MaterialsforChemical Sensing
Editors
ThiagoRegisLongoCesarPaixão DepartmentofFundamentalChemistry
InstituteofChemistry
UniversityofSãoPaulo
SãoPaulo,SP
Brazil
SubrayalMedapatiReddy ChemistryDivision,SchoolofPhysical SciencesandComputing UniversityofCentralLancashire Preston,Lancashire UK
ISBN978-3-319-47833-3ISBN978-3-319-47835-7(eBook) DOI10.1007/978-3-319-47835-7
LibraryofCongressControlNumber:2016954604
© SpringerInternationalPublishingAG2017
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ThiagoRegisLongoCesarPaixãodedicates thisbooktohiswife,JulianaNaozukaand son,ThiagoAkioNaozukadaPaixão who haveinspired,encouragedandhelpedhimin everythinghehasdone.
SubrayalMedapatiReddydedicatesthisbook tohiswifeandchildrenfortheirfriendship, supportandunderstanding.
Preface
Overtheyears,thenumberofacademicpublicationsonthedevelopmentof chemicalsensorshasincreaseddramaticallyfrom76articlesin1983to7461 articlesin2015(accordingtoaScopusWebsearchusingthephrase “chemical sensor”).Additionally,thecommercialmarketisincreasingitssearchfornew chemicalsensorsinordertomonitoraplethoraofanalytesinrealtimeandinan efficientandcost-effectiveway.Areasofapplicationincludeenvironmentalmonitoring,foodscienceandsafety,andwearabletechnologiesforpoint-of-care devicestrackingmedicalstatus.Thereisanongoingworkinthedevelopmentand applicationofcheap,biodegradablematerialstomonitordiseasesinareaswithpoor infrastructure.Thedevelopmentofnewchemicalsensorsandtheimprovementof existingoneshavebecomeacollaborativeendeavor,integratingmultipledisciplines,suchaschemistry,physics,engineering,biology,materialsscience,mathematics,andbioinformatics.Thisbookstartswithadefinitionofthechemical sensorinChap. 1,followedbyhowthischemicalsensorcouldextractchemical informationusingopticalandelectrochemicaltechniquesinChap. 2.
Thecombinationofchemicalsensorwithlow-costmaterials,morespecifi cally paper-baseddevices,hasprogressedinthelast10yearsandhasbeenstimulating recentresearchactivitiesinthedevelopmentofpoint-of-caredevicetobeusedin developingcountries.Chapter 3 isdedicatedtothistypeofdevicewiththe exploitationofbiomaterials,suchasenzymesandantibodies,torecognizethe chemicalinformation.Thekeypointforthedevelopmentofachemicalsensoris relatedtothematerialsusedtorecognizeandtranslatethechemicalinformation throughachemicalinteractionorreaction.However,thereisnotaquantitative theoryormodeltobeusedwhichdescribesphysicalandchemicalparametersto obtainimprovedmaterialsthatwillrecognizethespeciesbeinganalyzed,andthe stepstoobtainabettermaterialarelargelyempiricalandcouldbesometimesseen morelikeanartthanascience.However,theuseofenzymestotranslatethe informationiscommon,andresearchersareattemptingtomimictheenzymeand antibodyenvironmentsinordertocreateartifi cialreceptorsandotherbiomimetic compounds,suchasmolecularlyimprintedpolymers.Inthisapproach,theyare
tryingtocopynatureinordertoachievethesamelevelofrecognitionandaffi nity, andChap. 4 willdiscussthistopic.
Chapters 4–8 willshowhowdifferentnaturalandsyntheticmaterialsinfluence thedevelopmentofchemicalsensors.Chapter 5 willdiscusshowclassesofcarbon, suchasgrapheneanditsderivatives,offerdifferentnanofeatures,structures,and dimensions,whichprovidetransductionrecognitionwithpotentiallynovelsensing properties.Chapter 6 willshowhownanomaterialscouldbeturnedintoapersonalizedmonitoringplatformintheformofwearableandimplantablesensor networksystems,whichwouldallowpeople’sactivitiestobemonitored.Chapters 7 and 8 willdemonstratehowothersyntheticornaturalmaterials,suchas self-assembled filmsandphthalocyanines,canbeusefulforenhancingsensing performance.Chapter 9 willdemonstratehowarray-basedchemicalsensors, combinedwithchemometricdataprocessingtools,canbeusedtomimicthehuman tongue,oneofourmoresophisticatedsensors.
Wewouldliketothankallthecontributingauthorsfortheirenthusiasmand participationinthepreparationofthisbook.Wealsowouldliketoexpressour gratitudetothestaffofSpringer,inparticular,AniaLevinsonandBrianHalm,for theirassistanceinbringingthisbooktoprintandpublication.
SãoPaulo,BrazilThiagoRegisLongoCesarPaixão Lancashire,UKSubrayalMedapatiReddy
1IntroductionofMaterialsUsedinChemicalSensors 1 WilliamReisdeAraujo,SubrayalMedapatiReddy andThiagoRegisLongoCesarPaixão
2InformationExtractionTechniquesinChemicalSensing 7 ThiagoMatheusGuimar ãesSelva,TiagoLuizFerreira andThiagoRegisLongoCesarPaixão
3(Bio)ChemicalSensorsBasedonPaper ....................... 29 NipapanRuecha,KentaroYamada,KojiSuzukiandDanielCitterio
4MembraneTechnologiesforSensingandBiosensing ............
SubrayalMedapatiReddy
5InterfacingGrapheneforElectrochemicalBiosensing 105 OnurParlak
6NanomaterialsasImplantableSensors 123 RogerJagdishNarayanandNishantVerma
7Self-assemblyThinFilmsforSensing
CelinaMassumiMiyazaki,AnerisedeBarros, DanielaBrancoTavaresMascagni,JulianaSantosGraça, PaulaPereiraCamposandMarystelaFerreira
8PhthalocyaninesasSensitiveMaterialsforChemicalSensors ..... 165 DebdyutiMukherjee,RevanasiddappaManjunatha, SrinivasanSampathandAsimKumarRay
9MaterialsforElectronicTongues:SmartSensorCombining
ManeldelValle
IntroductionofMaterialsUsed inChemicalSensors
WilliamReisdeAraujo,SubrayalMedapatiReddy andThiagoRegisLongoCesarPaixão
1.1FromSensorstoChemicalSensing
Sincetheadventofsmartphonetechnologies,theword “sensor” hasbecomemore andmorecommonplaceoutsideoftheacademicenvironment.Nowadays,itiseasy to findsmartphoneswithavarietyofsensors,forexample,proximity,motion, ambientlight,gyroscopic,andmagnetic.Thesesensorsaredevicesthatdetect inputsfromthephysicalenvironment,inordertogenerateanoutputsignalthatcan bereadandunderstoodbyahumanand/orcanbetransmittedelectricallyby someoneoramachine.Asimpleexampleofasensoristhemercury-basedglass thermometerthathasaheatasinputandasconsequenceofthechangeintemperaturetheliquidmercuryexpands,orcontracts,indicatingavalueofthetemperaturemeasuredinacalibratedmarkedgaugethatcanbedetectedbyanatural sensor,thehumaneye.Basically,thephysicaldeviceshighlightedabovetranslate physicalpropertiesintoahuman-readableoutputjustassomehumananaloguescan dothrough,forexample,touch,vision,orhearing.However,naturehasgivenus sensorialsystemsresponsiblenotonlytotranslatephysicalquantitiesasaninterpretationoftheoutsideworld,butalsotheabilitytosensechemicalsthroughtaste andolfactionsystems.Combinatorially,chemicalinformationcanbetranslated togetherwithphysicalinformation,tobetterunderstandtheenvironment[1, 2].
W.R.deAraujo T.R.L.C.Paixão
DepartamentodeQuímicaFundamental,InstitutodeQuímica,UniversidadedeSãoPaulo, AvenidaProf.LineuPrestes,748,05508-000SãoPaulo,SP,Brazil e-mail:trlcp@iq.usp.br
S.M.Reddy(&)
ChemistryDivision,SchoolofPhysicalSciencesandComputing,UniversityofCentral Lancashire,Preston,LancashirePR22HE,UK e-mail:SMReddy@uclan.ac.uk
© SpringerInternationalPublishingAG2017
T.R.L.C.PaixãoandS.M.Reddy(eds.), MaterialsforChemicalSensing, DOI10.1007/978-3-319-47835-7_1
Naturalchemicalsensors,likethemammaliantastesystems,performanevaluationofthecontentoffoodandbeveragesduringallthemealsexperiencesthatwe have,anddependingonhowwelltrainedisthisnaturalsensor,thebetterarethe chancesto qualitatively detecttheingestionoftoxicsubstancespresentinfoodand beveragesoranyadulteration(deliberateorotherwise)ofthefoodtaste.This recognitionofthemammaliantastesystemisduetoreceptorsinourtongue mediatingsweet,bitter,umami,andsourtasteandgeneratingamoleculartools decodedbyourbraintocreatethebasisofthetaste[1]andFig. 1.1 showsasimple schematicrepresentationofhowthemammaliantastesystemworkstodecodethe chemicalinformation.
Thisschematicrepresentationagreeswiththecommondefinitionofchemical sensorsfoundintheliterature,andintroducedbyWolfbeisin1990[3],as “adevice comprisingarecognitionelement,atransductionelement,andasignalprocessor.” Thisdefinitionshowsthatarti ficialchemicalsensorsmimicthesignalprocessing flowreportedinFig. 1.1 andcanbesimpli fiedasFig. 1.2.
Thiscommonsensedefinitionisincompleteasthedecodingsystemorthe computerinformationprocessingstageneedstobebetterdefi nedinordertosimulatehowahumangoesaboutreadingtheinformation,thendecodingtheinformation,andthenlinkingsomeinterpretiveunderstandingtothereadableoutput. Wolfbeis[3]extendedthede finitionofchemicalsensorsas “small-sizeddevices comprisingarecognitionelement,atransductionelement,andasignalprocessor capableofcontinuouslyandreversiblyreportingachemicalconcentration.” New requirementsappearinthisnewdefinition.Oneistheattributeofreversibility, whichmainlybecomesimportantifthesamechemicalsensorisrequiredtomake
Fig.1.1 Simpleschematicrepresentationofhowthechemicalinformationisdecodedbythe mammaliantastesystem,forexample,mammaliantaste
Fig.1.2 Schematicviewofhowtheinformationisdecodedbythearti ficialchemicalsensor system
multipleuninterruptedmeasurementsinaclosed-loopsystem.Thisreversibility stopsnormallyduetoexternalinterferencesthateitherpassivate(orfoul)thesensor orthesensorisusedoutsideitstolerancesresultingindamageand/ordeactivation. Whenthereisnorequirementforthesensortobereversible,itbecomesasingle-use device.Disposablesensordevices,suchasthosebasedonpaper(willbediscussed inChap. 3),arebeingusedmoreandmoretotransducechemicalinformationdue totheraftofdifferenttransducermethodsandrecognitionmaterialsthatcanbe integratedwithpapertoproducedevicestotranslatechemicalinformation[4, 5] Basedonthisdefinition,thesedevicescouldnotbecalledchemicalsensors,aswell asdeviceswithoutarecognitionelement,likedevicestomonitorbloodoxygenationbyreflectometry1 [6].Referringtothelatterasachemicalsensorwascontroversialatthetimeastherewasnotanacceptabledefi nitionforthetechnique undertheumbrellaofanalyticalchemistrytechniques,suchasforexample, spectroscopy.
Additionally,theattribute “reportingachemicalconcentration” inthedefinition isequallyinterestingandthereadableoutputismoreapparent.Inthiscase,the functionoftheoutputdevice(e.g.,computer,smartphone,ortablet)istoreadthe electricalinformationandthentoprocesstheinformationtoreporttheactual concentrationoftheanalyzedchemicalspeciesbasedonacalibrationcurve,i.e., interpolatingtheunknownsignalofthesampleto findthechemicalconcentration basedonplotoftheelectricalresponseversusconcentrationofthestandards.With thisinmindandreturningtotheparalleldiscussionaboutnaturalandartifi cial chemicalsensors,noneofournaturalchemicalsensingsystemsreturnstheactual concentrationvalueofagivenchemicalspecies,forexample,caffeineandother ingredientswhenwedrinkcoffee,butweknowqualitativelythatitiscoffeeand couldevenalludetothetypeofcoffee.Additionally,wethinkintermsof thresholdsofstrengthandthereforereturnasemiquantitativemeasureofthatrather thananactualvalue.Basedonthis,thecomputer-basedartificialsensorcouldalso beusedtocomparechemical “fingerprints” extractedforthesamples,likea chemicalspectrum,andcomparethisanalyticallyusefulinformationwithadatabaseinordertogiveayesornoanswer,i.e.,qualitativeinformationthrough discriminationofdifferenttypesofcoffeebasedontheextractedinformationora qualitativecompositionanalysis.Suchdeviceswere firstreportedintheliteraturein 1982[7]andcalledelectronicnose,withtheideatouseanarrayofsensorsto extractchemicalinformationinordertodiscriminatequalitativelythesamples.The termelectronictongue(Chap. 9),todiscriminateliquidsamplesinsteadofgas samples(electronicnose),wasintroducedin1996intheliterature[8].
In1991,IUPAC[9]madeamoregeneraldefinitionforchemicalsensorsas “ a devicethattransformschemicalinformation,rangingfromconcentrationsofa speci ficsamplecomponenttototalcompositionanalysis,intoananalyticallyuseful signal.” Thisdefinitionenhancedthepossibilitiesofchemicalsensorsaswellas includingsomedevices,reportedabove,notcomprisedinthepreviousdefinitions reportedhere.Hence,achemicalsensorcouldbedefinedasadevicewhich respondstoananalytebasedonachemicalreaction(orrecognition)andcanbe usedforqualitativeorquantitativedeterminationsofthespeciesbeinganalyzed(to
giveananalyticallyusefulsignal),andthisdefinitioncouldbegeneralizedforan arrayofchemicalsensors(Chap. 9).
OnemaininformationbehindtheIUPACdefinitionisthesignaltransduction. Thechemicaltransductionoccursbymonitoringaphysicochemicalpropertyofthe analytethatisrelatedtoitsconcentration,liketheabsorbanceorpeakcurrentfora reversiblesystemmeasuredbyspectrophotometryorcyclicvoltammetry,respectively,thatisrelatedwiththeBeer–Lambert(Eq. 1.1)andRandles–Sevcik (Eq. 1.2)equations:
where A isabsorbance(dimensionless), e isthemolarabsorptivity(Lmol 1 cm 1), b isthepathlengthofthesample(cm) thatis,thepathlengthofthecuvettein whichthesampleiscontained,and c istheconcentrationoftheanalyteinsolution (molL 1).
where Ip ispeakcurrent(A), n isnumberofelectrons, A isareaoftheelectrode (cm2), D isdiffusioncoefficient(cm2 s 1), v isscanrate(Vs 1),and Co isbulk concentrationofanalyteinsolution(molcm 3).
Mainly,thisbookwillfocusonopticalandelectrochemicalmeasurementsto extractthechemicalinformationbyanalyticaltechniques;suchextractionmethods willbediscussedintheChap. 2
Thekeypointforthedevelopmentofachemicalsensorisrelatedwiththe materialsusedtorecognizeandtranslatethechemicalinformationthrougha chemicalinteractionorreaction.However,thereisnotaquantitativetheorywhich willdescribephysicalandchemicalparameterstoobtainthebettermaterialthatwill recognizethespeciesbeinganalyzed,andthestepstoobtainabettermaterialare largelyempiricalandcouldbesometimescomparedmorelikeanart. Notwithstandingthis,forbiosensors(chemicalsensorsinwhichtherecognition systemisbasedonthebiochemicalorbiologicalmechanism),researchersare attemptingtomimictheenzymeandantibodyenvironmentsinordertocreate artificialenzymesandotherbiomimeticcompounds,suchasmolecularlyimprinted polymers(Chap. 4)orbiomimeticsensors[10].Inthisapproach,wearetryingto copynatureinordertoachievethesamelevelofrecognitionandaffinity.
Chapters 4–8 willdemonstratetheuseofnewandsmartmaterialsastransducing elementsforchemicalsensors,summarizedinFig. 1.3.Analyticalapproachesand strategiestoobtainandextract/convertthechemicalinformationintoareadable signalwillbediscussedindetailinChap. 2.Chapter 3 isdedicatedtoportable devices(paper-basedsensors)withlowcostandeasyoperationmode.Finally, Chapter 9 demonstratestheuseof(electrochemical)sensorarrayscombinedwith chemometricdataprocessingtoolstoimproveoverallsensorperformance.
Partofthedifficultytodefineachemicalsensorandcreatechemicalsensorsis becauseresearchmusthavemultidisciplinarycollaborationsofresearcherswith
Fig.1.3 Schematicrepresentationofchemicalsensorcomponentsfocusedontransducers’ materials
differentcomplementaryexpertisesuchaschemists,electronicandmechanical engineers,materialsscientists,andspecialistsinchemometrics(todecodethelarge amountofinformation,likethebraindoesseeminglyeffortlessly).Hence,theidea ofthisbookistopulltogetheralloftheseprofessionalswithdifferentexpertisein ordertoshowcasethecomplementarityleadingtoadvancesinthedevelopmentof differentmaterialsforchemicalsensing.
References
1.ChandrashekarJ,HoonMA,RybaNJP,ZukerCS(2006)Thereceptorsandcellsfor mammaliantaste.Nature444(7117):288–294.doi:10.1038/nature05401
2.FiresteinS(2001)Howtheolfactorysystemmakessenseofscents.Nature413(6852):211–218.doi:10.1038/35093026
3.WolfbeisOS(1990)Chemicalsensors?Surveyandtrends.Fresenius’ JAnalChem337 (5):522–527.doi:10.1007/BF00322857
4.CateDM,AdkinsJA,MettakoonpitakJ,HenryCS(2015)Recentdevelopmentsin paper-basedmicrofluidicdevices.AnalChem87(1):19–41.doi:10.1021/ac503968p
5.NeryEW,KubotaLT(2013)Sensingapproachesonpaper-baseddevices:areview.Anal BioanalChem405(24):7573–7595.doi:10.1007/s00216-013-6911-4
6.VuceaV,BernardPJ,SauvageauP,DiaconuV(2011)Bloodoxygenationmeasurementsby multichannelreflectometryonthevenousandarterialstructuresoftheretina.ApplOpt50 (26):5185.doi:10.1364/AO.50.005185
7.PersaudK,DoddG(1982)Analysisofdiscriminationmechanismsinthemammalian olfactorysystemusingamodelnose.Nature299(5881):352–355.doi:10.1038/299352a0
8.HayashiK,YamanakaM,TokoK.YamafujiK(1990)Multichanneltastesensorusinglipid membranes.SensActuatB:Chem2(3):205–213.doi:10.1016/0925-4005(90)85006-K
9.HulanickiA,GlabS,IngmanF(1991)Chemicalsensors:definitionsandclassification.Pure ApplChem63(9).doi:10.1351/pac199163091247
10.LeeJH,JinH-E,DesaiMS,RenS,KimS,LeeS-W(2015)Biomimeticsensordesign. Nanoscale7(44):18379–18391.doi:10.1039/c5nr05226b
Chapter2 InformationExtractionTechniques inChemicalSensing
ThiagoMatheusGuimarãesSelva,TiagoLuizFerreira andThiagoRegisLongoCesarPaixão
Inordertoproduceananalyticallyusefulsignal,asmentionedintheprevious chapter,weneedtotransducechemicalinformationusinganinstrumentaltechnique.Numerousanalyticalchemistrytechniquesexistfortheextractionof chemicalinformation,e.g.spectrometry,separationtechniquescoupledwith spectroscopicdetection,orelectrochemicalandothermethods.However,mainly duetothecostandnecessityofportability,electrochemicalandcolorimetric techniquesarefrequentlyusedtotranslatechemicalinformationintoareadable outputfortheanalystsandusersinin-fi eldapplicationsofchemicalsensors.This chapterwillintroducetheconceptsinvolvedinthesetechniques,whicharemainly usedtoextractinformationforfabricatingchemicalsensors.
2.1ElectrochemicalSensors
Essentially,theelectrochemicalsensorsareclassi fiedas:conductimetric, potentiometric,andamperometricorvoltammetricsensors.
T.M.G.Selva InstitutoFederaldeEducação,CiênciaeTecnologiadePernambuco, AvenidaProf.LuizFreire,500,50740-540Recife,PE,Brazil
T.M.G.Selva T.R.L.C.Paixão(&) DepartamentodeQuímicaFundamental,InstitutodeQuímica, UniversidadedeSãoPaulo,AvenidaProf.LineuPrestes,748, 05508-000SãoPaulo,SP,Brazil e-mail:trlcp@iq.usp.br
T.L.Ferreira(&) InstitutodeCiênciasAmbientais,QuímicaseFarmacêuticas, UniversidadeFederaldeSãoPaulo,RuaProf.ArturRiedel,275, 09972-270Diadema,SP,Brazil
e-mail:tlferreira@unifesp.br
© SpringerInternationalPublishingAG2017
T.R.L.C.PaixãoandS.M.Reddy(eds.), MaterialsforChemicalSensing, DOI10.1007/978-3-319-47835-7_2
2.1.1ConductimetricSensors
Conductimetricsensorsarebasedonmeasuringtheionicconductanceofsolutions. Thisconductanceresultsfromtheindividualcontributionsofeachioninthe solution;itisthereforeapropertythatdoesnotdependonthespeci ficreaction levelsofanelectrode,asopposedto,forexample,voltammetricsensors.
Conductimetricsensorscanbeemployedindirectconductometry,whereelectrolyteconcentrationisdeterminedbyasingleconductancemeasurement,orrelativeconductometry,whereconductanceismonitoredduringtitration,andtheend pointisdeterminedfromthecollecteddata.
Sincethesesensorsmeasureelectricalconductancearisingfromallionicspecies presentinsolution,theydonotrespondtospeci ficions.Thisspeci ficitymaybe achievedbyaseparationtechnique,suchaschromatographyorcapillaryelectrophoresis,employingconductimetricsensorsforthedetectionofchargedspecies ofinterest.
TheconductanceofasolutionorasolidmaterialcanbeexpressedbyOhm´ s law,Eq. 2.1:
where I istheelectriccurrent flowingthroughasolutionorasolid, E isthepotential difference,and G istheconductance.Generally,Ohm´slawisexpressedintermsof resistance,Eq. 2.2:
¼ R I
whichleadstothedefinitionof G astheinverseofresistance,Eq. 2.3:
Theresistanceorconductanceofaspecimendependsonitstemperature, chemicalnature,homogeneity,size,andshape.Forsolutions,theconductancealso dependsonthenumberofionspresent.
Foraspecimenuniformoveritswholelength:
where A isthecross-sectionalareaofthespecimen,and l isitslength.Theproportionalityconstant j iscalledconductivity(Fig. 2.1).Thesamerelationisvalid forasolutionbetweentwoelectrodes(Fig. 2.1).
Experimentally,eithertheresistanceortheconductanceofasolutionismeasuredtodetermine j.Thebasicexperimentallymeasuredparameterinbothcasesis thesolutionresistance,butmodernconductancebridgesarecalibratedtodirectly
provideconductanceread-outs.Theexperimentalset-upisbasedonaWheatstone bridgeapparatus.
Whenstudyingtheconductivityofasolution,itisessentialthattheconcentrationremainsconstantthroughoutthemeasurement.Passageofanelectriccurrent throughasolutioninduceschemicalreactionsattheelectrodes,resultinginchanges inthesolutionconcentration.Thesereactionsortheireffectsmustbeavoidedin conductivitymeasurements.Thisisachievedbychangingthedirectionofthe current,sothatthereactionsarecontinuallyreversedandhavenonetchemical effect.Alternatingcurrentisgenerallyused,e.g.onehavingafrequencyof 1000Hz.
Thecurrentpassingthroughasolutioniscreatedbythemovementofions containedtherein.Theamountofionspresentislikelytobeinverselyproportional totheresistanceofthesolutionanddirectlyproportionaltoitsconductivity, j.Experimentsshowthat j variesconsiderablywithconcentration.
Fig.2.1 Schematicrepresentationofsolidandelectrolyticconductors
Furtherdiscussionandprocedurescanbefoundintextbooksonconductimetric titrations,determinationofdissociationconstantsofweakelectrolytes,etc.[1, 2].
Thisnotwithstanding,therearetwoimportantcasesofinterestingapplicationsof conductimetricsensors:(i)measurementofsolutionconductanceemployingelectrodesoutsidethesolution(oscillometry)and(ii)measurementofconductanceof anarrayofmodi fiedsensorsasanelectronicnose.
Oscillometryisoftenemployedforconductancemeasurementsincorrosive solutions,e.g.onesthatcoulddamagethecellelectrodes.Inordertomeasurethe conductanceofasolutionusingthisprocedure,theelectrodesarepositionedoutside thesolution,ontheexternalwalloftheconductancecell.Toperformthesemeasurements,equipmentcapableofoperatingathighfrequenciesisrequired(ca. 106 Hz).Aninterestingapplicationofoscillometryistheuseofcontactlessconductimetriccellsincapillaryelectrophoresistodetectdifferentchargedspeciesastheyare separatedbyelectroosmotic flow.Inthesecases,theconductimetriccellispositioned onanappropriateportionoftheexternalwallofthecapillary[3].
Thesecondcasedealswithsmellidenti ficationusinganarrayofinterdigitated electrodesmodifi edwithaconductingpolymer,whereeachelectrodeismodi fied withadifferentpolymer.Whengaseousmoleculesareabsorbedbythepolymer,its electricalconductivityisaffected.Differentgasesaffecttheconductivityindifferent ways.Thesesignalvariationscanprovidea “digitalimpression” or “chemical fingerprint” ofthestudiedvapour[4].Theelectronicnose(E-nose)mustbe “trained” torecognizedifferentsmellsthroughelectricalconductivityusing chemometricapproachesdescribedinChap. 9
2.1.2PotentiometricSensors
Potentiometricsensorsworkbymeasuringtheequilibriumpotential(potentialof zerocurrent)ofthesensorversusareferenceelectrode.Thesepotentialsarea functionoftheactivityofthespeciesinsolution.Theequalityofactivityand concentrationisreasonabletoassumeonlyfordilutesolutions[5].
Typicalinstrumentationforpotentiometricmeasurementsincludesareference electrodeandanindicatorelectrode(potentiometricsensor)connectedtoahigh outputimpedancevoltmeter(1012 X)[6].
Therearetwoclassesofpotentiometricsensors:(i)metallicelectrodes,i.e. electrodesthatdevelopapotentialdeterminedbyredoxequilibria(Nernstequation) attheelectrode–solutioninterface(e.g.platinumelectrode)and(ii)ion-selective electrodes,wherethedifferenceofpotentialsacrossamembraneismeasured, whichisinfluencedbytheactivityofthespeciesoneithersideofthemembrane.
Theuseofmetallicelectrodescommonlybringspoorselectivityifmorethanone redoxcoupleispresentinsolution,sinceallcouplescontributetotheoverall equilibriumpotential.Ontheotherhand,thepotentialgeneratedforion-selective electrodes(ISEs)isduetoaselectiveinteractionbetweentheelectrodemembrane andanion.
Fig.2.2 Schematic representationofamembrane electrode. Arrows symbolize theexchangeofionsacross themembranebetweenthe internalandexternalsolutions
ISEsmeasurethepotentialdifferencecreatedbythemovementofionsbetween aninternalandanexternalsolutionphase,delimitedbythemembrane(Fig. 2.2).
Themembranepotential, Emembrane,isgivenbyEq. 2.5:
where R istheuniversalgasconstant, T isthetemperatureinKelvin, F isthe Faradayconstant,and a istheactivityofanion i ofcharge zi.Astheactivityofthe ion i intheinternalsolutionisconstant:
where c isaconstant.
Themembranepotentialismeasuredbycalculatingthepotentialdifference betweenaninternalreferenceelectrodeandanexternalreferenceelectrode.Thus, themembraneservesasalinkbetweentwohalvesofaconcentrationcell.
AperfectISErespondstoonlyoneioninasolutioncontaining “any” ions.This idealsituationcannotbeachieved,particularlywhenionswithsimilarproperties arepresentinsolution.Theinterferenceeffectsofotherionsdependontheir potentiometricselectivitycoeffi cients, Kij,accordingtotheNicolsky–Eisenman Eq. 2.7:
where j istheinterferingspecieswithcharge zj.
Theseselectivitycoeffi cientscanbeevaluatedbya fixedinterferencemethod (varyingtheprimaryionactivityataconstantlevelofinterferent)oraseparated method(comparingtheresponseoftheelectrodeinprimaryionsolutionwiththat inasolutioncontainingonlytheinterferentionwiththesameactivity)[5].
Selectiveelectrodesaredividedintothreeclasses:
(i)primaryion-selectiveelectrodes;
(ii)compoundormultiple-membraneion-selectiveelectrodes;
(iii)all-solid-stateion-selectiveelectrodes.
2.1.2.1GlassElectrodes
Glasselectrodeswerethe firstISEstobedevelopedandareusedmainlytomeasure pH.GlassisanamorphoussolidconsistingpredominantlyofsilicatesandispermeabletoH+,Na+,andK+.Thecompositionofglassdeterminesthepermeability toeachtypeofion,butsomeinterferencealwaysoccurs.
Theglassmembranemustbeconductivetoserveasapotentiometricsensor. ConductionwithinthehydratedgellayerinvolvesthemovementofH+.Sodium ionsarethechargecarriersinthedryinteriorofthemembrane.Thissensor functionsbyexchangeofsolutionprotonswithsodiumionsinthesurfaceregion,to adepthofca.50nm.
So,forlowprotonandhighsodiumconcentrationsinsolution,thisexchangeis notcompleteandtheobservedpotentialishigher(pHislower)thanexpected, accordingtoEq. 2.9 (Eisenmanequation)fortheinterferenceofNa+
Instronglyacidicoralkalinesolutions,theactivitycoefficientsofH+ andNa+
cansignificantlydependontheenvironment,possiblyleadingtoadeviating potential.ThesedeviationsathighactivitiesoccurinallISEs.
InpHmeasurement,thepotentialdifferencebetweentworeferenceelectrodeson bothsidesoftheglassmembraneismonitored.Thetwoelectrodesareoften combinedwiththeglassmembrane(Fig. 2.3).
Itisveryimportanttocalibratetheglasselectrodepriortomeasurementsdueto thedifferencesbetweenitsinnerandoutersurfaces,whichleadtodifferencesinthe monitoredpotential.Thispotentialcontribution(oftencalledasymmetrypotential)
canalsochangewithtimewhentheelectrodeisused,makingperiodiccalibration necessary[1, 6, 7].
2.1.2.2CrystallineMembraneElectrodes
Thesepotentiometricsensorsarebasedonasolid-statecrystallinemembrane.The homogeneousmembraneisanionicsolidwithalowsolubilityproduct,andthe sensedioncorrespondstothecationicoranionicconstituentoftheabovemembrane.Thepotentialiscreatedbyionexchangebetweenthesolutionandthesurface oftheioniccrystal.Migrationofcrystalstructuredefectsaccountsforthecharge transportthroughthemembrane.
Asthesesensorsrespondtoboththecationandanionofthesolidmembrane,it isexpectedthattheywouldalsobethemaininterferingspecies.Theelectrodeis alsosensitivetoionsthatcanbindthemembranecomponents,especiallyifthe bindingproductshavelowersolubilitythanthemembranematerial.
Othercrystallinemembranes,calledheterogeneousmembranes,arebasedonan inertplasticmatrix(e.g.PVC,siliconerubber,orconductingepoxyresin)with incorporatedsmallcrystalsoftheionicsolid.
Generally,thiskindofpotentiometricsensordoesnotuseaninternalreference electrode,butanohmiccontact[1].
Fig.2.3 CombinedglasselectrodeforpHsensing
2.1.2.3Non-crystallineMembraneElectrodes
Non-crystallinemembraneelectrodesarebasedonapolymer-supportedmembrane containingsolventandanionexchangerorneutralcarrier(commonlyachelating agent)selectiveforthespeciestobedetermined.Transportacrossthemembraneis achievedbyexchangeofthespeciesofinterestbetweenadjacentchelatingagents [1].
2.1.2.4Gas-SensingElectrodes
ThesepotentiometricsensorsaresimpleISEswithasecondgas-permeablemembrane,whichallowscertainmoleculestopass.Usually,asmallamountofelectrolytesolutionisplacedbetweentheselectivemembraneandtheoutermembrane. TheselectivemembraneiscommonlyapHglassmembrane,andthevariationof pHisrelatedtothepartialpressureofthegas[1].
2.1.2.5PotentiometricEnzymeElectrodes
Thesesensorsalsohaveasecondmembrane,whichcontainsanimmobilized enzyme.Sinceenzymesarehighlyspecificcatalysts,oneoftheproductsofan enzymaticreactioncanbemonitoredandtheanalyteindirectlydetermined[1].
2.1.2.6Ion-SelectiveField-EffectTransistors
Inordertominiaturizepotentiometricsensorsachievingreproduciblesignalswitha highsignal/noiseratio,ion-selective fi eld-effecttransistors(ISFETs)[8]were developedusingsemiconductortransistortechnology(Fig. 2.4).Thefunctionofa conventional fi eld-effecttransistoristorespondtotinyvoltagedifferencesofa metallicgatebetweenthesourceanddrain,convertingthemintoalow-impedance outputsignal(currentsignal).
InISFETs,themetallicgateisreplacedbyanion-selectivemembrane,whichis incontactwithsolution.Thedrainsignal(output)isdirectlyrelatedtotheactivity ofionsinsolution[8].
2.1.3VoltammetricSensors
Voltammetricsensorsarebasedonmeasuringtherelationshipbetweenthecurrent andtheappliedpotential.Therearetwomainapproachestocarryoutvoltammetric experiments:(i)measurethecurrentresponseasafunctionofappliedpotentialand (ii)monitorthepotentialresponseasafunctionofappliedcurrent.Most
Fig.2.4 ISFETforpHsensing.ReprintedfromJimenez-Jorqueraetal.[8].Copyright(2010), withthepermissionfromMDPI®
voltammetricsensorsarebasedonpotentialcontrol.Amperometricsensorsarea specialkindofvoltammetricsensors,wheredeterminationofelectroactivespecies isperformedatconstantpotential[5, 6, 9, 10].
Theinstrumentationforvoltammetricsensorsismorecomplexthanthatfor conductimetricandpotentiometricsensors.Threeelectrodesarenecessarytoavoid currentpassagethroughthereferenceelectrode,whichwouldchangeitspotential. Thecurrentpassesthroughanelectricalcircuitbetweentheworkingelectrodeand anauxiliaryelectrode,withthereferenceelectrodeusedtocontrolthepotentialof theworkingelectrode.Apotentiostatisnecessarytocontroltheappliedpotential andregisterthecurrentattheworkingelectrode.Togaininformationon current-controlledexperimentsandmonitorchangesinthepotentialoftheworking electrode,agalvanostatisrequired.
Thecurrentofanalyticalinterestinvoltammetryisthefaradaiccurrent,whichis generatedbyoxidationorreductionoftheanalyteatthesurfaceoftheworking electrode.Anothercurrent,calledacapacitivecurrent,interfereswitheachmeasurement.Forexample,whenthepotentiostatforcestheelectrontransferfora reductionprocesstooccurontheworkingelectrode,bringingthepotentialtomore negative(orlesspositive)values,thecationsinthesolutionareattractedtothe electrodesurface,whereasanionsarerepelled.This fluxofionsandelectrons,i.e. thecapacitivecurrent,isnotacontributionfromtheredoxreactionandmustbe minimizedinordertoachievelowervoltammetricdetectionlimits.
Involtammetry,thepotentialexcitationsignalcanbeimposedonaworking electrodeindifferentwaveforms,witheachpotentialwaveformelicitingacharacteristiccurrentresponse(Fig. 2.5).
Aclassicalvoltammetryexcitationsignalisalinearpotentialscan,wheretheDC potentialappliedtotheelectrochemicalcellvarieslinearlyasafunctionoftime.
Fig.2.5 Potentialexcitationwaveformsfor a linearsweepvoltammetryand b pulsevoltammetry c showsthebehaviouroffaradaicandcapacitivecurrentsasafunctionoftimeduringapotential pulse
Thecurrentthat flowsinthecellisrecordedasafunctionoftimeandthusasa functionoftheappliedpotential,resultinginavoltammogram.Amongthe parametersthatneedtobespeci fiedtorecordavoltammogram,thepotentialsweep rateiscrucial.Thisparametercontrolstheslopeofthepotentialvariationasa functionoftime.
Atypicalresponsetoalinearpotentialsweepisapeak-shapedvoltammogram. Thecurrentstartstorisewhenthepotentialvaluesmatchthoseofanelectrode process.Thiscreatesaconcentrationgradientofelectroactivespeciesbetweenthe electrodesurfaceandbulksolution,withthelackofelectroactivespeciesonthe electrodesurfacemakingthecurrentfall.
2.1.3.1CyclicVoltammetry
Thepotentialcanalsobecycledmultipletimesbetweentwovalues,e.g. fi rstbeing increasedlinearlyandthenloweredatthesamerate(Fig. 2.6).
Fig.2.6 Cyclicvoltammetry potentialexcitationsignal
Fig.2.7 Atypical peak-shapedcyclic voltammogram.The parameters t0, t1,and t2 are showninFig. 2.6
Incyclicvoltammetryofreversiblesystems(i.e.oneswithfastelectrodekinetics relativetothepotentialsweeptimescale),theproductofinitialoxidationor reductioncanberegeneratedbyreversingthescandirection(Fig. 2.7).Thefollowingequationrelatesthepeakcurrentwithotherparametersoflinearsweep voltammetry:
where Ip isthecurrentpeakin A, n isthenumberofelectronstransferredinthe electrodeprocess, A istheelectrodeactiveareaincm2, D isthediffusioncoeffi cient ofelectroactivespeciesincm2 s 1 , C istheconcentrationofelectroactivespeciesin molcm 3,and v isthepotentialscanrateinVs 1
Forreversiblesystems,(i)theratioofoxidationandreductioncurrentpeak values(anodicandcathodiccurrentpeaks, IPA and IPC,respectively)isclosetoone and(ii)theseparationbetweenthecathodicandanodicpotentialpeaks(EPC and EPA,respectively)isequalto59.0/n mV,orequivalently:
Forcompletelyirreversiblesystems,onlytheoxidationorreductionprocessis detected,withnopeakinthereversedsweep.Mostoftheredoxcouplesare positionedbetweenthecompletelyreversibleandirreversiblesystems(called quasi-reversiblesystems).Inthesecases,thereversepeakappears,butissmaller thantheforwardpeak[5, 6, 10].
2.1.3.2HydrodynamicVoltammetry
Hydrodynamicelectrodescanbeemployedasvoltammetricsensors,subjectto controlledconvectionimposedbysolutionorelectrodemovement.Convection enhancesthemasstransportofelectroactivespeciestotheelectrodesurface,sothat thediffusionlayer,withaconcentrationgradientpresenttherein,isthinnerthanin theabsenceofconvection.Consequently,thecurrentresponseisenhanced. Hydrodynamicelectrodesareimportantvoltammetricsensorsthatoperateunder steady-stateconditions.Foranalyticalpurposes,thesensorswithhighestsensitivity arethosewhereapotentialcorrespondingtothelimitingcurrentregionisapplied.If therateofconvectivetransportisconstant,togetherwithalltheothercontrol parameters,thecurrentresponseoftheelectrodeisalsoconstant.Theseelectrodes areusuallyoperatedunderlaminar flowconditions(intheabsenceofturbulence). Thebest-knownhydrodynamicelectrodeistherotatingdiscelectrode.Thelimiting currentforthiselectrodeisgivenby:
where r istheradiusoftheelectrodeincm, F istheFaradayconstant, C isthe concentrationofelectroactivespeciesinmolcm 3 , D isthediffusioncoeffi cientin cm 2 s 1 , t isthekinematicviscosity,and x isthespeedofrotationoftheelectrode inHz[6, 10].
Insomesituations,electrodesareusedin flowsystems.Therearemanyelectrochemical flowcelldetectorsbasedonwall-jetorchannel-tubeelectrodes. Generally,thesecellsaredesignedforchromatography,capillaryelectrophoresis, flowinjectionanalysis,orbatchinjectionanalysis.
2.1.3.3MicroelectrodeVoltammetry
Microelectrodesareelectrodeswithatleastonedimensioninthemicrometrerange. Thisminutedimensionleadstolowcapacitivecurrentcontributionsandthe possibilityofregisteringsteady-statecurrentsinashorttime(Fig. 2.8). Microelectrodeshavemanyadvantagescomparedtoconventionalelectrodes:(i)insertionofmicroelectrodesinplaceswhereotherelectrodesaretoolarge;(ii)high 18T.M.G.Selvaetal.
Fig.2.8 Atypical microelectrodecyclic voltammogram
signal/noiseratio;(iii)possibilityofregisteringvoltammogramsinhighlyresistive mediawithouttheadditionofaninertelectrolyte;and(iv)relativeinsensitivityto forcedconvectionofthesolution.Foramicrodiscelectrode,thesteady-statecurrent inthelimitingcurrentregionisgivenbythefollowingequation[10]:
2.1.3.4PulsedVoltammetricTechniques
Pulsetechniquesarebasedonthecurrentresponsetoasequenceofpotentialsteps intheforwardand/orreversedirections.Thisresponseisapulseofcurrentthat decreaseswithtimeastheelectroactivespeciesisconsumedintheregionnearthe electrodesurface.
Theregisteredcurrenthasacontributionfrombothfaradaicandcapacitive processes.Thecapacitivecurrentdecreasesfasterthanthefaradaiccurrent.Thus, thecurrentisusuallysampledafterthecapacitivecontributionbecomesverylow. Pulsewidthsareadjustedtoachievethiscondition(Fig. 2.5).
Themostfrequentlyusedpulsetechniquesaredifferentialpulsevoltammetry andsquarewavevoltammetry.Conceptually,thetwotechniquesareverysimilar. Thedetectionlimitsareoftheorderof10 7 molL 1 fordifferentialpulse voltammetryand10 8 molL 1 forsquarewavevoltammetry[5, 10].
2.1.3.5MembraneandModifi edElectrodes
Therearemanysituationswhencontrollingthepotentialisnotsufficienttogain selectivityinvoltammetricexperiments.Responseoverlapcanoccurduetothe proximityofelectroactivespeciespotentialsorelectrodekineticprocesses.Insome cases,thecurrentresponsedecreaseswithtimeduetoblockingoftheelectrode surfacebystronglyadsorbedspecies.Theseproblemscanbecircumventedby usingmodi fiedelectrodesasvoltammetricsensors.Usually,themodi ficationof electrodesbringsselectivityby:(i)creationofphysicalbarriers/membranesthat blockinterferingspeciesor(ii)depositionofmaterialthatreactswiththeanalyte moreselectivelyoractsasamediatorforelectrontransfer.
Porousmembranescanbeusedinvoltammetricsensors,coveringtheelectrode surfacedirectlyorhavingathinlayerofseparatingelectrolyte.Thesemembranes canactassizeexclusionseparators(blockinglargerspecieslikeproteins)orasa gas-permeablemembrane(asintheClarkoxygenelectrode).
Theuseofenzymesimmobilizedontheelectrodesurfacedirectlyorwithina membranecoveringtheelectrodealsoallowstoachievehighspeci ficity.These biosensorscombineelectrochemicalsignaltransductionwithabiologicalsensing component.
Inmodi fiedelectrodes,changesarepromotedinthesurfacelayersoftheelectrode.Alternatively,anewlayerattheelectrodesurfaceisformedtogainselectivity.Thegeneralintentionistoenhanceorfacilitatesomeelectrodeprocesses whileinhibitingotherones.Therearemanystrategiesforvoltammetricsensor modifi cation,includingadsorption,chemicalmodi fication,electrodeposition,and surfacetreatment[11].
2.1.3.6OtherTechniques
Importantinformationabouttheelectroactivespecies,suchasthenumberof transferredelectronsandthediffusioncoefficient,canbegainedusingtechniques suchaschronoamperometry(recordingcurrentasafunctionoftime)and coulometryorchronocoulometry(recordingchargeasafunctionoftime).
InACvoltammetry,asmallamplitudesinewaveissuperimposedonaprogrammedpotentialvariation.Theperturbationofthesystemresultsincurrent responsesthatvaryinamplitudeandphaseangle.Theobtainedvoltammograms canprovideinformationonthekinetics,andtheresponsecanbeusefulforanalyticaldeterminations.Thein-phaseandout-of-phasecurrentcomponentsare relatedtofaradaiccurrentsandtheseparationofchargingcurrents,respectively. Generally,thistechniqueallowstoachievelowdetectionlimits,butprocesseswith slowelectrodekineticsresultinthelossofsensitivity[5, 6, 10].
2.2ColorimetricSensorsandStrategiesforExtracting ColorimetricInformation
Initsearlyyears,chemicalanalysis,eitherqualitativeorquantitative,wasperformedusingreagent-basedcolorimetrictests.Aftertheriseoftheinstrumental methodsofanalysis,quantitativetitrationwaspracticallyabandoned.Ontheother hand,qualitativeand/orsemi-quantitativespottestsarestillpopular[12],withthe useofpHcolour-fixedindicatorsbeingapopularexample.
Themosttypicalinstrumentalwaytoobtaininformationonsolutioncolouristo useaspectrophotometeroraphotometeroperatinginthevisiblespectralrange (400–800nm),basedontransmittance/absorbancemeasurement.Briefly,in absorptionspectroscopy,theradiationintensityfromasourceoflightataspeci fic wavelengthisattenuatedbypassingthroughacolouredsolutioninacuvettethatis situatedbetweenthelightsourceandadetector.Themathematicaldescriptionof thisprocessisknownastheBeer–Lambertlaw(Eqs. 2.14 and 2.15):
where T isthetransmittanceoftheradiationpassingthroughthesolution, I0 isthe totalintensityoftheradiationsource, I istheintensityoftheradiationafterpassing throughthesolution,and A isthesolutionabsorbance.Thechoiceofwavelength (k)tomonitorcolouredspeciesisdeterminedfromthevisibleabsorptionspectra plot(A vs. k),whichmaybeperformedsweepingthewavelengthsinthevisible spectralrange.Often,themaximumabsorptionwavelength(kmax)isused,mainly becauseitprovideshighersensitivity.Absorptionisproportionaltotheconcentrationandlengthofthecuvetteopticalpath,ascanbeinferredfromEq. 2.16. Longerbeampathsoftheincidentradiationincreasetheprobabilityofthecoloured speciesabsorbingapartofthisradiation:
where ɛ isthemolarabsorptionincm 1 mol 1 L, b istheopticalpathincm,and c is theconcentrationofthecolouredspeciesinmolL 1.Figure 2.9 showsasimple schemeoftheinstrumentationused.
Aclassicalapplicationofspectrophotometricmeasurementsistheindirectcolorimetricquantificationofglucoseinbiological fluids.Thismethodisbasedonthe reactionofglucosewithglucoseoxidasetoproducehydrogenperoxide.Thelatter reactswithachromogenicoxygenacceptorinthepresenceofperoxidase,producinga chromogenicspecies,whichisspectrophotometricallymonitoredat460nm[8].
Evenaftertheriseofportablespectrophotometersandthedecreaseoftheirprice, thesearchforalternativewaysofanalyticalcolorimetricmeasurementscontinued.
Fig.2.9a Schemeofthevisibleradiationabsorptionprocess. b Resultantabsorbance(A)versus wavelength(k)plot
Recently,variousresearchgroupshaveuseddesktoporportablescanners[14], digitalcameras,webcams[15],cellphones,andsmartphones[16]tocollectanalyticalinformationbasedoncolourmeasurement.Inthesemethods,thereflection ofthesystemwasused,insteadofthetraditionalwayofmeasuringtransmittance/ absorbance.Thisis,therefore,anadvantage,allowingtheanalysisofturbidsamples [17].
Amongthecoloursystemsused,themostcommonwaytoprocessanalyticaldata fromdigitalimagesisbasedontheRGBcolourmodel,whichisusedincomputer screensandutilizestheprimarycoloursoflight.Thisisanadditivemodel,andthe namecomesfromthethreeprimarycolours:red(R),green(G),andblue(B),also calledchannels.Incomputers,eachofthesethreechannelsisrepresentedbyaninteger numberfrom0to255,andeachcombinationrepresentsaparticularcolour,makingit possibletorepresentmorethan16millioncombinations(2563 =16,777,216).White colour,forexample,isobtainedwhenallthreechannelshavevaluesof255,while blackcorrespondstoallchannelsequaltozero.SomeresearchersusetheHparameter (hue)oftheHSVcolourspace[18, 19],whichcanbecorrelatedwiththeRGBsystem, tomonitoraspecifi ccolouredreaction.TheextractionoftheRGBcodeofadigital imagecanbeperformedbysoftware[20, 21]orbyasmartphoneapp.Theseappsmay behome-made[22]ordownloadedfrompopularvirtualstores[23]oftheoperating system,suchasiOSandAndroid.Inaddition,itispossibletoconverttheRGBcolour modeltoothermodels,suchasgrayscaleandCMYK(cyan,magenta,yellow,and black/key).TheCMYKcolourmodel,usedforcolourprinting,isasubtractivesystem andusessecondarycolourscreatedbymixingtwoprimaryones(RGBmodel).For example,mixingredandbluecoloursgivesmagenta[24].Analternativewayto recordanalyticalcolourinformationwithouttheuseofanysophisticatedinstrumentationwasproposedbyCateetal.[25].Theauthorsexploredthemicrofluidic propertiesof filterpapertoproposeapaper-basedanalyticaldevice(µPAD),shapedas astripandlimitedbyawaxbarrier,whichwasspottedwithreagentsgivingacoloured reaction.Asimplemeasurementofthereactionextentdistance,usingaruler,was correlatedwiththeconcentrationoftheanalyte[25].TheuseofGoogleGlassto performdiagnosticcolorimetrictestshasalsobeenproposed[26]. VariouswaysoftreatingtheRGBdatainadigitalimagearedescribedinthe literature.Awidespreadmethodisto findachannel(R,G,orB)thatcorrelates
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