c and d) TEM images of Cu₂O/Ag-rGO nanocomposites.

XRD diffraction pattern shows that the main diffraction peaks of Cu₂O/Ag-rGO in the range of 10–90° are consistent with the previous reports [34], as shown in Figure 2. The strong intensity of the diffraction peaks indicates that the synthesized Cu₂O/Ag-rGO nanocomposites have a well-ordered structure. The results prove the formation of the ternary heterojunction structure of Cu₂O/Ag-rGO. No other impurities are found in the figure, which demonstrates that the synthesized Cu₂O/Ag-rGO nanocomposite is relatively pure.

Figure 2

XRD diffraction patterns of Cu₂O/Ag-rGO nanocomposites.

The chemical composition and valence distribution of Cu₂O/Ag-rGO nanocomposites were characterized by XPS, as shown in Figure 3. C 1s, O 1s, Ag 3d, and Cu 2p in the full spectrum prove the existence of C, O, Ag, and Cu elements, respectively (Figure 3a). In Figure 3b, the binding energy peaks at 374.15 and 368.15 eV correspond to Ag 3d3/2 and Ag 3d5/2, respectively [35,36]. The binding energy peaks at 288.1, 285.95, 284.8, and 284.3 eV in the C 1s spectrum indicate that the structure contains C═O, C‒O, C‒C, and C═C [37], as shown in Figure 3c. In Figure 3d, the binding energy peaks at 532.95, 531.45, and 530.25 eV in the O 1s spectrum indicate that the structure contains C═O, C‒OH, and Cu‒O [38]. In Figure 3e, the binding energy peaks at 952.1 and 932.3 eV correspond to Cu 2p1/2 and Cu 2p3/2, respectively [39]. The results prove the formation of Cu₂O/Ag-rGO nanocomposites.

Figure 3

(a) XPS full spectrum of Cu₂O/Ag-rGO nanocomposites. High-resolution XPS spectra of (b) Ag 3d, (c) C 1s, (d) O 1s, and (e) Cu 2p.

3.2 Optimization of H₂O₂ catalytic conditions

In order to explore the optimal conditions for the obtained Cu₂O/Ag-rGO nanocomposites to catalyze H₂O₂, control experiments were carried out. The catalytic performance of the nanocomposites was adjusted by controlling the amount of Cu(NO₃)₂ added into the reaction. Under the working potential of −0.4 V, H₂O₂ with different concentrations was added slowly into the working system to observe the current response. It can be seen from Figure 4 that the nanocomposites prepared with the addition of 0.04 M Cu(NO₃)₂ have the best catalytic effect on H₂O₂.

Figure 4

Current–time (i–t) curves of the GCE modified by the Cu₂O/Ag-rGO nanocomposites prepared by adding different amounts of Cu(NO₃)₂ into the reaction solution.

To prove that the prepared nanocomposites have good electrocatalytic activity, the catalytic behaviors of unmodified GCE, rGO-modified GCE, and the Cu₂O/Ag-rGO nanocomposite-modified GCE for H₂O₂ were compared, as shown in Figure 5. It can be seen that the unmodified GCE has no obvious oxidation–reduction process after adding 10 μM H₂O₂, while the diffusion current of GCE increased after rGO modification. In contrast, the current of the GCE modified by Cu₂O/Ag-rGO nanocomposites is significantly increased, indicating that the nanocomposites have reduced H₂O₂ on the electrode surface and form an amplified current signal. These results show that the Cu₂O/Ag-rGO nanocomposites have obvious electrocatalytic reduction activity for H₂O₂. The introduction of Cu₂O makes the local energy level in the nanocomposite change correspondingly, which is conducive to the transfer of electrons. The synergistic effect between Cu₂O and Ag could enhance the electrical conductivity of the electrode. Additionally, Cu₂O/Ag nanoparticles can disperse rGO and make it difficult to agglomerate, which increases the active surface areas of the modified electrode.

Figure 5

Cyclic voltammograms of bare GCE, rGO modified GCE, and Cu₂O/Ag-rGO nanocomposite-modified GCE toward the detection of H₂O₂.

3.3 Sensitivity analysis of the detection system

Figure 6a shows a typical i–t curve with the Cu₂O/Ag-rGO nanocomposite-modified GCE on successive additions of H₂O₂ into the 0.1 M of PBS solution. It is clear that the addition of 1 µM H₂O₂ causes a rapid and stable current response as a result of the reduction of H₂O₂. The corresponding calibration of the i–t data indicates a regular response toward H₂O₂ with a linear detection range from 1 to 310 µM (Figure 6b). According to the linear fit, it can be concluded that the linear regression equation is I (mA) = −0.001c (μM) – 0.036. The calculated detection limit of this sensor is 0.34 µM (S/N = 3).

Figure 6

Electrochemical sensing H₂O₂: (a) i–t response of the Cu₂O/Ag-rGO nanocomposite-modified GCE by adding H₂O₂ with various concentrations, and (b) corresponding calibration line of current against the concentration of H₂O₂.

The selectivity and anti-interference of this electrochemical sensor were evaluated by adding glucose, AA, UA, and DA. Figure 7a shows that no interference is observed in the measured potential. Therefore, it can be concluded that this sensor has good anti-interference ability and high selectivity. The reuse stability of Cu₂O/Ag-rGO nanocomposite-modified GCE was further investigated. Figure 7b shows that the fabricated sensor can be reused at least five times. The long-term stability of Cu₂O/Ag-rGO nanocomposite-modified GCE within 14 days was studied. The prepared electrodes were stored in a refrigerator at 4℃ and measured once after 1, 2, 3, 7, and 14 days. Figure 7c shows that after 14 days, the reduction current response of 10 μM H₂O₂ remains at 85.4% of the initial value, revealing that the sensor based on Cu₂O/Ag-rGO nanocomposites has acceptable stability.

Figure 7

Performance of the H₂O₂ sensor based on the Cu₂O/Ag-rGO nanocomposites: (a) anti-interference capability, (b) reuse stability, and (c) long-term stability.