Electrochemistry Communications 12 (2010) 557–560

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Electrochemistry Communications

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Nanomolar detection of dopamine in the presence of ascorbic acidat b-cyclodextrin/graphene nanocomposite platform

Lin Tan, Kai-Ge Zhou, Yong-Hui Zhang, Hang-Xing Wang, Xue-Dong Wang, Yun-Fan Guo, Hao-Li Zhang *

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 12 November 2009Received in revised form 20 January 2010Accepted 27 January 2010Available online 1 February 2010

Keywords:Graphene sheetb-CyclodextrinDopamineElectrochemical sensorNanocomposite

1388-2481/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.elecom.2010.01.042

* Corresponding author. Tel./fax: +86 931 8912365E-mail address: Haoli.Zhang@lzu.edu.cn (H.-L. Zha

Nanocomposite of b-cyclodextrin and graphene sheet (b-CD/GS) was successfully prepared, which exhib-ited high stability in aqueous solution. When used in electrochemical detection of dopamine, the b-CD/GSmodified carbon electrode showed low detection limit, broad linear range, along with good ability to sup-press the background current from large excess ascorbic acid. The electrochemical reaction of dopamineon the b-CD/GS showed a mass diffusion-controlled process, which was different from the adsorption-controlled process on the unmodified graphene sheet.

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1. Introduction

Graphene sheet (GS), a two-dimensional monolayer graphite,has received tremendous attention recently because of its extraor-dinary electronic, thermal, and mechanical properties [1]. Theultrathin structure of GS allows its band structure readily affectedby the adsorbents, and hence induces changes in its electronic orspectroscopic properties. Therefore, GS has been considered as anattractive new material in the design of novel sensors [2,3]. How-ever, the strong tendency of monolayeric GS to agglomerate intomultilayeric graphite presents a great challenge to the productionof GS and thus severely restricts its applications [4,5].

b-Cyclodextrin (b-CD) has been widely used as a dispersing re-agent for insoluble chemicals and nanomaterials, including carbonnanotubes [6,7]. However, the effect of b-CD on the properties ofGS has not been previously investigated. In this communication,we firstly explored the feasibility of dispersing GS using b-CDand then applied the b-CD/GS nanocomposite in detecting bioac-tive molecule dopamine (DA). By combining the unique electronicproperties of GS with the good water solubility of b-CD, the b-CD/GS nanocomposite showed significantly improved electrochemicalsensing performance compared to unmodified GS.

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.ng).

2. Experimental

GS was prepared by reducing graphene oxide (GO) using NaBH4

via method similar to that reported before [8]. For preparing b-CD/GS nanocomposite, a mixture containing 0.1 wt.% GS and 1 wt.% b-CD in water was sonicated for 30 min to give a stable black solu-tion. The glassy carbon electrode (GCE) coated with GS (GS/GCE)or b-CD/GS (b-CD/GS/GCE) was prepared by dropping 5 lL GS orb-CD/GS solution on the GCE, and dried in air for 24 h at roomtemperature.

Electrochemical experiments were performed with a CHI660Belectrochemical workstation (CHI, USA) with a conventionalthree-electrode cell. The working electrode was a 3 mm diameterGCE. A saturated calomel electrode (SCE) and a platinum electrodewere used as the reference and the counter electrodes, respec-tively. An accumulation potential of �0.1 V was applied for 3 minbefore cyclic voltammetry (CV) was performed [9]. Measurementswere carried out in pH 7.4 phosphate buffer solution (PBS) at roomtemperature under nitrogen atmosphere.

3. Results and discussion

The GS and b-CD/GS prepared in this work were firstly charac-terized by Raman spectroscopy (Fig. 1a) and were compared withHOPG and natural graphite flake. Both GS and b-CD/GS show sig-nificantly stronger D band, indicating more edge-plane defects[10]. The FESEM image (Fig. 1b) of the b-CD/GS film clearly shows

Fig. 1. (a) The Raman spectra of HOPG, graphite, GS and b-CD/GS; (b) FESEM image of b-CD/GS thin film deposited on silicon; AFM images of (c) GS and (d) b-CD/GS depositedon mica from highly diluted solutions. Representative profiles along with height histogram and bearing analysis are provided.

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a lot thin flake structures, confirming that the GS are not aggre-gated. To evaluate the stabilities of the GS and b-CD/GS in aqueouscondition, AFM measurements were conducted after the solutionswere aged under ambient condition for three weeks. Fig. 1c showsthat GS forms large microscopic patterns on mica, and only a fewindividual sheets can be resolved. Profile analysis gives film thick-ness higher than 4 nm, suggesting that many GS layers haveagglomerated into multilayeric graphite. Statistic information ob-tained from both the height histogram and bearing analysis (i.e.Abbott–Firestone curve) reveals an average film thickness around6 nm. In contrast, well separated sheets are observed on the b-CD/GS sample (Fig. 1d), and most of the sheets give a uniformthickness of 1 nm, which is consistent with the statistic result fromthe height histogram and bearing analysis. The slightly increasedthickness of b-CD/GS as compared to unmodified single layergraphene (measured as 0.8 nm by our instrument) is attributedto the adsorbed b-CD on the GS surface. The AFM results reveal thatthe incorporation of b-CD significantly prohibits the aggregation ofGS and helps to maintain the single-layered state of GS in aqueousenvironment. This conclusion is also supported by the experimen-tal observation that the b-CD/GS solution remains homogeneousfor several weeks under ambient condition, which is much morestable than that of the unmodified GS.

DA is a neurotransmitter and its concentration detection isimportant for the early diagnosis of many neurological illnesses[11,12]. Fig. 2a compares the CV response of 100 lM DA on thebare GCE, GS/GCE and b-CD/GS/GCE. On the bare GCE, the anodicpeak current (Ipa) and cathodic peak (Ipc) of DA are found to be5.5 and 3.9 lA, respectively. By contrast, the GS/GCE gives Ipa andIpc of 117.0 and 81.6 lA, respectively, around 20� higher than thatof the bare GCE, indicating a much larger active area. The b-CD/GS/GCE electrode shows Ipa and Ipc of 63.3 and 49.7 lA, respectively,slightly lower than that of the GS/GCE, suggesting that the ad-sorbed b-CD occupies some active surface of GS. The potential dif-ference (DEp) between the anodic and cathodic peaks is 115 mV on

the GS/GCE, and is only 73 mV on the b-CD/GS/GCE. The smallerDEp observed on the b-CD/GS/GCE reveals a faster electron transferprocess compared to the GS/GCE [6].

As DA often coexists with high concentration of ascorbic acid(AA) in biological systems [13], it is necessary to evaluate the influ-ence of assess AA. On conventional electrodes, the oxidation peaksof DA and AA are normally overlapped, making the detection of DAimpossible. Fig. 2b shows the CV curves of different concentrationsof DA on b-CD/GS/GCE in the presence of 1 mM AA. The oxidationpeaks of AA and DA are well separated from each other for morethan 200 mV. As a result, the Ipa of DA can be readily measuredeven when the AA concentration is 1000� higher. The high selec-tivity of b-CD/GS nanocomposite can be attributed to the negativecharge on the defective sites of GS, which attracts positively-charged DA while repels the negatively-charged AA [3,14].

The CVs of GS/GCE and b-CD/GS/GCE in various concentrationsof DA are shown in Fig. 2c and d, respectively. The peaks on theGS/GCE become very broad and show dramatically increased DEp

when the DA concentration is higher than 100 lM. In contrast,the redox peaks on the b-CD/GS/GCE are much narrower and giveonly slightly increased DEp even when the DA concentrationreaches 1 mM, revealing a much more reversible electrochemicalreaction. Calibration curves of the GS/GCE electrode (inset ofFig. 2c) shows that the Ipa is linearly related to the DA concentra-tion in a range of 0.3–7.0 lM. The experimentally observed detec-tion limit is around 0.2 lM. For the b-CD/GS/GCE, the Ipa versus DAconcentration curve shows two linear ranges with a transitionpoint at 2.0 lM. The experimental detection limit of b-CD/GS/GCE is found to be 5.0 nM, two orders of magnitude lower thanthat of the GS/GCE. The two linear range phenomenon has alsobeen observed on carbon nanotube modified electrode [15], andone possible origin may be the transition from monolayer adsorp-tion state into more complicated multilayer adsorption state. It isworth noting that the two linear ranges of b-CD/GS/GCE cover from9.0 nM to 12.7 lM. The overall linear range and sensitivity of b-CD/

L. Tan et al. / Electrochemistry Communications 12 (2010) 557–560 559

GS/GCE is also better than that of most reported carbon materials-based electrodes [14–17]. According Compton’s suggestion that thecatalytic activity of graphitic carbon electrodes mainly comes fromdefect sites [18], we attribute the high sensitivity of b-CD/GS/GCEto two factors. First, as shown by the strong D band in the Ramanspectrum, the b-CD/GS/GCE has large amount of defects act as cat-alytic sites. Second, the adsorbed b-CD prevents the GS to agglom-erate; hence the b-CD/GS/GCE has more accessible active sites thanGS/GCE to facilitate higher catalytic activity.

The reaction kinetic was investigated by studying the effects ofscan rate on the peak currents (Fig. 3). A linear relationship is ob-served on the GS/GCE (Fig. 4a), indicating a typical adsorption-con-

Fig. 2. (a) Cyclic voltammograms of 100 lM DA on bare GCE, GS/GCE and b-CD/GS/GCE;presence of 1 mM AA; (c) CVs of 0.3, 0.7, 1.0, 2.0, 4.0, 7.0, 50, 100, 500, 1000 lM DA on theGS/GCE. Insets of (c) and (d) shows the relation between Ipa and DA concentration in th

Fig. 3. (a) Plots of the Ipa and Ipc on the GS/GCE as functions of the scan rates; (b) plots ofmeasured in 100 lM DA.

trolled process. However, on the b-CD/GS/GCE (Fig. 3b), the peakcurrents are proportional to the square root of the scan rate inthe range of 0.1–2 V s�1, showing a mass diffusion-controlledprocess.

Fast DA detection capabilities of the two modified electrodeswere assessed by recording the current–time responses upon thesuccessive addition of various concentrations of DA (Fig. 4). Afterthe addition of DA, the GS/GCE electrode needed around 45 s toreach the 95% value of the steady-state current. The b-CD/GS/GCEshowed a much faster response, which achieved 95% of the stea-dy-state current in 30 s. Besides, the amperometric response ofDA was more reproducible on the b-CD/GS/GCE. The linear range

(b) cyclic voltammograms of 1, 5, 10, 20, 40, 70 lM DA on the b-CD/GS/GCE in theGS/GCE; (d) CVs of 0.1, 0.5, 1.0, 2.0, 7.0, 12.7, 50, 100, 500, 1000 lM DA on the b-CD/

e linear ranges. CV was carried out at a scan rate of 100 mV s�1.

the Ipa and Ipc on the b-CD/GS/GCE as functions of the square root of the scan rates,

Fig. 4. Current–time curves for GS/GCE and b-CD/GS/GCE at 0.1 V with successiveaddition of DA (curve of b-CD/GS/GCE is offset by 3 lA for clarity). Arrow shows theconcentrations of DA added in solution. Inset: calibration curves for DA on GS/GCEand b-CD/GS/GCE in their linear ranges.

560 L. Tan et al. / Electrochemistry Communications 12 (2010) 557–560

for DA detection by using the b-CD/GS/GCE (0.9–200 lM) is muchwider than that of GS/GCE (3–58 lM) and that of the electrodemodified by carbon nanotube b-CD composite (10–80 lM) [6].

4. Conclusions

The electrode modified by b-CD/GS nanocomposite exhibitedmore reversible electrochemical response to DA than that of theunmodified GS. By using CV method, the linear current responserange of the b-CD/GS/GCE covered the range from 9.0 � 10�3 to12.7 lM, with a detection limit of 5.0 nM, showing a superior sens-

ing performance than GS/GCE. Working under amperometricmode, the b-CD/GS/GCE exhibited a linear range from 0.9 to200 lM, which was much wider than that of GS/GCE. Our resultsdemonstrated that the b-CD/GS nanocomposite have good poten-tial to become a new platform for designing both voltammetricand amperometric biosensors.

Acknowledgements

The authors are grateful to the financial support from the Na-tional Natural Science Foundation of China (NSFC. 20621091,J0730429), Chunhui project and ‘‘111” project.

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