All of the polymer films on a set of electrodes sensors：所有在一组电极的聚合物薄膜传感器

Allofthepolymerfilmsonasetofelectrodes(sensors)startoutatameasuredresistance,theirbaselineresistance.Iftherehasbeennochangeinthecompositionoftheair,thefilmsstayatthebaselineresistanceandthepercentchangeiszeroe-e-e-e-e-e-BASELINERESISTANCE

Ifadifferentcompoundhadcausedtheairtochange,thepatternofthepolymerfilms'changewouldhavebeendifferent:

Eachpolymerchangesitssize,andthereforeitsresistance,byadifferentamount,makingapatternofthechange

THEELECTRONICNOSESMELLSSOMETHINGe-e-e-e-e-e-e-e-e-e-e-e-SistemadesarrolladoporNASAparalosshuttles,etc.Theelectronicnoseconsistsoftwocomponents,anarrayofchemicalsensors(usuallygassensors)and

(2)apattern-recognitionalgorithm.Thesensorarray"sniffs"thevaporsfromasampleandprovidesasetofmeasurements;thepattern-recognizercomparesthepatternofthemeasurementstostoredpatternsforknownmaterials.Gassensorstendtohaveverybroadselectivity,respondingtomanydifferentsubstances.Thisisadisadvantageinmostapplications,butintheelectronicnose,itisadefiniteadvantage.Althougheverysensorinanarraymayrespondtoagivenchemical,theseresponseswillusuallybedifferent.Figure1showssetsofresponsesofatypicalsensorarraytodifferentpurechemicals:ProblemsWheretheE-NoseCanHelpTheE-Noseisbestsuitedformatchingcomplexsampleswithsubjectiveendpointssuchasodororflavor.Forexample,whenhasmilkturnedsour?Or,whenisabatchofcoffeebeansoptimallyroasted?TheE-Nosecanmatchasetofsensorresponsestoacalibrationsetproducedbythehumantastepanelorolfactorypanelroutinelyusedinfoodscience.TheE-Noseisespeciallyusefulwhereconsistentproductqualityhastobemaintainedoverlongperiodsoftime,orwhererepeatedexposuretoasampleposesahealthrisktothehumanolfactorypanel.AlthoughtheE-Noseisalsoeffectiveforpurechemicals,conventionalmethodsareoftenmorepractical.Project:HeadspaceAnalysisofCulturesofE.coliSamplesweretakenfromagrowingcultureofE.coliuntilstationaryphasewasreached.Cellswerefilteredfromthesamples,andthefiltratesweresealedintoautosamplervials.ThesamplesweremeasuredontheMOSESIIElectronicNose(LennartzElectronicGmbH)usingmetal-oxideandquartz-microbalancesensors.ThepatternrecognitionalgorithmwasabletodistinguishsamplesofEcoli-alteredgrowthmediumafter5hours.Continuedworkwillemphasizesamplingandsensorarrayimprovementstoincreasethesensitivityofdetection.ProblemsThattheE-NoseDoesBest: Identificationofspilledchemicalsincommerce(forU.S.Coast

Guard). Qualityclassificationofstoredgrain. Waterandwastewateranalysis. Identificationofsourceandqualityofcoffee.Monitoringof

roastingprocess. Ranciditymeasurementsofoliveoil(duetoaccumulationof

short-chainaldehydes). Detectionanddiagnosisofpulmonaryinfections(e.g.,TBor

pneumonia). Diagnosisofulcersbybreathtests. Freshnessoffish. Processcontrolofcheese,sausage,beer,andbreadmanufacture. Bacterialgrowthonfoodssuchasmeatandfreshvegetables.Figure2.PrincipalcomponentplotofsensorarraydatafromheadspacesamplesofnutrientbrothinwhichE.colihadbeengrownforvariousperiodsoftime.Includedisadataset("Nogrowth")inwhichthemediumwasinoculatedwithnonviablebacteria;nogrowthwasobservedoverthecourseoftheexperiment.SensorsAsindicatedintheaccompanyingtable,eachmanufacturerusesitsownproprietarysensortechnology."Thetypeofsensorsandhowthesensorsaremadeandusedintheinstrumentiswhatreallydifferentiatestheinstrumentsonthemarkettoday,"saysKenWeber,technicalsalesrepresentativeforNeotronics."Oursensorsareelectrochemicallygrownconductingpolymersmadewithpolypyrroleresins,althoughpolyanilineresinsarealsocommonlyused."AccordingtoWeber,engineerselectrochemicallygrowtheresinsatadefinedpotentialacrossanelectrodegap.Bychangingthesolventsystemorthecounterionassociatedwiththeirmanufacture,itispossibletomodifythepolymerchainandgetsubtledifferencesinthereactivityofeachsensor.WhileNeotronicselectrochemicallygrowseachindividualsensor,AromaScanusesan"inking"or"masking"processwherebyitputs32sensorsononecomputerchiporboard,saysWeber.Neotronicsusesanarrayof12differentsensorswhicharebasicallysolderedontoasensorhead.AlphaM.O.S.,anotherinstrumentmanufacturer,usesprimarilytinoxide-basedsensorsinitsarray,butitalsocanconfiguretheinstrumentwithalimitednumberofconductingpolymersensors.Metaloxidesensorsdemonstrategoodsensitivitytoorganicvapors(ppmorevenppbdetectionlimits)foraverybroadrangeofchemicalcompounds.Duetotheirpoorselectivity--i.e.,allsensorscanrespondtoasinglevolatilecompoundbutindifferentmagnitudes--sensorarraysmustbeemployed.Metaloxidesensorsaremadebydepositingathinlayer(505microns)ofoxidefilmonaceramicmaterial.Forproperfunctioning,metaloxidesensorsareusuallyheatedtobetween175°and425°C.Theelectricalresistanceofthesensordecreasesinthepresenceofanodor,withthemagnitudeoftheresponsedependentonthenatureofthedetectedmoleculeandthetypeofmetaloxideusedinpreparingthesensor.Responsetimeofmetaloxidesensorsisbetween10and120seconds.AromaScanandNeotronicsinstrumentsusesensorsmadeofconductingpolymerresinsinsteadofmetaloxides.MicheleGreaney,scienceandtechnologymanagerforAromaScan,explains:"Theyhaveaninherentchargeorbaseresistance,andasvolatilecomponentsareabsorbedontothesurfaceoftheconductivepolymers,theychangetheirbaseresistance.YougetwhatwecallanFRvalue,andthisFRisuniqueacrossthesensorarray."ByplottingtheFRvalueforeachsensor,ahistogramorbarchartcanbegenerated.Thechartcouldforinstance,representagood-tastingcoffeesample.GraphicoutputsSensor-arrayresponsestoorganicvaporscanbepresentedinnumerouswaysusingavarietyofstatisticalandpattern-recognitionapproaches.Oneformofoutputisascaledpolarplot.Thiswayofvisuallydisplayingdataissimpletointerpret.Eachvectoronthepolarplotrepresentstheoutputfromonesensor.Astherelativeresponseofeachsensorchangeswhenthesensorarrayisexposedtovaporsfromdifferingfoodsamples,theoverallshapeandappearanceofthepolarplotvary.Anothertypeofdatapresentationcommonlyusedisthe