Microchimica Acta

, Volume 177, Issue 3, pp 485–490

Facile one-step electrochemical fabrication of a non-enzymatic glucose-selective glassy carbon electrode modified with copper nanoparticles and graphene

Authors

    • The Key Laboratory of Food Colloids and Biotechnology, Ministry of EducationSchool of Chemical and Material Engineering, Jiangnan University
  • Hongyan Zhang
    • The Key Laboratory of Food Colloids and Biotechnology, Ministry of EducationSchool of Chemical and Material Engineering, Jiangnan University
  • Sisi Jiang
    • The Key Laboratory of Food Colloids and Biotechnology, Ministry of EducationSchool of Chemical and Material Engineering, Jiangnan University
  • Jinqiang Jiang
    • The Key Laboratory of Food Colloids and Biotechnology, Ministry of EducationSchool of Chemical and Material Engineering, Jiangnan University
    • The Key Laboratory of Food Colloids and Biotechnology, Ministry of EducationSchool of Chemical and Material Engineering, Jiangnan University
Short Communication

DOI: 10.1007/s00604-012-0795-4

Cite this article as:
Luo, J., Zhang, H., Jiang, S. et al. Microchim Acta (2012) 177: 485. doi:10.1007/s00604-012-0795-4

Abstract

We have developed a non-enzymatic glucose sensor by using a composite prepared from copper nanoparticles (CuNPs) and graphene which can be prepared by simple 1-step electrochemical reduction using graphene oxide (GO) and copper ion as the starting materials. The GO is electrochemically reduced to graphene at a voltage of −1.5 V, and this is accompanied by the simultaneous formation of CuNPs on the surface of the graphene. This novel nanocomposite combines the advantages of graphene and of CuNPs and displays good electrocatalytic activity toward glucose in alkaline media. The performance of the respective glucose electrode was evaluated by amperometric experiments and revealed a fast response (<2 s), a low detection limit (200 nM), and high sensitivity (607 μA mM−1). The sensor also exhibits good reproducibility and very good specificity for glucose over ascorbic acid, dopamine, uric acid, fructose, lactose and sucrose.

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Figure

(A) CVs of Cu NPs/graphene electrode (a), graphene electrode (b),and Cu/GC electrode (c) in 0.1 M NaOH solution with 0.5 mM glucose; (B) The response of the Cu NPs/graphene electrode to successive addition of glucose from 5 μM to 0.2 mM.

Keywords

GrapheneCopper nanoparticlesElectrochemical reductionGlucoseSensor

Introduction

In recent years, graphene has shown great potential as sensing element due to their unique advantages such as large surface area, excellent electrical conductivity, fast electron transportation, ease of functionalization and mass production [1]. Since every atom in a graphene is a surface atom, molecular interaction and thus electron transport through graphene can be highly sensitive to absorbed molecules [2]. The introduction of graphene into electrochemical sensors has indeed resulted in dramatic evolution of various organic compounds such as ascorbic acid [3], H2O2 [4], hydrazine [5], NADH [6] and dopamine [7].

Copper- and copper oxide-based materials were of great interest for the electro-oxidation of glucose for a long time [8, 9]. Recently, there is a great amount of interest on copper and copper oxide nanomaterials for the non-enzymatic detection of glucose [1012]. For instance, Kang fabricated a Cu-carbon nanotubes composite sensor by electrochemically depositing copper nanoclusters on a multiwall carbon nanotube modified glass carbon electrode [10]. Zhang also reported Cu nanocubes and Cu2O nanoparticles modified CNTs via electrodeposition and sputtering deposition to create nonenzymatic glucose biosensor [11, 12]. High sensitive and fast amperometric detection of glucose with low detection limit was achieved by using the above mentioned copper or copper oxide/CNTs nanomaterials due to the increased electrocatalytical area and promoted electron transfer for the reaction of the glucose oxidation. Comparing with carbon nanotubes, graphene has shown the advantages of high conductivity, ease of production and function, good biocompatibility and abundance of inexpensive source material [13]. Considering these attractive properties of graphene, it is quite expected that graphene could provide an excellent support of the Cu nanoparticles towards the oxidation of glucose. We recently prepared a novel Cu-graphene nanocomposite based electrode by electrodepositing Cu nanoparticles on a graphene modified glass carbon electrode [14]. The obtained Cu-graphene electrode showed outstanding electrocatalytic property for glucose detection and detection, which can be attributed to the combination of the advantages of graphene and Cu nanoparticles. However, the preparation route was tedious and inconvenient, which included chemical reduction of graphene oxide (GO) to graphene, dispersing graphene in water and then dropping the graphene dispersion on GC electrode to get the graphene electrode and finally electrodepositing Cu nanoparticles on the graphene electrode. In addition, due to the hydrophobic property of graphene, it is not so easy to get graphene dispersion. Furthermore, the chemical reduction of GO to graphene needed a large amount of toxic hydrazine. Therefore, it is quite necessary to develop a facile and environmental-friendly approach to prepare Cu nanoparticles/graphene nanocomposite.

Electrodeposition method is one of the mostly useful approaches to prepare copper nanostructures. By this means, copper NPs can be facilely synthesized on the surface of conducting surfaces, and the sizes and shapes of the prepared nanoparticles can be easily controlled by altering the conditions of electrochemical deposition [10, 11]. Meanwhile, it is reported recently that high quality graphene can be prepared by electrochemical reduction of GO at −1.5 V, which eliminate the use of excessive reducing agents which will contaminate the resultant products and can completely reduce the oxygen functionalities on GO [15, 16]. While the deposition of Cu NPs starts at much more positive potentials (normally at −0.25 V), it is expected that electrochemical reduction of GO and electrodeposition of Cu NPs can be simultaneously performed at −1.5 V vs. SCE in a solution of copper ion. This one step synthesis approach is simple, green and fast, and has been used to fabricate graphene-supported Pt NPs which displayed good electrocatalytic activity towards methanol electrooxidation [17].

Herein, in the current work, we describe a simple one-step electrochemical reduction approach to prepare a novel Cu NPs/graphene nanocomposite for enzyme-free glucose sensing, which has not been reported. In this one step synthesis approach, simultaneous electrochemical reductions of Cu ion and GO are performed, generating Cu NPs on graphene sheets with an average size of ca. 8 nm. Encouragingly, this new Cu NP/graphene nanocomposite material takes advantage of salient properties of both Cu NPs and graphene, exhibiting improved electrocatalytic properties for glucose oxidation and detection compared to the single graphene or Cu. The resulted Cu NP/graphene modified electrode presents fast response, high sensitivity, low detection limit, and excellent selectivity, which can open up new opportunities for fast, simple and sensitive detection of glucose.

Experimental

Chemicals and apparatus

Graphite powder, copper sulfate (CuSO4), sodium sulfate (Na2SO4), glucose, dopamine, ascorbic acid, uric acid, fructose, lactose and sucrose were purchased from the Shanghai Aladdin Chemical Reagent Company (Shanghai, China, http://www.aladdin-reagent.com).

XRD profiles were obtained (XD-3A, Shimadzu, Japan, http://www.shimadzu.com) with high-intensity Cu Kα radiation (λ = 1.5406 nm). Morphologies and nanostructures of Cu NPs/graphene nanocomposites were determined using scanning electron microscopy (SEM) (Hitachi S-3700N, Tokyo, Japan, http://www.hitachi.com). Electrochemical measurements were performed on a CHI 660C electrochemical analyzer (CH Instruments, Chenhua Corp., Shanghai, China, http://www.chinstruments.com/schi.html). A conventional three-electrode system was employed, involving a modified GC electrode as the working electrode, a platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.

Preparation of Cu NPs/graphene electrode

Graphene oxide (GO) was synthesized from graphite by the Hummers method [18]. All other reagents used in this study were of analytical grade. Deionized water (>18.4 MΩ·cm) was used for all solutions’ preparation. The prepared GO powder was exfoliated by ultrasonication to form a 1.0 mg mL−1 GO colloidal dispersion. GC electrode was polished with 1 and 0.5 μm Al2O3 powder and then rinsed by ethanol and deionized water, dried in N2. A 5 μL portion of the resulting GO dispersion was spread on the pretreated bare GCE and dried at room temperature to afford the GO-modified GC electrode. To fabricate the Cu nanoparticles modified graphene electrode, a one step electrochemical reduction was carried out: using the potentiostatic technique, the GO modified electrode was reduced at a potential of −1.5 V (vs. SCE) in 10 mM CuSO4 and 100 mM Na2SO4 for 500 s with N2 bubbling. The bare graphene electrode was prepared by electrochemical reduction of GO-modified GC electrode at −1.5 V in aqueous solution without CuSO4. The controlled Cu modified GC electrode was prepared under the same experimental condition with that of Cu NPs/graphene electrode except that bare GC electrode was used instead of the GO modified GC electrode.

Results and discussion

The morphology of the resulting Cu NPs/graphene was characterized by SEM. As can be seen from Fig. 1a and b, crumble graphene sheets are densely covered by Cu nanoparticles as indicated by the bright dots on graphene sheets, and the average particle size is around 8 nm. Obviously, the generated Cu NPs were homogenously distributed onto the graphene sheet matrix, constructing a Cu NPs-decorated graphene nanocomposite.
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Fig. 1

SEM images of Cu NPs/graphene at low (a) and high (b) magnification. Raman spectra (c) and XRD pattern (d) of GO (b) and Cu NPs/graphene (a)

The Cu NPs/Graphene nanocomposite prepared by using the one-step method was characterized by Raman spectroscopy as shown in Fig. 1c. For comparison, the Raman spectrum of GO is also displayed (curve b). Both the GO and the electrochemically reduced GO display a strong D peak (defect peak due to intervalley scattering) at about 1,340 cm−1 and a strong G peak (the graphene peak) at about 1,600 cm−1 [19]. However, the D/G ratio of the electrochemically reduced GO (1.38) is higher than that of GO (1.17), indicating an increase in the number of smaller graphene domains upon reduction of GO [20]. In addition, 2D band (at 2,637 cm−1) and 2G band (2,915 cm−1) appeared after electrochemical reduction. These results demonstrate that graphene oxide has been successfully electrochemically reduced to graphene.

The XRD patterns of the GO and Cu NPs/graphene nanocomposite are illustrated in Fig. 1d. The disappearance of the peak at about 10.4° in the pattern of GO (Fig. 1d, curve b) revealed that it was successfully reduced to graphene [17, 21]. As shown in the XRD pattern of Cu NPs/Graphene nanocomposite (Fig. 1d, curve a), the strong diffraction peaks at 43.5, 50.5 and 74.5° correspond to the (111), (200) and (220) facets of the face-centered cubic structures of Cu crystal, respectively [10]. The broad peak centered at 24° is related to the diffraction of loosely stacked graphene [22].

Figure 2 presents the electrochemical oxidation of glucose investigated with (A) a bare graphene electrode (a, b), and (B) the as-prepared Cu NP/graphene electrode (d, e). Figure 2a (curve a) shows that the graphene electrode exhibits a capacitive charging response with a large oxidation tail corresponding to the onset of water breakdown observed at +0.65 V. The addition of glucose almost has no influence on the CV of the graphene electrode and no oxidation peak for glucose can be observed (curve b). For Cu NPs/graphene electrode, in the absence of glucose, its CV response is quite similar to that of bare graphene electrode (Fig. 2b, curve d). However, as shown from curve e in Fig. 2b, upon the addition of glucose, a single oxidative peak starting at about + 0.35 V with the oxidation peak at around +0.50 V, corresponding to the irreversible oxidation of glucose which is quite similar to Yang’s report [11, 12], is observed at the Cu NPs/graphene electrode. This suggests that Cu nanoparticles played a major role in the oxidation of glucose with its catalytic activity against glucose. To investigate the role of graphene sheet, a control Cu/GC electrode with Cu electrodeposition on bare GC electrode was also prepared and its electrocatalytic response to glucose is also investigated as shown in Fig. 2b (curve c). The Cu/GC electrode showed an irreversible glucose oxidation peak at +0.68 V. By comparing curve e and curve c, it is obviously observed that Cu NPs/graphene electrode exhibited much higher current than Cu/GC electrode. In addition, the onset of potential for the electrooxidation of glucose on Cu NPs/graphene electrode starts at 0.35 V, which is 200 mV negative than 0.55 V on the Cu/GC electrode. This clearly shows the Cu NPs/graphene electrode exhibits a much stronger electrocatalytic activity on the direct oxidation of glucose. The enhanced electrocatalytic performance of Cu NPs/graphene electrode compared to that of Cu/GC electrode is considered to be the result of a large surface area and high conductivity as well as fast electron transfer provided by graphene sheets, confirming the important role the graphene plays in the electrocatalytic performance towards glucose oxidation. From the above results, it can be concluded that the Cu NPs/graphene electrode shows a synergistic combination of copper nanoparticles and graphene on the electrochemical oxidation of glucose.
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Fig. 2

Cyclic voltammograms measured with (a) graphene electrode (a, b), (b) Cu NPs/graphene electrode (d, e) and Cu/GC electrode (c) 0.1 M NaOH without (a, d) or with (b, e, c) glucose (0.5 mM). Scan rate: 50 mV s−1

Furthermore, the CVs of oxidation of glucose on Cu NPs/graphene electrode at different scan rates were recorded (Fig S1, Electronic Supplementary Material, ESM), showing the anodic peak current of oxidation glucose is proportional to the scan rate. The results indicate that the electrochemical kinetics is controlled by the absorption of glucose. The CVs of the Cu NPs/graphene electrode in different concentrations of glucose were also measured, as indicated in Fig S2 (ESM), and a gradual increase in the current was observed with increasing the glucose concentration, pointing to its potential for developing Cu NPs/graphene electrode as a non-enzymatic glucose sensor.

Figure 3 displays the amperometric responses of Cu NPs/graphene electrode for a successive addition of glucose at 25 s in 0.1 M NaOH at optimal potential of 0.50 V. It is observed that Cu NPs/graphene electrode responds quickly to the change of glucose concentration and reaches a steady-state current in less than 2 s (inset of Fig. 3a), which indicates an extraordinarily fast rapid and sensitive response to glucose. This extraordinarily fast response concurs with fast diffusion of glucose molecules in the 3D network of Cu NPs/graphene and fast electron tranfers. This response is much faster than those reported for Cu nanocluster MWCNTs/GC (<5s) [10], SWCNTs/Cu/Nf (10s) [23], Cu nanobelt (<100s) [24], NiO/MWCNT (>30s) [25]. As observed in Fig. 3b, the calibration plot is linear over the wide concentration range of 5 μΜ to 1.4 mM with a slope of 0.6072 (sensitivity) and a correlation coefficient of 0.998. The detection limit of the electrode was found to be 0.2 μΜ at a signal-to-noise ratio of 3. The analytical performance characteristics of the Cu NPs/graphene electrode were compared with several typical non-enzymatic and enzymatic glucose sensors reported previously [10, 2332], which are summarized in Table S1 (ESM). Among these, our Cu NPs/graphene electrode exhibits faster response speed, lower detection limit and higher sensitivity, which is due to the promoted electron transfer and superb catalytic activity afforded by the nanocomposite structure of Cu nanoparticles and graphene. More importantly, this Cu NPs/graphene nanocomposite has the advantage of low production cost, facile preparation procedure compared to other sensing materials. The Cu NPs/graphene electrode also shows high stability and reproducibility in detection of glucose, with an error about 8 % in more than 30 times measurements.
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Fig. 3

a The response of the Cu NPs/graphene electrode to successive addition of glucose from 5 μM to 0.2 mM. The inset shows the response time of Cu NPs/graphene to 5 μM of glucose addition; b The linear relationship between the current and glucose concentration. (Detection potential: 0.50 V)

To evaluate the selectivity of the proposed sensor, a number of oxidisable interfering species such as dopamine (DA), ascorbic acid (AA), uric acid (UA) and other carbohydrate compounds such as fructose, lactose and sucrose, which could co-exist with glucose in many samples, were examined at the Cu NPs/graphene electrode. Considering the concentration of glucose is at least 30 times of interfering species in human blood, the interference experiment was carried out by successive addition of 1.0 mM glucose and 0.1 mM interfering species to 0.10 M NaOH solution. As shown in Fig. 4, well-defined glucose responses were obtained, whereas very small responses were observed for interfering species, which can be neglected compared to glucose. These results indicate that Cu NPs/graphene electrode has high selectivity for glucose detection.
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Fig. 4

Interference test of the Cu NPs/graphene electrode in 100 mM NaOH at +0.5 V with 1 mM glucose and 0.1 mM other interferents (dopamine (DA), ascorbic acid (AA), uric acid (UA) and fructose, lactose and sucrose) as indicated

The reproducibility and stability of the Cu NPs/graphene electrode were performed by measuring the current response of the electrode upon 0.5 mM glucose in 0.1 M NaOH. The average relative standard deviation (RSD) was no more than 4.5 %. In a series of 8 sensors prepared in the same way, a RSD of 3.8 % was obtained, indicating the reliability of this method. The prepared electrode was stored in air at ambient conditions. In order to investigate the stability of the sensor, the current response to 0.5 mM of glucose was recorded each 3 days. It was found that the current could retain 90 % of its original signal after 2 weeks storage, showing long-term stability.

Conclusion

We have demonstrated a fast, green, effective and one-step electrochemical method to synthesize a novel Cu NPs/graphene nanocomposite as a new non-enzymatic glucose sensor. Graphene oxide (GO) was electrochemically reduced to graphene, accompanied by the simultaneous formation of Cu nanoparticles (NPs) via cathodic electrochemical deposition. The resulted Cu NPs/graphene nanocomposite exhibited a synergistic combination of the advantages of Cu NPs and graphene toward the electrochemical oxidation of glucose. We have demonstrated that this Cu NPs/graphene nanocomposite based sensor shows fast response, low detection limit and high sensitivity to glucose detection. These excellent performance characteristics are combined with ease of fabrication, good reproducibility and perfect specificity to glucose in the presence of various common interferents, which makes our Cu NPs/graphene electrode as a potential candidate for glucose analysis.

Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (under Grant Nos. 21174056 and 51103064) and the Fundamental Research Funds for the Central Universities (JUSRP31003 and JUSRP11108).

Supplementary material

604_2012_795_MOESM1_ESM.doc (471 kb)
ESM 1(DOC 471 kb)

Copyright information

© Springer-Verlag 2012