Room temperature ethanol sensor with sub-ppm detection limit: Improving the optical response by using mesoporous silica foam

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SensorsandActuatorsB243(2017)1205–1213

Contents lists available atScienceDirect

Sensors

and

Actuators

B:

Chemical

j o u r n a l h o m e 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

Room

temperature

ethanol

sensor

with

sub-ppm

detection

limit:

Improving

the

optical

response

by

using

mesoporous

silica

foam

Dániel

Seb ˝ok

a,∗

,

László

Janovák

a

_,

_Dániel

_Kovács

a

_,

_András

_Sápi

b

_,

_Dorina

_G.

_Dobó

b,c

_,

Ákos

Kukovecz

b,c

_,

_Zoltán

_Kónya

b,d

_,

_Imre

_Dékány

a,e

a_Department_of_Physical_Chemistry_and_Materials_Science,_University_of_Szeged,₁_Rerrich_square,_H-6720_Szeged,_Hungary b_Department_of_Applied_and_{Environmental}_Chemistry,_University_of_Szeged,₁_Rerrich_square,_H-6720_Szeged,_Hungary c_MTA-SZTE_{“Lendület”}_Porous_{Nanocomposites}_Research_Group,_University_of_Szeged,₁_Rerrich_square,_H-6720_Szeged,_Hungary d_MTA-SZTE_Reaction_Kinetics_and_Surface_Chemistry_Research_Group,_University_of_Szeged,₁_Rerrich_square,_H-6720_Szeged,_Hungary e_MTA-SZTE_{Supramolecular}_and_{Nanostructured}_Materials_Research_Group,_University_of_Szeged,₈_Dóm_square,_H-6720_Szeged,_Hungary

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received25August2016 Receivedinrevisedform 17December2016 Accepted19December2016 Availableonline21December2016 Keywords: Thinﬁlm RIfS Mesoporoussilica Roomtemperature Sub-ppm Ethanolsensor

a

b

s

t

r

a

c

t

Inthispaper,theimprovementinroomtemperatureethanolsensingcharacteristicsofzincperoxide

(ZnO2)basedhybridthinﬁlmsispresentedbythecombinationofthebeneﬁcialsensingproperties

ofmesoporous materialsandreﬂectometricinterference spectroscopy(RIfS).Thehybridthin ﬁlms

werepreparedbyLayer-by-Layer(LbL)self-assemblymethodfromZnO2nanoparticles,polyelectrolyte

[poly(acrylicacid),PAA]and/ormesoporoussilica(MPS).Theexpectedimprovedsensingpropertieswere

attributedtothefractalpropertiesandhighspeciﬁcsurfacearea(as₎_of_the_mesoporous_{coating/interlayer}

material,whichwasevidencedbysmallangleX-rayscattering(SAXS)andN2sorptionmeasurements

(as_>₆₅₀_m2_/g)._The_sensor_tests_showed_that_the_detection_limit_of_the_thin_ﬁlms_is_in_the_sub-ppm_range

(<500ppb).Applyingsilicafoam(SF)assurfacecoatingorinterlayermaterialinthesandwich-structured

thinﬁlm(ZnO2/SF)improvedtheopticalresponse(l:wavelengthshift)comparedtotheZnO2/PAAthin

layer,butthesensitivityshowednon-linearcharacteristicandsignaldrift.Thethinﬁlmwithmixed

struc-ture(ZnO2/PAA/ZnO2/SF)showedlinearsensitivity(␭/c=0.6nm/ppm)inthe0.5–12ppmrangewith

anacceptableselectivityandstablebaseline.Testingthesensorinextended(upto40ppm)concentration

rangeshowedonlyaslightquadraticdeviationfromlinearbehaviorwithR2₌_0.9987.

1. Introduction

Sensorsforvolatileorganiccompounds(VOCs) (e.g.alcohols, benzeneetc.)playanimportantroleineverydaylifeand indus-trialsafety. Indisputable fact is that thesechemical agents are harmfulandunhealthy,sothedetectionofthesemoleculeshas agreatimportanceinenvironmentalandhealthprotection,such asinairandwaterqualitycontrol,foodindustryor–especiallyin thecaseofethanol–the“drivingunderinﬂuence”(DUI)control. Considering a comprehensive, although not complete overview ofthearticlespublishedinrecentyearsinethanolsensorstopic (Fig.1)wecanconcludethattheprinciples,technicalsolutions, thematerialsused, theoperating temperatureranges and con-centrationlevelsarefairlydiversiﬁed.Themostcommonlyused

∗ Correspondingauthor.

E-mailaddress:[email protected](D.Seb ˝ok).

sensor materialsare SnO2 [1–3], ZnO [4–7], SiO2 [8],In2O3 [9] TiO2[10],Fe2O3 [11,12]andothernanostructures[13–16], com-posites[17–25]orcoatings[26,27].Themeasurementprincipleis mainlybasedontheresistivemethod,butalsocapacitive[8,15], optical[2,26–28],quartzcrystalmicrobalance(QCM)[26,28]and piezo(self-poweringdevice)[7,21,22]applicationscanbefound. Fig.1showstheprinciples,studiedconcentrationrangesand oper-atingtemperaturespresentedintheworkscitedabove.Itcanbe seenthatthestudiescanbedividedintotwomajorgroups:room temperature(RT)andhightemperature(around200and300◦C) applications.Mainlyelectricalmethodsandmesoporoussensing materialsarepreferredinthelattercase,therebybroad concen-tration ranges withexcellent detection limits can be achieved. However,ithastobenotedthatVOCpollutantseasilyevaporate atroomtemperatureand canbeveryharmfulandcarcinogenic alreadyatlow concentration.ItcanbeseenonFig.1thatmost oftheRTtechnicalsolutions[2,7,8,15,16,26–28]areabletodetect ethanol vapour onlyabove 10ppm concentration.In this work, http://dx.doi.org/10.1016/j.snb.2016.12.097

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Fig.1.Diagramsummarizingthecomprehensive,althoughnotcompleteoverviewofthearticlespublishedinrecentyearsinethanolsensorstopic(groupedbythe measurementprinciples,thestudiedconcentrationrangesandoperatingtemperatures;note:roomtemperaturehasnotdetailedscale!).

wemadeanattempttocombinethebeneﬁcialsensingproperties ofmesoporousmaterials[3,29,30]andreﬂectometricinterference technique[31]toconstructahighlysensitiveethanolsensor oper-atingatroomtemperature.

Reflectometricinterferencespectroscopy[28,31–36]isan opti-cal method which is based on the spectral (red) shift of the interferencepatternreflectedfroma(fewhundrednanometersof layerthickness)thinfilm.Thewavelengthshiftiscausedbythe adsorptionoradhesionofmoleculesorcolloidalunits,soitcanbe utilizedinantigen-antibodyreactionsortodetecttheadsorptionof volatilecompounds[].ThesensorsurfaceofRIfStechniquecanbe preparedbyusingthewetcolloidchemicalprocedure,theso-called Layer-by-Layermethod[37,38].TheLbLmethodis widelyused forthinfilmpreparationdirectlyfromcolloidalsystems (nanopar-ticles, polymer solutions, etc.): it is an easy, non-instrumental techniqueanditresultsahomogeneoussurfaceandwell-ordered, transparentstructurewithcontrollablefilmthicknessandafine andporousmicrostructure[39],likethesimilarLangmuir-Blodgett method[40,41].TheseareessentialconditionsforapplyingRIfS technique,andthesensitivity,aswellas,thelimitofdetectioncan beimprovedbytheadditionofvarioussurface-modifyingagents [28,31].

In the present work we demonstrate the beneficial effect of using mesoporous silica materials on the sensitivity and detection limit of RIfS sensor in the gas phase. We show that applyingmixed(nanoparticle/polyelectrolyte/mesoporoussilica) nanostructureresultslinearsensitivityandsub-ppmethanol detec-tionlimitwithoutresponsedrift,whileboththeresponsetimeand selectivityremainstableandadequate.Furthermore,firstlyinthis workwecarriedoutreflectionintensitymeasurementinaddition tothewavelengthshiftmonitoring:thetwotypesofresponses dif-fersignificantly,whichmayhighlight–byfurtherstudies–the

differencesbetweentheadsorptionmechanismsontothevarious surfaces.

2. Experimental

2.1. Materials

Zincperoxidenanoparticleswithanaveragediameterof80nm were synthesized by the photolysis of zinc acetate dehydrate (C4H6O4Zn·2H2O,Fluka,a.r.)describedin[38].Poly(acrylicacid) (PAA,MW=100000,Sigma,a.r.)wasusedasanegativelycharged polyelectrolyte.Furthermore,SBA-15andsilicafoamwereusedas coatingsornegatively chargedinterlayermaterials.Synthesisof SBA-15silicaiswell-known[42].MesoporousSFwereprepared byamodifiedsol-gelroutebasedonthetechniquesuggestedby Bagshaw[43].Inatypicalsynthesis,13.9gTEOSwasslowlyadded to30mL 10 w% TritonX114 aqueoussolutionand the synthe-sismixturewerevigorouslystirredfor24h.Theobtainedsilica suspensionwascollectedbyvacuumfiltrationandlefttodryat roomtemperaturefor24h.Thedriedsamplewasintroducedinto aTeflon-linedstainlesssteelautoclavewithavolumeof100mL where 10mLwater was alsoadded separatelyto ensurewater vapourenvironment.Aftertheassemblyoftheautoclave,itwas heldat140◦Cfor24h.Finally,thesilicafoamwascalcinedinairat 450◦Cfor4h.

2.2. Thinﬁlmpreparation

Five types of hybrid thin ﬁlms were prepared byusing the ZnO2 nanoparticles,negatively chargedPAApolyelectrolyte and themesoporous silicasamples (see Fig. 2): (1.) 20 zinc perox-ide/poly(acrylicacid)bilayers([ZnO2/PAA]20);(2–3.)[ZnO2/PAA]20 ﬁlms with silica foam and SBA-15 coatings ([ZnO2/PAA]20 +SF

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D.Seb ˝oketal./SensorsandActuatorsB243(2017)1205–1213 1207

Fig.2. Theschematicviewofthepreparedandappliedhybridthinﬁlms.

and [ZnO2/PAA]20 +SBA); (4.) a thin film containing 20 bilay-ers of zinc peroxide/silica foam ([ZnO2/SF]20) and (5.) a thin film containing 10 mixed, zinc peroxide/poly(acrylic acid)/zinc peroxide/silicafoammultilayers([ZnO2/PAA/ZnO2/SF]10).During pre-experiments,SBA-15wasnotsuitableasinterlayermaterial inthinfilmsconsistingof40layers,therefore,itwasonlyapplied asacoating.ThethinfilmswerepreparedbytheLbLdeposition methodbythealternatedadhesion/adsorptionofZnO2 nanoparti-cles,poly(acrylicacid)[44]andmesoporoussilicananostructures (SBA-15andSF)onthesurfaceofglasssubstrate(microscopeslides, MarienfeldSuperior,Germany).Thethinfilmpreparationwas car-riedoutbyusingc=8g/LZnO2,c=0.1g/LPAAandc=10g/Lsilica solutions.Theimmersiontimewas10minforeachstep,whichwas followedbyrinsingwithdeionizedwatertoremovethesurplus (non-electrostaticallyattached)colloidunits.Duringthecoating processthepreviouslypreparedzinc-peroxide/poly(acrylicacid) hybridfilmswereimmersedintothesilicasuspensionandwere driedwithoutrinsingthesurplus.

2.3. Instrumentalmethods

TransmissionElectronMicroscopy(TEM)measurementswere carriedoutbyaFEITECNAIG2₂₀_X-Twin_{high-resolution} trans-missionelectronmicroscope(equippedwithelectrondiffraction) operatingatanacceleratingvoltageof200kV.Thesampleswere drop-castontocarbonﬁlmcoatedcoppergridsfromethanol sus-pension.

Thespeciﬁcsurfacearea(BETmethod)andthetotalporevolume weredeterminedbytheBJHmethodusingaQuantachromeNOVA 2200gassorptionanalyzerbyN2gasadsorption/desorptionat77K. Beforethemeasurements,thesampleswerepre-treatedinvacuum at200◦Cfor2h.Thedensityofthesilicapowderswasmeasured usingahelium

gaspycnometer(Micromeriticstype1305).

SAXStechniquewasusedtoinvestigatethefractalproperties andstructuralparametersofthemesoporoussilicacomponents. SAXScurves were recorded witha slit-collimated Kratky com-pact small-angle system(KCEC/3 Anton-PaarKG,Graz, Austria) equippedwitha position-sensitive detector(PSD50Mfrom M. Braun AG Munich, Germany) containing 1024 channels 55␮m inwidth.CuK_␣radiation(␭CuK␣=0.1542nm)wasgeneratedbya PhilipsPW1830X-raygeneratoroperatingat40kVand30mA.The fractaldimensionof atwo-phasesystemcanbedeterminedby usingthefollowingequation:I(h)=I0h−p,whereh=4sin␭−1 isthescatteringvector,isone-halfofthescatteringangle,isthe wavelengthofCuK_␣radiation,I(h)isthescatteringcurve,I0isthe scatteredintensityath=0,andpistheslopeoftheﬁttedlinein thehigherh-range(Porodregime)inlog-logplotofthescattering curve.If3<p<4thenthesampleissurfacefractal,andthe sam-plehasmassfractalpropertiesinthecaseof1<p<3.Thespeciﬁc surfacearea(as₎_values_were_calculated_by_using_equations_in_[45].

The optical properties of the thin films were studied by a Nanocalc2000spectrophotometer withADC1000-USBA/D con-verter (Ocean Optics). The reflectionspectra of the films were measuredina special,home-builttestcellatdetectionangleof 45◦.Thethickness(d)andeffectiverefractiveindex(n1)ofthethin filmswerecalculatedbasedonthemodelpresentedinFig.3.aand byusing(andfitting)Eq.(1)(moredetailsin[38]):

R (,ne,d)=c1+c2·cos

_4n 1dcos ε1

(1) whereε1 istheangleofrefractionatair/thinﬁlminterface,is thewavelength,c1andc2areconstantswhichcontainthetijandrij transmissionandreﬂectionamplitudescalculatedbyFresnel equa-tions(i,j=0,1,2,seeFig.3.a).

Thesametestcellwasusedduringthesensorialtestsindynamic conditions(Fig.3.b):itwasconnectedtoagasflowsystemwhich consistsofthecarriergas(N2)holder,thetemperaturecontrolled liquidsampleholderandanumberofflowcontrollers(MFC) (Cole-Parmer,USA). The vapourconcentration inthe test cellcan be controlledbytheMFCunitsandV1-V5valvesviathemixingrateof pureandvapourcontainingN2flows(theflowcontrollerscan reg-ulatemaximum3922,844and49mL/mingasflow).Theaccuracy oftheflowadjustingontheMFCscaleis±0.5division,sothe preci-sionoftheconcentrationis±0.04ppmor±0.68ppminthecaseof themax.49mL/minorthemax.844mL/mindevices,respectively. Theethanol dosageandrinsing(N2)timeswere3–3min, alter-nately.Thesensorresponses,␭(nm)andR(a.u.)weredefined asthewavelengthshiftofthegivenextremeofthereflection spec-traandthechangeofthereflectionvaluecorrespondingtothis extreme,respectively.The measurementsineach concentration steps(475-11880ppb)wererepeatedthreetimes,theresponses weredeterminedastheaverageofthethreevalue.During selec-tivitymeasurements2␮Lofliquids(methanol,ethanolasalcohols; n-hexaneasaliphatic;toluene,xyleneasaromaticmolecules)was droppedintotheliquidsampleholder(inthiscaseT=70◦C)with a mixingrateof4.9mL/minsub-branchand1000mL/minmain branchflowrates.

3. Resultsanddiscussion

3.1. Characterizationofthemesoporoussilicamaterials

The porosity,pore systemcharacteristic and speciﬁcsurface areaare of great importancein thecase of mesoporous adsor-bentsusedinsensorialapplications,thereforeseveralstructural parametersweredeterminedandcalculatedbyusingSAXS tech-nique.Asitisknownintheliterature,SBA-15has2Dhexagonally orderedporesystem[38],whichcanbeidentiﬁedbyTEMandSAXS technique(Fig.4.a:AandB).Bothmeasurementsclearlyshowthe porestructure;inthelattercasethepeaksath=0.705,1.235and 1.41nm−1correspondtothe1:√3:2ratio,therebytheP6mm

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sym-Fig.3.aTheusedthinfilmmodelforthicknessandrefractiveindexcalculations(indices:0–air,1–thinfilm,2–substrate).bSchemeoftheusedexperimentalsetup(gasflow systemandreflectometrictestcell)andthemeasurementprinciple.

metryisclearlyidentiﬁed[46].Thedoublelogarithmicplotofthe scatteringcurveissuitablefordeterminingthefractalproperties ofthematerial.InthecaseofSBA-15theslopeisapproximately −2inthehigherh-range,whichindicatesaframe-likesurface frac-talstructure,aswellas,p≈−3.5forSFsampleischaracteristicfor thesmoothsurfacefeatureofthemesocellularfoamstructure(see Fig.4.a:C).ThespeciﬁcsurfaceareavaluesdeterminedbySAXS measurementsare820and730m2_/g_for_SBA-15_and_SF, respec-tively.

TheN2adsorption/desorptionstudiesofthesilicasamplesshow isothermswithhysteresisloopandporesizedistributionsbetween 3and10nmcharacteristicformesoporousmaterials(Fig.4.b).The specificsurfaceareasandaverageporediametersare798m2_g−1_, 4.2nmand666m2_g−1_,_4.6_nm_for_SBA-15_and_SF,_{respectively.}_In summary,theSBA-15hashigherspecificsurfacearea,howeverthe SFshowedhighertotalpore volume(Vp=0.76and 1.30cm3g−1 for SBA-15and SF,respectively). The overall conclusionis that althoughthestructural and fractal natureofthese mesoporous materialsaresignificantlydifferent,buttheaverageporediameters andspecificsurfaceareasaresimilar,andthesevaluesappeartobe sufficientlyhighforconsiderableadsorptioncapacityandsensorial applications.

3.2. Thinﬁlmcharacterization

Therecordedandfitted(inthe␭=550–850nmrange)reflection spectraandthecalculatedrefractiveindexcurvesofthreetypesof thinfilms(withoutcoating)canbeseenonFig.5.Thelayer thick-nessesandeffectiverefractiveindices(at␭=589nm)are782nm and1.286,894nmand1.258,989nmand1.251for[ZnO2/PAA]20, [ZnO2/PAA/ZnO2/SF]10 and [ZnO2/SF]20, respectively. It can be establishedthatusingSFsilicaasinterlayermaterialincreasesthe thickness(d)anddecreasestheeffectiverefractiveindex(n1)ofthe thinfilms.Inouropinion,themainreasonsforthisisthe follow-ing:thesetypeofhighlyporoussilicananostructuressignificantly increasethemicro-andmacro-levelporosityofthehybridfilms, therebyconsiderablydecreasetheeffectiverefractiveindex,while thepolyelectrolyteformsultrathinlayersinthemultilayer struc-ture,therebyensuringacloserpackingfortheZnO2nanoparticles. Increasingtheporosityandformingthickerandporousinterlayers inthesandwich-likestructureexplaintheslightlylowerrefractive indices,higherfilmthicknessesandtheadvantageouseffectin sen-sorialtests(presentedlaterin3.3).Inthecaseof[ZnO2/PAA]20+SF and[ZnO2/PAA]20+SBAfilmsthesilicamonolayerhasno signifi-cantcontributiontotherefractiveindexandlayerthickness,and giventhefactthatforthesesamplesthecoatinghadnosignificant

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D.Seb ˝oketal./SensorsandActuatorsB243(2017)1205–1213 1209

Fig.4.a(A)ArepresentativeTEMimageofthemesoporousSBA-15sample,(B)theSAXScurvesofSBA-15andSFpowdersamplesinlog–logrepresentation(thepower-law exponentsareindicatedbydottedlines)and(C)theTEMimageoftheSFsilicasample.b(A,C)BETisothermsfortheSBA-15andSFsilicasamplesshowthemesoporous characteristic,(B,D)poresizedistributionofSBA-15andSFsamples,respectively.

Fig.5.(A)Themeasured(solidlines)andcalculated(dottedlines)reﬂectionspectraof[ZnO2/PAA]20,[ZnO2/PAA/ZnO2/SF]10and[ZnO2/SF]20thinﬁlms,and(B)thecalculated

refractiveindexcurves.

effectinthesensorialtests,thedetaileddiscussionoftheoptical propertiesisignoredinthisparagraph.

3.3. Sensorialtestofthehybridthinﬁlms

Thethin filmsweresubjected to reflectometricinterference measurements for testing sensorial applications. The measure-mentswerecarriedoutbymeasuringtheshiftofthelocalminimum ofreflectedintensitynear␭=500nmwavelength.Itis␭min=457nm in thecase of [ZnO2/PAA]20,and ␭min=507nm and 568nm for [ZnO2/PAA/ZnO2/SF]10and[ZnO2/SF]20,respectively(thesevalues arevalidinthet=0measurement point).Therawresults (sen-sorgrams),i.e., the vs. t and R vs. t curves are presented in Fig. 6.a and b, respectively. Conspicuous differences can be noticedviewing either thetwo sensorgrams or thecurves one byone.It canbeimmediatelyconcludedthatinthecaseof

curves the responsesare positive becauseof the optical thick-nessincreasesduetothevapouradsorption.Significantsignaldrift canbeobservedinthecaseofZnO2/PAA,ZnO2/PAA+coatingand ZnO/SFthinfilms,whichisratherdisturbingphenomenon:should bedrift compensationapplied? In thiscase, not in general.f it isignoredtheneach measurementstep(atthesame concentra-tion)resultsinhigher responsethan thepreviousoneand this factmakesimpossibletoaccuratelydetermineunknown concen-trations.As it can beseen onthe calibration (responsevs. concentration)curves(Fig.7.a),wedidnotapplycompensation,the responsesincreasewithconcentration,evenifnotlinearly. How-ever,inthecaseof[ZnO2/PAA/ZnO2/SF]10hybridthinfilmresponse driftwasnotobservedwhichresultedalinearcalibrationcurve (0.586nm/ppm).InthecaseofRvs.tcurves(Fig.6.b)positive responsecanbeobservedonlyinthecaseof[ZnO2/PAA]20 sen-sor,butif thethinfilm containsMPS (asinterlayer materialor

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Fig.6. aEthanolsensingtests:vs.tcurvesforthetestedthinﬁlms(labels: structureofthethinﬁlmandtheethanolconcentrationsteps);inset:responseof [ZnO2/PAA/ZnO2/SF]10mixedstructureforc=475ppbEtOH.bEthanolsensingtests:

Rvs.tcurvesforthetestedthinﬁlms(labels:structureofthethinﬁlmandthe ethanolconcentrationsteps).

coating)thenthereflectivitydecreasesduetotheethanoldosage. Furthermore,theresponsesofthecoatedthinfilmsarelowerthan theoriginal,aswellas,thesignalisnotevaluableinthecaseof [ZnO2/SF]20sensorsurface.ItcanbestatedthatevaluableR sig-nalandlinearcalibrationcanonlybeattributedtoPAAcontaining multilayers(withoutcoating),andtheresponsecanbeimprovedby interlayeredmesoporoussilicafoam(Fig.7.b:forclarity,the abso-lutevaluesoftheresponsesareplotted).Insummary,itwasfound thatapplying[ZnO2/PAA],[ZnO2/PAA]+MPScoatingor[ZnO2/SF] structuredthinlayersmostlyfailedduetothesignaldriftand non-linearsensitivity.Themixedstructureof[ZnO2/PAA/ZnO2/SF]was devoidofdriftandshowedlinearcalibrationcurves,sothistype ofhybrid(nanoparticle/polyelectrolyte/mesoporoussilica) multi-layerisan appropriate structure toapplyassensing surface in reflectometricinterferencesensoringasphase.Furthermore,we canconcludethatduringRIfSmeasurementsonsolid/gasinterface themonitoringofR(t)besidetheconventional(t)signalmay revealthecomplexityoftheadsorptionprocessesandmechanisms inmeso-andmacroporoushybridthinfilms.

Basedonthepresentedresultsthe[ZnO2/PAA/ZnO2/SF]10thin ﬁlmwasselectedforfurtherexperiments,suchas reproducibil-ity,responsetimeanalysisandselectivity(Fig.8).Itcanbestated that the sensors signal is well reproducible (Fig.8A),the sen-sorsresponsereachesthe90%ofmaximumvaluewithin40sand it is relaxed to10% within 80s(t90% and t10% on Fig.8B) (the totalresponseandrecoverytimeswere180–180s).Selectivitywas testedbydropping2␮Lofdifferentliquidsintoa70◦Cliquid

sam-Fig. 7.a Ethanol sensing (nm) vs. c(ppm) calibration curves for the pre-paredthinlayers(labels:structureofthethinﬁlmsandcalibrationequationfor [ZnO2/PAA/ZnO2/SF]10mixedstructure).bEthanolsensingRvs.c(ppm)calibration

curvesforthepreparedthinlayers(labels:structureofthethinﬁlmsandcalibration equationsfor[ZnO2/PAA]20and[ZnO2/PAA/ZnO2/SF]10mixedstructures).

pleholder, which wasconnected into the 1000mL/min carrier gasstreamtowardstheRIfStestcell.Theliquidsweremethanol, ethanol,n-hexane,tolueneandxylene.Thestudiedthinﬁlmswere [ZnO2/PAA]20and[ZnO2/PAA/ZnO2/SF]10toinvestigatetheeffect ofsilicafoamontheselectivity.Itwasestablishedthatinthecase ofZnO2/PAA/SFmixedstructure2–3timeshigherresponse was observedforethanolthanfortheothervolatileorganiccompounds (VOC),althoughalsotheafﬁnitytoaromaticmoleculesincreased comparedtoZnO2/PAAstructure(Fig.8C).

3.4. Extendingtheconcentrationrange

ThesensorsignalandcalibrationcurveofZnO2/PAA/ZnO2/SF mixedstructureinthelowppmrange(∼0.48-11.9ppm)was pre-sentedonFig.6.aand7.a,respectively.Itwasdemonstratedthat thelowest set and detected ethanol vapour concentration was 475±40ppb,andthevs.ccalibrationcurvewaslinearinthe c=475–11880ppbrange.Nextthe[ZnO2/PAA/ZnO2/SF]10thinﬁlm wassubjectedtosensorialtestinahigher,c=2.46-37(±0.68)ppm range(Fig.9.a).Theconcentrationsteps(0–50min)wererepeated aftera50minlongbaselinestabilitytest.Thestatementwasthat thesensorhasafairlystablebaseline(withoutdrift),butthe␭ calibrationcurveshowedaslightquadraticdeviationfromlinear behavior(Fig.9.b).Themostimportantparametersaresummarized inTable1.

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D.Seb ˝oketal./SensorsandActuatorsB243(2017)1205–1213 1211 Table1

Structure,layernumberandlayerthicknessofthinﬁlmsusedinsensorialapplication,andconcentrationrange,calibratingequation(asthefunctionofconcentration, c),R2_parameter_and_drift_properties_obtained_from_sensorial_tests_(*:_LOD_and_LOD_assuming_error-free_calibration_values_were_calculated_by_[47]_for_[ZnO₂_/PAA/ZnO₂_/SF]₁₀

inc=0.48-11.9ppmrange).Thetablecontainsearlierreflectometricinterferenceresultsforcomparison(originalandsurfacemodified–bybutyltrichlorosilane,BTS– [ZnO2/poly(styrenesulfonate)]20thinfilms).

Thinﬁlm d(nm) crange(ppm) Calibration,␭=f(c) R2 _Drift _LOD_(ppm)

[ZnO2/PAA]20 782 0.48–11.9 −0.0086c2+0.339c 0.997 yes –

[ZnO2/SF]20 989 3rdorder – yes –

[ZnO2/PAA/ZnO2/SF]10 894 0.586ca 0.996 no 0.61b [ZnO2/PAA/ZnO2/SF]10 894 0.573cc 0.996 no 0.63d [ZnO2/PAA/ZnO2/SF]10 894 2.46–37 0.0107c2+0.427c 0.999 no – [ZnO2/PSS]20e 514 0−128 0.038c+0.536 0.998 no 29.2 [ZnO2/PSS]20+BTe 0−106 0.331c–0.652 0.997 no 10.3 [ZnO2/PSS]20+BTSe 0−50 0.293c–0.140 0.999 no 5.5

Theresultsindicatedbyboldfontarethemostimportantresultsofthisarticle,andasigniﬁcantpartofthediscussionisdetailedaboutthis(ZnO2/PAA/ZnO2/SF)typeof

nanostructure.

a_calibration_by_using_continuously_selected_amounts_of_{concentrations.} b_LODefc_(assuming_error-free_calibration)₌_0.27_ppm;_[47].

c _calibration_by_using_randomly_selected_amounts_of_{concentrations.} d _LODefc_(assuming_error-free_calibration)₌_0.29_ppm;_[47].

e_D._{Seb ˝ok,}_I._Dékány,_Sensor._Actuat._B-Chem.₂₀₆₍₂₀₁₅₎_435–442._[28].

Fig.8.Responseanalysisfor[ZnO2/PAA/ZnO2/SF]10mixedstructure:(A)

repro-ducibility, (B) response and recovery times and (C) selectivity compared to [ZnO2/PAA]20thinﬁlm.

Fig.9.aEthanolsensingtestinextendedconcentrationrange:vs.tcurvesfor [ZnO2/PAA/ZnO2/SF]10thinﬁlminthe2.46–37ppmrange(blackline)compared

tothe0.48-11.9ppmconcentrationrange(grayline)(labels:ethanolconcentration steps).bEthanolsensingtestinextendedconcentrationrange:(nm)vs.c(ppm) calibrationcurvesfor[ZnO2/PAA/ZnO2/SF]10thinﬁlminthe2.46–37ppmrange

(whitesquares)comparedtothe0.48-11.9ppmconcentrationrange(blackcircles) (labels:calibrationequations).

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structures were subjected to sensorial tests in the gas phase. We showed that thedetection limit of the sensor is sub-ppm (<500ppb),but onlythemixed(ZnO2/PAA/ZnO2/SF) nanostruc-tureshowedlinearsensitivityinthe0.4-11.9ppmrangewithout responsedrift,whileboththeresponsetimeandselectivityremain reasonablegood.Testingthesensorin extended(upto37ppm) concentrationrangeshowedaslightquadraticdeviationfrom lin-ear behavior. In the future the functionalization of the sensor surfacebydifferentmodifyingagentsisexpectedtoenhancethe selectivityandsensitivityofthesensor.

Acknowledgements

Theauthorsareverythankful forthefinancialsupportfrom TheHungarianScientificResearchFund(NKFIHOTKA)PD116224 andGINOP-2.3.2-15-2016-00013.Theworkwasearlierpartially supported by the European Union and the State of Hungary, co-financed by the European Social Fund in the framework of TÁMOP4.2.4.A/2-11-1-2012-0001‘NationalExcellenceProgram’. ASisgratefulforthesupportofJánosBolyaiResearchScholarship oftheHungarianAcademyofSciences.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.snb.2016.12.097.

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Biographies

DánielSeb ˝ok(physicist)isaresearchfellowattheDepartmentofPhysical Chem-istryandMaterialsScienceattheUniversityofSzeged.HereceivedhisPh.D.degree inChemistryattheUniversityofSzegedin2012.Scientificfieldsofinterest: inves-tigationoftheinteractionofnoblemetalnanoparticlesandbiomoleculesbySmall AngleX-rayScattering(SAXS)andSurfacePlasmonResonance(SPR),developingof gas-andbiosensorsusingnanohybridthinfilms.

LászlóJanovákisanassistantprofessorattheDepartmentofPhysicalChemistry andMaterialsScienceattheUniversityofSzeged.HereceivedhisPh.D.degreein polymerchemistryattheUniversityofSzegedin2010.Scientiﬁcﬁeldsof inter-est:polymerbasedcompositematerialsforbiomedicalapplications,investigation ofsuperhydrophobicandphotocatalytichybridsurfaces.TeachingofColloid Chem-istryandMacromolecularChemistry.

DánielKovácsisanMScstudentofChemistryattheFacultyofScienceand Informat-icsoftheUniversityofSzeged.Scientiﬁcﬁeldsofinterest:synthesis,stabilization andindustrialapplicationofnanomaterials;preparationandcharacterizationof drugdeliverycarriers;sensors.

AndrásSápiisanAssistantProfessorofChemistryattheDepartmentofApplied andEnvironmentalChemistry,UniversityofSzeged.HereceivedhisPhDin2012in Chemistry.Dr.Sápihas>50scientiﬁcpublicationintheﬁeldofnanomaterialsand materialsciences.Hisresearchfocusesonsizecontrolledmetallicnanoparticles andmesoporousmetal-oxides,aswellasatomicandmolecularlevelinvestigation ofsuchmaterialsunderreactionconditionsinheterogeneouscatalysis.Heisan expertofsurfacescienceandcatalysis.

DorinaG.DobóisaPhDcandidateattheDepartmentofAppliedand Environ-mentalChemistryoftheUniversityofSzeged.MemberoftheMTA-SZTE“Lendület” PorousNanocompositesResearchGroupattheUniversityofSzeged.Scientiﬁcﬁelds ofinterest:Preparation,characterizationandcatalyticapplicationofmesoporous oxidessynthesizedviathesoftandhardtemplatemethod.

ÁkosKukoveczisassociateprofessorofchemistryattheDepartmentofApplied andEnvironmentalChemistry,UnivesityofSzeged,Hungary.HereceivedhisPh.D. in2001.Hisresearchinterestsareinterfacephenomenainnanoporousmaterials, synthesisoflow-dimensionalmaterialsandsensors.Hismaininstrumentalskills areimagingmethodsandRamanspectroscopy.Heisalsointerestedinprocessplant safetyandcontractR&Dworkfortheindustry.HecurrentlyheadstheMTA-SZTE “Lendület”PorousNanocompositesResearchGroup.

ZoltánKónyawasborninHungaryin1971.HereceivedhisM.Sc.in1994, (Chem-istry,UniversityofSzeged,Hungary)andPh.D.in1998(Chemistry,Universityof Szeged,Hungary).Heisprofessorofchemistryandsince2010,heisheadofthe ChemistryInstituteoftheUniversityofSzeged,Hungary.Dr.Kónyahaspublished 250+papersinrefereedscientiﬁcjournalsandco-authored15bookchapters.He holds12nationalandinternationalpatents.Hisresearchinterestisthe devel-opmentofnewnanostructuredmaterialsandtheirapplicationtorealchemical, environmental,biologicalandbiomedicalobjects.

ImreDékányisfullprofessorofchemistryattheDepartmentofPhysical Chem-istryandMaterialsScience,andtheheadoftheresearchgroupontheDepartment ofMedicalChemistry,FacultyofMedicine,UniversityofSzeged.Heismemberof HungarianAcademyofSciencessince2001.Researchinterest:colloidandinterface chemistryandnanostructuredmaterials.MemberoftheeditorialboardofApplied ClayScience.Hehaspublishedover360papers,12bookchapters,heisinventorof over25patentapplications.Thetotalnumberofcitationofhispapersandbooksis 10000+andhisHindexis50.