The influence of the surface morphologies of Langmuir Blodgett (LB) thin films of porphyrins on their gas sensing properties

ContentslistsavailableatScienceDirect

Sensors

and

Actuators

B:

Chemical

jou rn a l h o m e pag e :w w w . e l s e v i e r . c o m / l o c a t e / s n b

The

influence

of

the

surface

morphologies

of

Langmuir

Blodgett

(LB)

thin

films

of

porphyrins

on

their

gas

sensing

properties

D.

C¸

aycı

a

_,

_S.G.

_Stanciu

b

_{, ˙I.}

_C¸

_apan

a,∗

_,

_M.

_Erdo˘gan

a

_,

_B.

_Güner

a

_,

_R.

_Hristu

b

_,

_G.A.

_Stanciu

b a_{BalikesirUniversity,ScienceandArtsFaculty,PhysicsDepartment,10100Balikesir,Turkey}

b_{CenterofMicroscopy-MicroanalysisandInformationProcessing,University‘Politehnica’ofBucharest,313SplaiulIndependenteti,060032Bucharest,Romania}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received24August2010

Receivedinrevisedform30April2011 Accepted11May2011

Available online 18 May 2011 Keywords:

LangmuirBlodgett(LB)thinfilms Porphyrins

QuartzCrystalMicrobalance(QCM) Gassensingmechanism

AtomicForceMicroscopy(AFM)

a

b

s

t

r

a

c

t

Inthiswork,2,3,7,8,12,13,17,18-Octaethyl-21H,23H-porphine(OEP)initsfreebaseformandmetalated withiron(III)chloride(FeOEP),magnesium(II)(MgOEP)andcobalt(II)(CoOEP)havebeenusedto fabri-cateLangmuir–Blodgett(LB)thinfilms.Usingthesurfacepressure–surfacearea(˘–A)isothermgraphs optimumconditionsforthinfilmdepositionhavebeendeterminedandbychangingthedeposition parametersvariousthinfilmshavebeendeposited.QuartzCrystalMicrobalance(QCM)systemwasused toinvestigatetheirgassensingperformancesduringexposuretoVolatileOrganicCompounds(VOCs) includingchloroform,benzeneandtoluene.ThesurfacepropertieshavebeeninvestigatedusingAtomic ForceMicroscopy(AFM)andanalyzedtogetherwiththeQCMresultstounderstandtheeffectofthe sur-facepropertiesongassensingmechanism.Itisobservedthatlargersurfacearealeadstohigherresponse ingassensingapplicationsintermsofresonancefrequencychange.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Theporphyrincompoundswhichareknowntobehighly sen-sitivein gassensingapplicationsareof great interestin recent yearsindevelopinggassensingtechnology.Theirperformancein detectinginorganiccompoundshasalreadybeenproven[1–4].The detectionof organictoxic compoundsis ofcommercialinterest becauseoftheirtoxicityand frequentusein theindustry[5,6]. Theinvestigationshavebeenperformedbyusingdifferent tech-niquesincludingelectrical[7–9],gravimetric[10]oroptical[11,12] measurementofthegassensingresponse.

The Langmuir Blodgett (LB) thin film fabrication technique whichisanexcellentmethodinfabricatingwell-ordered mono-layerormultilayeredthinfilmsisusedtoinvestigategassensing performancesofthinfilms.LBthinfilmdepositiontechniqueallows thefabricationofvariousthinfilmsbychangingthethinfilm fabri-cationconditionssuchasdepositiondipperspeed,surfacepressure, dippermovementdirection[13].

In gas sensingresearch the investigationof thegas sensing mechanismisstill aphenomenontoclarify.Astothegas sens-ing mechanism for porphyrin and its derivatives’ thin films, a numberofmechanismshavebeenproposedincluding␲–␲* inter-actionsbetweentheporphyrinmacrocycles[7,14],affinitybetween porphyrinlayersandtheinteractingvapourspecies[10,15],

pho-∗ Correspondingauthor.Tel.:+902666121000/1301;fax:+902666121215. E-mailaddresses:inci.capan@gmail.com,ibasaran@balikesir.edu.tr(˙I.C¸apan).

toassisted ligandexchange route [12], the changes in van der Waals force betweenthe film and theadsorbed gasmolecules [8].Astheanalyzedtoxicgasfirstlyinteractswiththesurfaceof thethinfilm,themorphologyofthethinfilmsurfacemayplay animportantroleinthegassensingmechanism.However,only a limitednumber ofresearcheshavebeenconductedto under-standtheinfluenceofthesurfacepropertiesof thesethin films [16–19].

Thisworkisastudyoffourdifferentporphyrins:(OEP)inits freebaseform,whenmetalatedwithiron(III)chloride(FeOEP), withmagnesium(MgOEP)andwithcobalt(CoOEP).Theselected porphyrinswereusedtofabricatesolidstateLangmiur–Blodgett (LB) thin filmswhich were subsequently exposed to saturated organicvapors.Tobetterunderstandthesensingmechanismand toobtaindifferentarchitectures,thethinfilms’fabricationhasbeen performedwithdifferent surfacepressure values. AtomicForce Microscopy(AFM)andQuartzCrystalMicrobalance(QCM) tech-niquehavebeenemployedtoexplorethesurfacepropertiesand gassensingpropertiestowardssaturatedvaporsofselectedvolatile organicanalytesrespectively.Theselectedanalytesarecommonly produced in industrial processes.Benzene is formed fromboth naturalprocessesandhumanactivities.Itisproducedfrom volca-noesandforestfires,andisanaturalpartofcrudeoil,gasoline, and cigarette smoke. Toluene is a toxic ingredient in solvents, paints,andotherhouseholdproducts.Chloroformcancontribute totheformationofharmfulground-levelozoneandmajorrelease of chloroform happens asa result of itsproduction and usein thechemicals industry.Smallerreleasesresult fromthe

chlori-0925-4005/$–seefrontmatter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.05.033

porp2,porp3andporp4respectively.Thechemicalstructuresof theseporphyrinsweregiveninapreviouspublication[20].Other chemicals,usedingassensingexperiments,chloroform,toluene andbenzenewerealsopurchased fromSigmaAldrichandused withoutfurtherpurification.

2.2. Thinfilmfabrication

ForthefabricationoftheLBthinfilmsAlternateLayerNima622 modelLBfilmtroughprovidedwithafilterpaperWilhemlybalance hasbeenemployed.The surfacepressure–area (˘–A)isotherm graphswere recorded to determinethe surface pressure value whichwillbeheldconstantduringthetransferofthemonolayers onthewatersurfaceontothesolidsubstrates.Porphyrinsolutions thathavebeenpreparedbydissolvingtheporphyrinsinchloroform withaconcentrationof0.2mgml−1havebeenspreadontothe pre-cleanedwatersubphaseusingaHamiltonsyringeallowing10min forthesolventtoevaporate.Theisothermswererecordedwiththe compressionspeedof30cm2_min−1_{atroomtemperature.Y-type}

LBfilmshavebeenfabricatedatconstantsurfacepressurevalues changingbetween12.5and17.5mNm−1.Transferspeedsforboth downstrokeand upstrokedepositionwere25mmmin−1 witha transferratiovalueofapproximately0.90.11layersofthinfilms havebeendepositedontoquartzcrystalsubstratesforgassensing experimentsandontoglasssubstratesforAFMmeasurementsfor allfourkindsofporphyrins.

2.3. Gassensingexperiments

QCMsystemmeasurestheresonancefrequencyofaquartz crys-talwhichissandwichedbetweentwometalelectrodesandinserted intoanelectronicunitthatenablestorecordtheresonance fre-quencyvalueatamasschange.InQCMsystemamasschangerange ofnanogramscanbedetectedwhichleadstoagravimetric sen-sitivity.Thepiezoelectricbehaviourofthequartzcrystalwasfirst describedbySauerbrey[21]andtheresonancefrequencyshift(f) onaquartzcrystalagainstamasschangeperunitarea(m)isgiven by: f= −2f 2 0 1/2q 1/2q A m (1)

wheref0 istheresonantfrequency ofthefundamentalmodeof

thecrystal,A(1.13cm2_{)isthepiezoelectricallyactivearea,} q is

thedensityofquartz(2.648g/cm3_)and

qistheshearmodulusof

quartz(2.947_×1011_dyncm2_).

Anin-housemadeQCMhasbeenemployedtoinvestigatethe gassensingpropertiesoftheporphyrinthinfilmsanditsschematic

Fig.1. ThenumberofLBdepositioncyclesofporp2thinfilmataconstantsurface pressurevalueof15mNm−1_.

illustrationcanbefoundelsewhere[20].TheAT-cutquartz crys-talwithanominalfrequencyof3MHzcoatedwiththeporphyrin thinfilmwasinsertedintotheelectronicunitandtheresonance frequency wasmonitored during exposuretotoxic gasand its recovery.Dilutedamountsofsaturatedtoxicgasrangingbetween 20%and100%wereintroducedintothegascellfor2minfollowed byflushingwithdryairforanother2min.Allmeasurementswere performedatroomtemperature.

2.4. AtomicForceMicroscopy(AFM)measurementsandSPIP programme

Thesurfaceinvestigationsweremadebyusinganatomicforce microscope,Quesant-350.Themicroscopeworkedinthecontact mode,thetiphadaheightof20_␮m,radiuslessthan20nm,and coneanglelessthan30◦attheapex.Atypicalforceconstantofthe cantileveris0.15Nm−1andtypicalresonancefrequencyis12kHz. Thescanratewas1Hz.

TheimageanalysiswasperformedwiththeuseofScanning ProbeImageProcessor(SPIPTM_{).SPIPisacomprehensivesoftware}

platformcontainingmanyanalyticalandvisualizationtoolswhich canbeusedinordertoextractandhighlighttypicaluseful informa-tionfromimagescollectedbyvariousscanningprobemicroscopy techniques.SPIPisacommonlyusedtoolforAFMbasedhigh pre-cisionsurfacecharacterization.

3. Resultsanddiscussion

3.1. Fabricationofthethinfilms

To fabricate the thin films of porphyrins, LB thin film fab-ricationparameters includingthedipperspeed and thesurface pressurevaluethatwillbeheldconstant duringthefabrication needtobedetermined.Theisothermgraphsoftheporphyrinthin filmsshowedthatthesurfacepressurevalueswhichform regu-larmonolayersonthewatersubphasenamelysolidphasewere 12.5–17.5mNm−1thathavebeenalreadyfoundinpreviousstudies [22,23].ByvaryingthatvalueLBthinfilmswithdifferenttextures canbeobtained.InthisstudyLBthinfilmswerefabricatedat con-stantsurfacepressurevaluesof12.5,15.0and17.5mNm−1.InFig.1 thedepositiongraphwhichgivesinformationonthereductionof thesurfaceareaofthemonolayerversusdipperpositionduring thedepositionoftheLBlayersontothesubstratesareplottedfor porp2thinfilmat15mNm−1constantsurfacepressure.The aver-agereductionofthesurfaceareawassimilarinallcyclesindicating

Fig.2. (a)Thekineticgraphintermsoffrequencyshiftversustimeinexposure tochloroformvapour.(b)Calibrationgraphforthechloroformexposureinallthin films.

thatsimilaramountofmoleculesweredepositedontothequartz ineachlayerdeposition.

3.2. QCMresultsandgassensingproperties

InFig.2(a)thekineticgasresponseoftheporphyrinthinfilmsin exposuretodifferentconcentrationsofchloroformvaporinterms of the changein resonance frequency (f) withtime is given. Theanalytegaswasinjectedintothegascellfor2minatdiluted amountsofsaturatedvaporconcentrationfollowedby2min recov-erywithdryair.Theconcentrationofanalytevaporwasarranged as 20%, 40%, 60%,80% diluted saturated gases with dry air for consequentexposuresrespectivelyandatthelastcyclesaturated concentrationofanalytevaporwasinjected.Thedecreasein res-onancefrequencyofthequartzcrystalresonatorisproportional totheincreaseinmassduetopresenceofvaporadsorbedonits surface.Allporphyrinthinfilmsgiveveryfastresponsetothe ana-lytevaporchloroformintherangeofafewseconds.Thethinfilms recoverthemselvestotheirinitialvalues aftertheyareflushed withdryairwhichmeansthattheycanbeusedmultipletimes. Fig.2(b)showsthedependenceoftheresonancefrequencychange (f)tothepercentofsaturatedvapor,namelycalibrationgraph.It isclearthatallthinfilmshavealinearlyincreasingresponsewith

Fig.3. Calibrationgraphforthechloroformexposureforporp2thinfilm fabri-catedindifferentfabricationsurfacepressurevaluesof12.5mNm−1_,15mNm−1

and17.5mNm−1_.

theincreasingconcentrationofthevapor.Threedifferentvapors: benzene,tolueneandchloroformareusedandsimilarresultshave beenrecorded.AlloftheresultshavebeenlistedinTable1.

Themainaimofthisworkistoverifythedependenceofgas sensorperformanceofthethinfilmsontheirsurfaceproperties. Tobetterunderstandthisphenomenonthinfilmshavebeen fabri-catedusingdifferentvaluesoffabricatingsurfacepressureranging between12.5mNm−1and17.5mNm−1formingdifferentsurface textures.Thecalibrationcurvesforporp2thinfilmfabricatedat sur-facepressurevaluesof12.5mNm−1,15mNm−1and17.5mNm−1 havebeengivenforchloroformexposureinFig.3.Thecomplete datacanbeobservedinTable1.AsitisseenbothinTable1and Fig.3themaximumresponseofthethinfilmsduetotheexposure hasbeenrecordedforthethinfilmsfabricatedatasurface pres-surevalueof15mNm−1.Additionally,highestresponsehasbeen observedforchloroformvaporamongthetestedanalytes.

3.3. AtomicForceMicroscopy(AFM)results

In Figs. 4–7, theAFM imagesof11 layers of porphyrinthin filmsfabricatedontoglasssubstratesatdifferentfabrication sur-facepressurevalueshavebeengiven.Asseenfromtheimagesthe thinfilmsfabricatedatdifferentdepositionsurfacepressurevalues havedifferenttexturesontheirsurfaces.

Itiswellknownthattheparametersthataffectthestrength ofgasinteractionoftheporphyrinthinfilmsincludecentralmetal atom[2,6,20,24]andthemolecularsizeoftheanalytegasmolecules [5].Anotherpossiblemechanismaffectingtheinteractionbetween thegasmoleculesandthin filmsmaybethesurface morpholo-giesofthethinfilmsasappropriatedthattheinteractionofthegas moleculesfirstlystartswiththeinteractionoftheactiveadsorption sitesonthesurfaceofthethinfilmandcontinueswithdiffusion intothebulk.Thismakessurfacemorphologyimportantinterms of sensing capabilities. In order tounderstand therelationship betweenthegasinteractionmechanismsandthesurface proper-tiesofthinfilms,thesurfacepropertieshavebeenanalyzedusing theSPIPTM_program.

Thesurfaceroughnesscharacteristicshavebeenanalyzedby usingoneamplitudeparameter,rootmeansquareroughnessSq

andonehybridparameterSdr–thesurfacearearatiothatexpresses

Porp3 12.5 Benzene 11.2 13.2 18.4 21.3 32.9 Toluene 6.7 8.1 12.4 15.05 23.3 Chloroform 10.8 19.04 31.2 35.3 45.3 15 Benzene 7.8 9.8 15.8 20.24 36.7 Toluene 8.5 15.2 21.9 25.9 34.6 Chloroform 13.8 24.9 45.6 58.6 94.9 17.5 Benzene 6.3 10.15 15.05 13.8 10.9 Toluene 6.4 10.8 11.4 8.6 6.9 Chloroform 18.6 24.9 40.5 54.5 70.8 Porp4 12.5 Benzene 10.4 13.2 18.11 21.5 32.2 Toluene 8.3 16.4 23.1 26.4 45.2 Chloroform 16.9 37.9 62.1 91.6 112.3 15 Benzene 4.9 8.6 13.9 15.3 28.06 Toluene 1.9 3.5 6.3 10.2 17.7 Chloroform 14.2 18.2 26.5 42.3 74.9 17.5 Benzene 8.1 11.4 18.3 20.2 36.4 Toluene 3.7 7.3 12.1 15.7 29.4 Chloroform 16.9 28.7 50.7 67 109.2

Fig.5. 2DAFMimagesofporp2thinfilmfabricatedatafabricationdepositionpressureof(a)12.5mNm−1_,(b)15mNm−1_{and(c)17.5mNm}−1_{atascaleof10␮m×10␮m.}

andtheareaoftheflatx,yplane.ForarectangularimageM× N,

theparametersaredefinedas:

Sq=









1 MN M−1



k=0 N−1



l=0 [z(xk,yl)−]2 (2)

whereisthemeanheightwhichisexpressedbelow.

= 1 MN M−1



k=0 N−1



l=0 z(xk,yl) (3)

Sdr–thesurfacearearatioisexpressedas

Sdr=



M−2 k=0



N−2 l=0Akl



−(M−1)(N−1)dxdy (M−1)(N−1)dxdy 100% (4)

whereAklisgivenbelow.

Akl= 1 4

dy2_+(z(x k,yl)−z(xk,yl+1))2 +

dy2_+(z(x k+1,yl)−z(xk+1,yl+1))2



×

dx2_+(z(x k,yl)−z(xk+1,yl))2 +

dx2_+(z(x k,yl+1)−z(xk+1,yl+1))2



(5)

TheSdrvalues,S3Avalueswhichistherealareaofthesample

andrms(rootmeansquareroughness)valuesofthesurfaceshave

beenlistedinTable2fortheimagesrecordedwiththedimensions

10␮m×10␮m.Thehighestsurfacearearatioleadstothelargest

Fig.7.2DAFMimagesofporp4thinfilmfabricatedatafabricationdepositionpressureof(a)12.5mNm−1_,(b)15mNm−1_{and(c)17.5mNm}−1_{atascaleof10␮m×10␮m.}

Table2

Therootmeansquareroughness,surfacearearatioandrealareavaluesofthe samples.

Thinfilms Depositionsurface

pressure(mNm−1₎ Sdr(%) S3A(nm 2_{) rms(nm)} Porp1 15 9.49 1.09E+8 48.86 17.5 1.86 1.02E+8 23.42 Porp2 12.5 0.341 1E+8 84.59 15 2.40 1.05E+8 57.59 17.5 0.457 1.01E+8 30.58 Porp3 12.5 6.5 1.065E+8 57.13 15 7.59 1.08E+8 54.96 17.5 5.77 1.08E+8 45.70 Porp4 12.5 3.6 1.04E+8 76.87 15 15.72 1.16E+8 41.06 17.5 26.1 1.26E+8 55.59

surfaceofthesample.Thevaluesareconsistentwiththeimagesof

thethinfilmsurfaces(Figs.4–7).Onecanseethatforasurfacewith

higherandlargerfeatures(asthedepositionpressuredecreases) theSdr ishigher,indicatinga roughersurface.Thereisonlyone

exceptioninthecaseofporp4,wherethevalueofrmsishigherfor 17.5mNm−1thatfor15mNm−1.

Forporp1–porp3thinfilmsboththehighestSdr(%)valuesand

thehighestresonancefrequencychanges wereobservedforthe thinfilmsfabricatedatasurfacepressurevalueof15mNm−1.For porp4thinfilmsimilarsituationiseffectiveatthesurfacepressure valueof17.5mNm−1wherethemaximumresponseforbenzene exposureonly.Itisalsoournewconcerntoexplorethedependency oftheresponseonthetypeofthegasmoleculeandcentralatom.

4. Conclusion

Thegassensingmechanismforporphyrinthinfilmshasbeen attractingmuchinterestoverthelastdecade.Withthisstudythe effectofthesurfacemorphologyofthethinfilmsonthegassensing propertieshasbeenpredicatedforporphyrinthinfilmsby compar-ingthesurfaceareaandQCMmeasurements.Theideareliesonthe possibilityofhighinteractionofthegasmoleculeswhentheyfind alargersurface,andinconsequencemoresitestobindonthe sur-facesofthethinfilms.Ithasbeenshown thatthegasresponse

detectedusingQCMsystemwashighwhenthesurfaceofthethin filmwasalsohigh.Howevertherearesomeothereffectswhich havenotbeentakenunderconsiderationsuchascentralatomand thicknessofthethinfilms.Our futureworkwillconcentrateon thesefactors.

Acknowledgements

ThisworkhasbeenfoundedbyTUBITAKBilateralCooperation Programmeprojectnumber109T612andANCSBilateral Coopera-tionProgramcontractnumber414/2010.

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Biographies

DeryaC¸aycıgraduatedfromBalikesirUniversityin2007asaphysicistandshehas finishedherMasterSciencein2010ongassensingdevices.

StefanStanciuhasgraduatedin2007theFacultyofElectronics, Telecommunica-tionsandInformationTechnologyofUniversityPolitehnicaBucharest,receiving adiplomainInformationEngineering.CurrentlyheisResearchAssistantwithin theFacultyofAppliedScience,hisresearchactivitiestakingplaceattheCenter forMicroscopy-MicroanalysisandInformationProcessingwithinthesame univer-sityhehasgraduated.Hisresearchinterestsareconnectedtoscanningmicroscopy techniquesandrelatedcomputervisionandimageprocessingapplications. ˙InciC¸apanreceivedMScdegreein2003andherPhDin2008attheBalikesir Uni-versityPhysicsDepartment,Balıkesir,Turkey.Hermaininterestsarefabricationof thinfilms,gassensorsforenvironmentapplicationsandtheelectricalandoptical propertiesoforganicthinfilmmaterials.Dr.Capanwasappointedasresearch assis-tantattheBalıkesirUniversitybetween2000and2008andshehasbeenworking asassistantprofessorsince2008attheBalikesirUniversityinTurkey.

MatemErdo˘gangraduatedfromCumhuriyetUniversity,Sivas-Turkeyin1990and receivedhisMScdegreeatIllinoisInstituteofTechnology,Chicago-USin1996 andhisPhDdegreeatIstanbulTechnicalUniversity,Istanbul-Turkeyin2003.He joinedPhysicsDepartment,BalikesirUniversityin2004.Heisdeeplyinvolvedin swelling,drying,shrinking,agingandslowreleaseprocessesinpolymericgelsby usingsteady-stateandtimeresolvedfluorescencespectrofluorometrictechniques. Heisalsoworkingonthemodellingofdiffusionprocessinpolymericthinfilms duringgassensorapplication.

BurcuGünergraduatedfromBalikesirUniversityin2009asaphysicistandsheis nowstudyingforherMasterSciencesince2010.

RaduHristuiscurrentlyworkingtowardsthePhDdegreeatDepartmentofPhysics oftheUniversityPolitehnicaofBucharest.Hisresearchinterestsareconnectedto scanningmicroscopytechniquesandscanningprobemicroscopy.

GeorgeA.StanciujoinedUniversity“Politehnica”ofBucharestin1974asresearch assistant.HegainedthePhDdegreeinTechnicalPhysicsin1981.Hebecame Profes-sorofPhysicsin1994.In2001hefoundedCenterofMicroscopy-Microanalysisand ImageProcessing.Hehasbeenworkinginthelaserscanningmicroscopyfieldfrom 2004.Hispresentresearchinterestisconnectedwithscanninglasermicroscopy techniques,scanningprobemicroscopyandnanomaterialsandoptoelectronics.