LSPR Review2.docx

LSPR Review2.docx

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LSPRReview2

LocalizedSurfacePlasmonResonanceSensors:

Review

Si-yiYang1,Xiao-xueSun1,ChengWang1,

Dong-hongCai2,LiZhang1,Guo-huaLiu1

（1.CollegeofInformationTechnologyandScience,NankaiUniversity,Tianjin300071,China；2.DepartmentofPhysics,Kashiteacherscollege,Xinjiang844000,China）

Abstract:

Recently,theresearchonthelocalizedsurfaceplasmonresonance（LSPR）sensorsisoneofthehot-spotsinthefiledofsensortechnology.TheLSPRsensorshaveobviouslytechnologyadvantageindetectingandanalyzingthepropertiesofphysicsandchemistry,especiallybiology.ThisreviewintroducesthetechnicalprinciplesandfeaturesofLSPRsensorsandcomparesthecurrentresearchofseveralLSPRsensorstructuresandproductionmethods.Meanwhile,itdiscussesthefuturedevelopmentsandcommercialviewsofLSPRsensors.

KeyWords:

LocalizedSurfacePlasmonResonance（LSPR）;Biosensor;Nanoparticle

1.Introduction

Inrecentyears,scientistshavehighinterestsinthenanomaterialsforthespecialcharactersinoptics,electromagneticsandmechanics.NanoparticlesofthenoblemetalsexhibitastrongUV-Visabsorptionbandthatisnotpresentinthespectrumofthebulkmetal[1-8].Scientistshavereportedthatthecharactersofnoblemetalnanoparticlesuspensionsarisefromtheirstronginteractionwithlightandtheadventofthefieldofnanoparticleopticshasallowedforadeeperunderstandingoftherelationshipbetweenmaterialpropertiessuchascomposition,size,shape,andlocaldielectricenvironmentandtheobservedcolourofametalsuspension.Anunderstandingoftheopticalpropertiesofnoblemetalnanoparticlesholdsbothfundamentalandpracticalsignificance.Fundamentally,itisimportanttosystematicallyexplorethenanoscalestructuralandlocalenvironmentalfactorsthatcauseopticalpropertyvariation,aswellasprovideaccesstoregimesofpredictablebehaviour.Practically,thetunableopticalpropertiesofnanostructurescanbeappliedasmaterialsforsurface-enhancedspectroscopy[9-13],opticalfilters[14,15],plasmonicdevices[16-19]andsensors.

ItwasrealisedthatthesensortransductionmechanismofthisLSPR-basednanosensorisanalogoustothatofSPRsensors.LSPRsensorscanbeseenasthecontinuationandexpansionofSPRsensors.Theformertookplaceinthelocalizedmetalnanoparticlesandthelattertookplaceinthesurfaceofmetalfilms.However,theopticalcharacterofLSPRisdifferentfromthatofSPR.LSPRhasgreatpotentialinsensingfieldsothatithasbeenwidelystudied[20-26].Itisknownthatnanoparticles,suchasgoldandsilver,havestrongabsorptioninthevisibleregion,oftenconsideredaslocalizedsurfacePlasmonabsorption[27,28].SuchLSPRoccurswhentheincidentphotonfrequenciesmatchthecollectiveoscillationsoftheconductionelectronsofmetalnanoparticlesormetalislands.TheparticlesinthenanoscaleexhibituniqueopticalresponseswithintheUV-Visregion[29,30],wheretheabsorbanceshowsanexponentialdecaywithdecreasingphotonenergy（theso-calledMiescattering）,ontowhichanLSPRband,specificfortheparticlematerial,issuperimposed.Thesurfaceplasmonenergyandintensityhavebeenfoundtobesensitivetoanumberoffactors,includingparticleconformation,immediatesurroundingmedia,etc.[31-35].Thereactionsbetweensolutionandprobeonthemolecularnanoparticlescancausechangethethicknessofthemolecularlayer,whichledtotheshiftofLSPRabsorption,sothismethodcanbeusedindetectingdynamicreal-timedetection[36-37].TakeAgnanoparticlesfabricatedusingnanospherelithography（NSL）asanexample.Consistentwavelengthshiftstotheredwithincreasingdensityandthicknessofadsorbatelayers.Thewavelengthshiftresponseisdeterminedbythesizeandpackingdensityofthemoleculesonthenanoparticlesurfaceandthelimitofdetectionofthesystemaredeterminedbythesurface-confinedbindingconstantbetweenthecaptureligandonthesurfaceandthetargetmoleculeinsolution.Asthesystemsrevealnononspecificbinding,theentireresponsecanbeattributedtothespecificinteractionsbetweenthemoleculesofchoice.OptimizationoftheLSPRnanosensorhasbeenrealizedbyadjustingthesizeandshapeofthenanoparticles[30,38].

Forthenanomaterialshavethesamescaleasbiopolymers,proteinsandnucleicacidsinsizes,theresearchesandoptimizationsofLSPR-basedsensingtechnologyhavebeencarriedoninthebiomedicalfield.Thebasicofapplicationsinbioresearch,biosensing,celllabeling,fixed-pointdiagnosis,moleculardynamicsresearch,andvectortreatmentistheinteractionbetweenbiomolecularsandnanomaterials.LSPRnanosensorsareusefulindetectingbiologicalmolecules[22,39,40].

2.TheprincipleofLSPRsensors

LSPRiscollectiveoscillationsoftheconductionelectronsconfinedtometallicnanoparticlesandmetallicnanostructures[41-45].Itoccursinmetallicnanostructures,suchasnanoparticles,nanotriangles[46],andnanoisland[42].LSPRisobservedwhenthefrequenciesofincidentphotonmatchthecollectiveoscillationsoftheconductiveelectronsofmetalnanoparticles.Excitationoflocalizedsurfaceplasmonsbyanelectricfieldatanincidentwavelengthwhereresonanceoccursresultsinastronglightscattering,intheappearanceinintensesurfaceplasmonabsorptionbands,andtheenhancementofthelocalelectromagneticfield.ThenanoparticlesexhibituniqueopticalresponseswithintheUV-Visregion[28],wheretheabsorbanceshowsanexponentialdecaywithdecreasingphotonenergy（theso-calledMiescattering）ontowhichanLSPRband.Thefrequencyandintensityofthesurfaceplasmonabsorptionbandshighlydependonthetypeofthematerial（gold,silver,platinum）,thesizeandtheshapeofnanostructuresaswellasontheirsurroundingenvironment[29,30,34,47].

LSPRoccurswhenthefrequencyoftheincidentphotonsresonateswiththecollectiveoscillationsoffreeelectronsinthemetals.ThesimplestmodelofnanoparticleopticalresponsesisMietheory.Itdescribesextinctionamountsofthelong-wavelengthsphericalmetalparticles.Theformisasfollow[35]:

E（λ）istheextinctionwhichisequaltothesumofabsorptionandscattering,NAisthearealdensityofnanoparticles,aistheradiusofthemetallicnanosphere,εmisthedielectricconstantofthemediumsurroundingthemetallicnanosphere（assumedtobeapositive,realnumberandwavelengthindependent）,λisthewavelengthoftheabsorbingradiation,εiistheimaginaryportionofthemetallicnanosphere’sdielectricfunction,andεristherealportionofthemetallicnanosphere’sdielectricfunction.ItiseasytoseethattheLSPRconditionismetwhen（εr+2εm）2approacheszero.

Inthisprimitivemodel,itisclearthattheLSPRspectrumofanisolatedmetallicnanosphereembeddedinanexternaldielectricmediumwilldependasfollows:

theradiusa,thenanoparticlematerial（εiandεr）,andthenanoenvironment’sdielectricconstant（εm）.Furthermore,whenthenanoparticlesarenotspherical,asisalwaysthecaseinrealsamples,theextinctionspectrumwilldependonthenanoparticle’sin-planediameter,out-of-planeheight,andshape.Inthiscasetheresonancetermfromthedenominatorof

（1）isreplacedwith:

Whereχ,ashapefactorterm[11],isatermthatdescribesthenanoparticle’saspectratio.Thus,theLSPRwillalsodependoninterparticlespacingandthesubstratedielectricconstant.ItisapparentfromEq.1thatthelocationoftheextinctionmaximumofnoblemetalnanoparticlesishighlydependentonthedielectricpropertiesofthesurroundingenvironment,andthatwavelengthshiftsintheextinctionmaximumofnanoparticlescanbeusedtodetectmolecule-inducedchangessurroundingthenanoparticle.

3.TheconfigurationofLSPRsensingsystem

LSPR-baseddevicecanbesetupwithoutusingthespecificconfigurations,forexample,theattenuatedtotalreflection（ATR）opticalsetuporwaveguidecoupling,itispossibletofabricateverysmalldevicesbasedbyNSLorothertechnologies.ThesepropertieshavepromptedanintenseinterestinLSPR-basedsensorsandbiosensors.

Generally,anLSPRopticalsensorcomprisesanopticalsystem,atransducingmediumwhichinterrelatestheopticalandbiochemicaldomains,andanelectronicsystemsupportingtheoptoelectroniccomponentsofthesensorandallowingdataprocessing.ThetransducingmediumtransformeschangesinthequantityofinterestintochangesintherefractiveindexwhichmaybedeterminedbyopticallyinterrogatingtheLSPR.TheopticalpartoftheLSPRsensorcontainsasourceofopticalradiationandanopticalstructureinwhichsurfaceplasmonwaveisexcitedandinterrogated.IntheprocessofinterrogatingtheLSPR,anelectronicsignalisgeneratedandprocessedbytheelectronicsystem.MajorpropertiesofanLSPRsensoraredeterminedbypropertiesofthesensor’ssubsystems.Thesensorsensitivity,stability,andresolutiondependuponpropertiesofboththeopticalsystemandthetransducingmedium.Theselectivityandresponsetimeofthesensorareprimarilydeterminedbythepropertiesofthetransducingmedium.Takethefiber-basedbiosensorsystemasanexample.Theconfigurationoffiber-basedbiosensorsystemisshowninFig.1.Thesystemconsistsofalaser,achopper,afibercoupler,asensingfiber,aliquidcell,aphotoreceiver,andalock-inamplifier[48].

Fig.1TheschematicrepresentationoftheLSPRbiosensorsystem

4.ThefabricationandtechnologyofLSPRsensor

MetalnanostructuresarethekeytofabricatetheLSPRbiosensors.Theymainlyincludenanowirearray,nanoparticles,nanoislandsandotherstructures,thenwewillintroducethefabricationandtechnolog