ModernSensors

Lecture11X.WuAReviewofLecture10Twotypesofcommonlyusedacousticsensors:Bimorphceramic---likebimetalthermistor/piezoelectricCrystal:ADP,sodium-potassiumtartrate

tetrahydrate----piezoelectricLimitationstoacousticsensingBeamspreadAttenuationduetofrictionwithairandwatermoleculesTargetreflectioncross-sectionPiezoelectricityMaterialconstitutiveequationBasicIntent:ChemicalSensorsOverviewbasicconceptsforsensingofchemicalquantities.Thisisobviouslyanextremelybroadtechnicalarea,sothisoverviewwillbeincomplete,butisintendedtocoverthebasicissuesandsomeapplications.HumanNoseThehumannasalsensingapparatuscontainsaremarkablyflexibleandsensitivedetectioncapability.Youarecapableofdetectinganddistinguishingthousandsofdifferentsmellswithalmostinstantaneousrecognition.Odordetectionismadeverycomplicatedbecauseofthelackofactualuniquenessinthechemicalbasisofmostsmells.Thereisno"garlicmolecule"thatisdistinctfromthe"enchiladamolecule";yetyoucaneasilydistinguishthesesmells.HumanNose(cont.)Allbiologicalodordetectionsystemsarebasedonafairlysmallnumberofdistinguishablesensors.Thesmellrecognitionsystemisbasedonpattern-matchingoftheresponseofthedifferentchemicalsensorsinthenosetovariousodors.Garlicandenchiladasproduceaslightlydifferentcollectiveresponseintheentiresetofsensorsinyournose,andyourbrainhasstoredanextensivecollectionofthesepatternswhichareusedforcomparisons.

Psychologistshavefoundthattheseodorpatternscanbeamongthestrongestofmemories,andsmellsareoftenusedtoaidinthereconstructionofmemories.ChemicalSensingSystemChemicalsensingsystemdesignersneedtodrawlessonsfromthesebiologicalsystems.Animportantlessonisthatamulti-functionalsystemwillprobablyneedtouseasmallsetofdistinctsensorsandapattern-matchingalgorithmtoidentifyodorsaccurately.AresponsivematrixPattern-MatchingalgorithmIdentifyOdorsDetectorsofParticularMoleculesIfachemicalsensingapplicationrequiresdetectionofaparticularmolecule,severaltechniquesareavailable.Thesetechniquesarebasedontheuniquepropertiesofparticularmolecules.Onesetofpropertiesisassociatedwiththevibrationalandrotationalmodesofmolecules.Theexactenergiesofthesemodesisgenerallyuniquetoaparticularmolecule,andmaybeusedforidentificationpurposes.Mostvibrationsandrotationsare"opticallyactive",meaningthattheymaybeexcitedbyabsorptionofaphoton,ormayrelaxbyemissionofaphoton.Thesephotonabsorptionsaregenerallymostlikelytooccurintheinfrared,soinfraredspectroscopyisagenerallyusefulwaytoidentifymolecules.

Forexample,COdetectionCO:Averysimplemolecule(2ballsandaspring),capableofoscillatingatasinglefrequencyrotationsabout2axes(bothperpendiculartothelineconnectingtheatoms).Inquantummechanics,avibrationisrepresentedasasinglefrequencyQuantummechanicsincludes"SelectionRules"whichstronglyfavorrelaxationasinglestepofh/2atatime.Thisfeatureshowsupintheinfraredspectrumasasingleabsorption.COdetection(cont.)InthespectrumoftheabsorptionofCO,weseeapairofabsorptionpeaks.ThispeaksplittingisduetothefactthatCarbonexistsinisotopeswhichhaveatomicmassof12or13.TheadditionalmassoftheC13reducesthevibrationalenergysimplybecauseitlowerstheresonantfrequency:2150-2180wave-numberOr~4.5m(midinfrared)CommercialCODetectorsThebio-mimeticgelsensor:designedtomimicthebody'sresponsetocarbonmonoxidebycontinuallyabsorbinggas.

Metaloxidesensorsaremoreaccurateandarethecommontypeofsensoremployedinhomemodels.Thistypeofsensorusessolidstatetin-dioxidecircuits,whichclearquicklyandcontinuallymonitortheairforpresenceofcarbonmonoxide.

InstantDetectionandResponse(IDR)electrochemicalsensingtechnology,whichisclaimedtobethemosteffectivedetectionmethod.Optical?

On-chipOpticalDetectionMicro-ringresonator:nPressureRotationalenergy:NH3Inadditiontovibrations,moleculesalsocanhaverotationalenergy.InthespectrumofNH3(ammonia),therotationalspectrumcontributesaseriesofclosely-spacedlinescenteredaboutthevibrationalpeaks.Theseexcitationsinvolvetheabsorptionofaphotonandthechangeofboththevibrationalandrotationalenergy.Method1:InfraredSpectroscopyEachmoleculehasadistinctinfraredspectrum.Infraredspectrometersmaybeusedtomeasuretheabsorptionspectrumofagassample,andtolookforfeatureswhichindicatethepresenceofparticulargasconstituents.Inthisinstrument,lightfromasourcepassesthroughasample-filledchamber,thenthroughachopperwheel,afilterselector,andisfocusedontoadetector.Thefilterselectorconsistsofapairoffilterswithverynarrowinfraredtransmissionbands.Oneofthefilter'stransmissionbandiscenteredonthemolecularabsorption,andtheotheriscenteredjustoffthepeakoftheabsorptionbandofinterest.Byalternatingbetweenthesetwofilters,adifferencesignalmaybedetectedonlywhenthemoleculeofinterestispresent.Thisapproachcanbeoptimizedonlyforasinglemoleculeatatime,butisveryreliableforsuchsingle-moleculeapplications.AnexampleofsuchanapplicationwouldbeaCOdetectorforaautomotiveexhaustinspection/monitoringsystem.DifferentiatemoleculesbytheirmassesMassSpectroscopyBesidesthevibrationsandrotations,moleculesmayalsoberecognizedbytheirmass(ormassspectra...).Therefore,massspectrometersarealsousedtodetectanddistinguishmolecules.AdiagramofamagneticmassspectrometerisshownhereMassSpectrometerInthisdevice,

Oneimportantdrawbacktosuchaninstrumentisthatthemoleculesmusttraveltheirentirepathwithoutscatteringfromothermolecules.Inairatatmosphericconditions,theaveragedistancebetweencollisionsis1micron.Sincetypicalmassspectrometersneed10-100cmoftrajectory,thepressuremustbe6-9ordersofmagnitudelowerthanatmosphericpressure.Therefore,massspectrometersgenerallyrequirevacuumpumps.MoleculesIonizedAcceleratedinE-fieldDeflectedinMagneticfieldByLorentzforceAseriesofdetectorsCollect/countparticularmoleculesChromatography:aclassicOperationasampleisaddedtoapressurizedcarriergas,andforcedtodiffusethrougha"column",whichisessentiallyaverylongnarrowtube.Thecomponentsofthesamplediffuseatdifferentratesthroughthecolumn,andthedetectorattheendrecordsasignal-vs.-timetracewhichcontainspeaksthatmaybeidentifiedasbelongingtoaspecificsample.ChemSensorsv.s.BiosensorsThetermbiosensorreferstoanysensorthatusesanactivebiological(orsometimesbiologicallyderived)

componentinthetransductionprocess.Thismaybeasensorycelltakenfromalivingorganism,andmountedonanelectrode.Alternatively,antibodiesmaybeused,whichwilllockontothematerialofinterest,andholditinanappropriatepositionforsensing.Afurtheroptionistouseanenzymethatcatalysesareactionthatcanbedetectedbysuitablemeans.Sincethereisconsiderableinterestinmonitoringbloodglucoselevels(toprovideclosedloopcontrolofbloodglucose,bymeansofanartificialpancreas,fordiabetics),bloodglucosesensorshavereceivedmuchattention.Oneoftheseisbasedontheglucoseoxidaseenzyme;sothissensorwillbeoutlined.Biosensors:bloodglucosesensorsAlotofresearchhasgoneintothesesensors;biosensorsandbloodglucosesensorsinparticular.Whilstprogresshasbeenmade,therearestillalotofproblemstobesolved.Onebigprobleminthisareaisthatthesensorperformancedriftsordegradesovertime,ofteninunpredictableways.Sothedevicehastobecalibratedregularly,orjustbeforeuse.Clearlyabloodglucosesensorthatonlygivesreliablereadingsoveraperiodofahundreddayscannotbeusedinanimplantedartificialpancreas.Thus,whiletherearemanypotentialusesforchemicalsensors,theiruseisoftencomplicatedbycalibrationrequirements.ISFETsensorsISFETssensetheconcentrationofaparticularioninasolution.generallybasedontheenhancementmodeMOSFETstructure.IntheISFET,thegatemetalisreplacedwithanionselectivemembranethedeviceisimmersedinasolution.Ionsinthesolutioninteractwiththeionselectivemembrane.Whenthereisahighconcentrationofpositiveionsinthesolution,alotofthemwillaccumulateonthegate,wideningthechannelbetweenthesourceanddrain.+VProblem:lowyieldforanadhesivemembraneChemFET/integratedISFETGlucoseoxidasebasedsensorTheenzymeisimmobilisedonaplatinumelectrode,andcoveredwithathinpolyurethanemembranetoprotecttheenzymelayer,andreducethedependenceofthesensoronbloodoxygenlevels.GlucoseoxidaseGluconicacidEnzymeconversionEnzymeInteractsw/OxygenionsWater+O2MeasurableResistanceProblem:SlowResponseb/cnomoleculemovementFiberopticchemicalsensors(FOCS)Operatebytransportinglightwhich,eitherbywavelengthorintensity,providesinformationaboutanalytesintheenvironmentsurroundingthesensor.Theenvironmentisusuallyairorwater.FOCScanbecategorizedasintrinsicorextrinsic.ExtrinsicFOCSsimplyuseanopticalfiberasameanstotransportlight.Anexampleisthelaserinducedfluorescence(LIF)conepenetrometer.TrinsicFOCSusethefiberdirectlyasthedetector.Aportionoftheopticalfibercladdingisremovedandreplacedwithachemicallyselectivelayer.Thesensoristhenplaceddirectlyintothemediatobeanalyzed.Interactionoftheanalytewiththechemicallyselectivelayercreatesachangeinabsorbance,reflectance,fluorescence,orlightpolarization.Theopticalchangeisthendetectedbymeasuringthechangeinlightcharacteristiccarriedbytheopticalfiber.FOCS(cont.)TypicalUses

PrimarilybeendevelopedtomeasurevolatilepetroleumconstituentssuchasBTEXandchlorinatedVOCs(volatileorganiccompounds)suchasTCE,PCE,andcarbontetrachlorideinwater,air,orsoilgas.ThesensorshavebeendevelopedtobeplaceddownmonitoringwellstoprovideinsitumeasurementsofVOCconcentrationsingroundwater.FOCStypicallymeasuretotalVOCconcentrationsandarenotcapableofdistinguishingindividualvolatileorganicchemicals.Withappropriatechemicallyselectivelayers,FOCSarecapableofmeasuringSVOCs(semiVOC)aswell.FOCS(cont.)ManyofcommercialFOCSarecoating-basedsensors.Aportionofthecladdingisreplacedbyaproprietarycoating,whichselectivelyandreversiblyadsorbstheorganictargetanalytes.Thecoatingadsorbsorganicswhilerepellingwater.Theinteractionbetweentheorganictargetanalytesandthecoatingchangestherefractiveindexofthecoating.hydrophobic/organophilicchemicalcoating.Thechangeintheindexdecreases

theamountoflightgeneratedbyanLEDtransmittingtothedetector.Theresultantlossoflightismeasuredinproportiontotheconcentrationoforganicspresentinthesample.FOCS:PerformanceDetectionLimitsTypicaldetectionlimitsarearound1ppmforVOCsinwater.Lowerdetectionlimitsarepossiblewithpre-concentrationoftheanalyte.CalibrationCalibrationconsistsofmeasuringtheresponseofthedetectortoacalibrationstandard.Calibrationstandardsshouldconsistofaseriesofknownanalyteconcentrationsinarepresentativesamplematrix.Detectorresponseisplottedagainstanalyteconcentrationtogenerateacalibrationcurve.Aswithmostmoderninstrumentation,calibrationanddataacquisitioniscontrolledandrecordedwithamicroprocessorSamplePreparationFOCSaretypicallyusedforinsitumeasurementsprecludingsamplepreparation.FOCS:advantagesAdvantages

ThedesignofintrinsicFOCSprovidesinsituand

realtimemonitoring.FOCSaresmallinsizeduetosmallfiberopticdiameters.Opticalfibersareflexiblewithinlimitsallowinggreateraccesstodifficultlocations.Transmissionoverlongdistancesallowmonitoringindeepwellsandprovideameasureofsafetyformonitoringofhazardousatmospheres.Multielementanalysisispossibleusingvariousfibersandasinglecentralunit.FOCS:LimitationsManyFOCSarenotcompoundspecificandwillreacttomanyVOCsandwillproduceareadingonlyfortheconcentrationoftotalVOCs.Thedetectionlimitscanbehighcomparedwithconventionalanalyticalmethodssuchasgaschromatography.mainlyusedtodetectgrosscontamination.Somesensorsaretemperatureandtimedependent.Becausesensorresponseisbasedondiffusion,themeasuredconcentrationmayvarywithtimethattheFOCSisincontactwiththetargetanalyte.Therefore,itiscriticalthatequilibriumbeachievedbeforeameasurementistaken.Slowresponse:MostFOCSreachequilibriumin5to10minuteswhichisindicatedbyasteadystatesignal.Thenumberofreversiblereactions,thatisadsorptionandsubsequentdesorption,islimited,soprobesmayhavetoberegeneratedafterextend