ORIGINAL PAPER

Biosensor for atrazin based on aligned carbon nanotubesmodified with glucose oxidase

Qing Yang & Yongxia Qu & Yang Bo & Yin Wen &

Shasheng Huang

Received: 13 September 2009 /Accepted: 12 December 2009 /Published online: 21 January 2010# Springer-Verlag 2010

Abstract A sensitive method was developed for thedetermination of the hebicide atrazine. It based on the useof glucose oxidase that is self-assembled on aligned carbonnanotubes on the surface of a copper electrode. The surfacemorphology and electrochemical properties of the electrodewere characterized by field emission scanning electronmicroscopy and cyclic voltammetry. The effects of buffersolution and incubation time on the response of theelectrode were investigated. Response to atrazine is linearin the range from 0.58 µM to 42 µM, and the detectionlimit is 39 nM. The performance of the biosensor wasverified by determination of atrazine in environmentalwater samples.

Keywords Atrazine . Aligned carbon nanotubes . Glucoseoxidase . Detection

Introduction

Modern agriculture depends considerably on the use ofherbicides to achieve greater and greater crop yields. But,most of herbicides, with slow rates of degradation, are verystable and has caused serious environmental and healthproblems [1]. As an inhibitor of photosynthesis [2], atrazinehas been most widely used to control annual grasses andbroad -leaf weeds in agricultural areas [3]. However,

because of its toxicity and persistence atrazine inducespotential serious health consequences for human being andanimals [4].

The determination of atrazine at trace level can becarried out with several analytical techniques such as high-performance liquid chromatography (HPLC) [5], gaschromatography with mass spectrometry identification(GC/MS) [6], reflectometric interference spectroscopy [7],etc. Although these methods have high sensitivity, lowdetection limit and good stability, they require extraction, insome cases enrichment steps prior to determination, timeconsuming, very expensive for routine analysis andcomplex manipulation.

Enzyme-based biosensors for pollutant determinationhave caused the public interest due to their reliability, fastresponse, high sensitivity and selectivity. Enzyme-linkedimmunosorbent assay (ELISA) for determination of atria-zine herbicides has been become one of the most activestudies in recent years [8–11]. Several reports on theenzyme immobilization and inhibition of atrazine on theactivity of enzyme have been published [12, 13]. Anh et alused the tyrosinase biosensor to detect atrazine, using theinhibition of atrazine on the activity of enzyme [14]. Theconcentration of atrazine in aqueous can be determined dueto its inhibitory power toward to the catalytic activity ofPPO [15].

Carbon nanotube (CNT) is an important group ofnanomaterials. The transducer constructed by the carbonnanotubes can amplify the electrochemical signal of theproduct of the enzymatic reaction [16–20].

Aligned carbon nanotubes (ACNTs), one of the newestkinds of carbon-based materials, have more distinctadvantages. Compared with loose and random CNTs ithas a broad range of potential application, better mechanicalstability, vertical orientation and larger surface-to-volume

Electronic supplementary material The online version of this article(doi:10.1007/s00604-009-0272-x) contains supplementary material,which is available to authorized users.

Q. Yang :Y. Qu :Y. Bo :Y. Wen : S. Huang (*)Life and Environmental Science College,Shanghai Normal University,Shanghai 200234, People’s Republic of Chinae-mail: sshuang@shnu.edu.cn

Microchim Acta (2010) 168:197–203DOI 10.1007/s00604-009-0272-x

ratio [21]. As a better substrate for immoblizing enzyme,ACNTs provide a favorable well-aligned, densely arrayedand mutually separated surface orientation [22] and canfacilitate electron transfer between the active site ofimmobilized molecules and the underlying electrochemicaltransducer. Ye et al have fabricated aligned CNTs biosensorssuccessfully [23].

In this paper we have fabricated a modified Cu electrodebased on well-aligned CNTs. The electrochemical charac-teristics of atrazine on the modified electrode wereinvestigated by electrochemical methods and scanningelectron microscopy. The results showed that the excellentelectrochemical performance of the well-aligned CNTs fordirect electrochemical oxidation of glucose oxidase oncopper electrode. The biosensor was verified by thedetermination of organic pesticide atrazine in the environmentwater.

Experimental

Regents and apparatus

Multiwalled carbon nanotubules (MWNTs, diameter<10 nm; purity >95%; length, 0.5–500 μm) obtainedfrom Shenzhen Nanotech Port Co. Ltd. (Shenzhen,China, www.nanotubes.com.cn/docc/default.html) wereused directly without further purification. The alignedCNTs (ACNTs) were provided by YinWen [24]. In this work,the aligned CNTs 10 μm in length and 200 nm in diametergrew on a Ta plate was used. Glucose oxidase (GOx) waspurchased from Sigma. The stock solution of 5 mg mL−1

GOx was freshly prepared with a 0.1 M phosphate buffersolutions (pH 7.0). Atrazine and cetyltrimethylammoniumbromide (CTAB, 0.85 mg mL−1) were purchased fromChemical Reagent Company, Shanghai, China. Phosphatebuffer solutions (PBS) with various pH were prepared with0.1M Na2HPO4 and 0.1M NaH2PO4. All other chemicalsemployed in this study were of analytical grade. Doublydistilled water was used throughout the experiments.

Cyclic voltammetric and square-wave voltammetryexperiments were performed on a CHI660B electrochemi-cal workstation (Shanghai Chenhua Instrument Co., China).An electrochemical cell, consisting of a three-electrodesystem, a platinum wire as auxiliary electrode, a modifiedcopper electrode as working electrode and a saturatedcalomel electrode (SCE) as reference electrode was used.The test solutions were phosphate buffer solutions, whichwere deoxygenated with highly pure nitrogen for 10 minbefore electrochemical experiments and a nitrogen atmo-sphere was kept over the solution during measurements. Allthe electrochemical experiments were carried out at roomtemperament.

Preparation of atrazine biosensor

The preparation of the modified electrode based on thealigned CNTs was carried as following: The copper (Cu)electrode (ф=3 mm) was polished with alumina slurry,soaked successively in sulfuric aicd and ethanol to producea smooth, shiny surface, and then cleaned ultrasonically inwater, dried under nitrogen and used for modification, thenthe aligned CNTs film was immobilized on the Cu electrodewith electric gumwater. After the MWNTs -Cu electrodewas immersed in the CTAB (0.85 mg·mL−1) solutioncontaining GOx (5 mL·mg−1) for 4 h, the resulting GOx-aligned CNTs-Cu electrode was rinsed thoroughly withdouble-distilled water. The addition of the CTAB canpromote the immobilization of GOx on the surface of thealigned CNTs [25]. When not in use, the modified electrodewas stored in PBS (pH 7.0) solution at 4°C.

Procedure for detection of atrazine

The biosensor was soaked in the PBS buffer solution for20∼30 min until a stable baseline output signal wasreached. The atrazine (0.58 μM of the final concentration)substrate was added into the measurement vessel, the sensorwas incubated in the pollution solution for 7 min, then theatrazine was measured by the square-wave voltammetrymethod (SWV) with the voltage range form −0.65 toaround −0.3 V. Each experiment was repeated at least threetimes.

Results and discussion

Field emission scanning electron microscopyof the electrodes

The morphology of aligned-CNTs modified on the surfaceof Cu electrode has been investigated using field emissionscanning electron microscopy (SEM). The SEM micro-graphs showed that the nanotubes used in this paper havediameters ranging from 200 to 500 nm and a length ofabout 10 μm (Fig. 1a). It can be seen from Fig. 1 that thediameter of the pores distributes mainly in the range from100 to around 120 nm, the surface of the nanotubes issmooth, and most carbon nanotubes were well-aligned,densely arrayed and mutually separated surface orientationin the substrate.

From Fig. 1b, it can be seen that randomly aligned-CNTscover uniformly on the entire surface of the substrate. Afterdeposition of GOx, the surface of the aligned-CNTs becamerough, a large number of granular particles appeared on thesurface of the electrode compared with the pristine aligned-CNTs membrane, especially at the tips of the carbon

198 Q. Yang et al.

nanotubules, indicating that the GOx molecules are mod-ified on surface of the aligned- CNTs.

Electrochemical characteristics of GOx-aligned CNTs -Cuelectrode

Voltammetry behaviour of the aligned CNTs -Cu electrode

In order to evaluate the electrochemical properties of thealigned CNTs -Cu electrode, the electrochemical perfor-mance of the aligned-CNTs-Cu electrode was investigatedby cyclic voltammetry. As shown in Fig. 2, a pair ofremarkable peaks were observed (curve a in Fig. 2) onaligned-CNTs-Cu electrode and a peak potential separation(ΔEp) 71 mV was obtained. The peak potential separation(ΔEp) 71 mV indicated that the electrode performed fastelectron transfer and almost ideal-reversible electron trans-

fer reaction. The ΔEp value at well-aligned CNT electrodeis similar with well-aligned CNT- glass carbon electrode(70 mV) [20], smaller than that obtained at tangled-CNTsCu electrode (91 mV) (curve b in Fig. 2) and CNTselectrode (96 mV) reported by Li et al [26]. The resultsindicated that aligned- CNTs-Cu electrode retain moresignificant electrical conductivity than tangled CNTs andcan be used for biosensor with further modify.

Figure 3 represents the cyclic voltammetric responsesobtained from different modified Cu electrodes in 0.1 MPBS (pH 7.0) solution. No redox peaks were observed inthe CVs for the aligned CNTs-Cu electrode (curve a inFig. 3) in this potential range. However, when GOx wasmodified on the aligned CNTs-Cu electrode (denoted asGOx-aligned CNTs-Cu), a remarkable peak was observed(curve c in Fig. 3), and the oxidation peak and reductionpeak potential were −0.444 V and −0.483 V at scan rate of

0.5 0.4 0.3 0.2 0.1 0.0 -0.1-30

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Fig. 2 Cyclic voltammograms of aligned-CNTs-Cu electrode (a)tangled-CNTs-Cu electrode (b) in 4 mM [Fe(CN)6]

3−/4−. Scan rate:100 mv·s−1

Fig. 1 SEM images of thedifferent modifiers: (a), aligned-CNTs membrane; (b), GOX-aligned-CNTs membrane

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Fig. 3 Cyclic voltammetric of the aligned CNTs-Cu electrode (a)tangled CNTs-Cu electrode (b) and GOx-aligned CNTs-Cu electrode(c) in 0.1 M PBS (pH 7.0) solution. Scan rate: 100 mv·s−1

Biosensor based on glucose oxidase modified aligned carbon nanotubes 199

100 mV·s−1, respectively. That the current of peaks of theGOx-aligned CNTs-Cu electrode was larger than that of atangled CNTs-Cu electrode (curve b in Fig. 3) suggestedthat the direct electron transfer have been achieved betweenGOx and the surface of electrode, and that aligned CNTs bemuch more effective for mediating electron transfer thantangled CNTs. This is probably due to the apparent surfacearea of an aligned CNTs is larger than that of a tangledCNTs electrode. That the background current of curve c isslightly larger than that of curve b may be also partially dueto their well- aligned, which can promote the direct electrontransfer faster.

The CVs of GOx-MWNTs-GC electrode were investi-gated in PBS solution (pH 6.8), with the increase of thescan rate the peak-to-peak separation remains constant of36 mV (Fig. 4), and both the anodic and cathodic peakcurrents are linear with the scan rates from 20 mV·s−1 to150 mV·s−1. The dependence curve of the peak currents onthe scan rate (Figure not shown), suggesting an adsorptionreaction model [27]. The equations of peak current and scanrate of the oxidation and reduction peaks were y(μA)=0.1280 × (mV) + 6.47 (relative coefficient, r=0.9977) and y(μA)=−0.1148× (mV) + 1.211 (r=0.9975), respectively.

Square wave voltammetry (SWV) of GOx-aligned CNTs -Cuelectrode

SWV is a sensitive and rapid pulse electrochemicaltechnique because of its ability to scan the potential rangeof interest over a mercury drop [28]. For obvious reasons(sensitivity, rapidity), square wave voltammetry will givesubstantial improvements for determination of atrazine.

The SWV curves of modified electrodes in differentsolution were investigated. No peak was observed on the

aligned CNTs -Cu, but an obvious peak on GOx-alignedCNTs -Cu electrode in PBS (pH 7.0) appeared when thepotential were scanned from −0.3 to −0.7 V. However, adecrease of peak current was observed on the modifiedelectrode when atrazine was added into the PBS solution.That the response current of GOx-aligned CNTs −Cuelectrode decreased linearly with increasing the concentra-tion of atrazine can be adopted to determine atrazine.

Generally, the current response of biosensor for thepesticides was achieved by the analytes participating in thereaction between enzyme and a substrate. Viswanathan andhis coworkers reported determination of methyl parathionand chlorpyrifos pesticides using a biosensor based oninhibition of these pesticides on the reaction betweenACHE and acetylcholine [29]. Here, the atrazine can beanalyzed under the condition of without substrate. In thissituation, SWV responses of the biosensor were based onthe inhibition of enzyme activity by pesticide added in thetest solution. The percentage inhibition of the biosensor canbe calculated with Eq. (1) [29]:

Inhibition %ð Þ ¼ Io� Isð Þ=Io½ � � 100 ð1Þwhere Io and Is were peak currents of 0.1M PBS bufferwithout and with atrazine inhibition respectivily. Aninhibition of atrazine 6% calculated according to theEq. (1) can be obtained when the concentration of atrazinewas 1.056×10−5M.

Analytical application

Optimum conditions for analysis

Influence of buffer solution

Different supporting electrolytes, such as Na2HPO4—citricacid, phosphate buffer, Tris buffer and citric acid—sodiumcitrate at same pH were tested in the voltammetric analysisof GOx-aligned CNTs -Cu electrode. The obtained dataindicated that the best response was obtained in phosphatebuffer.

The reaction of enzyme takes place in certain range ofpH 4.5–pH 9.0 [30]. The medium pH affects both theenzyme activity and the optimum of operating potential incase of the substrates electron transfer. The current responseof GOx-aligned CNTs -Cu electrode was measured in therange of pH 5.0–9.0 in 0.1 M phosphate buffer. The resultsshowed that the response current increased at first withincreasing pH of solution, the maximum response wasobtained at pH 7.0. But the current decreased when pH ofthe solution was larger than 7.0. In further experiments aphosphate buffer (pH 7.0) is recommended as suitable formost practical situations.

-0.2 -0.3 -0.4 -0.5 -0.6 -0.7-20

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Fig. 4 Cyclic voltammograms of GOx-MWNTs-GC electrode in atdifferent scan rates PBS solution (pH 7.0)

200 Q. Yang et al.

Influence of the incubation time in atrazine solutions

The inhibition time is another experimental parameteraffecting the response of the electrode. Evaluation of theinhibition time is very important for off-line measurementsand on-site analyses. If the inhibition time is insufficient theenzymatic activity can not be fully inhibited. In this case,the analyte at low concentration can not be detected. Incontrast, a long exposure time to the sample solution maydramatically damage the structure and properties of theenzyme. Moreover, the incubation time is a compromisebetween biosensor sensitivity and general time of analysis[14].

Effect of Inhibition time on response behavior of electrode

Inhibition time is an important parameter affecting thecharacteristics of the electrode. If the inhibition time isinsufficient the enzymatic activity is not fully inhibited andthe pollutant at very low concentrations may not bedetected [14]. The inhibition effect of atrazine on GOx isshown in Fig. 5. It can be seen in Fig. 5 that the optimalinhibition time for atrazine was 7 min.

Analytical performance of the electrode

Under the selected optimal conditions, the SWV responseof the biosensor for atrazine was investigated by succes-sively adding atrazine to PBS solution. The results showed

that current response decreased with increasing concentra-tion of atrazine. There was a excellent linear relation of thecurrent response and concentration of atrazine between5.80×10−7and 4.22×10−5M, the linear regression equationis i(μA)=3.096−0.008960 [Atrazine] (μM) with a relativecoefficient, r=0.9992, the detection limit, taken as theconcentration that gave a signal equal to three times thestandard deviation of the blank signal, was 3.9×10−8M. Intotal, the analytical performance of the electrode wascomparable to other recently report [12].

Interferences

Some common herbicides and heavy metal ions wereexamined as possible interferents in the determination of1.056×10−5M atrazine by adding the appropriate amountsof interferent to the test solution. Interference was taken asthe level causing an error in excess of 10%. The resultsshowed that a 1000-fold excess of 2, 4, 6-trichlorophenol,phenol and paraquat did not interfere the determination ofatrazine. However, Pb2+and Cu2+ interfered in concentra-tions up to 30 times interfered in the determination ofatrazine, possibly because they can inhibit the activity ofGOx.

Determination of atrazine

The GOx-aligned CNTs -Cu electrode can be used for thedetermination of atrazine by SWV. The precision of theSWV method was assessed by repeated measurements for1.58×10−5M atrazine, the relative standard deviation was0.7% (n=5). The biosensor retained more than 91% of itsinitial response to the atrazine after the electrode was usedfor two weeks, indicating that the GOx was immobilizedfirmly on the surface of aligned CNTs -Cu electrode. Whennot use, the enzyme electrode was stored in PBS (pH 7.0) at4°C.

The analytical applicability of the biosensor was evalu-ated by using a standard addition method (Sample N1andN2). Atrazine in the waste–water (Sample N3, N4, N5) inthe river near Shanghai suburb was determined using theGOx-aligned CNTs -Cu electrode. The water sampleswhich from different places of the river were settled for

Fig. 5 Evolution of the residual GOx activity versus inhibition timein solution of 1.056×10−5M atrazine

Sample Detection/ μMa R.S.D% Addition / μM Found / μM Recovery / %

1 0 4.5 4.37 97

2 0 27.0 27.5 101

3b 3.82 5.1 5 8.62 98

4b 3.15 4.2 10 13.28 101

5b 3.71 6.4 10 13.45 98

Table 1 Analytical results ofatrazine in waste-water samples

a Average value for threedetectionsb Sample N1 and N2 are synthe-sized samples; N3, N4 and N5are waste-water samples

Biosensor based on glucose oxidase modified aligned carbon nanotubes 201

2 h and filtrated. The pH of samples was adjusted to 7.0with HCl (0.1M) or NaOH (0.1M) before adding PBS, andthen the SWV response was recorded. The averageconcentration of atrazine in this river is 3.56 μM. Theresults were satisfactory with an average recovery of 99 %as listed in Table 1.

Conclusion

In summary, this work describes a biosensors based onGOx-aligned CNTs -Cu electrode for the determination ofatrazine. The GOx can be easily self-assembled on thealigned carbon nanotubules surface of the Cu electrode.The detection limit of the biosensor was as low as 3.9×10−8M. The method can be used in the determination ofatrazine in waste–water, and provide an alternative methodfor the detection of atrazine.

Acknowledgments This work was supported by the National High-tech R&D program (863 program, 2007AA06Z402), Project of theFoundation of Shanghai Municipal Government (08520510400),Shanghai Leading Academic Discipline Project (S30406), LeadingAcademic Discipline Project of shanghai Normal University(DZL706)and Key Laboratory of Resource Chemistry of Ministry of Education.

References

1. Zacco E, Pividori MI, Alegret S, Galve R, Marco M-P (2006)Electrochemical magnetoimmunosensing strategy for the detectionof pesticides residues. Anal Chem 78:1780

2. Hottenstein CS, Rubio FM, Herzog DP, Fleeker JR, Lawruk TS(1996) Determination of trace atrazine levels in water by asensitive magnetic particle-based enzyme immunoassay. J AgricFood Chem 44:3576

3. Wittmann C, Schreiter PY (1999) Analysis of terbuthylazine insoil samples by two test strip immunoassay formats usingreflectance and luminescence. J Agric Food Chem 47:2733

4. Graziano N, McGuire MJ, Roberson A, Adams C, Jiang H, BluteN (2006) 2004 National atrazine occurrence monitoring programusing the abraxis ELISA method. Environ Sci Technol 2006(40):1163

5. Steinwandter H (1991) Contributions to residue analysis in soils.II.Miniaturization of the on-line extraction method for the determina-tion of some triazine compounds by RP-HPLC. Fresenius’ J AnalChem 339:30

6. Thurman EM, Meyer M, Pomes M, Perry CA, Schwab AP (1990)Enzyme-linked immunosorbent assay compared with gas chroma-tography/mass spectrometry for the determination of triazineherbicides in water. Anal Chem 62:2043

7. Mouvet C, Amalric L, Broussard S, Lang G, Brecht A, GauglitzG (1996) Reflectometric interference spectroscopy for thedetermination of atrazine in natural waters. Environ Sci Technol30:1846

8. Mosiello L, Cremisini C, Segre L, Chiavarini S, SpanoM,Kimmel T,Baumner AJ, Schmid RD (1998) Dipstick immunoassay format foratrazine and terbuthylazine analysis in water samples. J Agric FoodChem 46:3847

9. Sanvicens N, Pichon V, Hennion MC, Pr MM (2003) Preparationof antibodies and development of an enzyme-linked immunosor-bent assay for determination of dealkylated hydroxytriazines. JAgric Food Chem 51:156

10. Dankwardt A, Hock B, Simon R, Freitag D, Kettrup A (1996)Determination of non-extractable triazine residues by enzymeimmunoassay: investigation of model compounds and soil fulvicand humic acids. Environ Sci Technol 30:3493

11. Parellada J, Narváez A, López MA, Domnguez E, Fernández JJ,Pavlov V, Katakis I (1998) Amperometric immunosensors andenzyme electrodes for environmental applications. Anal ChimActa 362:47

12. Bjarnason B, Chimuka L, Önnerfjord P, Eremin S, Jönsson J-A,Johansson G, Emnéus J (2001) Enzyme flow immunoassay usinga Protein G column for the screening of triazine herbicides insurface and waste water. Anal Chim Acta 426:197

13. Roberge MT, Hakk H, Larsen G (2006) Cytosolic and localizedinhibition of phosphodiesterase by atrazine in swine tissuehomogenates. Food Chem Toxicol 44:885

14. Tuan MA, Sergei V, Dzyadevych MCV, Nicole JR, Chien ND,Jean-Marc C (2004) Conductometric tyrosinase biosensor for thedetection of diuron, atrazine and its main metabolites. Talanta63:365

15. Mazzei F, Botre F, Lorenti G, Simonetti G, Porcelli F, Scibona G,Botre C (1995) Plane tissue electrode for the determination ofatrazine. Anal Chim Acta 316:79

16. Riu J, Maroto A, Rius FX (2006) Nanosensors in environmentalanalysis. Talanta 69:288

17. Wang J, Kawde AN, Jan MR (2004) Carbon-nanotube-modifiedelectrodes for amplified enzyme-based electrical detection ofDNA hybridization. Biosens Bioelectron 20:995

18. Joshi PP, Merchant SA, Wang Y, Schmidtke DW (2005)Amperometric Biosensors Based on Redox Polymer-CarbonNanotube-Enzyme Composites. Anal Chem 77:3183

19. Shik CT, Davis JJ, Green MLH, Hill HAO, Leung YC, Sadler PJ,Sloan J, Xaviers AV (1998) The immobilisation of proteins incarbon nanotubes. Inorg Chim Acta 272:261

20. Tang H, Chen JH, Nie LH, Yao SZ, Kuang YF (2006)Electrochemical oxidation of glutathione, at well-aligned carbonnanotube array electrode. Electrochimica Acta 51:3046

21. Yang J, Zhang RY, Xu Y, He PG, Fang YZ (2008) Directelectrochemistry study of glucose oxidase on Pt nanoparticle-modified aligned carbon nanotubes electrode by the assistance ofchitosan–CdS and its biosensoring for glucose. ElectrochemCommun 10:188

22. Ye XR, Chen LH, Wang C, Aubuchon JF, Chen IC, Gapin AI,Talbot JB, Jin S (2006) Electrochemical modification of verticallyaligned carbon nanotube arrays. J Phys Chem 110:12938

23. Ye JS, Wen Y, Zhang WD, Cui HF, Xu GQ, Sheu FS (2005)Electrochemical biosensing platforms using phthalocyanine -functionalized carbon nanotube electrode. Electroanalysis17:89

24. Wen Y, Ye JS, Zhang WD, Shen FS, Xu GQ (2008) Electro-catalytic oxidation of methanol on a platinum modified carbonnanotube electrode. Microchim Acta 162:235

25. Yan YM, Zheng W, Zhang M, Wang L, Su L, Mao L (2005)Bioelectrochemically functional nanohybrids through co-assembling of proteins and surfactants onto carbon nanotubes:facilitated electron transfer of assembled proteins with enhancedfaradic response. Langmuir 21:6560

26. Li J, Cassell A, Delzeit L, Han J, Meyyappan M (2002) Novelthree-dimensional electrodes: electrochemical properties of carbonnanotube ensembles. J Phys Chem B 106:9299

27. Bard AJ, Faulkner LR (1980) Electrochemical methods. Wiley,New York, 519

202 Q. Yang et al.

28. Walcarius A, Lamberts L (1996) Square wave voltammetricdetermination of paraquat and diquat in aqueous solution.Electroanal Chem 406:59

29. Viswanathan S, Radecka H, Radecki J (2009) Electrochemicalbiosensor for pesticides based on acetylcholinesterase immobi-lized on polyaniline deposited on vertically assembled carbon

nanotubes wrapped with ssDNA. Biosens Bioelectron 24:2772–2777

30. Sandberg RG, Van Houten LJ, Schwartz JL, Bigliano RP, Dallas SM,Silvia JC, Cabelli MA, Narayanswamy V (1992) In: Mathewson PR,Finley JW (eds) Biosensor design and application; ACS SymposiumSeries 511. American Chemical Society, Washington, pp 81–88

Biosensor based on glucose oxidase modified aligned carbon nanotubes 203