Influence of the gold nanoparticles electrodeposition method on Hg(II) trace electrochemical detection

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OULOUSE

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Eprints ID : 5953

To link to this article : DOI:10.1016/j.electacta.2011.10.101

URL : http://dx.doi.org/10.1016/j.electacta.2011.10.101

To cite this version : Hezard, Teddy and Fajerwerg, Katia and

Evrard, David and Colliere, Vincent and Behra, Philippe and Gros,

Pierre Influence of the gold nanoparticles electrodeposition

method on Hg(II) trace electrochemical detection. (2012)

Electrochimica Acta, vol. 73. pp. 15-22. ISSN 0013-4686

Any correspondence concerning this service should be sent to the repository

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Influence

of

the

gold

nanoparticles

electrodeposition

method

on

Hg(II)

trace

electrochemical

detection

Teddy

Hezard

a,b,c,d

_,

_Katia

_Fajerwerg

e,f,∗

_,

_David

_Evrard

c,∗∗

_,

_Vincent

_Collière

e,f

_,

Philippe

Behra

a,b

_,

_Pierre

_Gros

c

a_Université_de_Toulouse;_INPT,_LCA_(Laboratoire_de_Chimie_{Agro-industrielle);}_UMR_1010,_ENSIACET,₄_allée_Emile_Monso,_F-31030_Toulouse_Cedex_4,_France b_INRA;_LCA_(Laboratoire_de_Chimie_{Agro-industrielle),}_F-31030_Toulouse,_France

c_Université_de_Toulouse,_Laboratoire_de_Génie_Chimique,_UMR_{CNRS/INPT/UPS}_5503,_Université_Paul_Sabatier,_F-31062_Toulouse_Cedex_9,_France d_FCS_RTRA_“Sciences_et_Technologies_pour_{l’Aéronautique}_et_{l’Espace”,}₂₃_avenue_Edouard_Belin,_F-31400_Toulouse,_France

e_CNRS,_LCC_(Laboratoire_de_Chimie_de_{Coordination),}₂₀₅_route_de_Narbonne,_F-31077_Toulouse,_France f_Université_de_Toulouse,_UPS,_INPT;_LCC,_F-31077_Toulouse,_France

Keywords: Goldnanoparticles

Electrodepositionmodecomparison Modifiedelectrodecharacterization Hg(II)tracedetermination Anodicstrippingdetection

a

b

s

t

r

a

c

t

Goldnanoparticles(AuNPs)weredepositedonGlassyCarbon(GC)substratebyusingthree electrochem-icaltechniques:CyclicVoltammetry(CV),Chronoamperometry(CA)andPotentiostaticDouble-Pulse (PDP).Foreachelectrodepositionmethod,theresultingAuNPs-modifiedelectrodeswerecharacterized byCVinH2SO4andFieldEmissionGunScanningElectronMicroscopy(FEG-SEM).CAwasfoundtobe

thebestelectrodepositionmodeforcontrollingthemorphologyandthedensityofAuNPs.The modi-fiedelectrodeswereusedforlowHg(II)concentrationdetectionusingSquareWaveAnodicStripping Voltammetry(SWASV).AuNPsobtainedbyCAaffordedthebestamperometricresponsewhile involv-ingthelowestamountofchargeduringtheelectrodepositionstep(QAu(III)).Thisanalyticalresponseis

correlatedtoboththesmallestparticlesize(ca.17nmindiameter)andthehighestparticledensity (332particlesmm−2),thusdisplayinghighelectrodeeffectivesurfacearea.Intheseoptimalconditions, usingaHg(II)preconcentrationtimeof300s,thenanosensorarrayexhibitedalinearityrangefrom0.80 to9.9nMwithasensitivityof1.16mAnM−1.Adetectionlimitof0.40nM(s/n=3)wasreached.

1. Introduction

Sincethemiddleofthe20thcentury,mercury(Hg)pollution

hasemergedasamajorproblembecauseofitshightoxicity.Like

mostotherpollutants(chromium,lead,cadmium,arsenic,

pesti-cides,etc.)thepresenceofHginnaturalsystemscausessignificant

humanhealthproblemsandenvironmentalrisks[1,2].Among

inor-ganicandorganicHgspecies,methylmercuryandneutralHg(II)

complexspeciesappeartobeamongthemosttoxicformsmainly

duetotheirhydrophobicpropertiesandtheirstrongaffinityfor

biologicalcompoundsthroughoutsulphydrylgroups[3,4]andDNA

binding[5].Monoanddimethylmercuryaccumulateinvitalorgans

andtissues[6]andareresponsibleformanyseverediseasessuchas

∗ Correspondingauthorat:CNRS,LCC(LaboratoiredeChimiedeCoordination), 205routedeNarbonne,F-31077Toulouse,France.Tel.:+330561333130;fax:+33 0561553003.

∗∗ Correspondingauthor.LaboratoiredeGénieChimique,UMRCNRS/INPT/UPS 5503,UniversitéPaulSabatier,F-31062ToulouseCedex9,France.Tel.:+33056155 6073;fax:+330561556139.

E-mailaddresses:[email protected](K.Fajerwerg),

[email protected](D.Evrard).

kidneyinjury,respiratoryfailure,centralnervoussystemdisorders,

braindamages,andcanevenfinallyinducedeath[7].Moreover,due

tobioaccumulationandbioamplification,Hg(II)ishighlydangerous

evenatverylowconcentration[8–10].Thisisconsistentwiththe

guidelinevalueof1mgL−1(ca.5nM)deliveredbytheWorldHealth

Organizationandappliedinmanycountries[11].Thus,Hg(II)trace

detectionandquantificationintheenvironmentisavery

challeng-ingresearchfield,andthereisanincreasingneedforanalytical

systemsthatdeliverfastandreliabledata.Particularly,effortshave

tobefocusedonsensitivityenhancementsandontheloweringof

thedetectionlimitsoftotalHg.

Leopoldetal.recentlyreviewedthenumerousworkson

spec-troscopictechniquesthathavebeenreportedintheliteraturewith

respectto Hg(II)trace analysis[12]. These latteroffer

selectiv-ityandquitegoodsensitivity[13],themostcommonbeingCold

VapourAtomicAbsorptionSpectroscopy(CVAAS)andColdVapour

AtomicFluorescenceSpectroscopy(CVAFS).Coldvapour

genera-tionsystemcanalsobecoupledtoInductivelyCoupledPlasmaMass

Spectrometry(ICP-MS)andeventomulticollectorICP-MSfor

look-ingatHgisotopebehaviourinnaturalsysteminordertobetter

knowitscyclingandsources[14].Thedouble-spikeisotope

(3)

forHgspeciation[15,16].Althoughthesetechniquesallowvery

lowconcentrationsdownto1pgL−1_(ca.₅_fM)_to_be_reached,_they

involverelativelyexpensivematerial,speciallyforICP-MSmethod,

andrequirecomplicatedand/orheavyprocedures,thuslimiting

anyinsituoronlineandoperandoanalysis.

Comparatively,electrochemicalsensing devices representan

interesting alternative. The advantages of the electroanalytical

techniquesoverotherdetectionmethodsaremanifold:lowcost,

easeofuse,lowenergyrequirementsandsimpleprocedures,and

thepossibilitytobuildportablesystemsthataresuitableeitherfor

laboratoryoron-sitemeasurements.Mostofthetime,detection

limitsinthepicomolarrangecanbereached[17,18]bycombining

apreconcentrationsteplikeAnodicStrippingVoltammetry(ASV)

[19]andpulsedtechniquessuchasDifferentialPulse

Voltamme-try(DPV)[17,20]orSquareWaveVoltammetry(SWV)[18,21,22].

Upto now,many workingelectrode materialshave beenused,

suchasplatinum[23],carboninalmostallitsforms[20,24–28]

andgold(Au).However,becauseofitsstrongaffinityforHgthat

enhancesthepreconcentrationeffectduringtheaccumulationstep,

Auisthemostcommonlyusedelectrodematerial.Arapidglance

totheliteratureoffersagoodinsightofthewiderangeofsuch

sys-temsthathavebeenreported:Aucanbeusedasabulk[17,21],

film[29,30],microwire[18,31],microdisk[32],ormicrodiskarray

electrode[33].Anotherstrategythatisfrequentlyencounteredto

improvethe performancesof the electrochemicalsensor is the

useofchemicallymodifiedelectrode.Theelectroactivesurfaceis

thenfunctionalizedusingeitherpolymers[34,35],organic[36]or

biologicalcompounds[37,38]chosenfortheircomplexingaffinity

towardsHg(II).

Thedevelopmentof nanoscalematerialsin recent yearshas

beenextensive,particularlywithrespecttometallicnanoparticles.

Interestshavefocusedontheiruseinanalyticalchemistrybecause

oftheir specificphysicochemical properties [39]. These include

enhanceddiffusionofelectroactivespecies basedonhigh

effec-tivesurfaceareaofAunanoparticles(AuNPs),improvedselectivity,

catalyticactivity,highersignal-to-noiseratioanduniqueoptical

properties[40].Theseattractivepropertiescombinedtothestrong

affinityofAuforHgfavouredtheappearanceofanewkindof

elec-trodemodificationinvolvingAuNPs[40,41].AuNPscanbeprepared

bychemicalsynthesis[42,43]ordirectlybymeansof

electrochem-icaltechniques[44–46].WithrespecttoHg(II)tracedetermination,

veryfewworkshavebeenreportedtillnow:AuNPswere

electrode-positedbyChronoamperometry(CA)eitheronAu[47]orGlassy

Carbon(GC) [48].On theotherhand,Gong etal.[49]proposed

amorecomplicatedsystemmadeofbimetallicAu-Pt

nanoparti-clesdepositedontoorganicnanofibersusingCyclicVoltammetry

(CV).Veryrecently,wereportedmonometallicAuNPs-modifiedGC

electrode(AuNPs-GC)devotedtolowHg(II)concentration

determi-nation[50].Inthislatterwork,AuNPshavebeenelectrodeposited

byCVstartingfromHAuCl4andcharacterizedasafunctionofthe

chargeconsumedduringtheAu(III)reductionstep.TheAuNPs-GC

electrodesallowedalimitofdetectionof0.42nMtobereached,

andtheiranalyticalperformancesprovedtobestronglycorrelated

toboththedensityandsizeoftheNPs.Thebestresponsestowards

Hg(II)tracedeterminationhavebeenrecordedforAuNPs-GC

elec-trodeswithhighdensity of rather small,spherical-shapedNPs.

Fromthesepreviousresults,ithasbeenevidencedthatseveral

fac-torsincludingthecontroloftheelectrodepositionconditionsare

ofgreatimportance.Inordertostudymoreextensivelythisaspect,

theinfluenceoftheelectrodeposition methodontheanalytical

performancesofAuNPs-GCelectrodeshastobeconsidered.

Inthepresentwork,wedescribethecomparisonofthree

differ-entelectrodepositionmethods,namelyCV,CAandPotentiostatic

Double-Pulse(PDP).ThedifferentAuNPsdepositswere

character-izedbyCVinH2SO4 and FieldEmissionGunScanningElectron

Microscopy (FEG-SEM). For each electrodeposition mode, the

analyticalperformancesoftheresultingAuNPs-GCelectrodesare

discussedwithrespecttotheassayoflowHg(II)concentration.

2. Experimental

2.1. Chemicalsandapparatus

Allthesolutionswerepreparedusingultrapurewater(Milli-Q,

Millipore,18.2Mcm).HAuCl4·3H2O(proanalysisgrade)was

pur-chasedfromAcrosOrganics.NaNO3andHCl30%(suprapurgrade)

wereobtainedfromMerck.H2SO495%(normapurgrade)was

sup-pliedbyVWRProlabo.Astandardstocksolutionof4.99±0.01mM

Hg(II)waspreparedbydilutionof1001±2mgL−1_Hg(NO

3)2NIST

standardsolution(certiPURgrade,Merck)andacidifiedtopH2

withconcentratedHNO3 65%(suprapurgrade,Merck),andthen

usedasitforfurtherdilution.

Alltheelectrochemicalexperimentswereperformedatroom

temperaturein a TeflonPFA three-electrodecell(Metrohm) by

using a PGSTAT 128N potentiostat(Metrohm Autolab,Utrecht,

Netherlands)interfacedtoalaptopcomputerandcontrolledwith

NOVA1.7softwarepackage(Metrohm).AMetrohmAg/AgCl/KCl

3Melectrode,separatedfromtheelectrochemicalcellbyaTeflon

PTFEcapillarycontaininga0.1MNaNO3solutionandterminatedby

aceramicdiaphragm(Dtype),andaMetrohmGCwirewereused

asreferenceand counterelectrodes,respectively.Working

elec-trodeswereGCrotatingdiskelectrodesfromRadiometer(3mm

diameter,A=7.07mm2₎_or_AuNPs-GC._The_{electrochemical}_cell_was

maintainedinaFaradaycageinordertominimizetheelectrical

interferences.Whenindicated,thesolutionsweredeaeratedusing

aN2streamfor10min.AN2atmospherewasthenmaintainedover

thesolutionduringthecorrespondingexperiments.

2.2. Electrodepreparationandmodification

Priortoeachmodification,theGCsurfaceswerecarefully

pol-ishedsuccessivelybysiliconcarbidegrindingpaper(grit1200)for

5s,andbya9mm,5mmand1mmaluminaslurry(Presi)onacloth

polishingpadfor10min,5minand3min,respectively.Between

each polishingstep, thesurfaces werecleaned inan ultrasonic

ethanolbath(5min)inordertoremoveanyimpurity.Finally,they

wererinsedinanultrasonicultrapurewaterbath(5min)anddried

for1minusingaN2stream.Then,thequalityofthepolishingstep

wasverifiedbycheckingthesurfacestateusingaNikonEclipse

LV150opticalmicroscope.

AuNPsweredepositedontoGCelectrodesusingCV,CAorPDP

methodandstartingfromadeaerated0.1MNaNO3solution

con-taining0.25mMHAuCl4(pH3).TheprocedureusingCVhasbeen

previouslyreported[50].Briefly,AuNPswereobtainedbyscanning

theworkingelectrodefromtheopen-circuitpotential(ca.0.90V)

to0.00Vatascanrateof50mVs−1_for_a_given_number_of_scans_(N).

ElectrodepositionusingCAwasperformedatEd=0.00Vfora

giventimetdfrom1to600s.Forthesakeofclarity,the

abbrevi-ationCAwasusedalthoughthismethodisreferringtoaconstant

potentialelectrolysis.

Concerning PDP, the following conditions were applied for

thefirstpulse: E1=0.00V, t1=50ms; and for thesecondpulse:

Ed=0.75V,tdfrom1to600s.

Whatever the electrodeposition method used, the resulting

electrodeswereactivatedina0.5MH2SO4solutionbyrunning10

scansbetween0.20and1.40Vatascanrateof100mVs−1_.

2.3. AuNPscharacterization

TheAuNPs-GCsurfacewascharacterizedbyFEG-SEMusinga

JEOLJSM 6700Fequipmentwithaccelerating voltages of5 and

(4)

Fig.1. CVsrecordedduringAu(III)reduction(accordingtoReaction(1))ataGC electrodeinadeaerated0.1MNaNO3solutioncontaining0.25mMHAuCl4:first

(solidline)andsecond(dashedline)scans(scanrate:50mVs−1_).

onthesample.Imageanalysiswascarriedoutusingahomemade

programforparticlescounting(densityestimation)and average

diametermeasurementdevelopedusingMATLABimage

process-ingtoolboxsoftware.ThedensityandaveragesizeofAuNPswas

evaluatedfroma12.1mm2 GCsurfaceanalysis,countinga

mini-mumof390–4000particles(dependingonthechargeusedduring

theelectrodepositionstepQAu(III)).Foreachdeposit,theerrorwas

calculatedfromtheanalysisofatleastthreedifferentSEMimages.

ForPDPimages, onlyspherical-shapedNPswereconsideredfor

densityandsizemeasurements.

2.4. StrippingvoltammetricdetectionofHg(II)

Electrochemical detection and assay of Hg(II) on

AuNPs-GC electrodes were performed in a deaerated (N2) 0.01M HCl

solution by using Square Wave Anodic Stripping Voltammetry

(SWASV)inthefollowingconditions:cleaningpotential=0.80V,

cleaningtime=15s;preconcentrationcathodicpotential=0.00V,

preconcentration time=300s; pulse amplitude=25mV, step

amplitude=5mV, frequency=200Hz; anodic scan from 0.00 to

0.80V.Duringthepreconcentrationstep,thesolutionwasstirred

bymeansoftherotatingworkingelectrode(2000rpm).Asecond

scanwasrecordedimmediatelyafterthefirstoneusingthesame

conditionsexceptthepreconcentrationtimewhichwassetto30s,

andconsideredasablank.Hg(0)reoxidationpeakheights(1Ip)

weremeasuredfromthecurvesobtainedaftersubtractionofthe

blank.Thisprocedure,called“subtractiveASVmethod”,hasbeen

previouslyreportedintheliterature[17].Itallowstheanalytical

resultstobereleasedfrombackgroundvagaries(seeSection3for

furtherprecisions).It is noteworthythat thesubtractiveanodic

signalsrecordedwithlowHg(II)concentrationswerenoisysowe

neededtouseaSavitzky–Golaysmoothingfunction.

3. Resultsanddiscussion

3.1. AuNPselectrodepositiononGCelectrode

Fig.1showsthefirsttwoscansobtainedbyCVonGCwitha

0.25mMHAuCl4 solution.Theforwardscan(Fig.1,solidline)of

thevoltammogramexhibitsthereductionofAu(III)toAu(0)witha

Fig.2. CurrenttransientsrecordedataGCelectrodefromadeaerated0.1MNaNO3

solutioncontaining0.25mMHAuCl4:usingCAwithEd=0.00Vfortd=3s(dashed

line);usingPDPwithE1=0.00V,t1=50msandEd=0.75V,td=3s(solidline).

cathodicpeakat0.48V,inducingthedepositionofAuNPsontothe

electrodesurfaceaccordingtoReaction(1):

AuCl4−+3e−→Au+4Cl− (1)

Onthebackwardscan,acurrentcrossoveroccurredat0.62V:

beyondthispoint,thebackwardcathodiccurrentbecamehigher

than the forwardone. This is consistent withthermodynamics

whichpredictsaneasiergrowthofpreviouslyformedAuNPsthan

anucleationofnewAuNPsonGCelectrode.Onthesecondscan

(Fig.1,dashedline) theAu(III) reduction peakshiftedtoa less

cathodicpotential(ca.0.78V),indicatingthatinthepotentialrange

between0.48and0.78VAu(0)depositionoccurredmoreeasilyon

theNPscreatedduringthefirstscan.

Fromthisvoltammogram,apotentialofEd=0.00Vwaschosen

astheworkingpotentialforelectrodepositionexperimentsusing

CA.Suchavaluecorrespondstoanoverpotential1=−0.70Vlarge

enoughtofavourtheestablishmentofnewnucleationsitesoverthe

growthofpreviouslycreatednuclei[51,52].Theelectrodeposition

timetdwasvariedfrom1to600s.Whateverthetdvalueused,the

currenttransientsallexhibitedatypicalshapewhichhasbeen

pre-viouslyreportedbyEl-Deab[53]andKomsiyskaandStaikov[52]

(Fig.2,dashedline):first,aninitialhighcathodiccurrentduetothe chargingofthedouble-layer;initialnucleiareformedatthisearly

stage.Then,acurrentdecaywhichfollowsCottrell’slaw

(propor-tionaltot−1/2_),_and_corresponds_to_a_planar_diffusion_regime_arising

fromtheoverlappingofthegrowinghemisphericaldiffusionlayers

intheneighbouringoftheNPs[45].Inallthisseriesofexperiments

theoverpotentialusedwashighenoughsothatthe“hump-shaped”

responsefollowingthechargingcurrentspikedescribedbyFinot

etal.[51]wasnotobserved.

Fig. 2 (solid line) presents the current transient obtained

usingPDPfortheelectrodepositionmethod.Atshorttimevalues

(0<t<t1),thiscurveexhibitsthesameshapethantheCAone

corre-spondingtothechargingofthedouble-layerandtheformationof

thefirstnuclei.WhenswitchingfromE1=0.00toEd=0.75V(PDP),

abreakwasobservedonthecurrenttransient(Fig.2,inset)andan

anodiccurrentwasrecordedforashorttime.Thispositivecurrent

maybeexplainedastheresultofadestabilizationoftheNPsdue

tooverlappingdiffusionzones.WhenswitchingfromE1toEd,only

thelargestnucleiformedduringthefirstpulsearestableunderthe secondpulseandNPswhoseradiirarelowerthanrcriticaldissolve

[54,55].Thenthecurrenttransientbecamenegativeagainandthe

increaseinthecathodiccurrentobservedbetween0.06and0.36s

(5)

Fig.3. CVsrecordedina0.5MH2SO4solutiononaAuNPs-GCelectrodeprepared

byCAusingtd=15s(solidline)andonanunmodifiedGCelectrode(dashedline)

(scanrate:100mVs−1_).

thecurrenttransientdecreasedagainfollowingCottrell’slawinthe

samewaythantheCAcurve.

3.2. CharacterizationofAuNPs-GC

3.2.1. Electrochemicalcharacterization

AuNPsdepositswerecharacterizedbyperformingCV

experi-mentsina0.5MH2SO4solution,scanningthepotentialbetween

0.20and1.40V.Fig.3providesanexampleforAuNPs

electrode-posited by CA using td=15s (solid line). The voltammograms

exhibitedthewell-knownanodicpeaksbetween1.10and1.35V

whichcorrespondtotheformationofdifferentkindsofAuoxides

[56],mainlyAuOaccordingtoReaction(2):

Au+H2O →AuO+2H++2e− (2)

Onthebackwardscanareductionpeakat0.87Vwasobserved

whichwasassociatedtothesubsequentreductionoftheoxides

formedduringtheforwardscan.Comparativelyasmallresidual

currentwasrecordedinthesameexperimentalconditionsusing

anunmodified GCelectrode (Fig.3,dashedline).Whatever the

electrodepositionmethodused,theshapeofvoltammogramswas

foundtobesimilarforagivenelectrodepositionchargeQAu(III)used.

Increasingthischarge,theshapeoftheanodicpartof

voltammo-gramsslightlyevolved,accordingtoavariationoftherelativeratios

ofthedifferentcrystallographicfacesofAuNPs [53,57].Table1

summarizesthevaluesofthecharge correspondingtothe

elec-trodepositionstepQAu(III)usingeitherCA,PDPorCV,andthecharge

relatedtoAuoxidesreductionQoxidesobtainedfromFig.3.Onone

hand,QAu(III)recordedforCAisinthesameorderofmagnitude

thanthatobtainedusingPDPwhatevertheAuelectrodeposition

time.Thisresultisinaccordance withFig.2where thecurrent

transientsare thesamefor both electrochemicalmethods. It is

notsurprisingifitisconsideredthattheamountofcharge

con-sumedforthenucleiformationisnegligiblecomparedtotheone

necessaryfortheNPsgrowthinthedepositiontimerangestudied.

Themeandifferenceislogicallyobtainedfortheshortertotal elec-trolysistime,correspondingtothehighestratiot1/(t1+td)inPDP

mode.OntheotherhandtheamountofchargeQoxidesisstrongly

lowerthanQAu(III).Assumingthat3electronsareexchangedduring

Au(III)reductionand2forAuoxidesformation(accordingto

Reac-tion(2))andthatthefaradicyieldforAuelectrodepositionis100%,

andconsideringthatQoxidesisequaltotheamountofAuwhichis

oxidizedduringtheCVsinH2SO4,thislatteriscalculatedfromthe

following relation: nAuO/nAu=1.5×Qoxides/QAu(III). In the case of

PDP,thisratiodecreasesfromabout17to3.6%whenincreasing

theelectrodepositiontime.ConsequentlyonlytheAuatom

lay-ersattheAuNPs-electrolyteinterfacearebelievedtobeeffectively oxidized,thispartdecreasingwithincreasingparticlessize. 3.2.2. Densityandsizecharacterization

Inordertofurthercharacterizethedeposits,SEM-FEG

analy-seswereperformed.Fig.4showstypicalimagesobtainedusing

CA(framesA–D),PDP(framesE–H)andCV(framesI–L)for

differ-entelectrodepositionconditions.TheNPsdensityandsizearealso

summarizedinTable1.ThedepositsobtainedbyCAwere

homo-geneouswithsmalland nearlyhemispherical-shapedAuNPsfor

depositiontimestdlowerthan300s.Theoverpotential1=−0.70V

waslargeenoughtoachieve instantaneousnucleationandthen

effectivelyseedthesurfacewithnucleiatshortt_d.Thisisconsistent withthefactthatthedensityreachedanoptimum,i.e.332NPs/mm2

(Table1,entry3).Atthesametime,theaveragesizeincreasedwith

thedepositiontimeandthenQAu(III).Fortd=300s(frameC)faceted

andsomepolyhedralAuNPsstartedtoappearandattd=600sAu

“popcorn-like”nanoclusterswereformed(frameD).This

morphol-ogycouldresultfromasimultaneousaggregationofAuNPsalready

presentonthesurfaceandtheformationofnewnuclei.

ManystudiesclaimthatthePDPprocedureisanefficientway

tocontrolthegrowthofthenucleiformedduringthefirstpulse

E1 sincetheshorttime t1 associatedtothis stepshouldfavour

monodispersity of the deposits [52,55,58–60]. Indeed, the first

pulseisusedtoinitiatetheformation ofnucleiandthesecond

pulseE_d, more positive thanE1 is usedto favourtheirgrowth

[54,58–60].TheAuNPsmorphologyobtainedinthispresentstudy

usingthiselectrodepositionmodecanbedividedintotwokindsof

populations(framesE–H):thefirstoneisrelatedtoisotropicand

spherical-shapedNPsfortd≤30sandthesecondonetoanisotropic

nanowires [61,62]. Thepresence of thesetwo kinds of

popula-tionswasevenmoreevidentwhenthedepositiontimetdincreased

beyond300s.TheinsetsinFig.4(FandG)arehigh-magnification

SEM-FEGimagesofthenanowireswhichillustratethattheselatter

shouldresultfromacoalescencephenomenon.TheNPsdensity(ca.

200NPs/mm2₎_was_nearly_constant_in_all_the_t

drangeconsidered

(Table1).Thistendencyisnotsurprisingwhileconsideringthat

thestablenucleiformedduringthefirstpulse(largeoverpotential) shouldbepreserved[54].

TheevolutionofthemorphologyandthedensityofAuNPs

elec-trodepositedbyCA,PDPandCV[50]dependsonthechargefor

electroreductionstepsQAu(III),i.e.low (ca.100mC)andhigh(ca.

800mC)whatevertheelectrodepositionmethod.Briefly,itcanbe

clearlyobservedthat atlowQAu(III) thedensityrecordedforCA

(239NPs/mm2_,_Table₁_,_entry₄₎_is_in_the_same_order_of_magnitude

thanthatobtainedusingPDP(259NPs/mm2_,_Table₁_,_entry_10)._The

valueobtainedinthelattercaseseemstobeslightlyoverestimated.

FortheCVmethod,twodistinctpopulationsofelectrodeposited

AuNPscanbealsodistinguished(framesI–L).Thefirstpopulation

ofparticlesisattributedtosmallandspherical-shapedNPsandthe

secondonetolarger,aggregate-likeNPs.Inourpreviouswork[50],

anincreaseintheparticledensityhasbeennoticedupto4

poten-tialscans,andthenthedensitydecreaseddownto12cyclesand

finallyremainednearlyconstanttill20scans.Thisevolutionofboth

theAuNPsdensityandaveragesizeindicatesagradualaggregation

and/orcoalescencephenomenonuptoacriticalvaluemoreorless

pronouncedwhatevertheelectrodepositionmethodused.

3.3. ElectrochemicalresponseandoptimizationtowardsHg(II)

detection

InordertotestandoptimizetheresponseofAuNPs-GC

(6)

Fig.4.FEG-SEMimagesofAuNPs-GCelectrodespreparedfromadeaerated0.1MNaNO3solutioncontaining0.25mMHAuCl4usingCA(A,B,C,D),PDP(E,F,G,H)andCV(I,

J,K,L).ConditionsforCA:Ed=0.00Vfortd=3s(A),30s(B),300s(C)and600s(D);ConditionsforPDP:E1=0.00V,t1=50msandEd=0.75Vfortd=3s(E),30s(F),300s(G)

and600s(H);NumberofscansforCV:1(I),4(J),8(K)and20(L).Thescalebarsintheinsets(FandG)are100nm.

experimentswasperformedwiththedifferentdepositsobtained

byCAandPDP.Fig.5showsthetypicalSWASVrecordedfora4.0nM

Hg(II)concentrationin0.01MHClusingAuNPs-GCelectrode

pre-paredbyCA(td=15s).Thisconcentrationwaschosenbecauseit

iseasilydetectableandallowsfurtheroptimizationofthesystem.

Forapreconcentrationtimeof300s(E=0.00V),abroadbaseline

centredat0.30Vwasobservedtogetherwithasharppeakat0.58V

(Fig.5,solidline)whichcorrespondstoHg(0)reoxidationaccording toReaction(3):

Hg+2Cl−

→HgCl2+2e− (3)

Hg(II)wasfirstreducedtoHg(0)duringthepreconcentration

stepandthenreoxidizedat0.58Vduringthestrippingstep.

Con-trary towhat has beenpreviously reported by severalauthors

[22,48],werecentlyassumedthatthebroadbaselinewasnotdue

tocalomelformationbuttoanoxidationofAuNPssurfaceinthe

presenceofCl− _ions_[50]_._So,_as_the_background_depends_on_the

amountofaccessiblesitesofAuatoms,recordingaSWASVinthe

absenceofHg(II)inthesolutionwasnotsuitableasablank.Inorder

thepeakcurrentat0.58Vtobeusedforananalyticalpurpose,an

experimentwasthuscarriedoutwithapreconcentrationtimeof

30s(Fig.5,dashedline).Intheseconditions,asmallshoulderwas

onlyobservedintheregionofHg(0)reoxidation.Thiscurvewas

thenusedasananalyticalblankandsubtractedtotheformerone

(Fig.5,inset).

Theeffectofboththeelectrodepositionmethodandtheamount

of electrodeposited AuNPs (QAu(III)) was then examined using

SWASVfora4.0nMHg(II)solution.Fig.6showsthevariationof

thesubtractivepeakcurrent(1Ip)correspondingtoHg(0)

reoxi-dation(blanksubtracted)asafunctionofQAu(III)forCAandPDP.

For both methods, a rapidincrease witha maximum in 1Ip is

observedatlowQAu(III)values,ca.between36and119mC(td=3s

and30s,respectively)forCAand110mC(td=30s)forPDP.Then,

1IpgraduallydecreasedwhileQAu(III)increased.Thisisconsistent

withourpreviousfindingwithCVelectrodeposition(Table1)that

(7)

Table1

CharacterizationofAuNPsonGCand1IpofHg(0)reoxidationforCAandPDPatdifferentdepositiontimestdduringtheelectrodepositionstep,andforCVusingdifferent

numbersofscans(N).

Entry Method td(s) QAu(III)(mC)a Qoxides(mC)b NPsdensity(mm−2)c Averagediameter(nm)c 1Ip(mA)d

1 CA 1 21 1.9 151±4 15±3(1830) 5.7±0.8 2 3 36 1.4 326±2 16±4(3930) 6.7±0.2 3 15 78 4.5 332±5 17±4(4000) 6.8±0.3 4 30 119 4.7 239±3 20±4(2885) 6.6±0.2 5 300 496 11.6 220±5 31±7(2650) 4.6±0.4 6 600 825 23.2 119±9 47±16(1455) 3.0±0.2 7 PDP 1 15 1.7 199±7 13±3(2400) 4.0±0.7 8 3 26 2.0 193±3 13±3(2320) 5.6±0.4 9 15 72 4.2 193±2 19±5(2325) 5.9±0.1 10 30 110 4.9 259±5 18±5(3115) 6.7±0.2 11 300 445 12.0 192±5 32±9(2335) 5.0±0.2 12 600 883 21.4 159 ±7 34 ±10(1915) 3.0 ±0.1 13 CV N=1 99 2.7 53±2 27±16(635) 5.0±1.3 14 N=4 260 5.8 73±3 36±13(860) 7.0±0.4 15 N=8 437 9.4 64 ±3 41±15(805) 6.5±0.2 16 N=20 846 20 32±2 71±20(390) 4.3±0.2 a_Q

Au(III)isthechargeconsumedduringtheelectroreductionstepin0.25mMHAuCl4. b _Q

oxidesisthechargecorrespondingtothereductionofAuoxidesin0.5MH2SO4.

c _See_Section₂_for_details_on_NPs_density_and_average_diameter_estimation._The_values_in_brackets_correspond_to_the_average_number_of_NPs_considered_for_the_calculation. d _1I

pisthesubtractivepeakcurrentofHg(0)reoxidationmeasuredfroma4.0nMHg(II)solution(seeFig.5).

smallNPsasfarastheAuNPsdepositsmaybeviewedasanarray

ofspherical-shapedAu“nanoelectrodes”[50].Whileincreasingtd

forbothelectrodepositionmethods,growthprocessesarefavoured

tothedetrimentofnewnucleiformation,thusleadingtobiggerNPs

andprobablyaggregationand/orcoalescencephenomena,

respec-tively.Asaconsequence,thecurrentdecreasesduetothedecrease

inthenumberof“nanoelectrodes”.Alltheseresultssuggestthatthe

amperometricresponseofthemodifiedelectrodestronglydepends

ontheeffectivesurfaceareaoftheAuNPs.Thelatterincreaseswith

thenumberandthesmallsizeofAuNPs(highsurfacetovolume

ratio).TheeffectivesurfaceareaincreaseswiththenumberofNPs

forthelowestQAu(III)anddecreasesforthehighestQAu(III)because

ofcoalescencephenomena.Inotherwords,thehighestthetotal

AuNPseffectivesurfacearea,thebesttheperformances.However,

inthecaseofCA,themaximumin1Ipwasobservedinatdrange

from3s(QAu(III)=36mC)to30s(QAu(III)=119mC),suggestingthat

forlowelectrodepositionchargevalues,thismethodaffordsmore

homogeneousdepositsthanPDP.Moreover,intheoptimaltdrange,

Fig.5.TypicalSWASVsignalsrecordedataAuNPs-GCelectrodepreparedbyCA (td=15s,QAu(III)=78mC)ina0.01MHClsolutioncontaining4.0nMHg(II)fora pre-concentrationtimeof300s(solidline)and30s(dashedline).Inset:resultingsignal aftersubtractionofdashedlinefromsolidline.

thedensityof AuNPs ishigher for CAthanfor PDP.Finally,CA

providedsimilaranalyticalresponseswhileinvolvinglowerQAu(III).

Forthesereasons,CAwaschosenforlowHg(II)concentrationassay.

3.4. AnalyticalperformancestowardslowHg(II)concentration

assay

FromtheanalysisofFig.6,anAuNPs-GCelectrodeprepared

byCAusingtd=15s(optimalconditions)waschosentostudythe

responsetowardslowHg(II)concentration.Successiveamountsof

Hg(II)wereaddedtoa0.01MHClsolution,andthe

correspond-ingSWASVrecorded(Fig.7).Undertheoptimizedexperimental

conditions (see SWASV parameters as described in Section 2),

theAuNPs-GCelectrodeexhibited a goodlinearityin therange

0.80–9.9nM(7standardconcentrations)withacorrelation

coef-ficientof0.9995(Fig.7,inset).Fromtheslopeofthecalibration

plot,thesensitivityof theAuNPs-GCelectrode wasfoundtobe

Fig.6.Variationofthesubtractivepeakcurrent(1Ip)observedbySWASVat

E=0.58Vwitha4.0nMHg(II)concentrationin0.01MHClasafunctionofthe amountofchargeconsumedduringAuNPselectrodeposition(QAu(III))usingCA

withEd=0.00Vfortd=1–600s(black-colouredcircles)andPDPwithE1=0.00Vfor

(8)

Fig.7.SWASVresponsesobtainedina0.01MHClsolutionforincreasingHg(II) concentrations(blanksubtracted)onaAuNPs-GCelectrodepreparedbyCAusing td=15s.Frombottomtotop:0.8,1.0,2.0,4.0,5.9,7.9and9.9nMHg(II),respectively.

Inset:1Ipvs.[Hg(II)]andlinearregression.

1.16mAnM−1.Alimitofdetection(LD)of0.40nMwascalculated

for a signal-to-noise ratio of 3 [63]. This valuecould beeasily

improvedusinglongerHg preconcentrationtime. These results

areconsistentwithourpreviousfindings dealingwithan

elec-trodepreparedusingCV[50]inwhichwereportedasensitivityof

1.37mAnM−1inaconcentrationrangefrom0.64to4.0nM,andaLD

of0.42nM.BothLDandsensitivityexhibitcomparablevalues,the

latteronebeinghoweverslightlybetterforCV,consideringasame

valueofHg(II)preconcentrationtime(300s).Nevertheless,the

lin-earityrangeaffordedbyCAismorethantwicewider(upperlimit:

9.9nMforCA,vs.4.0nMforCV)sinceasaturationphenomenon

beginstooccurinthecaseofCVforHg(II)concentrationshigher

than4nM,whichisnotobservedhereforCA.Thissuggeststhat

CAallowsabettercontroloftheAudeposit,leadingtoahigher

effectivesurfaceareathanCVforthesameamountofcharge

con-sumedduringtheelectrodepositionstep.Moreover,theoptimal

conditionsforlowHg(II)concentrationdetectionwereobtained

forQAu(III)valuesbetweentwoandfivetimeslowerforCA(Table1,

entries2–4)thanforCV(Table1,entry14).

4. Conclusions

Inthiswork,twodifferentelectrodepositionmethodsofAuNPs,

namelyCAandPDP,wereusedforlowHg(II)concentrationassay

andcomparedwithpreviousresultsobtainedbyCV[50].The

differ-entAuNPsdepositsobtainedwerecharacterizedusingCVinH2SO4

andFEG-SEManalysis.Fromthesetofresults,CAwasfoundto

affordthebestmorphologicalanddensitycontroloftheAuNPs.

Indeed,morehomogeneousdepositswithmorespherical-shaped

NPs for rather short electrodeposition times (td≤300s) were

obtained. CAalso provided better analytical responsestowards

Hg(II) detection than PDP and CV, since comparable 1Ip were

recordedforlower QAu(III).We thussuggestthat theresultsare

stronglycorrelatedtotheeffectivesurfaceareaoftheAuNPs:the

highesttheeffectivesurfacearea,thebesttheperformances.Inthe

optimalconditions,theresponseoftheAuNPs-GCelectrodewas

linearintherange0.80–9.9nMwithasensitivityof1.16mAnM−1,

andaLDof0.40nMwasobtained.Furtherworkisnecessaryto

optimizeanalyticalparameterbycouplingexperimentsand

mod-elling.

Acknowledgments

Thisworkwaspartofa projectfinanciallysupportedbythe

Fondation STAE (Sciences et Technologies pour l’Aéronautique

et l’Espace) under the acronym “MAISOE” (Microlaboratoires

d’AnalysesInSitupourdesObservatoiresEnvironnementaux).The

authorsthankDr.YannickHallezandDr.JérémieViguiéfortheir

helpinMATLABprogramming.Theauthorsthanktherefereesfor

theirfruitfulcomments.

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