**1. Introduction**

The rapid and sensitive detection of H2O2 has attracted a lot of attention because of the applications of H2O2 in food [1], medicine [2], chemical industry [3], and environmental protection [4] as a common intermediate and oxidant, as well as its involvement in many biological events and intracellular pathways [5]. Conventional techniques for H2O2 determination have been developed, such as titrimetry [6], colorimetry [7], chemiluminescence [8], fluorescence resonance energy transfer-based upconversion [9], chromatography [10], and electrochemical methods [11]. Among these techniques, the electrochemical method is considered to be a prospective approach for its good selectivity, high sensitivity, and simple manipulation [4]. Although enzyme-based H2O2 sensors exhibit prominent advantages of high selectivity, the complexity of the enzyme curing process and instability to toxic chemicals limit their practical applications [12]. Therefore, a growing interest in developing enzyme-free sensors for detecting H2O2 has been aroused in this field [13,14]. Catalytic active nanomaterials, including noble metals [15], transition metal oxides [16], and other transition metal compounds [17,18], thanks to their selectivity and high activity, have been widely used to construct nonenzyme H2O2 sensors.

In recent years, as a typical transition metal oxide, cuprous oxide (Cu2O) has attracted increasing attention as a promising candidate for H2O2 sensors due to its proper redox potentials,

easy production process, and low cost [19,20]. Unfortunately, pristine Cu2O sensors demonstrate low sensitivity and narrow linear detection ranges [21,22]. Combination with other materials to prepare composites is one effective way to improve the performance of Cu2O-based H2O2 sensors. The metal nanoparticles, thanks to their good conductivity and high electrocatalytic activity, could largely facilitate the electron transfer on the surface of transition-metal oxides and improve their electrocatalytic activity [23]. Up to now, different metal particles have been introduced to transition-metal oxides for H2O2 sensors, such as Au/MnO2 [24], Au/Fe3O4 [25], Ag/MnO2/MWCNTs [26], Au/Cu2O [27], and Pt/Fe3O4/Graphene [28]. Particularly, Ag nanoparticles (AgNPs) exhibit higher conductivity and lower cost compared with Au and Pt, and could produce synergistic effects when combined with some metal oxides [26], thus they are a promising material for improving the catalytic performance of the transition-metal oxides. Therefore, it is promising to introduce Ag into Cu2O-based composites to fabricate H2O2 sensors.

Although these transition-metal oxide/metal nanocomposites mentioned above do fairly well in H2O2 sensing, the preparation of these materials is usually complicated, multistep, and time-consuming. The conventional routes would synthesize metal oxides first, and then modify metal particle to the surface of metal oxides. Therefore, it makes sense to simplify the synthesis steps for material preparation.

In this work, we introduced a facile one-step procedure to combine Cu2O with Ag to prepare Cu2O/Ag nanocomposites. The effects of experimental conditions on composition and morphology of the nanocomposites were studied. The electrochemical measurements were applied to elucidate the sensing application of Cu2O/Ag nanocomposites, and the anti-interference capability experiments and the H2O2 recovery tests indicate Cu2O/Ag nanocomposites could be a promising material for H2O2 detection.

#### **2. Materials and Methods**

#### *2.1. Reagents and Chemicals*

All reagents were of analytical reagen<sup>t</sup> grade and used without further purification. Cu(NO3)2·3H2O, AgNO3, hexadecyl trimethyl ammonium bromide (CTAB), and ethanol were purchased from Beijing Chemical Reagents Company (Beijing, China). *<sup>D</sup>*-glucose, NaOH, urea, fructose, *L*-ascorbic acid, Na2HPO4, and H2O2 solution (30%) were purchased from Tianjin Fuchen Chemical Reagent Co, (Tianjin, China). Ltd. K3[Fe(CN)6] and NaH2PO4·12H2O were purchased from Aladdin Reagent Co (Shanghai, China). All aqueous solutions were prepared with double-distilled water.

#### *2.2. Synthesis of Cu2O/Ag Nanocomposites and Modification of Electrode*

The preparation of Cu2O/Ag nanocomposites was carried out in aqueous solution using glucose as reducing agen<sup>t</sup> and CTAB as dispersing agent. A typical procedure is performed as illustrated in Figure 1. A 0.035 g portion of AgNO3 (0.2 mmol) dissolved in 20 mL double-distilled water was marked as solution A. Next, 0.5 g Cu(NO3)2·3H2O (2 mmol) and 0.5 g glucose (2.5 mmol) were dissolved in 50 mL double-distilled water, and then 10 mL aqueous solution of CTAB (0.014 mol <sup>L</sup>−1) was added into the mixture under stirring. The solution was marked as solution B. The molar ratios of AgNO3 and Cu(NO3)2 could be varied by changing the quantity of AgNO3 according to the requirement. A 0.5 g portion of NaOH (12.5 mmol) dissolved in 20 mL double-distilled water was marked as solution C. The solutions A (20 mL), B (60 mL), and C (20 mL) were added into a flask under stirring at room temperature. The solution was stirred for another 10 min and a gray precipitate formed. Then the reaction suspension was heated under vigorous stirring (500 rpm) at a temperature of 50 ◦C for 30 min and the mixture turned brown-gray gradually. Finally, the product was separated by centrifugation and washed with water and ethanol for three times. The amount of ethanol and water used to wash the products was 20 mL per 100 mg each time, respectively. The products were dried at 70 ◦C overnight. Note, it is important to recover any organic solvent to reduce the environmental burden and improve

the sustainability of the methodology [29]. The alcohol used to wash the products could be recovered by fractionation for secondary use.

**Figure 1.** Schematic illustration for the facile method to prepare Cu2O/Ag/GCE.

A glassy carbon electrode (GCE) was polished, cleaned, and dried for the fabrication of the sensor. Generally, 10 mg of Cu2O/Ag nanocomposites were dispersed into 1 mL double-distilled water and sonicated for 15 min. A 10 μL portion of the suspension was dropped onto the GCE and then dried in air at room temperature. The modified electrode was marked as Cu2O/Ag/GCE. The Cu2O sample without Ag was used similarly to modify the electrode, which was marked as Cu2O/GCE.

## *2.3. Electrochemical Experiments*

Electrochemical measurements were carried out with a PARSTAT 2273 potentiostat galvanostat (Princeton Applied Research, Oak Ridge, TN, USA) in a three-electrode system, with the modified GCE (0.3 cm in diameter) as working electrode, Ag/AgCl/KCl (sat.) as reference electrode, and a platinum sheet as the counter electrode. The cyclic voltammetry profiles (CVs) and current–time profiles were measured in an N2-saturated PBS solution (0.1 M, pH = 7.2) at room temperature. The electrochemical impedance spectroscopy (EIS) was tested in a 5 mM [Fe(CN6)<sup>3</sup>−] solution containing 0.1 M KCl with a frequency range of 10−2–105 Hz and an amplitude of 10 mV.

#### *2.4. Material Characterization Techniques*

The powder X-ray diffraction (XRD) patterns of the as-prepared materials were carried out on a D8 Advance X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.54178 Å). The scanning electron microscopy (SEM) images of the products were characterized using an FEI Quanta 600 field emission scanning electron microscope (FEI Company, Hillsboro, OR, USA). The transmission electron microscopy (TEM) images and electron diffraction (ED) patterns were obtained using an FEI T20 transmission electron microscope (FEI Company, Hillsboro, OR, USA) working at 180 kV. High resolution transmission electron microscopy (HRTEM) images and electron dispersive spectra mapping of the materials (EDS mapping) were obtained using an FEI Titan G2 spherical-aberration-corrected transmission electron microscope (FEI Company, Hillsboro, OR, USA) working at 200 kV. The X-ray photoelectron spectra (XPS) of materials were characterized by an ESCALAB 250Xi X-ray Photoelectron Spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a monochromatic Al Kα X-ray and a 500 μm nominal spot size, and the high-resolution scans were collected with a pass energy of 30 eV and a step size of 0.05 eV.

#### **3. Results and Discussion**

#### *3.1. Effect of Experimental Conditions on Composition and Morphology*

In this study, a simple one-step method was used to prepare Cu2O/Ag nanocomposites successfully. The dose of Cu(NO3)2 0.5 g (2 mmol) was kept unchanged, and the dose of AgNO3 was changed. Different molar ratios of AgNO3 and Cu(NO3)2 in the reactants (*<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 0, 1:20, 1:10, 1:5, respectively) were used to prepare nanomaterials with different compositions at the temperature of 50 ◦C. The XRD patterns of these nanocomposites prepared with different molar ratios of AgNO3 and Cu(NO3)2 are shown in Figure 2a, from which we can easily find that all the nanocomposites show the strong diffraction peaks of the cubic crystal structure of the Cu2O phase (space group: *Pn3m*, JCPDS 5-667 [30]) with fitted lattice parameter of *a* = 0.430 nm. The six peaks (square notations) with 2θ values of 29.68, 36.50, 42.40, 61.52, 73.70, and 77.57 were observed and could be assigned to diffraction from the (110), (111), (200), (220), (311), and (222) planes, respectively. In addition, the XRD pattern of products (*<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:5, 1:10, 1:20, respectively) showed extra peaks (round notations) because of the introduction of Ag, and the XRD peaks at 2θ degrees of 38.11, 44.28, 64.43, 77.47, and 81.54 can be attributed to the (111), (200), (220), (311), and (222) crystalline planes of the face-centered-cubic (fcc) crystalline structure of Ag, respectively (space group: *Fm-3m*, JCPDS 4-783 [31]) with fitted lattice parameter of *a* = 0.409 nm. In addition, with the molar ratio of *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 decreased, the intensity of the Ag peaks decreased obviously, which indicated that the Ag content in the nanocomposites was positively correlated with the amount of AgNO3 added.

The SEM was used to investigate the morphology of nanomaterials prepared with different molar ratios of AgNO3 and Cu(NO3)2 under the temperature of 50 ◦C, as is shown in Figure 2b–e, from which we can easily find that the size of Cu2O particles decreased obviously with the increase of molar ratio of AgNO3:Cu(NO3)2. The average particle size of pure Cu2O prepared without addition of AgNO3 was between 400 nm and 1.2 μm (see the size distribution histograms shown in Figure S1a, SI). However, when *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:20, Cu2O particles of the nanocomposites became much smaller in size (50–300 nm) compared with the pure Cu2O prepared; the size distribution histogram is in Figure S1b. The reason for the decrease in sizes for Cu2O particles is that a lot of Ag nanoparticles were formed and acted as seeds before the Cu2O nanoparticles appeared, which could be observed when the mixture quickly turned gray at room temperature in the process of synthesis. As shown in Figure S2, the size of Ag nanoparticles initially formed was smaller than 20 nm, and they would act as nucleation seeds for Cu2O to nucleate on and grow. Therefore, the Cu2O particles and Ag particles would form good contact in the step. Then, Cu2O particles became small-sized because of these large numbers of Ag seeds. In addition, it can be seen from the SEM images in Figure 2d,e that the size of Cu2O became very small (<100 nm) when *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10 and 1:5. However, when *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:5, the nanoparticles tended to agglomerate. Considering the uniformity of particle size and the dispersion of nanocomposites, 1:10 is the appropriate dosage ratio to prepare Cu2O/Ag nanocomposites.

**Figure 2.** (**a**) XRD patterns of the as-synthesized nanomaterials prepared in different molar ratio of AgNO3:Cu(NO3)2. *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 =0(**1**), 1:20 (**2**), 1:10 (**3**), and 1:5 (**4**), respectively. SEM images of nanomaterials prepared at different molar ratio of AgNO3:Cu(NO3)2. *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 =0(**b**), 1:20 (**c**), 1:10 (**d**), 1:5 (**e**), respectively.

The formation of nanocomposites was also influenced by the reaction temperature. From the XRD patterns in Figure S3, we can easily find that the reaction temperature plays an important role in the formation of Cu2O/Ag nanocomposites. At room temperature, only Ag was produced. In contrast, Cu(NO3)2 was partially reduced to Cu when the temperature was 70 ◦C, and a mixture of Cu and Ag was synthesized when the temperature raised to 100 ◦C. Only when the reaction temperature was around 50 ◦C were Cu2O/Ag nanocomposites synthesized.

Additionally, the XPS measurement for the pure Cu2O and Cu2O/Ag nanocomposites (*<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10) was further carried out to elucidate the valence states of the Cu and Ag element. Figure 3a shows the XPS survey spectra of pure Cu2O and Cu2O/Ag nanocomposites. The C, Cu, and O elements were detected for both samples [32,33], and the survey spectrum of Cu2O/Ag nanocomposites (red line) shows extra peaks which can be assigned to the AgNPs [34]. Figure 3b shows the XPS spectra in Cu 2p regions of the Cu2O/Ag nanocomposite, which indicate the existence of Cu2O (932.3 eV: Cu(I) 2p3/2, 952.1 eV: Cu(I) 2p1/2 of Cu2O) and the surface of Cu2O nanoparticles was slightly oxidized (933.6 eV: Cu(II) 2p3/2, 953.4 eV: Cu(II) 2p1/2). Figure 3c shows the Ag 3d region of Cu2O/Ag nanocomposites with doublet peaks at 374.5 eV and 368.3 eV, which were assigned to the Ag 3d3/2 and Ag 3d5/2 of Ag(0), respectively. Figure 3d shows the O 1s regions of the Cu2O/Ag nanocomposites. The O 1s peak is around 529.7–532.4 eV, which is consistent with the O peak of Cu2O reported [33]. We can see clearly from the XPS data above that the AgNPs was introduced to Cu2O/Ag nanocomposites successfully.

**Figure 3.** (**a**) XPS survey spectrum of the as-synthesized pure Cu2O and Cu2O/Ag nanocomposites obtained with *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10. (**b**) Cu 2p regions of Cu2O/Ag nanocomposites. (**c**) Ag 3d regions of the Cu2O/Ag nanocomposites. (**d**) O 1s regions of Cu2O/Ag nanocomposites.

Figure 4a shows the TEM image and selected-area electron diffraction (SAED) image of pure Cu2O particles. The SAED patterns were taken at the edge of the particle and demonstrate a typical fcc structure of Cu2O crystals which are of highly crystalline nature [35]. Figure 4b shows the TEM image and SAED pattern of the Cu2O/Ag nanocomposites. It can be seen clearly from the TEM image that the size of AgNPs in the nanocomposites is smaller than 20 nm. Meanwhile, the size of Cu2O nanocubes is smaller than 100 nm, which is about less than 1/10 the size of the pure Cu2O cubes prepared by the same way (Figure S1a). Figure 4c,d are HRTEM images of the Cu2O/Ag nanocomposites. The lattice fringes in the particle in Figure 4d are separated by 0.236 nm, in good agreemen<sup>t</sup> with the (111) lattice spacing of Ag. In addition, it can be seen clearly that Ag particles are closely attached to Cu2O cubes from the HRTEM images.

To further observe the combination of Ag and Cu2O, EDS mapping was employed as shown in Figure 5. The EDS mapping images confirmed the coexistence of Ag, Cu, and O elements in the Cu2O/Ag nanocomposites and further confirmed that the composite material is not a simple mixture of Ag particles and Cu2O particles, but a nanoscale composite which is tightly bound together.

**Figure 4.** TEM images of the pure Cu2O particles and the Cu2O/Ag nanocomposites obtained with *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10. (**a**) The TEM image of Cu2O particles (Inset: the SAED pattern of pure Cu2O particles); (**b**) The TEM image of Cu2O/Ag nanocomposites (Inset: the SAED pattern of Cu2O/Ag nanocomposites); (**c**) HRTEM images of Cu2O/Ag nanocomposites; (**d**) Enlarged HRTEM image of rectangular region of (**c**).

**Figure 5.** The images of the Cu2O/Ag sample obtained with *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10. (**a**) A scanning transmission microscopy image, and (**b**–**d**) the corresponding EDS mapping images: (**b**) Ag element, (**c**) O element, (**d**) Cu element.

#### *3.2. Electrochemical Sensing Performances of the Cu2O/Ag/GCE for H2O2 Detection*

The Cu2O/Ag nanocomposites were successfully prepared with the molar ratios of *<sup>n</sup>*AgNO3:*<sup>n</sup>*Cu(NO3)2 = 1:10 at 50 ◦C and used to fabricate a sensor (Cu2O/Ag/GCE). In order to study the interfacial properties of the electrodes, electrochemical impedance spectroscopy (EIS) experiments were conducted. A typical Nyquist plot consists of a semicircle controlled by the electron transfer process in the high-frequency region and a straight line controlled by the diffusion process in the low-frequency region. The semicircle diameter of the curve reflects the electron transfer resistance (Ret) at the interface between the electrode material and the electrolyte [36]. Figure 6a shows the Nyquist plots of GCE, Cu2O/GCE, Cu2O/Ag/GCE in 0.1 M KCl solution containing 5 mM [Fe(CN6) <sup>3</sup>−]. It is easy to find that the semicircular diameter of the Cu2O/Ag/GCE Nyquist plots is smaller than that of the Cu2O/GCE curves, which indicates that the introduction of Ag reduces the propagation resistance between the electrode material and the electrolyte improves the electron transfer rate and is beneficial to improving the electrocatalytic performance to some extent.

The electrochemical properties of the electrodes were studied by cyclic voltammetry (CV). Figure 6b shows CV response of the bare GCE, Cu2O/GCE, and Cu2O/Ag/GCE in the presence of 1 mM H2O2 in 0.1 M PBS (pH = 7.2) at scan rate of 100 mV/s. From Figure 6b, it can be seen that the responses of the bare GCE toward the reduction of H2O2 are quite weak. Cu2O/GCE exhibits electrochemical response and the cathodic peak ( −0.4~ −0.17 V) and anodic peak ( −0.17~0.1 V) can be ascribed to electrochemical reactions of conversion of Cu2O to CuO (oxidation) and CuO to Cu2O (reduction), respectively [22]. The electrode reactions involved in the reduction of H2O2 by the Cu2O/Ag nanocomposites can be proposed as follows [37]:

$$\text{Cu}\_2\text{O} + 2\text{OH}^- - 2\text{e}^- \rightarrow 2\text{CuO} + \text{H}\_2\text{O} \tag{1}$$

$$2\text{CuO} + \text{H}\_2\text{O} + 2\text{e}^- \rightarrow \text{Cu}\_2\text{O} + 2\text{OH}^- \tag{2}$$

$$\text{H}\_2\text{O}\_2 + 2\text{e}^- \rightarrow 2\text{OH}^- \tag{3}$$

In comparison, Cu2O/Ag/GCE showed much higher current response than Cu2O/GCE and bare GCE, which proved the point that the introduction of silver improves the electrochemical properties towards H2O2 of nanocomposites. The enhanced electrocatalytic activity could be ascribed to the synergistic effect of Cu2O and Ag. On the one hand, the appearance of a large number of silver seeds causes the Cu2O nanocubes to have a small size of less than 100 nm in the process of synthesis. On the other hand, the introduction of silver could enhance the charge transport channels and accelerate the transfer rate of electrons in the reaction [38]. Meanwhile, the active area of reaction is increased by the combination of silver on the Cu2O surface, which is beneficial to the adsorption and reaction of H2O2.

Figure 6c shows CV curves of Cu2O/Ag/GCE in the presence of different concentrations of H2O2. It is obvious that the reduction currents gradually increased with the increase of the H2O2 concentrations, indicating the good electrocatalytic activity of Cu2O/Ag/GCE toward H2O2 reduction. To investigate the possible kinetic mechanism, the effect of scan rate on the cathodic current was also investigated. As shown in Figure 6d, with the increasing scan rate from 50 to 150 mV s<sup>−</sup>1, the reduction current increased linearly. Figure S4 shows that the linear relationship between cathodic peak current versus square root of scan rate can be obtained (R<sup>2</sup> = 0.9898), indicating this process was diffusion-controlled.

**Figure 6.** (**a**) Electrochemical impedance plots (Nyquist plots) of Cu2O/GCE and Cu2O/Ag/GCE in 5 mM [Fe(CN6)<sup>3</sup>−] containing 0.1 M KCl (Inset: Nyquist plots of bare GCE). (**b**) CVs of bare GCE, Cu2O/GCE, and Cu2O/Ag/GCE in N2-saturated 0.1 M PBS (pH 7.2) in the presence of 1.0 mM H2O2 at a scan rate of 100 mV/s. (**c**) CVs of Cu2O/Ag/GCE in N2-saturated 0.1 M PBS (pH 7.2) at a scan rate of 100 mV s<sup>−</sup><sup>1</sup> in the presence of H2O2 with different concentrations of 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 mM. (**d**) CVs of Cu2O/Ag/GCE in N2-saturated 0.1 M PBS (pH 7.2) containing 1.0 mM H2O2 at different scan rates (50, 60, 70, 80, 90, 100, 110, 120, 130, 140, and 150 mV s<sup>−</sup>1). (**e**) Current–time curves of the Cu2O/Ag/GCE upon successive addition of 0.1 mM H2O2 into N2-saturated 0.1 M PBS (pH = 7.2) under different applied potential of −0.10, −0.20, −0.30, and −0.40 V (vs. Ag/AgCl). (**f**) The corresponding calibration curves of currents vs. H2O2 concentrations under different potentials (−0.10, −0.20, −0.30, −0.40 V).

It is incontrovertible that the detection potential has much influence on the sensitivity of electrochemical sensors. When choosing the detection potential, the peak voltages in CV (−0.4~−0.2 V vs. Ag/AgCl) is preferred for the best reduction performance for H2O2, while the interference of possible impurities should be considered. The electroactive impurities such as ascorbic acid and uric acid can also be oxidized under high voltages, making it highly likely that their concurrent presences in real applications will interfere with the detection of H2O2 [39]. Figure 6e shows the current response at different detection potentials upon the successive addition of 0.1 mM H2O2. Figure 6f shows the corresponding calibration curves of currents vs. H2O2 concentrations under different potentials. According to Figure 6e,f, though the sensitivity with −0.2 V is lower than that with −0.3 V and almost the same as that with −0.4 V, the profile is more stable and has less background noise. Therefore, the potential of −0.20 V was chosen as the working potential for the detection of H2O2.

#### *3.3. Linear Range, Detection Limit, and Sensitivity of the Cu2O/Ag/GCE for H2O2 Detection*

The Cu2O/Ag nanocomposites-modified electrode was chosen as the sensor electrode for further investigation of H2O2 sensing for the outstanding electrochemical behavior and the good electrocatalytic reduction performance towards H2O2 detection. Figure 7a shows the current–time curves of the Cu2O/Ag/GCE to the successive addition of H2O2 into the stirred N2-saturated PBS (pH = 7.2) solution at an applied potential of −0.20 V. It can be seen clearly from the enlargement of the current–time curve at low concentrations that the detection limit of Cu2O/Ag/GCE for hydrogen peroxide is as low as 0.2 μM (the signal-to-noise ratio of 3, S/N = 3). Figure 7b shows the calibration curve for the H2O2 sensor, and the linear regression equation was *I* (μA) = −0.0870 *C* (μM) −1.559 with a highly linear relationship (R<sup>2</sup> = 0.9972), in which *I* is the current and *C* is concentration of H2O2. Meanwhile, this sensor has a linear detection range from 0.2 to 4000 μM and a sensitivity of 87.0 μA mM−<sup>1</sup> cm<sup>−</sup>2. In summary, Cu2O/Ag/GCE exhibited excellent performance towards the reduction of H2O2.

**Figure 7.** (**a**) Steady-state current–time responses of the Cu2O/Ag/GCE upon successive addition of H2O2 in N2-saturated 0.1 M PBS (pH = 7.2) under an applied potential of −0.20 V (vs. Ag/AgCl). Insert: Enlarged image of circle region of (**a**). (**b**) The corresponding calibration curve of currents vs. H2O2 concentrations. Each dot in (**b**) shows the current value at the corresponding H2O2 concentration which was obtained in (**a**) and the line is a linear fitting for the experiment points with 0.2 < C < 4000 μM.

Table 1 demonstrates the comparison in the performances of the H2O2 nonenzyme sensors fabricated based on the use of similar materials as the electrodes in previous literature reports and in this work. It is shown that our Cu2O/Ag sensor has a good performance in terms of a high sensitivity, a low detection limit, and a wide linear range. The enhanced electrocatalytic activity could be ascribed to the introduction of silver, which probably provides reaction sites and promotes the electron transfer on the surface of Cu2O.


**Table 1.** The comparison of H2O2determination with differently modified electrodes.

#### *3.4. Interference Study*

To explore the anti-interference ability of the synthesized Cu2O/Ag/GCE (red line) and Cu2O/GCE (black line) for H2O2 detection, we added interfering impurities into a continuous testing system. As shown in Figure 8, between the injections of 0.1 mM H2O2 solutions, 1 mM NaCl, 1 mM glucose, 1 mM ascorbic acid, and 1 mM urea solutions were added into the 0.1 M PBS solution (pH = 7.2) at −0.20 V in turn. Notably, compared with the Cu2O/GCE, the Cu2O/Ag/GCE was more sensitive to H2O2.

**Figure 8.** Amperometric response of the Cu2O/Ag/GCE and Cu2O/GCE successive addition of H2O2 (0.1 mM), NaCl (1 mM), glucose (1 mM), ascorbic acid (1 mM), and urea (1 mM).

The currents for the Cu2O/Ag/GCE had obvious changes only when H2O2 was added. In contrast, the currents did not show any change when the interrupters mentioned above were added. The results indicate that these possible interfering substances do not yield a significant current response, which shows that Cu2O/Ag/GCE has a good selectivity for H2O2.

#### *3.5. Reliability and Recovery Test*

The reliability test of the Cu2O/Ag/GCE was performed by measuring the current response of the electrode upon 1 mM of H2O2 in 0.1 M PBS solution (pH = 7.2). The average relative standard deviation (RSD) was not more than 4.2%. In a series of eight sensors prepared in the same way, an RSD of 4.8% was obtained, indicating the reliability of this sensor.

To explore the application of the sensor in the practical environment, the recovery test was constructed by adding a certain amount of H2O2 into milk samples. Before the recovery test experiments were conducted, 5 mL milk purchased from a supermarket was diluted into 50 mL solution using 0.1 M PBS solution first. Then, H2O2 was added into the as-prepared milk sample with the amounts as shown in Table 2. The results indicate that Cu2O/Ag/GCE has the potential to be applied in practical environments.


**Table 2.** Determination of H2O2 in milk samples.

What we need to be careful about is that the sensors would be better kept in a cool and dry environment to prevent the material from being oxidized in moisture. The service life of the sensor might be improved by using curing materials such as Nafion [36].
