CuO/Cu₂O nanofibers as electrode materials for non-enzymatic glucose sensors with improved sensitivity

Abstract

CuO nanofibers (NFs) were fabricated via the traditional electrospinning technique and subsequent thermal treatment processes. Using CuO NFs as precursors and glucose as a reducing agent, CuO/Cu₂O NFs, with high surface areas and ultralong one dimensional (1D) nanostructures, were obtained by a partial reduction of CuO NFs. Comparing with pure CuO NFs, CuO/Cu₂O NFs, as non-enzymatic electrode materials, showed a much higher sensitivity of 830 μA mM⁻¹ cm⁻² and a much wider detection range from 0.5 mM to 10 mM for the amperometric detection of glucose. The excellent electrocatalytic performances could be ascribed to the following advantages: (1) the CuO/Cu₂O NFs with Cu(II)/Cu(I) multiple oxidation states system could promote the redox reactions between electrode materials and glucose, and the reactive sites became more active due to the synergic effect; (2) the surface of CuO/Cu₂O NFs became smoother after partial reduction, resulting in less adsorption of the intermediates during the oxidation of glucose, generating the enlarged detection range. Therefore, the CuO/Cu₂O composite NFs electrode materials, with a multiple oxidation states system, would be promising candidates for the development of non-enzymatic glucose sensors.

---

1. Introduction

Diabetes mellitus, a group of metabolic diseases, is one of the principal causes of death and disability worldwide.^1,2 In order to avoid diabetic emergencies and reduce disease-associated complications, patients need to frequently test the blood glucose level, which in normal standard is about 4.4–6.6 mM (80–120 mg dL⁻¹).^3,4 Therefore, the development of a highly sensitive and selective diagnostic tool with a wide detection range has attracted extensive attention for decades.^5,6 Currently, glucose oxidase (GOD)-based glucose sensors, with high sensitivity, good selectivity and reliability, are widely used, amounting up to 85% of the entire biosensor market.^1,7 However, the activity of GOD is quite sensitive to temperature, pH, humidity and toxic chemicals due to the intrinsic nature of enzymes, which can destructively affect the performance of enzymatic glucose sensors.⁸ To overcome these limitations, in recent years non-enzymatic glucose sensors have been investigated to improve the performance of the direct electrocatalytic oxidation of glucose.

Transition metals (Au,⁶ Pt,⁹ Pd,¹⁰ Ni¹¹ and Cu⁷) and alloys (Pt–Pd,¹² Pt–Au,¹³ Au–Ru,¹⁴ etc.) have been explored as electrode materials for non-enzymatic glucose sensors. It has been reported that the oxidation of glucose by the above materials is based on the multi-electron oxidation on surface metal oxide layers.¹⁵ Nevertheless, these materials show some drawbacks, such as low sensitivity, poor selectivity and poisoning by adsorbed intermediates or chloride ions.⁸ In contrast to these pure metals or alloy electrode materials, transition metal oxides, such as Co₃O₄,¹⁶ NiO,¹⁷ CuO,¹⁸ Cu₂O,¹⁹ etc., are more stable in air and show no observable self-poisoning effect in solution. Electrodes made from transition metal oxides, as mentioned above, show excellent sensitivity and selectivity for the oxidation of glucose. Among the transition metal oxides, copper oxides (CuO, Cu₂O) are of particular interests due to their excellent mechanical, thermal and chemical stabilities and low overpotential for electron-transfer reactions. Considerable attention has been paid to fabricating CuO or Cu₂O electrode materials for non-enzymatic glucose sensors, which showed high electrocatalytic activities.^20–22 However, as with other non-enzymatic glucose sensors, it is still a very important issue to further improve the sensitivity of the CuO or Cu₂O modified electrode, in order to achieve a simple and reliable detection in practical application.

As it is known to all, the sensitivity of non-enzymatic glucose sensors is closely related to the redox activation of the electrode materials in the electrocatalytic processes. Many researchers found that constructing a composite material with multiple redox couples could promote the redox reactions.^23–25 Therefore, constructing composite materials consisting of CuO [Cu(II)] and Cu₂O [Cu(I)] may be a good choice to improve the sensitivity of the non-enzymatic glucose sensor due to its stable multiple oxidation states and its remarkable catalytic capability for glucose detection. Especially for their nanostructure, with small size effect and high surface areas, the composite materials could be beneficial for fully contacting with glucose. It has also been reported that the surface structure of the electrode is another major contributing factor to the sensitivity of non-enzymatic glucose detections.²⁶ Therefore, it is reasonable to construct the nanostructured CuO/Cu₂O composite materials for non-enzymatic glucose sensors.

In this work, our interest was to fabricate CuO/Cu₂O composite NFs as electrode materials for non-enzymatic glucose sensors, which were obtained by partially reducing the electrospun CuO NFs via hydrothermal treatment. We expected to utilize the multiple oxidation states system of the CuO/Cu₂O composite NFs to promote the redox reactions and improve the electrocatalytic properties. Meanwhile, the high surface areas of the CuO/Cu₂O composite NFs could contribute to the mass transport rate and further improve the electrochemical activity.²⁶ Moreover, the ultralong 1D nanostructures of the CuO/Cu₂O composite NFs could provide the vectorial electron-transfer passages to enhance the electron-transfer rate and improve the sensitivity.¹⁵ The detailed characteristics and electrochemical detections of the CuO/Cu₂O NFs electrode materials were discussed in the following sections.

2. Experimental section

2.1. Reagents and materials

Poly(vinyl pyrrolidone) (PVP) powders (M_n = 1 300 000) and Nafion (5 wt%) were purchased from Sigma-Aldrich. Cupric acetate [Cu(CH₃COO)₂·H₂O], sodium hydroxide (NaOH), D-glucose, sodium chloride (NaCl), ascorbic acid (AA), uric acid (UA), N,N-dimethylformamide (DMF) and the remaining inorganic chemicals were purchased from Beijing Chemical Plant (Beijing, China). All chemicals were of analytical grade and used as received. Standard glucose samples, containing various concentrations of glucose and interfering species (AA, UA and NaCl), were prepared with deionized water before experiments.

2.2. Preparation of CuO NFs

First, 0.4 g of Cu(CH₃COO)₂·H₂O were dissolved in 10 mL of DMF solution. Then, 1.6 g of PVP powders were added to the above solution with vigorous stirring at room temperature for about 12 h. The precursor solution was transferred into a plastic syringe for electrospinning, with the applied voltage of 10 kV and the distance between the needle tip and the collector of about 10 cm. A dense web of electrospun NFs of PVP/Cu(CH₃COO)₂ composite was collected on an aluminum foil. The above composite NFs were calcined at 500 °C in air for 2 h with a rising rate of 1 °C min⁻¹.

2.3. Fabrication of CuO/Cu₂O composite NFs

CuO/Cu₂O composite NFs were prepared by a hydrothermal method as follows: CuO NFs (15 mg), glucose (12.5 mg) and 20 mL deionized water were put into a Teflon-lined stainless steel autoclave of 25 mL capacity. The mixture was stirred for 10 min to form a suspension, then, sealed and hydrothermally treated at 180 °C for 2 h. After that, the autoclave was cooled naturally in air. The suspension was isolated by filtration, washed with deionized water several times, and dried in the oven at 60 °C for 4 h.

2.4. Preparation of modified electrodes

The glassy carbon electrodes (3 mm in diameter, CH Instruments) were polished sequentially with 1, 0.3 and 0.05 μm Al₂O₃ powders, followed by thorough rinsing with deionized water. After sequential sonication in 1 M nitric acid, acetone, and deionized water, the electrodes were dried at room temperature. The CuO/Cu₂O NFs (3.5 mg) were dispersed into 0.1 mL of absolute ethanol and 5 μL of 5 wt% Nafion, and then ultrasonically treated to form a suspension. Approximately 10 μL of the suspension were dropped onto the clean electrode surface and dried in vacuum for 4 h at room temperature to obtain the CuO/Cu₂O NFs electrode. For a comparative study, a CuO NFs electrode was prepared with the same procedure.

2.5. Apparatus

X-ray diffraction (XRD) measurements were carried out using a D/max 2500 XRD spectrometer (Rigaku) with Cu Kα line of 0.1541 nm. The morphologies of the as-prepared NFs were observed by scanning electron microscope (SEM; XL-30 ESEM FEG, Micro FEI Philips). All electrochemical measurements were performed on a CHI 660D electrochemical analyzer (Shanghai, China) with a conventional three-electrode system composed of a Ag/AgCl (3 M KCl) reference electrode, a platinum wire as counter electrode, and a modified GCE as working electrode. The cyclic voltammetry and amperometric i–t curve measurements were carried out in 0.1 M NaOH electrolyte under ambient air. The amperometric i–t curve measurements required operation of the electrode at a constant applied potential of 0.6 V vs. Ag/AgCl.

3. Results and discussion

3.1. Characterizations of the as-prepared NFs

The XRD patterns of the as-prepared pure CuO NFs, CuO/Cu₂O NFs and the standard data of Cu₂O as reference (PDF 77-199) were shown in Fig. 1. As for the pattern of pure CuO NFs in Fig. 1a, diffraction peaks at about 2θ = 32.5°, 35.4°, 38.7°, 48.7°, 53.5°, 58.3°, 61.5°, 65.8°, 67.9°, 72.4° and 75.0° could be perfectly indexed to the (110), (002), (111), (202̄), (020), (202), (113̄), (022), (113), (311) and (004) crystal planes of monoclinic structured CuO (PDF 05-0661), respectively, indicating that well-crystallized CuO NFs were obtained by calcination of PVP/Cu(CH₃COO)₂ NFs at 500 °C for 2 h. After hydrothermal treatment in glucose solution at 180 °C for 2 h, as shown in Fig. 1b, additional diffraction peaks with 2θ values of 29.6°, 36.4°, 42.3°, 61.3°, 73.5° and 77.3° appeared, corresponding to (110), (111), (200), (220), (311) and (222) crystal planes of cubic structured Cu₂O (PDF 77-199), respectively, illustrating that part of CuO was successfully converted into Cu₂O. Therefore, it is obvious that this synthesis route is favorable for obtaining the multicomponent oxide composite, as glucose is a common weak reducing agent in aqueous solution.^28,29

Fig. 1 XRD patterns of pure CuO NFs (a), CuO/Cu₂O NFs (b) and the standard data of Cu₂O as reference (PDF 77-199).

Fig. 2A and B showed the SEM images of the as-prepared pure CuO NFs and CuO/Cu₂O NFs, respectively. Both of the images presented a large quantity of randomly deposited ultralong 1D NFs with the average diameter of about 170–190 nm and several micrometers in length. The individual NF was accumulated by CuO nanoparticles, which could offer more active sites for contact with a large amount of glucose and high surface energies for catalytic reactions. Further, the close inter-particle contacts and the vectorial charge-conduct passages on the ultralong 1D NFs may be favorable for the electron-transfer reactions in the electrode materials,²⁷ which could enhance their electrochemical properties. Otherwise, it could be observed that the nanoparticles of CuO/Cu₂O NFs in Fig. 2B were larger than those of pure CuO NFs in Fig. 2A, thus, the surface of CuO/Cu₂O NFs became smoother after the hydrothermal method. It could lead to a lower adsorption of the intermediates during the oxidation of glucose, generating a larger detection range.

Fig. 2 SEM images of pure CuO NFs (A) and CuO/Cu₂O NFs (B).

3.2. Electrocatalysis and amperometric responses of the modified electrodes

The electrocatalytic activities of the as-prepared electrodes towards the oxidation of glucose in alkaline medium were studied using cyclic voltammograms (CVs). Fig. 3A and B displayed the CVs of CuO/Cu₂O NFs and pure CuO NFs modified electrodes in 0.1 M NaOH in the absence (black curves) and presence (red curves) of 5 mM glucose. It could be observed that for both electrodes no oxidation peaks appeared in the absence of glucose, while single reduction peaks were presented at about 0.56 V, assigning to a Cu(III)/Cu(II) redox couple similar to previous reports.^20,30 When 5 mM glucose was added in NaOH solution, the response currents of both electrodes increased and exhibited obvious anodic oxidation peaks of Cu(II)/Cu(III) at about 0.6 V in response to the irreversible glucose oxidation to gluconic acid. Moreover, the increase of the anodic oxidation peak current for the CuO/Cu₂O NFs electrode was about 3 times higher than that for the pure CuO NFs electrode, indicating the improvement of electrocatalytic activity towards the oxidation of glucose on the CuO/Cu₂O NFs modified electrode.

Fig. 3 CVs of CuO/Cu₂O NFs (A) and pure CuO NFs (B) modified electrodes in 0.1 M NaOH in the absence (black curves) and presence (red curves) of 5 mM glucose with the scan rate of 100 mV s⁻¹; amperometric responses and corresponding calibration curves of CuO/Cu₂O NFs (C and E) and CuO NFs (D and F) modified electrodes with successive addition of 0.5 mM glucose into 0.1 M NaOH at +0.6 V vs. Ag/AgCl.

The amperometric response curves were investigated in 0.1 M NaOH solution with successive addition of 0.5 mM glucose at the applied potential of +0.6 V vs. Ag/AgCl. Fig. 3C and D showed that both CuO/Cu₂O NFs (C) and CuO NFs (D) modified electrodes could obtain the steady-state currents within 4 s, indicating that a rapid oxidation of glucose and fast electron transfer processes occurred on the surface of the electrodes. The corresponding calibration curves of CuO/Cu₂O NFs and CuO NFs modified electrodes were shown in Fig. 3E and F, respectively. It could be seen that the electrochemical performances of the CuO/Cu₂O NFs electrode were much better than those of the pure CuO NFs electrode. First, a much larger linear response region, up to 10 mM, was achieved for the CuO/Cu₂O NFs electrode, which could match the normal standard blood glucose level. Second, the sensitivity of the CuO/Cu₂O NFs electrode was about 830 μA mM⁻¹ cm⁻². It was about 3.36 times higher than that of the CuO NFs electrode, which was consistent with the CVs results. The key performance characteristics of some reported electrode materials for non-enzymatic glucose sensors based on CuO or/and Cu₂O were shown in Table 1, for comparison. It could be seen that the CuO/Cu₂O NFs electrode in this work not only had a relatively high sensitivity, but also possessed a low detection limit and a wide detection range. However, the linear ranges of the reported non-enzymatic glucose sensors based on CuO or Cu₂O were mostly very narrow, which could not meet the requirements of the routine tests.

Table 1 Comparison of the key performance characteristics of some reported electrode materials for non-enzymatic glucose sensors based on CuO or/and Cu₂O

Electrode	Sensitivity (μA mM^−1 cm^−2)	Detection limit (μM)	Linear range (up to mM)	Correlation coefficient	References
GO/CuO/GCE	262.5	0.69	2.03	0.9914	34
Nafion/CuO/GC electrode	404.5	1	2.55	0.999	22
CuO NFs/ITO electrode	873	0.04	1.3	0.9995	18
Cu2O/MWCNTs GCE	92.4	0.05	0.01	0.9958	35
Cu2O/Nf/GCE	155.0	1.3	0.35	—	36
Cu2O/SMWNTs GCE	2143	0.2	2.5	0.997	19
CuxO/PPy/Au electrode	232.2	6.2	8.0	0.994	37
CuxO/Cu electrode	1620	49	4.0	0.998	23
CuO/Cu2O NFs electrode	830	0.7	10	0.9991	This work

---

3.3. Plausible mechanism for the enhanced electrocatalytic oxidation of glucose at the CuO/Cu₂O NFs electrode

To further investigate the mechanism for the enhanced electrocatalytic oxidation of glucose at CuO/Cu₂O NFs electrode, the effect of scan rate on the oxidation and reduction peak currents was studied in 0.1 M NaOH containing 1 mM glucose at different scan rates from 40 to 160 mV s⁻¹ as depicted in Fig. 4A. It could be seen that the oxidation and reduction peak currents were proportional to the scan rates (Fig. 4B), indicating that the redox process at CuO/Cu₂O NFs electrode was a typical surface controlled electrochemical process similar to previous reports.¹⁹

Fig. 4 (A) CVs of the CuO/Cu₂O NFs modified electrode in 0.1 M NaOH in the presence of 1 mM glucose at different scan rates: 40, 60, 80, 100, 120, 140 and 160 mV s⁻¹; (B) the plot of oxidation and reduction peak currents at +0.6 V vs. Ag/AgCl versus scan rates; (C) CVs and (E) the corresponding differential curves of CuO/Cu₂O NFs (black curves) and pure CuO NFs (red curves) modified electrodes in 0.1 M NaOH in the absence of glucose with the scan rate of 100 mV s⁻¹; (D) CVs and (F) the corresponding differential curves of CuO/Cu₂O NFs (black curves) and pure CuO NFs (red curves) modified electrodes in 0.1 M NaOH in the presence of 5 mM glucose with the scan rate of 100 mV s⁻¹.

In order to facilitate comparative studies, the CVs of CuO/Cu₂O NFs (black curves) and pure CuO NFs (red curves) electrodes in 0.1 M NaOH, in the absence of glucose, with the scan rate of 100 mV s⁻¹ are presented in Fig. 4C. Fig. 4C showed that in the absence of glucose, the appearance of CV for CuO/Cu₂O NFs electrodes was very similar to that of pure CuO NFs electrodes. However, the enhanced oxidation and reduction currents indicated the better electrocatalytic properties at the surface of CuO/Cu₂O NFs electrode. To obtain a more visual representation of the current changes for both electrodes in the absence of glucose, the corresponding differential curves were displayed in Fig. 4E. The figure clearly presented two peaks during the scan from positive potential to negative potential, suggesting that two reduction processes might occur at the surface of the CuO/Cu₂O NFs electrode. In the absence of glucose, the possible redox processes for the CuO/Cu₂O NFs electrode could be summarized as follows:

Cu(I) − e ↔ Cu(II) (1)

Cu(II) − e ↔ Cu(III) (2)

The existence of Cu(I) and Cu(II) in the CuO/Cu₂O NFs electrode may lead to the transformation processes between different valences, such as those in formula (1) and (2), which could promote redox reactions and enhance the electrocatalytic properties. For the pure CuO NFs electrode, the transformation between Cu(II) and Cu(I) may be suppressed.

Fig. 4D and F revealed the CVs and corresponding differential curves of CuO/Cu₂O NFs (black curves) and pure CuO NFs (red curves) modified electrodes in 0.1 M NaOH in the presence of 5 mM glucose with the scan rate of 100 mV s⁻¹. As shown in Fig. 4D, CuO/Cu₂O NFs modified electrodes possessed a larger oxidation current, while the peak in the corresponding differential curves (in Fig. 4E) showed a negative shift compared to those of CuO NFs electrodes, illustrating a lower overpotential of the oxidation processes. These observations could also be attributed to the synergic effect of the multiple oxidation states in CuO/Cu₂O NFs.³¹ In the presence of glucose, the possible redox processes on the surface of the CuO/Cu₂O NFs electrode may be as follows:

Cu(III) + glucose → Cu(II) + gluconic acid (3)

glucose − e → gluconic acid (4)

Cu(II) + glucose → Cu(I) + gluconic acid (5)

Combining formulas (2) with (3), we could obtain formula (4), which was the final reaction towards the oxidation of glucose during the electrocatalytic processes. Generally, the mechanism for Cu-based glucose sensors was considered as the oxidation processes in formula (2) and (3). However, the reaction in formula (5), which could be derived from formula (1) and (4), had rarely been proposed for the electrocatalytic oxidation of glucose, since Cu(III) was more active than Cu(II). In fact, CuO could react with glucose and produce Cu₂O and gluconic acid in an aqueous solution, as reported by Cao et al.²⁹ Thus, we could deduce that the reaction in formula (5) may also participate in the electrocatalytic processes, which could enlarge the oxidation current and decrease the overpotential. However, the exact mechanism for the electrocatalytic oxidation of glucose in alkaline medium at the Cu-based electrode is still not very clearly known,³² and further studies still needs to be conducted on this issue.

Besides the effects of the multiple oxidation states of copper oxides in CuO/Cu₂O NFs, their nanostructure morphologies may also provide a high surface-to-volume ratio for the surface controlled electrochemical processes. However, from the SEM results, the surface of CuO/Cu₂O NFs was much smoother than that of pure CuO NFs, indicating their lower surface-to-volume ratio. Therefore, the enhanced sensitivity of CuO/Cu₂O NFs electrodes could be attributed to the more active catalytic sites due to the synergic effect of the multiple oxidation states. On the other hand, the smoother surface of CuO/Cu₂O NFs could decrease the adsorption of the intermediates, resulting in the wider linear range than that of pure CuO NFs. Furthermore, the close inter-particle contacts and the vectorial charge conductance passages along the ultralong 1D NFs may also facilitate the electron transfer from the materials surface to the electrode and promote the electrochemical reactions. The component, surface morphology, and the conductivity had significant effects on the electrochemical oxidation of glucose. Therefore, it is reasonable to further improve the performance of Cu-based electrode materials, and even other transition metals, by optimizing the above parameters.

3.4. Interference and stability study

Selectivity is very important for a sensor in practical applications. There are lots of co-existing interfering species with glucose in the human blood serum, such as ascorbic acid (AA), uric acid (UA), sucrose, lactose and soluble starch. Among these, AA and UA could be easily oxidized at a relatively positive potential, and consequently interfere with the detection of glucose.³³ The normal physiological level of glucose is about 3–8 mM, and the level of interfering species such as AA and UA is about 0.1 mM.¹⁸ In addition, most of the non-enzymatic glucose sensors based on precious metals or alloys can easily loose their activities due to poisoning with chloride ions.³² Therefore, interference tests of the CuO/Cu₂O NFs electrode were examined in 0.1 M NaOH solution with successive addition of 0.1 mM AA, 0.1 mM UA and 200 mM NaCl at the applied potential of +0.6 V vs. Ag/AgCl. As shown in Fig. 5, the response currents for these interfering species were insignificant, indicating that the CuO/Cu₂O NFs electrode had a good selectivity for glucose at the applied potential of +0.6 V vs. Ag/AgCl.

Fig. 5 Amperometric responses of the CuO/Cu₂O NFs electrode with successive addition of 1 mM glucose, 0.1 mM AA, 0.1 mM UA and 200 mM NaCl into 0.1 M NaOH at +0.6 V vs. Ag/AgCl.

4. Conclusions

In summary, CuO/Cu₂O NFs with high surface areas and ultralong 1D nanostructures had been successfully fabricated via an in situ reduction approach by using electrospun CuO NFs as precursors and glucose as a reducing agent. The electrochemical measurements indicated that CuO/Cu₂O NFs, as electrode materials, exhibited improved electrocatalytic activities for non-enzymatic glucose sensors. The improved performances could be attributed to the multiple redox couples, the more active catalytic sites and the surface morphology of the CuO/Cu₂O NFs. Therefore, it was expected that CuO/Cu₂O NFs with high electrocatalytic activities would greatly promote their practical applications as non-enzymatic glucose sensors.

Acknowledgements

The present work is financially supported by the National Basic Research Program of China (973 Program) (Grant no. 2012CB933703), the National Natural Science Foundation of China (no. 91233204, 51272041, 61201107, and 11304035), the 111 Project (no. B13013), the Fundamental Research Funds for the Central Universities (12SSXM001, 14ZZ2223), and the Program for Young Scientists Team of Jilin Province (20121802).

References

1. J. Wang, Chem. Rev., 2008, 108, 814–825.
2. V. Scognamiglio, Biosens. Bioelectron., 2013, 47, 12–25.
3. K. M. El Khatib and R. M. Abdel Hameed, Biosens. Bioelectron., 2011, 26, 3542–3548.
4. A. Heller and B. Feldman, Chem. Rev., 2008, 108, 2482–2505.
5. Q. Wu, L. Wang, H. J. Yu, J. J. Wang and Z. F. Chen, Chem. Rev., 2011, 111, 7855–7875.
6. T. M. Cheng, T. K. Huang, H. K. Lin, S. P. Tung, Y. L. Chen, C. Y. Lee and H. T. Chiu, ACS Appl. Mater. Interfaces, 2010, 2, 2773–2780.
7. J. Zhao, L. M. Wei, C. H. Peng, Y. J. Su, Z. Yang, L. Y. Zhang, H. Wei and Y. F. Zhang, Biosens. Bioelectron., 2013, 47, 86–91.
8. S. Park, H. Boo and T. D. Chung, Anal. Chim. Acta, 2006, 556, 46–57.
9. J. H. Yuan, K. Wang and X. H. Xia, Adv. Funct. Mater., 2005, 15, 803–809.
10. Q. Y. Wang, X. Q. Cui, J. L. Chen, X. L. Zheng, C. Liu, T. Y. Xue, H. T. Wang, Z. Jin, L. Qiao and W. T. Zheng, RSC Adv., 2012, 2, 6245–6249.
11. H. G. Nie, Z. Yao, X. M. Zhou, Z. Yang and S. M. Huang, Biosens. Bioelectron., 2011, 30, 28–34.
12. J. Wang, D. F. Thomas and A. Chen, Anal. Chem., 2008, 80, 997–1004.
13. J. Ryu, K. Kim, H. S. Kim, H. T. Hahn and D. Lashmore, Biosens. Bioelectron., 2010, 26, 602–607.
14. J. H. Shim, A. Cha, Y. Lee and C. Lee, Electroanalysis, 2011, 23, 2057–2062.
15. W. Wang, L. L. Zhang, S. F. Tong, X. Li and W. B. Song, Biosens. Bioelectron., 2009, 25, 708–714.
16. Y. Ding, Y. Wang, L. A. Su, M. Bellagamba, H. Zhang and Y. Lei, Biosens. Bioelectron., 2010, 26, 542–548.
17. Y. Q. Zhang, Y. Z. Wang, J. B. Jia and J. G. Wang, Sens. Actuators, B, 2012, 171, 580–587.
18. G. Y. Liu, B. Z. Zheng, Y. S. Jiang, Y. Q. Cai, J. Du, H. Y. Yuan and D. Xiao, Talanta, 2012, 101, 24–31.
19. X. M. Zhou, H. G. Nie, Z. Yao, Y. Q. Dong, Z. Yang and S. M. Huang, Sens. Actuators, B, 2012, 168, 1–7.
20. Y. Li, Y. Y. Wei, G. Y. Shi, Y. Z. Xian and L. T. Jin, Electroanalysis, 2011, 23, 497–502.
21. M. M. Liu, R. Liu and W. Chen, Biosens. Bioelectron., 2013, 45, 206–212.
22. E. Reitz, W. Z. Jia, M. Gentile, Y. Wang and Y. Lei, Electroanalysis, 2008, 20, 2482–2486.
23. C. L. Li, Y. Su, S. W. Zhang, X. Y. Lv, H. L. Xia and Y. J. Wang, Biosens. Bioelectron., 2010, 26, 903–907.
24. B. Q. Yuan, C. Y. Xu, L. Liu, Q. Q. Zhang, S. Q. Ji, L. P. Pi, D. J. Zhang and Q. S. Huo, Electrochim. Acta, 2013, 104, 78–83.
25. J. Song, L. Xu, R. Q. Xing, W. F. Qin, Q. L. Dai and H. W. Song, Sens. Actuators, B, 2013, 182, 675–681.
26. D. B. Luo, L. Z. Wu and J. F. Zhi, ACS Nano, 2009, 3, 2121–2128.
27. H. Wu, L. B. Hu, M. W. Rowell, D. S. Kong, J. J. Cha, J. R. McDonough, J. Zhu, Y. A. Yang, M. D. McGehee and Y. Cui, Nano Lett., 2010, 10, 4242–4248.
28. X. H. Sun, C. M. Zheng, F. X. Zhang, Y. L. Yang, G. J. Wu, A. M. Yu and N. J. Guan, J. Phys. Chem. C, 2009, 113, 16002–16008.
29. M. H. Cao, C. W. Hu, Y. H. Wang, Y. H. Guo, C. X. Guo and E. B. Wang, Chem. Commun., 2003, 1884–1885.
30. J. Zhang, X. L. Zhu, H. F. Dong, X. J. Zhang, W. C. Wang and Z. D. Chen, Electrochim. Acta, 2013, 105, 433–438.
31. Y. Q. Xie and C. O. Huber, Anal. Chem., 1991, 63, 1714–1719.
32. Z. J. Zhuang, X. D. Su, H. Y. Yuan, Q. Sun, D. Xiao and M. M. F. Choi, Analyst, 2008, 133, 126–132.
33. B. Fang, A. X. Gu, G. F. Wang, W. Wang, Y. H. Feng, C. H. Zhang and X. J. Zhang, ACS Appl. Mater. Interfaces, 2009, 1, 2829–2834.
34. J. Song, L. Xu, C. Y. Zhou, R. Q. Xing, Q. L. Dai, D. L. Liu and H. W. Song, ACS Appl. Mater. Interfaces, 2013, 5, 12928–12934.
35. X. J. Zhang, G. F. Wang, W. Zhang, Y. Wei and B. Fang, Biosens. Bioelectron., 2009, 24, 3395–3398.
36. L. Zhang, Y. H. Ni and H. Li, Microchim. Acta, 2010, 171, 103–108.
37. F. H. Meng, W. Shi, Y. N. Sun, X. Zhu, G. S. Wu, C. Q. Ruan, X. Liu and D. T. Ge, Biosens. Bioelectron., 2013, 42, 141–147.

---