Fe3O4–Ag heterostructure nanocrystals with tunable Ag

Published on 27 February 2013. Downloaded by TONGJI UNIVERSITY LIBRARY on 21/01/2014 04:32:47. Fe3O4–AgheterostructurenanocrystalswithtunableAgdomainsandmagneticproperties3

Citethis:CrystEngComm,2013,15,3575

YiwuMao,aPeiweiYi,bZongwuDengbandJianpingGe*aFe3O4–AgdimersandmultimerswithtunableAgdomainsandmagneticpropertieswerepreparedbyaseedinggrowthmethod.ThereactionmechanismbasedontheattractionbetweenAgprecursorandFe3O4particlesaswellasthemigrationofadsorbedAgspeciestowardstheinitiallydepositedAgdomainwasproposedtoexplaintheheterogeneousnucleationofAgonFe3O4andthetuningofAgcrystalnumbers.Ligandoleicacid,lowreactiontemperature,largeseedingparticleandnonpolarsolventarefoundtobefavourabletothegrowthofFe3O4–AgHNCswithmorediscreteAgdomains.TheFe3O4–AgHNCsshowcompositionandstructurerelatedmagnetizationbehaviourandMRcontrasteffect,whichcanbedevelopedintoadualfunctionmaterialforbio-therapywhenutilizingtheplasmonic,fluorescentorantibacterialpropertiesofAgatthesametime.

Received17thJanuary2013,Accepted27thFebruary2013DOI:10.1039/c3ce40095fwww.rsc.org/crystengcomm

Introduction

Magnetic/metallicheterostructurenanocrystals(HNCs)haveattractedtremendousinterestinthepastdecadeduetotheiruniquemagneticandopticalproperties,1–3andpotentialapplicationsinbiomedicaltherapyandimaging,4–6recover-ablecatalysis7,8anddrugdelivery.9Forexample,Cheonetal.usedFePt–AudimerparticlesconjugatedwithantibodiestoidentifyneuroblastomacellsbasedonthemagneticresonanceimagefromtheFePtcomponent.10Sunetal.foundthatthedumbbellPt–Fe3O4nanoparticlesshow20-foldincreaseinmassactivitytowardoxygenreductioncomparedwithcom-mercial3nmPtparticles.7Becauseofthesenovelandsuperiorproperties,researcherskeeponbroadeningthecompositionofHNCsandinvestigatingevenmorecomplexandfascinatingstructures.11HNCsaremainlysynthesizedbytheseed-mediatedmethod,althoughtheycanbeproducedbyone-stepdecom-positionofmixedmetalprecursorsathightemperature.12,13Inaseed-mediatedgrowth,thesecondcomponentheteroge-neouslynucleatesonthesurfaceofapreformednanocrystalandevolvestoacrystaldomainattachingorsurroundingtheseedingparticles,whichcanbesummarizedascore@shell,dimerormultimerparticles.Forinstance,Au–Fe3O4dimerparticleshavebeenpreparedusingdecompositionofFe(CO)5onAunanoparticles,followedbyoxidationin1-octadecene.14Bytuningtheligandsandreactionkinetics,thismethodcan

DepartmentofChemistry,TongjiUniversity,Shanghai200092,China.E-mail:gejianping@gmail.combSuzhouInstituteofNano-techandNano-bionics,ChineseAcademyofSciences,Suzhou215123,China

3Electronicsupplementaryinformation(ESI)available:TEMimagesofmultimer-1andcorrespondingstatistics.SeeDOI:10.1039/c3ce40095f

abedevelopedtoproduceAu@Fe3O4core/hollow–shell,dumb-bellorflower-likenanoparticles.15–17Fromanotherperspective,thesehybridparticlescanalsobeobtainedbygrowingmetaldomainonmagneticseed,suchastheproductionofFe3O4–Ag,Fe3O4–Pt,andFe3O4@AuHNCsinrecentreports.18–21AlthoughmanyHNCshavebeenachievedinthepast,thepreparationofHNCwithdiscrete‘‘metal’’domainsandtunabledomainnumbersisnotonlychallenginginsynthesis,butalsopromisingduetotheiradvantagesincatalysis,surfaceplasmonresonanceandbiomedicalcoupling.

Inthiswork,wepreparedFe3O4–Agdimersandmultimers,inwhichthesizeandnumberofAgdomainsinthehybridparticlescanbeflexiblycontrolledbyligands,reactiontemperature,seedingparticlesize,andsolventpolarities.AgrowthmechanismbasedontheattractionbetweenAgprecursorandFe3O4particlesaswellasthemigrationofsurfaceAgspeciestowardstheinitiallydepositedAgdomainwasproposedtoexplaintheheterogeneousnucleationofAgonFe3O4andthetuningofAgdomainsnumbersbyligands.ThismechanismwasalsosuccessfulintheexplanationofFe3O4–AgHNCspreparedinsolventswithdifferentpolaritiesorusingFe3O4withdifferentsizes.Theplasmonicabsorption,themagnetizationcurveandmagneticresonancecontrasteffectsofthesebifunctionalnanocrystalswereinvestigated,whichshowstrongrelationstotheircomponentsandstructures.

Experimental

Materials

1-Octadecene(ODE,technicalgrade,90%),oleicacid(OA,technicalgrade,90%),oleylamine(OAm,technicalgrade,

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70%),anhydrousferricchloride(FeCl3,97%)andtri-n-octyl-phosphineoxide(TOPO,technicalgrade,90%)wereobtainedfromSigma-Aldrich.4-tert-Butyltoluene(TBT,.95.0%),oleicacidsodiumsalt(NaOA,.97.0%),and1,2-dodecanediol(DDD,90%)werepurchasedfromTCI.Silvernitrate(.98%),cyclohexane,ethanol,tolueneanddimethylsulfoxide(DMSO)weresuppliedbySinopharmChemicalReagentCo.,Ltd(SCRC).Allchemicalsweredirectlyusedasreceivedwithoutfurtherpurification.

Synthesisofiron–oleateandsilver–oleateprecursors

Iron–oleatewaspreparedbyreactingironchloridewithsodiumoleate.Typically,FeCl3(8mmol)andNaOA(24mmol)SynthesisofFe3O4–Agdimer

CrystEngComm

21/01/2014 04:32:47. Fe3O4–AgdimerwassynthesizedbygrowingasingleAgdomainonFe3O4–OAseedsinthepresenceofaspecificligand,oleylamine.Inatypicalsynthesis,AgNO3(0.2mmol)andoleylamine(800mL,2.43mmol)werefirstdissolvedin15mLof4-tert-butyltoluene(TBT)at60uC.Afteradding1mLofFe3O4seedsolutionwhichcontains10mgof17nmFe3O4–OANCs,theblacksolutionwasdirectlyheatedtothepresettemperature(70–110uC)andkeptstirringforabout30min.Thetotalvolumeofthesolutionis16.8mLandtheinitialconcentrationofAg+is0.012molL21.Uponcoolingtoroomtemperature,thesolutionwasmixedwithexcessiveethanolorno Yweredissolvedinamixedsolventofethanol(16mL),DIwaterRA(12mL)andhexane(28mL),whichwasheatedto70uCandRBreactedfor4hours.AfterthemixturecooleddowntoroomIL temperature,iron–oleateintheupperorganicphasewasYTIwashedthreetimesbyDIwaterinaseparatoryfunnel.Then,SRiron–oleatecomplexinawaxform(6.25g)couldbeobtainedEVafterevaporatingallthehexaneinthesolution.Thesilver–INUoleatecomplexwaspreparedbyreactingsilvernitratewith IJGsodiumoleate.Inatypicalexperiment,35mLofNaOANOaqueoussolution(10mmol)wasmixedwith50mLofAgNO3T solution(10mmol),whichwasstirredfor0.5h.Theyb dprecipitatedsilveroleatewasseparatedbyfiltration,washededathreetimeswithDIwater,anddriedinairat60uCtoproduceolnawaxypowderof3.4g.Overall,theweightpercentagesofwoDelementalFeandAgintheprecursorare7.2%and31.7%, .3respectively.

102 ySynthesisofironoxidenanoparticleswithtunablesizeandraurcappingligand

beF Ironoxidenanoparticleswithtunablesizeanddifferent72 nsurfaceligandsweresynthesizedbythermaldecompositiono doftheiron–oleatecomplexinODEinthepresenceofvariousehscappingagents.Allreactionswereprotectedbyargonusingilbustandardair-freeoperations.ThehightemperaturereactionPwasperformedinathree-neckedflaskheatedbyahemi-sphericalheatingmantle,whosetemperaturecanbepreciselycontrolledfromroomtemperatureto400uCbyaCole-ParmerDigi-SenseTemperaturecontroller.Forexample,forthepreparationofFe3O4–OA/TOPOnanoparticles(17nm),iron–oleate(5mmol)wasdissolvedinamixtureofOA(0.70mmol),TOPO(0.70mmol)andODE(13.20mL)atroomtemperature.Thetotalvolumeofthesolutionwas13.5mL,whichgivestheinitialFeconcentrationof0.37MandthemolarratioofFe/OA/TOPOas7.14:1:1.Afterheatingthemixtureto120uCtoformatransparenthomogenoussolution,thereactionvesselwasvacuumedandfilledwithargontodecreasetheinfluenceofoxygeninreaction.Then,thereactionsolutionwasheatedto320uCandmaintainedatthattemperaturefor90min.Theas-synthesizedFe3O4nanoparticleswerepurifiedbyrepeatedadditionofethanol,centrifugationanddissolutioninhexane.Inthelastcycle,theparticlesweredispersedinTBT(15mL)asseedsforthefollowingpreparationofFe3O4–AgHNCs.Fe3O4–OAnanoparticleswithdifferentsizesweresynthesizedbysimilarprotocols.Magnetitenanocrystalswithaveragedia-metersof6and12nmwereobtainedwhentheconcentrationofironoleatewere0.3and0.2molL21,respectively.

3576|CrystEngComm,2013,15,3575–3581acetonetoproduceyellow-blackprecipitation,whichcouldbeseparatedbycentrifugationandfurtherdispersedinmanynonpolarsolventsincludinghexane,tolueneandchloroform.SynthesisofFe3O4–Agmultimer-1andmultimer-2

Fe3O4–Agmultimer-1waspreparedbygrowingmultipleAgdomainsonFe3O4–OAseedsinthepresenceofanotherligand,oleicacid.Typically,silver–oleate(0.2mmol),1,2-dodecane-diol(1mmol),andoleicacid(1mmol)weredispersedin15mLofTBTat60uCunderstirring.Thetotalvolumeofthesolutionis16.5mLandtheinitialconcentrationofAg+is0.012molL21.Afteradding1mLofFe3O4seedsolution,thereactionsolutionwasheatedto100uCatarateof5uCmin21andmaintainedat100uCfor15minbeforeitwascooleddowntoroomtemperature.Theas-preparedHNCswerepurifiedbysimilarproceduresasabove.InordertopreparehybridnanocrystalswithmoreAgdomains,suchasFe3O4–Agmultimer-2,Fe3O4–OA/TOPOinsteadofFe3O4–OAseedswereused,whiletheotherreactionconditionswerekeptunchanged.Characterization

TEMimageswereobtainedonaJEOLJEM2100transmissionelectronmicroscopeoperatedatanaccelerationvoltageof200kV.TEMsampleswerepreparedbydrop-castingnanocrystalsdispersedinhexaneonto300-meshFormvar-coatedcoppergrids.PowderX-raydiffraction(XRD)patternswererecordedonaRigakuD/max2550VB3+/PCX-raydiffractometerwithKaofCuasX-rayradiationsource(l=1.5418Å).UV-VisabsorptionspectraweremeasuredbyanOceanOpticsMaya2000Prospectrometerwithaspectralresolutionof0.03nm.MagnetichysteresisatroomtemperaturewasmeasuredbyaLakeshore7312vibrationsamplemagnetometer(VSM).TheconcentrationofFeinthesolutionofFe3O4–AgnanocrystalswaspreciselydeterminedbyAgilent3510atomicabsorptionspectroscopy(AAS)testedinanair-acetyleneflameatl=248.3nm.AworkingcurveconsistingofabsorptionofstandardFeconcentrationfrom0to10ppmwasplottedinadvancetodeterminetheunknownFeconcentrations.Themagneticresonanceimages(MRI)werecapturedbyaBrukermicro2.5micro-MRIsystemperformedat11.7Teslawithaconventionalspin–echoacquisition.Relaxivity(r2)withunitsofmM21s21wasthencalculatedthroughthelinearsimulationofrelaxa-tiontimereciprocalsandcorrespondingFeconcentrations(mM).

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21/01/2014 04:32:47. Fig.2(a)XRDpatternsand(b)UV-VisabsorbanceforFe3O4–Ag,(i)dimer,(ii)multimer-1and(iii)multimer-2.ThediffractionpeaksofFe3O4(black)andAgno YRARBILFig.1(a)SchemefortheproductionofFe3O4–AgHNCswithincreasingAg Ycrystalnumbers,andTEMimagesforFe3O4–Ag(b)dimer,(c)multimer-1and(d)TISmultimer-2.Allscalebarsare20nm.

REVINU IJResultsanddiscussion

GNOTFig.1ashowsthesyntheticroutefortheproductionofFe3O4– ybAgHNCswithcontrollableAgnanocrystals.Inatypical deprocess,theseFed3O4–AgHNCsweresynthesizedbytheaolreductionofAgprecursoranditsfurthergrowthonmagnetitenwoseeds.First,theoleicacid(OA)andoleicacid/tri-n-octylpho-D .sphineoxide(OA/TOPO)cappedFe3O4seedparticleswiththe310samesize(17nm)werepreparedbyaliteraturemethod.22The2 yrligandsstabilizingtheAgandFe3O4nanocrystalswerefoundaurtobethemostimportantparametertocontrolthecrystalbeFgrowth.WhenoleylamineandAgNO 3werechosentogrowAg72 onFen3O4–OAseed,onlydimerswithasingleAgdomainformo dnomatterhowlongthereactionlastsandhowmuchAgehsprecursorwasadded.However,whenoleicacidandsilver–ilbuoleatewereusedtocreateanacidicenvironmentwithweakPreducingability,multipleAgdomainsformonthesurfaceofseedparticles.ReplacingFe3O4–OAseedswithFe3O4–OA/TOPOseedsleadstotheformationofFe3O4–AgmultimerswithevenmoreAgdomains.AsshowninFig.1balltheFe3O4–AgHNCshaveuniformmorphologiesinthiswork.ItshouldbenotedthatalltheTEMimagesinthismanuscriptarehighmagnificationimageswithonlyafewHNCs,becausethesmallAgdomainsarehardtobeobservedinlowmagnificationimagesanditishardtodistinguishwhichFe3O4seedshouldaspecificAgdomainbelongtoiftherearemanyHNCsinahighmagnificationimage.Therefore,wehaverecordedadequateTEMimages(seeFig.S1inESI3)foreachsampleandprovideatypicalpictureinthepaper.

ThecontrolledgrowthofFe3O4–AgHNCsisalsoconfirmedbytheirXRDpatternsandUV-Visabsorbance(Fig.2).ThecorrespondingXRDpatternsofthreesamplesinFig.1weremeasured.TheXRDpatternsusuallycontaintwogroupsofdiffractionpeaks,whichareconsistentwiththestandarddataforfcc-structuredFe3O4andfcc-structuredAg,respectively.BecausethesameFe3O4nanocrystalswereusedasseedsandAgcrystalswerereducedatthesametemperature,their

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crystallinityareverycloseinthreesamples.Inthiscase,theintensityorintegralareaoftheXRDpeakswouldbeinproportiontotheamountofperiodicalcrystalsstructures,thatisthemassofFe3O4orAg.ItwasalsoproportionaltothenumberofFe3O4orAgnanocrystals,astheirsizes(Fe3O4~17nm;Ag~6nm)areidenticaltoo.Therefore,onecanestimatetherelativequantityofAgbycomparingtheirdiffractionintensities.Theincreasingratiooftheirmajorpeak,I111(Ag)/I311(Fe3O4),provestheincreasingofAgcrystalnumbers,whichisconsistentwiththeTEMobservations.Inaddition,thegradualenhancementofplasmonicabsorptionofAgcrystalsalsoprovesthistrend.Generally,thered-shiftofabsorptionpeaksofnoblemetalnanostructuresisusuallycausedbytheincreasedparticlesize,thecouplingbetweenadjacentplasmonswithshorterinter-particlespacingandtheparticleaggregation.Inourwork,thesizeofAgdomainsinFe3O4–Agdimer(~6nm),multimer-1(~5nm)andmultimer-2(~5nm)werealmostthesame,sothatthered-shiftofabsorptionfrom415to435nmwasattributedtotheapproachingoftheadjacentAgdomaininFe3O4–Agmulti-mers.23–25ThekeyquestionstothegrowthofFe3O4–AgHNCsarewhyAgatomswoulddepositonFe3O4seedstoformAgcrystalsandhowthecappingligandscontroltheAgdomainnumbers.Previousworkshaveproposedthatnanocrystalsstabilizedwitholeicacidhavepartialnegativecharge,15,26becausetheligandsdonatetheirlone-pairelectronstotheemptyorbitalsofsurfaceatomsofnanocrystals.SinceAgspeciesbothinAg+(OAm)andAg+(OA)2precursorshavepartialpositivecharge,theywereattractedandenrichedaroundtheOA-cappedFe3O4nanocrystals,andfurtherreducedtoformAgcrystaldomainsinthesynthesis.Oncethe‘‘initialAgdomain’’wasgenerated,theelectronsfromthereductantwouldchoosetoinjecttometalnanocrystals(Ag)butnottheoxideparticles(Fe3O4),similartotheelectrontransferinthegrowthofFe3O4@AuHNCs,andcombinewiththefollowingAgprecursorstofinallyformAgnanocrystals.14Sincethe‘‘initialAgdomains’’willeventuallygrowtoAgnanocrystalsaslongasthesupplyofprecursorsisadequate,theAgcrystalnumberistherebycontrolledbythenumberof‘‘initialAgdomains’’inthebeginningstage.Insynthesisusing

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Ag+(OAm)asprecursorandoleylamineasreductant,thestrongreductantcausesthefastformationofarelativelylargeAgdomain,whichacceptsmoreelectronstogenerateahighlynegativelychargeddipole.TheadsorbedAgspeciesthentransfertothisAgdomainbyexchangeofsurfaceligandduetothestrongattractionexertedbytheinitialAgdomain,producingFe3O4–Agdimerparticlesonly.Generally,astrongreductantwouldeitherproducesmallnanocrystalsduetoanintense‘‘seedburst’’oracceleratethecrystalgrowthtogeneratelargenanocrystals,dependingonthespecificreac-tionconditions.Here,AgprefersthenucleationonFe3O4particles,sothatthelatterfactorwasdominantinthegrowth.CrystEngComm

(Fig.3aand3b).WhenFe3O4particlesenlargeinthesynthesisusingAg+(OA)2precursors,theincreasingofbothAgsizeandnumberareobserved.Apparently,thelatterphenomenonwasnotsimplycausedbytheincreasedsurfaceareaoflargerFe3O4seeds.Otherwise,thisshouldbeobservedintheAg+(OAm)routeaswell.Itisbelievedthat,astheseedingparticleenlarges,thesurfaceadsorbedAgspeciesbecomemoredifficulttobetransferredoveralongdistancesothattheywerelocallyreducedtoproducemoreAgcrystaldomains(Fig.3cand3d).

SolventpolaritywasanotherimportantparametertocontrolthenucleationandgrowthofAgcrystals.Itisknown 21/01/2014 04:32:47. no WhenAg+(OAm)andOAmwerereplacedwithalessionicYRAprecursor(Ag+(OA)2)andweakerreductant(diol),smallRB‘‘initialAgdomains’’formandlesselectronswereinjected,IL sothattheattractionbecomesweaker,themigrationslowsYTIdown,andmoreAgdomainsmayformatspecificlocations,SRleadingtotheproductionofmultimers.AsforFeE3O4seedsVIcappedbyOAandTOPO,thecontinuousnegativesurfacewasNUdividedintomanyregionsduetotheneutralandstericTOPO, IJGwhichinhibitstheAgmigrationandforcesthereductionoccurNOlocallytoproducemultimerwithevenmoreAgcrystals.

T yTheproposedmechanismwassuccessfultoexplaintheb deinfluenceofFe3O4particlesizeupontheAgcrystalgrowth.daoHere,Fe3O4particleswithdiametersof6(Fig.3a),12(Fig.3blnwand3c)and17nm(Fig.3d)werechosenasseedstogrowoD variousHNCs.InthesynthesisusingAg+(OAm)asprecursor,a.310largerFe3O4particlewithlowercurvaturewillproducealarger2 y‘‘initialAgdomain’’,whichcanacceptmoreelectronsfromtheraurreductantandattractmoreAgprecursors.TheseaccumulativebeFadvantagesintheenrichmentandreductionofAgprecursors 72 leadtotheformationoflargerAgcrystalsindimerparticles

no dehsilbuPFig.3InfluenceofseedingparticlesizeuponthegrownAgcrystalsizeandnumber.Thescalebarsare20nm.

3578|CrystEngComm,2013,15,3575–3581thattheattractionbetweenAgprecursorandFe3O4nanopar-ticleinducesthenucleationofAgexactlyontheFe3O4seeds,whiletheattractionbetweenAgprecursorandelectron-injectedAgdomaincausesthefurthergrowthofAgnanocrys-tals.Innonpolarsolvent,theAgspeciespreferdepositiononnegativeFe3O4particlestoformHNCsbecausetheysensenoattractionfromsolventmolecules.However,asthepolarityincreases,thesolventwillscreenthenegativeFe3O4particlefromAgprecursor,whichpromotesthehomogeneousnuclea-tionandformationofisolatedAgnanocrystals.14Here,wechoosefourorganicsolvents,whosepolaritycouldbecharacterizedbyempiricalparameters(ET)27(Table1).Asthepolarityincreasedfromtoluene,diphenylether(DPE),dichlorobenzene(DCB)tooctanol(OCT),theAgcrystalnumbergraduallydecreaseduntilnoheterostructurewasproduced,whichwasfullyconsistentwiththepreviousprediction(Fig.4).

TheAgcrystalsindimersandmultimersgrowinacontrollablemanneratlowtemperatureswithanarrowsizedistributionwellmaintained(Fig.5).Takingthegrowthofdimerat90uCasatypicalexample,smallAgcrystalswithuniformdiametersof3nmwereproducedoncethepre-settemperaturehasbeenreached.Inthefollowing2hours,theygraduallygrewintolargerAgcrystals(6.4nm)withnarrowsizedistributionwellmaintained.ThetimeevolutionofAgcrystaldiametersat70,90and110uCdemonstrateafastergrowingspeedatthebeginningofdimergrowth,andalsoathightemperature.ThegraduallyslowedgrowthspeedaswellastheobservationoffreeAgcrystalsinsolutionindicatesthatthegrowthatlowtemperaturewaskineticallycontrolledbythecappingligandwithfewchancesofOstwaldripening.However,whenthereactionwasdirectlyheatedto150uC,thesphericalAggrowstoalargertrigonalplatewithedgelengthof30–40nm,andtheisolatedAgnanocrystalsinsolutionbecamefewer,whichcanbeattributedtoatypicalripeningprocess(Fig.6).Similarripeninghappensinthe

Table1Empiricalparameters(ET)andnormalizedempiricalparameters(ETN)forsolventswithdifferentpolarities.Highervaluesindicatestrongpolarity

Solvent

TolueneDPEDCBOCTEET33.935.338.048.1TN0.099

0.142

0.225

0.537

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21/01/2014 04:32:47. no YRARBIL YTISREVINUFig.4TEMimagesofFe3O4–AgHNCsorseparatedFe3O4andAgnanocrystals IJsynthesizedinsolvent(a)toluene,(b)DPE,(c)DCBand(d)OCTwithincreasedGNpolarity.Thescalebarsare20nm.

OT yb dedaolngrowthofFew3O4–Agmultimers,wheresmallerAgcrystalsonoDFe3O4particleswereconsumedforthegrowthoflargerones. .31Asaresult,theAgcrystalspartiallydisappearandthe02 remainingoneshaveanevenbroadersizedistribution.yrauTherefore,inordertoprepareFe3O4–AgHNCswithpossiblyrbemoreAgcrystals,appropriateligand,moderatereactionF 72temperature,largeseedingparticleandnonpolarsolventare noessentialtothesynthesis.

dehTheFe3O4–AgHNCsaresuperparamagneticatroomsilbtemperatureandthemagneticpropertiesarerelatedtotheiruPcompositionandstructures.Itisknownthatbelowacriticalsize,eachferromagneticnanocrystalcanonlysupportasinglemagneticdomainandbehavelikeasinglelargemagneticmoment.Atlowtemperature,theferromagneticbehaviourremainsasthemagneticmomentpointsalongtheenergeti-callyfavourablecrystallographicaxis.Butatroomtempera-ture,thethermalfluctuationwillrandomizethemagneticmomentandleadtosuperparamagnetismforthesenanocrys-tals.Here,wechosetostudytheFe3O4seedsandFe3O4–AgdimersandmultimersinFig.1.Fig.7ashowsthatthegrowthofdiamagneticAgonFe3O4particlewillnotchangeitssuperparamagneticcharacteristics.ThemagnetizationcurvesofFe3O4–Agdimerandmultimershasneitherremanencenorcoercivity,buttheirsaturatedmassmagnetization(Ms)doesdecreasefrom36.6,33.5,24.2to21.0emug21astheattachedAgcrystalnumbersincreased.SincethediametersofAgindimer,multimer-1,-2andthatofFe3O4particlearedeter-minedtobe6,5,5,and17nmfromFig.1,onecancalculatetheAg/Fe3O4massratiobasedontheirsizeanddensity.IfthedecreaseofmagnetizationisentirelyattributedtotheintroductionofAg,thenumberofAgcrystalscanbecalculated

ThisjournalisßTheRoyalSocietyofChemistry2013Fig.5(a–f)TEMimagesofdimerparticlesproducedatdifferentreactiontimesat90uC.Allscalebarsare10nm.(g)TimeevolutionofAgdiameteratdifferentreactiontemperature.

tobe1.23,11.8and17.1forFe3O4–Agdimer,multimer-1and-2byconsideringtheirsaturatedmagnetizationandAg/Fe3O4massratio,whicharebasicallyconsistentwiththeTEMresultsasonlyhalfoftheAgdomainscanbeobserved.ItisnotsurprisingtoseethedecreasingofMswhenAgcrystalindimersgrowslarger(Fig.7b).However,thedimerwithlargerAgcrystalwasfoundtobemagnetizedmuchquicker,meaninglargermagneticsusceptibility,whenaweakfieldwasapplied.ItisknownthatAgisadiamagneticmaterialwithverysmallnegativemagneticsusceptibility,butthegrowthoftheAgdomain,onthecontrary,enhancesitsmagneticsusceptibilityinweakfield,whichsuggesttheAggrowthhaschangedthespin–orbitalinteractionandtherebythemagneticproperties.2WefurtherinvestigatethecapabilityofFe3O4–AgHNCsformagneticresonanceimaging(MRI).Theas-madenanoparti-clesarefirsttransferredtowaterphasebymodificationwithamphiphiliccopolymers.28Theirspin–spinrelaxationtime(T2)weightedMRimagesarerecordedinamagneticfieldof11.7T,anditsreciprocals(1/T2)areplottedasalinearfunctionofFeconcentrationtodeterminetherelaxivitycoefficient(r2)

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21/01/2014 04:32:47. no YRARBIL YTISREVINU IFig.6TEMimagesof(a,b)Fe3O4–Agdimerand(c,d)multimernanoparticlesJGpreparedat100uCand150uC,respectively.Thescalebarsare20nm.

NOT yb ded(Fig.8).TheFe3O4–AgdimerparticlesshowsstrongerMRaolcontrasteffectwithahigherr2(139mM21s21)comparingnwowiththatofFe3O4–Agmultimer-1(90mM21s21),whichD .demonstratesthegrowthofmorediamagneticAgdomains3102aroundtheFe3O4nanocrystalwillweakentheinduced yrmagneticfieldofthemagneticparticle,leadingtolessaurbdisturbancetothemagneticrelaxationprocessofwatereF protonsandlowerMRcontrast.Consideringthattherelaxivity72 ncoefficient(r2)ofcommercialAMI-25(Feridex;Endorem)ando dAMI-227(Combidex;Sinerem)are100mM21s21and53mM21ehsis21,respectively,theas-preparedFe3O4–AgHNCsnotonlylbuPprovidegoodMRIcapabilities,29butalsopossesstwo-photonfluorescence6orplasmonicproperties,makingthemapotentialdualfunctionalmaterialinmedicaldiagnosisandtherapy.

Fig.7Room-temperaturemagnetizationcurvesofFe3O4–AgHNCswith(a)increasingAgnumbers,and(b)increasingAgsizeinthedimer.

3580|CrystEngComm,2013,15,3575–3581Fig.8(a)T2-weightedMRimagesofFe3O4–Agdimerandmultimer-1inamagneticfieldof11.7T,(b)relaxivitycoefficient(r2)obtainedbylinearsimulationofrelaxationtimereciprocalsandFeconcentration.

Conclusions

Inconclusion,Fe3O4–AgdimersandmultimerswithtunableAgcrystalsizeandnumberswereprepared.TheelectrostaticattractionbetweenAgprecursorandFe3O4particlesaswellasthemigrationofsurfaceAgspeciestowardstheinitiallydepositedAgdomainwasproposedtoexplainthehetero-geneousnucleationofAgonFe3O4andthetuningofAgcrystalnumbers.InordertoprepareFe3O4–AgHNCswithpossiblymoreAgcrystals,appropriateligand,lowreactiontempera-ture,largeseedingparticleandnonpolarsolventarerequired.TheFe3O4–AgHNCsshowcompositionandstructure-relatedmagnetizationbehaviourandMRcontrasteffect,andwillpossiblybedevelopedintoadualfunctionmaterialforbio-therapywhenutilizingtheplasmonic,fluorescentoranti-bacterialpropertiesofAgatthesametime.

Acknowledgements

J.G.thanksthesupportfromMajorStateBasicResearchDevelopmentProgramofChina(2011CB932404),NationalScienceFoundationofChina(21001083,21222107),ShanghaiPujiangProgram(10PJ1409800)andSRFforROCS(SEM).

Notesandreferences

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