N.Lietal./JournalofPhotochemistryandPhotobiologyA:Chemistry215 (2010) 185–190189

CLintensityappearedafterinjectingluminolintotheCuBrsolu-tion.ItissupportedthattheCu(I)complexsolutioncouldreactedwithluminoltogenerateCLemissionandthecoexistedAgNO3orAgNPscouldenhancetheCLintensity.Then,SODwasusedtodeter-minewhetherasuperoxideanionwasinvolvedintheCLreactionoftheCu(I)complexsolutionandluminol.SODwasaddedintotheCuBrsolutionbeforeinjectionofluminolandtheCLwasdecreasedwiththeincreaseoftheSODconcentration.TheseresultsrevealedthatasuperoxideanionwasinvolvedintheCLreaction.Onthisbasis,itwassuggestedthattheluminolCLinducedbyAgNPsinthepresenceofNaBrderivedfromCu(I)complex,oneofprod-uctsinthereactionofAgNPs–NaBr–CuSO4,whichreactedwiththedissolvedoxygentogeneratethesuperoxideanion;thenthesuper-oxideanionreactedwithluminoltoproduceCL.TheCLprocessmayproceedasfollows:

Cu(II)+AgNPs(0) Br→

Cu(I)+Ag(I)(1)Cu(I)+O2→O2 +Cu(II)(2)LH +O2 →AP2 +N2(3)AP2 →AP2 +hv

(4)

Basedontheproposedmechanism,Cu(I)complexisakeyintermediateproductintheCLreaction.Br isconsideredasanucleophileandtherearetwoimportantfunctionsintheluminolCL.

TheonefunctionofBr istoenhanceredoxcapabilityofAgNPs,whichreducesCu(II)togenerateCu(I)complex.Increasingthecon-centrationoftheBr-couldcausemoreAgBrtoprecipitate.BasedonNernst’sequation,precipitationofAg+withhalidescoulddecreasethereductionpotentialofAg(0).Therefore,thetendencyoftheoxi-dationofAgNPsbyCu(II)andthegenerationofCu(I)increasewiththeNaBrconcentration.

ThedismutationofCu(I)toCu(0)andCu(II)inwateriswell-known.Withoutnucleophiles,Cu(I)wasconsumedinthedis-mutationreactionbeforereactedwithoxygensothattheCLofCu(I)withluminolwouldbeterminated.Fortunately,Br intheCLreac-tionwasef cientligandstobindtoCu(I)andtopreventCu(I)fromdismutation:

Cu++2Br →CuBr2 (5) Kc=

[CuBr2][Cu+][Br ]2

=105.89

(6)

[CuBr 2][Br ]2

[Cu]

=105.89×=105.89×10 4=101.89=77.6

(7)

whereKcisthesteadyconstantofCuBr2 .With0.01mol/LBr inasolution,theconcentrationratio[CuBr2 ]/[Cu+]isabout78.Accordingly,Cu(I)mightbeintheformofCuBr2 intheCLreactionsystembythevirtueofBr .Br intheluminolCLisef cientligandsforCu(I)sothatCu(I)couldbeaccumulatedinthesolutionfortheCLreaction,whichisanotherfunctionofBr .3.3.TherespondofothernucleophilestotheCL

Sinceothernucleophiles,suchasCl ,I ,andthiosulfatearealsoef cientligandsforCu(I),itisreasonabletodeducethatthesimilarreactionscouldgeneratetheCLinthepresenceofthesenucle-ophiles.However,whenNaClsubstitutedforNaBr,AgNPscouldnotinducetheluminolCLunderthesamereactionconditions.ItisknownthatthesolubilityproductsofAgCl(Ksp,AgCl=1.8×10 10)aremuchlargerthanthatofAgBr(Ksp,AgBr=5.0×10 13).Accord-ingly,thereducingabilityofAgNPsinthepresenceofNaClisweakerthanthatinthepresenceofNaBratthesameconcentration.Dur-ingtheexperiments,whenCuSO4wasaddedintoAgcolloidinthe

presenceofNaBr,thecolorofAgcolloidfadedquicklyandobvi-ously.ButinthepresenceofNaCl,therewasnovisiblebleaching

whenCuSO4wasaddedintotheyellowAgcolloidinashorttime.Wededucedtheinef cientofCl wasrelativetotheunproductivereactionsothattheCLwastooweaktobedetected.Inordertoaccu-mulateenoughCu(I)inthereactionbetweenCuSO4andAgNPsinthepresenceofNaClfortheCLreaction,luminolwasinjectedintothemixtureafterwaitingaquitelongertime.Thewaitingtimewasinvestigated.Morethan600safterCuSO4mixedwithAgNPsinthepresenceofNaCl,aconsiderableCLappearedwheninjectinglumi-nolintothemixture.Furthermore,NaIwasalsoinvestigatedintheCLreactioninsteadofNaBrandtheCLemissioncouldbeobserved.Asamatteroffact,I-ismoreef cientligandforCu(I)thanBr .How-ever,I couldreactwithCu2+toyieldCuIandI2.AlthoughCuIcouldleadtotheCLemissionofluminol,I2–luminolsystemisanotherwell-knownCLsystem.Inotherwords,Cu2+–I –luminol–Agsys-temwithadifferentmechanismfromothersystemscouldnotbetakenasanexamplefordiscussion.Accordingly,Br waschosenasthemainnucleophileinthiswork.Besides,thiosulfate,agoodligandforCu(I)intheaqueoussolution,wasusedasanucleophiletobetestedintheCLreaction.TheresultswiththiosulfateweresimilartoNaBr.AgcolloidfadedwhenmixingwiththiosulfateandCuSO4andaconsiderableCLemissionwasgeneratedbyfurtherinjectingluminol.4.Conclusions

InthepresenceofnucleophilesandCu2+,AgNPscouldinitiateluminolCL.TheAgNPs–NaBr–CuSO4–luminolreactionwaschosenasamodelsystemtostudytheCLprocess.TheCLmechanismislikelyduetothefactthatAgNPsbytheaidofNaBrcouldbeoxidizedbyCuSO4toproduceCu(I)complex;Cu(I)complexreactedwiththedissolvedoxygentogeneratethesuperoxideanion;thenthesuper-oxideanionreactedwithluminoltoproduceCL.Cu(I)complexisakeyintermediateproductintheCLreaction.Inthepresenceofothernucleophiles,suchasCl ,I andthiosulfate,AgNPscouldalsoreactwithCu(II)toproduceCu(I)andnucleophilescouldalsobindtoCu(I)preventingthemfromdismutation,whichcouldinducetheluminolCL.

Acknowledgements

The nancialsupportoftheresearchbytheNationalNaturalSci-enceFoundationofPRChina(GrantNos.20573101and20625517)andtheOverseasOutstandingYoungScientistProgramofChinaAcademyofSciencesaregratefullyacknowledged.References

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