MaterialsScience
MonodisperseMagneticSingle-CrystalFerriteMicrospheres**
HongDeng,XiaolinLi,QingPeng,XunWang,JinpingChen,andYadongLi*
Ithasbeenthoughtthatmanynovelpropertiesandpotentialapplicationswouldemergefrommonodispersema
terialswithsmalldimensions.Therefore,thesynthesisofmonodispersenanoparticleshasbeenintensivelypurs
uedfortheirtechno-logicalandfundamentalscientificimportance.[1–7]Thesynthesisofnanostructuredmagneticmaterialshasbecomeaparticularlyimportantareaofresearchandisattracti
ngagrowinginterestbecauseofthepotentialapplicationssuchmaterialshaveinferrofluids,advancedmagnetic
materials,catalysts,coloredpigments,high-densitymagneticrecordingmedia,andmedicaldiagnostics.[8–
13]Spinelferrites(MFe2O4;M=Fe,Mn,Zn,orCo)areamongthemostimportantmagneticmaterialsandhavebe
enwidelyusedinelectronicdevices,informationstorage,magneticresonanceimaging(MRI),anddrugdeliverytechnology.[8,9,14]Magnetite(Fe3O4)hasrecentlybeenconsideredanidealcandidateforbiologicala
pplications,bothasatagforsensingandimaging,andasanactivityagentforantitumortherapy.[15–
17]Forhighperfor-manceinfunctionspecificbiologicalapplications,magneticparticlesmustbesphericalandhavesmoothsurfaces,narrowsizedistr
ibutions,largesurfaceareas(formaximalproteinorenzymebinding),highmagneticsaturation(ss)toprovidem
aximumsignal,andgooddispersioninliquidmedia.[6,18,19]
´reportedthepreparationofAfterSugimotoandMatijevic
magnetiteparticleswithanarrowsizedistributionintheearly1980s,[20]monodisperseferritehasbeenfabricat
edbyvariouschemistrybasedsyntheticmethods,includingcoprecipitation,thereversemicellemethod,microwaveplasmasynthesis,
sol-geltechniques,freezedrying,ultrasoundirradiation,hydrothermalmethods,laserpyrolysistechniques,andthermal
decompositionoforganometallicandcoordinationcom-pounds.[1,9,14,18,20–
27]However,mostoftheseapproacheswerefocusedonthesynthesisofferriteparticleslimitedtodiametersbel
ow30nm.Therearenoreportsonthesynthesisofwellcrystallizedferritenanoparticleswithsizessimilartoproteinmolecules.Thedevelopmentofafacileandeconom
icsyntheticstrategyforthesynthesisofhydrophilic,biocompatiblemagnetitenanoparticleswouldbenefittheirtechnicaluseinbiomedicalfields,especiallyforapplication
sinvivo.Hereinwereportageneralapproachforthefabricationofmonodisperse,
hydrophilic,andsinglecrystallineferritemicrospheresbyasolvothermalreductionmethod.Tothebestofourknowledge,thisisthefirst
reportonthesynthesisofsinglecrystallinemagneticmicrospheres.Theferritesphereshadmonodispersediametersthatweretunableinthera
ngeof200–
800nm.Thisworkresultedinanimportantmethodforobtainingvariousmonodisperse,magnetic,andsinglecrystallinemicrospheres,andprovidedanopportunitytofurtherapplythesepromisingmaterials.
TypicalsynthesesofFe3O4andferritemicrosphereswerecarriedoutinasolvothermalsystembymodifiedredu
ctionreactionsbetweenFeCl3andethyleneglycol.WeconfirmedtheproductionofFe3O4byconductingcontro
lledoxidationreactionsinwhicha-andg-Fe2O3wereproduced(SupportingInformation).[1b,28–
29]ThecrystallinestructuresofMFe2O4werecharacterizedbyXRD.AsshowninFigure1,the
[*]Dr.H.Deng,X.Li,Q.Peng,X.Wang,Prof.Y.Li
DepartmentofChemistryandtheKeyLaboratoryofAtomicMolecularNanosciences
(MinistryofEducation,China)
TsinghuaUniversity,Beijing,100084(P.R.China)and
NationalCenterforNanoscienceandNanotechnologyBeijing,100084(P.R.China)Fax:(+86)10-6278-8765
E-mail:ydli@tsinghua.edu.cnProf.J.Chen
DepartmentofPhysics
PekingUniversity,Beijing,100084(P.R.China)
[**]ThisworkwassupportedbytheNSFC(90406003,20401010,
50372030,20025102,20131030),theFoundationfortheAuthorofNationalExcellentDoctoralDissertationoft
heP.R.China,andthestatekeyprojectoffundamentalresearchfornanomaterialsandnanostructures(2003CB
716901).
SupportinginformationforthisarticleisavailableontheWWWunderhttp://www.angewandte.orgorfromthe
author.
Figure1.XRDpatternsofFe3O4,tracea;MnFe2O4,traceb;ZnFe2O4,tracec;andCoFe2O4,traced.
patternscanbeeasilyindexedtoFe3O4(JCPDS75-1609),MnFe2O4(JCPDS74-2403),ZnFe2O4(JCPDS221012),andCoFe2O4(JCPDS22-1068).TheFe3O4samplewasdeter-minedbyXrayphotoelectronspectroscopy(SupportingInformation).
ThesizeandshapeoftheproductswereexaminedbyTEMandscanningelectronmicroscopy(SEM).Figure2sho
wsrepresentativeimagesofthespinelferritecompounds,whicharesphericalandhaveverynarrowdiameterdis
tributions.ThestandarddeviationforFe3O4particlediametersis%5%,excludingagglomerateparticles(fordiamete
r-distri-butionhistogramsoftheMFe2O4microspheres,seeSupportingInformation).Furthermore,theshapeandsizeofthea-Fe2O3andgFe2O3productsremainedunchangedfromthoseoftheFe3O4precursor.Inatypicalsolvothermalprocess,the
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DOI:10.1002/anie.200462551
Chemie
CoFe2O4(SupportingInformation).TheatomicratioFe/OforFe3O4was3:4,andananalogousM/Fe/Oratioof%
1:2:4wasobservedforeachoftheotherMFe2O4products(M=Mn,Zn,Co),inagreementwiththeexpectedstoic
hiometryineachcase.
HighresolutionTEM(HRTEM)analysisprovidedmoredetailedstructuralinformationonthemicrospheres.Areprese
ntativeTEMimageoftheFe3O4microspheresisshowninFigure3a,inwhichtheHRTEMimage(boxedarea)furth
ersupportsthesinglecrystallinenatureoftheparticles(SupportingInformation).Theinterlayerdistancewascalculatedtobe%0.48nm,whichagreeswellwiththeseparationbetweenthe(101)latticeplanes.
Figure3.a)RepresentativeTEMimageofaFe3O4microsphere
(diameter%200nm);b)HRTEMimageoftheboxedregionofpart(a)andtheelectrondiffractionpatternoftheindividualFe3O4microsphere(inset).
Figure2.RepresentativeTEMimages(a,c,e,andg)andSEMimagesBoththeliterature[6,7]andourownexperim
entalevidence
(b,d,f,andh)ofmicrospheresofFe3O4,(aandb);MnFe2O4,(candhaveledustobelievethatethyleneglycolplays
animportantd);ZnFe2O4,(eandf);andCoFe2O4,(gandh).Microspherediameterrole
inferriteformation.Ethyleneglycolisastrongreducingis%800nmintheSEMimages,and%200nmintheTEMima
ges,inwhichtheinsetsshowthecorrespondingelectrondiffractionpatterns.
agentwitharelativelyhighboilingpoint,[7]andhasbeenwidelyusedinthepolyolprocesstoprovidemonodisper
sefinemetalormetaloxidenanoparticles.However,magneticspinelferriteparticleshaveastrongtendencytoa
gglomerateduringtheirformationintheliquidphaseprocess.ToobtaindiametersoftheMFe2O4ferritemicrosphereswereinflumonodisperseFe3O4particles,wedesignedamodifiedencedbyreactiontimeandtheconcentrationofstarting
syntheticrouteinwhichthreeaddedfeatureswerefoundtomaterial.Withprecursorconcentration(0.13m)an
dtemper-becritical.First,NaAcwasaddedforelectrostaticstabilizaature(2008C)heldconstant,microspherediametersweretiontopreventparticleagglomeration.Suchbenefits
ofNaAcobservedtobe%200nmatareactiontimeof8h(Figures2a,werefoundinsimilarsyntheseswithRu,Pt,an
dRhc,e,andg),%400nmat48h,and%800nmat72hparticles.[30,31]Inoursystem,NaAcwasevenmoreimporta
nt,(Figures2b,d,f,andh).Likewise,microspherediametersasitsadditionseemstoassistintheethyleneglycolm
ediatedincreasedwithanincreaseinprecursorconcentration.Howreductionof3Fe3O.Controlexperimentsshowedthatever,thecomplexnatureofthegrowthprocessmakesit3
+difficulttoobtainastraightforwardcorrelationbetweenparticlediameterandchangesinreactiontimeorprec
ursorconcentration.Electron-diffractionpatternstakenfromindividualMFe2O4microspheresrevealedthesingle-crystallinenatureofthesesamples.Energy-dispersiveXrayanalysis34(EDAX)wasalsomeasuredtodeterminethechemicalThemagneticpropertiesoftheferritemicro
sphereswerecompositionoftheferritesamples.ResultsfromEDAXinvestigatedwithavibratingsamplemagne
tometer.Figure4spectrafromindividualferritemicrospheresshowedthatshowsthemagnetizationcurvesme
asuredat300KforsamplescontainonlyFeandOforFe3O4;Mn,Fe,andOforMFe2O4microsphereswithdiamete
rsof%200nm.TheMnFe2O4;Zn,Fe,andOforZnFe2O4;andCo,Fe,andOfor
magneticsaturationvaluesare81.9emugÀ1forFe3O4,
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ExperimentalSection
Fe3O4microspheresynthesis:[32]FeCl3·6H2O(1.35g,5mmol)wasdissolvedinethyleneglycol(40mL)toforma
clearsolution,followedbytheadditionofNaAc(3.6g)andpolyethyleneglycol(1.0g).Themixturewasstirredvigo
rouslyfor30minandthensealedinateflon-linedstainlesssteelautoclave(50mLcapacity).Theautoclavewasheatedtoandmaintainedat2008Cfor8–
72h,andallowedtocooltoroomtemperature.Theblackproductswerewashedseveraltimeswithethanolanddr
iedat608Cfor6h.
TheaboveprocesscanbecanbeextendedtothesynthesisofMFe2O4(M=Co,Mn,Zn)microspheresbycoprecipi
tationofMIIandFeIIIchlorides(M2+/Fe3+=0.5).[32]Forexample,amixtureofMnCl2·4H2O(0.50g,2.5mmol)an
dFeCl3·6H2O(1.35g,5mmol)underthesamereactionconditionsasusedforthesynthesisofFe3O4microsphere
s,producedmicrospheresofMnFe2O4.
AllsampleswerecharacterizedonaBrukerD8AdvancepowderXraydiffractometerwithCuKaradiation(l=1.5418�).TheTEMandHRTEMimagesweregeneratedwithaHitachiH
-800trans-missionelectronmicroscopeandwithaJEOLJEM-2010Fhighresolutiontransmissionelectronmicroscope.SEMimagesweretakenwithaNORANLDO1530apparatus.Magn
eticstudieswerecarriedoutonaQuantumDesignSQUIDmagnetometer.Received:November9,2004Publishe
donline:March30,2005
Figure4.Room-temperaturemagnetizationcurvesofferrite
microspheres(diameter%200nm):Fe3O4,tracea;g-Fe2O3,traceb;aFe2O3,tracec;MnFe2O4,traced;ZnFe2O4,tracee;andCoFe2O4,tracef.
53.2emugÀ1forMnFe2O4,60.0emugÀ1forZnFe2O4,and61.6emugÀ1forCoFe2O4.Moredetailedinvestigati
onsofthemagneticpropertiesoftheseproductsarestillinprogress.TheproductofFe3O4oxidationinHNO3gav
eassvalueof72emugÀ1(Figure4),whichisclosetothevalueof74emugÀ1determinedfromtheanalysisofcomm
ercialg-Fe2O3powder(Aldrich,48-1-066-5),supportingtheformationofgFe2O3undertheseconditions.AftertheFe3O4samplewasoxidizedinairat4008Cfor3h,thessvalueoftheprodu
ctwasnegligible(Figure4),whichindicatesthatunderthesecon-ditionsFe3O4istransformedintoa-Fe2O3.
Toexaminethecolloidstabilityofferritesamples,magneticferritemicrospheres(20mg)weredispersedindoubl
ydistilledwater(80mL)bysonication.Themagneticparticlesremainedinsuspensionformorethan1day,which
demonstratesthattheycanbewelldispersedinaqueoussolution.Therefore,withtheappropriatesurfacemodifications,thesemicrospheresmaybesuitableforclinicaldiagnosisandinthetransportofdrugs,proteins,viruses,orbacteria.Thesyntheticstrategydevelopedinthisstudyoff
ersseveralimportantadvantageousfeaturesforthetechnicalapplicationofmicroparticles.First,theferritesph
eresaremagnetic,whichmeansthattheycanbemanipulatedbyanexternalmagneticfield.Second,ourstrategy
allowsthedirectproductionofhighlycrystalline,monodisperse,andhydrophilicspheres.Third,microspheresizeiscontrollable,withdiametersrangingfrom200to800nm.Finally,theraw
materialsareinexpensiveandtheyieldsarerelativelyhigh(92%),whichmakesthisprocessamenabletolargescalereactionsforindustrialneeds.
Insummary,thesolvothermalreductionmethodhasbeenusedtosuccessfullyproduceaseriesofferriteMFe2O
4(M=Fe,Mn,Co,Zn)microspheres.Thisapproachprovidesaonestep,simple,general,andinexpensivemethodforthepreparationofmonodispersemagneticmicrosphereswit
hatunablediameterrangeof%200–
800nm.Webelievethatthesehydrophilicandbiocompatiblemicrosphereswillhaveimportantapplicationsnot
onlyinadvancedmagneticmaterialsandferrofluidtechnology,butalsoinbiomedicalfieldssuchasbiomolecularseparations,targeteddrugdeli
very,cancerdiagnosisandtreatment,aswellasmagneticresonanceimaging.
.
Keywords:colloids·crystalgrowth·iron·magneticproperties·reduction
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