Electrochemistry Communications 13 (2011) 363–365

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Electrochemistry Communications

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Aligned SWCNT-copper oxide array as a nonenzymatic electrochemical probeof glucose

Feng Jiang a, Shun Wang a,⁎, Juanjuan Lin a, Huile Jin a, Lijie Zhang a, Shaoming Huang a, Jichang Wang b,⁎a Nano-materials and Chemistry Key Laboratory, Wenzhou University, Wenzhou, Zhejiang 325035, Chinab Department of Chemistry and Biochemistry, University of Windsor, ON, Canada N9B 3P4

⁎ Corresponding authors. Fax: +86 577 88373111.E-mail addresses: shunwang@wzu.edu.cn (S.Wang),

1388-2481/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.elecom.2011.01.026

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 January 2011Received in revised form 25 January 2011Accepted 26 January 2011Available online 4 February 2011

Keywords:SWCNT arrayCopper oxidesNanoparticlesGlucoseElectrochemical sensor

Copper oxide nanoparticles (COs) were electrochemically deposited on horizontally aligned single-walledcarbon nanotube arrays (COs-aSWCNT) on SiO2/Si wafer, where X-ray photoelectron spectroscopy shows thatcompositions of COs are Cu2O and CuO with a ratio of 2 to 1. The as-prepared COs-aSWCNT electrodeexhibited synergistic electrocatalytic activity on the oxidation of glucose in alkaline media with a rapidresponse time of less than 2 s and a high sensitivity of 16.2 μA μM−1. This new sensor has two useful linearregions of glucose concentration and has an experimental detection limit of 20 nM. In the presence ofphysiological level ascorbic acid (0.1 mM), the experimental detection limit of glucose increases to 50 nMwith a sensitivity of 1.31 μA μM−1.

jwang@uwindsor.ca (J. Wang).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, carbon nanotubes (CNTs) have shown greatpotential as a sensing element due to their unique advantagesincluding good biocompatibility, enhanced electronic properties, andrapid electrode kinetics [1,2]. Moreover, CNTs significantly increasethe apparent electroactive area and thus lead to higher signalintensity of the electrode [3]. The introduction of CNTs, includingboth single-walled carbon nanotube (SWCNT) and multi-walledcarbon nanotube (MWCNT), into electrochemical sensors has indeedresulted in dramatic evolution in the detection of various organicsubstances such as carbohydrate [4–9]. Aligned SWCNT arrays couldavoid percolation transport pathways and thus have the advantagesover the random networks [10,11]. SWCNT can also avoid variousfeatures that are undesirable for practical applications such as unusualscaling of device properties, tube/tube junction resistances, etc. [12].

Combination of CNTs with copper oxide nanoparticles has recentlybeen successfully applied as an enzyme-free glucose sensor [13–18].These electrodes show great sensitivity, reproducibility and stability.The Cu2O/MWCNT nanocomposites prepared by a new fixtured-reduction method showed higher sensitivity and lower detectionlimit as compared to the CuO/MWCNT [9,18]. To utilize the

advantages of SWCNT and CuO/Cu2O nanoparticles (NPS), in thispaper a novel COs-aSWCNT electrode was prepared electrochemical-ly. Notably, the as-prepared electrode exhibits a faster response and alower LOD in glucose analysis than those of existing reports.

2. Experimental

The superlong well-oriented single-walled carbon nanotube arrayon SiO2/Si wafers was prepared according to our earlier study [19].The deposition of copper oxide NPs on the SWCNT array was achievedvia using a three-electrode system by cyclic voltammetry (CV)scanning from +0.6 to −0.6 V, in which a Pt wire and a saturatedAg|AgCl electrode were used respectively as the auxiliary andreference electrodes. The deoxygenated solution contained 0.1 MNa2SO4 and 1.0 mM CuSO4. Our experiments indicated that 30 cyclesat a rate of 100 mVs−1 are the optimal parameters for obtaining COs-aSWCNT electrodes that have a great sensitivity and reproducibility.The as-prepared COs-aSWCNT electrode was used without furthertreatment.

All reagents used in this study were of analytical grade. D-(+)-Glucose, Na2SO4, CuSO4, NaOH, ascorbic acid (AA) and uric acid (UA)were purchased from Alfa Aesar. Deionized water was prepared witha Milli-Q system (18.2 MΩ cm, Millipore, USA). SEM images weretaken on a Nova NanoSEM 200 (FEI, Inc.). XPS measurements werecarried out on an ESCALABMarkII photoelectron spectrometer using amonochromatic Mg Kα X-ray source (VG Company, U.K). Electro-chemical experiments were performed at room temperature with anAutolab PGSTAT30 electrochemical workstation (France).

Fig. 2. Cyclic voltammograms measured with (A) SWCNT array electrode and (B) COs-aSWCNT electrode in a 0.02 M NaOH solution without (curve a) or with (curve b) thepresence of glucose (0.8 mM).

364 F. Jiang et al. / Electrochemistry Communications 13 (2011) 363–365

3. Results and discussion

SEM images in Fig. 1A illustrate that the obtained SWCNTs are over200 μm in length and lay parallel on the SiO2/Si substrate. Since theseSWCNTs are apart from each other, it is reasonable to consider everySWCNT as an isolated electrode. Moreover, because of the insulatornature of SiO2, the SWCNT array shall act like arrayed electrodes.Fig. 1B shows the SWCNT array after the deposition of copper oxideNPs. The visible increase in the diameter and surface roughness ofthese SWCNTs suggests the successful deposition, which is furtherconfirmed by XPS measurement in Fig. 1C. The binding energies at952.7 eV and 932.5 eV are attributed to Cu 2p1/2 and Cu 2p3/2,respectively. The Cu 2p3/2 peak can be further deconvoluted into twopeaks at 932.9 and 935.4 eV with an atom ratio of about 4 to 1, whichmay be attributed respectively to Cu+ and Cu2+, suggesting that thecopper oxide NPs are the mixture of Cu2O and CuO with a ratio of 2 to1. Peaks at 940.3 and 943.5 eV are the satellite peaks of Cu2+ 2p1/2.

Fig. 2 presents the electrochemical oxidation of glucose investi-gated with (A) a bare SWCNT array electrode, and (B) the as-preparedCOs-aSWCNT electrode. For the SWCNT array electrode glucose onlycaused a slight increase in the anodic current at the potential above0.5 V, whereas in the COs-aSWCNT electrode there was a significantincrease in the anodic current in the potential range from 0.1 V to theend of positive scan. This measurement suggests that the COs-aSWCNT has excellent electrocatalytic activity on glucose oxidation. Apeculiar behavior with the CO-aSWCNT electrode is that the cathodiccurrent is also greatly enhanced. Similar phenomena have also beenobserved in the oxidation of glucose by nickel oxides or MnO2

electrodes [8,20]. The cathodic current was subsequently character-ized by varying the scanning rate from 10 to 90 mV/s, where the peakcurrent yields a linear relationship with the scanning rate, suggestingthat the reduction is surface-controlled [21]. The surprising increasein the cathodic current may thus result from the increase of the activesites on the electrode surface that were originally occupied byhydroxide ions, but became available due to the enhanced consump-tion of hydroxide by the electrocatalytic oxidation of glucose. Moresystematic investigation is required to reach a conclusive explanation.

Fig. 1. SEM images of SWCNT array on SiO2/Si wafer (A) before, and (B) afterelectrochemically depositing Cu2O/CuO NPs. Insets in (A) and (B) aremagnified images.(C) is the XPS spectra of COs-aSWCNT.

Fig. 3A characterized the response current at different potentials. Itdemonstrated that as the applied potential was increased from −0.4to 0.8 V, the response current increased gradually. The maximumresponse current with a good signal/noise ratio was achieved at 0.8 V.In Fig. 3B, the anti-interference ability of this new electrode againstthe commonly coexisting species such as UA and AA were examinedat potentials: (a) 0.65, (b) 0.7, and (c) 0.8 V. The selectivity towardsglucose was tested against the normal physiological level of AA

Fig. 3. (A) Dependence of the response current on the applied potential; (B) Interferenceexperimentsmeasured at the appliedpotential of (a) 0.65, (b) 0.7, and (c) 0.8 V vsAg|AgCl|KCl (sat). In experiment A each time 0.8 mMglucosewas added, whereas in experiment Beach time 1.0 mM glucose, 1.0 mM UA or 0.1 mM AA was added into 50 mL of 0.02 MNaOH solution. The amount of solution added each time is less than 20 μL.

365F. Jiang et al. / Electrochemistry Communications 13 (2011) 363–365

(0.1 mM) [22]. This study shows that the interference current fromUAis negligible, whereas the current increase induced by AA is about1.5%, 15.4%, and 7.8% respectively at the three potentials studied.

Fig. 4A presents the amperometric response of a COs-aSWCNTelectrode to the successive addition of glucose. The plot shows that asteady signal could be attained in less than 2 s and the experimentaldetermination limit is less than 20 nM. This represents considerableimprovements [23–27]. The corresponding calibration curve in Fig. 4Bhas two linear regions of glucose concentration, which are, respec-tively, in 20 nM–117 μM with a correlation coefficient 0.994 andbetween 117 μM and 800 μM with a correlation coefficient 0.998.Notably, in the two linear regions of glucose concentration, the lineardependence of response current gives rise to a sensitivity of 16.2 μAμM−1 and 6.5 μA μM−1. Under the same conditions, sixteensuccessive measurements of glucose with one COs-aSWCNT electrodeyielded a relative standard deviation (RSD) of 4.7%, indicating that thissensor has a satisfactory reproducibility. The glucose detection limitwas also tested in the presence of the physiological level of AA. It wascarried out by comparing the difference in the response current when0.1 mMAA or 0.1 mMAA+glucose was added. Specifically, in curve aof Fig. 4C only AA was added each time, whereas in curve b a mixture

Fig. 4. (A) The response of the COs-aSWCNT electrode to successive addition of glucosefrom 20 nM to 25 μM; (B) The linear relationship between the catalytic current andglucose concentration; and (C) Current–time profile in which (a) only 0.1 mM AA wasadded each time; and (b) a mixture of 0.1 mM AA and 50 nM, 70 nM, or 90 nM ofglucose was added consecutively. The inset calculates the difference in the responsecurrent as a function of glucose concentration.

of glucose+AAwas added each time. The inset in Fig. 4C shows that alinear relationship exists between 50 and 90 nM with a sensitivity of1.31 μA μM−1 (R=0.999). Here, the experimental detection limit isaround 50 nM.

4. Conclusions

In this paper, a new COs-aSWCNT electrode was prepared andinvestigated as a potential nonenzymatic glucose sensor. Thecombination of Cu2O/CuO NPs and SWCNTs produced synergisticelectrocatalytic activity on glucose oxidation. As a result, this hybridelectrode exhibited an ideal rapid response and a lower detectionlimit than existing non-enzymatic glucose sensors. The significantlyimproved performance is most likely arising from the improvement ofthe electrochemical property of the electroactive surface area. After6 month storage at room temperature, the response current decayedby 6.1%, confirming the long-term storage stability of COs-aSWCNTelectrodes. In addition, this COs-aSWCNT electrode has also shownattractive features in terms of resistance to poisoning in a glucosesolution. The preliminary exploration conducted with real serumsamples suggests that this modified array electrode may be employedas an efficient amperometric sensor for routine glucose analysis inclinical samples.

Acknowledgements

This work is supported through the NSFC (21073133 and20843007) and the Zhejiang Provincial Natural Science Foundationof China (Y4080177, Y4090248, and Y5100283).

References

[1] D. Eder, Chem. Rev. 110 (2010) 1348.[2] C.B. Jacobs, M.J. Peairs, B.J. Venton, Anal. Chim. Acta 662 (2010) 105.[3] A.F. Holloway, D.A. Craven, L. Xiao, J.D. Campo, G.G. Wildgoose, J. Phys. Chem. C

112 (2008) 13729.[4] V. Abhay, H. Nighat, T. Chi, L. Donal, R. James, Electrochem. Commun. 11 (2009)

2004.[5] J. Chen, W.D. Zhang, J.S. Ye, Electrochem. Commun. 10 (2008) 1268.[6] L. Meng, J. Jin, G.X. Yang, T.H. Lu, H. Zhang, C.X. Cai, Anal. Chem. 81 (2009) 7271.[7] H.J. Lee, S.W. Yoon, Y.J. Kim, J. Park, Nano Lett. 7 (2007) 778.[8] C.Z. Zhao, C.L. Shao, M.H. Li, K. Jiao, Talanta 71 (2007) 1769.[9] X.J. Zhang, G.F. Wang, W. Zhang, Y. Wei, B. Fang, Biosens. Bioelectron. 24 (2009)

3395.[10] E. Artukovic, M. Kaempgen, D.S. Hecht, S. Roth, G. Grüner, Nano Lett. 5 (2005) 757.[11] S. Kumar, J.Y. Murthy, M.A. Alam, Phys. Rev. Lett. 95 (2005) 0668021.[12] M.S. Fuhrer, J. Nygard, L. Shih, M. Forero, Y.-G. Yoon, M.S.C. Mazzoni, H.J. Choi, J.

Ihm, S.G. Louie, A. Zettl, P.L. McEuen, Science 288 (2000) 494.[13] K.E. Toghill, R.G. Compton, Int. J. Electrochem. Sci. 5 (2010) 1246.[14] C.B. McAuley, Y. Du, G.G. Wildgoose, R.G. Compton, Sens. Actuators B Chem. 135

(2008) 230.[15] W. Wang, L. Zhang, S. Tong, X. Li, W. Song, Biosens. Bioelectron. 25 (2009) 708.[16] Z. Zhuang, X. Su, H. Yuan, Q. Sun, D. Xiao, M.M.F. Choi, Analyst 133 (2008) 126.[17] C.B. McAuley, G.G. Wildgoose, R.G. Compton, L.D. Shao, M.L.H. Green, Sens.

Actuators B Chem. 132 (2008) 356.[18] L.C. Jiang, W.D. Zhang, Biosens. Bioelectron. 25 (2010) 1402.[19] S.M. Huang, Y. Qian, J.Y. Chen, Q.R. Cai, L. Wan, S. Wang, W.B. Hu, J. Am. Chem. Soc.

130 (2008) 11860.[20] J. Chen, W.D. Zhang, J.S. Ye, Electrochem. Commun. 10 (2008) 1268.[21] A.J. Bard, L.R. Faulkner, Electrochemical Methods, John Wiley & Sons, 2001.[22] B.K. Jena, C.R. Raj, Chem. Eur. J. 12 (2006) 2702.[23] Z.G. Wang, Y. Wano, H. Xu, G. Li, Z.K. Xu, J. Phys. Chem. C 113 (2009) 2955.[24] X.E. Wu, F. Zhao, J.R. Varcoe, A.E. Thumser, C. Avignone-Rossa, R.C.T. Slade,

Bioelectrochemistry 77 (2009) 64.[25] K. Zhao, S.Q. Zhuang, Z. Chang, H. Songm, L.M. Dai, P.G. He, Y.Z. Fang,

Electroanalysis 19 (2007) 1069.[26] J.C. Clauseen, A.D. Franklin, A. Haque, D.M. Porterfield, T.S. Fisher, ACS Nano 3

(2009) 37.[27] S. Cosnier, R.E. Ionescu, M. Holzinger, J. Mater. Chem. 18 (2008) 5129.