ContentslistsavailableatScienceDirect
Sensors
and
Actuators
B:
Chemical
j o u r n a l ho me p a g e :w w w . e l s e v i e r . c o m /l o c a t e / s n b
Fabrication
of
Langmuir–Blodgett
thin
films
of
calix[4]arenes
and
their
gas
sensing
properties:
Investigation
of
upper
rim
para
substituent
effect
Mustafa
Ozmen
a,∗,
Zikriye
Ozbek
b,
Sumeyra
Buyukcelebi
a,
Mevlut
Bayrakci
c,
Seref
Ertul
a,
Mustafa
Ersoz
a,
Rifat
Capan
d,1aDepartmentofChemistry,UniversityofSelcuk,Konya42075,Turkey
bDepartmentofBioengineering,UniversityofCanakkaleOnsekizMart,C¸anakkale17100,Turkey cUlukislaVocationalSchool,UniversityofNigde,Nigde51100,Turkey
dDepartmentofPhysics,UniversityofBalikesir,Balikesir10145,Turkey
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received5April2013
Receivedinrevisedform1September2013 Accepted3September2013
Available online 12 September 2013 Keywords:
Calix[4]arene Vaporsensing Langmuir–Blodgett Quartzcrystalmicrobalance
a
b
s
t
r
a
c
t
ThisstudyreportsthecharacterizationandorganicvaporsensingpropertiesofLangmuir–Blodgett(LB)
thinfilmsofcalix[4]arenederivativesthatcontaindifferentnumbersoftertbutylgroupsontheirupper
rims.Surfacepressure–areaisothermsshowthatverystablemonolayersareformedattheair–water
interface.TheLBfilmsaredepositedontodifferentsubstrates,whichallowedustocharacterizethe
filmsbycontactanglemeasurements,quartzcrystalmicrobalance(QCM),scanningelectronmicroscopy
(SEM),andatomicforcemicroscopy(AFM).Theresultsindicatethatgoodquality,uniformLBfilmscan
bepreparedwithtransferratiosofover0.95.Meanwhile,ourQCMresultsshowthatthedeposition
ofLBfilmlayersdependsheavilyonthenumberofp-tert-butylgroupsandcalix[4]arenewithfour
p-tert-butylgroupsyieldsthehighestslopewithamassvalueof1145ngperlayer.Furthermore,ourAFM
andSEMstudiesrevealadensesurfacemorphologyforallpreparedLBfilms.Thekineticresponseof
calix[4]arenescontainingp-tert-butylgroupsandwithoutp-tert-butylgroupsasanLBfilmtochloroform,
benzene,toluene,andethanolvaporswereinvestigatedasafunctionoftime.Afterattachingtert-butyl
groupsontothecalix[4]arenestructure,theresponseofLBfilmtochloroformvaporincreased.LBfilms
ofcompounds1–4yieldaresponsetoallvaporsandmoreoftenselectchloroformwithalarger,faster,
andmorereproducibleresponse.Wethusconcludethatthesecalix[4]arenescouldbeappliedtoresearch
concerningvaporsensingdevicesoperatingatroomtemperature.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
ItiswellknownthatLangmuir–Blodgett(LB)thinfilm tech-niquemakesitpossibletoprepareorganic,functional,ultrathin filmswithacontrolledthicknessatamolecularlevelandwith dif-ferentmolecularorientations[1].Theimportanceof calixarenes issimilarlywellknownandhasbeensincethepioneering stud-iesofGutsche[2,3].Inshort,calixarenesremainattractivetohost moleculesandcanbeeasilyfunctionalizedintosuitablebinding sitesfor target guest species [4]. To briefly review, calixarenes arecyclicoligomersmadeofseveralphenolicunitsboundedwith methylenebridges[5,6]andareregardedasthethirdgenerationof hostmoleculesbecauseoftheyareabletobeincludedbycations,
∗ Correspondingauthor.Tel.:+903322233893;fax:+903322412499. E-mailaddresses:musozmen@gmail.com(M.Ozmen),rcapan@balikesir.edu.tr
(R.Capan).
1 Tel.:+902666121000;fax:+902666121215.
anions,andneutralmolecules[7–11].Calix[4]arenescanbe eas-ilyfunctionalizedbothatthephenolic–OHgroups(ofthelower rim)and, afterpartialremovalof tert-butylgroups, atthepara positionsofthephenolrings(oftheupperrim)[12,13].Thevast majorityofthesemodifiedcalixarenesexistinconicalformations, eachwithacavitysuitableforreceivingdifferentionicand neu-tralspecies[14].Calixarenesareappliedinenzymemimetics,ion sensitiveelectrodesorsensors,selectivemembranes,non-linear optics,andinhigh-performanceliquidchromatographystationary phases.Applicationsofcalix[n]arenesmacrocyclichostcompounds inmaterialscienceshavebecomewidespreadandincludemass [15],ion[16]andoptical[17]sensors,non-linearoptics,molecular tectons[18]incrystal engineering,andLBfilmsforgas separa-tion[19].Becauseofthestructuralcharacteristicsandstabilityof calixarene,theLBtechnique[20]hasfrequentlybeenusedin cal-ixarenestudies.Functionalizedamphiphiliccalixareneshavebeen preparedsothattheirmonolayers,LBfilms,andself-assemblyfilms maybefurtherexamined[21].ThisisduetothefactthattheLB thinfilmtechniqueisa usefulwayofformingsequentiallayers
0925-4005/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.
M.Ozmenetal./SensorsandActuatorsB190 (2014) 502–511 503
ofultrathinorganicfilms[22]andcanpreciselycontrolthe thick-nessand orderofa filmatthemolecularscale[23].Calixarene andtheirderivativescanbeusedasspecificligandsfor analyti-calchemistry,sensortechniques,medicaldiagnostics,andduring materialsynthesis[24].Differenttypesofcalixarenesensorshave beenwidelyreportedintheliterature.Opticalsensorsbasedon cal-ixareneshavebeendesignedtodetectvariousmetalions[24–30], gaseousammonia[31,32]andorganicamines[33].Piezoelectric quartzcrystalcalixarenesensorshavebeendesignedtomonitor volatileorganicpollutantsin thegasphase,aswellasin aque-oussolution[34–37].Additionally,thecalix[n]arenecavitieshave proventoworkwellassensitivematerialsinbulklayersnotonlyfor thedetectionofvolatileorganiccompounds,suchashalogenated andaromatichydrocarbons[38],aswellasinmonolayers [39], butalsofortheelectrochemicaldetectionofions[40]. Develop-mentsingassensingtechnologyhavebecomeaseriousaspectto considerbecausetheneedtocontrolairqualityhasbecomean envi-ronmentallyimportant issue.Improvingtheperformanceofthe gassensingdevicesmostlydependsonthesensitivityand selec-tivityofthesensingmaterials.Regardinggassensing,theuseof organicmaterialshasincreasedbecauseoftheirsimple,lowcost synthesis,aswellastheirwiderangeofphysicalandchemical prop-ertiesthatcanbetailoredbychangingtheircompositions.Recently, calix[n]arenesand theirderivativeshavebeenextensively stud-iedfortheirpossibleapplicationtosensorsandelectronicdevices, fortheycanbehighlyselectivemolecularreceptorsforvarious metalionsandorganiccompounds,whichallowstheirusein var-iousseparationandanalysisapplications[41,42].Thehost–guest interactionisoftena dynamicprocess inwhich adsorptionand desorptionofvapormoleculesoccurswhenasensingelementis exposed tovapors. It is wellknown thatwhen a gas molecule isadsorbedontothesurfaceofanorganicmaterial,the physico-chemicalproperties,includingthestructural,electrical,andoptical properties,ofthissensingmaterialcanchange.Itisimportantto understandthemechanismofinteractionbetweenthesensing ele-mentandtheorganicvaporsforthedesignandsynthesisofnew moleculestodetectandidentifyorganicvaporsatlow concentra-tion.Thechiefdifficultyingasidentificationcontinuestobethe fabricationofstablesensorswithahighsensitivityandselectivity towardthesubstancetobedetected.Severalmeasurement tech-niques,suchassurfaceplasmonresonance(SPR),UV–visandquartz crystalmicrobalance(QCM),areusedtodetectandmonitorvarious gasesbecauseoftheirarrayofpotentialapplications.One exam-pleisenvironmentalmonitoring,suchasdetectingthepresence andconcentrationoftoxicorotherwisedangerousgasesthatare releasedthroughspillageorleakage[43].
Inthisstudy,thepreparationofLBfilmsofcalixarenederivatives containfour(1),three(2),two(3)andno(4)p-tert-butylgroupson thecalix[4]areneupperrims(Fig.1)wasevaluatedattheair/water (A/W)interfaceusingisothermgraphs.Investigationsofthe com-positionandstructuralorganizationoffilmsonglasssubstratewere performedbycontactangle(CA),atomicforcemicroscopy(AFM) andscanningelectronmicroscopy(SEM).AQCMsystemwas imple-mentedtodemonstratethethinfilmdepositiononaquartzcrystal substrate.Thismethodwasalsoemployedtoinvestigatethepara substituenteffectsofcalix[4]arenecompounds(1–4)ofLBfilmson organicvaporssuchasbenzene,chloroform,toluene,andethanol.
2. Experimentaldetails 2.1. Materials
Highpuritywater(18.2Mcm)thathadbeenpassedthrough a Millipore Milli-Q Plus water purification system was used to preparewater subphase mixture.The glass substrateswere
purchasedfromFisherScientific.Chloroform(extrapure,Merck) wasemployedasspreadingsolvent.Benzene,tolueneandethanol were supplied from Aldrich. All materials were used without further purification. Starting calixarene compounds 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene (1), 5,11,17-tri-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene (2), 5,17-di-tert-butyl-25,26,27,28-tetrahydroxycalix[4]arene (3), 25,26,27,28-tetrahydroxycalix[4]arene (4) weresynthesized accordingtopreviouslypublishedprocedure[6,9,11].
2.2. Synthesisofcalix[4]arenecompounds
Althoughcompound 1iscommerciallyavailable,we synthe-sized our own in our laboratory to obtain a sample of higher purity.Thep-tert-butylcalix[4]arenewasdeterbutylatedbyusing AlCl3 and phenolintoluene toproducecompound4. Toobtain
selectivedeterbutylatedcalix[4]arenederivatives2and3,firstly compound1wasinteractedwithbenzoylchlorideinthepresence ofN-methylimidazoleintolueneforcompound2.Theproductwas thendeterbutylatedwithAlCl3 intolueneatroomtemperature.
Finally,thethreeesterfunctionswerehydrolyzedwithNaOHina water/ethanolmedium.Theresultingcompound2wasobtained ina90%yieldasawhitesolid[44].Compound2waseasily syn-thesizedfromcompound1andbenzoylchlorideusingMeCNas solventandK2CO3asbase.ThesubsequentFriedel–Crafts
deter-butylationstepwascarriedoutintolueneusingAlCl3,andthetwo
benzoategroupswerethenremovedinalcoholicNaOHatreflux. Thecompound4wasthusobtainedinanalmostquantitativeyield asawhite solid[45].Allofthestructureshavebeen character-izedthrough1HNMR,FTIR(ATR),andelementalanalyses.Inthe
solution,allofthestructuresappearedinconicalconformationas provenbytheappearanceofArCH2Ar,whichdisplaysatypicalAB
typeprotonsignalat3.20–4.20ppm(J=13.1–13.3Hz). 2.3. DepositionofLBfilms
ANIMA622alternateLBtroughwithautomatedsurface bal-ancewasusedtoinvestigatethebehaviorofthemoleculesatA/W andfabricateLBfilmmultilayerontoglasssubstrates.Beforeeach experiment,barriersandtheTeflontroughoftheLBfilmsystem wererinsedwithultrapurewaterafterbeingcleanedwithethanol. ThesurfacepressurewasmeasuredbyusingaWilhelmybalance, equippedwithastripofchromatographypapersuspendingatthe A/Winterface.Thetemperatureofthewatersubphasewas con-trolledusinga LaudaEcolineRE204modeltemperaturecontrol unitandallexperimentaldataweretakenat20◦C.Calix[4]arene moleculesweredissolvedinchloroformwithaconcentrationof 1mgmL−1 and weresubsequently spread onto ultrapurewater subphaseatpH6.SolutionswerespreadbyaHamiltonmicroliter syringeontothesubphasesolutionbydistributingthedropletsover theentiretroughareaat20◦C.Atimeperiodof15minwasallowed forthesolventtoevaporatebeforetheareaenclosedbythe barri-erswasreduced.Thepressure–area(–A)isothermgraphgiven inFig.2wasdeterminedwiththeaccuracyof0.1mNm−1.(–A) graphsofcalix[4]arenemoleculeswererecordedasafunctionof surfaceareausingthecompressionspeedofbarriersatavalueof 172mm2min−1.
AsshowninFig.2,anextrapolationofthelinearpartproduces thevaluesofareapermoleculeinthecondensedstate(1.71nm2,
1.21nm2,1.19nm2 and 0.75nm2 forcompounds1, 2,3, and4,
respectively).Itisclearthattheareapermoleculedependsonthe numberofp-tert-butylgroupsormolecularweight.Itshouldbe notedthat theexpectedapproximateareapermoleculefor the calix[n]resorcinareneunit is in therange 1–2nm2 reportedfor
similarlysizedcalixareneswithdifferentsidechains[46–48].The areapermoleculevaluesfoundinthis workcloselyagree with
Fig.1. Chemicalstructuresofcalix[4]arenemoleculesusedforLangmuir–Blodgettfilms.(i)Benzylchloride,N-methylimidazole,AlCl3,NaOH;(ii)Benzylchloride,K2CO3,
AlCl3,NaOH;(iii)AlCl3,phenol,toluene,rt,1h.
thereportedvaluesof1.16nm2 [49],1.1–1.6nm2 [50],1.02nm2 [51].Areapermoleculevaluesof1.1nm2and0.75nm2arefound
fortwocalixmolecules.Thevalueof1.1nm2forcalix1molecule
suggeststhatamonolayerisformedatA/Winterface.The corre-spondingvalueforcalix2is0.75nm2,whichsuggeststhat this
calix[4]resorcinareneaggregates in the spreading solution to a greaterextentthanthatofcalix1[52].
Monolayersofcalix[4]arenemoleculesatthewater’ssurface werefound tobestable. Surface pressures of 20mNm−1 were selectedforLBfilm depositionontheglasssubstratesfor QCM measurements.Y-typeLBdepositionmodeandaverticaldipping procedurewasperformedattheselectedsurfacepressurewitha speedof10mmmin−1 forboththedownandupstrokes.LBfilm samplesweredriedfor5minaftereachupstroke.
ThedepositionefficiencyoftheLBfilmsisdenotedbythe trans-ferratio,whichistheratiooftheareaofthemonolayerremoved fromtheair–waterinterfaceduringdepositiontotheareaof sub-stratetobedeposited.isgivenby:
= AL
AS (1)
whereAListhedecreaseintheareaoccupiedbythemonolayeron
thewatersurface,whileASisthecoatedareaofthesubstrate.Using
Eq.(1),isfoundtobe0.95.
2.4. QCMmeasurements
Ablockdiagram ofourhomemadeQCM measurement sys-temisshowninFig.3.AthinlyATcutquartzcrystalsandwiched betweentwo electrodes in an overlapping keyhole design was usedforQCMmeasurements.TheseQCMcrystalswithanominal resonancefrequencyof3.5MHzwerecommercializedfromGTE SYLVANIAcompany.Allmeasurementsweretakenatroom tem-perature(20◦C)usinganoscillatingcircuitthatwedesigned.The quartzcrystalwasinsertedintotheelectroniccontrolunit,andthe frequencyofoscillationwasmonitoredasafunctionoftimeusing dedicatedsoftware.Thevaluesoffrequencychanges,which indi-catethedegreeofresponse,aremeasuredwithanaccuracyof1Hz. Aftereachdepositioncycle,theLBfilmsamplewasdriedforhalf anhourandthemasschangewasmonitoredusingthiscomputer controlledQCMmeasurementsystem.Thissystemwasusedfor
M.Ozmenetal./SensorsandActuatorsB190 (2014) 502–511 505
Fig.2.–Aisothermgraphofallcalix[4]areneLBfilms.(Forinterpretationofthe referencestocolorinthisfigurelegend,thereaderisreferredtothewebversionof thisarticle.)
theconfirmationofthereproducibilityofLBfilmmultilayersusing therelationshipbetweentheQCMfrequencychangesagainstthe depositedmass,whichshoulddependonthenumberoflayersin theLBfilm.
Aspecialgascellwasconstructedtostudythekineticresponse ofcalixLBfilmsonexposuretoorganicvaporsbymeasuringthe frequencychanges.Thesemeasurementswereperformedwitha syringe.ThevariationoftheQCMfrequencywasmonitoredasa functionoftimewhenthesamplewasperiodicallyexposedtothe organicvaporsforatleast2minandwasthenallowedtorecover aftertheinjectionofdryair.Thisprocedurewascarriedoutduring severalcyclestoobservethereproducibilityoftheLBfilmsensing element.
Allorganicvapor measurementswere taken in dry air con-dition in a small gas cell which could eliminate the effect of watervaporontheresponsepropertiesofcalix[4]areneLBfilms. In the literaturecalix[4]arene molecules are used to studythe
watervaporeffectasahumiditysensorbecausetheyare macro-cyclicmoleculesthattheycanbeeasilyfunctionalizefromtheir upperandlowerrims.Humiditysensingpropertiesofcalix[4]arene filmsincludesbothcarboxylateandsulphonategroups are sen-sitivetowatervapormolecules.Thiswatersolublecalix[4]arene filmswelledduetowateruptakeandcanbeusedasahumidity sensor[53].Anotherstudywascarriedoutforhumiditysensing behaviorusingthecalix[4]areneand 25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene(THBC)thinfilms[54].Theresultssuggested that the–OH groups areprotected bybeingburied in the cav-ities ofthemolecules.Thismayexplain whythecalix[4]arenes werehighlyhydrophobic,andtheinteractionbetweenwaterand calix[4]arenewasweak.Therefore,THBCthinfilmwasmore sensi-tivetohumiditythanwascalix[4]arene.Inourstudycalix[4]arene derivativeshavenotcarboxylateandsulphonategroupswhichare sensitivetowatermolecules.Ontheotherhand,ourwetting mea-surementresultsindicatedthatourcalix[4]arenemoleculeshave morehydrophobicbehavior(around70◦)thanhydrophilic behav-ior.Asaresultofthesewebelievedthatthewatervaporeffectin ourexperimentalconditionsisminimizedandcanbenegligible. Thereforetheeffectofwatervaporontheresponsepropertiesis notmeasuredorstudiedinthiswork.
3. Resultsanddiscussion 3.1. Contactangleresults
Wemeasuredthecontactangleasanindirectconfirmationof thecoatingofthemoleculesontheglasssurface.Thecontactangle is verysensitivequantitativeindicatorof thewettabilityof the calix[4]arenefilms.Thewaterdropcontactangleonbareglass sur-facedependssignificantlyonthesurfacepretreatmentandcanvary between3◦ and15◦.Forourglasssurfaces,theequilibrium con-tactangleofMilli-Qwateroncleanedandactivatedglasssurface wasmeasuredtobe3.2◦±0.9◦,whiletheequilibriumcontactangle fortheglasscoatedwiththecalix[4]arenescontainingtetrabutyl, tributyl,anddibutylandwithoutbutylgroupswere83.7◦±2.5◦, 75.3◦±0.6◦,71.4◦±2.1◦and67.0◦±1.5◦,respectively.Thecontact anglechangesmarginallyasthecalix[4]arenecoatedsurfacesturn
Fig.4. AFM(leftside)andSEM(rightside)imagesofLangmuir–Blodgettfilmssubstrates:(a)and(f)bareglasssurface,(b)and(g)calix[4]arenecompound1,(c)and(h) calix[4]arenecompound2,(d)and(i)calix[4]arenecompound3,(e)and(j)calix[4]arenecompound4.(Forinterpretationofthereferencestocolorinthisfigurelegend,the readerisreferredtothewebversionofthisarticle.)
M.Ozmenetal./SensorsandActuatorsB190 (2014) 502–511 507
morehydrophilicduetothedecreasingtertiarygroups(see Sup-plementarydataformoreinformation).
3.2. AFMandSEManalysis
Fig.4presents thetopographicimagesof thesurfacesofan activeglasssurfaceandtheglasssurfacecoatedwithcalix[4]arene molecules.TheAFMimageoftheactiveglasssurface(Fig.4a) con-sistsofgrainystructuresincertainareas;otherwise,itisflatwith anarearoughnessof2.00nmandaroot-mean-square(rms) rough-nessof2.95nmona5×5mscale.Fig.4(b)–(e)showsthesurface topographiesofthecalix[4]arenemultilayerswithdecreasing ter-tiaryalkylgroupslocatedonthelowerrimontotheglasssurface, whicharesignificantlydifferentfromthetopographyinFig.4(a). This result suggests that, the formation of calix[4]arene films changedthetopographyoftheglasssurface.Theiraverage rough-nessandrootmeansquarevaluesare13.02nm,16.59nm;4.53nm, 6.11nm;3.24nm,4.09nmand2.38nm,3.05nmforLBfilmsof com-pound1,compound2,compound3andcompound4,respectively. Itcanbeseenthatthecalix[4]arenefilmsontheglasssurfaceare uniform,dense,andhomogeneouswithsomesurfaceaggregates.
TheSEMimageoftheclean glasssubstrateshows aflatand roughsurface(Fig.4f).TheSEMimageofcalix[4]areneLBfilm con-tainsfourtertiaryalkylgroups,showssomeglobularunitswithfew aggregates(Fig.4g).TheSEMimageofcalix[4]areneLBfilm con-tainsthreetertiaryalkylgroups,showsadenseform,andresembles flower-likebodies(Fig.4h).TheSEMimageofcalix[4]areneLBfilm containstwotertiarygroupsshowingaclusterformationandnot socompactstructuredfilm(Fig.4i).Fig.4jshowsthatcalix[4]arene LBfilmcontainsnotertiarygroupsaccumulatedcluster-likebodies aswellasmorecompact,homogeneousglobularunits.
3.3. QCMmeasurements
QCMmeasurementtechniqueiswidelyappliedtomonitorthe depositionqualityofthinfilmsonaquartzcrystalsubstratebecause theresonantfrequency,f,isextremelysensitivetoasmallmass changegivenby[55] f = −2f 2 0m 1q/21/2A (2) wheref0istheinitialfrequencyofthecrystal(Hz),misthemass
change(g),Aisthepiezo-electricallyactivearea(0.785cm2), qis
thedensityofquartz(2.648gcm−3),andqistheshearmodulus
ofquartz(2.947×1011gcm−1s−2).
TheQCMmeasurementscanconfirmthereproducibilityofLB filmmultilayerstransferusingtherelationshipbetweentheQCM frequencychangesagainstthedepositedmass.ForanLBfilm,f shouldbedirectlyrelatedtothelayernumberandthechangein resonantfrequencygivenby:
f =
− 2fo2m 1q/21/2A N (3)whereNisthenumberoflayersand,misthemassperdeposited layer.WhenEq.(3)isrearranged,themasschangecanbedescribed as: m=−f 1/2 q 1/2A 2f2 0N (4) ifthenumericalvaluesinEq.(4)arearrangedas
m=−f(2.648)1/2(2.947×10
11)1/2(0.785)
2(3582400)2N (5)
Fig.5.Frequencyshiftasanumberoflayers.(Forinterpretationofthereferencesto colorinthisfigurelegend,thereaderisreferredtothewebversionofthisarticle.)
Finally,thechangeinresonantfrequencyforourLBfilmsisgiven by m=27×10−8
f N (6) This equation clearly indicates that a linear relationship betweenthemassofthenumberoflayersandthechangein reso-nantfrequencyforLBfilmsconfirmstheuniformtransferprocess oftheLBfilm.Fig.5depictsthetransferofcalix[4]areneLBfilmsonaquartz crystal.Asystematicchangeinthefrequencywithanincreaseinthe numberofmonolayerisclearlyobserved.Thechangeinfrequency asafunctionofthenumberofmonolayeriscloselyassociatedwith theLBlayermasschange.Furthermore,theprocesswasshownto bereproducible.Thisisalineardependencechangethatrevealsthe uniformtransferofcalix[4]areneLBfilmsandthissuggestingthat theequalmassperunitareaisdepositedontothequartzcrystal duringthetransferofLBfilmlayers.
Thefrequencyshiftperlayer(f/N)ofcalix[4]areneLBfilms aredeterminedfromtheslopeshowninFig.5.f/Nandthemass depositedontheactiveareaofthequartzcrystalarepresentedin Table1.Similarresultsindicatethattheincreasingsurfacepressure increasestheamountofdepositedmass[56].
AsreportedinTable1,thedepositionofLBfilmlayersdepends heavilyonthenumberofp-tert-butylgroups.Thehighestslopeis givenbycompound3,whichcontainstwotert-butylgroups.The secondhighestslopeistakenfromcompound1withfourtert-butyl groups.Thereisnotanapparentrelationshipbetweenthe molec-ularweightofthemoleculesandthefrequencyshiftduetothe firstlayer.Thisresultcouldderivefromadifferentmacromolecular aggregationthatdependsontheinterplayamongtheconjugated molecules,thesolvent,andthesubstratesurface[57].
It is important tounderstand themechanism of interaction betweenthesensingelementandtheorganicvaporsforthedesign and synthesis of newmolecules to detectand identify organic Table1
FrequencyshiftperlayerofQCMresultsanddepositedmassvalues.
LBfilm f/N(Hzlayer−1) m(ng)
Compound1 29.44 795
Compound2 21.5 581
Compound3 42.41 1145
Fig.6. TheresponseofchloroformvaporstoallLBfilms.(Forinterpretationofthe referencestocolorinthisfigurelegend,thereaderisreferredtothewebversionof thisarticle.)
vapors. The host–guest interaction is often a dynamic process
whereadsorptionanddesorptionof vapormolecules willoccur
whenasensingelementisexposedtovapors.Itiswidelyknown
thatwhenagasmoleculeisadsorbedontothesurfaceofanorganic
material,thephysicochemicalproperties,includingthestructural,
electrical,optical,andmassproperties,ofthissensingmaterialcan
change.Intheliterature,therearemanystudiesonvaporsensing
propertiesofcalix[n]arenewithseveralsubstitutedgroups
con-tainingLBthinfilms.CalixareneLBfilmsconsistofananoporous
matrixformedbytheintrinsic calixarenecavities, aswellasby
thegapsbetweenthemoleculesandbetweenthesubstituentalkyl
chains.Organicvaporscanpenetratethroughtheseporesinside
thefilmmatrixandcondensethere[58].Thekineticresponseof
calix[4]arenecontainingp-tert-butylgroupasanLBfilmto chlo-roform,benzene,toluene,andethanol vaporswasexamined.In ordertodeterminethefrequencychangeoftheorganicvapor,the responseofanuncoatedQCMcrystalandaLBfilmcoatedQCM crystalwasinvestigatedinourpreviousstudyforthecalibrationof oursystem.Itisthusconcludedthattheresponseofanuncoated QCMcrystalissmallerthanthatoftheLBfilmcoatedQCMcrystal, whichisinsignificant[59].
UsingQCMmeasurementtechniquetheresonancefrequency wasrecordedasafunctionoftime.Fig.6showsthenormalized response as a function of time when the sample was periodi-callyexposedtochloroformvaporwithaconcentrationvalueof 2.78×108ppmfor2minandfollowedwithaninjectionofdryair
foranother2minperiod.Theconcentrationvaluesoforganicvapor (seeTable2)inppmarecalculatedbytheformulaasfollows[60]: c=V(22.4 L/mol)106 MV0 (7) c=22.4×V×106 MV0 (8) wherec(ppm)istheconcentrationofvapor,(gmL−1)isthe den-sityofvapor,V(mL)isthevolumeofvaporwhichisinjectedinto thegaschamber,M(gmol−1)isthevapormolecularweight,andV0
isthevolumeofthegaschamber(∼0.002L).Thevaporvolume val-uesareusedinthisstudyinthefollowingorder:20%forV=2mL, 40%forV=4mL,60%forV=6mL,80%forV=8mL,and100%for V=10mL.
ThenormalizedresponsedescribedinEq.(9)iscalculatedas thedifferencebetweentheobservedfrequencyresponse(f)and
Fig.7.Kineticmeasurementsofcompound2LBfilmwithincreasingconcentration ofvaporsasafunctionoftime.(Forinterpretationofthereferencestocolorinthis figurelegend,thereaderisreferredtothewebversionofthisarticle.)
thebaselinefrequencyresponse(fo).Theresultantquantityisthen
dividedbythebaselinefrequencyresponse. Normalizedresponse=
f−fofo
(9) The values of f[f=(f−fo)], which indicate the degree of
response,aremeasuredwithanaccuracyof1Hz.Itisveryclear thatallcalix[4]arene moleculesyield afastresponse to chloro-form vapor. The highest response is taken using compound 2, while compound 3 gives a response smallerthan that of com-pound2.Compound1and4givealmostthesameresponse.The mechanismsof interactionamong organicvapors in anLBfilm structurescanbeexplainedbythreesteps:theprocessesofsurface adsorption,diffusion,anddesorption.Thesurfaceadsorptioneffect betweenLBfilmstructureandvapormoleculescausessharp fre-quencychangewhenLBfilmisintroducedwithorganicvapors[61]. Afterthisinteraction,theincreaseinfrequencyslowsduetothe bulkdiffusioneffect,whichisalsocalleddynamicprocess.When thenumberofadsorbedanddesorbedmoleculesisequal,the fre-quencyshiftachievesthestablevalueuntilthedryairisflushed intothecell.Thefrequencychangeisdirectlyproportionaltothe numberofadsorbedvapormolecules.Itcanbethusconcludedthat theadsorptionofvapormoleculesiseasierontotheLBfilm struc-tureusingcompound2thanallothers.Whenthevapormolecules wereremovedfromthegascell,onlydesorptionprocessoccurs andthefrequencychangedecreasesrapidly.Inorderto investi-gateLBfilmsensingpropertiesregardingotherorganicvapors,the compound2LBfilmwasselected.QCMfrequencywasmeasured atthefirst2mininair,andfollowinganother2minperiod,organic vaporwasintroducedintothegascell.Afterthisprocedure,dryair wasinjectedintothegascelltochecktherecoveryofthesensing material.Thiskineticmeasurementwascarriedoutduring5cycles withincreasingconcentrationstoobservethereproducibilityofthe compound2LBfilm.Fig.7showsthekineticresponseofthe com-pound2LBfilmintheformoffrequencychangetoallvapors.They arealmostreversiblewithresponseandrecoverytimesreported byTable3intheorderofafewsecondswhenthegascellisflushed withdryair.ForareproducibleLBfilmgassensor,sensingmaterial shouldalwaysgivethesamepatternoftheoutputsignalwhenthe sensorisrepeatedlyexposedtoanorganicvaporatconstant inter-valsoftime.Itisclearthatcompound2yieldedarelativelystable repeatability,agoodreproducibility,andalmostuniformchanges infrequencyduetotheadsorptionanddesorptionprocesses.
M.Ozmenetal./SensorsandActuatorsB190 (2014) 502–511 509
Table2
Theconcentrationvaluesoforganicvapors.
Organicvapors (gcm−3) M(gmol−1) c(20%)×108ppm c(40%)×108ppm c(60%)×108ppm c(80%)×108ppm c(100%)×108ppm
Chloroform 1.483 119.38 2.78 5.56 8.34 11.12 13.90
Benzene 0.876 78.11 2.51 5.02 7.53 10.04 12.55
Toluene 0.870 92.14 2.11 4.22 6.33 8.44 10.55
Ethanol 0.789 46.11 3.83 7.66 11.49 15.32 19.15
These results show that the compound 2 LB film yields a
responsetoallvaporsandismoreselectivetochloroformbased
onahost–guestrecognitionmechanismwithCH2–interaction
[62].Theresponseofcompound2LBfilmtochloroformexposures isquitelarge,fast,andreproducible.Uponremovingthe chloro-formvapor,thecompound2LBfilmrecoverywassimilarlyfaster thanothers.
Thevalueofthevaporresponseisproportionaltothechanges infrequency oftheQCMmeasurements. Itcanbeseen thatits response to allvapors is very fastand reversible.Such behav-iorof sensorscanbeexplained bytheinteractionbetweenthe chemicalstructureofmaterialandtheorganicvapor.Thef in QCM generally increaseswith themolecular weight of organic vapors,whilethesensitivityofthefilmdependsonthe molecu-larweightandstructureoftheanalytemoleculesadsorbedonto thefilmsurface.Fortheinteractionmechanisms,itwasproposed that thefrequency response duringadsorption is eitherdue to dipole/dipoleorahydrogenbondinginteraction[63].Itis reason-abletoassumethatifthenumberofadsorbedmoleculesonan adsorbentislimitedandidenticalforvariousadsorbents,agreater molarmassofadsorbentwould leadtoalargerfrequencyshift. AsshowninFig.7,theresponseofcalix[4]areneLBfilmforthe organicvaporsatvarious concentrationvalues given inTable3 (20%,40%,60%,80%,and100%)areinthefollowingascendingorder: ethanol<toluene<benzene<chloroform.ThefinQCMgenerally increaseswithmolecularweightoforganicvapors;however,the molecularweightofbenzeneislowerthanthatoftoluenevapor. Itiswellknownthatf isdirectlyproportionaltothenumberof adsorbedvapormolecules.Thisresultcanbesummarizedthatthe numberofadsorbedbenzenevaporsishigherthantoluenevapors becausebenzenehasalowermolarvolumeandarelativelyhigh viscosityparameter,whichindicatesthatbenzenemoleculesare moremobilethanthetoluenevaporsandpenetrateeasilyintothe calix[4]areneLBfilmstructure[64].
AcomparisonoftheLBfilmofcompounds1to4,thecalixarene derivatives,especially contains tert-butyl groups, reports better resultsfororganicvapors.Thecavityofthecalixarenewithtert butylgroupsis largeenoughtoinclude organicmolecules.This situationagreeswiththeresultsofrelatedliteraturebecausethe structuralchangesinthecalixarenescaffold,suchasremovingthe para substituents affectthe molecularinteractions [65]. Gener-ally,removingtertbutylgroupsattheparapositionsignificantly decreasedthemolecularinteractionofcalixarenes.Inthisstudy, however,excellentexperimentalresultswereobservedfor com-pound2 containingthree p-tert-butylgroupsontheupper rim ofthecalix[4]arenescaffold.Thissituationprobablyresultsfrom therebeingonearylgroupofcalixareneskeleton,whichisrotated upwardtoproducethepartiallyconical conformation[66].This
Table3
ResponseandrecoverytimesofLBfilmsofcalix[4]arenemolecules.
LBfilms Responsetime(s) Recoverytime(s)
Compound1 6 10
Compound2 3 10
Compound3 3 8
Compound4 4 13
formationmayalsoaffectthegassensingefficiencyofcompound
2towardorganicvapors.
4. Conclusion
Thisstudyinvestigatedthecharacterizationandorganicvapor
sensingpropertiesofLBthinfilmsofcalix[4]arenederivatives
con-tainingdifferentnumberoftert-butylgroupsontheupperrim.They
areveryorderedattheA/WinterfaceasanLBmonolayer,whichis
transferredatseveralsubstrateswithtransferratiosofover0.95.
QCMresultsareusedtocalculatethedepositedmassvalues,which
dependingheavilyonthe numberof p-tert-butylgroups. Using
wettabilitymeasurements,thecontactanglewasdeterminedas
theaveragevalueofmeasurementsinfiveneighboringsitesoffilm,
whileLBfilmmodifiedwithpurecomponentcalix[4]areneswith
tetrabutyl,tributyl,anddibutylandwithoutbutylgroupsonupper
rimcalixareneskeletonwere83.7◦±2.5◦,75.3◦±0.6◦,71.4◦±2.1◦
and67.0◦±1.5◦,respectively.UsingAFMimageofcalix[4]areneLB
filmsthermsvaluesarecalculatedbetween2.38nmand13.02nm.
Itisthusconcludedthatthecalix[4]arenefilmsontheglass
sur-face are uniform, dense, and homogeneous withsome surface
aggregates.SEMimagesalsoshowedthatthemorphologyofLB
filmsweredifferentthanthatofbare glasssubstrate.To
inves-tigatethekineticresponse ofcalix[4]arenederivativesas anLB
filmtochloroform,benzene,toluene,andethanolvapors,the
res-onancefrequencyisrecordedasafunctionoftime.Thecompound
2yieldsthehighestresponsetochloroformvaporwithafastand
almostfullyreversibleresponseinamatterofafewseconds.When
exposedtoothervapors,compound2LBfilmismoreselectiveto
chloroformthanothervaporswithalarge,fast,andreproducible
response.Thisstudyconcludesthatp-tert-butylgroupinthe
struc-tureofcalix[4]arenemoleculecanplayasignificantroleforthe
organizationoftheLBfilmmonolayerattheA/Winterface,the
depositionofLBfilmsontothesolidsubstrates,andthedetection
oforganicvaporsforthesensorindustry.
Acknowledgments
TheauthorswouldliketothankTheResearchFoundationof
Sel-cukUniversity(BAP)forfinancialsupportofthiswork.Wearealso
thankfultoCansuOzkayaforherhelpduringisotherm
measure-ments.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in
theonlineversion,athttp://dx.doi.org/10.1016/j.snb.2013.09.008.
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Biographies
MustafaOzmenreceivedtheM.Sc.andPh.D.degreesinchemistryfromSelcuk University,Konya,Turkey,in2006,and2011,respectively.Hewasappointedasa researchassistantfrom2005to2011attheChemistryDepartmentofSelcuk Uni-versityinTurkey,andsincethenhewasasaresearchassistantdoctoratthesame department.Hisresearchinterestsincludeself-assembly,micro/nanopatterning techniques,synthesisofnanoparticles(magnetic,gold,silverandTiO2)andtheir
functionalization,organicthinfilmdepositionviaLangmuir–Blodgetttechniqueand thespectroscopicandopticalcharacterizationsoforganicthinfilmmaterialsand theirapplicationsasbiosensor.
ZikriyeÖzbekreceivedherM.Sc.andPh.D.degreesinphysicsfromtheUniversityof Balikesir,Turkeyin2007,and2012,respectively.Shehasappointedasanassistant professorfrom2013attheBioengineeringDepartmentofCanakkaleOnsekizMart UniversityinTurkey.HerresearchareaisfabricationofLangmuir–Blodgettthin films.
SumeyraBuyukcelebireceivedherB.Sc.degreeinchemistryfromtheUniversity ofSelcuk,Turkeyin2010.SheiscurrentlyworkingtowardtheM.Sc.degreeat UniversityofSelcuk,Turkey.
MevlutBayrakcireceivedhisB.Sc.degreeinDepartmentofChemistryfromNigde Universityin2004andM.Sc.andPh.D.degreesinchemistryfromSelcuk Univer-sity,Konya,Turkey,in2007,and2012,respectivelyunderthesupervisionofDr. ErtulandProf.Yilmaz.Hisresearchinterestsareinthedesignandsynthesisof macrocycliccompoundssuchascalixareneandcrownetherandtheiruseasdrug solubilizingagentsaswellastheirmetalcomplexes.Currently,hehasbeen work-ingasanassistantprofessorattheUlukislaVocationalSchoolofNigdeUniversityin Turkey.
SerefErtulreceivedhisB.Sc.inDepartmentofChemistryfromAtaturk Univer-sity,Erzurum,Turkeyin1989andM.Sc.degreeinDepartmentofChemistryfrom SelcukUniversityKonya,Turkeyin1991andPh.D.degreeinDepartmentof Chem-istryfromSelcukUniversityKonya,Turkeyin1997.Hehasbeenworkingasan assistantprofessorattheChemistryDepartmentofSelcukUniversityinTurkey.His researchinterestsincludedesignandsynthesisofsupramolecularstructuresbased calixarene,crownetherand/orSchiffbasesandtheiruseassensorstowardmetal cationsandtoxicanions.
MustafaErsozhasreceivedhisM.Sc.degreeatUniversityofSelcuk(Konya,Turkey) in1985.HeobtainedhisPh.D.atUniversityofGlasgowin1994.Following postdoc-toralexperienceatGKSSResearchCenter,Germany,hethenspent1yearwithinthe surfactantandcolloidresearchgroupatUniversityofHull,UnitedKingdom.Much ofhisresearchisfocusedtowardtheincorporationofself-assembledmonolayers withinultra-thinfilmsandtheirapplications,aswellaspatterningtechniquessuch asmicrocontactprinting.Heistheauthorofinexcessof120publishedpapers.He isamemberoftheTurkishAcademyofSciences(TUBA).
RifatCapanreceivedM.Sc.degreeatHacettepeUniversityPhysicsEngineering Departmentin1991,Ankara,TurkeyandhisPhDattheUniversityofSheffield(UK) in1998.HeestablishedfirstLangmuir–BlodgettThinFilmResearchGroupinTurkey. HehadaPhDscholarshipfromTurkishHighEducationCouncilbetween1993and 1998andhadOversea’sResearchStudentAward(UK)from1995to1998.Hismain interestsarepyroelectricheatsensor,gassensorforenvironmentapplications,the electricalandopticalpropertiesoforganicthinfilmmaterials.Hehasbeenworking asaprofessorsince2007attheUniversityofBalikesir.
