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SeniorScientists,Asemblon,Inc.

SelfAssembly:FromNaturetoTheLabInthe1980s,scientistsdiscoveredthatalkanethiolsspontaneouslyassembledonnoblemetals.Thisnewareaofscienceopenedthedoorstoasimplewayofcreatingsurfacesofvirtuallyanydesiredchemistrybyplacingagoldsubstrateintoamillimolarsolutionofanalkanethiolinethanol.Thisresultsincrystallinelikemonolayersformedonthemetalsurface,calledselfassembledmonolayers(SAMs).1

Overtheyears,themechanismoftheselfassemblyprocesshasbeenwellstudiedandelucidated.Researchershavefoundthatatypicalalkanethiolmonolayerformsa(33)R30structure2ongoldwiththethiolchainstiltedapproximately30degreesfromthesurfacenormal.36Theexactstructureofthemonolayerdependsonthechemistryofthechain.

Selfassemblyformsthebasisformanynaturalprocessesincludingproteinfolding,DNAtranscribingandhybridization,andtheformationofcellmembranes.Theprocessofselfassemblyinnatureisgovernedbyinterandintramolecularforcesthatdrivethemoleculesintoastable,lowenergystate.Theseforcesincludehydrogenbonding,electrostaticinteractions,hydrophobicinteractions,andvanderWaalsforces.

Aswithselfassemblyinnature,thereareseveraldrivingforcesfortheassemblyofalkanethiolsontonoblemetalsurfaces.Thefirstistheaffinityofsulfurforthegoldsurface.Researchershavefoundthatthesulfurgoldinteractionisontheorderof45kcal/mol,3formingastable,semicovalentbondincomparison,theCCbondstrengthis~83kcal/mol.

Thenextdrivingforceforassemblyisthehydrophobic,vanderWaalsinteractionsbetweenthemethylenecarbonsonthealkanechains.Foralkanethiolmonolayers,thisinteractioncausesthethiolchainstotiltinordertomaximizetheinteractionbetweenthechainsandlowertheoverallsurfaceenergy.Awellorderedmonolayerformsfromanalkanechainofatleast10carbons.Withcarbonchainsofthislength,hydrophobicinteractionsbetweenthechainscanovercomethemoleculesrotationaldegreesoffreedom.6,7

AsimplealkanethiolmoleculeisshowninFigure1.(nextpage)Analkanethiolcanbethoughtofascontaining3parts:asulfurbindinggroupforattachmenttoanoblemetalsurface,aspacerchain(typicallymadeupofmethylenegroups,(CH2)n),andafunctionalheadgroup.Asmentionedabove,thesulfuratomandthecarbonsinthemethylenegroupsactasthemaindrivingforcesforassemblyofthealkanethiols.Theheadgroupthenprovidesaplatformwhereanydesiredgroupcanbeusedtoproducesurfacesofeffectivelyanytypeofchemistry.

Figure1.Schematicdiagramofathiolmolecule.

Thesulfurgrouplinksthemoleculetothegoldsurface.Theheadgroupcanbedesignedtoprovidevirtuallyanysurfacechemistry,bindingcapacity,orproperty.

Bysimplychangingtheheadgroup,asurfacecanbecreatedthatishydrophobic(methylheadgroup),hydrophilic(hydroxylorcarboxylheadgroup),proteinresistant(ethlyleneglycolheadgroup),orallowschemicalbinding(NTA,azide,carboxyl,amineheadgroups).Thisenablesaresearchertocustomdesignasurfacetoserveanydesiredfunction.

SelfAssembly:PurityMattersSAMsaretypicallymadefroma1mMsolutionofthedesiredalkanethiolinethanol.Initialmonolayerformationisveryfast,withmonolayercoveragebeingachievedwithinsecondstominutes.Thisinitiallyformedmonolayerisnotwellorderedandcontainsmanygauchedefectswithinthechains.Overtime,thelayersbecomemoreorderedandwellpacked.Reportedassemblytimesvarythroughouttheliterature,buttypicallyare12hoursto2days.

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CategoriesAdsorptionElectrochemicalanalysisElectronicsMassspectrometryMicroarrayAnalysisNucleicacidhybridizationPAGEPurificationSpectroscopy

SelfAssembledMonolayers:AdvantagesofPureAlkanethiolsBy:Dr.MaxiBoeckl,Dr.DanielGraham,MaterialMatters2006,1.2,3.

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assemblytimesvarythroughouttheliterature,buttypicallyare12hoursto2days.

Theformationofawellassembledmonolayercandependonthepurityofthealkanethiolbeingused.Thepresenceofevenlowlevelsofcontaminantscanresultinadisordered,nonidealmonolayer.Manytypicalimpuritiesinthiolcompoundsarethiolatedprecursormoleculesthatwerenotseparatedduringthepurificationprocess.Theseprecursormoleculescaneitherbestraightchainalkanethiolsthatlacktheheadgroupsofthefinalproduct,ortheycanbemoleculesusedtointroducethethiolfunctionalgrouptoaprecursormolecule(suchasthioaceticacid).Sincethesecompoundsalsocontainthiolfunctionalities,theycancompetewiththealkanethiolofinterestforavailablesurfacelocations.Competitiveadsorptioncanbeparticularlyproblematicforalkanethiolswithcomplexorbulkyheadgroups.Bulkyheadgroupscanreducethedrivingforceforassemblybydisruptingtheclosepackedarrangementofthealkanechains.Withthisreduceddrivingforceforassembly,thestraightchainorsmallthiolcontaminantscanoutcompetethealkanethiolofinterestonthesurface.

Theseeffectswerenoticedinearlyalkanethiolselfassemblyresearchandseveraldetailedstudieswereperformedonthepreferentialadsorptionofonealkanethiolversusanother.Adsorptionfromadilutesolutioninethanolofamixtureoftwoalkanethiols,onewithapolartheotherwithanonpolarheadgroup,showedthatadsorptionofthealkanethiolwiththenonpolarheadgroupwasfavored.7Bainetal.alsonoticedthatalkanethiolswithequivalentheadgroups,butlongerchainsadsorbedpreferentiallyovershorteralkanethiols.4Whenonetriestointentionallyobtainamixedmonolayer,wheretwoormoreheadgroupsarepresentatadesiredratio,itisverydifficulttopredicttherelativesurfaceconcentrationfromthesolutionconcentrationandacalibrationcurveistypicallynecessary.4AnexampleisgivenbyNelsonetal.,whostudiedmixedmonolayersofbiotinylatedalkanethiolmixedwitheithermercaptohexadecaneor(1mercapto11undecyl)tetra(ethyleneglycol)andcorrelatedthesolutionconcentrationtothesurfaceconcentrationusingelectronspectroscopyforchemicalanalysis(ESCA)andTimeofFlightSecondaryMassSpectrometry(ToFSIMS).8Thisstudydemonstratedhowthesurfaceandsolutioncompositioninmixedmonolayerscanvarysignificantlyandwhyacalibrationcurveisnecessarywhenengineeringmixedmonolayersurfaces.

AsademonstrationoftheeffectsofthiolimpuritiesinSAMformation,Table1showsESCAsurfacecompositiondatafromaseriesofSAMspreparedfromthiolsolutionsthatwerepurposelyspikedwithimpurities.Forthisexperiment,solutionsof(1mercapto11undecyl)tetra(ethyleneglycol)(PEG4thiol,AldrichProd.No.674508)weremixedwith0%,1%and10%(v/v)thioaceticacid(TAA).Allmonolayerswerepreparedfrom1mMsolutionsinabsoluteethanol.Thepercentagesshownforthevariousthiolcomponentsarevolumepercentages.

Table1.ESCAcompositiondatafromPEG4thiolmonolayerswithandwithoutimpurities.Alldataisshowninatomicpercent.Twoanalysisspotsareshownforeachsample.

PEG4thiol=(1mercapto11undecyl)tetra(ethyleneglycol)AldrichProd.No.674508TAA=thioaceticacid

ThefirstobservationnotedfromTable1istheincreaseintheatomicpercentageofgoldwithincreasingamountofthioaceticacidintheassemblysolution.Thegoldsignalcomesfromthegoldsubstrateunderlyingthemonolayer.Thisgoldsignalisattenuatedbytheoverlyingmonolayerfilm.Athinnerormoredisorderedoverlayercausesadecreaseinsignalattenuation(increaseinthegoldsignal).Thissuggeststhattheadditionofeven1%thioaceticacidtotheassemblysolutioncausedamarkeddecreaseinthemonolayerthickness.Thioaceticacidisasmallmoleculewhichifinsertedintothemonolayerwoulddisruptthelocalmonolayerordercausingthenoteddecreaseinthelayerthickness.

Table2showsthedatafromTable1rescaledwithoutthegoldsignal.Thisallowscomparisonoftheatomicpercentageswiththatexpectedbasedonthesolutionmixtureatomiccomposition.AsseeninTable2,theexperimentalvaluescomparewellwiththecalculatedvalues.Theatomicpercentageofsulfuristypicallyobservedtobelowerthanexpectedduetoattenuationbytheoverlyingmonolayer.Thisisthecaseforthe100%PEG4thiol.Thefactthatthesulfurdatafromtheothertwomonolayersisclosetothepredictednumbers,suggeststhatthelayersaredisordered(consistentwiththegoldatomicpercentages)orthatthereisahighamountofthioaceticacidonthesurface(whichhasahighrelativepercentageofsulfurvs.theotheratoms).

Table2.ESCAcompositiondatafromPEG4thiolmonolayerswithandwithoutimpuritiesrescaledwithoutgoldsignal.Alldataisshowninatomicpercent.Twoanalysisspotsareshownforeachsample.Thevaluesinboldarethecalculatedatomicpercentagesbasedonthesolutionmixtureratioofthecompoundsused.

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PEG4thiol=(1mercapto11undecyl)tetra(ethyleneglycol)AldrichProd.No.674508TAA=thioaceticacid

ItisinterestingtonotethatwhenlookingatTable2,onemightthinkthatsincetherelativepercentagesofcarbonandoxygendonotchangesignificantlyfromsampletosample,thatthesurfacesarestillpurePEG4thiol.Itisonlythehighpercentageofsulfurthatsetsthesamplesapartandindicatesthatanimpurityislikelypresent.

Overall,thisdatademonstratesthatevensmallamountsofcontaminantsinasolutioncancausesignificantdifferencesinthemonolayercompositionandstructure.Thebestwaytoavoidtheseproblemsistousemoleculeswiththehighestpossiblepurities.

ResultssimilartothesewereseenwhenthioaceticacidwaspresentinPEG3andPEG6thiolsamples.Bothofthesesamplesshowedahigherthanexpectedgoldandsulfursignalsindicatingthelayersweredisorderedandasulfurcontaminantwaspresent.Themostimportantpropertyofpoly(ethyleneglycol)alkanethiols(PEGnthiols)istheabilitytoformanonfoulingselfassembledmonolayer,butthepresenceofjustsmallamountsofcarboxylicacidterminatedPEGnthiols(possiblyintroducedfrompoly(ethyleneglycol)startingmaterial)leadstosignificantproteinadsorption,makingthesurfacemorefoulingthanbaregolditself,asshownbyRobertsetal.9

TheinterferenceofthiolcontaminationwithmonolayerformationhasalsobeenreportedfortheassemblyofthiolatedDNAongold.10Itwasfoundthatthepresenceofathiolcontaminant,dithiothreitol,impededtheassemblyofthethiolatedDNAontothegoldsurface.TheauthorsshowedthatwhencontaminatedthiolatedDNAwasusedfortheassembly,theatomicpercentagesofcharacteristicDNAelements(P,N)didnotincreasewithincreasingassemblytimesastheydidwithpurersamples.Increasedassemblytimesinsteadresultedinincreasedamountsofthecontaminantmoleculesonthesurface.SamplespreparedfromrelativelypureDNAshowedlogicaltrendsfortheDNAelementsinatimeseriesassembly.

SelfAssembly:UnlimitedOpportunitiesSAMshavebeenusedforstudiesandapplicationsinmanyareas.Afewexamplesincludesurfacewetting,nonfoulingproperty,electrochemistry,surfacepassivation,proteinbinding,DNAassembly,corrosionresistance,biologicalarrays,cellinteractions,andmolecularelectronics.Theseandothertopicshavebeensummarizedinpreviousreviewarticles.1113

SAMshavetrulyopenedthedoorstowarddirectsurfaceengineering.Onlytheimaginationlimitsthepossibilitiesavailabletotheinterestedresearcher.Figure2showsschematicdiagramsofseveralpossibleapplicationsforalkanethiolmonolayers.ThesetypesofapplicationswilllikelyrelyonhighpurityalkanethiolswithheadgroupssuchasthoselistedinTable3.

Figure2.Possibleapplicationsforalkanethiolmonolayers.A)NonfoulingsurfacesB)SAMswithspecificbindingreceptorsC)CellsupportsfornativecellgrowthandstudiesD)MolecularelectronicsE)MicroarraysF)Separations.

Table3:SomeheadgroupexamplesusefulfortheapplicationsshowninFigure2.

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