Langmuir-Blodgett thin film for chloroform detection

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o ur na l ho me pa g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Langmuir–Blodgett

thin

ﬁlm

for

chloroform

detection

Rifat

C¸apan

a,∗

_,

_Hilal

_Göktas¸

b

_,

_Zikriye

_Özbek

c

_,

_Sibel

_S¸

_en

b

_,

Mehmet

Emin

Özel

d

_,

_Frank

_Davis

e

a_Balıkesir_University,_Science_Faculty,_Physics_Department,₁₀₁₄₅_Balıkesir,_Turkey

b_C¸anakkale_Onsekiz_Mart_University,_Science_Faculty,_Physics_Department,₁₇₁₀₀_C¸_anakkale,_Turkey

c_C¸_anakkale_Onsekiz_Mart_University,_Engineering_Faculty,_{Bioengineering}_Department,₁₇₁₀₀_C¸_anakkale,_Turkey d_Maltepe_University,_Vocational_School,_Dept_of_Medical_Imaging_Systems,_Maltepe,_Istanbul,_Turkey

e_Centre_for_Biomedical_Engineering,_Cranﬁeld_University,_Cranﬁeld,_Bedford_MK3_0AL,_UK

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received30October2014

Receivedinrevisedform11February2015

Accepted15February2015

Availableonline21February2015

Keywords: Calix[4]arene

Vaporsensing

Langmuir–Blodgett

Surfaceplasmonresonance

a

b

s

t

r

a

c

t

Calix[4]resorcinarene(C11TEA)moleculescouldbedepositedasanLBfilmbytheLangmuir–Blodgett (LB)techniqueontosuitablesubstrates.Surfaceplasmonresonance(SPR)andUV–visspectroscopywere employedforthecharacterizationoftheseLBfilms.AhighqualityanduniformLangmuirmonolayer fromthewatersurfacecanbetransferredontoaglassorgoldcoatedsubstrateswithatransferratioof over95%.ThicknessandrefractiveindexvaluesoftheresultantLBfilmsweremeasuredusingsurface plasmonresonancewithavalueof1.04nmperlayerand1.4respectively.Forthesensingapplication towardschloroform,thisLBfilmyieldsafastandalmostfullyreversibleresponsetochloroforminfew seconds.

1. Introduction

ThenamecalixarenewasproposedbyGutscheforaseriesof

cyclicoligomers,readilyobtainedbytreatmentofp-alkylphenol

withformaldehyde and a suitable base.As wellas these

com-pounds, a similarclass of materials,can besynthesized bythe

acidcatalyzedcondensationofanumberofaldehydetogivecyclic

tetramers, known as calixresorcinarenes [1]. Calix[n]arenes are

knowntobeexcellentLangmuirandLangmuir–Blodgettﬁlm

form-ingmaterialsandtheyareusedasspeciﬁcligandsforanalytical

chemistry, sensor techniques, and medical diagnostics, since a

numberofsynthesismethodsallowﬂexiblevariationoftheirring

size,theycanbefunctionalizedwitha widerangeoffunctional

groups,andhavestructuralcharacteristicssuchashighporosity

andgoodchemicalstability[2].Chemicalsensorsbasedon

rela-tivelylowcostsynthesiscalixarenesandcalixresorcinareneswere

designedtodetectvariousinorganicgases[3],explosivevapours[4]

andvolatileorganiccompounds(VOC’s)[5,6]becauseoftheir

ﬂex-iblestructure,aswellastheirwiderangeofphysicalandchemical

propertiesthatcanbetailoredbychangingtheircompositions.

The monitoring and detection of VOC’s has become a

seri-ous aspectto consider because theneed to control air quality

∗ Correspondingauthor.Tel.:+902666121000;fax:+902666121215.

E-mailaddress:rcapan@balikesir.edu.tr(R.C¸apan).

hasbecomean environmentally importantissue. The

construc-tion of gas and VOC sensors will allow the rapid detection of

harmfulorenvironmentallydamagingvapourswithouttheneed

fortechniquessuchasgaschromatographyormass

spectrome-trywhichoftenrequireexpensiveequipmentandhighlytrained

laboratorypersonnel.Improvingtheperformanceofgassensing

devices mostlydependsonthesensitivity andselectivityofthe

sensingmaterials.It iswellknownthatwhenagasmoleculeis

adsorbed onto thesurface of an organicmaterial, the

physico-chemicalproperties,includingthestructural,electrical,andoptical

properties,ofthissensingmaterialcanchange.Thishasledto

inten-siveresearchintonewmaterialswiththeabilitytobind,detect

andidentifyorganicvapours.Thechiefdifﬁcultyingas

identiﬁca-tioncontinuestobethefabricationofstablesensorswithahigh

sensitivityandselectivitytowardsthesubstancetobedetected.

Becauseofthesensitivityofcalixarenes toorganicvapoursand

theirexcellent LBﬁlm formingability, we haveinvestigated in

depththeuseof thesematerialstosenseVOCs. Previous work

hasdemonstratedsensitivityofcalixresorcinareneﬁlmstoBTEX

gases[5]andarangeofcommonorganicsolvents[6,7].Anumber

ofotherauthorshavealsocarriedoutstudiesonthesesystems,

for exampleSPR couldbeusedtomeasure theadsorptionof a

rangeoforganiccompoundsontocalixarenethinﬁlms[8].Other

workersdeposited anamine-modiﬁedcalixresorcinarenesas LB

ﬁlms onto piezoelectric quartz crystals and showed their

sen-sitivityto a range of organicvapours,demonstrating that both

http://dx.doi.org/10.1016/j.apsusc.2015.02.109

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Fig.1. ChemicalstructureofC11TEA.

thepHofthesubphaseandthepresenceofcopperionsaffected

thesensitivityof theﬁlms[9].In otherwork,calixarenes were

depositedbothasLBandcastﬁlms[10]andtestedfortheir

sen-sitivitytowardsaromatichydrocarbonsusingpiezoelectricmass

sensors, cast ﬁlms were shown to be more sensitive but LB

ﬁlmsdisplayedgreaterselectivity.Anovelcalixarene–porphyrin

compoundcouldalsobedeposited asanLBﬁlmand shownto

demonstratechangesinopticaladsorptiononexposuretoamine

vapours[11]. Oneof thematerials studiedwasspeciﬁc to

sec-ondaryaminesanddisplayednosensitivitytoprimaryandtertiary

amines.Similartetramersbasedonpyrroleratherthanphenolic

unitshave alsobeenshowntoformLBﬁlmsanddisplay

sensi-tivitytoalcoholswhenassessedusingsurfaceplasmonresonace

[12,13].

Tetraundecylresorcinarenewasderivatisedusinga Mannich

reactionwithparaformaldehydeanddiethylamine[14].The

result-ingtetramine wasthen reﬂuxed withﬁve equivalents of ethyl

bromideinethanolfor16h,volatilesremovedandthecalix

recrys-tallisedfromethanol.Calix[4]resorcinarene(C11TEA)waschosen

asitformsgoodqualityLBﬁlmsandsimilarmaterialshavealso

beenshowntoreacttoaqueousguests[15].Itwasalso

hypothe-sizedthatthepresenceofcationicammoniumsubstituentswould

effect both the size and electron density of the cavity,

poten-tiallyimprovingit’sbindingpropertiesandenhancingsensitivity

towardscertainvapours.Inthisstudy,thepreparationofLBﬁlmof

theC11TEAgiveninFig.1wasevaluatedatthewatersurfaceusing

isothermgraphs.Surfaceplasmonresonance(SPR)andquartz

crys-talmicrobalance(QCM) systemswere usedtodemonstratethe

thinﬁlmdepositiononagoldcoatedglasssubstrateandto

deter-minethethicknessandrefractiveindexofLBﬁlm.Furthermorewe

utilizedthisLBﬁlmastheactivecomponentwithinasensorfor

organicvapourssuchaschloroform,benzene,toluene andethyl

alcohol.

2. Experimentaldetails

2.1. LBﬁlmpreparationanddeposition

A computer controlled NIMA 622 alternate LB trough was

employedtostudythemolecularbehaviourofC11TEAmolecule

attheair–waterinterfaceandtoproduceLBﬁlmsontoglassand

goldcoatedglasssubstrates.Beforeeach experiment,the

barri-ersandtheTeﬂontroughoftheLBﬁlmsystemwererinsedwith

ultrapurewater(18.2Mcm)afterbeingcleanedwithchloroform.

ThesurfacepressurewasmeasuredbyusingaWilhelmybalance,

equippedwithastripofchromatographypapersuspendedatthe

air–waterinterfaceat20◦CwhichwascontrolledusingLauda

Eco-lineRE204modeltemperaturecontrolunit.C11TEAmoleculewas

dissolvedinchloroformwithaconcentrationof0.2mgmL−1 and

Table1

Theconcentrationvaluesoforganicvapours.

Organicvapours (g/cm3₎ _M_(g/mol) _c_×₁₀3_ppm

Chloroform 1.483 119.38 278.26

Benzene 0.876 78.11 251.35

Toluene 0.870 92.14 211.50

Ethanol 0.789 46.11 383.62

wassubsequently spread onto ultrapurewater subphase atpH

6.Solutions werespreadby aHamiltonmicroliter syringeonto

thesubphasesolutionbydistributingthedropletsovertheentire

trougharea. A time period of 20min was allowedfor the

sol-vent toevaporatebeforetheareaenclosedbythebarriers was

reduced.Thepressure–area(_␲–A)isothermgraphwasdetermined

withtheaccuracyof0.1mNm−1.Thisgraphwasrecorded asa

functionofsurfaceareausingthecompressionspeedofbarriers

atavalueof172mm2_min−1_._A_quartz_slide_for_UV–vis

measure-ment,a50nmgoldcoatedglassslideforSPRmeasurementand

aquartz crystalforQCMmeasurement wereusedas substrates

forvariousmeasurementmethods.Theﬂoatingmonolayeratthe

air–waterinterfacewasfoundtobestableatasurfacepressureof

22.5mNm−1;therefore,thissurfacepressurevaluewasselected

fortheLBﬁlmdepositionprocedure.Y-typeLBdepositionmode

andaverticaldippingprocedurewasperformedattheselected

surfacepressurewithaspeedof10mmmin−1forboththedown

andupstrokes.LBﬁlmsamplesweredriedfor5minaftereachup

stroke.

2.2. SPRmeasurements

A surface plasmon resonance spectrometer (BIOSUPLAR 6

Model)withalow powerlaserdiode(630–670nm)lightsource

wasemployedforthemeasurementswithanangularresolutionof

about0.003degrees.Aglassprism(n=1.62)mountedtoontoaholder

isavailableformeasurementinliquidorinair.Theglassslideshadthe

dimensionsof20mm×20mmandthetotalthicknesswas1mmare

coatedonthetopbya50nmthingoldlayer.Aftereachdeposition

cycle,theC11TEALBﬁlmsamplewasdriedforhalfanhourand

theSPRcurvewasmonitoredusingSPRmeasurementsystem.This

systemwasusedfortheconﬁrmationofthereproducibilityofLB

ﬁlmmultilayersusingtherelationshipbetweentheanglechanges

againstthedepositedthickness,whichshoulddependonthe

num-beroflayersintheLBﬁlm.WINSPALLsoftwaredevelopedinthe

groupofMax-Planck-InstituteforPolymerResearch(byWolfgang

Knoll),GermanywasutilizedforﬁttingofSPRcurvesareusedto

determinethicknessandrefractiveindexvaluesofLBﬁlm.

Chloroform(99%Merck),benzene,toluene and ethanol(99%

Aldrich)wereusedwithoutfurtherpuriﬁcationasorganicvapours

forthedetectionperformances.Allmeasurementsweretakenin

dryairconditioninasmallgascellwhichcouldeliminatetheeffect

ofwatervaporontheresponsepropertiesofcalix[4]resorcinarene

LBﬁlms[7,8].Aspecialﬂowcellfromtransparentplastic,

compat-iblefor vapororgasmeasurementswasconstructedtostudythe

kineticresponseofC11TEALBﬁlmonexposuretoorganicvapours

bymeasuringthereﬂectedlightintensitychanges.Thecellhastwo

channels,withinletandoutletconnectedtosiliconetubes.TheSPR

systemiscompletelycontrolledbytheBiosuplar-Softwarewhich

handlesthesettings,themeasurementsanddataacquisitionaswell

ascontrollingtomeasurementanddatapresentation.VOC

injec-tionswereperformedwithasyringe.Theconcentrationvaluesof

organicvapor(canbeseeninTable1)inppmcanbecalculatedby

theformulaasfollows[16]:

c=

24.055V

MV0

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0 5 10 15 20 25 30 35 40 45 0 0.5 1 1.5 2 2.5 3 Surface Pressure, (mNm -1)

Area per molecule,

(nm

2₎

Fig.2. TheisothermgraphofaC11TEAmonolayerattheair–waterinterface.

wherec(ppm)istheconcentrationofvapor,(g/mL)isthedensity

ofvapor,V(mL)isthevolumeofvaporwhichisinjectedintothe

gaschamber,M(g/mol)isthevapormolecularweight,andV0is

thevolumeofthegaschamber(∼0.02mL).

Theresponsewasrecordedasafunctionoftimewhenthe

sam-plewasperiodicallyexposedtotheorganicvapoursforatleast

2minandwasthenallowedtorecoverafterinjectionofdryair.

Thisprocedurewascarriedoutduringseveralcyclestostudythe

concentrationchangesandtoobservethereproducibilityoftheLB

ﬁlmsensingelement.

3. Resultanddiscussions

3.1. Depositionprocess

Apressure–area(␲–A)graphforaLangmuirmonolayershows

thecharacteristicsurfacebehaviourofanorganicmoleculeonthe

air–waterinterface.Theareapermoleculeforanorganicmonolayer

canbecalculatedusingthefollowingrelation[17]:

am=_cNAMw

AV (2)

whereAistheareaofthewatersurfaceenclosedbythetrough

bar-riers,Mwthemolecularweight,ctheconcentrationofthespreading

solution,NAtheAvogadro’snumber,andVisthevolumeofsolution

spreadoverthewatersurface.

Solutions(200␮l)werespreadbyaHamiltonmicrolitersyringe

onto the water surface by distributing the droplets over the

entire trough area. A time period of 15min was allowed for

the solvent toevaporate beforethe area enclosed by the

bar-rierswasreduced. The␲–A isothermgraph wasobtainedwith

theaccuracyof0.1mNm−1.Fig.2showstheisothermgraphof

C11TEA. After the gas phase, C11TEA molecule shows a sharp

increase until surface pressure of 40mNm−1 and then

mono-layerat theair–water interfacestartsto collapse. A deposition

pressurevalueof22.5mNm−1wasselectedformonolayer

trans-ferontothesolid substrates.Using Fig.2,thelimitingareaper

moleculeobtainedbyextrapolatingtheslopeoflow

compressibil-itytozeropressureinthecondensedstateandavalueof1.79nm2

forC11TEAwasdeterminedusingEq.(1).Similarresultsonthe

behaviourofareapermoleculevaluesof1.71nm2 _and_0.75_nm2

arefoundforcalix[4]arenederivativesthatcontaindifferent

num-bersoftertbutylgroupsontheirupperrims[7].Similarresearch

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 500 450 400 350 300 250 200 Absorbance Wavelength, (nm) Solution 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 410 310 210 Absorbance Wavelength, (nm) LB film 24 layer 20 layer 16 layer 12 layer 8 layer 4 layer

Fig.3. TheUV–visspectraofaC11TEAsolution.Inset:LBﬁlmofC11TEAwith

differentnumbersoflayers.

are found using the methodof extrapolating tozero pressure,

areas per molecule for calix[4]resorcinarenes between 1.1nm2

and 0.75nm2 _[15]_. _C11TEA _material _yields _a _typical _isotherm

behaviourwiththreeof thephasesasgas(0–1mNm−1), liquid

(∼1–18mNm−1), and solid phases (∼18–39mNm−1). The

col-lapsepointbegins at39mNm−1 wheretheorderof monolayer

is destroyed. This result is supported by McCartney [18]. The

areapermoleculeisstronglydependentonthechemical

struc-ture of the materials used and the orientation and packing of

molecules at theair–water interface.Several␲–A graphs were

taken with the same amountof volume value and our results

forC11TEAmaterialindicatedagoodstabilityand

reproducibil-ity.

TheefﬁciencyoftheLBﬁlmdepositionischeckedbythetransfer

ratio,whichistheratiooftheareaofthemonolayerremovedfrom

theair–waterinterfaceduringdepositiontotheareaofsubstrate

tobedeposited.isdescribedby:

= AL_AS (3)

whereAListhedecreaseintheareaoccupiedbythemonolayer

onthewatersurface,whileASisthecoatedareaofthesubstrate.

UsingEq.(2),isfoundtobearound0.95.Thisvaluecanbeused

toconcludethatsteady,reproducibleanduniformmonolayersof

C11TEAweredepositedfromtheair–waterinterfaceontoaquartz

crystalsubstrate.

3.2. UV–visspectraofthedepositedﬁlms

OpticalabsorptionspectrumofC11TEAsolutionandLBﬁlms

wererecordedinthewavelengthrangeof200–500nm.Fig.3shows

theUV–visabsorptionspectrumofC11TEAsolutioninchloroform

andthemajorabsorptionpeakbeingobservedat280nm.Thisis

a typicalUV spectrafor calixarenes and calixresorcinarenes[1]

althoughsubstitutioncanextendtheabsorptionpeak,for

exam-ple azobenzenecalixarenes display UV–vis spectrawith strong

absorptionregions around300–400nm [19].Theinset inFig.3

showstheabsorptionspectrumofC11TEALBﬁlmswiththicknesses

between4and24layers.TheUV–visspectraoftheLBﬁlmsare

similartothesolutionspectrabutUV–visspectraoftheLBﬁlms

showa widerband(300–400nm)thanthat oftheonein

solu-tion.UV–visspectrumoftheLBﬁlmshowsaredshiftfromthe

UV–visspectrumofsolutionthiscanbeduetoanumberoffactors.

Thenatureofthesolventusedcanaffecttheresultantspectrum

anditis alsoknownthataggregationofmoleculescanoccurin

LBﬁlmsleadingtobroadeningandred-shiftingofanyadsorption

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0 500 1000 1500 2000 2500 3000 43 44 45 46 47 48 49 50 Re fl ac tan ce , (a .u.)

Angle of incidence

gold 2 layer 4 layer 6 layer 8 layer 10 layer

Fig.4. SPRcurvesfortheC11TEALBﬁlmswithdifferentnumbersoflayers.

3.3. SPRcurvesandthinﬁlmthickness

Fig. 4 shows SPR curve resonance shifts of bare gold and

increasingnumberoflayersbetween2and10layersofC11TEA

LBﬁlms. Those curves are shiftedto largerangleswhen

num-beroflayersisincreasedasexpected.Theﬁlmthickness(d)and

refractiveindex (n) of C11TEA LB ﬁlmswere calculated by

ﬁt-ting the SPR curves with a Fresnel formula algorithm via the

WINSPALL software as mentioned above. It was assumed that

k=0forC11TEALBﬁlm,sincetheyaretransparentat=633nm

[20].Theﬁttingcalculationsproduceameanvalueof1.4forthe

refractiveindexofC11TEALBﬁlm.Otherstudieshavedetermined

the refractive index of differing calixarene thin ﬁlms as 1.494

by[21],1.48by[5]and between1.54and 1.43by[22]

respec-tively.

By using the ﬁtting calculations for ﬁlm thickness

demon-stratedthatwhenthenumberoflayersisincreased,ﬁlmthickness

valueisincreasinglinearlyasexpected.Theaverageﬁlm

thick-ness,determinedbytheslopeof thethicknessversus thelayer

numbershowninFig.5,isfoundtobe1.04nm/depositedlayer.

Therefractiveindexandthicknessofdifferentcalixarenematerials

havebeendeterminedtohaverefractiveindexesbetween1.47and

1.70alongwiththicknessvaluesfrom0.80to1.50nm[23].Capan

etal.[24]studiedtherefractiveindexandthicknessofcalix[8]acid

LB ﬁlms and obtained a refractive index value of 1.21±0.08,

along with a thickness value (determined by the slope of the

thicknessversuslayer numbers) tobe1.08±0.07nm/deposited

layer.

Fig.5. Thicknessofthethinﬁlmsasafunctionofthenumberoflayers.

1350 1850 2350 2850 3350 3850 0 200 400 600 800 1000 P ho todetector re sponce , (a .u. ) Time, (s)

Chloroform Benzene Toluene Ethyl alcohol

Fig.6. KineticresponseofsensorcoatedwithC11TEA(10layers)toinjectionof

vapoursintotheworkingcell.

3.4. Organicvaporsensingproperties

Itisimportanttounderstandtheresponsepropertiesbetween

theC11TEAmoleculeandorganicvapourstoallowforthedetection

andidentiﬁcationoforganicvaporforapplicationasroom

tempera-turevaporsensors.IntheSPRvaporsensingarrangement,usually,a

photodiodeoperatesasadetectorprovidinganoutputvoltage

pro-portionalincidentlightintensity.Themeasurementangleisﬁxed

atapointonthesteepestpartoftheresonancecurveandthe

photo-diodeoutputisamonitoredasafunctionoftime.Thisiscalleda

kineticmeasurement.ThekineticresponsesoftheC11TEALBﬁlm

tothechloroform,benzene,tolueneandethylalcoholvapoursis

giveninFig.6bymeasuringthereﬂectedlightintensityasa

func-tionoftimefor2min,followedbyinjectionofdryairforafurther

2minperiod.

ThekineticresponsesoftheC11TEALBﬁlmintheformoflight

intensitychangetoallvapoursarealmostreversible.Theirresponse

andrecoverytimesaregiveninTable1.Theirvaluesareintheorder

ofafewsecondswhenthegascellisﬂushedwithdryair.Theﬁrst

stageinVOCanalysisistoﬂushareferencegas(dryair)throughthe

sensortoobtainabaseline(I0).WhentheLBﬁlmsensormaterial

isexposedtotheorganicvapor,thiscauseschangesinthe

out-putsignaluntilthesensorreachesasteady-state(I1).Thevaporis

ﬁnallyﬂushedoutofthesensorusingthedryairandthesensor

responsereturnsbacktoitsbaseline.Thetimeduringwhichthe

sensorisexposedtothevaporiscalled“theresponsetime”while

thetimeittakesthesensortoreturntoitsbaselineresistanceis

“therecoverytime”.Iindicatesthedegreeofthetotalresponseof

C11TEALBﬁlmwhichisthedifferencebetweentheobservedlight

intensityresponse(I1)andthebaselinelightintensity(I0).Table1

summarizestheanalysisofkineticresponses.Here,C11TEALBﬁlm

showsafastresponseaswellasaﬁnedegreeofrecovery,with

responseandrecoverytimestypicallyofafewseconds.This

pro-cessisrepeatedfourtimesandeachvaporgivessimilarresponses

totheirinitialresponse.For areproducible andreliableLBﬁlm

gassensor,sensingmaterialshouldalwaysgivethesamepattern

oftheoutputsignalwhenthesensorisrepeatedlyexposedtoan

organicvaporatconstantintervalsoftime.ItisclearthatC11TEALB

ﬁlmyieldedarelativelystablerepeatability,agood

reproducibil-ity,andalmostuniformchangesinreﬂectedlightintensitydueto

theadsorptionanddesorptionprocesses.It canbeusedseveral

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Table2

AnalysisdataofthekineticresponseofC11TEALBﬁlm.

Cycle Response

time(s)

Recovery time(s)

I0(a.u.) I1(a.u.) I[I1−I0](a.u.) Themeanvalue

¯I=

n i=1Ii n Standarddeviation =

n i=1(I−¯I) 2 (n−1) Chloroform 1 2 3.5 2972.04 3114.12 142.08 159.85 _±_12.07 2 1.5 3.7 3008.52 3177.48 168.96 3 1.3 3.4 3035.40 3200.52 165.12 4 3.9 7.3 3056.52 3219.72 163.20 Benzene 1 1.1 2.5 1853.54 1927.61 74.07 79.27 _±_4.86 2 1.2 2.8 1811.88 1888.26 76.38 3 2.0 3.4 1844.29 1926.45 82.16 4 2.5 5.6 1850.07 1934.55 84.48 Toluene 1 1.4 4.3 2421.32 2494.77 73.45 67.50 ±5.49 2 1.4 4.5 2419.25 2484.43 65.18 3 2.2 3.5 2449.25 2510.29 61.04 4 2.1 3.9 2467.87 2538.22 70.35 Ethylalcohol 1 1.2 1.7 1544.74 1580.43 35.69 37.11 ±1.65 2 1.8 2.2 1561.81 1600.60 38.79 3 1.7 2.8 1567.50 1603.19 35.69 4 2.3 4.5 1569.05 1607.33 38.28

C11TEALBﬁlmfortheorganicvapoursinthefollowingascending

orderare:chloroform>benzene>toluene>ethylalcohol.This

cal-ixresorcinareneismoreselectivetochloroformvaporthantowards

theothervapours.Asimilarworkreportedthattheexposureof

chloroformvaporismoreeffectivethan benzeneontheoptical

parametersofspunﬁlmbecauseoftheinteractionintheformof

CH–␲bonding betweenchloroform vaporand theelectron-rich

aromaticringsofthecalixresorcinarenethinﬁlms[25].Benzene

vaporshowsahigherresponsethattoluenevapor;howeverboth

ofthemhavebenzenerings.Itiswellknownthatthenumberof

adsorbedbenzenevapoursishigherthantoluenevapoursbecause

benzeneismorevolatile,hasalowermolarvolumeanda

rela-tivelyhighviscosityparameter,indicatingthatbenzenemolecules

aremoremobilethanthetoluenevapoursandpenetrateeasilyinto

theC11TEALBﬁlmstructure[7].Anumberofothercalixarenes

havepreviouslybeenreportedtoshowsimilarsensitivitytowards

chloroform[26,27](Table2).

4. Conclusion

C11TEAmoleculewasinvestigatedintheformofLBthinﬁlms

usingUV–visandSPRmeasurements.Isothermanalysisindicated

that a stable and reproducible monolayer of the material was

formedintheair–waterinterface.Thelimitingareapermolecule

wasshowntohaveavalueof1.79nm2_and_a_uniform_LB_deposition

wasobservedwithatransferratioof≥95%.TheUV–visspectraof

theLBﬁlmsaresimilartothematerialspectrainchloroform

solu-tionalbeitwithsomebroadeningandred-shiftinthesolidform,

possiblyduetoaggregation.Thethinﬁlmthickness(1.04nm/layer)

andrefractiveindexvalue(1.4)wasobtainedusing SPRcurves.

SPRkineticmeasurementsshowthatC11TEALBﬁlmswere

sig-niﬁcantlysensitivetochloroformandtheresponseoftheﬁlmis

fast,large,reproducibleandhadashortrecoverytime,typically

ofa few seconds. Thetotal response oftheC11TEALB ﬁlmfor

theorganicvapoursisinthefollowingascendingorder:

chloro-form>benzene>toluene>ethylalcohol.Theseresultsdemonstrate

thepossibilitiesforthedevelopmentofopticalsensorsforC11TEA

ﬁlms.

Acknowledgement

ThisworkissupportedbyTurkishScientiﬁcandTechnological

ResearchCouncil(TÜBITAK).Projectno:TBAG-107T343.

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Langmuir-Blodgett thin film for chloroform detection

Applied

Surface

Science

Langmuir–Blodgett

thin

ﬁlm

for

chloroform

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