Luminol chemiluminescence induced by silver nanoparticles in(3)

Fig.3.EffectsofthereactionconditionsontheCLintensity.(A)EffectofthepH,reactionconditions:1×10 3mol/LCuSO4,0.01mol/LNaBr,1×10 4mol/Lluminolin0.1mol/LcarbonatebufferswithvaryingpHvalues.(B)Effectoftheluminolconcentration,reactionconditions:pH10.0,1×10 3mol/LCuSO4,0.01mol/LNaBr,luminolwithdifferentconcentrationsin0.1mol/Lcarbonatebuffers.(C)EffectoftheCuSO4concentration,reactionconditions:pH10.3,0.01mol/LNaBr,1×10 4mol/Lluminolin0.1mol/Lcarbonatebuffers.(D)EffectoftheNaBrconcentration,reactionconditions:pH10.3,1×10 3mol/LCuSO4,1×10 4mol/Lluminolin0.1mol/Lcarbonatebuffers.

b).Therefore,theCLaccompanyingtheluminol–CuSO4–AgcolloidreactioninthepresenceofNaBrwasrelatedtotheAgNPsratherthanassociatedspeciespresentinthesynthesis.TheCLspectrashowedthatthemaximumemissionwavelengthwasca.425nm,indicatingthattheluminophorwas3-aminophthalate,anoxida-tionproductofluminol[28].

TheeffectsofthereactionconditionsontheCLintensitywereinvestigated.AsshowninFig.3,theCLintensitywaslargelydepen-dentonthepHofthereactionandthereactantconcentrations,includingCuSO4,NaBr,andluminol.TheCLintensityincreasedwiththepHfrom9.5to10.3,whichwassimilartootherluminolsystems,butitdecreasedwhenthepHwashigherthan10.3(Fig.3(A)).ItmaybeduetothatCuSO4withhighconcentrationsunderultra-basicconditionwouldformCu(OH)2precipitatesbeforetheCLreactions.IncreasingtheconcentrationofthehydroxideioncausesmoreCu(OH)2toprecipitate,whichdecreaseconcentrationofCu2+fromthesolution.BasedontheresultsinFig.3(C),thedecreaseofCu2+concentration(0.5–0mmol/L)leadstothedecreaseoftheCLintensity.Accordingly,thedecreaseoftheCu2+concentrationinthereactionmightbethereasonforthedecreaseoftheCLintensityduringthehighpHrange.AsshowninFig.3(B),theCLintensityincreasedwiththeluminolconcentrationovertherangeof1.0×10 7to1.0×10 3mol/L.AsshowninFig.3(C),theCLintensityincreasedobviouslywiththeCuSO4concentrationover0–0.5mmol/L,butitdecreasedslightlywhentheCuSO4con-centrationwashigherthan0.5mmol/L.TheeffectoftheNaBrconcentrationontheCLintensitywassimilarastheeffectoftheCuSO4concentrationasshownin3(D).WiththeNaBrconcen-trationover0–2.0mmol/L,theCLintensityincreased.However,whentheconcentrationwashigherthan2mmol/L,theCLintensitydecreasedgradually.ItisreasonablethattheincreaseofCLinten-sityfollowedbytheinitialincreaseoftheconcentrationofCuSO4orNaBrbecausetheyarereactantsintheCLreaction.However,furtherincreaseoftheconcentrationofCuSO4orNaBralsoledtoanincreaseinionicstrength.Itwasreportedthattheincreaseof

ionicstrengthcouldcausetheCLdecrease[29].Asaresult,theCLintensitydecreasedslightlyorremainedintactbeyondaparticularconcentrationofCuSO4orNaBr.3.2.MechanismoftheCLreaction

ItreportedthatCu2+couldoxidizeAgcolloidinthepresenceofnucleophiles[12].Therefore,itwasdeducedthatCuSO4mightreactwithAgNPsbytheaidofNaBratthebeginning;then,luminolmightreactwiththeintermediateproductsofAgNPs–NaBr–CuSO4toyieldaCLemissionwhenluminolwasinjectedinthemixture.Tovalidatethishypothesis,supportingexperimentswerecarriedoutasfollows.

Firstofall,UV–visabsorptionspectrawereutilizedtomon-itorthereactionbetweenCuSO4andAgNPsinthepresenceofNaBr.AgNPsexhibitedSPRabsorptionbandsinthevisibleregion(ca.410nm)beforethereaction,ingoodagreementwithreportedresults[30].WhenNaBrandCuSO4wereaddedtotheyellowAgcolloid,thetime-dependentSPRabsorptionspectraofthemixtureareshowninFig.4.TheSPRabsorptionspectraweremeasuredatatimeintervalof0.5s.Although121spectrawereobtainedin60s,only5typicalonesatdifferentmomentsareshowninFig.4.Theinsetsshowthetemporalabsorbanceofca.410nm( max)atatimeintervalof0.5s.Clearly,withtheincreaseofreactiontime,theSPRabsorbancedecreasedgradually.Afterthereaction,theyellowcolorofthemixturedisappeared.TheseresultsindicatedAgNPscouldreactwithCuSO4inthepresenceofNaBrandtheconcentrationofAgNPsdecreasedduringthereaction.

Subsequently,toascertainthereactionproductofAgNPs–NaBr–CuSO4,thereactionsystemwasevaporatedandprecipitatesoccurredinthereactedmixture.Thebrownpow-dersderivedfromtheprecipitateswereinvolvedintoXRDandXPSanalysis.InFig.5,XRDpatternoftheprecipitatefromtheAgNPs–NaBr–CuSO4systemisingoodagreementwithXRDpatternofpureCuBrandAgBr[31,32].Thepeaksat30.9 ,44.4 ,