graphene nanocomposites for nonenzymatic electrochemical glucose biosensor applications

Electrochimica Acta 82 (2012) 152–157

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Synthesis of CuO/graphene nanocomposites for nonenzymatic electrochemical glucose biosensor applications Yu-Wei Hsu a , Ting-Kang Hsu a , Chia-Liang Sun a,b,∗ , Yung-Tang Nien c , Nen-Wen Pu d , Ming-Der Ger e a

Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan Biosensor Group, Biomedical Engineering Research Center, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan d Department of Photonics Engineering, Yuan Ze University, Chung-Li 32003, Taiwan e Department of Chemical & Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Tao-Yuan 335, Taiwan b c

a r t i c l e

i n f o

Article history: Received 1 December 2011 Received in revised form 19 March 2012 Accepted 20 March 2012 Available online 27 March 2012 Keywords: Graphene Copper oxide Nanocomposite Nonenzymatic electrochemical biosensor Glucose

a b s t r a c t Glucose detection is of great importance in the fields of biological, environmental, and clinical analyses. In this study, CuO/graphene nanocomposites have been synthesized for nonenzymatic electrochemical glucose biosensors to avoid the disadvantages of enzymatic sensors. Transmission electron microscopy images show that the spherical and size-selected CuO nanoparticles were well dispersed on the surface of graphene. For the amperometric glucose detection, the low detection limit of 1 ␮mol L−1 with wide linear range from 1 ␮mol L−1 to 8 mmol L−1 can be obtained using the CuO/graphene (CuOG)-modified glassy carbon (GC) electrode with a low loading. Under the applied potential of +0.60 V vs. Ag/AgCl, the optimal CuOG-modified GC electrode exhibits a sensitivity of 1065 ␮A mmol−1 Lcm−2 . With such good analytical performance from simple process, it is believed that CuO/graphene nanocomposites are promising for the development of cost-effective nonenzymatic electrochemical glucose biosensors. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved.

1. Introduction Since reliable and fast determination of glucose is important in many areas such as biotechnology, clinical diagnostics and food industry, the development of electrochemical glucose sensor has attracted extensive attention [1–3]. Blood glucose monitoring is a way of testing the concentration of glucose in the blood and particularly important in the care of diabetes. In the past, the research for the enzymatic glucose biosensor based on using glucose oxidase was very popular because of its high selectivity [4–9]. For instance, Lin et al. [10] reported that glucose oxidase was covalently immobilized on carbon nanotube (CNT) nanoelectrode ensembles for the selective detection of glucose. However, the glucose oxidase is easily being affected by the environmental factors such as temperature, humidity, pH values, etc. In addition, the immobilization of glucose oxidase is a complicated and expensive process. Therefore, the nonenzymatic glucose biosensors start to catch the scientists’ eyes and attention [11]. Metal alloys [12], noble metal nanoparticles (NPs) [13], CNT [14], and CNT-supported nanoparticle

∗ Corresponding author at: Department of Chemical and Materials Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan 333, Taiwan. Tel.: +886 3 2118800x5379; fax: +886 3 2118668. E-mail address: [email protected] (C.-L. Sun).

composite materials [15–19] have been extensively investigated in the development of nonenzymatic glucose biosensors. However, these electrodes still have some drawbacks such as poor selectivity, low sensitivity of less than 1000 ␮A mM−1 cm−2 , high cost, or poisoning from chloride ion, which greatly limit their applications. Thus the development of a cheap, highly selective, fast and reliable nonenzymatic glucose biosensor is still imperatively demanded. Graphene, which is a two-dimensional monolayer of graphite, has recently received tremendous attention because of its extraordinary properties [20,21]. Recently, graphene has been rapidly becoming the attractive electrode material due to their high surface area, unique structures, excellent electrical conductivity, ultra-strong mechanical properties and high stability. Graphenebased materials also exhibit a significant potential for gas sensor and electrochemical biosensor applications [22–25]. For example, Shang et al. [26] fabricated multilayer graphene nanoflake films on Si via microwave-assisted plasma chemical vapor deposition (MPCVD) for sensing of ascorbic acid (AA), dopamine (DA), and uric acid (UA). Kim et al. [27] adsorbed graphene onto glassy carbon (GC) to separate the electrochemical potentials of AA and DA. Tan et al. used ionic liquids to disperse graphene [28], and Wang et al. [29] added chitosan on graphene for the same purpose. As a p-type semiconductor with a narrow band gap of 1.2 eV, CuO has been widely studied because of its numerous applications

0013-4686/$ – see front matter. Crown Copyright © 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.03.094

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in catalysis, semiconductors, gas sensors, biosensors, and field transistors [30–32]. Some efforts have been made on amperometric determination of glucose using nanostructured CuO materials. For example, CuO/Cu(OH)2 nanoparticles deposited on graphite-like carbon films showed enhanced sensitivity and stability for glucose sensing [33]. CuO nanowires on a Cu rod were used for the detection of glucose [34]. However, the synthesis of CuO nanowires is pretty time-consuming and the copper substrate exposing to the environment may affect the sensor performance. More recently, the existence of CuO NPs as impurities in CNTs was claimed to be responsible for electrocatalytic oxidation of glucose [35]. However, there is little research on the CuO NP-graphene nanocomposite materials for the electrochemical detection of glucose. In order to fully utilize the novel properties of graphene, we use graphene to replace CNT as the support for size-selected CuO NPs for the glucose sensing. Additionally, the relationship between the particle size of CuO NPs and the sensitivity will be discussed in this study.

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2. Experimental 2.1. Chemicals and reagents Copper nitrate (Cu(NO3 )2 ), sulfuric acid (H2 SO4 ) and ethylene glycol were purchased from J.T. Baker. Nafion (DuPont, 5 wt.%) was used to generate the ink. NaOH, KCl, glucose, AA, DA, UA, fructose, lactose and sucrose were obtained from Sigma. All solutions were prepared with deionized water with a resistivity of 18 M cm−1 . 2.2. Preparation of graphene Graphite powders were used from Alfa (natural, briquetting grade, 10 mesh, 99.9995%) [36,37]. Graphene oxide powders were prepared following Staudenmaier’s method and reduced to graphene powders by annealing at 1050 ◦ C under an argon atmosphere [38,39].

Fig. 1. (a) TEM image of a single piece of the CuO/graphene composite material. (b) HRTEM image of a typical single CuO nanoparticle attached on graphene. (c) The corresponding EDX spectrum of the CuO/graphene composite material in (a). (d) The corresponding SAED pattern of CuO/graphene composite material in (a). XPS spectra of (e) C 1s and (f) Cu 2p of the CuO/graphene composite material.

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2.3. Preparation of CuO/graphene nanocomposites Copper nitrate (0.35 g) powders were dissolved in ethylene glycol (50 mL) with the pH values controlled by 0.1 mol L−1 NaOH. Because Bock et al. [40] reported that the pH value can influence the size of PtRu nanoparticles, the pH values in the solution of copper nitrate in ethyl glycol were controlled to make 7 types of nanocomposite materials in the last part of this study. Graphene (0.05 g) powders were added into the reaction solution and then heated to 190 ◦ C for 3 h. The obtained solution was filtered and dried to get the Cu/graphene powders. The resultant Cu/graphene powders were annealed in a tube furnace for 30 min under the air environment to form CuO/graphene powders. Unless otherwise mentioned, the data shown in this study is from CuO/graphene prepared at a pH of 12.1 except the last part for the size-dependent discussion. 2.4. Materials characterization Transmission electron microscopy (JEOL JEM 2100F, 200 kV or JEOL JEM-1230, 100 kV) was used to characterize sample morphologies. X-ray photoelectron spectrometer (XPS) (VG Scientific ESCALAB 250) was used to study the composition and bonding change. A Bruker X-ray diffractometer (D8 DISCOVER with GADDS) was used to identify the phase of nanoparticles on graphene.

reported with respect to the Ag/AgCl. The measurements were carried out in a 0.1 mol L−1 NaOH solution at a pH of 12 at room temperature. 3. Results and discussion 3.1. Characterizations Fig. 1(a) displays the typical TEM micrograph of several pieces of CuO-decorated graphene (CuO/graphene) sheets. The dark dots on the graphene sheets are high-density CuO NPs with a mean diameter less than 20 nm. It can be observed that the CuO NPs formed on graphene surfaces are well-separated. A high-resolution transmission electron microscopy (HRTEM) image with a higher magnification is also shown in Fig. 1(b). The clear lattice fringe of a CuO NP with the particle size of around 30 nm can be found. As shown in Fig. 1(c), energy dispersive X-ray (EDX) analysis was performed to detect the composition. The O signal in CuO/graphene is significantly increased after the annealing process. Fig. 1(d) is the corresponding selected area electron diffraction (SAED) pattern of the CuO/graphene, the ring pattern indicated these CuO NPs were randomly oriented. The five major crystalline ¯ ¯ planes here are (0 0 2), (111), (1 1 1), (2 0 0), and (202) that confirmed the existence of CuO phase after the annealing at 300 ◦ C. Fig. 1(e) and (f) shows the XPS C 1s and Cu 2p spectra of this CuO/graphene composite material. In the C 1s spectrum, the main

2.5. Preparation of modified electrodes

2.6. Electrochemical measurements

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The different powders (6 mg) were mixed with deionized water (3 mL), ethanol (2 mL), and Nafion (60 ␮L) and then ultrasonically treated to form the inks (Dleta DC300, 40 kHz and 300 W). Approximately 10 ␮L of each ink was dropped onto a clean GC electrode and dried at room temperature to form the modified GC electrodes.

Electrochemical experiments were performed with a CHI 700D electrochemical workstation (CHI, USA). A standard three electrode cell was used for the electrochemical experiments. A 3-mmdiameter GC was used as the working electrode (a working area of around 0.07 cm2 ). A silver/silver chloride (Ag/AgCl) electrode and a platinum electrode were used as the reference and the counter electrodes, respectively. All potentials in this study are

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peak located at about 285 eV is attributed to the sp2 -hybridized carbon bonds from the graphene. Because the peak shoulder in the high-binding-energy region is not very broad, it suggests that the graphene was not severely damaged by oxygen in air during the annealing process. The main peak of Cu 2p3/2 shifted to about 942.3 eV that is much higher than that of the metallic Cu. Hence, this fact concurs the formation of a CuO phase after the annealing process. The surface compositions of CuO/graphene were determined by taking C 1s, Cu 2p, and O 1s and their atomic sensitivity factors into account. The composition ratios of C, Cu, and O of this CuO/graphene composite material are 45.2%, 19.1%, and 35.7%, respectively. Fig. 2 shows the XRD pattern of the CuO/graphene composite material. It was observed that CuO phase on graphene is polycrystalline due to the randomly-oriented NPs and high-quality CuO phase would be obtained at the annealing at 300 ◦ C. The crystal system of this CuO (JCPDS 05-0661) phase is monoclinic which is also reported in the porous hollow architectures through a top-down chemical approach published in 2008 [41]. 3.2. Electrochemical measurements Graphene, CuO nanoparticles, and CuO/graphene (CuOG) composites were used to modify GC electrodes to detect AA, DA, and UA electrochemically using cyclic voltammetry (CV). Fig. 3(a) and (b) shows the cyclic voltammograms of various modified GC electrodes in NaOH and for sensing glucose. As shown in Fig. 3(a),

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the capacitance of CuOG-modified GC electrode is much larger than those of graphene- and CuO nanoparticle (CuO)-modified electrodes. Furthermore, we used the same CuOG-modified GC electrode to detect glucose with different concentrations between 2 and 14 mmol L−1 . As shown in Fig. 3(b), the oxidation current can be found between 0.34 and 0.48 V. The anodic peak is shifted to the high potential region while the glucose concentration is raised. For the detection of 12 mmol L−1 glucose, the peak current of a CuG/GC electrode can reach 967.0 ␮A. However, the peak current for 14 mmol L−1 glucose is lower than that for 12 mmol L−1 glucose. Thus it is suggested that there is a degradation or fatigue phenomenon for the repetitive measurements using the same one electrode. 3.3. Amperometric detection of glucose at a CuO/graphene-modified GC electrode The amperometric responses of various modified GC electrodes to glucose are depicted in Fig. 4. As shown in Fig. 4(a), 0.5 mmol L−1 glucose was added into 0.1 mol L−1 NaOH solution for three modified electrodes every 100 s. CuOG/GC electrode outperformed other two electrodes in terms of current densities at a fixed potential of +0.6 V. After adding glucose solution with different concentrations into stirred 0.10 mol L−1 KOH, the oxidation currents of a CuOG/GC electrode were monitored at a fixed potential of +0.6 V in Fig. 4(b). From the amperometric curve for glucose, the linear relationship between the oxidation current and the glucose concentration was obtained for concentrations

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Time / s Fig. 4. (a) Amperometric responses of the graphene-, CuO nanoparticle-, and CuO/graphene-modified GC electrodes after the subsequent addition of 0.5 mmol L−1 glucose in a 0.1 mol L−1 NaOH solution. (b) Amperometric response of a CuO/graphene-modified GC electrode after the subsequent addition of glucose in a 0.1 mol L−1 NaOH solution. Inset: corresponding calibration curve of 1 ␮mol L−1 glucose.

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Time / day Fig. 5. (a) Amperometric response to 1.0 mmol L−1 glucose as well as 0.10 mmol L−1 interferents of AA, DA, UA, fructose, lactose and sucrose at a CuO/graphene-modified GC electrode. (b) Long-term stability of a CuO/graphene-modified GC electrode for 1 mmol L−1 glucose measurement.

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ranging from 1 ␮mol L−1 to 8 mmol L−1 . The linear regression equation is given by Ipa (A) = 46.45 + 63.67Cglucose , with a correlation coefficient of r = 0.9825 (N = 3). The detection limit of glucose using a GuOG/GC electrode was found to be 1 ␮mol L−1 with the sensitivity of 1065.21 ␮A mmol−1 Lcm−2 . The inset in Fig. 4(b) is the calibration curve for 1 ␮mol L−1 glucose. 3.4. Interference and stability study Anti-interference property is crucial for the development of a nonenzymatic electrochemical biosensor. Because the species such as AA, DA, UA and other compounds that easily oxidize always exist with glucose in human blood, their electrochemical response

needs to be examined at a CuOG/GC electrode. Because the concentration of glucose is at least 30 times higher than those of interfering species in human blood, the interference experiment was carried out by adding 0.1 mmol L−1 interfering species into 1.0 mmol L−1 glucose in 0.10 mol L−1 NaOH. As shown in Fig. 5(a), no significant signals can be observed for interfering species when the well-defined glucose oxidation currents were obtained. Therefore, it indicates that a small amount of AA, DA, UA, fructose, lactose and sucrose can be neglected when CuOG/GC electrode exhibits high selectivity for glucose sensing. In the literature, the voltammetric peaks of the graphene and graphene/Pt-modified GC electrodes can be resolved into three peaks at approximately +0.04 V (AA), +0.23 V (DA), and +0.37 V (UA) [36]. Meanwhile, the amperometric response of the composite CuOG/GC electrode to glucose was recorded at a fixed potential of +0.6 V. Hence, it is clear that the composite electrode has low sensitivity for the interfering species such as AA, DA, and UA. The stability of a biosensor was evaluated at +0.6 V to compare the amperometric current responses of a CuOG/GC electrode within a 14-day period in Fig. 5(b). The electrode was exposed to air and tested every 2 days in 1.0 mM glucose. The current response of the CuOG/GC electrode was approximately 92% of its original counterpart. The good stability is owing to the chemically stable CuO phase in basic solution. 3.5. Size-dependent sensitivities of CuO NPs in CuO/graphene nanocomposites The pH values for the synthesis of Cu NPs on graphene can be controlled in the process as a key factor in order to influence their particle sizes. 7 pH values from 11.50 to 13.59 were used for the fabrication of size-selected NPs. After annealing at 300 ◦ C, the mean diameters of CuO in 7 products were decided from their TEM pictures. Two size histograms and their corresponding TEM images are illustrated in Fig. 6(a) and (b). Moreover, the size-dependent sensitivities of 7 products with different particle sizes are shown in Fig. 6(c). As the pH value is higher, it is found that the particle size of CuO on graphene gradually becomes smaller. Interestingly, the smallest CuO particles on graphene are not linked to the best sensitivity. In fact, the CuO NPs with an optimal mean diameter of 15.75 nm on graphene have the highest sensitivity. It has to be mentioned that the high precursor concentration and long annealing time for making CuO on graphene also generate the large NPs with the low sensitivities. Thus it suggests that the appropriate size of CuO on graphene is necessary for the excellent sensitivity. It is well known that the particle size of Pt nanocatalysts can influence various electrochemical reactions like oxygen reduction, as well as oxidation of CO, methanol, and formic acid [42–47]. Effects of geometric and surface electronic properties of various CuO NPs could be correlated to their glucose oxidation activities because the dispersion and surface percentage of atoms on different facets and on the edges between the facets are dependent on their sizes. 4. Conclusions

Fig. 6. (a) A size histogram of CuO nanoparticles with the mean diameter of 5.17 nm estimated from the inset TEM image. (b) A size histogram of CuO nanoparticles with the mean diameter of 15.75 nm estimated from the inset TEM image. (c) The summary of the pH-dependent diameters of CuO nanoparticles and sensitivities of different CuO/graphene-modified GC electrodes.

A unique CuO/graphene nanocomposite has been developed for the electrochemical detection of glucose. The process parameter for the size control of CuO on graphene has been investigated and the sensitivity of each composite material has been measured. The optimal particle size of CuO decorated on graphene is 15.75 nm and provides a superior sensitivity of 1065.21 ␮A mmol−1 L cm−2 compared with those with smaller or bigger CuO NPs. The observed detection limit for this composite material is 1 ␮mol L−1 with the linear range from 1 ␮mol L−1 to 8 mmol L−1 as well as the reaction time of 1 s. No severe interference and good stability of a CuOG/GC electrode can be obtained in amperometric current-time

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