3748uniquepropertiesofGrcanonlyberealizedafteritisintegratedintomorecomplexassemblies.SomeGr-basedhybridmaterialshaveshowngreaterversatilityasadvancedelectrodematerialsforthefabricationofelectrochemicalsensorsandbiosensors[13,14].TiO2isametaloxideandhasbeenwidelyusedinthefabricationofelectrochemicalsensorsandbiosensorsduetoitsgoodbiocompatibilityandhighconductivity[15,16].Mostrecently,wereportedtheTiO2–graphene(TiO2–Gr)nanocompositeusedinelectro-chemicalbiosensorconstructionandtheirapplicationinsomebiomoleculesensing,suchasdopamine,glucose,ad-enine,andguanine[17–19].TiO2–Gr-basedanalyticalmethodsshowedexcellentperformance,suchashighselec-tivity,broaddynamicrange,andlowdetectionlimit,whichowedtothecharacteristicsandadvantagesofTiO2–Grnanocomposite,anditopenedanewplatformforelectro-chemicalsensorsandbiosensorsdesign.

Herein,weemployedtheTiO2–GrandNiOcompositesasapropellantofdirectelectrontransferbetweenGODandtheelectrodesurface.Amediator-freeGOD-basedglucosebiosensorwasconstructedthoughalayer-by-layerassemblyapproach.Firstly,theNafion-stabilizedTiO2–GrcompositeswerecoatedonthesurfaceofGCE.Then,NiOwaselectro-depositedontheresultingelectrode.Finally,GODwasself-assembledonthelargeandspecificsurfaceofTiO2–Gr/NiO.Thebiosensorexhibitedspecificandsensitivedetec-tionforglucosewithshortresponsetime,lowdetectionlimit,andhighsensitivity.Preparation,characterization,performance,andfactorsinfluencingtheperformanceoftheobtainedbiosensorwereinvestigated.

ExperimentalMaterials

Graphitepowder,hydrazinesolution(50wt.%),andam-moniasolution(28wt.%)wereobtainedfromShanghaiChemicalReagentCorporation(Shanghai,China).GODwasobtainedfromSigma(SaintLouis,MO,USA).Ti-taniumisopropoxide(Ti(OiPr)4,98%)wasobtainedfromAladdinChemistryCo.,Ltd.Phosphate-bufferedsolutions(PBS)werepreparedbymixingthesolutionsofKH2PO4,Na2HPO4,andKCl.Double-distilledwaterwasusedthroughout.Instruments

CHI660Delectrochemicalworkstation(CHInstruments,Shanghai,China)andastandardthree-electrodecellwhichcontainedaplatinumwireauxiliaryelectrode,asaturatedcalomelreferenceelectrode(SCE),andthemodifiedelectrodeasworkingelectrodewereemployedforelectrochemical

JSolidStateElectrochem(2012)16:3747–3752

studies.AllofthepotentialsinthisarticlewerewithrespecttoSCE.ThepHmeasurementsweremadewithapHmeter(MP230,Mettler-Toledo,Greiffensee,Switzerland).PowderX-raydiffraction(XRD)datawerecollectedonaRigakuMiniFlexIIX-raydiffractometer.Scanningelectronmicros-copy(SEM)imageswereobtainedonaHitachiS-4800scan-ningelectronmicroscope.

PreparationofTiO2–Grnanocomposite

Grapheneoxide(GO)waspreparedfromgraphitepowderbythemodifiedHummersmethod[20].Graphitewasputintoamixtureof12mLconcentratedH2SO4,2.5gK2S2O8,and2.5gP2O5.Thesolutionwasheatedto80°Cwithcontinuousstirringfor5husingoilbath.Next,themixturewasdilutedwithdeionizedwater(500mL).Theproductwasobtainedbyfilteringusing0.2-μmNylonfilmanddriednaturally.Theproductwasre-oxidizedbyHummersandOffemanmethodtoproducethegraphiteoxide.Exfoliationwascarriedoutbysonicating0.1mgmL 1graphiteoxidedispersionfor1h.TiO2–Grnanocompositewaspreparedaccordingtothepreviouswork[17].Inshort,20mgofGOwasdispersedinamixedsolutionofH2O(10mL)andethanol(5mL)underultrasonicationfor1htogetaho-mogenouscolloidalsuspensionofexfoliatedGO.Then,0.2mLofTi(OiPr)4wasaddedtotheGOsuspensionandultrasonicatedforanother1h.Theresultantmixturewastransferredtoa25-mLTeflon-sealedautoclaveandkeptat130°Cfor12h.Thefinalproductwasisolatedbyfiltrationandrinsedthoroughlywithwaterandethanol,respectively.Then,theproductwasdriedinvacuum.TheTiO2–Grnano-compositewasobtainedintheformofblackpowder.Fabricationofmodifiedelectrode

Atotalof1.0mgoftheas-preparedTiO2–Grnanocompo-sitewasdispersedin10.0mL0.25-wt.%Nafionsolutionsunderultrasonicationfor30mintoobtainahomogeneous,well-distributedsuspensionofNafion-TiO2–Grcomposite.Priortothemodification,thebareGCE(3mmindiameter)wascarefullypolishedtoobtainamirror-likesurfacewith0.3and0.05μmaluminaslurry,followedbysuccessivesonicationinwaterandethanolfor5minanddryinginair.Subsequently,20μLofpreparedNafion-TiO2–GrwasdroppedonthecleanedGCEanddriedunderaninfraredlamptopreparetheTiO2–Gr/GCE-modifiedelectrode.Modificationoftheelectrodewasaccomplishedbytrans-ferring10μLof2mMnickelnitratesolutiontothesurfaceofTiO2–Gr/GCEanddryingunderaninfraredlamp.Theelectrodewasthenconditionedbypotentialcyclinginalimitedrange(0.1–0.6V)in0.10MNaOHsolution,assupportingelectrolyte,untilasteadystatevoltammogramwasobtained.ThemodifiedelectrodeNiO/TiO2–Gr/GCE