actionnano6

LaserInducedGraphene:EnRoutetoSmartSensingLibeiHuang1,JianjunSu1,YunSong1,RuquanYe1,2**RuquanYe,ruquanye@cityu.
edu.
hk1DepartmentofChemistry,CityUniversityofHongKong,Kowloon,HongKong,People'sRepublicofChina2StateKeyLabofMarinePollution,CityUniversityofHongKong,Kowloon,HongKong,People'sRepublicofChinaHIGHLIGHTSSummarizingthestrategiesforthesynthesisandengineeringoflaser-inducedgraphene,whichisessentialforthedesignofhigh-performancesensors.
IntroducingLIGsensorsforthedetectionofvariousstimuliwithafocusonthedesignprincipleandworkingmechanism.
DiscussingtheintegrationofLIGsensorswithsignaltransducersandconveyingtheprospectsofsmartingsensingsystemstocome.
ABSTRACTThediscoveryoflaser-inducedgraphene(LIG)frompolymersin2014hasarousedmuchattentioninrecentyears.
Abroadrangeofapplications,includingbatteries,catalysis,sterilization,andseparation,havebeenexplored.
TheadvantagesofLIGtechnologyoverconventionalgraphenesynthesismeth-odsareconspicuous,whichincludedesignablepatterning,environmentalfriend-liness,tunablecompositions,andcontrollablemorphologies.
Inaddition,LIGpossesseshighporosity,greatflexibility,andmechanicalrobustness,andexcel-lentelectricandthermalconductivity.
Thepatternableandprintablemanufactur-ingprocessandtheadvantageouspropertiesofLIGilluminateanewpathwayfordevelopingminiaturizedgraphenedevices.
Itsuseinsensingapplicationshasgrownswiftlyfromasingledetectioncomponenttoanintegratedsmartdetectionsystem.
Inthisminireview,westartwiththeintroductionofsyntheticeffortsrelatedtothefabricationofLIGsensors.
Then,wehighlighttheachieve-mentofLIGsensorsforthedetectionofadiversityofstimuliwithafocusonthedesignprincipleandworkingmechanism.
Futuredevelopmentofthetechniquestowardinsituandsmartdetectionofmultiplestimuliinwidespreadapplicationswillbediscussed.
KEYWORDSLaser-inducedgraphene;Smartsensor;Printableelectronics;DesignprincipleISSN2311-6706e-ISSN2150-5551CN31-2103/TBREVIEWCiteasNano-MicroLett.
(2020)12:157Received:23April2020Accepted:9July2020Publishedonline:3August2020TheAuthor(s)2020https://doi.
org/10.
1007/s40820-020-00496-0FabricationandengineeringMechanicpropertiesNon-specificbindingSpecificbindingPressStretchBendLIGsynthesisMechanicsensorChemicalsensorBiomimeticnosePathogenRNAGlucoseMotioncaptureSoundOHHOHOHOHHHHOOHHFabricationandengineeringMechanicpropertiesNon-specificbindingSpecificbindingPressStretchBendLIGsynthesisMechanicsensorChemicalsensorBiomimeticnosePathogenRNAGlucoseMotioncaptureSoundOHHOHOHOHHHHOOHHNano-MicroLett.
(2020)12:157157Page2of17https://doi.
org/10.
1007/s40820-020-00496-01IntroductionThereportofhighelectronmobilityandstabilityofhigh-qualityfew-layergrapheneexfoliatedbythe"ScotchTape"in2004wasreputedagroundbreakingexperimentinmaterialsscience[1].
Sincethen,manyresearchershavebeendevotedtoexploringitsfundamentalpropertiesanddevelopingappli-cationsinbroadfields.
Tocommercializegraphene,varioussynthesisprotocolshavebeendeveloped,suchasmechanicalexfoliation,chemicalvapordeposition,andchemicalreduc-tionofgrapheneoxide[2].
Thesemethodshavetheadvan-tageinmanufacturinggrapheneofdifferentgrades,yettheirscale-upproductionscouldbehamperedbytheweaknesssuchaslowproductivity,highenergyconsumption,andmas-sivewastesgeneration.
In2014,itwasfoundthatpolymerssuchaspolyimide(PI)couldbedirectlyconvertedtoporousgrapheneusinganinfraredCO2laser,amachinethatiscom-monlyfoundinindustry[3].
BesidesinfraredCO2(10.
6μm)laser,visiblelaser[4–9]andultravioletlaser[10]havealsobeensuccessfullyusedtosynthesizeLIG.
Forinfraredlaser,thephotothermaleffectwassuggestedtoaccountforthetran-sition.
Underinstantaneouspyrolysis,thechemicalbondsintheprecursorwouldbebrokenandrecombinedwiththereleaseofgas[3].
Forultravioletlaser,aphotochemicalpro-cesswasmorelikelytohappen.
Sincethephotoenergyofultravioletlaserisclosetothatofchemicalbonds,itcoulddirectlybreakthechemicalbondsinprecursorandgenerateLIG[10].
Forvisiblelaser,bothphotothermaleffectandpho-tochemicaleffectcontributetotheLIGformation[4,9].
Thelaserirradiationprocesswasperformedinambientconditionswithminisculewastesgenerated.
Inaddition,theshapeofLIGcouldbeeasilycontrolledbythecomputerdesign,whichholdsagreatpromisetowardthedevelopmentofprintableelectronics.
TheLIGhasasurfaceareaof428m2g1andresistanceof≤10Ω/[11,12],whicharecomparabletothegraphenesynthesizedbytheconventionalmethods.
PriortoLIGtechnology,manymanufacturingmethodsthatarealsopatternablehavebeendevelopedtofabricategraphene,suchasscreenprinting[13–15],3Dprinting[16–21],andphoto-lithography[22–24].
Table1analyzesthesetechnologies'meritsandflaws.
TheuniqueadvantagesofLIGhavemadeitapopulargraphenesynthesismethodnowadays.
SinceLIG'sdiscovery,tremendousresearcheffortsacrosstheglobehavebeenpaidtoimprovethesynthesisofLIGandtransitingitintoaplethoraofapplicationareas.
Forthesynthesis,theprecursorshavebeenextendedfromPItoalmostallkindsofsubstratessuchasvariouscommercialpolymers[3,25],metal/plasticcomposites[26,27],andnaturallyoccurringmaterials[12,28].
Inaddition,LIGcanbeeasilyembeddedinotherhostmaterialstoformfunctionalcomposites[29,30],whichimprovesthemechanicalflexibilityandstretchability.
Thescale-upmanufacturingofLIG,suchaslaminatedprintingandroll-to-rollproductioncouldbeachievedviatheoptimiza-tionoflasersettingsandthedesignofanautomationstream-line[31–33].
TheadvancesinLIGsynthesisandengineer-inghaveexpandeditsuseindiversefields.
Forexample,thehigh-performancemicro-supercapacitorsbasedonLIGcouldbeattainedbyengineeringeffortssuchasseriesorparallelconfiguration[34]andchemicalspathwaysuchasheteroatomdopingormakingacomposite[27,35,36].
Theself-sterilizingpropertyofLIGwasstudiedforwatertreatment[29,37,38].
Avarietyofchemicalreactions,suchasoxygenreductionreaction[26],oxygenevolutionreaction[28],andhydrogenperoxidegeneration[38],couldbecatalyzedbythemetal/LIGandmetaloxide/LIGcomposites.
Inadditiontotheabove-mentionedapplications,theabilitytocontroltheshapeofLIGandtheexcellentprop-ertiesofLIGhavemadeitapowertechniqueindevelopinghighlysensitiveandrobustsensorsforthedetectionofadiversityofstimuli.
Thedevelopmentofhighlysensitivesensorsisimperativeinourdailylives.
Forinstance,theoutbreakofcoronavirusdisease(COVID-19)sweptglob-allyandhasbeendesignatedaglobalhealthemergencybytheworldhealthorganization(WHO)[39].
Almosttenthousandcholeraandotherwater-bornediseasecasesandtheongoingglobalwarmingaffectpeople'sdailylife[40].
Totackletheseproblems,sensorsplayanessentialrole.
Forexample,healthcaresensorsformonitoringbodytemperatureandrespiratorycouldreflecttheconditionsofpatients.
Thedetectionofenvironmentalqualitysuchasairandwaterareimportantformaintainingahealthyandsafelivingenvironment.
Fortunately,withglobalresearchefforts,theLIGtechniquehasbeendevelopedtodetectabroadrangeofstimuli.
Thisreviewfocusesontheadvance-mentofsensorsfabricatedfromtheLIGtechnology.
Wefirstbrieflyintroducethefabricationandstructuralmodi-ficationofLIG.
Thedesign,mechanism,andtheperfor-manceofLIG-basedsensorsarethensummarizedinthefollowingsection.
Finally,wewilldiscusstheimpactofLIGanditsfuturedevelopment.
Nano-MicroLett.
(2020)12:157Page3of171572SynthesisofLIGInthissection,wewilloverviewthesyntheticeffortsforthesynthesisofLIGanditsmodificationspertainingtothefabricationofLIG-basedsensors.
2.
1FabricationandEngineeringofLIGIn2014,JianLinfoundthatthePIcouldtransformintoporousgraphenewhenitwaslasedbyaCO2laserinambi-entconditions[3].
Theshapeofgraphene,asdemonstratedbythe"owl"shapedLIGinFig.
1a,wascontrolledbytheprogrammablecomputerdesignwithoutusingamask.
Thescanningelectronicmicroscopy(SEM)andhigh-resolutiontransmissionelectronmicroscope(HRTEM)inFig.
1bshowgraphene'shighporosityandcharacteristiclatticespaceof~3.
4.
Withthecontrolofatmosphericcompositions,thegrouptunedthesurfacepropertiesofLIGwithacontactanglerangingfrom0°to>150°(Fig.
1c)[41].
Bychangingtheradiationenergy,operationalmodes,pulsesdensity,andlaserdutycycle,thegraphenemorphologycouldvaryfromsheettofiber(Fig.
1d)andtodroplets[42],aswellasspheri-cal[25]andtubularstructure[43].
Themorphictransitionhelpsthemanipulationofproperties.
Forexample,theLIGchangedfromhydrophilictosuperhydrophobicduetothedifferentsurfacetension[43].
InadditiontoPI,otherbioma-terialsorsyntheticpolymerssuchaspolyetherimide(PEI)[3],wood[12],food[44],andpolysulfone(PSU)[37]havealsobeensuccessfullyconvertedtoLIGbymultiplestep-wiselasing,additionoffireretardantordefocusedlasing.
ThecompositionengineeringofLIGhelpstoimproveitschemicalandmechanicalproperties.
Thisincludestheheteroatomdoping,formationofhybridcomposites,andtheembeddedstructures.
TheheteroatomdopingofLIGcouldbeachievedbyusingadditives-containingprecur-sororpolymercompositesandchangingcarriergasesinthelasingatmosphere[27,37].
TheLIGhybridmaterialcouldbeobtainedbysubsequentdepositionoffunctionalmaterialsontoLIG.
Thiscanbeachievedbyelectrodepo-sition[45]orbyasecondlasingofthemetalsalts-loadedLIG[35].
TheembeddedstructurewasattainedbyfirstsynthesizingLIGonPIsubstrateandtheninfiltratingfillersuchasPVAandPDMS.
Aftercuring,thePIsubstratewaspeeledoffandtheLIGwouldbeleftinthefillers.
ThisembeddedstructurecouldgreatlyimprovetheadherencebetweensubstrateandLIG[29].
2.
2MechanicPropertiesofLIGTheachievementincontrollingthestructureandcomposi-tionofLIGhasfurtherimproveditspropertiesandexpandeditsuse.
Sensorsworkingindifferentenvironmentrequiredifferentmechanicproperties.
Forexample,forwearableelectronics,themechanicflexibilityandstretchabilityshouldbeafforded.
Figure2a,bshowsthemechanicflexibilityofaLIGsupercapacitorfabricatedonaPIsubstrate.
Benefit-ingfromthemechanicalstrengthofPIandtheintegrityofLIGstructure,thecapacitanceretentionofboron-dopedLIGcapacitancestillachievednearly100%atabendradiusof17mm[27].
Figure2c,dshowstheimagesandstretchabilitytestofasinglestretchablemicro-supercapacitors(S-MSC)madefromLIGcomposites.
S-MSCunderdifferentstretch-ingstatesshowedsimilarcapacitivepropertiesandonly15%lossoftheinitialcapacitanceafterrepeating100%stretch-ing[46].
Forsensorsusedinconstruction,themechanicintegrityandrigidityaremoreimportantforadaptingtothesurroundingextremeenvironment.
ALIGembeddedwithcementgassensorwasfabricatedbytheprocessshowninFig.
2e.
ThecementwaswellintercalatedwithintheLIGlargeporesandtheLIGpatternkeptintactaftertransferringLIGfromPItocement(Fig.
2f).
ThissystemcouldworkTable1Comparisonofscreenprinting,3Dprinting,photolithogra-phy,andLIGScreenprinting3Dprint-ingPhotoli-thographyLIGPatternableMask/mold-free***Highresolution(40μm)(150nm)(atomic)(12μm)Highyield*Lowcost*GO-free***Directcontrolofsurfacemorphologyandproperties**Nano-MicroLett.
(2020)12:157157Page4of17https://doi.
org/10.
1007/s40820-020-00496-0underultrahightemperaturewhilemaintainingthestructuralintegrity[47].
3LIGBasedChemicalSensorsChemicalsensorsarebroadlyusedintheexaminationoffoodsafety,thecontaminantsinaquaculture,andportablewater,airqualityaroundindustrieswithhazardgasemis-sions,andthemetabolitessuchasglucose,lacticacid,anddopamineinpointofcare.
Theworkingmechanismofchemicalsdetectionusuallyreliesonthevariationinelec-tricsignalsincludingresistance,capacitance,andthechargetransferresistanceinducedbythestimuli.
Thedetectionofsuchvariationcouldbecatalogedintotwomaingroups,oneisbasedonthespecificbindingofchemicalstothesurfaceofLIG,andtheotheristhenon-specificbindingdetectionpathway.
3.
1SpecificBindingofChemicalSensorsThespecific-binding-typechemicalsensorsareestablishedonthesurfacefunctionalizationoftheLIGwithprobessuchasantibodies(animmunoglobulinwhichcouldrecognizeauniquemoleculeofthepathogen),enzyme(biologicalcatalysts),andaptamers(ashortDNAsequencewhichcanspecificallycombinewiththrombin).
Duetotheprecisecombinationbetweenrecognitionelementsandtargetedchemicals,thesensorsoftenshowextraordinarysensingsensitivity.
Cardosonetal.
preparedLIGelectrodecom-binedwithabiorecognitionelementtodetectchloramphen-icol(CAP)[48].
Figure3ashowsthefabricationofthreeelectrodesbyone-stepandmask-freeLIGtechnologyandthemodificationoftheworkingelectrode.
TostabilizetheloosenLIGparticlesandreceivemoresensinglayers,the3,4-ethylenedioxythiophene(EDOT)waselectrochemicallyAirorO23%H2/ArAr(Chamber)H2(Chamber)16012080400Contactangle(°))d()c((a)LaserPILIG3.
37(b)all0.
50.
512460.
5124Laserdutycycle(%)6Fig.
1TunablestructureandcompositionsofLIG.
aSchematicofthesynthesisprocessofLIGfromPI.
bSEMandHRTEM(inset)imageofLIG,scalebaris10μmand5nm,respectively.
a,bAdaptedwiththepermissionfromRef.
[3],Copyright2014SpringerNature.
cContactanglesofLIGsamplespreparedunderdifferentgasatmosphereswithdifferentlaserdutycycles.
AdaptedwiththepermissionfromRef.
[41],Copyright2017WILEY-VCHVerlagGmbH&Co.
KGaA,Weinheim.
dSEMimageofLIGfiber,scalebaris500μm.
Adaptedwiththeper-missionfromRef.
[42],Copyright2018ElsevierNano-MicroLett.
(2020)12:157Page5of17157polymerizedtoformPEDOTintheworkingelectrode.
ThesamestrategyofthePEDOTdepositionwasappliedinLIG-baseddopaminesensor[49],whichsignificantlyenhancedtheelectrontransferresponsesandthesensingperformance.
EriochromeblackT(EBT)waselectropolymerizedinthepresenceoftheCAPtemplate.
Molecularlyimprintedpoly-mer(MIP)wasthenformedandservedastherecognitionelementofCAPsensor.
ThesensorwhichwasassembledwithoutCAPtemplatewasreferredtoNIP.
Whenthecon-centrationofCAPincreased,therewouldbemoreinter-actionbetweenCAPandMIP,whichinterferedwiththeinteractionbetweentheelectrodesurfaceandelectrolyte.
Thechargetransferresistance(Rct),aparameterreflectingthechargetransferbetweentheelectrodesurfaceandelec-trolyte,couldthereforebeusedtomeasuretheconcentra-tionofCAP.
TheanalyticalperformanceofCAPsensorwasdemonstratedinFig.
3b,c.
TheMIPmaintainedthelinearbehaviorattheconcentrationfrom1nMto10mMwithanaverageslopof162.
5Ω/decadeandalimitofdetec-tion(LOD)aslowas0.
62nM,whileNIPhadnospecificresponsewitharandomrelationshipbetweenRctandCAPconcentration.
ThedifferentresponsebehaviorbetweenMIPandNIPunderlinedthekeyroleofspecificbinding.
Inaddition,theselectivityofMIPwasstudiedbyinter-feringwithoxytetracycline(OTC),sodiumsulfadiazine,andamoxicillin(AMC).
Thelowvalueofrelativestandarddeviation(RSD)forinterferencespeciesOTCandAMCindicatedtheexcellentselectivityofMIP.
ThoughRSDforsodiumsulfadiazinereached24.
79%,itwasbecauseofthechemicalreactionbetweenCAPandsulfadiazineratherthantheinterferingeffectofsensingsurface.
Theresponsesen-sitivityandselectivityofLIG-basedMIPsensorwerecom-parablewithsensorsmadebycommercialgraphene-andcarbon-basedscreen-printedelectrode.
TheworkshowedahighpotentialofprintableLIG-basedMIPsensorforon-siteanalysis.
11010090807060501.
00.
60.
20.
20.
61.
0Retention(%)510Bentradius(mm)2015(b)(d)(c)(e)(f)LIGLaseCasecementPeelPICompositeCementPILIGGasSensorCement1cm(a)0%25%50%100%0.
20.
20.
6Potential(V)Current(mA)1.
00%25%50%100%Fig.
2MechanicLIGanditscomposites.
aDigitalphotographofabentboron-dopedLIGatabendingradiusof10mm.
bCapacitancereten-tionofboron-dopedLIGcapacitanceatdifferentbendingradii.
a,bAdaptedwiththepermissionfromRef.
[27],Copyright2018ElsevierLtd.
ImagescandstretchabilitytestdofaS-MSCat0,25,50,and100%stain.
c,dAdaptedwiththepermissionfromRef.
[46],Copyright2019WILEY-VCHVerlagGmbH&Co.
KGaA,Weinheim.
eSchematicshowingtheprocessofembeddingaLIG-basedsensorintocement.
fOpti-calimageoftheLIGsensor-embeddedincement.
e,fAdaptedwiththepermissionfromRef.
[47],Copyright2019AmericanChemicalSocietyNano-MicroLett.
(2020)12:157157Page6of17https://doi.
org/10.
1007/s40820-020-00496-0Usingsimilarspecificbindingmechanism,ahostofmate-rialshavebeensuccessfullydetected,rangingfromsmallmoleculestobiomoleculesandevenpathogen.
Forexam-ple,smallmoleculessuchasthrombin(anenzymeemerg-inginclottingprocessthatpromotesplateletactivationandaggregation)[50]andbisphenolA(BPA)[51]weredetectedbyimmobilizingspecificaptamersontoLIG.
Glucose[52],biogenicamines[53],andurea[54]sensorswerefabricatedbyanchoringenzymes,andtherecognitionofionsandthemeasurementofconcentrationswereachievedbythefunc-tionalizationofionophores[55].
Thesesensorsarebasedonthechangeofsurfacepropertiesafterinteractingwiththechemicals,whichcanbetransducedintoelectricsignalsrangingfromsurfacecapacitancetoredoxcurrentdensi-ties,andresistance.
Figure4aillustratestheassemblypro-cessofaptamerfunctionalizedLIGelectrodeforthrombindetectionusingtheredoxcurrentdensity.
Thefunctionaliza-tionof1-pyrenebutyricacid(PBA)onLIGprovideselec-trodeenoughcarboxylgroups,whichcouldberapidlyandcovalentlybondedwiththeamino-functionalizedaptamer.
Accordingtothedifferentialpulsevoltammetry(DPV),thebareLIGelectrodewithoutPBAmodificationshowedalmostnochangebeforeandafteraptamerfunctionalization,under-liningtheimportantroleofPBA.
LIGpossesseshighspe-cificsurfacearea,ithasalargenumberofedgeplane/defectsitesandhighheterogeneouselectrontransferrate.
PotassiumNCNOOOpolyimidegrapheneencapsulationelectrodefabricationPlGraphenePEDOTdepositionAminationTemplateremovalElectropolymerizationPlasticSilverinkGrapheneEBTAminePEDOTCAPElectrodefabrication(a))c()b(ElectrodemodificationCO2lasery=162.
5x+1813.
3R2=0.
9923180090001470CAPCAP+AMC11.
96%3.
44%24.
79%CAP+OTCCAP+sulfadiazineLog(CAP,M)Rct()12006000Rct()Fig.
3FabricationprocessandsensingperformanceofCAPsensor.
aSchematicrepresentationoftheworkflowemployedontheproductionoftheLIGelectrodes(top)andtheMIPfabrication(bottom)fortheelectrochemicalbiosensorfordetectionofCAP.
bDependenceofRctonCAPconcentration.
cSelectivitybehaviorofthebiosensorforCAPagainstOTC,AMCandsulfadiazine.
a–cAdaptedwiththepermissionfromRef.
[48],Copyright2018ElsevierB.
VNano-MicroLett.
(2020)12:157Page7of17157ferricyanide(Fe(CN)63/4)wasthenusedastheinner-sphereredoxspeciestoindicatethesurfacepropertyofelectrodes[56,57].
Inshort,thepeakcurrentofredoxcoupledecreaseswiththereductioninedgeplanecontentofelectrodes.
Theincreaseinthrombinconcentrationleadedtothereductioninpeakcurrent,whichwasbecausethatthethrombincapturedbyaptamerreducedtheedgeplaneareaanddecreasedtheheterogeneouselectrontransferrateofFe(CN)63/4[50].
Asaresult,thehigherthrombinconcentration,thelessLIGelectrodesurfacewasavailableforhexacyanoferrate(III),andthus,thelowerpeakcurrentrenderedinDPV.
Besidestheredoxsignalasusedbythethrombinsensors,thevaria-tioninsurfacecapacitanceofLIGuponspecificbindingisanothereffectivemediatorfordetection.
ThiswasshownbyChengetal.
,whodevelopedaLIGsensorforthedetectionofBPA(Fig.
4b)[51].
WhenBPAbondedtoaptamer,astheBPAparticlesarenon-conductive,itinhibitedtheinterfacialchargeaccommodationandhencereducedthecapacitance.
TheauthorsalsofoundthatintroducingalternatingcurrentcanspeedupthetransportationofBPAmolecules,whichsignificantlycurtailedtheresponsetime.
Thesuperhighsen-sitivityofthisBPAsensorwereascribedtotheporousnano-structureofLIGandthespecificbindingbetweenaptamerandBPA.
Athirdtypeofspecificdetectionmethodsisfromthecatalyticreactionofenzyme.
Figure4cshowsthedetec-tionmechanismofanenzymaticglucosesensorfromcascadereactions[52].
Ag/AgClandLIG(rGO)servedasreferenceelectrodeandworkingelectrode,respectively.
Thedepositionofsilvernanowires(AgNW)onLIGwastoimprovethecon-ductivityofLIGundermechanicaldeformation.
Thefiltra-tionofPDMSwasforfurtherpeelingoffelectrodesfromPI.
AndtheadditionalAuandPtnanoparticles(AuPtNP)wereusedasthecatalyststogreatlyincreasetheelectrochemicallyactivepropertiesanddeformability.
Atthepresenceofglu-cose,theglucoseoxidase(GOX)willproducegluconicacidandhydrogenperoxide(H2O2).
ThegeneratedH2O2willthenbedetectedbytheLIGworkingelectrodefromtheampero-metriccurrentresponseinducedbytheoxidationreactionofH2O2.
Fromthecurrentdensity,theglucoseconcentrationcouldbereflected[58].
Theglucosecouldbedetectedwithhighsensitivityandnotaffectedbytheadditionofascorbicacid(AA),uricacid(UA),andNaClsolution,astheglucoseoxidaseinteractsspecificallywithglucose.
Thedetectionofglucosecouldalsobeachievedusingothersensingelementssuchasfluorescentprobes[59],whichalsoshowhighselec-tivityandsensitivityindetection.
YettheLIGsensorsmighthaveadvantagesincertainscenarioasitdoesnotrequirespecificinstrumentation.
Otheranalytes,suchasurea[54],canalsobeselectivelymonitoredbyusingtheircorrespond-ingenzymes.
Inadditiontosmallmoleculesandbiomolecules,thedetectionofpathogenfromthevariationofelectrodeimped-ancewasreportedbyWang'sgroup[60].
Theantibodyandbovineserumalbumin(BSA)wereanchoredontoLIGforthespecificabsorptionofpathogenE.
coliO157:H7(Fig.
4d).
WhenE.
ColicoveredtheLIGsurface,itinterferedwiththechargetransferbetweentheelectrodeandtheelectrolyteandincreasedtheresistance.
Therefore,astheconcentrationofE.
colirangedfrom1*102to1*108cfumL1,thesemicirclediameterofNyquistplotsincreasedandalinearrelationshipbetweentheE.
coliconcentrationandtheelectrontransferresistancewasfound(Fig.
4e,f).
Yetthenon-targetbacteriahadnosignificantresponse.
Theauthoralsocompareddif-ferentelectricsignalsinducedbytheadsorbedE.
coliandfoundthatthechargetransferresistancehadamuchhigherdetectionsensitivitythansheetresistanceanddoublelayercapacitance.
Insignificantimpedancechangeof≤10%afterhundredsofbendingcyclesconfirmedtheexcellentflex-ibilityoftheLIG-basedpathogensensor.
3.
2NonspecificBindingofChemicalSensorsNon-specificbindingchemicalsensorsalsoplayanimpor-tantpartinchemicalsensors.
Withouttheuseofrecognitionelementssuchasantibodyandaptamer,thecostofthenon-specificbindingsensorsisusuallylower.
Boththeintrinsicchemicalredoxreactionsandthephysicalpropertiesofthechemicalsareinformativesourcesforsensing.
3.
2.
1ChemicalRedoxReactionThechemicalredoxreactionhasbeencommonlyusedforthedetectionofsolutesandevengasmolecules.
Thedetec-tioncouldbebothqualitativeandquantitative.
Forexample,theredoxpotentialshelptodifferentiatedifferentanalytes,andthecurrentdensityrelatedtotheredoxreactioncanpro-videinformationontheconcentrationsofanalytes.
Gao'sgroupreportedawearablesensorforuricacid(UA)andtyrosine(Tyr)detectioninsweat[61].
DPViscapabletoevaluatedifferentanalytesbyextrapolatinginformationfromNano-MicroLett.
(2020)12:157157Page8of17https://doi.
org/10.
1007/s40820-020-00496-0theoxidationcurrentpeakintensitiesandoxidationpoten-tials.
TheoxidationpeaksofUAandTyrlocatedat~0.
39and~0.
64V,respectively,whichsimultaneouslydetecteddifferentmetabolites.
TehraniandBavarianfabricatedadis-posalglucosesensorusingdirectlaserengravedgraphene(DLEG)withdecompositionofcoppernanocubes(CuNCs)[62].
Whenaddedglucosewithdifferentconcentration,thecurrentincreasedwithdifferentamplitude(Fig.
5a),show-ingthefeasibilityofquantitativedetection.
Figure5billus-tratesthecurrentwereinlinearrelationshipwiththeglucose[Fe(CN)6]4(a)(c)(d)ASBydobitnAE.
coliNafionEnz.
layerPtAuNPLIGAgNWAg/AgClPDMS(b)FluidflowBPAAptamerBPS(interference)6-Mercaptohexanol[Fe(CN)6]4[Fe(CN)6]3ThrombinbindingLIGGlucoseOxidaselayerAu/rGO/AuPtNPNafionMembraneGlucoseGluconicAcidAptamer1-Pyrenebutyricacid[Fe(CN)6]3eO2H2O2H2O2GOx(Ox)GOx(Red)1501209060300150250(e)(f)4501*102-1*108cfumL1Realimpedance()Imaginaryimpedance()350420360300240180Electrontransferresistance()Y=131.
06+29.
93XR2=0.
9551234Concentrationofbacteria(LogcfumL1)5678Fig.
4Variousspecificbindingsensors.
Schematicofathrombinsensor,bBPAsensor,andcenzymaticglucosebiosensorsensor.
AdaptedwiththepermissionfromaRef.
[50],Copyright2017AmericanChemicalSociety;bRef.
[51],Copyright2016AmericanChemicalSociety;cRef.
[52],Copyright2018ElsevierB.
V.
dSchematicillustrationoftheAuNPs-LIG-basedimmunosensorforthedetectionofE.
coliO157:H7.
eNyquistplotsofE.
colisensor.
fCalibrationcurveoftheimpedanceresponsewiththeconcentrations.
d–fAdaptedwiththepermissionfromRef.
[60],Copyright2019ElsevierB.
VNano-MicroLett.
(2020)12:157Page9of17157concentration,andtheexcellentsensitivityof4532.
2A/mM/cm2andlinearrangefrom25Mto40mMwereachieved.
Non-enzymaticH2O2sensor[63]anddopaminesensor[64]basedonthereductioncurrentandconcentrationofH2O2wasalsosuccessfullymade.
3.
2.
2PhysicalPropertiesThephysicalpropertiessuchastheresistanceofLIGuponinteractingwithanalyteandtheconductivityorimpedanceofanalytesolutionarealsousedtoprobetheresponsefromstimuli.
Forexample,anartificialnosebasedonthechemi-calbondingbetweenpalladium(Pd)andhydrogen(H2)forhydrogendetectionwasmadebytheParkgroup[65].
Theturbinateplaysanimportantroleforodorperceptionduetothelargesurfaceareanatureandtheabilitytopropelairtowardtheolfactionnervereceptors.
Inspiredbythetur-binatestructure,biomimeticturbinate-likeLIG-basedH2sensorwasdeveloped.
ThesensormadeuseofLIG'shighporosityandelectricconductivity,whichhelpedtoimprovethesensitivityofthedevice.
ThePdnanoparticles(NPs)wereusedasthemediumforhydrogensensingbecauseofthehighaffinityofhydrogentoPd.
Figure6aillustratesthecatalyticreactionmechanismofLIG/Pdsenor.
Theas-preparedLIGshowedn-typebehaviorduetoconsiderableoxygenandnitrogenatomsonLIG.
TheabsorptionofH2byPdNPschangedtheFermienergylevelofPdandreducedtheworkfunctionofPd.
ThechargesthentransferredfromPdtoLIG,andthus,thechargecarrierdensityofn-typeLIGincreases,leadingtothedecreaseinLIG'sresistance.
TheresistancevariedwithH2concentrationlinearly.
TheauthorsfurthertransferredtheLIG/Pdcompositesintoflexiblepolyethyleneterephthalate(PET)substrateandmeasuredtheresistanceresponseunderdifferentbendingstates(Fig.
6b).
Thenegligiblevariationinresistanceresponseunderdif-ferentbendingstrengthevidencedtheexcellentmechanicflexibilityoftheH2sensor.
SimilarworkingmechanismwasemployedinNO2detectionbyHogroup[66].
Thethermalconductivityofgasisanotherusefulparam-eterforthefabricationofgassensors,asreportedbyTour'sgroup[47].
ThesensorwasfabricatedbylinkingaLIGfila-mentwithawidthof57mtotwoplanarLIGelectrodes(Fig.
2f).
WhenthedevicewasJoule-heated,mostofheatlocalizedaroundthefilamentbecauseofitslargeresistance.
Whenthesensorwasexposedtogas,theheatedfilamentcooleddownduetotheconvectiveheatlosstothegas.
Gaswithhigherthermalconductivitydecreasesthetemperatureoffilamentmoresignificantly.
Sincetheresistanceofthefilamentistemperature-dependent,thevariationinresist-ancewouldthereforehelptoidentifythegas.
Thekathar-ometer-likegassensorcouldbeusedtomonitorvariousgasoncethethermalconductivity,andthetemperaturerelation-shipoftestedgaswasunequivocal.
Variousgasessuchasair,helium,oxygen,andcarbondioxidehavebeendetected(Fig.
6c).
Figure6dshowsresponseofairsensorbendingwitharadiusofcurvatureof7mmwithin1000cycles,andtheminorvariationsimpliedthattheLIG-basedgassensorpossessedrobustresponseandgoodmechanicflexibility.
Inaddition,theauthorembeddedthegassensorintocementanddemonstratedtheviabilityofusingthissmartbuildingmaterialforthemonitoringofthecompositionsinfluegas.
Therearealsoothernon-specificbindingchemicalsen-sorsbuiltontheextrinsicpropertiesofanalytes.
Forexam-ple,Nag'sgroupexploitedsalinity(sodium)sensor[67]25M0.
1mM0.
25mMDLEG-CuNCsDLEG1mM4.
54.
03.
53.
02.
52.
01.
51.
00.
50Current(mA)0100200Time(s)0.
350.
310.
270.
230.
190.
15300400Current(mA)5070t(s)901100.
00450.
00400.
00350.
00300.
00250.
00200.
00150.
00100.
00050Current(A)00.
0020.
0040.
0060.
0080.
010Glucoseconcentration(M)y=0.
6795x+0.
0002R2=0.
9936(a)(b)Fig.
5aAmperometriccurrentresponsewithsuccessiveadditionofdifferentglucoseconcentrations.
bCalibrationcurveoftheglucosesensor.
a,bAdaptedwiththepermissionfromRef.
[62],Copyright2016SpringerNatureNano-MicroLett.
(2020)12:157157Page10of17https://doi.
org/10.
1007/s40820-020-00496-0andnitratesensor[68]fromtheresistanceofthesolution.
Theimpedanceofasolutionconsistsofinternalcapaci-tance(Cint),resistanceofsolution(Rsol),andcapacitanceofsolution(Csol).
RsolandCsolareinfluencedbythesolu-tionmedium.
TherealpartofimpedanceRsolwasusedtoinvestigatetheionconcentrations.
Whentheconcentrationofsolutionincreased,theRsolreducedduetotheenhancedionicconductivity.
Figure6edepictsthelinearimped-anceresponsetowardnitrateconcentration,andthesensorachievedawidedetectionrangeof1–70ppm.
Sincetheionicconductivitycouldalsobeaffectedbytemperature,theauthorfurtheraddedaLIG-basedtemperaturesensortocorrectthetemperatureeffect.
ThetemperaturesensorwasdesignedfromthesamemechanismastheTour's[47],whichwasbasedonthecorrelationbetweentheresistanceofLIGandthesurroundingtemperature.
Figure6fshowsthatthemeasuredtemperaturefromtheLIG-basedsensorwasconsistentwiththeactualtemperature.
Thecompensationoftemperatureinterferencegreatlyimprovedtheprecisionofsensing.
TheLIG-basedhumiditysensorsalsoutilizedtheextrinsicproperties(changeofcapacitance)[69,70].
AlthoughextrinsicpropertiesofanalyteprovideasimpleEcH2H2Currentflow(e)(f)n-typeEfEvρ=+∞ρ=3cmρ=2cm0.
60.
91.
21.
51.
82.
12.
4Response(R/R0%)0123456782010010203040505040302010043210R/RHe(%)R/RHe(%)0.
2(b)(a)(c)(d)0.
40.
6Sensitivity(a.
u.
)Concentration(%)0.
81.
01.
2VacuumAirAirHeHeO2O2VacuumVacuumVacuumVacuumVacuumVacuum0100Time(s)200300400y=667.
97x+9196.
9R2=0.
96y=1.
0122x0.
2736R2=0.
99500010002004006008001000Bendingcycles200300Air1000bendingcycles0bendingcyclesTime(s)Realimpedance(k)01020304050607001020304050Actualtemperature(°C)Calculatedtemperature(°C)Nitrae-Nconcentration(ppm)CintCsolRsolFig.
6Non-specificbindingsensorsfromtheintrinsicandextrinsicproperties.
aBandenergyanalysisoftheH2gasactingontoLIG(top)andcatalyticreactionofH2onLIG/Pd(bottom).
bResponseversusH2concentrationwithdifferentbendingstates.
a,bAdaptedwiththepermis-sionfromRef.
[65],Copyright2019AmericanChemicalSociety.
cResponsesofgassensortowardavarietyofgases.
dMagnitudeofresponseofgassensortoairafterbendingitwitharadiusofcurvatureof7mm.
Insetfigureshowstheresponseofthegassensortoairafter0and1000bendingcycles.
c,dAdaptedwiththepermissionfromRef.
[47],Copyright2019AmericanChemicalSociety.
eNitratesensorresponsetothenitrateconcentration.
Insetistheequivalentcircuitofsensorimmersedinsolution.
fComparisonofactualandmeasuredtemperature.
e,fAdaptedwiththepermissionfromRef.
[68],Copyright2017ElsevierB.
VNano-MicroLett.
(2020)12:157Page11of17157detectionpathway,thistypeofchemicalsensorusuallyhasinferioraccuracyandprecisionwhencomparetothespecific-bindingsensorsandthenon-specific-bindingonesbasedontheintrinsicandcharacteristicchemicalandphysi-calpropertiesofanalytes.
Forexample,ionconcentrationsensorwillbeinterferedbyotherionsinacomplexsystemwherethereareallsortsofchemicalsratherthanasinglespecies.
4LIGBasedMechanicSensorsMechanicsensorsarewidelyusedinsubtlehumanmotiondetection,signlanguagetranslation,andsoftroboticgripper.
TheLIG-basedmechanicsensorsareusuallybuiltonthepiezoresistiveeffect,whichdetectsthechangeofresistanceduetotheshapedeformationinducedbythestimuli.
Forexample,Zhao'sgroupcombinedthe3DprintingtechniquewiththeLIGprocesstofabricatesmartcomponents(SC),whichhelpedtoreflecttheconditionssuchastheworkingprocessandabrasion(Fig.
7a)[71].
Withcomputer-controldesign,theyfabricatedsmartgearfrompolyetheretherketone(PEEK)withLIGpatterns.
ThePEEK-LIGSCrespondedtoboththebendingandstretchingofPEEKcomponents,asshowninFig.
7b,c.
TheresistanceresponseofstrainsensorwascorrelatedwiththeconnectionandcompactnessofLIGonPEEK.
WhentheSCwasbendedoutwardorstretched,theresistanceincreasedduetothealoosenedconnectionofLIG.
Incontrast,bendinginwarddensifiedtheLIGandthereforereducedtheresistance.
Thegaugefactor(GF)was212.
35and155.
36forstretchingandbending,respectively,whichsuggestedahighersensitivityofplanarstrain.
Theresponsetimeandrecoverytimewereshort(Fig.
7d),whichwasascribedtothehighelasticitymodulusofPEEK.
AsshowninFig.
7e,theresistanceofgearswascorrelatedwiththeconditionsoftheLIG.
Whenthegearwasabrased,theresistanceincreasedaccordingly.
Theproposedsmartgearcoulddetectitsrotationandabrasionwhileitwasworking,showinggreatpromiseforself-monitoringsystems.
Byrecordingthepiezoresistiveeffectchronologically,LIG-basedmechanicsensorscanbeusedfortheinsitudetectionofavarietyofstimulisuchasheartbeat,motions,andsounds.
Forexample,byattachingtheLIGmechanicsensorstodif-ferentlocationsofhumanbody,Lin'sgrouphassuccessfullydetecteddifferentelectrophysiologicalprocessessuchaselec-troencephalograms(EEGs),electrocardiograms(ECGs),andelectromyograms(EMGs)[72].
ThemechanicsensorsweremadebytransferringLIGintoanelastomerinakirigamidesign,whichimprovedthestretchabilityofthedevices.
AsshowninFig.
8a,alpharhythmwithfrequencycenteredat10Hzfromsensorsonforeheadimpliedthatthebrainwavesweresuccessfullyrecorded.
ThecharacteristicP-wave,QRScomplex,andT-waveofECGwereidentifiedclearly.
AndtheEMGsignalsrespondedtofingerbending,whichcanbeusedforhuman–machineinterfaceapplication.
Tao'sgroupreportedthefabricationofaLIG-basedartificialthroatforsoundsensing(Fig.
8b)[7].
WhenthethroatwasattachedwithaLIGsensor,thevibrationofthroatcordschangedtheresist-anceofLIGsynchronously.
Asdifferentsoundsgenerateddif-ferentwaveshapesofresistance,byrecordingadatabaseandcombiningwithmachinelearning,therecognitionofsoundwasattainable.
ThereportofLIG-basedsoundsourcewasalsofoundintheliterature[73,74].
Withsimilardetectionprinci-ple,somegroupsreportedtheimprovementintheperformanceofLIG-basedpiezoresistivesensorsbymodifyingthestruc-tureandcompositionofthedevices.
Forexample,Luoetal.
foundthatthelaserconditionsdictatingthemorphologyandstructureofLIGhadgreateffectonpiezoelectricsensor'sper-formance,andtheoptimizedLIGsensorshowedhighergaugesensitivitythancommercialstraingaugebynearly10times[75].
Chhetryandco-workersdesignedaMoS2/LIGstrainsen-sorforthedetectionofvoice,eye-blinking,andpulsewave[76].
ThedecorationofMoS2significantlyreducedthecrackinLIGandimprovedthemechanicalstrengthofthesensor.
ByreplacingthePIfilmwithaPIpaperasthesubstrateforLIGsynthesis,Wangetal.
improvedthehomogeneityandintegrityoftheLIGanddemonstratedtheapplicationasastrainsensortocapturethemotionsofhumanfingerandsoftrobot[77].
UtilizingthemechanicalandacousticalperformanceofLIG,Taoetal.
fabricatedadual-functionaldeviceforphysiologi-calsignals(wristpulseandrespiratory)detection,andselfalarming,whichoffersabrandnewideaforhealthmonitoringsensors[78].
5SummaryandOutlookSincethediscoveryofLIGin2014,theadvancesinsyn-thesisofLIGtechnologyhavesignificantlyimprovedthepropertiesofgrapheneandaddedtotheversatilityofapplications.
Forinstance,thewavelengthoflaserextendsfrominfraredtovisibleandevenultraviolet,whichhelpstoNano-MicroLett.
(2020)12:157157Page12of17https://doi.
org/10.
1007/s40820-020-00496-0improvethespatialresolutionoftheLIGstructureto~12m[6].
ThestrategiesfortheformationofLIGcomposites,suchastheinsituandexsitumodificationprocess,canenhancethephysicalpropertiesofLIGsuchasmechanicalstrengthandconductivity,aswellasthechemicalpropertiesbyincorporatingfunctionalmaterials[32,33].
ThelowcostofLIGtechnologyandsimplicityinsynthe-sispromotesthedevelopmentofaserialofLIGsensorsandmakesitapotentialcandidateforindustrialproduction.
Withtherationaldesignofsensingmechanism,alargediversityofstimulihasbeendetectedrangingfromvariouschemicalstosounds,motions,andtemperature.
Thesesensorsoftenshowhighsensitivityandhighstabilityduetothehighsur-faceareaandchemicalstabilityofLIG.
Inaddition,thehighconductivityofLIGmakesitanidealtransducerforconvert-ingthestimuliintoelectricalsignal.
PristineLIGmadefrompolymersisoftenflexible,anditstransfertoothersubstratessuchaselastomersorcementscouldconferitstretchabilityorrigidity,whichmakesLIGfeasibleforuseindifferentscenariossuchaswearableelectronicsandsmartbuilding.
ThedevelopmentofLIGsensorshasevolvedfromasin-gledetectioncomponentintointegratedsystems.
Real-timeandcontinuousdetectionofstimulihasbeenachievedbytheintegrationofwirelesstransmissionandmicrocontrollerFig.
7aSchematicofthe3DprintingofthePEEKcomponentandthesynthesisprocessofLIGfromthe3DprintedPEEKgear.
bWorkingmechanismofPEEK–LIGSCforbidirectionalbendingandstretching.
cRelativechangeinresistanceofthesensorversustheappliedstrains.
(Thedatawereobtainedaftermorethan1000unloadingcyclesforthebendingandstretching.
)dRespondtimeandrecoverytimeforbending(0–5%strain).
eCircuitresistanceincreasesbecauseofabradingofthegear.
Theinsetphotographsshowedthreedifferentabrasiondegreesofthesmartgear.
(I)Notabrased,(II)partlyabrased,and(III)fullyabrased.
a–eAdaptedwiththepermissionfromRef.
[71],Copyright2019AmericanChemicalSocietyNano-MicroLett.
(2020)12:157Page13of17157moduleswithsensorsforInternetofThings(IoT)applica-tions[47,65,67,79].
Forexample,Gao'sgroupincor-poratedflexibleprintedcircuitboard(FPCB)andmicro-controllerwithLIG-basedUAandTyrsensor[61].
Thisintegratedsystemcanwirelesslyrecordsensingsignalsandconvertthedigitalsignaltoanalogoutput,whichpavesawayfortheinsituandnoninvasivemonitoringofhealthcon-ditions.
TheBrukittgroupassembledthesensorwithsmartmicrocontrollersystemandwirelessconnection,whichhelpstoformadistributedsensingnetworkforthereal-timemoni-toringofthewaterquality[68].
Beingapatternableandprintablemanufacturingtech-nique,theLIG-basedsensorsilluminateanewpathwayfordevelopingintegratedminiaturizeddevices.
YettherearestillsomeroomsforimprovingtheLIGtechnologyforprac-ticalapplications.
Forexample,thebondingofLIGlayerandprecursorsubstrateisnotstrongenoughinsomescenarios.
ThoughcircumventionofLIGsuchasitsfunctionalizationwithviscouspolymerortransferringtheLIGtoelastomercanresolvesuchissue,theconsumptionofchemicalsandadditionalmanufacturingstepsisnotdesirableforproduc-tion.
SomeproposedLIGsensorswerenotdemonstratedforinvivooron-sitedetection,whichmightnotreflectthefeasibility,stabilityanddurabilityofthesensorsinrealsitu-ations.
This,however,isimportantforpracticalapplications,asinterferencesfromtheenvironmentandthevariationinconditionsfromlaboratorycouldpotentiallyaffectthesensitivityandreliabilityofthesensors.
Nonetheless,withresearcheffortsfromtheglobe,thediversityofthetransi-tionsofLIGintovarioussensorshasbeenrewardinganddelightfultobehold.
Withfuturedevelopment,LIGsensorswillfindacommonplaceinwidespreadapplications.
AcknowledgementsWeacknowledgethefundingsupportfromtheCityUNewResearchInitiatives/InfrastructureSupportfromCentralunderGrantAPRC-9610426andtheStateKeyLaboratoryofMarinePollution(SKLMP)SeedCollaborativeResearchFund121110981501005001.
00.
50.
00.
51.
0210Fequency(Hz)012Time(min)30.
0LIGResistance(a.
u.
)Time(s)0.
5012Time(s)3456789Amplitude(mV)PSTRQV2(Hz)EEGECGEMGAmplitude(mV)0510CoughHumScreamSwallowNodTime(s)20253015(a)(b)Fig.
8aEEG,ECG,andEMGmeasurements.
AdaptedwiththepermissionfromRef.
[72],Copyright2018WILEY-VCHVerlagGmbH&Co.
KGaA,Weinheim.
bLIG-basedartificialthroatwithsound-sensing.
AdaptedwiththepermissionfromRef.
[7],Copyright2017SpringerNatureNano-MicroLett.
(2020)12:157157Page14of17https://doi.
org/10.
1007/s40820-020-00496-0underSKLMP/SCRF/0021.
TheauthorsgratefullyacknowledgeMr.
ZhengtongLifordrawingTOCgraphic.
OpenAccessThisarticleislicensedunderaCreativeCommonsAttribution4.
0InternationalLicense,whichpermitsuse,sharing,adaptation,distributionandreproductioninanymediumorformat,aslongasyougiveappropriatecredittotheoriginalauthor(s)andthesource,providealinktotheCreativeCommonslicence,andindicateifchangesweremade.
Theimagesorotherthirdpartymaterialinthisarticleareincludedinthearticle'sCreativeCom-monslicence,unlessindicatedotherwiseinacreditlinetothematerial.
Ifmaterialisnotincludedinthearticle'sCreativeCom-monslicenceandyourintendeduseisnotpermittedbystatutoryregulationorexceedsthepermitteduse,youwillneedtoobtainpermissiondirectlyfromthecopyrightholder.
Toviewacopyofthislicence,visithttp://creativecommons.
org/licenses/by/4.
0/.
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