# Facile Synthesis of Copper-Coated-Reduced-Graphene-Oxide and Its Application as a Highly Sensitive Electrochemical Sensor for Hydroquinone

Authors: Youzhi Yao
Journal: Journal of Chemistry
 (2022)

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## Abstract

A facile step-by-step approach for synthesizing copper nanoparticles (CuNPs) loaded on the wrinkled surface of reduced-graphene-oxide (Cu/rGO) was conducted using a reductant at room temperature. Multiple characterization methods were applied to specify the morphology and composition of the nanocomposites. The scanning electron microscope and transmission electron microscope of Cu/rGO show that spherical CuNps were dispersed uniformly on the surface of rGO. In addition, the characteristic peaks of Cu and carbon in energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses spectra proved that Cu/rGO nanocomposites were synthesized. Soon afterwards, a new hydroquinone electrochemical sensor was prepared with Cu/rGO and a glassy carbon electrode. The sensor was characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Hydroquinone was detected by differential pulse voltammetry using the composite electrode. Under the optimal condition, the linear response range was from 0.05μM to 90 μM; the detection limit is 0.02 μM (S/N = 3) for hydroquinone. The electrochemical sensor exhibited high sensitivity in practical environmental water sample detection.


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## Body

## 1. Introduction

Compared with other metals, copper has attracted extensive attention because of its wide source, low cost, and rich morphology. Copper nanoparticles (CuNPs) can promote electron transfer at low overpotential and excellent electrocatalytic activity [1]. On the other hand, due to the high surface activity, CuNPs are easy to be oxidized by oxygen in the air at room temperature [2]. The catalytic activity of copper is inhibited due to the formation of an oxide layer on the surface of copper.In order to solve the problem that CuNPs are easy to be oxidized, various copper nanocomposites have been prepared, such as Cu/polypyrrole [3], Cu/carbon nanotube [4], and Cu/reduced graphene oxide (rGO) [5]. In fact, Cu nanoparticles fixed on two-dimensional flakes or other nanomaterials can improve their vulnerability effectively [6]. Graphene is one of the best materials to prevent CuNPs from being easily oxidized at room temperature [7]. Since 2004, due to high electron transport, large surface area, high mechanical strength, and high electrocatalytic activity [8], the preparation of various composites with graphene has been widely studied, for instance, the preparation of graphene and metal nanoparticles Au/GO [9], Ag/GO [10], and Cu/GO [11–13].With high stability, excellent conductivity, catalytic performance [14], and good electrochemical activity, Cu nanocomposites have been employed in wastewater determination and treatment [15, 16]. Various approaches have been used to prepare Cu/GO nanocomposites with different morphologies [17, 18]. For example, Cu/GO with nacre structure was prepared by molecular level mixing and the self-assembly method, and Cu/GO with multilayer structure was prepared by using the ball milling method [19]. Recently, Kappagantula et al. fabricated macroscopic Cu/GO composites with enhanced electrical properties at 450°C and nitrogen atmosphere [20].Hydroquinone is employed extensively in coal tar production, dye synthesis, paper industry, photographic developer, cosmetics, drugs, and so on [21]. Due to the low degradation rate in the ecological environment, HQ exists in daily necessities for a long time, affecting people’s health. Hydroquinone has a strong corrosive effect on the skin and mucosa, which can inhibit the central nervous system or damage liver and skin functions. In 2017, the list of carcinogens published by the International Agency for Research on Cancer of the World Health Organization was preliminarily sorted out for reference. Hydroquinone was included in the list of three types of carcinogens. The acceptable emission in the Chinese national standard is less than 0.5 mg·mL−1 [22]. There are many methods for HQ detection, such as electrochemical sensors [23], gas chromatography [24], spectrophotometry [25], high-performance liquid chromatography, and fluorescence [26]. Among them, electrochemical sensors have attracted extensive attention because of their superiorities of low cost, small size and mass, fast detection speed, and high sensitivity. With the change of the environment and the progress of society, the stability and sensitivity of HQ electrochemical sensors still need to be further improved [27–29].So far, it is still a great challenge to prepare Cu/GO nanocomposites with high conductivity and good electrocatalytic performance. It is still a meaningful problem to realize the potential of high-performance Cu/GO nanocomposites by improving the dispersion of Cu in graphene matrix. In this work, a method of synthesizing highly dispersed graphene at room temperature through a simple experimental process was explored. An electrochemical sensor was prepared using the synthesized nanocomposites. The sensor was used to detect HQ as the target of toxic pollutants. It shows the characteristics of high stability and sensitivity for HQ compared to other sensors. The determination results are satisfactory.


## 2. Experimental

### 2.1. Materials

Commercial graphite (99.9%) is provided by Beijing Jingte Carbon Technology Co., Ltd. Sodium citrate (C6H5Na3O7), sodium borohydride (NaBH4), copper sulfate (CuSO4), hydroquinone (HQ), sodium dihydrogen phosphate (NaH2PO4), and disodium hydrogen phosphate (Na2HPO4) were purchased from Shanghai Chemical Reagent Co., Ltd. Phosphate buffer solution (PBS 0.10 M) was prepared with NaH2PO4 and Na2HPO4 (pH 3.0–9.0). All chemicals were used directly without further purification.


### 2.2. Characterization

The scanning electron microscope (SEM) (Hitachi, s-4800) and energy dispersive X-ray spectroscopy (EDS) (GG314-JPS-9200) were used to investigate the surface morphology and elemental content distribution spectrum of nanocomposites. The transmission electron microscope (TEM) was used to characterize the microstructure of Cu/rGO nanocomposites; lattice structure and spacing of carbon and copper on the sample surface were examined by using a high resolution transmission electron microscope (HRTEM) (JEOL, JEM 2100F).In a conventional three electrode electrochemical workstation (CHI-660E Shanghai Chenhua Co., Ltd.), the bare glassy carbon electrode and the modified glassy carbon electrode were examined as work electrodes for electrochemical research. The resistance of various working electrodes was analyzed by electrochemical impedance spectroscopy (EIS). According to the curve shape of cyclic voltammetry (CV), the peak current and its reversibility of electrode reaction were discussed.


### 2.3. Synthesis of Cu/rGO Nanocomposite

The purchased commercial graphene (99.9%) was prepared into water-soluble graphene oxide by using the improved Hummers method [30]. In a typical experiment, firstly, C6H5Na3O7 (100 μl. 0.15 M) was added into 30 ml of GO solution (3 mg/mL) in order to prevent the Cu2+ reduction reaction from being too fast. The diameter of the prepared copper particles cannot reach the nanometer level without C6H5Na3O7. Secondly, CuSO4 (2 ml, 0.15 M) was added to the mixture and stirred for 30 minutes so that Cu2+ could be fully combined with the functional groups on the GO surface. Thirdly, under the stirring condition, newly prepared NaBH4 (0.06 M) was added in the mixed solution by a drop every 5 seconds, and the total dosage was about 5 ml. According to the following reaction formula 4Cu2+ + BH4− + 8OH− = 4Cu + BO2− + 6H2O, copper ions were converted into copper, and GO was transformed into rGO, simultaneously. Subsequently, the solution was set for 24 hours to obtain a black precipitate. After washing and centrifugation (CT14D, Shanghai, China), the obtained samples were placed in a vacuum oven at 20°C for drying. All steps were performed at room temperature, as shown in Figure 1.Figure 1
Schematic of the experimental setup and the growth model for Cu/rGO.


### 2.4. Preparation of the Modified Electrode

The glassy carbon electrode (GCE) was treated with aluminum powder before the experiment, washed by ultrasonic, and dried at room temperature for standby. 2.0 mg Cu/rGO nanocomposite, 3.0 mL ethanol, and 15μL Nafion were mixed and sonicated for 30 minutes to obtain a uniform suspension. 2.0 mg rGO was prepared into suspension under the same conditions. The as-prepared two suspensions (Cu/rGO nanocomposite, rGO) of 6 μL were coated on several polished surfaces of the GCE, respectively. At room temperature, modified working electrodes were dried naturally.


## 2.1. Materials

Commercial graphite (99.9%) is provided by Beijing Jingte Carbon Technology Co., Ltd. Sodium citrate (C6H5Na3O7), sodium borohydride (NaBH4), copper sulfate (CuSO4), hydroquinone (HQ), sodium dihydrogen phosphate (NaH2PO4), and disodium hydrogen phosphate (Na2HPO4) were purchased from Shanghai Chemical Reagent Co., Ltd. Phosphate buffer solution (PBS 0.10 M) was prepared with NaH2PO4 and Na2HPO4 (pH 3.0–9.0). All chemicals were used directly without further purification.


## 2.2. Characterization

The scanning electron microscope (SEM) (Hitachi, s-4800) and energy dispersive X-ray spectroscopy (EDS) (GG314-JPS-9200) were used to investigate the surface morphology and elemental content distribution spectrum of nanocomposites. The transmission electron microscope (TEM) was used to characterize the microstructure of Cu/rGO nanocomposites; lattice structure and spacing of carbon and copper on the sample surface were examined by using a high resolution transmission electron microscope (HRTEM) (JEOL, JEM 2100F).In a conventional three electrode electrochemical workstation (CHI-660E Shanghai Chenhua Co., Ltd.), the bare glassy carbon electrode and the modified glassy carbon electrode were examined as work electrodes for electrochemical research. The resistance of various working electrodes was analyzed by electrochemical impedance spectroscopy (EIS). According to the curve shape of cyclic voltammetry (CV), the peak current and its reversibility of electrode reaction were discussed.


## 2.3. Synthesis of Cu/rGO Nanocomposite

The purchased commercial graphene (99.9%) was prepared into water-soluble graphene oxide by using the improved Hummers method [30]. In a typical experiment, firstly, C6H5Na3O7 (100 μl. 0.15 M) was added into 30 ml of GO solution (3 mg/mL) in order to prevent the Cu2+ reduction reaction from being too fast. The diameter of the prepared copper particles cannot reach the nanometer level without C6H5Na3O7. Secondly, CuSO4 (2 ml, 0.15 M) was added to the mixture and stirred for 30 minutes so that Cu2+ could be fully combined with the functional groups on the GO surface. Thirdly, under the stirring condition, newly prepared NaBH4 (0.06 M) was added in the mixed solution by a drop every 5 seconds, and the total dosage was about 5 ml. According to the following reaction formula 4Cu2+ + BH4− + 8OH− = 4Cu + BO2− + 6H2O, copper ions were converted into copper, and GO was transformed into rGO, simultaneously. Subsequently, the solution was set for 24 hours to obtain a black precipitate. After washing and centrifugation (CT14D, Shanghai, China), the obtained samples were placed in a vacuum oven at 20°C for drying. All steps were performed at room temperature, as shown in Figure 1.Figure 1
Schematic of the experimental setup and the growth model for Cu/rGO.


## 2.4. Preparation of the Modified Electrode

The glassy carbon electrode (GCE) was treated with aluminum powder before the experiment, washed by ultrasonic, and dried at room temperature for standby. 2.0 mg Cu/rGO nanocomposite, 3.0 mL ethanol, and 15μL Nafion were mixed and sonicated for 30 minutes to obtain a uniform suspension. 2.0 mg rGO was prepared into suspension under the same conditions. The as-prepared two suspensions (Cu/rGO nanocomposite, rGO) of 6 μL were coated on several polished surfaces of the GCE, respectively. At room temperature, modified working electrodes were dried naturally.


## 3. Results and Discussion

### 3.1. Surface Morphological Analysis

The morphology of the fabricated Cu/rGO nanocomposite was examined by SEM and TEM analyses (Figure2). Figure 2(a) shows the SEM micrograph of rGO. There are obvious folds on its surface, but no other substances are attached to it. As shown Figure 2(b), it can be clearly seen that a large number of spherical particles are attached to the folded surface of graphene, indicating the distribution of massive Cu nanoparticles on rGO. Figure 2(c) shows the energy dispersive X-ray spectroscopy (EDX) spectrum of Cu/rGO. The peak values of copper and carbon are obvious, and the peak value of a small amount of oxygen may be from the silicon matrix. The TEM morphology of the Cu/rGO nanocomposite is shown in Figure 2(d), and spherical nanoparticles with uneven sizes are stacked on the surface of rGO. The HRTEM pattern of rGO is shown in Figure 2(e). The lattice spacing of two spherical copper nanoparticles measured by digital micrograph tool is approximately 0.21 nm. This finding indicates that the nanoparticles are {111} twin crystals that usually appear in fcc crystals [31]. The selected area electron diffraction (SAED) image inserted in Figure 2(d) is circular, indicating that copper on rGO is polycrystalline. The above characterization results show that Cu coated rGO nanocomposites have been synthesized successfully.Figure 2
SEM, EDX, TEM, and HRTEM images of Cu/rGO.
(a)(b)(c)(d)(e)The chemical composition of the nanocomposites was analyzed using X-ray photoelectron spectroscopy (XPS). In the typical XPS spectra of GO and Cu/rGO, as shown in Figure3(a), the peaks of C1s and O1s change significantly, especially the peak of O1s. At the same time, it can also be seen that the peak positions of C1s and O1s shift slightly, which is attributed to the different oxidation states of GO and rGO. The characteristic peak of Cu appeared at 932 eV. As shown in Figure 3(b), the peak value of C–C (sp2 bonding in the GO) at 284.5 eV is almost unchanged, and the intensity of C–O (epoxy and acid groups) at 286.6 eV and C=O (carbonyl groups) peak at 288.4 eV is a significant decline, further indicating that GO is reduced rGO. In the Cu2p spectra (Figure 3(c)), the peaks of Cu2p3/2 and Cu2p1/2 appear at 932.7 and 952.5 eV, originating from metallic Cu0. The peaks at 942.6 and 961.1 eV originate from Cu2+ and correspond to Cu2p3/2 and Cu3d, respectively [32, 33]. Cu nanoparticles on the graphene surface mainly exist as Cu0, and graphene prevents further oxidation of copper [24]. The peak at 531.8 eV in the O1s spectra could be assigned to the adsorbed oxygen species relating to –OH and H2O or absorbed molecule O2 (Figure 3(d)) [34].Figure 3
XPS spectra of GO and Cu/rGO: (a) survey; (b) C1s; (c) Cu 2p; (d) O1s.
(a)(b)(c)(d)Electrochemical impedance spectroscopy (EIS) analysis was conducted to investigate the suitable process for sensor preparation. Figure4 presents the Nyquist plots of GCE, rGO/GCE, and Cu/rGO/GCE in phosphate buffer solution (pH = 7.0) containing 5.0 mM K4Fe(CN)6. The impedance of each modified electrode is different, indicating that the modified nanomaterials change the conductivity of the electrode. Cu/rGO exhibits higher electronic conductivity than that of bare GCE or rGO. The increase of the electron conduction rate of the modified electrode may be attributed to the network pore structure of rGO and the small size effect of copper nanoparticles. The latter facilitates faster charge transport across graphene sheet junctions, increase the specific surface area, and provide abundant active sites for catalytic reduction [35]. The EIS pattern can be fitted by the equivalent circuit shown in the inset of Figure 4. By fitting the data, the Ret of bare GCE is calculated to be 310 Ω, indicating that the interfacial electron transfer is relatively difficult. When rGO is added to the electrode surface, due to the accelerated electron transfer of rGO, the Ret value is significantly reduced to 186 Ω, and the Ret value of Cu/rGO/GCE is smaller, which is 162 Ω, indicating that the electron transfer is faster on the electrode surface modified by Cu/rGO/GCE. These results showed that rGO and Cu/rGO were successfully modified on GCE.Figure 4
Nyquist plots of GCE, rGO/GCE, and Cu/rGO/GCE in 1.0 mM [Fe(CN)6]3−/4− solution, inset: equivalent circuit.


### 3.2. Electrochemical Behavior of HQ on Various Electrodes

The electrocatalytic activity of the three prepared electrochemical sensors was evaluated by CV in 0.10 M PBS (pH 7.0) containing 5.0μM HQ at 120 mV·s−1 scanning rate, and the response spectra of the three sensors to HQ are shown in Figure 5. The redox peak of HQ appeared in all three curves, in which the peak value in the curve of Cu/rGO/GCE is maximum.Figure 5
CV of GCE (a), rGO/GCE (b), and Cu/rGO/GCE (c) in PBS containing 5.0μM HQ; scanning rate, 120 mV·s−1.According to the Laviron formula: Ip = nFQv/4RT,n is calculated to be equal to 2.2, conforming to the double electron reaction formula.The area of the modified electrode was calculated according to the formula:IP = 2.69 × 105 n3/2 Acd1/2 v1/2 (Randles–Sevcik formula); the effective surface areas of the three electrodes were AGCE = 0.023 cm2, ArGO/GCE = 0.032 cm2, and ACu/rGO/GCE = 0.042 cm2, respectively.The peak in the curve of Cu/rGO/GCE exhibits higher value because of the synergistic effect of Cu and rGO. In addition, the above calculation results indicate that Cu nanoparticles enhanced the effective surface area of the electron transport and the number of highly active catalytic sites [36], leading to a higher CV peak current. Thereby, Cu/rGO/GCE has the best electrocatalytic activity among the three sensors.


### 3.3. Optimization of Experimental Parameters for the Determination of HQ

The coating amount and acidity of Cu/rGO suspension are the key parameters affecting the performance of the modified electrode. The effect of HQ on the current of reduction peak was investigated by changing the amount of Cu/rGO suspension. As shown in Figure6, the volume of Cu/rGO suspension coated on the surface of GCE increased from 1 μL to 6 μL. The current increased significantly, indicating that Cu/rGO increased the active area and accumulation efficiency of the GCE surface. As the volume of Cu/rGO suspension increased from 6 μL to 10 μL, the peak current decreased gradually. This result may be due to the thick Cu/rGO film hindering the conductivity [37]. Therefore, the surface of GCE modified by 6 μL Cu/rGO suspension was appropriate to achieve the highest sensitivity.Figure 6
Effect of the amount of Cu/rGO suspension on the current of reduction peak (5.0μM HQ at 120 mV·s−1 scanning rate).According to C6H4(OH)2 − 2e− = C6H4O2 + 2H+, protons participate in the electrode reaction. Therefore, the peak current generated by HQ electrochemical process will change significantly under different acidities. The effect of pH on HQ oxidation was studied carefully by CV in the acidity range of pH 3.0–9.0 with Cu/rGO/GCE. As shown in Figure 7(a), the anodic peak potential of HQ shifts negatively with the increase of pH value from 3.0 to 9.0, indicating that protons are involved directly in the electrochemical redox process. The linear equation between Epa and pH is EpaV=−0.05712pH+0.4206R=0.9986, and the slope of HQ is 57.26 mV·pH−1, which is close to the theoretical value of 59.0 mV·pH−1, indicating that the electrochemical redox process of HQ with equal proton and electron numbers has taken place on the electrode surface [38].Figure 7
(a) The relationship between pH and the peak potentials. (b) The relationship between pH and the peak currents of HQ (5.0μM HQ at 120 mV·s−1 scanning rate).
(a)(b)As can be seen from Figure7(b), the peak current of HQ oxidation increases with the increase of pH value from 3.0 to 6.5, while when the pH value is higher than 6.5, the peak current decreases. The possible reason is that the pKa value is 9.85. With the increase of pH value, the protonated aromatic isomer is easy to deproton and transform into an anion. The surface of Cu/rGO/GCE is negatively charged, the electrostatic repulsion between them is enhanced, and the adsorption amount of isomer on the electrode surface is reduced, resulting in the decrease of current peak value [39]. Considering the measurement sensitivity, the electrolyte with a pH value of 6.5 was selected as the best experimental condition for subsequent detection of HQ.In order to study the kinetic reaction of the electrode/electrolyte, the influence of the scanning rate on the electrochemical behavior was examined in 0.1 M PBS solution containing 5.0μM HQ to obtain the optimized Cu/rGO/GCE by CV. As shown in Figure 8, with a change in the scanning rate from 10 to 240 mVs−1, the peak current of the cathode and anode increases accordingly. It can also be seen that the peak potential of anodic oxidation (Epc) and cathodic reduction (Epa) moves slightly to the positive potential and negative potential, respectively, indicating that the electrochemical reaction process is a quasireversible process. Generally speaking, the potential change observed is attributed to the corresponding change of catalytic point reaction activity on the material surface with the change of the scanning rate.Figure 8
Cyclic voltammograms at Cu/rGO/GCE in 0.1 M PBS (pH 6.5) containing 5.0μM HQ with different scan rates of 10, 20, 40, 60, 80, 100, 120, 150, 180, 200, 220, and 240 mV·s−1, respectively. Plots of peak currents vs. the square root of scan rates for HQ (inset).The value of Ipc, Ipa, and the square root of the scanning rate in the range of 10 to 240 mVs-1 scanning rate (v 1/2) is linear, as shown in the insertion diagram of Figure8, which can be described by the following linear regression equation:(1)Ipc=−3.16538+5.4009v1/2,R2=0.99637,Ipa=−2.8029+5.922802v1/2,R2=0.99665.Therefore, the electrochemical sensing of HQ is a diffusion-controlled process.


### 3.4. Determination of HQ

Under the optimum experimental conditions, HQ on Cu/rGO/GCE was determined quantitatively by differential pulse voltammetry (DPV) because it has high current sensitivity and good resolution. As shown in Figure9, there is a good linear relationship between the oxidation peak current of HQ and its concentration. When the concentrations of HQ were in the range of 0.05 μM to 90 μM, as shown in the inset of Figure 9, the linear regression equation is Ipc = 1.46521 + 0.9311C, R = 0.9986; the detection limit for HQ is estimated to be 0.02 μM (S/N = 3).Figure 9
DPV curves of different concentrations of HQ (0.05–90μM) at Cu/rGO/GCE in 0.10 M PBS (pH 6.5). The insert shows the relationship between peak current and concentration of HQ.The results of HQ determination with different modified electrodes were also compared. The linear detection limits and ranges are shown in Table1. The results show that the linear range of HQ is wide, and the detection limit is lower than that of most published papers. Therefore, compared with other modified electrodes, Cu/rGO/GCE is a competitive candidate electrochemical sensor for HQ determination.Table 1
A comparison of the reported electrochemical sensors for hydroquinone.
ElectrodeTechniqueLinear range (μM)Lod (μM)Ref.Zn@ZnOCyclic voltammetry10–900.10443[27]Nano-co/poly-L-gluCyclic voltammetry3.58–1300.497[21]Polypyrrole/carbon black/ZnOAmp0.9–6.50.0229[40]AuNPs/EGPDPV0.07–1000.0273[41]TiO2–ZnO-rGODPV0.1–5000.031[42]Cu/RGODPV0.05–86.50.02This work


### 3.5. Reproducibility, Repeatability, Stability, and Selectivity

The electrochemical performance of the prepared sensor in solution containing 5.0μM HQ was assessed to evaluate the reproducibility of Cu/rGO/GCE. Five independently fabricated electrodes exhibited similar responses, with a relative standard deviation (RSD) of 3.2%. The repeatability of the sensors was examined by continuous measurements using the same concentration of HQ (5.0 μM). The sensor exhibited good reproducibility (2.3% RSD) after 10 repeated measurements. These results indicate the excellent stability and repeatability of the sensors. The sensors based on Cu/rGO/GCE were stored in a constant temperature drying oven when they were not used. As seen in Figure 10(a), after 30 days of segmented testing, the current response to HQ decreases to 95.18%, indicating that Cu/rGO/GCE exhibits good stability.Figure 10
(a) Storage stability of Cu/rGO/GCE stored in a constant temperature drying oven, (b) the selectivity of the proposed method for HQ determination. The concentration of HQ is 5μM, concentrations of phenol, resorcin, catechol, glucose, and benzoic acid are 0.5 mM, concentration of inorganic ions is 1 mM; the error bars illustrate the standard deviations of six independent measurements.
(a)(b)As shown in Figure10(b), the selectivity of the sensor was evaluated through DPV by adding some potential interfering compounds including phenol, resorcin, catechol, glucose, and benzoic acid in HQ solution. In the presence of these interfering substances, no obvious current response was detected. Some inorganic ions (Na+, Ca2+, Fe3+, Zn2+, Cu2+, Cl−, CO32−, and NO3−) were also added in the solution. These ions also did not interfere significantly with the detection process.


### 3.6. Sample Analysis

The Cu/rGO/GCE was applied to determine HQ in practical water samples (local tap water and water from a local reservoir). Based on DPV analysis, target analytes were not found in the water samples, indicating that the concentration of HQ may be lower than the detection limits. Therefore, the standard addition technique was performed by adding known concentrations of HQ prior to analysis. The experimental results are listed in Table2. The recoveries of the samples are within 97.0%–102.0% for HQ, which is in good agreement with the real addition. The RSD values are less than 5%, which is acceptable.Table 2
Determination results for HQ in local tap and reservoir water.
SamplesSerial numberAdded (μM)Found (nM)Recovery (%)RSD (%)Found (nM)Recovery (%)RSD (%)Cu/rGO/GCE sensorHPLC-UVTap water110.09.898.03.29.999.04.1220.019.798.52.319.897.53.2330.030.5101.74.130.2100.73.6Reservoir water110.010.2102.04.29.797.02.8220.019.497.03.919.698.03.5330.029.698.72.529.598.32.7The same samples were also determined by using the HPLC-UV technique. It can be seen from the table, the data obtained by the two methods are almost identical, which indicates that the method of determining HQ by using the Cu/rGO/GCE sensor is reliable. The Cu/rGO/GCE sensor can be efficiently used to determine HQ in real samples.It is an average value of eight measurements.


## 3.1. Surface Morphological Analysis

The morphology of the fabricated Cu/rGO nanocomposite was examined by SEM and TEM analyses (Figure2). Figure 2(a) shows the SEM micrograph of rGO. There are obvious folds on its surface, but no other substances are attached to it. As shown Figure 2(b), it can be clearly seen that a large number of spherical particles are attached to the folded surface of graphene, indicating the distribution of massive Cu nanoparticles on rGO. Figure 2(c) shows the energy dispersive X-ray spectroscopy (EDX) spectrum of Cu/rGO. The peak values of copper and carbon are obvious, and the peak value of a small amount of oxygen may be from the silicon matrix. The TEM morphology of the Cu/rGO nanocomposite is shown in Figure 2(d), and spherical nanoparticles with uneven sizes are stacked on the surface of rGO. The HRTEM pattern of rGO is shown in Figure 2(e). The lattice spacing of two spherical copper nanoparticles measured by digital micrograph tool is approximately 0.21 nm. This finding indicates that the nanoparticles are {111} twin crystals that usually appear in fcc crystals [31]. The selected area electron diffraction (SAED) image inserted in Figure 2(d) is circular, indicating that copper on rGO is polycrystalline. The above characterization results show that Cu coated rGO nanocomposites have been synthesized successfully.Figure 2
SEM, EDX, TEM, and HRTEM images of Cu/rGO.
(a)(b)(c)(d)(e)The chemical composition of the nanocomposites was analyzed using X-ray photoelectron spectroscopy (XPS). In the typical XPS spectra of GO and Cu/rGO, as shown in Figure3(a), the peaks of C1s and O1s change significantly, especially the peak of O1s. At the same time, it can also be seen that the peak positions of C1s and O1s shift slightly, which is attributed to the different oxidation states of GO and rGO. The characteristic peak of Cu appeared at 932 eV. As shown in Figure 3(b), the peak value of C–C (sp2 bonding in the GO) at 284.5 eV is almost unchanged, and the intensity of C–O (epoxy and acid groups) at 286.6 eV and C=O (carbonyl groups) peak at 288.4 eV is a significant decline, further indicating that GO is reduced rGO. In the Cu2p spectra (Figure 3(c)), the peaks of Cu2p3/2 and Cu2p1/2 appear at 932.7 and 952.5 eV, originating from metallic Cu0. The peaks at 942.6 and 961.1 eV originate from Cu2+ and correspond to Cu2p3/2 and Cu3d, respectively [32, 33]. Cu nanoparticles on the graphene surface mainly exist as Cu0, and graphene prevents further oxidation of copper [24]. The peak at 531.8 eV in the O1s spectra could be assigned to the adsorbed oxygen species relating to –OH and H2O or absorbed molecule O2 (Figure 3(d)) [34].Figure 3
XPS spectra of GO and Cu/rGO: (a) survey; (b) C1s; (c) Cu 2p; (d) O1s.
(a)(b)(c)(d)Electrochemical impedance spectroscopy (EIS) analysis was conducted to investigate the suitable process for sensor preparation. Figure4 presents the Nyquist plots of GCE, rGO/GCE, and Cu/rGO/GCE in phosphate buffer solution (pH = 7.0) containing 5.0 mM K4Fe(CN)6. The impedance of each modified electrode is different, indicating that the modified nanomaterials change the conductivity of the electrode. Cu/rGO exhibits higher electronic conductivity than that of bare GCE or rGO. The increase of the electron conduction rate of the modified electrode may be attributed to the network pore structure of rGO and the small size effect of copper nanoparticles. The latter facilitates faster charge transport across graphene sheet junctions, increase the specific surface area, and provide abundant active sites for catalytic reduction [35]. The EIS pattern can be fitted by the equivalent circuit shown in the inset of Figure 4. By fitting the data, the Ret of bare GCE is calculated to be 310 Ω, indicating that the interfacial electron transfer is relatively difficult. When rGO is added to the electrode surface, due to the accelerated electron transfer of rGO, the Ret value is significantly reduced to 186 Ω, and the Ret value of Cu/rGO/GCE is smaller, which is 162 Ω, indicating that the electron transfer is faster on the electrode surface modified by Cu/rGO/GCE. These results showed that rGO and Cu/rGO were successfully modified on GCE.Figure 4
Nyquist plots of GCE, rGO/GCE, and Cu/rGO/GCE in 1.0 mM [Fe(CN)6]3−/4− solution, inset: equivalent circuit.


## 3.2. Electrochemical Behavior of HQ on Various Electrodes

The electrocatalytic activity of the three prepared electrochemical sensors was evaluated by CV in 0.10 M PBS (pH 7.0) containing 5.0μM HQ at 120 mV·s−1 scanning rate, and the response spectra of the three sensors to HQ are shown in Figure 5. The redox peak of HQ appeared in all three curves, in which the peak value in the curve of Cu/rGO/GCE is maximum.Figure 5
CV of GCE (a), rGO/GCE (b), and Cu/rGO/GCE (c) in PBS containing 5.0μM HQ; scanning rate, 120 mV·s−1.According to the Laviron formula: Ip = nFQv/4RT,n is calculated to be equal to 2.2, conforming to the double electron reaction formula.The area of the modified electrode was calculated according to the formula:IP = 2.69 × 105 n3/2 Acd1/2 v1/2 (Randles–Sevcik formula); the effective surface areas of the three electrodes were AGCE = 0.023 cm2, ArGO/GCE = 0.032 cm2, and ACu/rGO/GCE = 0.042 cm2, respectively.The peak in the curve of Cu/rGO/GCE exhibits higher value because of the synergistic effect of Cu and rGO. In addition, the above calculation results indicate that Cu nanoparticles enhanced the effective surface area of the electron transport and the number of highly active catalytic sites [36], leading to a higher CV peak current. Thereby, Cu/rGO/GCE has the best electrocatalytic activity among the three sensors.


## 3.3. Optimization of Experimental Parameters for the Determination of HQ

The coating amount and acidity of Cu/rGO suspension are the key parameters affecting the performance of the modified electrode. The effect of HQ on the current of reduction peak was investigated by changing the amount of Cu/rGO suspension. As shown in Figure6, the volume of Cu/rGO suspension coated on the surface of GCE increased from 1 μL to 6 μL. The current increased significantly, indicating that Cu/rGO increased the active area and accumulation efficiency of the GCE surface. As the volume of Cu/rGO suspension increased from 6 μL to 10 μL, the peak current decreased gradually. This result may be due to the thick Cu/rGO film hindering the conductivity [37]. Therefore, the surface of GCE modified by 6 μL Cu/rGO suspension was appropriate to achieve the highest sensitivity.Figure 6
Effect of the amount of Cu/rGO suspension on the current of reduction peak (5.0μM HQ at 120 mV·s−1 scanning rate).According to C6H4(OH)2 − 2e− = C6H4O2 + 2H+, protons participate in the electrode reaction. Therefore, the peak current generated by HQ electrochemical process will change significantly under different acidities. The effect of pH on HQ oxidation was studied carefully by CV in the acidity range of pH 3.0–9.0 with Cu/rGO/GCE. As shown in Figure 7(a), the anodic peak potential of HQ shifts negatively with the increase of pH value from 3.0 to 9.0, indicating that protons are involved directly in the electrochemical redox process. The linear equation between Epa and pH is EpaV=−0.05712pH+0.4206R=0.9986, and the slope of HQ is 57.26 mV·pH−1, which is close to the theoretical value of 59.0 mV·pH−1, indicating that the electrochemical redox process of HQ with equal proton and electron numbers has taken place on the electrode surface [38].Figure 7
(a) The relationship between pH and the peak potentials. (b) The relationship between pH and the peak currents of HQ (5.0μM HQ at 120 mV·s−1 scanning rate).
(a)(b)As can be seen from Figure7(b), the peak current of HQ oxidation increases with the increase of pH value from 3.0 to 6.5, while when the pH value is higher than 6.5, the peak current decreases. The possible reason is that the pKa value is 9.85. With the increase of pH value, the protonated aromatic isomer is easy to deproton and transform into an anion. The surface of Cu/rGO/GCE is negatively charged, the electrostatic repulsion between them is enhanced, and the adsorption amount of isomer on the electrode surface is reduced, resulting in the decrease of current peak value [39]. Considering the measurement sensitivity, the electrolyte with a pH value of 6.5 was selected as the best experimental condition for subsequent detection of HQ.In order to study the kinetic reaction of the electrode/electrolyte, the influence of the scanning rate on the electrochemical behavior was examined in 0.1 M PBS solution containing 5.0μM HQ to obtain the optimized Cu/rGO/GCE by CV. As shown in Figure 8, with a change in the scanning rate from 10 to 240 mVs−1, the peak current of the cathode and anode increases accordingly. It can also be seen that the peak potential of anodic oxidation (Epc) and cathodic reduction (Epa) moves slightly to the positive potential and negative potential, respectively, indicating that the electrochemical reaction process is a quasireversible process. Generally speaking, the potential change observed is attributed to the corresponding change of catalytic point reaction activity on the material surface with the change of the scanning rate.Figure 8
Cyclic voltammograms at Cu/rGO/GCE in 0.1 M PBS (pH 6.5) containing 5.0μM HQ with different scan rates of 10, 20, 40, 60, 80, 100, 120, 150, 180, 200, 220, and 240 mV·s−1, respectively. Plots of peak currents vs. the square root of scan rates for HQ (inset).The value of Ipc, Ipa, and the square root of the scanning rate in the range of 10 to 240 mVs-1 scanning rate (v 1/2) is linear, as shown in the insertion diagram of Figure8, which can be described by the following linear regression equation:(1)Ipc=−3.16538+5.4009v1/2,R2=0.99637,Ipa=−2.8029+5.922802v1/2,R2=0.99665.Therefore, the electrochemical sensing of HQ is a diffusion-controlled process.


## 3.4. Determination of HQ

Under the optimum experimental conditions, HQ on Cu/rGO/GCE was determined quantitatively by differential pulse voltammetry (DPV) because it has high current sensitivity and good resolution. As shown in Figure9, there is a good linear relationship between the oxidation peak current of HQ and its concentration. When the concentrations of HQ were in the range of 0.05 μM to 90 μM, as shown in the inset of Figure 9, the linear regression equation is Ipc = 1.46521 + 0.9311C, R = 0.9986; the detection limit for HQ is estimated to be 0.02 μM (S/N = 3).Figure 9
DPV curves of different concentrations of HQ (0.05–90μM) at Cu/rGO/GCE in 0.10 M PBS (pH 6.5). The insert shows the relationship between peak current and concentration of HQ.The results of HQ determination with different modified electrodes were also compared. The linear detection limits and ranges are shown in Table1. The results show that the linear range of HQ is wide, and the detection limit is lower than that of most published papers. Therefore, compared with other modified electrodes, Cu/rGO/GCE is a competitive candidate electrochemical sensor for HQ determination.Table 1
A comparison of the reported electrochemical sensors for hydroquinone.
ElectrodeTechniqueLinear range (μM)Lod (μM)Ref.Zn@ZnOCyclic voltammetry10–900.10443[27]Nano-co/poly-L-gluCyclic voltammetry3.58–1300.497[21]Polypyrrole/carbon black/ZnOAmp0.9–6.50.0229[40]AuNPs/EGPDPV0.07–1000.0273[41]TiO2–ZnO-rGODPV0.1–5000.031[42]Cu/RGODPV0.05–86.50.02This work


## 3.5. Reproducibility, Repeatability, Stability, and Selectivity

The electrochemical performance of the prepared sensor in solution containing 5.0μM HQ was assessed to evaluate the reproducibility of Cu/rGO/GCE. Five independently fabricated electrodes exhibited similar responses, with a relative standard deviation (RSD) of 3.2%. The repeatability of the sensors was examined by continuous measurements using the same concentration of HQ (5.0 μM). The sensor exhibited good reproducibility (2.3% RSD) after 10 repeated measurements. These results indicate the excellent stability and repeatability of the sensors. The sensors based on Cu/rGO/GCE were stored in a constant temperature drying oven when they were not used. As seen in Figure 10(a), after 30 days of segmented testing, the current response to HQ decreases to 95.18%, indicating that Cu/rGO/GCE exhibits good stability.Figure 10
(a) Storage stability of Cu/rGO/GCE stored in a constant temperature drying oven, (b) the selectivity of the proposed method for HQ determination. The concentration of HQ is 5μM, concentrations of phenol, resorcin, catechol, glucose, and benzoic acid are 0.5 mM, concentration of inorganic ions is 1 mM; the error bars illustrate the standard deviations of six independent measurements.
(a)(b)As shown in Figure10(b), the selectivity of the sensor was evaluated through DPV by adding some potential interfering compounds including phenol, resorcin, catechol, glucose, and benzoic acid in HQ solution. In the presence of these interfering substances, no obvious current response was detected. Some inorganic ions (Na+, Ca2+, Fe3+, Zn2+, Cu2+, Cl−, CO32−, and NO3−) were also added in the solution. These ions also did not interfere significantly with the detection process.


## 3.6. Sample Analysis

The Cu/rGO/GCE was applied to determine HQ in practical water samples (local tap water and water from a local reservoir). Based on DPV analysis, target analytes were not found in the water samples, indicating that the concentration of HQ may be lower than the detection limits. Therefore, the standard addition technique was performed by adding known concentrations of HQ prior to analysis. The experimental results are listed in Table2. The recoveries of the samples are within 97.0%–102.0% for HQ, which is in good agreement with the real addition. The RSD values are less than 5%, which is acceptable.Table 2
Determination results for HQ in local tap and reservoir water.
SamplesSerial numberAdded (μM)Found (nM)Recovery (%)RSD (%)Found (nM)Recovery (%)RSD (%)Cu/rGO/GCE sensorHPLC-UVTap water110.09.898.03.29.999.04.1220.019.798.52.319.897.53.2330.030.5101.74.130.2100.73.6Reservoir water110.010.2102.04.29.797.02.8220.019.497.03.919.698.03.5330.029.698.72.529.598.32.7The same samples were also determined by using the HPLC-UV technique. It can be seen from the table, the data obtained by the two methods are almost identical, which indicates that the method of determining HQ by using the Cu/rGO/GCE sensor is reliable. The Cu/rGO/GCE sensor can be efficiently used to determine HQ in real samples.It is an average value of eight measurements.


## 4. Conclusions

This work describes a facile method for preparation of Cu/rGO at the temperature. The novel nanocomposite was used to develop a new electrochemical sensor for simple, sensitive, and stable determination of HQ. The electrochemical activity of the modified electrode can be greatly improved by combining Cu nanoparticles immobilized on rGO. The fabricated Cu/rGO/GCE was applied for HQ detection and showed wide linear range and low detection limit. The proposed sensor was also applied for real-sample detection of HQ, and satisfactory results were obtained. Overall, the proposed potential platform can be applied for simple and effective detection of HQ.



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