7738J.Phys.Chem.C2010,114,7738–7742
Ag-CoatedFe3O4@SiO2Three-PlyCompositeMicrospheres:Synthesis,Characterization,andApplicationinDetectingMelaminewithTheirSurface-EnhancedRamanScattering
HaiboHu,ZhenghuaWang,*LingPan,SupingZhao,andShiyuZhu
AnhuiKeyLaboratoryofFunctionalMolecularSolids,CollegeofChemistryandMaterialsScience,AnhuiNormalUniVersity,Wuhu241000,People’sRepublicofChina
ReceiVed:January6,2010;ReVisedManuscriptReceiVed:March24,2010
Agnanoparticleswithaveragesizesof20nmwerewell-dispersedonthesurfacesofFe3O4@SiO2compositemicrospheresthroughasimplewet-chemicalmethodemployingtheAg-mirrorreaction.Theas-synthesizedAg-coatedFe3O4@SiO2three-plycompositemicrospheresaremonodisperseandbifunctional,withferro-magneticandsurface-enhancedRamanscattering(SERS)properties.TheproductswerecharacterizedbymeansofX-raydiffraction(XRD),scanningelectronmicroscopy(SEM),transmissionelectronmicroscopy(TEM),andenergydispersiveX-rayanalysis(EDX).SERSsignalsoftypicalanalytessuchasrhodamine6G(Rh6G)wereobservedonAgnanoparticlesfromtheAg-coatedFe3O4@SiO2microspheres,eventhoughtheconcentrationoftheanalytewasaslowas1×10-15M(Rh6G).TheAg-coatedFe3O4@SiO2microsphereswereappliedtodetectingmelamine,andstrongSERSsignalswereobtainedwithmelamineconcentrationof1×10-6M.Thisworkmayprovideapotentialanduniquetechniquetodetectmelamine.
1.Introduction
Inrecentyears,muchattentionhasbeenattractedbythefabricationofmagneticnanoparticlesandnanocompositesbecausetheyhavebothfundamentalandpracticalvaluesduetotheirpotentialapplicationsinareassuchasferro?uids,medicalimaging,drugtargetinganddelivery,cancertherapy,separations,andcatalysis.1-5Especially,magneticnanocom-positeswhichcontaintwoormoredifferentnanoscalefunc-tionalitiesareofspecialinterest.Duetotheirparticularstructureandinterfaceinteractions,thesemagneticnanocompositescanexhibitnovelphysical,chemical,andbiologicpropertiesthatwillbeessentialforfuturetechnologicalapplications.Themagneticnanocompositeswithcore-shellstructuresarewell-knowncompositesystemsthathaveshownmultiplepropertiessuchasadsorptive,optical,magnetic,antibacterial,andcatalyticpropertiescomparedtotheirindividualsingle-componentmaterials.6-9Forexample,Maceiraetal.havesynthesizedsemiconductor-coatedmagneticsilicasphereswithbothopticalandmagneticproperties.6Liuandco-workershaveshownthatChitosanmodi?edmagnetitenanoparticlescanremoveabove85%heavymetalionssuchasPb2+,Cu2+,andCd2+fromwastewater.7Wangetal.reportedthatAgimmobilizedmagneticnanoparticlescouldeasilykillEscherichiacoli(gram-negativebacteria),Staphylococcusepidermidis(gram-positivebacteria),andBacillussubtilis(sporebacteria)andcouldeasilyberemovedfromwaterwithamagnetic?eldtoavoidcontamina-tionofsurroundingsbecausetheyhavesuperparamagneticandantibacterialproperties.8Therefore,thesynthesisofmultifariousmagneticnanocompositeswithmultiplepropertiesattractsmoreandmoreattentions.
Sincesurface-enhancedRamanspectroscopy(SERS),whichcanenhanceRamansignalintensitybyupto6ordersofmagnitudedependingonthepreparationofthenanometersizedmetallicstructures(typicallyAgandAu),10,11wasdiscoveredbyFleischmannandHendra,12itarousesintenseinterestbecause
*Towhomcorrespondenceshouldbeaddressed.Phone:+86-553-3869303.Fax:+86-553-3869302.E-mail:zhwang@mail.ahnu.edu.cn.
ithaswidepotentialapplicationinmany?elds.13,14Varioussynthesismethodsofsuchmetalshavebeenreportedwhichincludevapordeposition,15chemicalreactions,16andion-beam-sputteredtechnique.17However,moreworkisstillneededtosimplifythesynthesismethodandtoimprovethesensitivity.Inthiswork,asortofthree-plycompositemicrospherewaspreparedbycoatingAgnanoparticlesonthesurfacesofFe3O4@SiO2compositemicrospheresthroughthewell-knownAg-mirrorreaction.ThepreparedAg-coatedFe3O4@SiO2microspherespossessbothferromagneticandSERSproperties.Ontheonehand,theyshowobviousferromagneticpropertyatroomtemperatureandcanbeeasilymagnetizedbyanexternalmagnetic?eld.Ontheotherhand,theycanserveasanef?cientSERSsubstrate.ThetypicalanalyteRhodamine6G(Rh6G)whichhasbeenfull-characterizedbySERSwasselectedasthemodeltovalidatetheeffectofthissubstrate;theresultindicatesthatthissubstrateisultrasensitive.Furthermore,thisSERSsubstratewasappliedtodetectmelamineandtheSERSsignalsofmelaminearestrongeventhoughtheconcentrationofmelamineisaslowas1×10-6M.2.ExperimentalSection
AllreagentswerecommerciallyavailablefromShanghaiChemicalRegentsCo.withanalyticalgradeandwereusedwithoutfurtherpuri?cation.
2.1.MonodisperseFe3O4MicrospheresSynthesis.Thesynthesiswascarriedoutaccordingtoapreviousreportwithalittlemodi?cation.19Inatypicalprocedure,1.35gofFeCl3·6H2Owasdissolvedin40mLofethyleneglycoltoformaclearsolution,then1.0gofpolyethyleneglycol20000and3.6gofNaAc·3H2Owereadded.Themixturewasstirreduntilthereactantswerefullydissolved.Afterthat,themixturewastransferredintoaTe?on-linedautoclavewithacapacityof50mLandheatedat200°Cfor8h.Theproductswerecollectedandrinsedwithdeionizedwaterandethanolseveraltimeseach,thendriedundervacuumat60°Cfor6hforfurtheruse.
10.1021/jp100141c?2010AmericanChemicalSociety
PublishedonWeb04/14/2010
Ag-CoatedFe3O4@SiO2Three-PlyCompositeMicrospheresJ.Phys.Chem.C,Vol.114,No.17,20107739
Figure1.SynthesisrouteofAg-coatedFe3O4@SiO2compositemicrospheres.
2.2.MagneticFe3O4@SiO2CompositeMicrospheresSyn-thesis.ThesynthesiswascarriedoutaccordingtothenotedSto¨bermethodwithalittlemodi?cation.20Typically,0.2gofpreparedFe3O4microsphereswasdispersedinamixtureof20mLofethanoland4mLofdeionizedwaterbyultrasonicationforabout10min.Thenundercontinuousmechanicalstirring,1mLofammoniasolution(25%)and0.8mLoftetraethylorthosilicate(TEOS)wereconsecutivelyaddedtothemixture.Thereactionwasallowedtoproceedatroomtemperaturefor3hundercontinuousmechanicalstirring.Theresultingproductswerecollectedandwashed,andthendriedundervacuumat60°Cfor3hforfurtheruse.
2.3.MonodisperseAg-CoatedFe3O4@SiO2CompositeMicrospheresSynthesis.First,0.05gofFe3O4@SiO2com-positemicrosphereswasdispersedinto30mLofa0.1MAg(NH3)2+solutionandthesolutionwasstirredbyamechanicalstirrerfor0.5htoensuresuf?cientadsorptionofAg(NH3)2+bytheFe3O4@SiO2compositemicrospheres.Then,themicro-sphereswerecollectedandwashedwithdeionizedwatertwotimes.Next,themicrospheresweredispersedinto30mLofa0.5Mglucosesolution.Thesolutionwasheatedwithawaterbathat50°Cfor1h.Duringtheheatingprocess,thesolutionwasalsostirredbyamechanicalstirrer.The?nalproductswerecollectedandwashed,thendriedundervacuumat60°Cfor3h.ThewholesynthesisprocedureisshowninFigure1.2.4.PreparationofSERSSubstrate.Apieceofcopperfoil(8mm×8mm)waswashedbyultrasonicationinethanolandacetoneonetimeeach.Thenasmallcopperboxwithoutacover(5mm×5mm×1mm)wasfabricatedbytailoringthecopperfoil.TheAg-coatedFe3O4@SiO2compositemicrospheres(about0.005g)wereputintothecopperboxandpressed.
2.5.CharacterizationMethods.Thestructureandmorphol-ogyofsampleswerecharacterizedbyX-raydiffraction(XRD;XRD-6000),scanningelectronmicroscopy(SEM;HitachiS-4800),andtransmissionelectronmicroscopy(TEM;JEOL-2010withenergydispersiveX-rayanalysis(EDXA)system).Infrared(IR)spectraofthesampleswereobtainedonaVectortm22Fouriertransforminfrared(FT-IR)spectrometer(Bruke,Germany).Themagneticpropertiesofthesampleswereinvestigatedbyusingavibratingsamplemagnetometer(VSM)withanapplied?eldbetween-5000and5000Oeatroomtemperature(BHV-55,Riken,Japan).Rh6GandmelamineweredetectedviaSERSwithaLabram-HRconfocallasermicro-Ramanspectrometerequippedwithanargonionlaserwithexcitationof514.5nm.Anair-cooledCCDwasusedasthedetector,theaccumulationtimewas20s,andtheincidentpowerwas3mW.Thespotsizeofthelaserwas1μmindiameter,usinga50×objective.
Figure2.TypicalXRDpatternsof(a)Fe3O4microspheres,(b)Fe3O4@SiO2compositemicrospheres,and(c)Ag-coatedFe3O4@SiO2compositemicrospheres.
3.ResultsandDiscussion
3.1.CharacterizationoftheObtainedSamples.Thephaseandpurityoftheas-obtainedsampleswereexaminedbyXRD.Figure2ashowsatypicalXRDpatternoftheobtainedFe3O4sample;allofthediffractionpeakscouldbereadilyindexedtotheorthorhombicphaseofFe3O4(JCPDScardNo.75-1609),whichmatchedwellwiththeresultofLietal.19Figure2bisatypicalXRDpatternoftheFe3O4@SiO2sample,whichshowsalmostthesamefeatureasthatshowninFigure2a:nodiffractionpeakscorrespondingtoSiO2wereobservedbecausethepreparedSiO2isamorphous.Figure2cshowsatypicalXRDpatternoftheAg-coatedFe3O4@SiO2sample.InadditiontothediffractionpeaksthatcorrespondtoFe3O4,therealsoexistthreeotherdiffractionpeaks(labledwiththesymbol#).ThesediffractionpeakscouldbeeasilyindexedtothecubicphaseofAg(JCPDScardNo.04-0783).
ThesampleswerealsocharacterizedbyIR.Figure3ashowstheIRspectrumofFe3O4microspheres,inwhichonlyapeakat582cm-1canbeseen.ThispeakisascribedtotheFe-Ostretchingvibration.21Figure3bshowstheIRspectrumoftheFe3O4@SiO2sample.Inadditiontothepeakat582cm-1ascribedtoFe-Ostretching-1vibration,thereisanewstrongbandaround1093cmthatoriginatesfromtheSi-Obondofsilica.21ThisresultindicatesthatSiO2isimmobilizedonthesurfacesofFe3O4microspheres.Figure3cshowstheIRspectrumoftheAg-coatedFe3O4@SiO2sample.BecauseAgnanoparticlesdonothaveabsorptionintheinfraredregion,theIRspectrumoftheAg-coatedFe3O4@SiO2sampleisalmostthesameasthatoftheFe3O4@SiO2sample.
ThesizeandshapeofthesampleswereexaminedbySEMandTEM.PanelsaandbofFigure4showtheSEMimagesoftheFe3O4sample,fromwhichitcanbeclearlyseenthatthesampleiscomposedofmanynearlymonodispersesphericalparticleswithadiameterofabout400nm.Furthermore,Figure
7740J.Phys.Chem.C,Vol.114,No.17,2010Figure3.IRspectraof(a)Fe3O4microspheres,(b)Fe3O4@SiO2compositemicrospheres,and(c)Ag-coatedFe3O4@SiO2compositemicrospheres.
Figure4.(a,b)SEMimagesofFe3O4microspheres,(c,d)SEMimagesofFe3O4@SiO2compositemicrospheres,and(e,f)SEMimagesofAg-coatedFe3O4@SiO2compositemicrospheres.
4bindicatesthatthemagneticmicrospheresarecomposedofmanysmallerparticlesandtheirsurfacesarenotsmooth.SEMimagesoftheFe3O4@SiO2sampleareshowninFigure4c,d,whichindicatethattheFe3O4@SiO2compositemicrosphereshaveasmoothsilicashell.PanelseandfofFigure4showtheSEMimagesoftheAg-coatedFe3O4@SiO2sample.ManylittleAgnanoparticleswithadiameterofabout20nmareadheredtothesurfacesoftheFe3O4@SiO2microspheres.
TheTEMimages(Figure5a-c)furthercon?rmtheaboveresults.ItcanbeclearlyseenfromFigure5athattheFe3O4@SiO2compositemicrosphereshavethecore-shellstructure.TEMimagesofFigure5b,cshowdistinctlythattheAg-coatedFe3O4@SiO2compositemicrosphereshavethree-plystructureandtheoutermostlayeriscomposedofmanytinyAgnano-particles.Figure5distheEDXspectrumoftheAg-coatedFe3O4@SiO2compositemicrospheres,inwhichFe,O,Si,andAgareallpresent.TheCandCuwerederivedfromthecarbon-coatedcopperTEMgrid.
Figure6showsthemagnetichysteresisloopsoftheas-obtainedFe3O4microspheres,Fe3O4@SiO2compositemicro-
Huetal.
Figure5.(a)TEMimageofFe3O4@SiO2compositemicrospheres,(b,c)TEMimagesofAg-coatedFe3O4@SiO2compositemicrospheres,and(d)EDXspectrumofAg-coatedFe3O4@SiO2compositemicrospheres.
Figure6.Magnetichysteresisloopsof(a)Fe3O4microspheres,(b)Fe3O4@SiO2compositemicrospheres,and(c)Ag-coatedFe3O4@SiO2compositemicrospheres.
spheres,andAg-coatedFe3O4@SiO2compositemicrospheres.Allofthemshowferromagneticbehavioratroomtemperature.Themagneticsaturation(Ms)valuesoftheFe3O4microspheres,Fe3O4@SiO2compositemicrospheres,andAg-coatedFe3O4@SiO2compositemicrospheresare80.0,36.1,and34.5emu·g-1,respectively.ItisnoticedthattheMsvalueofFe3O4micro-spheresishigherthanthatofFe3O4@SiO2compositemicro-spheresandAg-coatedFe3O4@SiO2compositemicrospheres,becauseintheFe3O4@SiO2compositemicrospheresandAg-coatedFe3O4@SiO2compositemicrospherestheFe3O4iscoatedwithalayerofamorphousSiO2.ThesmalldecreaseoftheMsvalueofAg-coatedFe3O4@SiO2compositemicrospherescomparedtothatofFe3O4@SiO2compositemicrospherescanbeattributedtotheslightincreaseofsizeandmassduetotheadherenceofAgnanoparticlesonthesurfaceofthemagneticcomposites.Suchanexcellentmagneticpropertymeansthatallofthepreparedsampleshavestrongmagneticresponsivityandcanbeseparatedeasilyfromsolutionwiththehelpofanexternalmagneticforce.
Ag-CoatedFe3O4@SiO2Three-PlyCompositeMicrospheresFigure7.SEMimagesofAg-coatedFe3O4@SiO2compositemicro-spheresobtainedwithdifferentconcentrationsofAg(NH3)2+:(a)0.1Mand(b)0.01M.
Figure8.SERSspectrumofa1×10-15MR6GethanolsolutionadsorbedonAgnanoparticlesoftheAg-coatedFe3O4@SiO2compositemicrospheres.
3.2.EffectofAg(NH3)2+ConcentrationontheFormationofAg-CoatedFe3O4@SiO2CompositeMicrospheres.ItmustbenoticedthattheconcentrationofAg(NH3)2+solutionplaysanimportantroleintheformationofAg-coatedFe3O4@SiO2compositemicrospheres.AlongwiththeconcentrationofAg(NH3)2+solutiondecreases,theamountofAgnanoparticlesisdecreasedandtheirdistributionbecomesnonuniformonthesurfacesofAg-coatedFe3O4@SiO2compositemicrospheres.ThisresultcanbeobtainedfromthecomparisonofthesamplepreparedwithAg(NH3)2+concentration+of0.1M(Figure7a)andthesamplepreparedwithAg(NH3)2concentration+of0.01M(Figure7b).HigherconcentrationsofAg(NH3)2provedtobecrucialforaugmentingtheyieldoftheAgnanoparticlesandtheiruniformity.
3.3.ApplicationofAg-CoatedFe3O4@SiO2CompositeMicrospheresinSERS.ThepreparedAg-coatedFe3O4@SiO2SERSsubstratewas?rstusedtodetectthetypicalSERSactiveanalytesuchasRh6Gtotestitseffect.Figure8showstheSERSspectrumobtainedbyadding25μ15LofRh6Gethanolsolutionwithaconcentrationof1×10-Monthesubstrate.Fourpeaksexistinthisspectrumandthesepeaksarecharac-teristicpeaksofRh6G.16,18,22Thepeakat1188cm-1isassociatedwithC-Cstretchingvibrations,whilethepeaksat1303,1356,and1575cm-1areassociatedwitharomaticC-Cstretchingvibrations.23,24AlthoughtheconcentrationofRh6Gisverylow,thesignalsoftheSERSarestillstrong.TheseresultsindicatethatthissubstrateforSERSdetectioniseffectiveandultrasensitive.
Itisreportedthattherearetwomajormechanismsthatcontributetotheenhancementeffect:16,18oneistheelectro-magneticeffectassociatedwithlargelocal?eldsduetoresonancesoccurringinthemicrostructuresonthemetalsurface,theotheristhechemicaleffectinvolvingascatteringprocessassociatedwithchemicalinteractionbetweenthemoleculeandthemetalsurface.Theenhancementduetotheformerisbelieved
J.Phys.Chem.C,Vol.114,No.17,20107741
Figure9.SERSspectraofdifferentconcentrationsofmelaminemethanolsolutionsadsorbedonAgnanoparticlesoftheAg-coatedFe3O4@SiO2compositemicrospheres:(a)1×10-6M,(b)1×10-7M,(c)1×10-8M,and(d)1×10-9M.
tobeafewordersofmagnitudemorethanthelatter.AccordingtoGarcia-VidalandPendry’smodel,25thesmallermetallicparticlesgivehigherenhancement.Butwhentheparticlesizeislessthan15nm,theenhancementsaturatesbecausetheseparationbetweenmetalnanoparticlesisequaltotheirdiameter.Inthiswork,theaveragesizeoftheAgnanoparticlescoatedonFe3O4@SiO2microspheresisclosetothissize,whichmayberesponsiblefortheultrasensitivityoftheAg-coatedFe3O4@SiO2SERSsubstrate.
Melamine(2,4,6-triamino-1,3,5-triazine)isanitrogen-richchemicalcommonlyusedtoproducemelamineresin,asyntheticheat-tolerantpolymer.Althoughmelamineisnotinherentlyacarcinogenandtoxicchemical,illegallarge-doseadulterationinroutinedairyproductscanresultinurinarycalculi,acuterenalfailure,andeveninfantdeath.Thusdevelopinganaccurateandrapidon-sitemelaminescreenmethodisaveryimportantandsigni?canttask.Herein,weusedAg-coatedFe3O4@SiO2compositemicrospheresasaSERSsubstratetodetectmelamine.Figure9showstheSERSspectraobtainedbyadding25μLofmelaminemethanolsolutionwithdifferentconcentrations6onthesubstrate.TheSERSspectrumofthe1×10-Mmelaminemethanolsolution-1(Figure9a)clearlyshowsthepeaksat686and582cm.26Themostprominentpeakaround686cm-1isassignedtotheringbreathingmodeIIandinvolvesin-planedeformationofthetriazineringinmelaminemolecules.27Spectrab-dinFigure9showtheSERSspectraofthemelaminemethanolsolutionwithconcentrationsof1×10-7,1×10-8,and1×10-9M,respectively.Inthesespectraonlytheweakpeakat686cm-1canbeseen.TheseresultsdemonstratethatmelaminecanbewelldetectedwiththeAg-coatedFe3O4@SiO2SERSsubstrateinconcentrationsaslowas1×10-6M.4.Conclusions
Insummary,bifunctionalandmonodisperseAg-coatedFe3O4@SiO2compositemicrosphereswithbothferromagneticandSERSpropertieshavebeensuccessfullysynthesizedthroughasimplesolution-phasemethodemployingtheAg-mirrorreaction.TheAg-coatedFe3O4@SiO2compositemicrospherescanbeusedasaSERSsubstratetodetectmelaminewithahigh-6sensitivityevenwhentheconcentrationisaslowas1×10M.Thismethodiseffectiveandconvenientfordetectingmelamineatverylowconcentrationandwithashortdetectiontime.
Acknowledgment.FinancialsupportformtheNationalNaturalScienceFoundationofChina(20701001)andAnhui
7742J.Phys.Chem.C,Vol.114,No.17,2010KeyLaboratoryofControllableChemistryReaction&MaterialChemicalEngineeringisgratefullyacknowledged.ReferencesandNotes
(1)Ledezma,R.;Bueno,D.;Ziolo,R.F.Macromol.Symp.2009,283-84,307.
(2)Zhang,L.;Wang,W.Z.;Zhou,L.;Shang,M.;Sun,S.M.Appl.Catal.,B2009,90,458.
(3)Hou,C.H.;Hou,S.M.;Hsueh,Y.S.;Lin,J.;Wu,H.C.;Lin,F.H.Biomaterials2009,30,3956.
(4)Zhao,W.R.;Chen,H.R.;Li,Y.S.;Li,L.;Lang,M.D.;Shi,J.L.AdV.Funct.Mater.2008,18,2780.
(5)Wang,L.;Neoh,K.G.;Kang,E.T.;Shuter,B.;Wang,S.C.AdV.Funct.Mater.2009,19,2615.
(6)He,X.X.;Chen,Y.J.;Wang,K.M.;Wu,P.;Gong,P.;Huo,H.L.Nanotechnology2007,18,285604.
(7)Liu,X.W.;Hu,Q.Y.;Fang,Z.;Zhang,X.J.;Zhang,B.B.Langmuir2009,25,3.
(8)Salgueirino-Maceira,V.;Correa-Duarte,M.A.;Spasova,M.;Liz-Marzan,L.M.;Farle,M.AdV.Funct.Mater.2006,16,509.
(9)Guo,S.J.;Dong,S.J.;Wang,E.Chem.sEur.J.2008,14,4689.(10)Nie,S.M.;Emery,S.R.Science1997,275,1102.
(11)Kneipp,K.;Wang,Y.;Kneipp,H.;Perelman,L.T.;Itzkan,I.;Dasari,R.;Feld,M.S.Phys.ReV.Lett.1997,78,1667.
(12)Fleischmann,M.;Hendra,P.J.;McQuillan,A.J.Chem.Phys.Lett.1974,26,163.
(13)Shadi,I.T.;Chowdhry,B.Z.;Snowden,M.J.;Withnall,R.Chem.Commun.2004,1436.
Huetal.
(14)Borodko,Y.;Humphrey,S.M.;Tilley,T.D.;Frei,H.;Somorjai,G.A.J.Phys.Chem.C2007,111,6288.
(15)Ruan,C.M.;Eres,G.;Wang,W.;Zhang,Z.Y.;Gu,B.H.Langmuir2007,23,5757.
(16)Shao,M.W.;Lu,L.;Wang,H.;Wang,S.;Zhang,M.L.;Ma,D.D.D.;Lee,S.T.Chem.Commun.2008,2310.
(17)Chattopadhyay,S.;Lo,H.C.;Hsu,C.H.;Chen,L.C.;Chen,K.H.Chem.Mater.2005,17,553.
(18)Michaels,A.M.;Nirmal,M.;Brus,L.E.J.Am.Chem.Soc.1999,121,9932.
(19)Deng,H.;Li,X.L.;Peng,Q.;Wang,X.;Chen,J.P.;Li,Y.D.Angew.Chem.,Int.Ed.2005,44,2782.(20)Sto¨ber,W.;Fink,A.;Bohn,E.J.ColloidInterfaceSci.1968,26,62.
(21)Gao,Q.;Chen,F.H.;Zhang,J.L.;Hong,G.Y.;Ni,J.Z.;Wei,X.;Wang,D.J.J.Magn.Magn.Mater.2009,321,1052.
(22)Shao,M.W.;Zhang,M.L.;Wong,N.B.;Ma,D.D.D.;Wang,H.;Chen,W.W.;Lee,S.T.Appl.Phys.Lett.2008,93,233118.
(23)Hildebrandt,P.;Stockburger,M.J.Phys.Chem.1984,88,5935.(24)Watanabe,H.;Hayazawa,N.;Inouye,Y.;Kawata,S.J.Phys.Chem.B2005,109,5012.
(25)Garcia-Vidal,F.J.;Pendry,J.B.Phys.ReV.Lett.1996,77,1163.(26)He,L.L.;Liu,Y.;Lin,M.S.;Awika,J.;Ledoux,D.R.;Li,H.;Mustapha,A.Sens.Instrum.FoodQual.2008,2,66.
(27)Lin,M.;He,L.;Awika,J.;Yang,L.;Ledoux,D.R.;Li,H.;Mustapha,A.J.FoodSci.2008,73,T129.
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