Amino Acid Based Metal-OrganicNanofibers

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AminoAcidBasedMetal-OrganicNanofibersInharImaz,†MartaRubio-Martínez,†WojciechJ.Saletra,‡DavidB.Amabilino,‡andDanielMaspoch*,†Centred’InVestigacio´enNanocie`nciaiNanotecnologia(CIN2,ICN-CSIC),EsferaUAB,CampusUAB,08193Bellaterra,Spain,andInstitutdeCie`nciadeMaterialsdeBarcelona(CSIC),CampusUAB,08193Bellaterra,Spain

ReceivedOctober13,2009;E-mail:daniel.maspoch.icn@uab.es

Self-assemblyofsmallmoleculesand/ormetalionsintonano-structures,suchasspheres,tubes,rods,tapes,fibers,andcrystallineparticles,hasevolvedasanattractivestrategyforfabricatingmaterialswithtunablephysicalandchemicalpropertiesviacontroloftheircomposition,size,andshape.Thisversatilityhasopenedupabroadrangeofpotentialapplicationsincatalysis,electronics,sensing,medicaldiagnostics,tissueengineering,datastorage,anddrugdelivery.1Recently,conventionalcoordinationchemistryhas

alsostartedtoattractagreatamountofinterestforsynthesizingthesenanoscalestructures.Inparticular,severalrecentstudieshavedemonstratedthataninfinitecoordinationpolymerizationofmetalionsthroughorganicligandsfollowedbyaprecipitationcangenerateinfinitecoordinationpolymerparticles(ICPs).2,3Thusfar,

theseICPshaveshownnotonlypromisingfunctionalities,suchasmagnetism,4porosity,5ionexchange6oropticalproperties,6but

alsoacapacityforuseasnovelencapsulatingmatrices.7

Eventhoughthelatterapproach,inprinciple,canprovideinterestingICPs,nanoscaleone-dimensional(1-D)coordinationpolymers,includingrods,tubes,andfibers,arestillscarce.In2004,Martinetal.synthesizedthefirstdiscretemetal-organicnanotubesbyusingtemplatedlayer-by-layergrowth.8Soonafter,Linetal.

preparedGd(III)-basednanorodsabletoactascontrastagentsthroughawater-in-oilmicroemulsion-basedtechnique,9whereas

Ohetal.usedhydrothermalsynthesistoobtainporousIn(III)-basedhexagonalnanorods.10Morerecently,coordinationpolymer-based

gelsandnanofibershavebeendescribedbyYouandLoh’sgroups,respectively.11,12Thesenanofibersareefficientinlightharvesting,

becausefluorescenceresonanceenergytransferisfavoredinthe1-Dstructure.12Therefore,designingnovelsyntheticroutesto

fabricate1-Dcoordinationpolymernanostructureswillcertainlyopenupexcitingopportunitiesfordevelopinganewclassofelectronically,optically,andbiologicallyactivematerials.Herewereportafaciletemplate-freestrategytoverylong(uptoonecentimeter)discretechiralcoordinationpolymernanofibersunderambientconditionsusingaqueous/organicinterfacialcoor-dinativepolymerization(Figure1).Thenanofibershavediametersbetween100and200nm.Thesynthesisisbasedonthewell-knowncapacityoftheL-orD-asparticacids(Asp)togeneratebiorelated

coordinationpolymerswhentheyarereactedwithtransitionmetalions.13ThedicarboxylateformoftheAspandCu(II)metalions

areseparatedbytheinterfacebetweenanorganicsolventcontainingthesodiumaspartateandanaqueousphasecontainingthetransitionmetalions.TheCu(II)-Aspproduct,whichstartstopolymerizeattheinterface,containsexclusivelylongnanofibersthatgroworientedalongthediffusiondirection.Nanofiberswithshorterlengthsontheorderofmicrometersarealsoproducedwhenthephasesaremixedunderstirring.Thisobservationindicatesthattheformationofnanofibersisnotdeterminedbytheinterfacebutmorelikelyby

thenatureofthecoordinationpolymerizationoftheCu-Aspsystem.However,itisclearthatthelengthoftheresultingnanofibersishighlydependentonthemethodusedfortheirsynthesis.Inatypicalexperiment,L-orD-asparticacidandNaOHwere

dissolvedinanethanol/watermixture(5/1),whereasCu(NO3)2

·

6H2Owasdissolvedinwater.Thesolutionswerethentransferredcarefullytoatesttube,generatinganinterfacebetweenthetwolayers(Figure1b).After3days,darkbluefibersstarttoformattheliquid-liquidinterface(Figure1c).Then,astheethanoliclayerslowlydiffusedintotheaqueousphase,thesefibersgraduallygreworientedalongwiththediffusiondirection.After2weeks,theentireaqueousphasewasfilledhomogeneouslywithdark-bluemetal-organicnanofibers(Figure1d).Figure2ashowsaphotomicrographofbundlesofthesefibers,illustratingtheir∼1cmlengthandhighorientation.Transmissionelectronmicroscopy(TEM)imagesofthesefibersshowaveryhomogeneoussampleoffiberswithdiametersof100-200nm(Figure2c).Thesampleuniformityandnarrowdiameterdistributionwerealsoconfirmedbyfield-emissionscanningelectronmicroscopy(FESEM).Figure2bshowsatypicalFESEMimageofananofiberthinfilmcastonanaluminumsubstratefromacolloidalsuspension.Thesampleappearstobeexclusivelynanofibers,consistentwithTEMimages.X-raypowderdiffractionperformedontheCu-Aspnanofibersconfirmedtheircrystallinecharacter,buttheirsinglecrystalstructuredetermination,andthereforeadetailedanalysisofthestructuralconnectivity,wasnotfeasibleduetotheirsub-200nmdiameter.However,someadditionalcharacterizationappearstoindicatethatCu(II)ionsandAspligandsarearrangedin1-Dchainswiththegeneralformula[Cu(Asp)(H2O)x]n.14Thechemicalcompositionof