Severaltechniqueshavebeendevelopedforfabricationofthe well-definedgeometrymicroelectrodesbasedonCNTs,inwhich thesupportsubstratewasisolated,attherangeofnano[7,9,15] tomicroscale[8,13,14,1
Trang 1Talanta
j ourna l h o me p a g e : w w w e l s e v i e r c o m / l o c a t e / t a l a n t a
Fabrication of new single-walled carbon nanotubes microelectrode for
electrochemical sensors application
a School of Materials Science, Japan Advanced Institute of Science and Technology (JAIST), 1-1 Asahidai, Nomi City, Ishikawa 923-1292, Japan
b Faculty of Chemistry, Hanoi University of Science, VNU, 19 Le Thanh Tong, Hoan Kiem District, Ha Noi, Viet Nam
c The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
d Research Center for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, VNU, 334 Nguyen Trai, Thank Xuan District, Ha Noi, Viet Nam
a r t i c l e i n f o
Article history:
Received 31 October 2011
Received in revised form 12 January 2012
Accepted 12 January 2012
Available online 20 January 2012
Keywords:
Single-walled carbon nanotube
Microelectrode
Electrochemical sensors
a b s t r a c t
Inthispaper,wedescribetwosimpledifferentwaystofabricateanarrayofsingle-walledcarbon nano-tubes(SWCNT)microelectrodesfromSWCNTnetwork,grownonSisubstrate,throughmicro-fabrication process.Twokindsofmaterial,photoresist– organiccompoundandsputteredSiO2,wereusedasan insulatorlayerforthesearraysofSWCNTmicroelectrodes.TheSWCNTmicroelectrodeswere character-izedbyscanningelectronmicroscopy(SEM),Ramanspectroscopy,andelectrochemicalmeasurements TheSWCNTmicroelectrodeswithsputteredSiO2asaninsulatorexhibitsomeprioradvancestothese usedphotoresistlayerasinsulatorsuchasmuchstableinharshcondition(highactiveorganicsolvents) andhighcurrentdensity(24.94Amm−2 comparedto2.69Amm−2,respectively).Inaddition,the well-definedgeometryofSWCNTmicroelectrodesisnotonlyusefulforinvestigatingkineticsofelectron transfer,butalsopromisingcandidateinelectrochemicalsensorsapplication
© 2012 Elsevier B.V All rights reserved
Carbonnanotubes(CNTs),discoveredbyIjimain1991[1],isthe
nextgenerationofcarbonmaterials.Theyhavedistinctstructural
andelectronicpropertiescomparedtoconventionalcarbon
mate-rialsthathavebeenwidelyusedinelectrochemistrysuchasglassy
carbon(GC),graphite,andcarbonfiber[2,3].Thecombinationof
highaspectratio,nanometersizeddimensions,goodelectrical
con-ductivity,andlowcapacitance[4]inthepristinestatedictatesthat
CNTshavethecapabilitytomakeexcellentelectrodematerials[5]
Randomnetworksof single-walled carbon nanotubes
(SWC-NTs),whichlieflatontheinsulatorsupportsurface,havemultiply
interconnectedpointsathighdensityofnetworks.Theyhave
con-ductivelengthscalesmuchlongerthantheindividualnanotubes
Infact,theyhaveactingeffectlikethinconductingfilm[6].These
SWCNTnetworkshaveshowninterestingpropertiesforelectrical
andelectrochemicalapplications[5,6]
ThereismuchattentioninusingSWCNTtofabricateelectrode
inthesmalldimension,becausethereareseveralbenefitsofusing
small-scaleelectrodesinelectrochemicalsensors.Firstly,since
cur-rent(i)isproportionaltotheelectrodearea,small-scaleelectrodes
willfurtherreducetheOhmic(iR)dropdistortionandcanbeusedto
∗ Corresponding author Tel.: +81 761 51 1661; fax: +81 761 51 1665.
E-mail address: yztakamura@jaist.ac.jp (Y Takamura).
detectelectrochemicalreactionsinpoorlyconductingmedia,even
intheabsenceofasupportingelectrolyte[7].Secondly, double-layercapacitancesareproportionaltoelectrodearea.Thustheyare greatlyreducedforsmall-scaleelectrodeswhichhavesmall sur-facearea,resultingsmallRC(R:resistance,C:capacitance)time constants inelectrochemical cells [8].Thirdly, therate ofmass transporttoandfromtheelectrode(andtherelatedcurrent den-sity)increasesastheelectrodesizedecreases[9,10]
Normally,CNTelectrodesarepreparedessentiallybyrandomly dispersingorconfiningtheCNTsonaconductingsubstrate,most commonly glassycarbon.Thisis achievedbydropcasting[11], abrasiveattachment[12]ordirectlygrownrandomCNTsnetwork
onmetalsubstratesbychemicalvapordeposition(CVD)[13,14] But,intheseCNTelectrodes,thesupportingconductivesubstrates arenotelectricallyisolatedtheelectrolytesolution,sothatthe elec-trochemicalsignalscamefrombothCNTsandsupportingsubstrate Thus,theyareunsuitableforfundamentalelectrochemicalstudies,
asitisdifficulttoquantitativelyisolatetheSWCNTresponsefrom thatofthesupportsubstrate
Severaltechniqueshavebeendevelopedforfabricationofthe well-definedgeometrymicroelectrodesbasedonCNTs,inwhich thesupportsubstratewasisolated,attherangeofnano[7,9,15]
tomicroscale[8,13,14,16].Koehneetal.fabricatednanoelectrode arrays using vertically aligned multi-walled carbon nanotubes (MWCNT)embeddedwithinSiO2matrix.OnlytheendofMWCNT wasprotrudedtoexposetosolutionbyusingchemicalmechanical
0039-9140/$ – see front matter © 2012 Elsevier B.V All rights reserved.
Trang 2layersontheelectrochemicalpropertiesofSWCNTmicroelectrodes
wereinvestigated.ThecoupleK3[Fe(CN)6]/K4[Fe(CN)6]wasused
asabenchmarktocharacterizetheelectrontransferpropertiesof
SWCNTmicroelectrodes
2.1 Reagent
Fe(NO3)3·9H2O, Mo(acac)2, KCl, K3[Fe(CN)6] were purchased
fromWakoPureChemicalIndustries(Japan).Otherreagentswere
ofanalyticalgrade,and allsolutionswerepreparedanddiluted
usingultra-purewater (18.2Mcm) fromtheMilliporeMilli-Q
system
2.2 Devicefabrication
2.2.1 ElectriccontactonSiwafer
Platinumcontacts,thicknessof30nmwith10nmTiadhesive
layers,werethermallyevaporatedontotheSisubstratethrough
aconventionalphotolithographyprocessandaplasmasputtering
method.TheSisubstrateisthep-typewith100nmofthermally
intrinsicSiO2.Inordertoseparatebetweenelectrodes,achromium
layer(60nmofthickness)wasalsosputteredinthemiddleof
plat-inumcontacts.Thislayerthenwasremovedoffafterthecatalytic
processbychromiumetchantsolutiontoleavetheblankspaceof
SiwaferbetweenelectrodesonthesameSiwafer
2.2.2 GrowthofSWCNTnetwork
SWCNTnetwork wasgrownonSi substratebyethanol CVD
method[17].TheSisubstratewasimmersedinthecatalystsolution
consistingofFe(NO3)3·9H2O,Mo(acac)2,andalumina
nanoparti-clesfor10min,anddriedintheairfor10min[18].Thecatalyst
coatedSi substrate was baked at 130◦C in 3minbefore
trans-ferringintoCVDmachine.Thesubstratewasheatedto825◦Cin
CVDmachineunderargongasandthenargongaswasreplacedby
ethanolvapor.Thegrowthprocesswasconductedfor10min
2.2.3 ApplytheinsulatorlayeronSWCNTnetwork
a)SiO2layer
The procedure for fabricating SWCNT microelectrodes is
showninFig.1a.Achromiumlayer(200nm)was,firstly,
ther-mallyevaporatedontotheSWCNTnetworkusingtheplasma
sputteringmethod.Aphotoresistlayerwiththicknessof15m
(PMERphotoresist)wassubsequentlyspunoverthechromium
layer.Disktypepatternsof180mdiameterinsidethePt
con-tactswereformedbyexposingtothe458nmHelightfor30sand
developinginPG-7solution.Theexposedchromiumlayerwas
removedbychromiumetchantsolutionin2min.Thenathermal
SiO2 layerof250nm wassputtered onexposed SWCNT
net-workbytheplasmasputteringmethod.Finally,theresiduedisk
quentlywashedbypurewatertogettheSWCNTmicroelectrode ThisSWCNTmicroelectrodeisnamedelectrodeP
Inboth ways,24 SWCNTmicroelectrodeswerearrayedona singleSisubstrateof25mm×30mm
2.2.4 Measurement Scanning electron microscopy (SEM) images were obtained usingHitachiS-4100withacceleratingvoltage20kV.Raman spec-trawereperformedwithlaserexcitationenergyof514.5nmon Micro-Ramanmachine
Electrochemical measurements were performed on ALS/CH Instrumentselectrochemicalanalyzer,model730C(USA)asshown
inFig.1c,inwhichthreeelectrodessystemwereusedwithPtwire
ascounter,AgCl/Agmicro-electrodeasreference(Microelectrodes Inc.,USA)andSWCNTmicroelectrodesasworkingelectrode.Adrop
ofKCl 0.1Msolution(25l) containinginterested electroactive specieswasplacedinthePDMSchamberovertheexposedSWCNT area
Cyclicvoltammetricmeasurement onas-grown SWCNT net-workwasconducted byusing conecapillary cell, in whichthe topandbottominnerdiameterofcapillaryare3mmand0.5mm, respectively[19].Thecounterandmicro-referenceelectrodewere placedinsidethecapillary.Theapproachofcapillarytothesurface
ofSWCNT network wasperformedbyusingthe manually con-trolledx-y-zpositioner.Bythisway,thebottomtipofcapillary willnotcontactwiththesurfaceofSWCNTnetwork
Theelectrodeposition of AuonSWCNT microelectrodes was conductedbychronoamperometrictechnique.Theconstant poten-tialof−0.4VversusAg/AgClwasappliedonthesemicroelectrodes foraperiodof20sin0.1MphosphatebuffersalinePBSsolution containing10mMHAuCl[19]
3.1 CharacterizationofSWCNTnetworkandmicroelectrode TheSEMimageofas-grownSWCNTnetwork,showninFig.2a, clearlyillustrateshighdensityofas-grownSWCNTandtheyexist
intheindividualorsmallbundletubes.TheRamanspectrumof as-grownSWCNTisdepictedinFig.2b.Theshapeandpositionofpeaks
inRamanspectrumaround1600cm−1(Gband)allowtoidentify theSWCNT[20].Thecharacterizedpeakofdisordergraphiteat around1350cm−1(Dband)indicatesthatthereislittleamorphous carbonpresentandthattheas-grownSWCNTishighquality,that thereareveryfewdefectsinSWCNT[17,18,20]
ThedistributionofSWCNTdiametercanbeinducedfromthe radialbreathingmode(RBM)followingequation[20]
d= 248
Trang 3Fig 1.Schematic of procedure for fabricating SWCNT microelectrodes (a) Using SiO 2 layer as insulator (b) Using photoresist layer as insulator (c) The electrochemical
Trang 4Fig 2.(a and b) SEM image and Raman scattering spectrum of as-grown SWCNT network, respectively.
whereistheRBMpeakpositionandd(nm)isthenanotube
diam-eter.FromEq.(1)thenanotubediameteriscalculatedandgiven
intheupperx-axisandthevalueofnanotubediameterisinrange
1.0–2.0nm,ingoodagreementwiththoseobtainedfrompublished
literatures[14,17]
Fig.3ashowstheSEMimageofanobtainedSWCNT
microelec-trode.ThediskdiameterofSWCNTmicroelectrodeswascalculated
fromtheSEMimagesandgotthevalueof185±2.6m(averageof
8individualsamples)
The SEM images of SWCNT network inside electrode S and electrodePareshowninFig.3 andc,respectively.TheSWCNT networksofelectrodePafterposttreatmentprocesseslookslightly thickerthanthatinas-grownSWCNTnetworkandelectrodeSas well.TheSEMimagesexhibitthatthedensityofSWCNTnetworks
Trang 5Fig 4. Cyclic voltammograms in 0.1 M KCl solution containing 0.05 mM K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] mixture (1:1 molar ratio) of (a) as-grown SWCNT network (b) electrode S and electrode P All potential scan rates are 4 mV s−1.
afterpostprocessesseemashighasthatofas-grownSWCNT(see
Fig.2a).ThisisbecausetheindividualorsmallbundleofSWCNT
entangleseachothertoincreasethestabilityofnetworkduring
postprocesses.IncaseofelectrodeS,thesurfaceofSWCNT
net-workwascoveredentirelybyasputteringCrlayer.TheCrlayer,
however,finallyremovedbychromiumetchantsolution,which
specifiestoCronlyanddoesnothave anyharmfuleffectstoSi
substrateandSWCNTnetwork.ThisCrlayeralsoactedasashield
topreventdirectcontactoforganiccompounds(photoresist,
sol-vents)totheSWCNTnetwork,whichisbelievedstronglyeffectson
theelectrochemicalpropertiesofSWCNT[9]
Ontheotherhand,theSiO2insulatorlayerisdirectlyappliedon
theSWCNTnetworkwithouttheCrlayer.WhenSiO2isremoved
fromSWCNTsurfacebyeitherwetetchingwithHFsolution[9]
ordryingetching[14].Bothetchingprocessescouldnotexactly
controlremovingtheupperlayerofSiO2insulator.Onecanleadto
overetchingwhichbreaktheSisubstratedestroyingtheSWCNT
network,anotherisnotcompletelyremovedwhichmakeSWCNT
networkinactiveatsomeareasduetotheblockingofSiO2onthe
surfaceofSWCNT(datanotshown)
TheSiO2insulatorexhibitssomeadvantagestophotoresist
insu-lator[8]includingmuchthermalstableandinertinDMF,DMSO
environment.Thesesolventsare usuallyusedasthesolventfor
linkermolecular duringthe immobilizationof antibody onthe
surfaceofSWCNT[14,21]inthesensingprocesscomparedto
pho-toresistlayer
3.2 ElectrochemicalpropertiesoftheSWCNTmicroelectrodes
Fig.4ashowsthetypicalcyclicvoltammogram(CV)ofas-grown
SWCNT electrode in 0.05mMK3[Fe(CN)6]/K4[Fe(CN)6] mixture
solution(1:1molarratio)atpotentialscanrate4mVs−1
TheEp,peakpotentialseparationofforwardandbackward
curvesinaredoxcouple,is67mVsuggestingaquasireversibleat
theelectrode
For a microelectrode using SWCNT network, the
den-sity/separationofindividualorsmallbundleSWCNTiscritical[7]
AsobservedwiththehighdensitySWCNTnetwork,thediffusion
layersofneighborindividualSWCNTlikelyoverlap.Asaresult,the
CVofthiskindofelectrodesinmeasuringtheredoxspeciesinbulk
solutionissimilartoasolidplanarmacroelectrode[22]
ThehighelectroactivityofSWCNThasbeenrecentlyattributed
bythepresentofnano-graphitic“impurity”[23,24],whichisrich
edge planematerial and is thehighest electrochemicalactivity
amongcarbonmaterials[25–27],inas-grownSWCNTratherthan
bySWCNTitself
ItisclearthattheCNTelectrodeisdrivingtheelectron trans-fer(ET)reactionfasterthanmanyothercarbonelectrodessurfaces observed[3],withverysmall apparentactivationbarrierat the electrodesurface
Ontheotherhand,theSWCNTmicroelectrodesshowthevery differentCVbehavior.Fig.4 showstypicalCVsoftwodifferent SWCNTmicroelectrodesin0.05mMK3[Fe(CN)6]/K4[Fe(CN)6] mix-turesolution,recordedatapotentialscanrateof4mVs−1.TheCVs showawell-definedsigmoidalshape,characteristicofsteadystate behavioratultramicroelectrode (UME).Thesteady statefeature
inCVisthestrongevidenceindicatingthatmostexposedSWCNT behaviorasindependentnanoelectrodeafterpostgrownprocesses ThecurrentmagnitudeatSWCNTelectrodeSismuchhighthan that ofelectrode P.Thecurrent densityis inducedfromCVs to yieldavalueof24.49,and2.69Amm−2forSWCNTelectrodeS andP,respectively.ThisillustratesthattheelectrodeSismuch electrochemicalactivethanelectrodeP
Therearesomepossibilitiescausingthisphenomenon(i)the reductionindensityofSWCNTnetworkduetolosingofSWCNT
in postgrown processes (ii)partialblocking surface ofSWCNT afterthecontactingofSWCNTwithphotoresist,whichcausesthe contaminationonSWCNTand(iii)thereductionofnano-graphitic particlesinSWCNTnetwork.ForelectrodeSonlythereasoniand iiicanaffecttothechangeofCVcharacterization
Because the density of SWCNT networks do not change so much,wetentativelyattributethistothereductionof“impurity” nano-graphiticparticles densityin SWCNT afterposttreatment processes.InthecaseofelectrodeP,thepartialblockingsurfaceof SWCNTbyorganiccompoundsfromphotoresist,whichcausethe contamination,alsoaffectsstronglyontheelectrochemical prop-erties[9].InFig.5showstheSEMimagesofAuelectrodepositedon electrodeSandP.WeconfirmedthatthewhitespotsintheSEM imagesareAubyX-raydiffractometry(SEM-EDS)(datanotshown) ThedensityofAuparticlesonelectrodeSisclearlyhigherthanthat
onelectrodeP.ThisisalsotheevidencethattheelectrodeSismore activethanelectrodeP
ThecurrentdensityobtainedatelectrodeSalsomuchhighthan thatoftheMWCNTnanoelectrodearray[7],SWCNTUMEusing photoresistasinsulatorlayer[8],andmicrocarbonfiberelectrode
[28],0.4,2.04and9.84Amm−2,respectively.Thecurrentdensity
atelectrodePisslightlyhigherthanthatattheUME,whichwas fabricatedusingsimilarmethod[8]
Thehysteresis(shiftinhalfwavepotential)betweentheforward andbackwardwavesindicatesaslightdeparturefromatruesteady stateresponse,attributedtothepartialoverlapofdiffusionlayers
atsomelocationsintherandomlydistributednetwork[7]
Trang 6Fig 5. SEM images of Au particles electrodeposited on (a) electrode S (b) electrode P, respectively.
Fig 6. (a) Cyclic voltammograms of electrode S in 0.1 M KCl solution containing 0.025, 0.05, 0.25, 0.5 and 1.0 mM K 3 [Fe(CN) 6 ]]/K 4 [Fe(CN) 6 ] mixture (1:1 molar ratio), scan rate is 4 mV s −1 (b) Plot of limiting currents (at 0.4 V) vs mediator concentration and linear fit for K 3 [Fe(CN) 6 ].
Fig 6a shows the CVs of electrode S in the
K3[Fe(CN)6]/K4[Fe(CN)6] mixture solution (1:1 molar ratio) in
therangeof0.025–1mMwiththepotentialscanrateof4mVs−1
Thediffusion-controlledsteady-statelimitingcurrent,iss,ata
coplanardisk-shapedmicroelectrodeisgivenbyEq.(2)[29]
wherenisthenumberofelectronstransferredperredoxevent,Fis
theFaradayconstant,ristheradiusofthediskelectrodeandCand
Darethebulkconcentrationanddiffusioncoefficientofthe
elec-troactivespecies,respectively.TheuseofEq.(2)isvalidbecausethe
degreetowhichtheelectrodeisrecessedisminimalcomparedtoits
lateraldimension.Fig.6 showsplotoflimitingcurrents(at0.4V)
versusredoxspeciesconcentration.Thelinearfitthroughthepoints
indicatesanearperfectscalingofisswithmediatorconcentration
(r2is0.9903).Thediffusioncoefficientextractedfromtheslopeof
theplotis8.8×10−6cm2s−1,whichisverygoodagreementwith
literaturevalues[29,30]
Theobservedplateauincyclicvoltammetricresponseandthe
scalingof iss withconcentrationofredoxmediatorin the
man-nerpredictedbyEq.(2)indicatesthattheSWCNTmicroelectrode
behavesasaconventionalmetallicUME(e.g.,Pt,Au)
Wehave successfully fabricatedtheSWCNT microelectrodes
based SWCNT network on insulator substrate through
micro-fabricationprocess.TheSWCNTmicroelectrodesusedsputterSiO2
asinsulatorexhibitsomeprioradvancestotheseusedphotoresist
layeras insulatorsuchasmuch stablein harshcondition(high
activeorganicsolvents)andhighcurrentdensity(24.94compared
with2.69Amm−2,respectively).Thesebenefitscomefromusing
ofCrlayertocoverentirelySWCNTnetworkduringthefabricating processes.Inaddition,thewell-definedgeometryofSWCNT micro-electrodes is not only useful for investigating kineticselectron transfer,butalsopromisingcandidateinelectrochemicalsensors application
Acknowledgement
ThisworkwaspartiallysupportedbyaGrant-in-Aidfor Scien-tificResearchonPriorityAreas(No.19054011)andtheCooperative ResearchProgramof“NetworkJointResearchCenterfor Materi-alsandDevices”fromtheMinistryofEducation,Culture,Sports, ScienceandTechnologyofJapan
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