Preparation of cuprous oxide - supported silver - modi ﬁ ed reduced graphene oxide nanocomposites for non - enzymatic electrochemical sensor

: We constructed a non - enzymatic H 2 O 2 sensor based on cuprous oxide - supported silver - modi ﬁ ed reduced graphene oxide nanocomposites. It was found that the sensor exhibited good performances for sensing H 2 O 2 with a detection limit of 0.34 μ M and a wide detection range of 1 – 310 μ M. The combination of graphene with silver and cuprous oxide improved the sensor ’ s sensitivity for detecting H 2 O 2 , with good repeatability, selectivity, and stability. The synthesis method of this nanocomposite provides a new idea for the green preparation of gra phene - based nanocomposites and a new method for the construction of a new electrochemical sensor platform.


Introduction
Rapid and sensitive methods for detecting H 2 O 2 have broad application prospects in the fields of food, medicine, chemical industry, and environmental protection and have attracted much attention in recent years [1][2][3]. Among many methods for detecting H 2 O 2 , the electrochemical method is considered promising because of its high sensitivity and simple operation [4]. With the development of materials science and nanotechnology, nanomaterials with mimetic enzymatic activities have been widely used for the modification of chemical electrodes to fabricate non-enzymatic sensors and biosensors. It has been reported that some nanomaterials such as twodimensional (2D) materials, metallic nanoparticles, metal oxides, and others reveal potential enzymatic activity that is similar to natural enzymes, and therefore, could catalyze the oxidization and reduction of H 2 O 2 that could be done by natural enzymes previously [5][6][7][8].
Graphene is a material with sp 2 hybrid connected carbon atoms closely packed into a single-layer 2D cellular lattice structure. Due to the excellent optical, electrical, and mechanical properties of graphene, graphene and graphene-based hybrid materials have been widely applied to materials science, micro-nano processing, energy, biomedicine, and drug delivery [9][10][11]. Graphene is considered to be a revolutionary material for the future. Previously, graphene-based non-enzymatic electrochemical sensors for high-performance detection of H 2 O 2 and glucose have been reported [12]. In some cases, to improve the sensing performance, other metallic and metal oxide nanoparticles have been incorporated with graphene to form 2D nanocomposites, which exhibited synergistic effects for enhancing the electrochemical sensing efficiency [13][14][15].
Silver (Ag) nanomaterials have been widely applied in the fields of surface-enhanced Raman scattering (SERS), transparent conductive films, catalysts, sensors, and antibacterial sterilization due to their unique electrical, optical, thermal, catalytic, and elastic properties [16][17][18][19][20]. Cuprous oxide (Cu 2 O) is a low-cost, environment-friendly p-type semiconductor with a narrow band gap [21], and therefore Cu 2 O-based materials have been used widely in catalysis, sensors, solar cells, lithium-ion batteries, and water splitting [22][23][24][25]. The combination of Ag with Cu 2 O has attracted strong attention, and Ag-Cu 2 O nanocomposites for different purposes have been synthesized [26][27][28]. For instance, Luo et al. used Ag nanoparticles to decorate the surface of the prepared Cu 2 O cubes. The Cu 2 O/Ag heterostructures within the cellulose nanofibril network were employed as an SERS substrate for efficient sensing of methylene blue [29]. Hu and coworkers prepared a series of Cu 2 O/Ag heterojunctions using a simple microwave-assisted green route. The study of antibacterial behavior against Escherichia coli and Staphylococcus aureus indicated that the heterojunction exhibited much better antibacterial performance than the pristine Cu 2 O due to the synergistic effect of Cu 2 O and Ag [30]. The combination of graphene with Ag and Cu 2 O may lead to unique nanomaterials with novel functions [20,31,32]. For example, Sharma et al. synthesized ternary nanocomposites composed of Ag-Cu 2 O and reduced graphene oxide (rGO) using Benedict's solution and glucose solution without any toxic reagent, surfactant, or special treatment [33]. The resulting Ag-Cu 2 O/rGO nanocomposites exhibited good photocatalytic performance for the photodegradation of methyl orange.
In this work, we used the catalytic activity of Ag nanoparticles on H 2 O 2 to modify rGO and then combined the products with Cu 2 O to form Cu 2 O/Ag-rGO nanocomposites. It was found that the nanocomposites exhibited good electrocatalytic properties for H 2 O 2 . The results showed that our method was simple and green, and the sensor based on the Cu 2 O/Ag-rGO nanocomposites had the advantages of high sensitivity, high selectivity, and a wide detection range. This method provides not only a highly selective and sensitive electrochemical detection platform for the detection of H 2 O 2 , but also a new idea for the application of graphene-based metal nanocomposites in the field of electrochemical biosensors.

Preparation of Cu 2 O/Ag-rGO nanocomposites 2.2.1 Preparation of the Ag-rGO composite
After ultrasonic treatment of 0.1 mg·mL −1 of monolayer GO solution, 30 mL of the treated solution was taken and poured into a three-port flask, and then water was added up to 200 mL. Next, 36 mg of silver nitrate was added to the three-port flask and heated until the solution boiled. Then, 4 mL of sodium citrate solution was quickly added and stirred. Finally, the temperature was adjusted to 85℃ and heated for 40 min to obtain the Ag-rGO composite structure.

Preparation of the Cu 2 O/Ag-rGO composite
One gram of PVP was added to 50 mL of 0.04 M copper nitrate solution and stirred until PVP was fully dissolved. Then, 20 mL of the prepared Ag-rGO solution was added to this solution. Next, 68 μL of hydrazine hydrate was added to this solution and stirred. The obtained product was centrifuged and washed with water.

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 Escalab-MkII energy spectrometer. 3 Results and discussion

Characterizations of Cu 2 O/Ag-rGO nanocomposites
The morphology and crystal structure of Cu 2 O/Ag-rGO were characterized by SEM and TEM, as shown in Figure 1. SEM images show that Cu 2 O/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 Cu 2 O/Ag nanocomposites were further analyzed by TEM (Figure 1c and d). It can be seen from Figure 1c   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 Cu 2 O nanoparticles, indicating that the exposed crystal facet of Cu 2 O nanocrystals is (111) crystal facets. The fringe spacing of 0.238 nm corresponds to the (111) crystal planes of Ag nanoparticles [31]. XRD diffraction pattern shows that the main diffraction peaks of Cu 2 O/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 Cu 2 O/Ag-rGO nanocomposites have a well-ordered structure. The results prove the formation of the ternary heterojunction structure of Cu 2 O/Ag-rGO. No other impurities are found in the figure, which demonstrates that the synthesized Cu 2 O/Ag-rGO nanocomposite is relatively pure.
The chemical composition and valence distribution of Cu 2 O/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]. 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 Cu 2 O/Ag-rGO nanocomposites.

Optimization of H 2 O 2 catalytic conditions
In order to explore the optimal conditions for the obtained Cu 2 O/Ag-rGO nanocomposites to catalyze H 2 O 2 , control experiments were carried out. The catalytic performance of the nanocomposites was adjusted by controlling the amount of Cu(NO 3 ) 2 added into the reaction. Under the working potential of −0.4 V, H 2 O 2 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(NO 3 ) 2 have the best catalytic effect on H 2 O 2 .
To prove that the prepared nanocomposites have good electrocatalytic activity, the catalytic behaviors of unmodified GCE, rGO-modified GCE, and the Cu 2 O/Ag-rGO nanocomposite-modified GCE for H 2 O 2 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 H 2 O 2 , while the diffusion current of GCE increased after rGO modification. In contrast, the current of the GCE modified by Cu 2 O/Ag-rGO nanocomposites is significantly increased, indicating that the nanocomposites have reduced H 2 O 2 on the electrode surface and form an amplified current signal. These results show that the Cu 2 O/Ag-rGO nanocomposites have obvious electrocatalytic reduction activity for H 2 O 2 . The introduction of Cu 2 O makes the local energy level in the nanocomposite change correspondingly, which is conducive to the transfer of electrons. The synergistic effect between Cu 2 O and Ag could enhance the electrical conductivity of the electrode. Additionally, Cu 2 O/Ag nanoparticles can disperse rGO and make it difficult to agglomerate, which increases the active surface areas of the modified electrode.   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 Cu 2 O/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 Cu 2 O/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 H 2 O 2 remains at 85.4% of the initial value, revealing that the sensor based on Cu 2 O/Ag-rGO nanocomposites has acceptable stability.

Conclusions
In this work, we proposed a method based on the Cu 2 O/Ag-rGO nanocomposites to construct a nonenzymatic H 2 O 2 sensor with high sensitivity and selectivity. We studied and analyzed the electrochemical performance of the sensor based on the Cu 2 O/Ag-rGO nanocomposites. The sensor exhibits a wide detection range of 1-310 μM and a relatively low detection limit of 0.34 μM. Compared with the traditional method, our method has the advantages of being a simple experimental process, using a simple instrument, and being environmentally friendly. Furthermore, this method provides a new idea for the construction of a fast, sensitive, economical, and highly selective non-enzymatic H 2 O 2 sensor, which has theoretical and practical significance. Author contributions: Yuhong Qian: writingoriginal draft, methodology, data analysis; Yiting Wang: formal analysis; Li Wang: writingoriginal draft, writingreview and editing, project administration.

Conflict of interest:
The authors state no conflict of interest.