N

NanoelectrodesinSECM439●NNNADH—Reducedformofnicotinamideadeninedin-ucleotide(NAD+).NADHandNAD+areimportantcoenzymesincellularenergyproduction.Withabio-chemicalstandardpotential,E,of−.VNADHisakeyredoxandhydrogencarrierinthemetabolicsub-strateoxidation.Locatedinthecellplasmaitservesasacofactor,e.g.,inthecitricacidcycleandglycolysisincellrespiration.TogetherwithNADPHNADP+(nicoti-namideadeninedinucleotidephosphate)itisessentialforthefunctioningofNAD(P)+-dependenthydroge-nases,as,e.g.,lactatedehydrogenase,alcoholdehydro-genase,andglucosedehydrogenase.InthisfunctionitisalsoessentialwhenNADH-dependentenzymesareusedforbiosensorsandbiofuelcells.Atunmodifiedelec-trodes,theredoxbehaviorofNADHNAD+isratherir-reversible.WhereastheoxidationofNADHtoNAD+requiresapplyingalargeoverpotential(electrochemi-calirreversibility)thereductionofNAD+oftenleadstotheformationratherofthedimerthanofNADH.Onepossibilitytoincreasethestabilityandtheelectrochem-icalreversibilityistousemodifiedelectrodesthatfacili-tatethetwo-electronredoxprocesses(Fig.).NADH—Figure.StructuresofNADHandNAD+,andthereductionofNAD+[i]Ref.:[i]EgginsBR()Biosensors:anintroduction.Wiley,ChichesterUSNafion◯R—registeredtrademarkofE.I.DuPontdeNemours&Co.Nafion◯Risacopolymerconsistingofsulfonate-terminatedperfluorovinylethergroupsonatetrafluoroethylene(Teflon)backbone(Fig.).Itrepre-sentsoneoftheprototypesofmodernpolyelectrolytes(ionomers)withahighprotonconductivitythermal,chemical,andmechanicalstability.Thehighioniccon-ductivityofNafionisbasedonamicroscopicmorphol-ogycomprisinginterconnectedmicro-channelsandsul-fonateclusters[i,ii].Nafionmembranesarebroadlyused,examplesare,e.g.,polymerelectrolytemembranefuelcells(PEMFC)andtechnicalelectrolysisprocesses.Solutionsoflow-molecular-weightNafioninalcoholshavebeenusedtocastfilmsonelectrodesurfacestopro-ducepolyelectrolytepolymer-modifiedelectrodes[iii,iv]andtobindcatalysttomembranesinPEMFCs.Nafion◯R—Figure.StructuralformulaofNafion◯RRefs.:[i]Heitner-WirguinC()JMembraneSci:;[ii]Mau-ritzKA,MooreRB()ChemRev:;[iii]MurrayR()Moleculardesignofelectrodesurfaces.Wiley,NewYork;[iv]InzeltG()Mechanismofchargetransportinpolymer-modifiedelectrodes.In:BardAJ(ed),Electroanalyticalchemistry,vol..MarcelDekker,NewYorkUSNanodes—Occasionallyusedtermforultramicro-electrodeswithnmdimensions.Ithasseveralmeaningsoutsideofelectrochemistry.Thetermnanoelectrodesisprobablymoredescriptiveandpreferred.FS,AJBNanoelectrode—Electrodewithacharacteristicdi-mensionbelowμm.Seeultramicroelectrode(UME).CGZNanoelectrodesinSECM—Fortheimprovementofthelateralresolutioninscanningelectrochemicalmi-croscopy(SECM)thesizeoftheultramicroelectrode440Nanoparticles●Nusedastiphastobedecreased.Moreover,duetothede-pendenceofthetip-to-sampledistancefromthetipsize,smallerelectrodesunequivocallyrequireanaccuratepo-sitioningoftheSECMtipinconstantdistancethrough-outscanning.Inordertodecreasethesizeoftheactiveelectrodesurfacebelowtheminimumsizeofthecom-merciallyavailablenoble-metalwiresmainlyelectro-chemicaletchingproceduresweredescribedleadingtoconicallyshapedultramicroelectrodes.Insulatedetchedtipswerepreparedusingapiezonwax,electrophoreticpaint,electrochemicallydepositedself-limitingpoly-merfilmsorpoly(ethylene).Recently,theuseofalaserpullerforthefabricationofultramicroelectrodeswasproposed.Theobtainednanoelectrodeswerecharacter-izedbymeansofelectronmicroscopy,cyclicvoltamme-try,andz-approachcurvesinSECM.Incombinationwithshearforce-dependentconstant-distanceposition-inginSECM,nanoelectrodesweresuccessfullyappliedfortheimagingoflocalelectrochemicalactivityofsam-plesathighresolution.Refs.:[i]FishG,BouevitchO,KokotovS,Lieberman,PalankerD,TurovetsI,LewisA()RevSciInstr:;[ii]ShaoYH,MirkinMV,FishG,KokotovS,PalankerD,LewisA()AnalChem:;[iii]SunP,ZhangZQ,GuoJD,ShaoYH()AnalChem:;[iv]KatemannBB,SchuhmannT()Electroanalysis:;[v]ZoskiCG()Electroanalysis:;[vi]LugsteinA,Bertag-nolliE,Kranz,KuengA,MizaikoffB()ApplPhysLett:;[vii]SklyarO,WittstockG()JPhysChemB:;[viii]Kate-mannBB,SchulteA,SchuhmannW()Electroanalysis:;[ix]SklyarO,TreutlerTH,VlachopoulosN,WittstockG()SurfSci:;[x]FanFR,BardAJ()PNASUSA:;[xi]UfheilJ,HessC,BorgwarthK,HeinzeJ()PhysChemChemPhys:WSchuNanoparticles—Nanoparticlesaresmallsolid-stateparticleswithdimensionsbelowca.nm.Duetothesmallsize,nanoparticlesexhibitarangeofinter-estingphysicalandchemicalpropertiesdifferentfromthoseofthecorrespondingbulkmaterial.Themoststrikingphenomenonobservedwithelectricallycon-ductivenanoparticlesisthetransitionfromhighden-sityofstatesinmacroscopicparticlestomoresepa-rated(quantized)energylevelsinnanoparticleswithdi-mensionsdecreasingbelowca.nm.Whileamacro-scopicparticlehasphysicalpropertiesinvariantwithparticlesize,ananoparticlemayshowphysicalprop-ertiesdramaticallychangingwithitssize,forinstance,Cunanoparticleswithdiameterϕnmbecomesuperhard,Ptnanoparticleswithϕnmloosetheircatalyticpropertiestowardoxygenreductionand