onWeb10/12/2010 SynthesisofCore-ShellStructuredDual-MesoporousSilicaSphereswithTunablePoreSizeandControllableShellThickness DechaoNiu,†ZhiMa,‡YongshengLi,*,†andJianlinShi*,†,§KeyLaboratoryforUltrafineMaterialsofMinistryofEducation,SchoolofMaterialsScienceandEngineering,East ChinaUniVersityofScienceandTechnology,Shanghai200237,China,ShanghaiInstituteofOrganicChemistry,ChineseAcademyofSciences,Shanghai200032,China,andStateKeyLaboratoryofHighPerformanceCeramicsand SuperfineMicrostructure,ShanghaiInstituteofCeramics,ChineseAcademyofSciences,Shanghai200050,China ReceivedAugust6,2010;E-mail:ysli@;jlshi@ Abstract:Core-shellstructureddual-mesoporoussilicaspheres(DMSS)thatpossesssmallerpores(2.0nm)intheshellandlargertunablepores(12.8-18.5nm)inthecorehavebeenessfullysynthesizedbyutilizinganamphiphilicblockcopolymer(polystyrene-b-poly(acrylicacid),PS-b-PAA)andcetyltrimethylammoniumbromide(CTAB)ascotemplates.ThethicknessoftheshellsandthelargerporesizeinthecorecouldbeeasilytunedbychangingtheamountsofTEOSandthehydrophobicblock(PS)lengthduringsynthesis,respectively.Byencapsulatinghydrophobicitenanoparticlesintothecores,icdual-mesoporoussilicasphereswereobtained.DrugstorageandreleasetestingresultsshowedthatthediffusingrateofthestoreddrugcouldbeefficientlycontrolledbychangingtheshellthicknessofDMSS. Hierarchicallyporousmaterialswithmultimodalporesystemshaveattractedincreasingattentionbecauseoftheirunique,adjustable,andwell-definedporestructureandgreatpotentialforapplicationsincatalysis,sorption,separation,andbiomedicalfields.1Uptonow,varioustemplatingapproacheshavebeenadoptedtosynthesizehierarchicallyporousmaterialsbyusingemulsions,colloids,andsurfactantsastemplates.2-4Especially,hierarchicallyporousmaterialswithbimodalmesoporositycanbesynthesizedbya“dualtemplating”approachorchemicalandhydrothermalposttreatmentsofmesoporoussilicamaterials.5However,inallthesecases,structuralcontrolwasdifficult,andmostofthesebimodalmesoporousmaterialslackawell-definedporestructureonatleastonelengthscaleintermsofporeshapeorsize.Inaddition,themacroporesizeswereusuallylarge,atleastat200-300nm,thoughsmallermacropores(<100nm)wouldbemoredesirable.6Recently,anovelkindofhierarchicallybimodalmesoporematerialswithtunableporesizeandstructurehasbeensynthesizedessfullybyusingvariousblockcopolymersasalargeporetemplateandasmallersurfactantasasmallporetemplate;7however,theobtainedparticlessufferedfromtheirregularmorphologyandbroadenedparticlesizedistribution.Consequently,developinganovelapproachonthesynthesisofdual-mesoporousmaterialswithwell-definedparticlemorphologyandsizeinthenanoscale(∼200nm)isstillagreatchallenge. Herein,wereportaverysimplemethodonthesynthesisofcore-shellstructuredandmonodisperseddual-mesoporoussilicaspheres(designatedasDMSS)withlargerporesinthecoreand †SchoolofMaterialsScienceandEngineering,EastChinaUniversityofScienceandTechnology. ‡ShanghaiInstituteofOrganicChemistry,ChineseAcademyofSciences.§ShanghaiInstituteofCeramics,ChineseAcademyofSciences. 151449J.AM.CHEM.SOC.2010,132,15144–15147 Figure1.TEMimages(a,b)aswellasN2adsorption-desorptionisothermsandBJHporediameterdistributioncurveatadsorptionbranch(inset)(c)ofDMSS-PS100. smallerporesintheshellusingamphiphilicblockcopolymerpolystyrene-b-poly(acrylicacid)(PS-b-PAA)andcetyltrimethylammoniumbromide(CTAB)asdualtemplates.Differentfrompreviouslyreportedblockcopolymer-templatingmethods,8anionicblockcopolymerPS-b-PAAisusedinthepresentstudyasatemplatetoformthelargermesopores.Moreimportantly,thesizeofthelargerporesandthethicknessoftheshellcanbeeasilytunedbychangingthelengthofPSblocksofPS-b-PAAandtheposition,respectively(TableS1).Itisimportanttonotethatdual-mesoporoussilicaspherespreparedbythismethodhaveuniformparticlesizesandawell-definedcore-shellstructure.Also,functionaldual-mesoporoussilicasphereswereessfullypreparedbyencapsulatinghydrophobicnanoparticles(suchasicFe3O4particles)inthelargepores.Drugstorageandreleasetestingresultsshowedthatupto500mgofibuprofencouldbestoredin1gofDMSS,andthediffusionrateofdrugmoleculescanbeeffectivelytunedbyvaryingthethicknessoftheshell. Figure1apresentstypicalTEMimagesofDMSSpreparedwithblockcopolymerPS100-b-PAA16asonetemplate(denotedasDMSSPS100).Well-definedcore-shellstructurednanospheresof∼250nmindiameterareclearlypresented.Itisinterestingtonotethatthecoreisneithersolidnorhollowbutposedoforderedmesoporeswithaporediameterofaround14nm.AnHRTEMimageofDMSS-PS100,asshowninFigure1b,furtherconfirmsthecore-shellstructureddual-mesoporousstructurewithlargerporesinthecoreandsmallerpores(∼2.0nm)intheshell.Inordertoconfirmthemesostructuralordering,smallangleX-rayscattering(SAXS)analysiswasadopted(FigureS1).AsshowninFigureS1,theSAXSpatternofDMSS-PS100showswell-resolvedscatteringpeaksintheqrange0.03-0.13A-1,indicatinganorderedhexagonalmesostructureinthecores.Inaddition,theXRDpatternofDMSS-PS100showninFigureS2alsoindicatestheorderedarrangementofthelargermesopores.However,thebroadenedscatteringpeakintheqrange0.15-0.20A-1inFigureS1andtheequallybroadeneddiffractionpeakinthe2θrange2°-3°showthedisorderedarrangementofthesmallermesopores,whichisalsoinordancewiththeaboveTEMobservations(Figure1b).In 10.1021/ja10706532010AmericanChemicalSociety COMMUNICATIONS Figure2.TEMimagesofDMSSpreparedbyusingblockcopolymersofdifferentlengthsofPSblockastemplates:(a,b)PS78-b-PAA16,(c,d)PS154b-PAA16. ordertounderstandthedual-mesoporousstructureofDMSSmoreclearly,anHRTEMimage(FigureS3)ofanultrathinmicrotomesectionofDMSS-PS100wasexamined.Itshouldalsobenotedthatsmallermesoporesarepresentnotonlyintheshellsbutalsointhelargeporeframeworkinthecores,whilelargermesoporescanonlybefoundinthecores. Inordertofurtherdemonstratethedual-mesoporousstructureofDMSS-PS100,N2sorptionanalysiswasperformed(Figure1c).AsshowninFigure1c,theadsorption-desorptionisothermsshowtwomajorcapillarycondensationstepsintherelativepressureranges0.1-0.3and0.80-0.95,implyingthattwosetsofmesoporesrespectivelyat∼2.0nmand∼14nmindiameterhavebeenobtainedascanbeidentifiedfromthecorrespondingdistributioncurveofDMSS-PS100,whichisobtainedbytheBarrett-JoynerHalenda(BJH)method.Inaddition,thespecificsurfaceareaandtotalvolumeofDMSS-PS100iscalculatedtobe697m2·g-1and0.95cm3·g-1,respectively.Noticeably,itisknownthattheBJHmethodisquitegoodforlargeporesbutmayunderestimatethesizeofsmallpores.AstheCTAB-templatedmesoporesareusuallysmallintherange3-3.5nm,toobtainamoreurateporesize,anovelnonlocaldensityfunctionaltheory(NLDFT)methodwasusedforthecalculationofporesizeanddistributionfromadsorptionisotherms.9AsshowninFigureS4,thesmallerporesizeofDMSSPS100bytheNLDFTmethodisaround2.94nm.Therefore,thesmallporesinourDMSSinthepresentworkcanbeunderestimatedbyabout1nmbytheBJHmethod. UndersimilarconditionstothoseforDMSS-PS100,DMSSwithvariedlargerporesizes,i.e.DMSS-PS78,DMSS-PS154,andDMSSPS260,wereobtainedbychangingthelengthofthePSblock.TEMimagestogetherwiththeN2sorptionanalysisresultsdemonstratethecore-shelldual-mesoporousstructureandsphericalmorphologyofDMSS-PS78andDMSS-PS154(Figures2andS5).OnincreasingthenumberofPSblockunitsinPS-b-PAAfrom78to154,thelargerporediameterinthecoreswasenlargedfrom12.8to18.5nm,respectively,whilethesmallermesoporeskeepconstantatabout2.0nm.ThismeansthatthenumberofPSblocksdeterminesthelargerporesizeofDMSS,toalargeextent.However,mesoporoushollowsilicasphereswereobtainedwiththenumberofPSblockunitsincreasingto260(FiguresS5andS6).Suchachangeofparticlestructurecanbemostprobablyattributedtothemicellemorphologytransformationfromrod-liketovesicleorotherpoundmicelles.AsamphiphilicblockcopolymerPS-bPAAmayformmicellesofvariedmorphologies(e.g.,spherical,rod-like,vesicle,andpoundmicelles),10itisanticipatedthatvariousparticlemorphologiesofdifferenthierarchicalporestructurescouldbepreparedwiththepresentmethodology.In Figure3.TEMimagesofDMSSwithdifferentshellthicknesses:(a,d)5nm,(b,e)25nm,and(c,f)60nm.Scheme1.SchematicIllustrationfortheFormationofDual-MesoporousSilicaSpheresa a(a)THFsolutionofPS-b-PAA;(b)CTAB-coatedPS-b-PAAaggregatesformedbytheelectrostaticinteractionbetweenCTA+andPAA-;(c)CorepartformedfromtheassemblybetweenCTAB-coatedPS-b-PAAaggregatesandTEOS;(d)Mesoporousshellformationviatheself-assemblybetweentheremainingCTABandtheadditionalTEOS;(e)Finalcore-shellstructureddual-maeoporoussilicaspheresaftercalcination. addition,theshellthicknessofDMSScouldbewelladjustedbychangingtheamountofTEOSduringsynthesis.AsshowninFigures3andS7,theshellthicknessofDMSS-PS100couldbevariedintherange5to60nm,withtheparticlesizevaryingfrom200to400nm,correspondingly. Basedontheaboveobservations,apossiblemechanismwasproposedtoexplaintheformationofdual-mesoporoussilicaspheres(Scheme1).AsthemorphologyofPS-b-PAAmicellescouldbechangedfromspherestorod-likeaggregatesbytheadditionofanicionsorsmallmoleculesurfactants,11inourexperiments,rod-likeaggregateswereassumedtoforminthepresenceofasmallmoleculesurfactant(CTAB)andammonia(NH4+)atthebeginningstage,whichwasconfirmedbytheTEMobservationofaprecursorsolutionbeforeaddingethanolandTEOS(FigureS8).Atthesametime,thehydrophilicPAAblocksofrod-likeaggregatescouldcouplewiththeCTABmicellesviaCoulombforceandelectrostaticinteractionbetweenCTA+andPAA-toformCTAB-coatedrodlikeaggregates,positemicelles,insolution.AftertheadditionofTEOSandethanol,theanicself-assembledhybridmicelles,whichposedofsilicateoligomersandCTABcoatedPS-b-PAArod-likeaggregates,werefirstformed.Then,these
J.AM.CHEM.SOC.9VOL.132,NO.43,201015145 COMMUNICATIONS rod-likepositemicellespacktogetherinanorderedfashiontoformthecorepartdual-mesoporousstructure,whichissimilartothemodelproposedbyCaiandco-workers.12Finally,theelectrostaticinteractionbetweenpositivelychargedCTA+andnegativelychargedsilicaspeciesandthecondensationoftherodlikesilicatemicellesfacilitatethedepositionofthemicelles,resultingintheformationofmesoporoussilicawithasphericalmorphology.AsacertainlimitedamountofCTABcancoatthePS-b-PAAmicellestopositemicelles,theremainingCTABmoleculeswillbeexpelledoutofthecoreportionofthestructure,andoncethecoreparteslargeenough,theimmediateouterregionofthecore(or,theinterfacebetweenthecoreandtheliquidphase)willesufficientlyrichinCTAB,leadingtothecooperativeself-assemblybetweentheseCTABmoleculesandadditionalTEOSandconsequentlytheformationofanoutershellofsmallerporessurroundingthecores. Fromthisproposedmechanism,itisclearthatCTABplaysanimportantroleintheformationofthestructure.WithoutCTAB,CTAB-coatedPS-b-PAAaggregatescannotformandthecorrespondingself-assemblybetweentheCTAB-coatedPS-b-PAAaggregatesandTEOScouldnottakeplace,sothatmesoporousmaterialswillnotbeobtained.Inaddition,ethanolsolventplaysanessentialroleintheformationofstableCTAB-coatedrod-likeaggregates.AsshowninFigureS9,withoutorwiththeadditionofasmallamountofethanol(e.g.,40mL),dual-mesoporoussilicaspherescannotbeobtained;onlyconventionalmesoporoussilicasphereswithasingleporesystemat2.3nm,orsometimeswithfewlargeporechannels,wereformed.Incontrast,whenamuchlargeramountofethanol(120mL)wasemployed,onlylarge-poremesoporoussilicasphereswithaporesizeof15nmwereobserved.Thereasonforthedisappearanceofsmallerporesisprobablyduetothelargelyincreasedcriticalmicelleconcentration(CMC)ofCTABwiththeincreasedamountofethanol,13whichishigherthanthatofCTABmicelleconcentration,resultinginthedissociationofCTABmicelles.ThistransformationwasalsoconfirmedbyN2sorptionanalysis(FigureS10). Inaddition,thismethodalsoprovidesopportunitiesfordevelopingdual-mesoporoussilicasphereswithspecificfunctions.Here,wedemonstratedtheessfulincorporationofitenanoparticlesintothelargerporesofDMSSforicfunctionalization.FromtheTEMimagesofite-loadeddual-mesoporoussilicaspheres(denotedasFe3O4@DMSS,FigureS11),itcanbeseenthatthemonodispersityanddual-mesoporousstructureofparticleswerenotaffectedwiththeencapsulationofitenanoparticles.Duetothesmallsizeofitenanoparticles(∼4nm),itishardtodistinguishtheFe3O4nanoparticlesfromthematrixintheTEMimage.However,theenergydispersivespectrum(EDS)andXRDpatternofFe3O4@DMSSconfirmtheexistenceofFe3O4nanoparticlesintheparticles(FiguresS12andS13).N2adsorptiondesorptionanalysis(FigureS14)showstwosetsofmesoporescenteredat2.1and11.2nm,respectively,andahighspecificsurfaceareaof1154m2·g-1andporevolumeof1.21cm3·g-1wereobtainedforFe3O4@DMSS.TheicpropertiesofFe3O4@DMSSat300Kwererecordedusingavibrating-sampleometer(VSM),aspresentedinFigureS11c,demonstratingtheiricfeaturewithasaturationizationvalueof2.5emu/g,whichisdesirablefortheirbiomedicalapplications,suchasinicseparation,icresonanceimaging(MRI),targeteddrugdelivery,andhyperthermiatreatmentofcancer.14TheicseparabilityofFe3O4@DMSSwasfurthervisuallytestedinwaterbyplacinganeartheglassbottle.Theparticleswereattractedtowardthewithinseveralminutes,asshowninFigureS11d,demonstratingdirectlythatthe 15146J.AM.CHEM.SOC.9VOL.132,NO.43,2010 core-shellnanosphereshavebeenendowedwithicproperties.Followingthesameprocedure,variouskindsofhydrophobicnanoparticlescanbeencapsulatedinthecoreofthedualmesoporoussilicaspheres,resultinginmultifunctionalsilicasphereswiththedual-mesoporousstructure. Toexploreitscapabilityasadrugcarrier,ibuprofen,atypicalanti-inflammatorydrug,wasintroducedintotheporesofDMSSofvariedshellthicknessesof5,25,and60nmanddenotedasDMSS100-ST5,DMSS100-ST25,andDMSS100-ST60,respectively.Theuptakeamountsofibuprofenareca.26.3,34.6,and29.5wt%,asassessedbyTGanalysis.ThereleasebehaviorofibuprofeninPBSsolution(pH)7.4)wasmeasuredovera24hperiod(FigureS15).ItisclearthatthereleaseratesofdrugmoleculescouldbecontrolledbychangingtheshellthicknessofDMSS.Inaddition,anapparentthree-stageibuprofenreleaseprofilefromsamplesDMSS100-ST25andDMSS100-ST60isclear:thefirststageinvolvestherapidreleaseofibuprofenwithinthefirst60min,owingtothereleaseofIBUadsorbedontheexternalsurface.Then,thereleaseamountofibuprofenincreasesagainfrom2to6handreachesaplateauat73%forsampleDMSS100-ST25,forexample.Itisreasonabletoattributethisreleasetothosetrappedinthesmallpore.Finally,theibuprofenconcentrationincreasesfrom6to24handeventuallyreachesanotherplateauat80%forDMSS100-ST25.Thereleaseinthelaststagecouldprobablybeattributedtotheadsorbeddruginthelargepores.Thethree-stepreleaseprofileofthedual-mesoporousstructureissimilarwiththatformesoporousandyolk-shellstructurereportedpreviously.15ThisindicatesthatDMSScanbeaneffectivecarrierfordrugdeliverandrelease. Insummary,anovelkindofcore-shellstructureddualmesoporoussilicasphereswithahierarchicalporestructure,whichposedofsmallermesoporesintheshellsandorderedlargermesoporesinthecores,havebeenfabricatedbyasimpledualtemplatingmethod.ThelargerporesizeofDMSScanbeeasilytunedbychangingthePSblocklengthofPS-b-PAA.Inaddition,theshellthicknessofdual-mesoporoussilicaspherescanbewellcontrolledfrom5to60nmbyvaryingtheconcentrationofTEOS.Athree-stepreleaseprofileofdrugswasobservedwiththeseparticulardual-mesoporousstructures.Also,icitenanoparticlescanbeencapsulatedindual-mesoporoussilicaspheres,resultinginpositespheres,whichmayfindpotentialapplicationsincatalysis,sorption,separation,andbiomedicalfields. Acknowledgment.ThisworkwasfinanciallysupportedbytheNationalHi-TechProjectofChina(GrantNo.2007AA03Z317);theNationalNaturalScienceFoundationofChina(GrantNo.20633090and21001043),ScientificInnovationProjectofShanghaiMunicipalEducationCommission(GrantNo.11ZZ53),andShanghaiPujiangProgram(GrantNo.09PJ1403000). SupportingInformationAvailable:Experimentaldetailsforthesynthesisandadditionalfigures.Thismaterialisavailablefreeofchargeviatheat. References
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