纳米材料在生物医学中的应用

138ChemicalReviews,2010,Vol.110,No.1Figure10.Schematicdiagramshowingthebandstructureandresultingambipolar eldeffectingraphene.ConductionandvalencebandsmeetattheDiracpointwithoutanexternal eld.Undergatebias,theFermilevelmovesaboveorbelowtheDiracpointtointroduceasigni cantnumberoffreecarriers.[ReprintedwithpermissionfromNature(),ref10.Copyright2007NaturePublishingGroup.]

hasmajorimplicationsforthesemiconductorindustrybecauseitenables,inprinciple,fabricationofall-ballisticdevicesevenattoday’sintegratedcircuit(IC)channellengths(currentlydownto45nm).

Thesecondimportantpointaboutchargetransportingrapheneisambipolarity.Inthe eld-effectcon guration,thisimpliesthatcarrierscanbetunedcontinuouslybetweenholesandelectronsbysupplyingtherequisitegatebias.Thiscanbeeasilyvisualizedgiventheuniquebandstructureofgraphene(seeFigure10).10Undernegativegatebias,theFermileveldropsbelowtheDiracpoint,introducingasigni cantpopulationofholesintothevalenceband.Underpositivegatebias,theFermilevelrisesabovetheDiracpoint,promotingasigni cantpopulationofelectronsintotheconductionband.

Besidesmotivatingacademicinterest,accesstoatrulyambipolarsemiconductorenablesanumberofnoveldevicestructures.Thesearefundamentallydifferentfromsilicon-basedlogicbecausedopinglevelscanbedynamicallycontrolledentirelybygating.Momentarilyprovidinglocalgatebiasestodifferentpartsofthesame akecanformjunctionsorevenmorecomplicatedlogic.Subsequentlyrearrangingthebiasescanthencompletelyrede nethedevicewithoutmakinganyphysicalchangestothechannelmaterial.

4.2.SingleMoleculeDetection

Thesecondexcitingproof-of-conceptimplementationofmechanicallyexfoliatedgraphenewasinchemicalsensors.Severalimportantfeaturesofgraphenemadeitanexcellentcandidateforthesensingactivearea.Firstandforemost,the2Dstructureofgrapheneconstitutesanabsolutemaximumofthesurfaceareatovolumeratioinalayeredmaterial,whichisessentialforhighsensitivity.Infact,thishasbeenthemajormotivationbehindimplementationofothernano-structuredmaterialsinsensors.Inthecaseoftraditionalmaterials,bulkpropertiessuchasresistivityarenotsubstan-tiallyin uencedbysingleadsorptioneventsontheirsurface.Ingraphene,however,thereisnosuchdistinctionbetweensurfacesitesandthebulkmaterial,soeveryadsorptioneventissigni cant.

Versatilityofgrapheneasthebasisofasensorresultsfromitsuniqueelectronicstructure.Theambipolaritymeansthat

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al.

Figure11.Thelackofsurfacestatesingraphenemakespossiblethedetectionofevensingleadsorbate.Thedirectionofchangeinthe gureindicatesthesignofinducedcarriers(holesforH2OandNO2;electronsforCOandNH(),3).[ReprintedwithpermissionfromNatureref21.Copyright2007NaturePublishingGroup.]

adsorptionofeitherelectronwithdrawingordonatinggroupscanleadto“chemicalgating”ofthematerial,whichcanbeeasilymonitoredinaresistive-typesensorsetup.

In2007,singlemoleculesensitivitytoNObytheManchester21,22groupforthe2andNH rstgraphene-3wasdemonstratedbasedsensor(seeFigure11).Inthecaseofeitheranalyte,adsorptioneventsinducedsomepopulationoffreecarriersandtheresistivityofasinglelayer akedecreaseduponexposure.AHall-typecon gurationcon rmedtheoppositesignofcarriersgeneratedbythetwogases,withanelectron-withdrawingspecies(e.g.,NO2)inducingconductionbyholes(p-type)andanelectron-donor(e.g.,NH3)inducingconduc-tionbyelectrons(n-type).

Withsuchexcitingearlyresults,experimentswithgraphene-basedsensorshavecertainlyjustbegun.Whiletherearefewquestionsaboutthelimitsofsensitivityforthesedevices,therealdrawbackthusfarisalackofselectivity.Asensorisratherimpracticalifitrespondsinasimilarwayuponexposuretoanyanalyte.Thisisanexcellentareaofopportunityforchemists.Modi cationofthebasalplaneoritsedgescouldcertainlyincorporateanalyte-speci clock-and-keytypebindingsites.Suchanapproachwouldnotonlyprovideselectivesensitivitytoalargevarietyofchemicalspeciesbutperhapsalsoenabledetectionofbiologicalagentsaswell.Similarschemeshavebeensuccessfullydemon-stratedwithcarbonnanotubesandquantumwires.92-95

5.AlternativestoMechanicalExfoliation

Excitingprogressinthe eldofgraphene,andespeciallysosoonafteritsinitialdiscovery,begantosuggestabrightfuture.Thelimitingstepformostexperimentswassimplyobtaininggoodsinglelayersbymechanicalexfoliation.Thiswouldhavegreaterimplicationsforreal-worlddevicesbecausetheprocessislowthroughputandunlikelytobeindustriallyscalable.Withthisinmind,thechallengeof ndinganalternateroutetosingle-layergraphenebecamethefocusofagreatdealofresearch.

Oneshouldkeepinmindthreeimportantfactorsbeyondscalabilitywhenconsideringthepro ciencyofanysyntheticroutetographene.Firstandforemost,aprocessmustproducehighqualityinthe2Dcrystallatticetoensurehighmobility.Second,themethodmustprovide necontrolovercrystallite