Electrochemical Activation of Self-Assembled Monolayers for the Binding of E ff ectors - Markovic et al. - 2020 - Unknown

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pubs.acs.org/LangmuirArticleElectrochemicalActivationofSelf-AssembledMonolayersfortheBindingofEffectorsAleksandraMarkovic,LeonBuschbeck,IzabellaBrand,CarstenDosche,JensChristoffers,andGuntherWittstock*CiteThis:Langmuir2020,36,14623−14632ReadOnlineACCESSMetrics&MoreArticleRecommendations*sıSupportingInformationABSTRACT:Aself-assembledmonolayer(SAM)ongoldwaspreparedfromadiaminoterephthalate(DAT)derivativeasfunctionalmoleculeand1-decanthiolasabackfiller.TheDATderivativeisN-protectedbyatert-butyloxycarbonyl(Boc)groupandisanchoredtothegoldsurfaceviaaliponicacidasastableanchorgroup.TheterminalDATmoietyexhibitsinterestingeffectorpropertiessuchasfluorescenceandelectrochemicalactivity.Irreversibleoxidationofthemonolayerat0.4V(Hg|Hg2SO4)in0.1MHClO4triggersdeprotectionoftheDATgroupandsubsequentchemicalreactions,duringwhich10%oftheDATgroupsoftheoriginalSAMaretransformedtoanewsurface-bound,quasi-reversibleredoxcouplewithaformalpotentialof0.0V(Hg|HgSO)andastandardrateconstantof8s−1in0.1MHClO.244ImmersionofthemixedSAMin0.1MHClO4atopencircuitpotentialoroxidationin0.1MH2SO4didnotproducethissurface-boundredoxcouple.ThemonolayerswerethoroughlycharacterizedbyX-rayphotoelectronspectroscopy(XPS)andpolarizationmodulationinfraredreflectionabsorptionspectroscopy(PMIRRAS)afterthedifferentpreparationstepsindicatingonlyminorchangesintheoverallcompositionofthemonolayer,inparticular,thepreservationoftheheteroatoms.Thenewredoxcoupleislikelyadiimine,inagreementwithitsabilitytobindnucleophilessuchasanilinesbyconjugateadditionthatcouldbefollowedbymulticyclevoltammetryandXPS.TheDATeffectorgroupisespeciallyinterestingbecauseitcanalsoreportthebindingreactionbychangedelectrochemicalandfluorescencesignals.■INTRODUCTIONsubsequentcycles.ElectrochemicalquartzcrystalmicrobalancedataandgrazingincidentangleFouriertransforminfraredSelf-assemblyofmoleculesonsurfacesisanattractivemethod1(FTIR)spectraprovedtheelectrochemicalconversionofforsurfacemodificationbecauseofitssimplicity.Today,self-1213SAMs.Kimetal.alsousedananalogouselectrochemicalassembledmonolayers(SAMs)areusedasanchorsforactivationmechanismforhydroquinone-cagedbiotin.Afterattachingsurfacemoleculardevicesthatcanbeusedforcleavingthehydroquinoneester,biotinbecameavailableforDownloadedviaUNIVOFCALIFORNIASANTABARBARAonMay16,2021at12:07:09(UTC).Seehttps://pubs.acs.org/sharingguidelinesforoptionsonhowtolegitimatelysharepublishedarticles.differentsensingpurposes,commonlycalledintegrated2associationwiththeproteinstreptavidin.molecularsystems.Thesemolecularsystemsaremainly3−56−8Probablymechanismsinwhicheffectors,i.e.,moleculeswithusedforsensingpH,inorganic,andorganiccompounds9−11aspecificfunctionalitywithrespecttoanapplication,couldbeaswellasforbiosensors.SomeoftheseintegratedboundtosurfacesuponelectrochemicalactivationofaSAMmolecularsystems“turnedon”thebindingpropertiesofthe15aremoreversatile.YousafandMrksichuseda1,4-terminalgroupsondemandwhenapplyingapotentialinanhydroquinone-terminatedSAMasaplatformtowhichelectrochemicalcelltotheAuelectrodeonwhichtheSAMwascyclopentadienewasimmobilizedbyDiels−Alder(DA)formed,aprocessoftencalled“electrochemicalactiva-12−1412reactiononlyafterthehydroquinonehadbeentransformedtion”.Kimetal.electrochemicallydeprotectedSAMstothecorrespondingquinone.AftertheDAadditionofforsite-selectiveimmobilizationofbiomolecules.Theydevelopedasystembasedonamonocarboxylicesterofhydroquinone.AfteroxidationofthetetheredhydroquinoneReceived:August16,2020group,thehydroquinone/quinonegroupwascleavedofftheRevised:October31,2020surfacebynucleophilicacylsubstitutionofquinonebyHO.Published:November24,20202Theelectrochemicalactivationcanbeobservedasanirreversibleanodicpeakat0.2VvsHg|Hg2SO4incyclicvoltammogramsoftheSAM.Thispeakdisappearsinthe©2020AmericanChemicalSocietyhttps://dx.doi.org/10.1021/acs.langmuir.0c0242614623Langmuir2020,36,14623−14632

1Langmuirpubs.acs.org/LangmuirArticleScheme1.PreparationofDATDerivative2cyclopentadienetotheSAM,adecreaseinthepeakcurrentsinGermany).Thegoldsubstrateswerefurthermodifiedbyself-assemblycyclicvoltammogramsfortheoxidation/reductionoftheimmediatelyaftertheirretrievalfromthevacuumchamber.Theyarehydroquinone/quinonecouplewasindicativeoftheprogressdenotedasAu/glass.Self-AssemblyofDATsonAuSurfaces.Freshlypreparedgoldofthereaction.ThereactioncanbedescribedasanECsampleswereimmersedin0.1mMethanolicsolutionofcompound2mechanismonsurfaces.Inarelatedsystem,theelectrochemi-(absoluteethanol,analyticalreagentgrade,FisherChemicals,callyformed,tetheredquinonewasusedfortheformationofaSchwerte,Germany)for2daysforself-assembly.Afterward,1617Schiffbase.Shamsipuretal.studiedelectrochemicalsubstrateswereplacedina0.1mMethanolicsolutionof1-conjugateadditionreactionsonsurfacesofhydroquinone-decanethiol(96%,Sigma-Aldrich)for1day,followedbyrinsingterminatedSAMswithglutathione.ThereversibleoxidationofwithethanolanddryinginanArstream.Forbrevity,wedenotethisthehydroquinonemoietytoquinonewasfollowedbyamonolayeras“initialSAM”.nucleophilic1,4-additionofglutathionebyanECmechanismMicrocontactPrinting.Microcontactprintingwasperformedwithpolydimethylsiloxane(PDMS)stamps.Further,0.25cm2oftheonsurfaces.Involtammograms,thepeakofthesurface-boundstampswaswettedby10μLofa0.1mMsolutionofcompound2inhydroquinone/quinonecouplediminisheswithtime,whichisethanol.Afterdryingforabout20minunderaplasticcover,thenotthecaseintheabsenceofglutathione.ThisreactionwasPDMSstampwasplacedonthegoldsamplefor10s.Afterward,thealsodemonstratedinalaterallyconfinedfashionusinggoldsamplewasplacedina0.1mMsolutionof1-decanethiolfor18scanningelectrochemicalcellmicroscopy.backfillingfor10h,rinsedwithethanol,anddriedinanArstream.Inthispaper,wereportanewbindingsystemthatisbasedVoltammetry.ThemodifiedgoldslideswerepressedagainstanonSAMscontainingderivativesof2,5-diaminoterephthalateO-ringatthebottombelowacylindricalliquidreservoirofacustom-(DAT).TheDATmotifcanbeeasilyfunctionalizedwithuptomadeelectrochemicalcell.TheopeningoftheO-ringdefinedthe19circularelectrodeareaA=0.283cm2ofadiameterof6mm.Allfourdifferentfunctionalgroups.Bythesesubstitutions,thegeomemissionspectraofthegroupchangeinacharacteristicway,19solutionswerepurgedwithArfor45minbeforethemeasurements.20,21Cyclicvoltammograms(CVs)wererecordedat295Kusingawhichwasusedfortheconstructionofturn-ondyes.Thepotentiostatwithananalogscangenerator(MetrohmAutolab,combinationofuptofoursubstitutionpositionsandtheUtrecht,theNetherlands)andathree-electrodeassemblycomprisingconcomitantsystematicchangeofspectroscopicandelectro-theSAM-modifiedAusurfaceastheworkingelectrode(WE),aPt22chemicalpropertieshavealreadybeenutilizedforapplicationsheetastheauxiliaryelectrode(Aux,areaof1cm2),andaHg|HgSO19,23,2424inlifesciencesandmaterialsscience.TheuseofDATsreferenceelectrode(KoslowScientific,Englewood,NJ).astheterminalgroupinSAMs,asexploredforthefirsttimeinX-rayPhotoelectronSpectroscopy(XPS).XPspectrawerethispaper,isveryattractivebecauseitisexpectedthattherecordedwithanESCALAB250XiX-rayphotoelectronspectrometerSAMcanbindeffectorstothesurfaceafterelectrochemical(ThermoFisher,EastGrinstead,U.K.)usingmonochromatizedAlKαactivationandthatthisbindingeventcanbemonitoredbyradiation(hν=1486.6eV),apassenergyof30eV,adwelltimeof50ms,andanenergystepsizeof0.05eV.Thescannumberwasadjustedcyclicvoltammetryand/orbyfluorescencedetection.toobtainreasonablesignal-to-noiseratios(15scansforS2p,30scans■forN1s,15scansforF1s,15scansforO1s,10scansforC1s).AllEXPERIMENTALSECTIONspectrawerefittedusingthesoftwareAvantage(v.5.932,ThermoCompoundsandMaterials.Compound2waspreparedin25%Fisher)afterapplyinga“smartbackground”(acombinationofayieldbyamidecouplingofα-liponicacid(ALA)withaDATpolynomialandaShirleybackground)andaconvolutionofGaussderivative1(Scheme1)equippedwithanaminopropyllinkerunitandLorentzfunctionsforeachspectralcomponent.TheXPspectrausingstandardreagents(COMU−DIPEA)[COMU=(1-cyano-2-areplottedwiththemeasureddatapointsasdots,thebackgroundasaethoxy-2-oxoethylideneaminooxy)(dimethylamino)(morpholino)-light-grayline,thefittedcomponentsascoloredlines,andthesum25carbeniumhexafluorophosphate,DIPEA=ethyldiisopropylamine].curveasadark-grayline.26Thepreparationofcompound1hasbeenreportedbefore.BocPolarizationModulationInfraredReflectionAbsorption[tert-butyloxycarbonyl,tBuO(CO)]isastandardcarbamate-Spectroscopy(PMIRRAS).PMIRRAspectraweremeasuredprotectinggroupforamines.SyntheticdetailsareprovidedintheusingaVertex70spectrometerandanexternalreflectionsetupSupportingInformation(SI-1).(Bruker,Ettlingen,Germany)containingaphotoelasticmodulatorAllsolutionswerefreshlyprepared.Deionizedwaterwithaspecificwithafrequencyof50kHzandademodulatorPMA50(Hindsresistanceof18.2MΩcm(ELGALabWater,Celle,Germany)wasInstruments,Hillsboro,OR).Thehalf-waveretardationwasseteitherusedforallsolutionandrinsingprocedures.Theelectrolytewas0.1Mto1600or2900cm−1.Theincidentanglewas80°,and1200spectraHClO(HClO,puriss.p.a.,70.0−72.0%,Sigma-Aldrich,Steinheim,witharesolutionof4cm−1wererecordedforeachsample.PMIRRA44Germany)inwater.Thesolutionof0.1MH2SO4(H2SO4,95.0−spectrawereprocessedusingtheOPUSv5.5software(Bruker,97.0%,Sigma-Aldrich)inwaterwasusedinsomecontrolexperi-Ettlingen,Germany).Spectrainthispapershowtheabsorbanceasaments.Microscopeglassslides(VWRInternationalBVBA,Leuven,functionofwavenumberafterbackgroundsubtractionbyspline-227Belgium)werecutintopiecesof2×2.5cmandrinsedwithwaterinterpolationandnormalizationoftherawPMIRRASsignal.andethanol(analyticalgradeFisherChemicals,Schwerte,Germany).FluorescenceMicroscopy.Aninvertedmicroscope(DMIRE2,Afterward,theglassslidesweredriedinastreamofAr.Layersof0.7Leica,Wetzlar,Germany)equippedwithanEMCCDcamera(LucaRnmCrasanadhesionpromoterand200nmAuwereevaporatedonDL604M,Andor,Belfast,U.K.)andaFluotar50×/0.8objectivewasthecleanedglasssurfacebyelectronbeamevaporationandresistiveusedforfluorescencedetection.Sampleswereexcitedwithatungstenheatinginvacuumwhilemonitoringthelayergrowthusingaquartzlampfilteredwithadichroicfiltersetforblue/green(bandpass450−crystalmicrobalance(MiniCoater,TectraGmbH,Frankfurt/Main,490nmforexcitation/510nmdichroic/longpass515nmfor14624https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

2Langmuirpubs.acs.org/LangmuirArticleemission).TheabsenceofRamanscatteringwasverifiedwithaofareversiblepeakinamonolayerwithoutlateraladsorbate−green/redfilterset(bandpass515−560nmforexcitation/580nmadsorbateinteractionmeasuresdichroic/longpass590nmforemission).Forallmeasurements,theEMgainwassetto200.RawdatawereexportedasaTIFFfileand90.6mVprocessedusingtheFitswork4software(Hohmann-EDV,BadTölz,fwhm=n(1)Germany).■at298K,wherenisthenumberofelectronstransferredperRESULTSmoleculeintheredoxreaction.ThefwhmissmallerthanforElectrochemicalCharacterizationofSAMs.Amono-anideal1e−systemandlargerthan45.3mVexpectedforanlayerofcompound2backfilledwith1-decanethiolwasself-ideal2e−system.ThissuggeststhattwoelectronsareinvolvedassembledontheAuelectrodesurface.Thebackfillingwith1-intheoxidationoftheDATmoietywithsomebroadeningofdecanethiol,selectedtomatchthespacerchainlengthinthepeakduetokineticlimitation29orkineticdispersion.30Acompound2,shouldimprovethepackingofcompound2inpossible2e−,2H+oxidationofcompound2intheSAMistheSAM.Figure1showsCVsoftheSAMofcompound2inshownasthereactionbetweenScheme2a,b.31aqueous0.1MHClO4.Aftertheirreversibleoxidationreaction,areversiblepairofpeaksIIa/IIcatE=0.00VandanirreversibleoxidationpeakIIIaatE=0.22VappearedintheCVinFigure1(cycles2−4),whichwerenotrecordedinthefirstanodichalfscan.ThepeakIIIadecreasedduringfurthercyclingwhilepeaksIIa/IIcretainedtheirintensityduringprolongedcycling.Forbrevity,wedenotethistransformedmonolayeras“activatedmono-layer”.TheappearanceofnewpeaksindicatedthatnewspecieswereformedintheSAMaftertheelectrochemicaloxidationinthefirstpositivehalf-cycle.Inanacidicenvironment,theBocgroupisindeedsusceptibletoanucleophilicattackbywaterandcanbecleavedofftheoxidizedDATmoiety(Scheme12,132c).Thesubsequentchemicalreactionstepsyieldunsubstituted,redoxactiveDATheadgroupsasshowninScheme2d,whichgivesrisetotheredoxtransitionbetweenFigure1.InitialfourCVsoftheSAMofcompound2backfilledwith−1thediamineandquinoneimineform(Scheme2d,e)andthe1-decanethiolonaAuelectrodein0.1MHClO4,v=0.05Vs.Numbersindicatethepotentialcycle.associatedpeaksIIa/IIcinFigure1.Thecurrentinthesubsequentpotentialcycleswasonlyabout10%ofthatintheinitialoxidation,indicatingthatonlyafractionofthemoleculesInthefirstpotentialcycle(curve1inFigure1),anofcompound2isconvertedtotheunsubstitutedDATirreversibleoxidationpeakIawasrecordedatapotentialE=accordingtoScheme2d.ThenatureofpeakIIIacouldnotbe0.38VvsHg|Hg2SO4.Thefollowingcyclesdiffereddeterminedwiththetoolsavailabletous.Tentatively,wesignificantlyfromthefirstone.ThesecondpotentialcycleassignthisirreversiblepeaktotheoxidationprocessesofDAT(curve2)containedthreeoxidationpeaksIIa,IIIa,andIaandgroupsthatunderwentconjugateadditionswithmolecularonereductionpeakIIc.ThecurrentofsignalIasignificantlyfragmentscleavedoffthemonolayerorwithneighboringdecreasedcomparedtocycle1andfinallydisappearedinthemoleculeswithinthemonolayerduringthepotentialcycle.thirdcycle.Thefullwidthathalf-maximum(fwhm)oftheDuetothechemicalsimilarityofgroupsinvolvedinthe28signalIais75mV.AccordingtoLaviron’stheory,thefwhmoxidationreactionsandthelowintensityofpeakIIIacomparedaScheme2.ReactionMechanismoftheElectrochemicalActivationofCompound2aThepeakassignmentreferstothevoltammograminFigure1.14625https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

3Langmuirpubs.acs.org/LangmuirArticletotheoxidationofthefullmonolayer(peakI),theunderthepeakIinFigure1asΓ=(2.5±0.5)×10−10molaa2identificationofthespeciesyieldingpeakIIIwasnotpossible.cm−2(SI-3).Thisvaluecorrespondstotheaverageareaof0.67aWewouldliketopointoutthatasimpleexposureofthenm2perredoxactivemolecule.Theestimatedcross-sectionalSAMofcompound2toaqueous0.1MHClOsolutionwasareaofthebulkyterminalgroupwascloseto0.80nm2when4thearomaticringwasparalleltothesubstrateand0.47nm2notsufficienttocleaveofftheBocgroupincontrasttotheexpectedreactivityofcompound2whendissolvedinsolutionwhenthearomaticringhadaperpendicularorientationtothe(SI-6).substrate(SI-4).TheexperimentalvalueisbetweenthetwoTheirreversibletransformationofhydroquinonemonoesterslimitingorientationsandindicatesatiltedorientationofthe12,13wasalsousedbyKimetal.fortheactivationofaSAM.InDATrings.Thisresultagreeswithanorderedanddenselytheircase,theSAMwasformedfrom12,12′-dithio-bis-packedmonolayerofcompound2ontheAusurface.Dueto[dodecanoicacid(4-hydroxyphenyl)ester].Afteroxidationofthecompactpacking,anymovementoftheterminalhead12thehydroquinoneatE=0.20VvsHg|Hg2SO4andgroupintheinitialmonolayerisrestricted.Inaddition,almostnucleophilicacylsubstitution,amonolayerofmercaptounde-allterminalgroupsparticipatedinthefirstoxidationreaction.canoicacidwasobtained,whichcouldbeagainactivatedThesurfaceconcentrationofredoxactivespeciesintheactivatedmonolayer[Γ=(2.6±0.5)×10−11molcm−2]waschemicallybyN-[3-(dimethylamino)propyl]-N′-ethylcarbodii-actmine(EDC)andN-hydroxysuccinimide.TheactivatedesterobtainedfromthechargeunderthepeakIIa.Thissurfacewasusedtobindamino-terminatedoligonucleotides.Intheconcentrationisabout10%ofthesurfaceconcentrationΓ2ofsecondexample,abiotin-terminatedSAMwasused,inwhichcompound2.ThestronglydecreasedpeakIainthesecondandbiotinwassubstitutedbyamonoesterifiedhydroquinone.AfterthirdpotentialcyclesinFigure1indicatesthatalmostallBoc-oxidationofthehydroquinonegroupandsubsequentcleavingprotectedDATmoleculesof2oftheinitialmonolayerwereofthequinoneandCO2,thebiotingroupbecameavailableforoxidizedbutonlyasmallfractionreactedtothenew13redoxactivemoiety.Mostlikely,themajorityofthemoleculesbindingtostreptavidin.Comparedtotheseexamples,compound2doesnotrequireanadditionalchemicalactivationof2weredeactivatedinotherreactionpathspossiblybythesteplikethemercaptododecanoicacidlayerinref12orfollow-upreactionbetweenneighboringmoleculesintheSAM.elaboratedsynthesislikethecagedbiotinusedtoformtheTherelativerateofsuchreactionsmaystronglydifferfrommonolayerinref13.analogousreactionsinorganicsolventsduetothehighSurprisingly,wewereonlyabletoactivatethebackfilledconcentrationofthemoleculesinaSAM(closetothatoftheSAMofcompound2inHClO4.Attemptstoperformsolidstate),aswellasduetotherelativeorientationandanalogousactivationsinothermineralacids,e.g.,in0.1Malignmentoftheindividualmoleculesinthelayer.ThefwhmH2SO4,failed(SI-2,FigureS1).TheirreversibleoxidationpeakofthepeakpairIIa/IIcequaled65and77mV(Figure2a),IawasobservedinFigureS1inthefirstpotentialcycleatE=respectively,indicatingthattwoelectronswereinvolvedintheredoxreactionII/II.A2e−,2H+reactionagreeswiththe0.41VvsHg|Hg2SO4,i.e.,atasimilarpotentialandwithaacsimilarshapeasinFigure1.However,nonewpeakpairIIa/IIcexpectationfortheredoxreactionofthediiminegroup38wasdevelopedin0.1MH2SO4.Thus,thefirstoxidation(Scheme2d,e)inanacidicaqueoussolution.TheformalreactionoftheterminalBoc-protectedDATgrouptookplacepotentialEads°’ofthissurface-boundredoxcoupleis(0.000±intheSAMinbothmineralacids.Furtherchemicalsteps,such0.004)VvsHg|Hg2SO4.Thekineticsofthisreactiondependsasthenucleophilicattackofwater,proceededdifferentlyinonthepHoftheelectrolytesolution,andsimilartothebothacidsolutionsandyieldedanew,redoxactiveDATquinone/hydroquinoneredoxcouple,theoverallreactionrate38derivativeofcompound2(causingthesurfaceredoxwaveIIa/iscontrolledbythechemicalstep,theprotonationreaction.II)onlyintheHClOsolution.TheClO−ionischaotropic,Anideallyfastsurface-boundredoxcoupleshouldhavenoc44weaklyhydrated,andhastheabilitytopenetrateintopeakseparationbetweentheanodicpeakIIaandtheassembliesofamphiphilicmolecules.32,33ThesepropertiesofcorrespondingcathodicpeakII.However,practically,allctheClO−iondisruptthepackingandconformationofthesurface-immobilizedsystemsshowanonvanishingpeak4hydrocarbonchainoflipidmoleculesinbilayerassemblies32separationevenforveryslowscanrates,whichmaybedueandmonolayerfilms.33−35Forexample,inalkanethioltouncompensatedsolutionresistanceandotherimperfections.29monolayersstudiedin0.1MHClO4electrolytesolution,atAccordingtoLaviron’srecommendations,theintrinsicpotentialsmorepositivethan0.1VvsHg|Hg2SO4,anincreasekineticlimitationsofaspecificsystemshouldbestudiedininthemonolayercapacitywasobserved.35InsituIRspectratherangewherethepeakseparationvariessystematicallywithclearlyindicatedthatthealkanethiolmonolayerbecomesscanrate.CVsoftheactivatedSAM(peakpairIIa/IIc)atpermeabletowaterandClO−ionsatpositivepotentials,variousscanratesvwereusedtodeterminethestandardrate429causingachangeinthetiltofthehydrocarbonchainintheconstantsks°ofthesurface-boundredoxsystem.Figure2bfilm.35,36Incontrast,theSO2−isakosmotropicanion,well-showstheplotoftheanodic(Ep,a)andcathodic(Ep,c)peak4hydrated,and,therefore,doesnotenterassembliesofpotentialsasafunctionoflog10(v).Thevalueslog10(va)andamphiphilicmolecules.Basedonourobservationandliteraturelog10(vc)atwhichtheregressionlinefortheCVssatisfiesthereports,wepostulatethatthepackingandorientationoftheconditionn(Ep,a−Ep,c)>0.2V(n=2)were−0.71824andterminalfunctionalgroupsincompound2intheSAMdifferin−0.64155fortheanodicandcathodicbranch,respectively.thetwoelectrolytesatpositivepotentialsandslightlyfavortheTheywererelatedtothetransfercoefficientαandks°byprogressofthereactionindifferentways.Oneshouldalsokeep◦◦inmindthatonlyca.10%ofcompound2wasconvertedtotheRTksRTks=νc,=νaDATderivativethatcausespeaksIIa/IIcinFigure1(videnFαnF(1−α)(2)37infra).Thesurfaceconcentrationofredoxactivecompound2(Γ)Solvingthecoupledeq2forαandk°yieldedk°=8s−1andα2ssintheactivatedSAMwascalculatedfromthetotalcharge=0.54.Themeasurementuncertaintyofthisprocedureshould14626https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

4Langmuirpubs.acs.org/LangmuirArticlecompatiblewithaquasi-reversiblesurface-boundcouplesuchastheDATgroup.CharacterizationofStructuralChangesduringtheActivationProcess.ExsitusurfacespectroscopictechniqueswereusedforstructuralanalysisoftheSAMbeforeandafterelectrochemicalactivation.ItmustbekeptinmindthattheoverallchargemeasuredduringtheirreversibleoxidationinpeakIofcurve1inFigure1isfullyconsistentwitha2e−aprocessofanentiremonolayer,whileonly10%oftheinitiallyoxidizedmoleculesformthenewredoxcouplethatcausesthenewquasi-reversiblepeakpairIIa/IIc.Consequently,surfacespectroscopictechniquesofallkindsdetectnotonlysignalsofthenewredoxcouplebutaredominatedbythemajorityofmoleculesthatweretransformedintoother,redox-inactiveproducts.XPSwasusedtocomparetheelementalcompositionandbindingstateoftheinitialandactivatedSAMs.ThesurveyXPspectrumrevealedthepresenceofAu,C,O,N,andSforbothSAMsamples,indicatingthepresenceofthemonolayerontheAusurface.High-resolutionXPspectraoftheC1s,O1s,N1s,andS2pregionsforbothSAMsareshowninFigure3.OnlyminorspectralchangeswereobservedaftertheelectrochemicalactivationoftheSAMofcompound2.TheS2pspectrashowastrongdoubletatthebindingenergiesEB=161.8eV(S2p3/2)andEB=162.9eV(S2p1/2),whichcorrespondedtotheSatomsingoldthiolate(Figure3).Thesecond,weakdoubletatEB=163.3eV(S2p3/2)andEB=Figure2.(a)FirstthreeCVsoftheelectrochemicallyactivatedSAM164.4eV(S2p1/2)originatedfromradiationdamageknownofcompound2onAuin0.1MHClO4inthepotentialrangeoftheforalkanethiolatesonAu.39−41Compound2containsthreeNpeaksII/II,v=0.05Vs−1.(b)PlotofpeakpotentialsEandEacp,ap,catoms,allofwhichcontributetotheN1slineatEB=399.5forsignalIIa/IIcvslog10(v).ThepeakpotentialsusedfortheeV.ThepositionoftheN1ssignalinFigure3didnotchange29regressionlinessatisfiedtheconditionn(Ep,a−Ep,c)>0.2VandareaftertheelectrochemicalactivationoftheSAM.TheC1splottedinblacksymbols.spectrumoftheinitialmonolayerwascomposedoffourcomponents,whichreflectdifferentenvironmentsofCatomsincompound2andin1-decanethiolintheSAM.AfterthenotbeunderestimatedbecausetheIRdropmayadditionallyactivation,asmalldecreaseinthecomponentforsp3Catomsincreasethesplittingbetweentheanodicandthecathodicat285.1eVwasobserved,whichagreeswiththelossoftheBocpeaksathighscanrates.Furthermore,threeparameters(ks°,α,moietyduringtheelectrochemicalactivation.TheO1sEads°’)areunknownhere.Nevertheless,therateconstantisphotoemissionsignalsinbothSAMshadtwocomponentsatFigure3.High-resolutionXPspectraoftheinitialSAMofcompound2backfilledwith1-decanethiolonAuandtheelectrochemicallyactivatedSAM.14627https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

5Langmuirpubs.acs.org/LangmuirArticleEB=531.7eV(C−O)andEB=533.3eV(COandpossiblyH2Otrappedinthemonolayeraftertreatmentinaqueoussolution).TheassignmentofthecomponentsissummarizedinTable1.Table1.AssignmentoftheSpectralComponentsinFigure3fwhm/eVfwhm/eVEB/eVinitialSAMactivatedSAMassignmentref284.41.071.06sp2Cinaromaticrings42285.11.491.60sp3Cinhydrocarbons42286.81.671.60C−Oester,C−N42289.01.671.60(O−CO,N−CO),C42Na290.90.503.49shakeup42399.51.631.55N1s(carbamate,amide)43531.71.531.67O1s(C−O)44533.31.531.31O1s(OC,H2O)44161.81.000.95S2p3/2thiolate45162.91.000.95S2p1/2thiolate46163.31.000.95S2p3/2thiolateafter45radiationdamage162.41.000.95S2p1/2thiolateafter46radiationdamageaToolowinintensity.TheatomicratiosnormalizedtosulfurwereC/O/N/S=16.5:4.2:1.7:1inthefreshlypreparedSAMand16.2:4.2:1.7:1Figure4.ExsituPMIRRAspectraoftheSAMofcompound2intheactivatedSAM.Theexperimentaldataofbothsamplesbackfilledwith1-decanethiolonAuindifferentstagesoftheagreewithinexperimentaluncertaintywiththeexpectedratiopreparation:(1)initialSAM,(2)SAMafteractivationatE=0.38V,forcompound2of14:3.5:1.5:1for(C28H43N3O7S2).The(3)reducedformoftheactivatedSAM,and(4)oxidizedformoftheslightlyhigheramountofcarbononthesurfacecanbeactivatedSAM.(a)RegionofCHstretchingvibrationsand(b)regionexplainedbythepresenceof1-decanethiolandpossiblysomeofthecarbonylandamidebandsaswellasthefingerprintregion.ubiquitoussurfacecontamination.Theamountof1-decanethiolusedforbackfillingtheSAMofcompound23008cm−1wasassignedtotheν(CH)inthearomaticring.seemstobeinsignificant.TheN/SatomicratiodoesnotshowThemethylgroupsintheBocandtheethylesterresiduesgaveanyincreaseintheScontentafteractivation,whichwouldberisetotwoν(CH)modesat2983and2972cm−1,as3expectedintheSAMofcompound2backfilledwitharespectively,aswellastheν(CH)modeat2875cm−1.Thes3significantcontentof1-decanethiol.Thisfindingisnotmethylenegroupscausedtheν(CH)modeat2930cm−1as2unexpectedbecausetheadsorptiontimeforcompound2andtheν(CH)modeat2855cm−1.Theywereassociateds2wassufficientlylongtoallowalmostcompletecoverage.Duetowiththealiphaticchainofthespacerincompound2and1-twoanchoratomspermoleculeincompound2,theexchangedecanthiol.TheelectrochemicalactivationoftheSAMleadstoreactionofcompound2againstdecanethiolatthesurfaceisasmalldecreaseintheintensitiesoftheν(CH)modeat3008notexpectedtobeveryefficient.Thisresultagreeswithcm−1andtheν(CH)modes(Figure4a,curve2).as3electrochemicalstudies,whichshowthattheaverageareaperFurthermore,themethylenestretchingmodeintheethylredoxactivemolecule2equals0.67nm2,correspondingtoaesterresiduesunderwentabathochromicshiftfrom2972todensepackingoftheBoc-protectedDATmoietiesin2962cm−1intheactivatedSAM(Figure4a,curves3and4),compound2withintheSAM.Intheabsenceofexchangeindicatingamorehydrophobicenvironmentoftheterminalreactions,theseleavelittlespaceforintegrating1-decanethiolmethylenegroupsintheactivatedSAMcomparedtotheinitialintotheSAM(probablyatpackingdefectsonly).TheXPSAMofcompound2.spectraconfirmedthepresenceofcompound2intheSAMThespectralregionof1800−1100cm−1showedlargeanddidnotshowanysignificantchangesaftertheactivationchangesaftertheelectrochemicalactivation(Figure4b).Forprocedure.ConsideringthefactthatthepeaksIIa/IIcinFiguretheinitialSAM,ofthetwoν(CO)stretchingmodes,one2awerecausedbyabout10%ofthemoleculesof2containedcenteredat1730cm−1wasassignedtotheesterandtheoneatintheinitialSAM,thisfindingwasnotverysurprising.1688cm−1totheamideandcarbamatemoietiesincompoundPMIRRASwasusedtoexaminestructuralchanges2.SharpIRabsorptionmodesoftheinitialSAMinthe1570−occurringintheSAM(Figure4).ThePMIRRAspectrum1400cm−1spectralregionarosefromthein-planestretchingoftheinitialSAMdisplaysseveralstrongandwell-resolvedIRmodesinthearomaticring.Thestrongν(C−O)modeatabsorptionmodes(curves1inFigure4).TheassignmentoflowerwavenumbersfortheinitialSAMismostlycausedbytheIRabsorptionmodesofcompound2SAMiscollectedincarbamate(1266cm−1)andestermoieties(1243cm−1).47TableS1.TheinitialSAMgavesixIRabsorptionmodesThespectraoftheactivatedSAMshowedanew,weakIRbetween3100and2750cm−1ascribedtotheCHstretchingabsorptionmodeat1665cm−1(Figure4b,spectrum2).Thismodes(Figure4a,curve1).TheweakIRabsorptionmodeatmodewasassignedtoν(CN),indicatingtheoxidationof14628https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

6Langmuirpubs.acs.org/LangmuirArticletheDATresidue.TheintensitiesofallotherIRabsorptionFluorescenceReportingofSAMConversion.DATandmodesoriginatingfromtheterminalfunctionalresidueDATderivativesshowfluorescencewhoseintensityanddrasticallydecreasedaftertheelectrochemicalactivationemissionrangedependsonthesubstitution.Consequently,(Figure4b,curves2−4).Thesespectralchangessuggestedfluorescenceimagingcanreportthestateofthesurfaceduringthattheelectrochemicalactivationofcompound2wasactivationandbinding.Figure5showsepifluorescenceimagesaccompaniedbyadramaticchangeintheorientationoftheplaneofthearomaticring.Ingeneral,theintegralIRabsorptionintensityoforderedSAMsdependsonthesurfaceconcentrationoftheabsorbinggroupandtheorientationoftherespectivetransitiondipolemoment(andthustheorientationoftheabsorberitself)withrespecttothesurface48,49normal.Foragivensurfaceconcentrationoftheabsorber,theintensityofanIRabsorptionreachesitsmaximumforparallelorientationofitstransitiondipolemomentandthesurfacenormal(i.e.,thedirectionoftheelectricfieldvectorresultingfromtheoverlapofincipientandreflectedwaves).InthedenselypackedSAM,itisexpectedthatthemoleculesofcompound2haveauniformorientation.Thus,anintensityFigure5.FluorescencemicroscopyofSAMscontainingcompound2decreaseofanIRabsorptionmodemayindicateeithera(a)beforeand(b)afterelectrochemicalactivation.Graphsbelowdecreaseinthesurfaceconcentrationofagivenfunctionalshowcrosssectionsalongthedirectiondenotedbythearrowsinthegrouporachangeintheorientationoftransitiondipoleupperimages.momentofthecharacteristicIRmodetoamoreperpendicularonewithrespecttothesurfacenormal.Inthepotentialrangerecordedunderidenticalopticalsettings.Thepatternwasoftheelectrochemicalactivationofcompound2,thethiolobtainedbymicrocontactprintingofneatcompound2inmoleculesareadsorbedonthegoldsurface.Reductivestripesof10μmwithacenter-to-centerspacingof20μm.ThedesorptionoccursonlyatE<−1.60VvsHg|Hg2SO4,whileinitiallyemptyareaswerebackfilledwith1-decanethiol,asoxidativedesorptionisasluggishprocess,whichinacidicdescribedintheExperimentalSection.Thefluorescencemediatakesplaceinthepotentialrangeof0.8−1.6VvsHg|imagesinFigure5showthatfluorescencewasobserved36,50Hg2SO4.Inaddition,theanalysisoftheXPspectra(Figuredespiteapossiblequenchingbythegoldsurfacethatleadsto3)oftheactivatedSAMindicatedonlyminorchangesintheanoveralllowintensity,whichwasonlyfaintlydistinguishedelementalcompositionandbindingenergiesoftheactivatedfromthebackgroundnoise.TheimagesalsoshowedhigherSAM.Thus,thethiolanchorgroupsremainedattachedtotheintensitiesintheprintedarea.Furthermore,thefluorescenceAusurfaceduringtheelectrochemicalactivation.Figure4aintensitywassignificantlyreducedaftertheelectrochemicalshowsthattheintensitiesofthemethylenemodesdonotactivation(Figure5bvsa).Thisobservationisinagreementchangesignificantlyaftertheelectrochemicalactivationof26withdatapublishedbyBuschbecketal.forcompound2compound2.Thus,thehydrocarbonchainofthespacerwithindissolvedindichloromethanewherethefluorescenceyieldtheSAMalsohadastableorientation,whichwasaffectedbydecreasedtoca.25%aftertheactivationstep.theactivationoftheBoc-protectedDATgrouptoasmallAdditionReactionontheSurface.Summarizingtheextentonly.AlargedecreaseintheintensityoftheIRconclusionoftheelectrochemicalandspectroscopiccharacter-absorptionmodesoccurredonlyforthemodesoftheterminalizations,we,firstofall,couldconfirmthattheinitialmonolayerfunctionalgroups,indicatingredox-state-dependentreorienta-containsonlyaminorfractionofthebackfiller1-decanethiol.tions,loss,ortransformationinthispartoftheSAM.ForFortheactivatedSAM,wefoundthat(i)theanchorgroupsofexample,intensitiesofthein-planeringstretchingmodestheinitialmonolayerremainedunchanged,(ii)theinitialdecreasedaftertheelectrochemicalactivation(by88%at1548monolayerwascompletelyconvertedtoprobablyseveralcm−1andby77%at1422cm−1).Thevectorsofthetransitiondifferentproducts,(iii)theelementalcompositionofthesedipolemomentsofthesemodeswerelocatedintheplanewithproductmixturesintheactivatedSAMunderwentonlyminorthearomaticring.InthefreshlypreparedSAM,largechanges,(iv)asmallfractionofcompound2oftheinitialintensitiesofthesemodesindicatedthatthearomaticringsmonolayerwastransformedtoanewredoxactivecompoundofcompound2tendedtowardaverticalorientationwiththatshowedverystable,wellbehaved,quasi-reversibleredoxrespecttotheAusurface.Aftertheactivation,theplaneofthereactionsat0.00VvsHg|Hg2SO4,and(v)theentirearomaticringstiltedtowardthegoldsurfaceprobablyenabledmonolayerafteractivationseemedquiterobustduringbythelossofafewrings.AcomparisonoftheO1sandN1sprolongedfurtherexperimentation.ThisstabilitywaslikelyaspectraoftheSAMbeforeandafteractivationindicatedthatresultofcovalentbondsbetweenneighboringmoleculeswithinmostoftheheteroatomswerestillcontainedinthefilm.theSAMthatoccurredwithoutdramaticchangesintheoverallAcomparisonofthePMIRRAspectrafromthereducedandelementalcomposition.Whiletheavailablespectroscopicoxidizedformsoftheactivatedfilm(Figure4b,spectrum3vsinformationdidnotallowacompletestructuralanalysisofspectrum4)revealedonlyminorchanges.IntheoxidizedformtheproductmixtureintheactivatedSAM,itsstabilityisaoftheactivatedSAM,asmallincreaseintheabsorbancefavorablepropertyforpotentialapplications.Weconsideraintensityataround1665cm−1wasobserved(Figure4b,deprotectedDATterminalgroupasdepictedinScheme2dspectrum3).Thisweakmodecouldarisefromtheν(CN)embeddedintoacross-linkedinertmatrixasthemostlikelymodeandconfirmthattheoxidationreactioninvolvesDAT.molecularmotifresponsiblefortheobservedstablepeakpair14629https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

7Langmuirpubs.acs.org/LangmuirArticleIIa/IIcinFigure1a.SystematicvariationoftheratiobetweenDATderivative2andchemicallydifferentdiluentsorbackfillersintheSAMisastartingpointforfurtherstudiestogainamoresystematicunderstandingofpossiblecross-linkingreactionswithinthemonolayer.Below,weconcentrateonexploitingtheredox-activityandpossiblebindingpropertiesofthesesmallfractionsoftheSAMconstituentsthatcausethepeakpairIIa/IIc.IfindeedthepeakpairIIa/IIcwascausedbytheoxidizedanddeprotectedDATderivativeinScheme2d,itcouldundergoconjugateadditionwithasoftnucleophile.Tobeabletounequivocallydetecttheproductofsuchareactionevenforthesmallfractionofmoleculesthatweretransformedinthisway,weselected3-(trifluoromethyl)anilineasthenucleophileandthemodeleffectormoleculemainlybecauseoftheclearidentificationofthebindingbyXPS,asfluorineisnotcontainedintheinitialmonolayer.ThereactionschemeisshowninSchemeS3inSI-7.TheactivatedmonolayerwasFigure7.F1sXPspectraofanactivatedSAMofcompound2onAusubjectedtopotentialcyclinginthepotentialrangeofpeaks(a)afterpotentialcyclingin0.1M3-(trifluoromethyl)anilinein0.1MIIa/IIcin0.1MHClO4.WhiletherewasnochangeoftheHClO4and(b)afterstoragein0.1M3-(trifluoromethyl)anilineinsignalinthebareelectrolyte(Figure2a),thepeakheight0.1MHClO4atopencircuitpotential.decreasedineachpotentialcycleaftertheadditionof0.1M3-(trifluoromethyl)anilinetotheelectrolyte(Figure6).First,theunspecificallyadsorbtotheactivatedSAMnorreactwithotherfunctionalgroupsthatwerepresentinthefilm.Instead,theoxidationoftheredoxcouplecausingpeaksIIa/IIcisanecessaryconditionforbinding.TheS/FatomicratioafterbindingtotheactivatedSAMequals1:0.15.Resultsoftheelectrochemicalactivationstudiesshowthatca.10%ofthemoleculesofcompound2undergoreactionsyieldingredoxactivespecies.Fromthisestimation,weneglecttheinsignificantlysmallcontributionofthebackfilled1-decanethioltotheS2psignalandslightattenuationoftheS2psignalbythemonolayer.Becauseca.10%ofcompound2isabletoreactwith3-(trifluoromethyl)aniline,theyieldoftheconjugateadditionatthesurfaceiscloseto100%.Theconjugateadditionwasalsotestedwith4-aminobenzoicacid,wherethereactionprogresswasmonitoredbyvoltammetry(SI-8).Figure6.First20cyclicvoltammogramsoftheactivatedSAMof2onAuinaqueous0.1M3-(trifluoromethyl)anilineand0.1MHClO4,v=0.02Vs−1.■CONCLUSIONSAnewliponicacid-conjugateofthetert-butyloxycarbonyl(Boc)-protected2,5-diaminoterephthalate(DAT)chromo-redoxactivemoleculeswithintheactivatedSAMwereoxidizedphorewassynthesized,structurallycharacterized,andusedtothequinonediimineandwerethenabletobind3-forself-assemblyonAusurfaces.TheresultingSAMs(trifluoromethyl)anilinefromtheelectrolytesolution.TheunderwentanirreversibleoxidationreactionfollowedbyaproductofthisreactionwasnotredoxactiveinthepotentialcleavageoftheBocgroupleadingtoanewquasi-reversiblerangebetween−0.2and+0.3V.Inthestronglyacidicaqueousredoxcoupleintheactivatedlayer,whichisprobablyDATandmilieu,onlyasmallfractionoftheanilinewasnotprotonateditscorrespondingtwo-electronoxidationproduct,adiiminesothattheprogressofthereactionwasretardedtoanextent(Scheme2d).Thisreactionhasalowyieldinadenselypackedthattheconversioncouldpreciselybeadjustedbymonitoringmonolayer.Ina0.1MHClO4solution,itiscloseto10%,thegradualdecreaseofthepeakheightforthesignalsIIa/IIcinwhereasthenewredoxcoupleisnotdetectedafterthesamesuccessiveCVs(Figure6).activationstepina0.1MH2SO4solution.ThenewredoxHigh-resolutionF1sspectrawererecordedfortheactivatedcouplewasstableforprolongedredoxcyclingin0.1MHClO4SAMafterpotentialcyclinginthepresenceof3-andtheentireactivatedSAMwasrobustduringfurther(trifluoromethyl)aniline(Figure7a).TheF1ssignalwasexperimentaltreatmentprobablyduetovariouscross-linkingsclearlyidentifieddespitethelowfractionofmoleculesofthebetweentheconstituents.TheoxidizedformofthenewcoupleinitialSAMthatwastransformedintheDAT-terminatedisabletobindnucleophiliceffectormoleculesfromsolutionbymoleculescausingpeaksIIa/IIc.Inacontrolexperiment,theaconjugateadditionreactionwithanalmostquantitativeactivatedSAMwasimmersedin3-(trifluoromethyl)anilineinconversion.SAMsfromcompound2canbeusedtobuild0.1MHClO4withoutpotentialcyclingfor15min.Thesensinglayersorintegratedfunctionalsystems.TheparticularsubsequentlyrecordedF1sspectruminFigure7bdidnotadvantagesaretherobustnessoftheactivatedlayerandtheshowanysignsofFpresentinthissample.Thisprovidedpossibilitytotriggerfurtherfunctionalizationsimplybystrongevidencethat3-(trifluoromethyl)anilinedoesnotoxidationofthiscouple(i.e.,withoutfurtherchemical14630https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

8Langmuirpubs.acs.org/LangmuirArticleactivation).TheprogressofthebindingreactionscanbeResearchTraininggroup“SensoryBiology”(DFGGRK1885)monitoredbyvoltammetryandpotentiallyalsobyfluorescenceandA.M.wasamemberofGraduateResearchTraininggroupspectroscopyoftheDATunit.Thelatterisalsoapplicableif“ChemicalBondActivation”(DFGGRK2226).Theacquis-suchlayers(withdifferentanchorgroups)areformedonitionoftheXPspectrometerattheUniversityofOldenburginsulatingsubstratesandoxidationisperformedchemically.waspartiallyfundedbytheDFGthroughitsMajorResearch■InstrumentationProgram(INST184/144-1FUGG).ASSOCIATEDCONTENT*sıSupportingInformation■REFERENCESTheSupportingInformationisavailablefreeofchargeat(1)Ulman,A.FormationandStructureofSelf-Assembledhttps://pubs.acs.org/doi/10.1021/acs.langmuir.0c02426.Monolayers.Chem.Rev.1996,96,1533−1554.Organicsynthesis(SI-1);oxidationoftheSAMin(2)Gooding,J.J.;Mearns,F.;Yang,W.;Liu,J.Self-Assembledsulfuricacid(SI-2);calculationofinterfacialconcen-Monolayersintothe21stCentury:RecentAdvancesandtration(SI-3);estimationofspacerequirementoftheApplications.Electroanalysis2003,15,81−96.DATgroup(SI-4);assignmentofIRabsorptionmodes(3)Bardea,A.;Katz,E.;Willner,I.BiosensorswithAmperometric(SI-5);exposureoftheSAMto0.1MHClOatopenDetectionofEnzymaticallyControlledpH-Changes.Electroanalysis4circuitpotential(SI-6);determinationoftheyieldofthe2000,12,731−735.(4)Beulen,M.W.J.;vanVeggel,F.C.J.M.;Reinhoudt,D.N.additionreactiontotheSAM(SI-7);additionalexampleCouplingofAcid-BaseandRedoxFunctionsinMixedSulfideforadditionreaction(SI-8);initialthreeCVsoftheMonolayersonGold.Chem.Commun.1999,35,503−504.SAMofcompound2backfilledwith1-decanethiolona(5)Hickman,J.J.;Ofer,D.;Laibinis,P.E.;Whitesides,G.M.;Auelectrodein0.1MHSO,v=0.05Vs−1(Figure24Wrighton,M.S.MolecularSelf-AssemblyofTwo-terminal,S1);schematicrepresentationofthecross-sectionalareaVoltammetricMicrosensorswithInternalReferences.Science1991,oftheDATmoietyofcompound2orientedparallelto252,688−691.thesubstratesurface(SchemeS1);andassignmentof(6)Yang,W.;Jaramillo,D.;Gooding,J.J.;Hibbert,D.B.;Zhang,R.;theIRabsorptionbandsoftheinitialSAMofcompoundWillett,G.D.;Fisher,K.J.Sub-pptDetectionLimitsforCopperIons2fromFigure4ofthemaintext(TableS1)(PDF)withGly-Gly-HisModifiedElectrodes.Chem.Commun.2001,1982−1983.(7)Yang,W.;Gooding,J.J.;Hibbert,D.B.RedoxVoltammetryof■AUTHORINFORMATIONSub-partsperBillionLevelsofCu2+atPolyaspartate-ModifiedGoldCorrespondingAuthorElectrodes.Analyst2001,126,1573−1577.GuntherWittstock−SchoolofMathematicsandScience,(8)Yang,W.;Gooding,J.J.;Hibbert,D.B.CharacterisationofgoldChemistryDepartment,CarlvonOssietzkyUniversityofelectrodesmodifiedwithself-assembledmonolayersofL-cysteineforOldenburg,D-26111Oldenburg,Germany;orcid.org/theadsorptivestrippinganalysisofcopper.J.Electroanal.Chem.2000,0000-0002-6884-5515;Email:wittstock@uol.de516,10−16.(9)Gooding,J.J.;Erokhin,P.;Losic,D.;Yang,W.;Policarpio,V.;AuthorsLiu,J.;Ho,F.M.;Situmorang,M.;Hibbert,D.B.;Shapter,J.G.AleksandraMarkovic−SchoolofMathematicsandScience,ParametersImportantinFabricatingEnzymeElectrodesUsingSelf-ChemistryDepartment,CarlvonOssietzkyUniversityofAssembledMonolayersofAlkanethiols.Anal.Sci.2001,17,3−9.Oldenburg,D-26111Oldenburg,Germany;orcid.org/(10)Gooding,J.J.;Hibbert,D.B.TheApplicationofAlkanethiolSelf-AssembledMonolayerstoEnzymeElectrodes.TrAC,Trends0000-0002-5706-0325Anal.Chem.1999,18,525−533.LeonBuschbeck−SchoolofMathematicsandScience,(11)Guiomar,A.J.;Guthrie,J.T.;Evans,S.D.UseofMixedSelf-ChemistryDepartment,CarlvonOssietzkyUniversityofAssembledMonolayersinaStudyoftheEffectoftheMicroenviron-Oldenburg,D-26111Oldenburg,GermanymentonImmobilizedGlucoseOxidase.Langmuir1999,15,1198−IzabellaBrand−SchoolofMathematicsandScience,1207.ChemistryDepartment,CarlvonOssietzkyUniversityof(12)Kim,K.;Yang,H.;Kim,E.;Han,Y.B.;Kim,Y.T.;Kang,S.H.;Oldenburg,D-26111Oldenburg,Germany;orcid.org/Kwak,J.ElectrochemicalDeprotectionforSite-SelectiveImmobiliza-0000-0002-7710-0021tionofBiomolecules.Langmuir2002,18,1460−1462.CarstenDosche−SchoolofMathematicsandScience,(13)Kim,K.;Yang,H.;Jon,S.;Kim,E.;Kwak,J.ProteinPatterningChemistryDepartment,CarlvonOssietzkyUniversityofBasedonElectrochemicalActivationofBioinactiveSurfaceswithOldenburg,D-26111Oldenburg,GermanyHydroquinone-CagedBiotin.J.Am.Chem.Soc.2004,126,15368−JensChristoffers−SchoolofMathematicsandScience,15369.(14)Choi,I.;Kim,Y.-K.;Min,D.-H.;Lee,S.;Yeo,W.-S.On-ChemistryDepartment,CarlvonOssietzkyUniversityofDemandElectrochemicalActivationoftheClickReactiononSelf-Oldenburg,D-26111Oldenburg,GermanyAssembledMonolayersonGoldPresentingMaskedAcetyleneCompletecontactinformationisavailableat:Groups.J.Am.Chem.Soc.2011,133,16718−16721.https://pubs.acs.org/10.1021/acs.langmuir.0c02426(15)Yousaf,M.N.;Mrksich,M.Diels−AlderReactionfortheSelectiveImmobilizationofProteintoElectroactiveSelf-AssembledNotesMonolayers.J.Am.Chem.Soc.1999,121,4286−4287.Theauthorsdeclarenocompetingfinancialinterest.(16)Dutta,D.;Pulsipher,A.;Yousaf,M.N.SelectiveTetheringofLigandsandProteinstoaMicrofluidicallyPatternedElectroactive■FluidLipidBilayerArray.Langmuir2010,26,9835−9841.ACKNOWLEDGMENTS(17)Shamsipur,M.;Kazemi,S.H.;Alizadeh,A.;Mousavi,M.F.;A.M.andL.B.gratefullyacknowledgesupportbyfellowshipsWorkentin,M.S.Self-AssembledMonolayersofaHydroquinone-throughtheGraduateProgramme“Nanoenergy”oftheTerminatedAlkanethiolontoGoldSurface.InterfacialElectro-MinistryforScienceandCultureoftheStateofLowerchemistryandMichael-AdditionReactionwithGlutathione.J.Saxony.Moreover,L.B.wasamemberoftheGraduateElectroanal.Chem.2007,610,218−226.14631https://dx.doi.org/10.1021/acs.langmuir.0c02426Langmuir2020,36,14623−14632

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