Selective Detection of Dopamine in Presence of Ascorbic Acid by Use of Glassy-Carbon Electrode...

DOI: 10.1002/elan.201400477

Selective Detection of Dopamine in Presence of AscorbicAcid by Use of Glassy-Carbon Electrode Modified withAmino-b-Cyclodextrin and Carbon NanotubesAymen Gaied,[a] Nejmeddine Jaballah,[a] Moncef Tounsi,[a] Mohamed Braiek,[b] Nicole Jaffrezic-Renault,[b]

and Mustapha Majdoub*[a]

1 Introduction

Dopamine (DA) is a vital neurotransmitter in mammaliancentral nervous systems [1]. Indeed, changes in DA levelshave proved to be essential in effective brain function,such as for learning and memory development [2]. Thesymptoms of Parkinson�s disease arise when the DA con-taining neurons in the brain start to deteriorate [3]. How-ever, an excess of DA in certain brain regions causes ter-rible symptoms associated to schizophrenia [4].Therefore,the detection of DA in biological media has been thefocus of many recent researches. In particular, there hasbeen much interest in the development of DA electro-chemical sensors. In fact, DA is electroactive and his oxi-dation can be followed using electrochemical techniques.Besides, electrochemical sensors are usually inexpensive,highly sensitive and easy to employ. Numerous worksconcerning the electrochemical detection of DA usinga wide range of membranes were reported. Cosnier et al.described an electro-enzymatic biosensor based on poly-phenol oxidase; an extremely sensitive detection limit of5�10�8 M was obtained [5]. Comparable performanceswere reached using a glassy carbon electrode modifiedwith acid-treated multi-wall carbon nanotubes [6]. Acarbon paste electrode treated with graphite oxide wasalso used for determination of DA and exhibit a detectionlimit of 1.5 �10�8 M [7].

However, the detection of DA is particularly challeng-ing using electrochemical methods because DA co-existswith many interfering compounds in biological samples.These interfering compounds are usually present in con-centrations higher than DA. This is particularly true forthe ascorbic acid (AA) which is the main interfering com-pound. Indeed, AA oxidized at the same potential of DA

for most solid electrodes, which make very difficult itselectrochemical detection. In order to overcome thisproblem, the modification of the electrode is the mostpopular strategy, including electrodes modified by con-ducting polymers, [8,9] carbon nanotubes [10] or receptormolecules as b-cyclodextrin [11].

The b-Cyclodextrin (b-CD) is an oligosaccharide con-sisting of seven glucose units which present a toroidalform with a hydrophobic inner cavity and a hydrophilicouter side. The most characteristic property of b-CD is itsremarkable ability to form inclusion complexes witha wide variety of guest molecules [12]. It has been report-ed that inclusion of DA in b-CD could be a result of hy-drophobic and hydrogen-bonding interaction [13].Carbon Nanotubes (CNTs) have been widely recognizedas the typical nanomaterial since their discovery in1991[14]. Owing to their chemical stability, catalytic effi-ciency, high mechanical strength and good electrical prop-erties, CNTs have been intensively used in electrode sur-face modification [15,16]. The CNT-modified electrodeshowed improved performances due to its ability of pro-moting electron transfer with electroactive species pres-ent in electrolyte solution [17].

In the present work, a simple and reliable method wasdeveloped for determination of dopamine in presence of

[a] A. Gaied, N. Jaballah, M. Tounsi, M. MajdoubLaboratoire des Interfaces et des Mat�riaux Avanc�s(LIMA), Universit� de Monastir, Facult� des SciencesBd. de l�Environnement, 5019 Monastir, Tunisie*e-mail: [email protected]

[b] M. Braiek, N. Jaffrezic-RenaultUniversity of Lyon, Institute of Analytical SciencesLa Doua Street, 69100 Villeurbanne, France

Abstract : A glassy carbon electrode modified with per-6-amino-b-cyclodextrin (b-CDNH2) and functionalizedsingle-walled carbon nanotubes (SWCNT-COOH) waselaborated. This structure was investigated for the detec-tion of dopamine acid (DA) in presence of ascorbic acid(AA). The sensor behavior was studied by cyclic voltam-metry, square wave voltammetry and electrochemical im-pedance spectroscopy. The analysis results show that the

electrode modification with CD derivative improves thesensitivity and selectivity of the DA recognition; the elec-trochemical response was further improved by introduc-tion of SWCNT-COOH. The sensor shows good and re-versible linear response toward DA within the concentra-tion range of 7 �10�7–10�4 M with a detection limit of 5 �10�7 M.

Keywords: Cyclodextrins · Carbon nanotubes · Dopamine · Sensor · Square wave voltammetry

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ascorbic acid. It consists on the use of glassy carbon elec-trode modified with per-amino-b-cyclodextrin and carbox-ylic acid functionalized single-walled CNTs. The sensitivi-ty, linear range, limit detection and electrochemical prop-erties of this sensor were studied by the cyclic voltamme-try (CV), square wave voltammetry (SWV) and electro-chemical impedance spectroscopy (EIS).

2 Experimental

2.1 Reagents and Apparatus

b-Cyclodextrin (b-CD), triphenylphosphine (Ph3P), diio-dine, ammonia aqueous solution, dimethylformamide(DMF), dopamine acid, l-ascorbic acid and carboxylicacid functionalized single-walled carbon nanotubes(SWCNT-COOH) were purchased from Sigma�Aldrich.The 1H NMR and 13C NMR spectral data were obtainedon a Bruker AV300 spectrometer. Phosphate buffersaline solutions (PBS, 0.1 M) with different pH were pre-pared by mixing stock standard solutions of KH2PO4,K2HPO4, NaCl and KCl, then adjustment of the pH valuewith HCl or NaOH; double-distilled water was used.Electrochemical measurements were performed with anEC-LAB version 9.9.All experiments were performedwith a conventional three-electrode system comprisinga modified GCE working electrode (3 mm in diameter),a platinum wire auxiliary electrode and a saturated calo-mel electrode (SCE) as the reference electrode.

2.2 Synthesis of the Per-6-Amino-b-CD

The b-CDNH2 was prepared through three successive re-actions according to the elsewhere reported method(Scheme 1) [18]. Firstly, treatment of b-CD with I2 andPPh3 in DMF at 70 8C afforded the per-6-iodo-b-cyclodex-trin (b-CDI). Next, the b-CDI was treated with NaN3 inDMF at 60 8C to give the per-6-azido-b-cyclodextrin (b-

CDN3). Finally, b-CDN3was treated with the PPh3 inDMF for one hour and then an aqueous solution ofNH3was added to the mixture to give the b-CDNH2.

b-CDI: 1H NMR (DMSO-d6) d ppm 3.28 (t, J=9 Hz,7H), 3.34–3.48 (m, 14H), 3.54–3.68 (m, 14H), 3.80 (d, J=9 Hz, 7H), 4.99 (d, J=3 Hz, 7H), 5.94 (d, J=2 Hz, 7H),6.05 (d, J=6.5 Hz, 7H); 13C NMR (DMSO-d6) d ppm 9.3(C�6), 70.8 (C�2), 71.8 (C�3), 72.3 (C�5), 86.3 (C�4),101.8 (C�1).

b-CDN3:1H NMR (DMSO-d6) d ppm 3.30–3.42 (m,14H), 3.53–3.65 (m, 14H), 3.68–3.82 (m, 14H), 4.91 (d, J=3 Hz, 7H), 5.77 (d, J) 2 Hz, 7H), 5.92 (d, J=7 Hz, 7H);13C NMR (DMSO-d6) d ppm 51.4 (C�6), 70.4 (C�2),72.1(C�3), 72.7(C�5), 83.3(C�4), 102.1(C�1).

b-CDNH2: 1H NMR (D2O) d ppm 3.26 (dd, J=7,13 Hz, 7H), 3.44 (dd, J=3, 13 Hz, 7H), 3.57 (t, J=9 Hz,7H), 3.66 (dd, J=3.5, 9.5 Hz, 7H), 3.98 (dd, J=9,9.5 Hz, 7H), 4.15–4.25 (ddd, J=3, 7, 9 Hz, 7H), 5.15 (d,J=3.5 Hz,7H); 13C NMR (D2O) d ppm 42.9 (C�6), 70.5(C�2), 74.3 (C�3), 74.8 (C�5), 84.8 (C�4), 104.1 (C�1).

2.3 Preparation of the GCE/b-CDNH2-SWCNT Sensor

A glassy carbon electrode (GCE) was carefully polishedsuccessively with 1.0 mm, 0.3 mm and 0.05 mm a-Al2Opowder slurries to produce a mirror-like surface. It wasthen sonicated for 1 min in ethanol and doubly distillatedwater. 1 mg of SWCNT-COOH was dispersed by ultra-sonic agitation for 5 min in 1 mL of b-CDNH2 aqueoussolution (2 %). Then, 10 mL of the obtained suspensionwas deposited onto the GCE surface. After solvent evap-oration, the electrode surface was thoroughly rinsed withdoubly distilled water and dried under ambient condition.

2.4 Analytical Procedures

In the square wave voltammetry (SWV) and cyclic vol-tammetry (CV) experiments, the modified GCE was im-mersed in 0.1 M of PBS solution (pH7.2) containinga known amount of DA. The SW curves were recordedfrom �0.5 to 0.6 V, using a scan rate of 100 mV s�1 ata pulse amplitude of 100 mV and a pulse duration of20 ms. The CV plot was recorded from �0.5 to 0.8 V witha scan rate of 100 mVs�1. The electrochemical impedancespectroscopy (EIS) of GCE before and after modificationwas performed in 5 mM Fe(CN)6

3�/4� (1/1) solution con-taining 0.1 M of KCl. The frequency range was from100 mHz to 100 kHz and the polarization potential was of0.2 V.

3 Results and Discussion

3.1 Impedance Analysis of the Modified GCE

Electrochemical impedance spectroscopy (EIS) recordsthe impedance changes of the electrode surface in themodification process and therefore provides detailed in-Scheme 1. Synthetic route to the b-CDNH2.



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formation on the change of the modified electrode sur-face properties [19].

A typical Nyquist plots for different modification pro-cesses of GCE are shown in Figure 1. The EIS data forGCE bare, GCE/SWCNT sensor and GCE/b-CDNH2-SWCNT sensor were fitted with the Randles equivalentcircuit; the obtained electrochemical parameters data aresummarized in the Table 1. At the GCE bare, the redoxprocess of the [Fe(CN)6]

3�/4� probe showed a Rct of92.0 W, indicating a huge interface electron transfer resist-ance. The Rct value decreased to 8.3 W after modificationby the SWNCT, suggesting an improvement of the con-ductivity and the electron transfer process. The GCE/b-CDNH2-SWCNT shows a significant increasing in the Rct

value to 25.3 W. Indeed, the b-CDNH2 layer deposited onthe electrode surface hinders the electron transfer fromthe [Fe(CN)6]

3�/4�redox probe to the electrode [20].

3.2 Wetting Properties

The wettability of the b-CDNH2 membrane was evaluat-ed using contact angle measurements. Figure 2 shows thedecrease in the contact angle for the b-CDNH2-SWCNTmembrane in comparison with the SWCNT. This resultproves that the surface became more hydrophilic by in-corporation of the b-CDNH2 in the membrane composi-tion. The reduction of the surface hydrophobicity can beattributed to the b-CDNH2 amino groups. The surface en-ergies of the SWCNT and b-CDNH2-SWCNTmembraneswere determined by the van Oss model [21]. The resultsshowed a decrease in the surface energy by adding theCD derivative (from 57.7 mJ m�2 for SWCNT to42.8 mJ m�2 for b-CDNH2-SWCNT).

Fig. 1. Nyquist plots for GCE bare (a), GCE/SWCNT (b), GCE/b-CDNH2-SWCNT (c); electrolyte solution: [Fe (CN)6]3�/4� (5 mM)/

KCl (0.1 M).

Table 1. Fitting data of GCE and modified GCE.

Rs (W) Rct (W) CPE (mF) n

GCE 90.5 92.0 101 0.68GCE/SWCNT 89.2 8.3 267 0.86GCE/b-CDNH2-SWCNT 91.1 25.3 41 0.75

Fig. 2. Contact angle: SWCNT membrane (a) and b-CDNH2-SWCNT membrane (b).



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3.3 Electrochemical Performance of the Modified GCE

The CV responses of DA at GCE/b-CDNH2-SWCNT,GCE/SWCNT and GCE were stabilized after five cycles;the obtained voltammograms are shown in Figure 3. Theelectrochemical response of the bare GCE to DA waspoor; broad and small oxidation/reduction peaks corre-spondent to the dopaquinone (DOQ)/DA [22] were ob-served at 0.22 V and 0.15 V (curve a). When the electrodewas modified with the SWCNT, the redox peaks in-creased substantially (curve b), due to the strong electro-catalytic activity of the SWCNT-COOH. In fact, it isknown that the delocalization of the p-electron inSWCNT-COOH improves its electron transport proper-ties [23]. Besides, the p�p interactions between thephenyl group of DA and the hexagonal carbon structureof MWCNT enhance the electron transfer [24]. The oxi-dation peak was further increased by adding the b-CDNH2 to SWCNT (curve c). This behavior can be as-cribed to the improvement of the DA accumulation byformation of [b-CDNH2/DA] inclusion complex at theelectrode surface. The curve (c) also showed the appear-ance of other redox peaks. In fact, in the first scan, DA isoxidized to DOQ at 0.21 V. A part of the formed DOQundergoes cyclization reaction to form dihydroxyindoline(DHI), which is immediately oxidized to aminochrome(AC) (Scheme 2) [25].

Upon scan reversal, two peaks attributed to the reduc-tion of DOQ and AC were observed at 1.35 and �0.28 V,respectively. In the following cycle, oxidation peak ap-pears at �0.24 V, corresponding to the oxidation of DHIto AC.

Figure 4 represents the CV responses of DA at theGCE/b-CDNH2-SWCNT under various scan rates. Theplot of the anodic peak current (Ip) as function of scanrate is shown in the insets of Figure 4. The Ip was propor-tional to scan rate in the range from 20 to 100 mV s�1.

This behavior proves the surface immobilization of DAand it was assumed that the interaction of DA with GCE/b-CDNH2-SWCNT was a typical adsorption-controlledprocess [12]. The anodic peak slightly shifts toward thehighest potential with increasing scan rates. These resultsindicate that the electron transfer rate is not fast and theelectrode process is quasi-reversible at high scan rates[25]. Therefore, the average surface concentration (G) ofDA on the GCE/b-CDNH2-SWCNT surface could be es-timated using the following equation:

Ip ¼ n2 F2 GA v=4RT

Where A is the electrode surface area, n represents thenumber of transferred electrons, F is the Faraday con-stant, u is the scan rate, R is the ideal gas constant and Tis the temperature. Hence, a DA average surface concen-tration of 1.35�10�9 molcm�2 was calculated using theslop value in the Ip(v) plot.

Fig. 3. Cyclic voltammograms of 10�4 M DA at a GCE bare (a), GCE/SWCNT (b) and GCE/b-CDNH2-SWCNT (c) in PBS (pH 7)(scan rate: 100 mV s�1).

Scheme 2. Mechanism of DA oxidation.



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3.4 Determination of DA at the GCE/b-CDNH2-SWCNTby SWV

Square wave voltammetry has been performed to investi-gate the sensitivity of GCE/b-CDNH2-SWCNT sensor tothe DA. The voltammograms reveal the DA oxidation at0.16 V as shown in Figure 5. The oxidation peak currentincreased proportionally to the DA concentration in therange of 0.7–100 mM and a detection limit of 0.5 mM wasestimated. The peak current (IDA) against the DA concen-tration [DA] shows a linear regression with IDA =0.514[DA]+2.6 equation (Inset of Figure 5), indicating a sensi-tivity of 0.514 mA mM�1.

The determination of the DA in presence of ascorbicacid (AA) was carefully investigated. The AA concentra-

tion was kept constant at 100 mM, whereas the DA con-centration was varied between 1 mM and 100 mM; theFigure 6 shows the obtained voltammograms. The DA ox-idation currents were increased with increasing DA con-centration, but the anodic current of the AA was constantwith successive DA additions. In fact, at a traditional bareelectrode, the AA can be oxidized at a potential close tothat of DA, resulting in overlapping voltammetric re-sponses [26,27]. The use of the electrode modified withthe b-CDNH2-SWCNT effectively overcomes this prob-lem. It enabled the differentiation of the responses: twoanodic peaks were observed at �0.04 V and 0.15 V, forAA and DA, respectively (Figure 6). Indeed, the mem-brane surface is rich on CD ammonium groups (pH 7PBS), which facilitate the movement of AA anions to the

Fig. 4. Cyclic voltammograms obtained from the GCE/b-CDNH2-SWCNT in 0.1 M PBS (pH 7) containing 10�4 M DA, at differentscan rates, from inside to outside: 20, 40, 60, 80 and 100 mVs�1. Inset: Plot of anodic peak current against the scan rate.

Fig. 5. The SWVs of GCE/b-CDNH2-SWCNT in 0.1 M PBS(pH 7), containing different concentrations of DA. The DA con-tents from (a) to (k) are: 0, 0.7, 1, 4, 8, 10, 20, 40, 60, 80 and100 mM. Inset: Calibration curve of peak current to the DA con-centration.

Fig. 6. The SWVs of GCE/b-CDNH2-SWCNT in 0.1 M PBS(pH 7), containing different concentrations of DA in presence ofAA (100 mM). The DA contents from (a) to (l) are: 0, 1, 2, 4, 6,8, 10, 20, 40, 60, 80 and 100 mM. Inset: Calibration curve of peakcurrent to the DA concentration.



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surface of the modified electrode in comparison to theDA cations. Thus, the DA was oxidized at higher anodicpotential. The DA oxidation peak current increased line-arly with increasing the DA concentration in the range of1–100 mM, with a limit detection of 0.5 mM. The linear re-gression equation is: IDA =0.228 [DA]+25.5, indicatinga sensitivity of 0.228 mAmmol�1 L for the GCE/b-CDNH2-SWCNT sensor, in the detection of DA in presence ofAA.

In all electrochemical measurements, a stable currentintensity was registered, indicating no leakage of b-CDNH2. This behavior can be attributed to the electro-static interactions of the positively charged CD derivativewith the negatively charged SWCNT-COOH, which limitits solubility in aqueous media.

In Table 2, the detection limit of the proposed modifiedGCE is compared with previously reported values for thedetection of DA in presence of AA by using b-CD basedsensors. We noticed a low detection limit for GCE/b-CDNH2-SWCNT comparing to several works [27–30]. Alower detection limit was obtained by b-CD/MWCN/chi-tosan modified GCE [25], due to the effect of chitosan onthe sensor sensitivity to the DA. Izaoumen et al. [26]modified the working electrode using the sonogelmethod; this modification exhibits also a low detectionlimit, but it is more difficult compared to the methodused in this work.

4 Conclusions

A simple modification of a glassy carbon electrode withper-6-amino-b-cyclodextrin and carboxylic acid function-alized single-walled carbon nanotubes was elaborated andcharacterized. It was successfully applied for the detec-tion of dopamine in presence of the ascorbic acid usingthe square wave voltammetry technique. The modified

GCE combines the electrocatalytic properties of theSWCNTs and the selective inclusion of the cyclodextrinderivative, which exhibits an attractive ability for highlysensitive dopamine recognition. The electrochemicalsensor showed good performances in terms of detectionlimit, linearity range and selectivity.

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Table 2. Detection limit of DA in presence of AA for differentb-CD-modified electrodes. SWV: square wave voltammetry,DPV: differential pulse voltammetry, CV: cyclic voltammetry.

Modified electrode Detectionlimit(mmol)

Method References

Per-6-amino-b-CD/ func-tionalized SWCN modi-fied GCE

0.50 SWV This work

b-CD/MWCN/chitosanmodified GCE

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b-CD-Sonogel-carbonelectrode

0.16 DPV [29]

Polypyrrole-sulfonated b-CD modified Pt electrode

3.20 Amperometry [30]

Carbon paste electrode/poly(b-CD)

4.10 DPV [31]

Polypyrrole-b-CD modi-fied GCE

6.00 CV [32]

MWCN/ b-CD modifiedGCE

6.70 Amperometry [33]



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Received: August 29, 2014Accepted: September 20, 2014

Published online: && &&, 2014



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A. Gaied, N. Jaballah, M. Tounsi,M. Braiek, N. Jaffrezic-Renault,M. Majdoub*

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Selective Detection of Dopamine inPresence of Ascorbic Acid by Useof Glassy-Carbon ElectrodeModified with Amino-b-Cyclodextrin and Carbon Nanotubes



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