Electrochemically selective determination of dopamine in the presence of ascorbic and uric acids on the surface of the modified nafionsingle wall carbon nanotubepoly(3 methylthiophene) glassy carbon electrodes

Sampath, Selective determination of uric acid in presence of ascorbic acid and dopamine at neutral pH using exfoliated graphite electrodes, Electroanalysis 16 2004 866–869.. Lin, Simulta

jo u rn al h om epa g e :w w w e l s e v i e r c o m / l o c a t e / c o l s u r f b

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

Pham Hung Vieta

a Research Centre for Environmental Technology and Sustainable Development, Hanoi University of Science, 334 Nguyen Trai Road, Ha Noi, Viet Nam

b Institute of Materials Science, Vietnam Academy of Science and Technology, 18, Hoang Quoc Viet Road, Ha Noi, Viet Nam

Article history:

Received 13 May 2011

Received in revised form 11 August 2011

Accepted 12 August 2011

Available online 22 August 2011

Keywords:

Nafion (NF)

Single-walled carbon nanotubes (SWCNT)

Poly(3-methylthiophene) (PMT)

Dopamine (DA)

Electrochemical methods

AvoltammetricmethodbasedonacombinationofincorporatedNafion,single-walledcarbonnanotubes andpoly(3-methylthiophene)film-modifiedglassycarbonelectrode(NF/SWCNT/PMT/GCE)hasbeen successfullydevelopedforselectivedeterminationofdopamine(DA)intheternarymixtureofdopamine, ascorbicacid(AA)anduricacid(UA)in0.1Mphosphatebuffersolution(PBS)pH4.Itwasshownthat

todetectDAfrombinaryDA–AAmixture,theuseofNF/PMT/GCEwassufficient,buttodetectDAfrom ternaryDA–AA–UAmixtureNF/SWCNT/PMT/GCEwasrequired.Thelatermodifiedelectrodeexhibits superiorelectrocatalyticactivitytowardsAA,DAandUAthankstosynergiceffectofNF/SWCNT (com-bininguniquepropertiesofSWCNTsuchashighspecificsurfacearea,electrocatalyticandadsorptive properties,withthecationselectivityofNF).OnthesurfaceofNF/SWCNT/PMT/GCEAA,DA,UAwere oxidizedrespectivelyatdistinguishablepotentialsof0.15,0.37and0.53V(vs.Ag/AgCl),toform well-definedandsharppeaks,makingpossiblesimultaneousdeterminationofeachcompound.Also,ithas severaladvantages,suchassimplepreparationmethod,highsensitivity,lowdetectionlimitand excel-lentreproducibility.Thus,theproposedNF/SWCNT/PMT/GCEcouldbeadvantageouslyemployedforthe determinationofDAinrealpharmaceuticalformulations

© 2011 Elsevier B.V All rights reserved

1 Introduction

3,4-Dihydroxyphenyl ethylamine, commonly known as

dopamine (DA), has been of interest to neuroscientists and

chemistssinceitsdiscoveryinthe1950s[1].Asthemost

signifi-cantneurotransmitter,theamountofDAdistributedinorganshas

greatinfluencesonhumanemotionsandis directlyrelatedtoa

varietyofdiseasesderivingfromtheabnormallowconcentration

levelofDAwhichhasbeenlinkedtoseveralneurological

disor-ders, e.g., schizophrenia, Huntington’s disease, and Parkinson’s

disease(thethirdmostpopularoneintheworld),andeventhe

HIVinfection[2–4].ElectrochemicaldetectionofDAisafeasible

approachbecauseofitsgoodelectroactivityand easyoxidation

It hasbeen shown that the oxidation of DA is a two-electron

irreversibleprocesswithtransferoftwoprotons.Uricacid(UA)

isthemajorfinalproductofpurinecatabolisminhumanbody.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.vn

(T.D Lam).

ahealthyhuman,thenormallevelofUAinurineisinmMrange whereasinserumitisin␮Mrange.AbnormallevelsofUAare symptomsofseveraldiseasessuchasgout,hyperuricemina,and Lesch–Nyan disease [5] Ascorbicacid (AA)is present in many vegetables,citrusfruitsand biologicalfluidswhereitactsasan anti-oxidantandfree-radicalscavenger.AAconcentrationinthe bodyfluidscanbeusedtoassessthelevelofoxidativestressis related to diseases like cancers, diabetes mellitus and hepatic disorders UA and AA is co-present in biological fluids suchas bloodandurine

Fromtheabovereasons,itisessentialtodeveloprapidand sim-plemethodstodetect/determinetheDAconcentration.Oneofthe biggestchallengesofelectrochemicaldetectionofDAinreal bio-logicalmatrixesisthecoexistenceofmanyinterferingcompounds

Inbiologicalsystems,AAusuallycoexistswithDAin extracellu-larfluidata highconcentrationlevel,nearly1000timeshigher thanDA.Moreover,DA,AAandUAcanbeoxidizedatpractically thesamepotentialatbareelectrodes,resultinginthepeak over-lappingaswellaspoorresponseresolutioninDAdetermination HomogeneouscatalyticoxidationofAAbyoxidizedDA,interaction

ofAAandtheproductsofDAoxidationareotherdifficultiesinthe

DAdetermination,severelylimitingtheaccuracyofdetection 0927-7765/$ – see front matter © 2011 Elsevier B.V All rights reserved.

elec-trodesinsteadofbareonesispreferred.Variousmaterials,such

asmetalormetaloxidenanoparticles(NPs)andmetalcomplexes

(AuNPhybridfilmandnanogoldmodifiedcarbonfiberelectrodes,

palladium nanoparticle loaded carbon nanofibers, zinc oxide

compositefilm,rutheniumoxide,magneticFe3O4 NPsdeposited

on gold electrode, titanate nanotubes, LaFeO3 nanoparticles

[6–15]); organic polymers and composites (polymeric films of

aniline, pyrrole, 3-methylthiophene and p-nitrobenzene

resor-cinolarereported tobeusefulin theselectivedetectionof DA

in excess of AA [16–19], poly(cresol-red) modified

electrode-spoly(oracet blue) modified electrode, poly(eriochrom black T)

modified electrode, poly(naphthalene sulfunic acid) modified

electrode,poly(Evansblue)modifiedelectrode,poly(vinylalcohol)

and poly(chromotrope 2B)-modified electrodes are also used

to detect DA and/or DA, AA and UA, simultaneously;

poly(4-amino-1,1′-azobenzene-3,4′-disulfonic acid)-coated electrode

hasbeenreportedfortheselectivedetectionofDAinthe

pres-enceofAA, UAand NADH, poly(3-(3-pyridyl) acrylicacid), and

3-(5-chloro-2-hydroxyphenylazo)-4,5-dihydroxynaphthalene-2,7-disulfonic acid have been reported for simultaneous

determination of AA, DA and UA [20–26])can be used to

fab-ricate modified electrode due to their excellent properties to

decrease the over-potential, accelerate the electron and mass

transfer rate or greatly enrich the substrates onthe electrode

surface

This article reports the successful combination of

poly(3-methylthiophene) (PMT) with Nafion (NF) and single wall

carbonnanotubes(SWCNTs)inordertoprovideNF/SWCNT/PMT

modified electrodes with enhanced sensitivity and

selectiv-ity towards DA in the presence of high excess of AA and

UA

Glassycarbonelectrodes(GCE)areveryversatileaselectrode

materialfortraceleveldeterminationoforganicmoleculesasthey

providehighsensitivity,negligibleporosity,andgoodmechanical

rigidity.GCEshavebeenmodifiedbymeansofvariousnanosized

additives[27,28]

PMTis a widelyinvestigated electronicallyconducting

poly-mer,whichcanbeeasilyelectrodepositedontoelectrodesurface

byelectro-oxidationofitsmonomer.TheapplicationsofPMThave

beenextensivelyreportedandshowedexcellentelectrocatalytic

effect,neurotransmitterspecies[29,30]

ThechoiceofSWCNTtodevelopelectrochemicaltransducers

is based on its subtle electronic properties, strong

electrocat-alyticeffect,rapidelectrontransferrate,hightensilestrengthand

chemicalstability,andultra-smallsizeeffect[27].AsforNafion,

aperfluorinatedsulfonatedcationexchanger,havinghydrophilic

andhydrophobicdomains(inwhichthelaterconsistsprimarily

ofTeflonwhiletheformerconsistsofsulfonicgroups,presented

attheendof theside chainsof theTeflonbackbone), excellent

antifoulingcapacity,chemicalinertnessandhighpermeabilityto

cations,hasbeenextensivelyemployedasanelectrodemodifier

fororganicmolecules[31,32].Thesensitivityenhancementofthe

NFmembraneisbelievedtoassociatewithaccumulationsofDA

inthehydrophilicregionsortheionchannelsofNF.Thankstoits

amphiphilicstructure, SWCNTcanbehomogeneouslydispersed

inNF solution.Thus,asynergisticeffectofbothNF-SWCNTfilm

modified can beexpected tofurther enhance sensitivity in DA

determination

The primary objective of this work is not to introduce

novel materials for modified electrodes but to emphasize the

efforts ondesigning/optimizing a sensitive and selective

inter-facefor simultaneouslyelectrochemicaldetermination ofDA in

thepresenceofAAandUAbydifferentvoltammetrictechniques

likecyclicvoltammetry(CV)anddifferentialpulsevoltammetry

(DPV)

2 Experimental 2.1 Chemicals 3-methylthiophene (MT), 5% Nafion® 117 and tetrabuthy-lammoniumperchlorate (TBAP)were fromFluka (Switzerland) Ascorbic acid, dopamine hydrochloride (DA·HCl), uric acid and otherchemicalswerefromSigma(Germany)andwereused with-out further purification SWCNT was purchased from Chengdu OrganicChemicals Co.Ltd (Chengdu,China).Aqueoussolutions werepreparedwithde-ionized(DI)water.Theotherreagentswere

ofanalyticalreagentgrade.Highlypuritynitrogenwasusedfor deaeration

2.2 Electrodepreparation Priortotheelectropolymerization,thesurfaceoftheGlassy car-bonelectrodewaspolishedwith15␮mand0.3␮maluminaslurry andcleanedbyultrasonicationinDIwater PMTwas electrode-positedonaGCE(togetPMT/GCE)fromasolutioncontaining0.1M

MTand0.1MTBAP(dissolvedinacetonitrile(CH3CN))for20sata constantpotentialof1.75V(vs.Ag/AgCl).Afterwards,itwastreated

at0.7Vfor10s

Twomaintypesofmodifiedelectrodes(denotedasNF/PMT/GCE andNF/SWCNT/PMT/GCE)areprepared.First,7␮Lof2.5%NF solu-tionwasscrupulously dropped onto thePMT/GCEsurfaces,the solventwasevaporatedinairtoobtainNF/PMT/GCE.Second,4␮L

ofSWCNTdispersion(pre-carboxylatedbythemixtureof concen-tratedH2SO4andHNO3(1/3,v/v),thenmixedultrasonicallywith 0.25wt%NFsolutionfor30min)wasscrupulouslydroppedonto thesurfacesofPMT/GCE,then,thesolventwasevaporatedinairto obtainNF/SWCNT/PMT/GCE.Allelectrodeswerecarefullyrinsed withDIbeforefurthercharacterization

2.3 Electrochemicalmeasurements

CV and DPV were performed with Autolab PGSTAT-30 Potentiostat/Galvanostat with GPES software (EcoChemie, The Netherlands) The three-electrode system was employed with Ag/AgCl/saturatedKClreferenceelectrodeandPtwireasauxiliary electrode,theworkingelectrodewaseitherabaredGCEora mod-ifiedGCE.Theelectrochemicaldetectionwasperformedin0.1M

pH4.0PBScontainingDA(inabsence/presenceofAA,UA),purged

byhigh-puritynitrogen.Allexperimentswerecarriedoutatroom temperature.AllDPVsweremeasured/recordedintriplicate(DPV peakheight(Ipa)remainsitsinitialvaluewitharelativestandard deviation(R.S.D)lessthan2–3%for3successivescans).Meanvalues wereusedforfurthercalculationsoflinearregressionequation

3 Resultsanddiscussion 3.1 ElectrochemicalbehaviorofDAatthemodifiedelectrode

NF/SWCNT/PMT/GCEwasstudiedbyCV.Fig.1showsthe voltam-mogramsof(a)bareGCE;(b)PMT/GCE;(c)NF/SWCNT/GCEand(d) NF/SWCNT/PMT/GCE,respectivelyin0.1MPBS(pH4.0),atscan rateof100mVs−1.Curves(a)and(b)confirmedthatthe electro-chemicalresponsetoDAatbareGCEandPMT/GCEwasverypoor, whilethatatNF/SWCNT/GCE(curvec)andNF/SWCNT/PMT/GCE (curve d) was much better: a well-defined redox couple was recorded, theanodic current value wasmuch higher than that

of thetwo previouscases (curvesa and b).The reason forthe peak currentenhancement mayoriginatein thefaster electron transferrateand/orlargersurfaceareaofSWCNTaswellaseasier masstransferthankstoaninclusioncomplexofNFandDA,which

0.8 0.6 0.4 0.2 0.0 -0.2

-40

-30

-20

-10

0

10

20

30

(a) GCE

E /V vs Ag/AgC l

Fig 1 CVs of 100 ␮M DA at: (a) bare GCE; (b) PMT/GCE; (c) NF/SWCNT/GCE; (d),

NF/SWCNT/PMT/GCE in 0.1 M PBS of pH 4.0 with the scan rate of 100 mV s −1

couldbedissociatedandrapidlydiffusedmorerapidlythroughthe

porouslayerofSWCNTtothemodifiedelectrodesurface)

Next,toelucidatetheprocesskineticofDAelectro-oxidation

atelectrodesurface,theeffectofthescanrateonthepeak

cur-rentsattheNF/SWCNT/PMT/GCEwasinvestigated(Fig.2).Theinset

showedthatthecathodicandanodicpeakcurrentsincreased

lin-earlyalongwiththesquarerootofscan rates(v1/2), suggesting

thattheelectrochemicalelectro-oxidationofDAwasnon-surface

controlledbutdiffusion-controlledprocess,owingtoaslow

elec-tronhopingacrossthematrixofthecompositefilminthestudied

rangeofpotentialsweeprates,accordingtothefollowing

equa-tions:Ipa(␮A)=−13.3+3.03×v1/2andIpc(␮A)=10.3−2.30×v1/2(I

in␮Aandv1/2inmV1/2s−1/2),withthecorrelationcoefficientsof

0.99815and0.99801,respectively.Thisresultwaswellconsistent

withotherpreviousstudies[30]

ItiswellknownthatinDPV,currentismeasuredattwopoints

foreach pulse,thefirstpoint just beforetheapplicationofthe

pulseandthesecondattheendofthepulse.Thesesamplingpoints

areselectedtominimizenonfaradiccurrent,thusDPVmethodis

consideredmoresensitivethanCV[33]).SoDPVwasusedhereto

detectDAatdifferentmodifiedelectrodes.Fig.3A–Cpresentsthe

relationshipbetweenDAconcentrationandcurrentsignalrecorded

at (a) PMT/GCE; (b) NF/PMT/GCE and (c) NF/SWCNT/PMT/GCE,

respectively It is clear that only at two later cases the linear

dependenceofcurrentIon[DA]concentrationwasobserved.The

0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0

-0,1

-40

-30

-20

-10

0

10

20

30

40

50

60

10

200

v (mV/s)

E /V vs Ag/AgCl

Fig 2 Electrochemical response of 100 ␮M DA in 0.1 M pH 4.0 PBS at

NF/SWCNT/PMT/GCE with different scan rates, from 10 to 200 mV s −1 Inset: Ipa − v1/2

and Ipc 1/2 plots.

0.8 0.6

0.4 0.2

0.0 -0.2

0 2 4 6 8 10 12 14 16 18 20 22

11.4 16.2 23.4 32.9 44.5 57.9 73.2 90.1 108

PMT/GCE

E /V vs Ag/AgCl

0.8 0.6

0.4 0.2 0.0 -0.2 0 2 4 6 8 10 12

1.5

2.5 4.5

7.5 11.4

23.4 32.9

NF/PMT/GCE

E /V vs Ag/AgCl

0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 -0,1 -2 0 2 4 6 8 10 12 14 16 18 20

NF/SWCNT/PMT/GCE

E /V vs Ag/AgCl

A

B

C

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.

linearregressionequationswerewrittenasIpa(␮A)=0.267×[DA] (␮M)(R2=0.998)andIpa(␮A)=0.452×[DA](␮M)(R2=0.999)for NF/PMT/GCE and NF/SWCNT/PMT/GCE, respectively Moreover,

it is worth noting that NF/SWCNT/PMT/GCE is not only more sensitive (with higher slop value) but also more operational (withtwicebroaderlinearrangeofI–[DA])compared tothatof NF/PMT/GCE,Fig.3D).Tothebestofourknowledge,thevalueof sensitivity (0.452␮A×␮M−1)is comparable tothebestresults, recently reported in literature using polypyrrole film doped with sulfonated ␤-cyclodextrins (0.886␮A×␮M−1) or poly(3, 4-ethylenedioxythiophene-co-(5-amino-2-naphthalenesulfonic acid))modifiedelectrode(∼1␮A×␮M−1)[34,35]

0.8 0.6

0.4 0.2

0.0 -0.2

0

5

10

15

20

25

DA AA

a

c b

E /V vs Ag/AgCl

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

Fig 4 DPVs of mixture of 100 ␮M DA and 1000 ␮M AA in 0.1 M PBS of pH 4.0 at (a)

bare GCE; (b) PMT/GCE; (c) NF/PMT/GCE.

3.2 SelectivedeterminationofDAinbinaryDA–AAmixtureon

NF/PMT/GCE

Asmentionedabove,AAcoexistswithDAintheextracellular

fluidofthecentralnervoussystemandtheirconcentrationsare

muchhigherthanthatofDA.TheinterferenceofAAtoDAdetection

arisesfromtwoaspects:oneistheverysimilaroxidationpotential

ofAAandDAatbareGCE;theotheristheelectrocatalytic

oxida-tionofDAbyAA.Thus,onecouldexpectthattheoxidationwaveof

DAwillbeaffectedbythepresenceofAA.Effectively,aspresented

inFig.4,currentpeaksofDAandAAwerepoorlydistinguishedat

bareGCE(curvea).However,thepeakoverlappingwasreduced

atPMT/GCE(curve b),whichcanbeassigned totherole ofthe

electroactivePMT.ThedifferentmechanismofinteractionofPMT

withDAandwithAAmadetheirdiffusiontotheelectrodesurface

differentiated.Further,withNFadditioninNF/PMT/GCE,the

inter-ferenceofAAtoDAcouldbesignificantlyeliminated,asshown

incurvec.AtpH4DA(pKa=8.87)exits incationicform,while

AA(pKa=4.17)canbefoundalmostequallyinanionicaswellas

cationicforms.EventheconcentrationofAAwas10timeslarger

thanthatofDA,muchsmallerdetectedpeakofAA,comparedtothat

ofDA(curvec),clearlydemonstratedthatpHof4isagoodchoice

Itshouldbeemphasizedthatinmanystudiesreportedinliterature

thedetectionoccursinPBSmediumofpH6.5–7[29,35].Exceptfor

maintainingthephysiologicalenvironment,theimportantreason

fortheirchoicewasgenerallybasedonpurelyelectrostatic

inter-action,suggestingthatatthispHthenegativelychargedNFwill

excludeAAanions(pKa=4.17<pH7)andprovidesapreferential

transportchannelforDAcations(pKa=8.87>pH7)

Further,thequalitativedeterminationofDAinthepresenceof

AAcouldbeexcellentlyperformedintherangeof5.0–177.0␮M

ofDA(Fig.5,inset).Thelinearregressionequationwascalculated

as:Ipa (␮A)=−0.13+0.26488×[DA](␮M)withR2=0.999

More-over,thepeakintensityandpositionofAAdidnotchangewiththe

variationoftheconcentrationofDAintheabovementionedrange,

signifyingthatAAcouldnotinterferetothesensitivityoftheDA

atNF/PMT/GCEandtheselectivedetectionofDAispossiblefrom

binaryDA–AAmixtureatthemodifiedNF/PMT/GCE

3.3 SelectivedeterminationofDAinternaryDA–AA–UAmixture

onNF/SWCNT/PMT/GCE

LikeAA,UAalsocoexistswithDAintheextracellularfluidof

thecentralnervoussystem.UAisthesecondmajorinterference

forDAdetectionastheybothareoxidizedatthesamepotential

0,8 0,6

0,4 0,2

0,0 -0,2

10 20 30 40 50 60 70

200 150 100 50 0

0 10 20 30 40

50

R = 0.99921

C

DA /µM

[AA] = 3.7 mM 5 µM

DA

E /V vs Ag/AgCl

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,thisinvestigationfurtherextended,ontheverification

ofUAeffectontheDAoxidationatmodifiedelectrodes

FromFig.6A, itcan beseenthat theoxidationpotentials of

AAand UAatPMT/GCEwerequiteclosetoeachother,making theoxidationpeaksofAA–DAandDA–UAmergedsignificantly

AtNF/PMT/GCE,theoverlappingwaslesspronouncedbutstill pre-sented(Fig.6B),implyingthatpurelyelectrostaticrepulsion,based

onNF component wasnotenough toavoid theinterferenceof

0.8 0.6

0.4 0.2

0.0 -0.2

0 1 2 3 4

DA

UA AA

E /V vs Ag/AgCl

0.8 0.6

0.4 0.2

0.0 -0.2

4 6 8 10 12 14 16 18

5 µM

3 µM

7 µM

DA + UA

AA

E /V vs Ag/AgCl

A

B

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

DA

UA

0.700 0.600 0.500 0.400 0.300 0.200 0.100 0

-4

0.20x10

-4

0.23x10

-4

0.25x10

-4

0.28x10

-4

0.30x10

-4

0.33x10

-4

0.35x10

-4

0.38x10

-4

0.40x10

-4

0.43x10

-4

0.45x10

E / V

0.700 0.450

0.200 -0.050

-0.300

-4

-0.03x10

0

-4

0.03x10

-4

0.05x10

-4

0.08x10

-4

0.10x10

-4

0.13x10

-4

0.15x10

E / V

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 in

0.1 M PBS of pH 4.0 at NF/SWCNT/PMT/GCE (B) DPVs of mixture solution of 0.1 mM

DA, 2.5 mM AA and 0.415 mM UA in 0.1 M PBS at NF/SWCNT/PMT/GCE, in function

of pH.

UAtothedetectionofDAinthemixturesolution(effectively,in

termsofpurelyelectrostaticinteraction,thenegativelychargedNF

cannotexcludeUA(pKa=5.4>pH4).Therefore,itisimpossibleto

determinetheindividualconcentrationofeachcompoundfromthe

mergedvoltammetricpeakonthesurfaceofNF/PMT/GCE

In contrast to NF/PMT/GCE, the situation was substantially

improvedwhenNF/SWCNT/PMT/GCE wasused.Fig.7Ashowed

thatallthreecompoundsofAA,DAandUAwereoxidizedat

distin-guishablepotentialsof0.15,0.37and0.53Vvs.Ag/AgCl,forAA,DA

andUA,respectivelytoformwell-definedandsharppeaks.This

largepeakseparationisexpectedtoallowtheselective

determi-nationofDAevenattheconcentrationofAAandUAof20times

largerthanthatofDA.Asmentionedabove,thesefindingscannot

beinterpretedonthebasisoftheaboveelectrostaticinteraction

mechanism.Themoreprobableexplanationforthisisthe

syner-geticeffectofNFandSWNTatNF/SWCNT/PMT/GCEwhichcontains

thecationexchanger,NF,havingaselectivecationexchange

enrich-ingpropertyduetotheelectrostaticinteractionaswellasSWCNT,

displaying attractive characteristics, such as much larger

spe-cificsurfacearea,excellentadsorptiveabilityandcatalyticability

Thus,itcanbeinferredthattheelectro-oxidationoftheanalyzed

moleculesbecamefacileatthesurfaceofNF/SWCNT/PMT/GCE

ItisclearthatultimatepurposeofDAdetectionistoeffectively discriminateitagainstAAandUA.Inourstudy,weemphasizethe effortsonmaximizingthepeakseparationthusminimizingpeak overlapping.TwoimportantexperimentalparameterssuchasNF amountandpHwillbediscussedandtheirchosenvalueswillbe justifiedbelow

First,itshouldberemindedthatNFhastheabilitynotonlyto extractDAbutalsotodecreasethemasstransferrate.Thus,the additionofasmallamountofNFformsathinfilmresultinginpoor sensitivitytoDA,whilealargeramountofNFformsarelatively thickfilm,decreasingthemasstransferrateofDAandthetransfer rateofelectronswithintheNFfilm.Thusinordertoenhancethe performanceofthehybridfilmmodifiedelectrodetheNFamount shouldbeoptimized.Investigatingtherelationshipbetweenpeak currentsandtheamountofNF,experimentallyvariedfrom0%to 2.5% incan beinferredthat atthe beginning,thepeak current increases withincreasingamountof NF, but whenthe amount exceeds0.25%thepeakcurrentdecreases(figurenotshown).With increasingamountofNF,thesitesofionexchangeincrease,and theadsorptionontheNF-SWCNTmodifiedelectrodeisalso ampli-fied.Hence,thepeakcurrentincreases.But,whentheamountof

NFisincreasedbeyondacertainvalue,theNFthicknesswillinduce higherresistancefortheelectrochemicalprocess,thereforehinder theelectronexchangebetweenDAandNF/SWCNT/PMT/GCE, lead-ingtoadecreaseoftheelectrodesensitivity.Thus,inourstudy, 4.0␮Lof1.0mgmL−1NF-SWCNT(0.25wt.%)wasconsideredasan optimalvalueandwaschosenforNF/SWCNT/PMT/GCEpreparation

inallfurtherexperiments

Second,thepHoftheelectrolytesolutionhasastrong influ-enceontheoxidationofAA,DAandUAatelectrodesurface,when varyingboththepeakcurrentandpotential.TheeffectofpHofthe electrolytesolutiononthepeakcurrentandpeakpotentialwas examinedbyrecordingDPVofAA, DAandUAof concentration 0.1mM,2.5mMand0.415mM,respectivelyinaseriesinthepH rangefrom3to7.TheresponseofpeakcurrenttopHisshownin Fig.7B.ForDAtheanodicpeakcurrentwashigheratpH3and decreasesgraduallywithincreasingpH.Similarly,UA alsogave higherpeakcurrentatpH4.ThepeakpotentialofDAandUAwere linearlyshiftedtopositivesidewithdecreasingpHasshownin Table1withaslopeof−55.3mVperpHunitforDAconfirming twoprotonsandtwoelectronswereinvolvedinoxidationprocess respectively.Fromthistable,inviewofsimultaneous determina-tionofAA,DAandUA,itisobviousthatthelowerpHvalue,the largerpeakseparationandthesharperpeakformsthushigher sen-sitivityandhigherselectivitywillbe.Forabovereasons,pH4was preferredtophysiologicalpH7

Next, the typical DPVs of the ternary mixture at the NF/SWCNT/PMT/GCE for the DA concentrationrange from 2 to

120␮MwereshowninFig.8.ItcanbefoundthattheDPVpeak height was linearly related to the DA concentration over two concentrationregions,namely,of1.5–20␮Mand20–120␮M,for whichthelinearregressionequationswerewrittenrespectivelyas follows:

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 tonotethat theslope variation for thetwo regionsmaybeanevidenceofmechanismchangeofDAtransport towardstheelectrodesurface,fromadsorptivetodiffusionalmode, accordinglycharacterizedforthelowerandhigherregionsofDA concentration(Fig.8,inset)

3.4 InterferencestudiesontheNF/SWCNT/PMT/GCE Theinterferencefromselectedorganiccompoundsandmetal ionswasevaluated.InterferencetestswereinvestigatedbyDPV,at

Table 1

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

(mV)

E pa,DA–UA

(mV)

electrode

peakcurrentandDAconcentration,demonstratedthatthecontent

ofDA·HClinthedrugwas39.851±0.213mgmL−1.Theaverage

contentofDA·HCl, calculatedas 39.851mgmL−1,wasless than

0.4%differentfromthelabeledcontent,meaningthattheproposed

NF/SWCNT/PMT/GCEcouldbeapplicablefordirectDA

determina-tioninrealsamples

0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0

-0,1

20

22

24

26

28

30

32

34

36

38

40

42

44

46

48

50

50 40 30 20 10 0

-10

0

2

4

6

8

10

12

14

UA DA

AA

E /V vs Ag/AgCl

Fig 8 DPVs of mixture solution of different concentration of DA, 1 mM AA, 1 mM

UA in 0.1 M PBS of pH 4.0 at NF/SWCNT/PMT/GCE Inset: the linear regression curve

0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 -1 0 1 2 3 4 5 6 7 8 9

Sample 1st Add 2nd Add 3rd Add 4th Add

E /V vs Ag/AgCl

Fig 9 DPVs for dopamine injection drug sample Inset: the linear regression curve

of peak current vs DA concentration.

4 Conclusions

Inthepresentstudy,NF/PMTandNF/SWCNT/PMTwere syn-thesized at thesurface of GCE By means of theCV and DPV, theselective DAdetermination in thesolution withtheexcess

AAandUAofbinary(DA–AA)andternary(DA–AA–UA)mixtures hasbeenshown.Itwasfoundthatowingtothesynergeticeffect

ofNFandSWNTatNF/SWCNT/PMT/GCEtheelectro-oxidationof each moleculebecamefacileand distinguishable at thesurface

ofNF/SWCNT/PMT/GCEand thebiosensor showedtheexcellent features,suchaswidelinearresponserange,highsensitivityand selectivity,goodreproducibilityandlongtimestability.Thissensor provedtobesuccessfullyusedforDAdeterminationin pharmaceu-ticalandclinicalpreparations

Acknowledgements ThisworkwassupportedbyVietnam’sNationalFoundationfor Scienceand TechnologyDevelopment (NAFOSTED)under Grant 107.04.108.09

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