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Colloids and Surfaces B: Biointerfaces 88 (2011) 764– 770

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces B: Biointerfaces

j our na l ho me p age: www.elsev ier .com/ locate /co lsur fb

lectrochemically selective determination of dopamine in the presence ofscorbic and uric acids on the surface of the modified Nafion/single wall carbonanotube/poly(3-methylthiophene) glassy carbon electrodes

o Phuc Quana,∗∗, Do Phuc Tuyena, Tran Dai Lamb,∗, Phan Thi Ngoc Trama, Nguyen Hai Binhb,ham Hung Vieta

Research Centre for Environmental Technology and Sustainable Development, Hanoi University of Science, 334 Nguyen Trai Road, Ha Noi, Viet NamInstitute of Materials Science, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Ha Noi, Viet Nam

r t i c l e i n f o

rticle history:eceived 13 May 2011eceived in revised form 11 August 2011ccepted 12 August 2011vailable online 22 August 2011

eywords:afion (NF)ingle-walled carbon nanotubes (SWCNT)

a b s t r a c t

A voltammetric method based on a combination of incorporated Nafion, single-walled carbon nanotubesand poly(3-methylthiophene) film-modified glassy carbon electrode (NF/SWCNT/PMT/GCE) has beensuccessfully developed for selective determination of dopamine (DA) in the ternary mixture of dopamine,ascorbic acid (AA) and uric acid (UA) in 0.1 M phosphate buffer solution (PBS) pH 4. It was shown thatto detect DA from binary DA–AA mixture, the use of NF/PMT/GCE was sufficient, but to detect DA fromternary DA–AA–UA mixture NF/SWCNT/PMT/GCE was required. The later modified electrode exhibitssuperior electrocatalytic activity towards AA, DA and UA thanks to synergic effect of NF/SWCNT (com-bining unique properties of SWCNT such as high specific surface area, electrocatalytic and adsorptive

oly(3-methylthiophene) (PMT)opamine (DA)lectrochemical methods

properties, with the cation selectivity of NF). On the surface of NF/SWCNT/PMT/GCE AA, DA, UA wereoxidized respectively at distinguishable potentials of 0.15, 0.37 and 0.53 V (vs. Ag/AgCl), to form well-defined and sharp peaks, making possible simultaneous determination of each compound. Also, it hasseveral advantages, such as simple preparation method, high sensitivity, low detection limit and excel-lent reproducibility. Thus, the proposed NF/SWCNT/PMT/GCE could be advantageously employed for thedetermination of DA in real pharmaceutical formulations.

. Introduction

3,4-Dihydroxyphenyl ethylamine, commonly known asopamine (DA), has been of interest to neuroscientists andhemists since its discovery in the 1950s [1]. As the most signifi-ant neurotransmitter, the amount of DA distributed in organs hasreat influences on human emotions and is directly related to aariety of diseases deriving from the abnormal low concentrationevel of DA which has been linked to several neurological disor-ers, e.g., schizophrenia, Huntington’s disease, and Parkinson’sisease (the third most popular one in the world), and even theIV infection [2–4]. Electrochemical detection of DA is a feasiblepproach because of its good electroactivity and easy oxidation.

t has been shown that the oxidation of DA is a two-electronrreversible process with transfer of two protons. Uric acid (UA)s the major final product of purine catabolism in human body. In

∗ Corresponding author. Tel.: +84 4 37564129; fax: +84 438360705.∗∗ Corresponding author. Tel.: +84 4 38588152; fax: +84 438587964.

E-mail addresses: doquan@vnu.edu.vn (D.P. Quan), lamtd@ims.vast.ac.vnT.D. Lam).

927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.colsurfb.2011.08.012

© 2011 Elsevier B.V. All rights reserved.

a healthy human, the normal level of UA in urine is in mM rangewhere as in serum it is in �M range. Abnormal levels of UA aresymptoms of several diseases such as gout, hyperuricemina, andLesch–Nyan disease [5]. Ascorbic acid (AA) is present in manyvegetables, citrus fruits and biological fluids where it acts as ananti-oxidant and free-radical scavenger. AA concentration in thebody fluids can be used to assess the level of oxidative stress isrelated to diseases like cancers, diabetes mellitus and hepaticdisorders. UA and AA is co-present in biological fluids such asblood and urine.

From the above reasons, it is essential to develop rapid and sim-ple methods to detect/determine the DA concentration. One of thebiggest challenges of electrochemical detection of DA in real bio-logical matrixes is the coexistence of many interfering compounds.In biological systems, AA usually coexists with DA in extracellu-lar fluid at a high concentration level, nearly 1000 times higherthan DA. Moreover, DA, AA and UA can be oxidized at practicallythe same potential at bare electrodes, resulting in the peak over-

lapping as well as poor response resolution in DA determination.Homogeneous catalytic oxidation of AA by oxidized DA, interactionof AA and the products of DA oxidation are other difficulties in theDA determination, severely limiting the accuracy of detection.

aces B

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D.P. Quan et al. / Colloids and Surf

To solve these problems, the use of chemically modified elec-rodes instead of bare ones is preferred. Various materials, suchs metal or metal oxide nanoparticles (NPs) and metal complexesAu NP hybrid film and nanogold modified carbon fiber electrodes,alladium nanoparticle loaded carbon nanofibers, zinc oxideomposite film, ruthenium oxide, magnetic Fe3O4 NPs depositedn gold electrode, titanate nanotubes, LaFeO3 nanoparticles6–15]); organic polymers and composites (polymeric films ofniline, pyrrole, 3-methylthiophene and p-nitrobenzene resor-inol are reported to be useful in the selective detection of DAn excess of AA [16–19], poly(cresol-red) modified electrode-poly(oracet blue) modified electrode, poly(eriochrom black T)odified electrode, poly(naphthalene sulfunic acid) modified

lectrode, poly(Evans blue) modified electrode, poly(vinyl alcohol)nd poly(chromotrope 2B)-modified electrodes are also usedo detect DA and/or DA, AA and UA, simultaneously; poly(4-mino-1,1′-azobenzene-3,4′-disulfonic acid)-coated electrodeas been reported for the selective detection of DA in the pres-nce of AA, UA and NADH, poly(3-(3-pyridyl) acrylic acid), and-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-,7-disulfonic acid have been reported for simultaneousetermination of AA, DA and UA [20–26]) can be used to fab-icate modified electrode due to their excellent properties toecrease the over-potential, accelerate the electron and massransfer rate or greatly enrich the substrates on the electrodeurface.

This article reports the successful combination of poly(3-ethylthiophene) (PMT) with Nafion (NF) and single wall

arbon nanotubes (SWCNTs) in order to provide NF/SWCNT/PMTodified electrodes with enhanced sensitivity and selectiv-

ty towards DA in the presence of high excess of AA andA.

Glassy carbon electrodes (GCE) are very versatile as electrodeaterial for trace level determination of organic molecules as they

rovide high sensitivity, negligible porosity, and good mechanicaligidity. GCEs have been modified by means of various nanosizeddditives [27,28].

PMT is a widely investigated electronically conducting poly-er, which can be easily electrodeposited onto electrode surface

y electro-oxidation of its monomer. The applications of PMT haveeen extensively reported and showed excellent electrocatalyticffect, neurotransmitter species [29,30].

The choice of SWCNT to develop electrochemical transducerss based on its subtle electronic properties, strong electrocat-lytic effect, rapid electron transfer rate, high tensile strength andhemical stability, and ultra-small size effect [27]. As for Nafion,

perfluorinated sulfonated cation exchanger, having hydrophilicnd hydrophobic domains (in which the later consists primarilyf Teflon while the former consists of sulfonic groups, presentedt the end of the side chains of the Teflon backbone), excellentntifouling capacity, chemical inertness and high permeability toations, has been extensively employed as an electrode modifieror organic molecules [31,32]. The sensitivity enhancement of theF membrane is believed to associate with accumulations of DA

n the hydrophilic regions or the ion channels of NF. Thanks to itsmphiphilic structure, SWCNT can be homogeneously dispersedn NF solution. Thus, a synergistic effect of both NF-SWCNT film

odified can be expected to further enhance sensitivity in DAetermination.

The primary objective of this work is not to introduceovel materials for modified electrodes but to emphasize thefforts on designing/optimizing a sensitive and selective inter-

ace for simultaneously electrochemical determination of DA inhe presence of AA and UA by different voltammetric techniquesike cyclic voltammetry (CV) and differential pulse voltammetryDPV).

: Biointerfaces 88 (2011) 764– 770 765

2. Experimental

2.1. Chemicals

3-methylthiophene (MT), 5% Nafion® 117 and tetrabuthy-lammonium perchlorate (TBAP) were from Fluka (Switzerland).Ascorbic acid, dopamine hydrochloride (DA·HCl), uric acid andother chemicals were from Sigma (Germany) and were used with-out further purification. SWCNT was purchased from ChengduOrganic Chemicals Co. Ltd. (Chengdu, China). Aqueous solutionswere prepared with de-ionized (DI) water. The other reagents wereof analytical reagent grade. Highly purity nitrogen was used fordeaeration.

2.2. Electrode preparation

Prior to the electropolymerization, the surface of the Glassy car-bon electrode was polished with 15 �m and 0.3 �m alumina slurryand cleaned by ultrasonication in DI water. PMT was electrode-posited on a GCE (to get PMT/GCE) from a solution containing 0.1 MMT and 0.1 M TBAP (dissolved in acetonitrile (CH3CN)) for 20 s at aconstant potential of 1.75 V (vs. Ag/AgCl). Afterwards, it was treatedat 0.7 V for 10 s.

Two main types of modified electrodes (denoted as NF/PMT/GCEand NF/SWCNT/PMT/GCE) are prepared. First, 7 �L of 2.5% NF solu-tion was scrupulously dropped onto the PMT/GCE surfaces, thesolvent was evaporated in air to obtain NF/PMT/GCE. Second, 4 �Lof SWCNT dispersion (pre-carboxylated by the mixture of concen-trated H2SO4 and HNO3 (1/3, v/v), then mixed ultrasonically with0.25 wt% NF solution for 30 min) was scrupulously dropped ontothe surfaces of PMT/GCE, then, the solvent was evaporated in air toobtain NF/SWCNT/PMT/GCE. All electrodes were carefully rinsedwith DI before further characterization.

2.3. Electrochemical measurements

CV and DPV were performed with Autolab PGSTAT-30Potentiostat/Galvanostat with GPES software (EcoChemie, TheNetherlands). The three-electrode system was employed withAg/AgCl/saturated KCl reference electrode and Pt wire as auxiliaryelectrode, the working electrode was either a bared GCE or a mod-ified GCE. The electrochemical detection was performed in 0.1 MpH 4.0 PBS containing DA (in absence/presence of AA, UA), purgedby high-purity nitrogen. All experiments were carried out at roomtemperature. All DPVs were measured/recorded in triplicate (DPVpeak height (Ipa) remains its initial value with a relative standarddeviation (R.S.D) less than 2–3% for 3 successive scans). Mean valueswere used for further calculations of linear regression equation.

3. Results and discussion

3.1. Electrochemical behavior of DA at the modified electrode

The electrochemical behavior of the modifiedNF/SWCNT/PMT/GCE was studied by CV. Fig. 1 shows the voltam-mograms of (a) bare GCE; (b) PMT/GCE; (c) NF/SWCNT/GCE and (d)NF/SWCNT/PMT/GCE, respectively in 0.1 M PBS (pH 4.0), at scanrate of 100 mV s−1. Curves (a) and (b) confirmed that the electro-chemical response to DA at bare GCE and PMT/GCE was very poor,while that at NF/SWCNT/GCE (curve c) and NF/SWCNT/PMT/GCE(curve d) was much better: a well-defined redox couple wasrecorded, the anodic current value was much higher than that

of the two previous cases (curves a and b). The reason for thepeak current enhancement may originate in the faster electrontransfer rate and/or larger surface area of SWCNT as well as easiermass transfer thanks to an inclusion complex of NF and DA, which

766 D.P. Quan et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 764– 770

0.80.60.40.20.0-0.2

-40

-30

-20

-10

0

10

20

30

(d) NF/SWCN T/PMT/GCE(c) NF/SWCN T/GCE(b) PMT/GC E(a) GCE

I /µA

E /V vs. Ag/AgC l

FN

cp

arsetctrti0w

fpacdrard

FNa

0.80.60.40.20.0-0.20

2

4

6

8

10

12

14

16

18

20

22

7.44 µM11.4

16.223.4

32.944.557.973.290.1108

PMT/GCE

I /µA

I /µA

I /µA

E /V vs. Ag/AgCl

0.80.60.40.20.0-0.20

2

4

6

8

10

12

0.5 µM1.5

2.5

4.57.5

11.4

23.4

32.9

NF/PMT/GCE

E /V vs. Ag/AgCl

68

101214161820

11.4 µM

23.4 µM

32.9 µM

57.9 µM

A

B

C

ig. 1. CVs of 100 �M DA at: (a) bare GCE; (b) PMT/GCE; (c) NF/SWCNT/GCE; (d),F/SWCNT/PMT/GCE in 0.1 M PBS of pH 4.0 with the scan rate of 100 mV s−1.

ould be dissociated and rapidly diffused more rapidly through theorous layer of SWCNT to the modified electrode surface).

Next, to elucidate the process kinetic of DA electro-oxidationt electrode surface, the effect of the scan rate on the peak cur-ents at the NF/SWCNT/PMT/GCE was investigated (Fig. 2). The insethowed that the cathodic and anodic peak currents increased lin-arly along with the square root of scan rates (v1/2), suggestinghat the electrochemical electro-oxidation of DA was non-surfaceontrolled but diffusion-controlled process, owing to a slow elec-ron hoping across the matrix of the composite film in the studiedange of potential sweep rates, according to the following equa-ions: Ipa(�A) = −13.3 + 3.03 × v1/2 and Ipc(�A) = 10.3 − 2.30 × v1/2 (In �A and v1/2 in mV1/2 s−1/2), with the correlation coefficients of.99815 and 0.99801, respectively. This result was well consistentith other previous studies [30].

It is well known that in DPV, current is measured at two pointsor each pulse, the first point just before the application of theulse and the second at the end of the pulse. These sampling pointsre selected to minimize non faradic current, thus DPV method isonsidered more sensitive than CV [33]). So DPV was used here toetect DA at different modified electrodes. Fig. 3A–C presents theelationship between DA concentration and current signal recorded

t (a) PMT/GCE; (b) NF/PMT/GCE and (c) NF/SWCNT/PMT/GCE,espectively. It is clear that only at two later cases the linearependence of current I on [DA] concentration was observed. The

0,80,70,60,50,40,30,20,10,0-0,1-40

-30

-20

-10

0

10

20

30

40

50

60

10

200

v (mV/s)

I /µA

E /V vs. Ag/AgCl

ig. 2. Electrochemical response of 100 �M DA in 0.1 M pH 4.0 PBS atF/SWCNT/PMT/GCE with different scan rates, from 10 to 200 mV s−1. Inset: Ipa − v1/2

nd Ipc − v1/2 plots.

0,70,60,50,40,30,20,10,0-0,1

-2024 7.4 µM

NF/SWCNT/PMT/GCE

E /V vs. Ag/AgCl

Fig. 3. DPVs for different concentration of DA in 0.1 M PBS of pH 4.0 at (A) PMT/GCE;(B) NF/PMT/GCE; (C) NF/SWCNT/PMT/GCE; (D) the linear regression curve of peak

current vs. DA concentration at NF/SWCNT/PMT/GCE.

linear regression equations were written as Ipa(�A) = 0.267 × [DA](�M) (R2 = 0.998) and Ipa(�A) = 0.452 × [DA] (�M) (R2 = 0.999) forNF/PMT/GCE and NF/SWCNT/PMT/GCE, respectively. Moreover,it is worth noting that NF/SWCNT/PMT/GCE is not only moresensitive (with higher slop value) but also more operational(with twice broader linear range of I–[DA]) compared to that ofNF/PMT/GCE, Fig. 3D). To the best of our knowledge, the value ofsensitivity (0.452 �A × �M−1) is comparable to the best results,recently reported in literature using polypyrrole film dopedwith sulfonated �-cyclodextrins (0.886 �A × �M−1) or poly(3,

4-ethylenedioxythiophene-co-(5-amino-2-naphthalenesulfonicacid)) modified electrode (∼1 �A × �M−1) [34,35].

D.P. Quan et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 764– 770 767

0.80.60.40.20.0-0.2

0

5

10

15

20

25

DA

AA

a

c

b

I /µA

E /V vs. Ag/AgCl

a: bare GCE b: PMT/GCE c: NF/PMT/GCE

Fb

3N

flmaotDibaewdfiActoItmfaet

Aoaovsab

3o

tf

0,80,60,40,20,0-0,210

20

30

40

50

60

70

2001501005000

10

20

30

40

50 I = -1.3 + 0.26488 * C (µA)R = 0.99921

I /µA

CDA /µM

[AA] = 3.7 mM 5 µM

30 µM

50 µM

80 µM

111 µM

145 µM

177 µMDA

I /µA

E /V vs. Ag/AgCl

the oxidation peaks of AA–DA and DA–UA merged significantly.At NF/PMT/GCE, the overlapping was less pronounced but still pre-sented (Fig. 6B), implying that purely electrostatic repulsion, basedon NF component was not enough to avoid the interference of

0.80.60.40.20.0-0.2

0

1

2

3

4

DA

UA

AA

I /µA

I /µA

E /V vs. Ag/AgCl

0.80.60.40.20.0-0.24

6

8

10

12

14

16

18

5 µM3 µM

7 µM

11 µM

15 µM25 µM

40 µM

60 µMDA + UA

AA

E /V vs. Ag/AgCl

A

B

ig. 4. DPVs of mixture of 100 �M DA and 1000 �M AA in 0.1 M PBS of pH 4.0 at (a)are GCE; (b) PMT/GCE; (c) NF/PMT/GCE.

.2. Selective determination of DA in binary DA–AA mixture onF/PMT/GCE

As mentioned above, AA coexists with DA in the extracellularuid of the central nervous system and their concentrations areuch higher than that of DA. The interference of AA to DA detection

rises from two aspects: one is the very similar oxidation potentialf AA and DA at bare GCE; the other is the electrocatalytic oxida-ion of DA by AA. Thus, one could expect that the oxidation wave ofA will be affected by the presence of AA. Effectively, as presented

n Fig. 4, current peaks of DA and AA were poorly distinguished atare GCE (curve a). However, the peak overlapping was reducedt PMT/GCE (curve b), which can be assigned to the role of thelectroactive PMT. The different mechanism of interaction of PMTith DA and with AA made their diffusion to the electrode surfaceifferentiated. Further, with NF addition in NF/PMT/GCE, the inter-erence of AA to DA could be significantly eliminated, as shownn curve c. At pH 4 DA (pKa = 8.87) exits in cationic form, whileA (pKa = 4.17) can be found almost equally in anionic as well asationic forms. Even the concentration of AA was 10 times largerhan that of DA, much smaller detected peak of AA, compared to thatf DA (curve c), clearly demonstrated that pH of 4 is a good choice.t should be emphasized that in many studies reported in literaturehe detection occurs in PBS medium of pH 6.5–7 [29,35]. Except for

aintaining the physiological environment, the important reasonor their choice was generally based on purely electrostatic inter-ction, suggesting that at this pH the negatively charged NF willxclude AA anions (pKa = 4.17 < pH 7) and provides a preferentialransport channel for DA cations (pKa = 8.87 > pH 7).

Further, the qualitative determination of DA in the presence ofA could be excellently performed in the range of 5.0–177.0 �Mf DA (Fig. 5, inset). The linear regression equation was calculateds: Ipa (�A) = −0.13 + 0.26488 × [DA] (�M) with R2 = 0.999. More-ver, the peak intensity and position of AA did not change with theariation of the concentration of DA in the above mentioned range,ignifying that AA could not interfere to the sensitivity of the DAt NF/PMT/GCE and the selective detection of DA is possible frominary DA–AA mixture at the modified NF/PMT/GCE.

.3. Selective determination of DA in ternary DA–AA–UA mixturen NF/SWCNT/PMT/GCE

Like AA, UA also coexists with DA in the extracellular fluid ofhe central nervous system. UA is the second major interferenceor DA detection as they both are oxidized at the same potential

Fig. 5. DPVs for different concentration of DA in 0.1 M PBS of pH 4.0 at NF/PMT/GCE.Inset: the linear regression curve of peak current vs. DA concentration.

[25]. Hence, this investigation further extended, on the verificationof UA effect on the DA oxidation at modified electrodes.

From Fig. 6A, it can be seen that the oxidation potentials ofAA and UA at PMT/GCE were quite close to each other, making

Fig. 6. (A) DPVs for 1 mM AA, 1 mM UA and 50 �M DA, in 0.1 M PBS of pH 4.0, atPMT/GCE. (B). DPVs for different concentrations (2–60 �M) of DA, 2 mM of AA and0.47 mM of UA, in 0.1 M PBS of pH 4.0, at NF/PMT/GCE.

768 D.P. Quan et al. / Colloids and Surfaces B:

AA

DA

UA

0.7000.6000.5000.4000.3000.2000.1000-40.20x10

-40.23x10

-40.25x10

-40.28x10

-40.30x10

-40.33x10

-40.35x10

-40.38x10

-40.40x10

-40.43x10

-40.45x10

E / V

i / A

0.7000.4500.200-0.050-0.300-4-0.03x10

0

-40.03x10

-40.05x10

-40.08x10

-40.10x10

-40.13x10

-40.15x10

E / V

i / A

pH7

pH3 4

5 6

A

B

Fig. 7. (A) DPVs of mixture solution of 0.1 mM DA, 2.5 mM AA and 0.415 mM UA in0.1 M PBS of pH 4.0 at NF/SWCNT/PMT/GCE. (B). DPVs of mixture solution of 0.1 mMDA, 2.5 mM AA and 0.415 mM UA in 0.1 M PBS at NF/SWCNT/PMT/GCE, in functiono

Utcdm

itgalnlbmgtidcTm

3.4. Interference studies on the NF/SWCNT/PMT/GCE

f pH.

A to the detection of DA in the mixture solution (effectively, inerms of purely electrostatic interaction, the negatively charged NFannot exclude UA (pKa = 5.4 > pH 4). Therefore, it is impossible toetermine the individual concentration of each compound from theerged voltammetric peak on the surface of NF/PMT/GCE.In contrast to NF/PMT/GCE, the situation was substantially

mproved when NF/SWCNT/PMT/GCE was used. Fig. 7A showedhat all three compounds of AA, DA and UA were oxidized at distin-uishable potentials of 0.15, 0.37 and 0.53 V vs. Ag/AgCl, for AA, DAnd UA, respectively to form well-defined and sharp peaks. Thisarge peak separation is expected to allow the selective determi-ation of DA even at the concentration of AA and UA of 20 times

arger than that of DA. As mentioned above, these findings cannote interpreted on the basis of the above electrostatic interactionechanism. The more probable explanation for this is the syner-

etic effect of NF and SWNT at NF/SWCNT/PMT/GCE which containshe cation exchanger, NF, having a selective cation exchange enrich-ng property due to the electrostatic interaction as well as SWCNT,isplaying attractive characteristics, such as much larger spe-

ific surface area, excellent adsorptive ability and catalytic ability.hus, it can be inferred that the electro-oxidation of the analyzedolecules became facile at the surface of NF/SWCNT/PMT/GCE.

Biointerfaces 88 (2011) 764– 770

It is clear that ultimate purpose of DA detection is to effectivelydiscriminate it against AA and UA. In our study, we emphasize theefforts on maximizing the peak separation thus minimizing peakoverlapping. Two important experimental parameters such as NFamount and pH will be discussed and their chosen values will bejustified below.

First, it should be reminded that NF has the ability not only toextract DA but also to decrease the mass transfer rate. Thus, theaddition of a small amount of NF forms a thin film resulting in poorsensitivity to DA, while a larger amount of NF forms a relativelythick film, decreasing the mass transfer rate of DA and the transferrate of electrons within the NF film. Thus in order to enhance theperformance of the hybrid film modified electrode the NF amountshould be optimized. Investigating the relationship between peakcurrents and the amount of NF, experimentally varied from 0% to2.5% in can be inferred that at the beginning, the peak currentincreases with increasing amount of NF, but when the amountexceeds 0.25% the peak current decreases (figure not shown). Withincreasing amount of NF, the sites of ion exchange increase, andthe adsorption on the NF-SWCNT modified electrode is also ampli-fied. Hence, the peak current increases. But, when the amount ofNF is increased beyond a certain value, the NF thickness will inducehigher resistance for the electrochemical process, therefore hinderthe electron exchange between DA and NF/SWCNT/PMT/GCE, lead-ing to a decrease of the electrode sensitivity. Thus, in our study,4.0 �L of 1.0 mg mL−1 NF-SWCNT (0.25 wt.%) was considered as anoptimal value and was chosen for NF/SWCNT/PMT/GCE preparationin all further experiments.

Second, the pH of the electrolyte solution has a strong influ-ence on the oxidation of AA, DA and UA at electrode surface, whenvarying both the peak current and potential. The effect of pH of theelectrolyte solution on the peak current and peak potential wasexamined by recording DPV of AA, DA and UA of concentration0.1 mM, 2.5 mM and 0.415 mM, respectively in a series in the pHrange from 3 to 7. The response of peak current to pH is shown inFig. 7B. For DA the anodic peak current was higher at pH 3 anddecreases gradually with increasing pH. Similarly, UA also gavehigher peak current at pH 4. The peak potential of DA and UA werelinearly shifted to positive side with decreasing pH as shown inTable 1 with a slope of −55.3 mV per pH unit for DA confirmingtwo protons and two electrons were involved in oxidation processrespectively. From this table, in view of simultaneous determina-tion of AA, DA and UA, it is obvious that the lower pH value, thelarger peak separation and the sharper peak forms thus higher sen-sitivity and higher selectivity will be. For above reasons, pH 4 waspreferred to physiological pH 7.

Next, the typical DPVs of the ternary mixture at theNF/SWCNT/PMT/GCE for the DA concentration range from 2 to120 �M were shown in Fig. 8. It can be found that the DPV peakheight was linearly related to the DA concentration over twoconcentration regions, namely, of 1.5–20 �M and 20–120 �M, forwhich the linear regression equations were written respectively asfollows:

Ipa1 (�A) = 0.37 + 0.42×[DA] (�M) (R2=0.9974), (1.5−20 �M)Ipa2 (�A) = 0.9 + 0.14 × [DA] (�M) (R2 = 0.9897) (20 − 50 �M)

It is interesting to note that the slope variation for the tworegions may be an evidence of mechanism change of DA transporttowards the electrode surface, from adsorptive to diffusional mode,accordingly characterized for the lower and higher regions of DAconcentration (Fig. 8, inset).

The interference from selected organic compounds and metalions was evaluated. Interference tests were investigated by DPV, at

D.P. Quan et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 764– 770 769

Table 1Epa,DA–AA and Epa,DA–UA (mV) in function of pH.

pH Epa,DA (mV) Epa,AA (mV) Epa,UA (mV) Epa,DA–AA

(mV)Epa,DA–UA

(mV)

3 401 175 562 226 1614 371 157 532 214 161

425 220 143377 214 131318 209 131

NfDceCiDc

3

srpsie

3

otGTtpoc0Nt

FUo

0,70,60,50,40,30,20,10,0-1

0

1

2

3

4

5

6

7

8

9

Sample

1st Add

2nd Add

3rd Add

4th Add

I /µA

E /V vs. Ag/AgCl

5 282 68

6 246 32

7 187 -22

F/SWCNT/PMT/GCE. DA concentration was fixed at 100 �M. Inter-ering species of 50 or 400 times higher concentration than that ofA were added to the solution. It was found that no interferenceould be observed for glucose (concentration ratio of interfer-nce/DA, n = 400), citric acid (n = 50), NaCl (n = 400), KCl (n = 400),aCl2 (n = 400), MgCl2 (n = 400), NaNO3 (n = 400), NH4NO3 (n = 400),

ndicating that NF/SWCNT/PMT/GCE has excellent selectivity forA and its determination was insignificantly affected by the mostommon interfering species.

.5. Reproducibility and stability of the NF/SWCNT/PMT/GCE

Another advantage of the NF/SWCNT/PMT/GCE was its workingtability, which was tested by measuring the voltammetric cur-ent decay during repetitive DPV cycling. It was found that the DPVeak height (Ipa) remains practically its initial value with a relativetandard deviation less than 5% for 30 successive measurements,ndicating an excellent reproducibility of above proposed modifiedlectrode.

.6. Determination of DA in real samples

In order to verify the reliability of the method for analysisf DA in pharmaceutical product, SWCNTs/PMT/GCE was appliedo determine DA·HCL (dopamine hydrochloride, Rotexmedica,ermany) injection (labeled 40 mg mL−1, used in heart treatment).he result from 10 random samples, analyzed by the standard addi-ion method (Fig. 9) and the relationship between the height of theeak current and DA concentration, demonstrated that the contentf DA·HCl in the drug was 39.851 ± 0.213 mg mL−1. The average

−1

ontent of DA·HCl, calculated as 39.851 mg mL , was less than.4% different from the labeled content, meaning that the proposedF/SWCNT/PMT/GCE could be applicable for direct DA determina-

ion in real samples.

0,80,70,60,50,40,30,20,10,0-0,120222426283032343638404244464850

50403020100-10

02468

101214

I /µA

C / µM

UADA

AA

I /µA

E /V vs. Ag/AgCl

ig. 8. DPVs of mixture solution of different concentration of DA, 1 mM AA, 1 mMA in 0.1 M PBS of pH 4.0 at NF/SWCNT/PMT/GCE. Inset: the linear regression curvef peak current vs. DA concentration.

Fig. 9. DPVs for dopamine injection drug sample. Inset: the linear regression curveof peak current vs. DA concentration.

4. Conclusions

In the present study, NF/PMT and NF/SWCNT/PMT were syn-thesized at the surface of GCE. By means of the CV and DPV,the selective DA determination in the solution with the excessAA and UA of binary (DA–AA) and ternary (DA–AA–UA) mixtureshas been shown. It was found that owing to the synergetic effectof NF and SWNT at NF/SWCNT/PMT/GCE the electro-oxidation ofeach molecule became facile and distinguishable at the surfaceof NF/SWCNT/PMT/GCE and the biosensor showed the excellentfeatures, such as wide linear response range, high sensitivity andselectivity, good reproducibility and long time stability. This sensorproved to be successfully used for DA determination in pharmaceu-tical and clinical preparations.

Acknowledgements

This work was supported by Vietnam’s National Foundation forScience and Technology Development (NAFOSTED) under Grant107.04.108.09.

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