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Sensors and Actuators B 196 (2014) 328–335

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo u r nal homep age: www.elsev ier .com/ locate /snb

welling behavior of pyrene-labelled polystyrene LB thin filmxposed to various volatile organic vapors

ikriye Ozbeka,∗, Matem Erdoganb, Rifat Capanb

Department of Bioengineering, Faculty of Engineering, Canakkale Onsekiz Mart University, 17020 Canakkale, TurkeyDepartment of Physics, Faculty of Science, University of Balikesir, Cagis Campus, 10145 Balikesir, Turkey

r t i c l e i n f o

rticle history:eceived 23 July 2013eceived in revised form 1 February 2014ccepted 6 February 2014vailable online 15 February 2014

eywords:angmuir–Blodgettolystyrene

a b s t r a c t

In this work thin films of pyrene-labelled polystyrene (PS) prepared Langmuir–Blodgett (LB) film tech-nique are characterized by UV–visible spectroscopy, Atomic Force Microscopy (AFM) and Surface PlasmonResonance (SPR). The sensing behaviors of the films were investigated with respect to volatile organiccompounds (VOCs) at room temperature. The sensing responses of the films against VOCs (chloroform,benzene, toluene and ethanol) were measured by SPR method. In addition, the effects of film morphol-ogy and gas sensing properties were discussed. It was found that the PS films exhibited good response,reversibility, stability and faster response and recovery characteristic to VOCs. The changes in reflectivityimplied the swelling behavior of PS thin film during adsorption and can be explained by the capture of

iffusion coefficientOCs

organic vapor molecules. Fick’s law for early-time diffusion was adopted to quantify real time SPR datafor the swelling processes. It was observed that diffusion coefficients (D) for swelling obeyed the t1/2

law and could be correlated with the VOCs used. The response of PS films to the choosed VOCs has beeninvestigated in conditions of physical properties of the solvents, and the films were obtain to be largelysensitive to chloroform vapor compared to other studied vapors.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Volatile organic compounds (VOCs) are widely used in indus-ry and our daily life. Measures and experiments must be taken toontrol VOC diffusion because VOC vapors are flammable, harmfulo human health, or detrimental to the ozone layer. For the in situetection of VOCs, highly sensitive, portable and relatively inexpen-ive sensors are demanded. Gas sensors that have been investigatedor perception application include quartz crystal microbalanceQCM), surface plasmon resonance (SPR) sensor, metal oxide sen-or (MOS), conductive polymer (CP) resistor, field effect transistorFET), etc. [1–9]. Polymeric materials can be often used as sensi-ive material for the detection of VOCs because of their ability tobsorption and desorption of different VOCs [10]. For the purpose ofensing gas molecules, the Langmuir–Blodgett (LB) thin film muste functionalized with a gas sensitive layer. Polymeric materialsan be easily deposited and produced using the LB [6,11,12], drop

oating [13,14] or spin coating [14,15] techniques. A large fieldf polymers has been utilized in gas sensors for VOCs detection16–20]. Polymer chains are supple, and molecules can simply be

∗ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 41.E-mail address: zikriye@comu.edu.tr (Z. Ozbek).

ttp://dx.doi.org/10.1016/j.snb.2014.02.024925-4005/© 2014 Elsevier B.V. All rights reserved.

absorbed into polymer items. This absorption process results in aswelling and a mass increase of the polymeric materials [21–24].

To better measure and quantify the above mentioned propertychanges, these polymer films may first be coated onto substratesas thin films or ultra thin films by using the LB methods. TheLB technique, with which the thickness, compositions as well asmolecular orientation of the produced film can be well controlledas compared with cast film, is often employed to the production ofnanoscale thin films. LB film is an ultra thin assembly of orientedorganic monolayers obtained by transferring monolayers from aliquid surface onto a solid support, e.g. glass or gold-coated glassslides (used in SPR), which are highly sensitive to mass change andare capable of detection in the sub-nanogram range. Gold-coatedglass slides which used for measurement in SPR produced withLB films, when exposed to VOCs undergo a change in intensity ofreflected light readings, which is linearly-related to the correspond-ing mass change of the adsorbed molecules on the LB film-coatedsubstrate [25–29]. The sensors based on the above technique will bemore sensitive, fast responsive, portable and convenient to observethe property changes. It is very important to investigate response

to VOCs of organic thin films and its mechanism. The importantdifficulty in gas determination is the fabrication of stable sensorswith high sensitivity and very good selectivity of the substance tobe defined. There are various surface analytical techniques. A few

Z. Ozbek et al. / Sensors and Actuators B 196 (2014) 328–335 329

yrene

mirbeibsawSitmscrabm

ptSlvrsFtsa

2

foPcta

t

TSp

d

Fig. 1. Synthesis of pyrene labelled polyst

easurement techniques such as SPR and QCM are used to mon-tor and detect various chemical gases because of their wideange of potential applications. The SPR phenomena has recentlyeen applied to bio- and chemical-sensors in thin metal films isxtremely sensitive to optical and structural properties of the metalnterface and has been utilized in the basic Kretschmann SPR haseen used for both bio- and chemical sensors due to it high sen-itivity and high resolution. On the other hand, gas sensors havelso been fabricated by taking advantage of the SPR phenomenon,hich enables ultra-high-resolution detection. In this study, the

PR system is used for gas measurements. When gas cell which usedn kinetic measurement changed, we worked to observe affectso which the extent diffusion coefficients. The SPR system has a

uch smaller volume according to the gas cell used in the QCMystem. As seen of the diffusion coefficient values, diffusion coeffi-ient is interaction is proportional with the surface interaction. As aesult, sensing surface in gas sensors is an important function. Thedvantage of the SPR system, the sensitivity of against the VOCs cane examination even smaller surfaces, ability to perform real-timeeasurement and exceptional sensitivity.In this work, polymeric film formed from pyrene-labelled

olystyrene (PS) chains were dependent to various concentra-ions of partly saturated VOCs to study swelling mechanism. UsingPR measurement system, variations in the intensity of reflectedight was monitored in real time during swelling in which organicapor is introduced into a gas cell. The organic vapor uptakeeasons an increase in the transparency of polymeric film thathows the changes in reflected light intensity during swelling.ick’s second law for diffusion [30] was deployed to determinehe diffusion coefficients for the swelling process. Furthermore, thetructure–property relationship and the interaction mechanism arelso analyzed.

. Experimental details

PS was synthesized by using a pyrene functional Atom Trans-er Radical Polymerization (ATRP) initiator in the polymerizationf styrene which was polymerized in bulk at 110 ◦C using 1-yrenylmethyl-2-bromopropanoate (PMBP) and CuBr/Bpy as theatalyst. The chemical structure of PS is shown in Fig. 1, the syn-

hesis of polymer has been reported previously [31]. Reactant rationd molecular weight of PS molecule are summarized in Table 1.

Typical coating techniques are based on vertical movement ofhe plate. Monolayers deposited during immersion are described

able 1ynthetic conditions and molecular weight characteristics of pyrene labelledolystyrene.

Materiala Reactant ratiob Mn (PDI)c

PS 130:1:1:3 4980 (1.18)

a Temperature 110 ◦C; reaction time 4 h; bulk.b [PS]0/[PMBP]0/[CuBr]0/[Bpy]0.c Determined by gel permeation chromatography relative to polystyrene stan-

ards.

by atom transfer radical polymerization.

as X-type, monolayers deposited during removal are describedas Z-type, and monolayers deposited during both immersion andremoval are described as Y-type. Moreover, there are three diversetypes of layer deposition: the first layer, upstroke transfer duringmultilayer deposition, and downstroke transfer during multilayerdeposition. The first layer may be different from next layers sincethe molecules interact with the bare substrate instead of the head-groups or tails of the previous layer. The different energetics canimpact the dynamics of transfer as well as the structure of the firstlayer. During multilayer deposition, the upstroke and downstroketransfer are clearly different firstly because of the essentiality ofwater drain during the upstroke layer which raises the specter ofpossible non-equilibrium kinetic effects upon the structure. Wepreferred to investigate the X-type film (having and even num-ber of layers) deposition method on account of how an impact willbe occur when coating is started with the tail on the solid surface.However, other parameters were kept constant. It is known that theformation of the LB thin film contribute positively to by increasinghydrophobic interactions of the long tail group. The X-type thinfilm is prepared as follows (a) As the substrate passes through thesurface, hydrophobic ends stick to the hydrophobic substrate (e.g.,glass slide). (b) As the substrate is withdrawn hydrophilic endsstick to the hydrophilic ends of the deposited film. Our previouswork revealed that PS derivatives coated as Y-type LB thin filmon the surfaces [31]. In the present work, PS containing pyrenethin films with low molecular weight (Fig. 1) have been synthe-sized and characterized. In light of the objectives of this study, PScontaining pyrene low molecular weight selected due to the inves-tigation of the dissolution and the sensitivity against VOCs of smallhydrophobic molecules in the core of the micelle.

The films were fabricated with LB technique using and X-type LBfilm was prepared using a NIMA 622 type alternate layer computer-controlled LB trough on a pure water subphase (25 ◦C, resistance18.2 M� cm, pH 5.6). In this study we used quartz slides for UV–vismeasurements and gold-coated slides for SPR measurements sub-strates. Quartz and gold-coated slide substrates were cleaned byultrasonication in acetone for 10 min followed by ultrasonicationin ethanol for 10 min. The solution was prepared by dissolving2.0 mg PS in 10.0 ml chloroform to give corresponding concen-trations of 0.2 mg ml−1. This solution was spread onto the watersurface using a Hamilton syringe and ∼20 min were allowed forthe chloroform to evaporate. LB film monolayers onto the watersurface were deposited onto quartz glass slides for UV–vis andatomic force microscope (AFM), gold-coated slides for SPR mea-surements respectively. UV–vis spectra of the solution and thefilm deposited on quartz glass slides were recorded on an OceanOptics UV light source (DH-2000-BAL Deuterium Tungsten lightsource) spectrophotometer in a range from 180 to 900 nm. The sur-face morphology of the film deposited on glass was investigatedby a tapping-mode AFM. The surface thickness and gas sensingproperties of thin film on gold-coated slides were analyzed on

the Biosuplar 6 SPR measurement system. This X-type is consid-ered to suppress the hydrophobic interaction which induces thenon-uniform structure in layers. The designed X-type SPR sensorinclude the structural advantages in the sensing layers for SPR

330 Z. Ozbek et al. / Sensors and Actua

mw

3

3

stiia

sfhfippiopa

3

affiat

rao(eivnTws

Fig. 2. Isotherm graph of PS.

easurements. These advantages are higher sensitivity and theider detection range.

. Results and discussion

.1. Isotherm graph of LB films

The isotherm graph recorded for PS is given in Fig. 2. The solutionpread onto the water surface was 100 �l for PS. It is clearly shownhat the PS molecules form regular monolayers on the air–waternterface which has been shown before [31]. The solid phase whichs suitable for the thin film fabrication is found to be between 10nd 20 mN m−1 for all chemicals.

The isotherm graph given in Fig. 2 was recorded at a compres-ion rate of 20 mm min−1. With regard to the solid phase range,rom the isotherm graph the surface pressure value of 16 mN m−1

as been chosen for the LB thin film fabrication and the floatinglms of PS were transferred onto the substrates by vertical dip-ing method at a dip speed of 10 mm min−1. X-type LB films wererepared with a surface pressure of 16 mN m−1 which this value

s a good agreement with the surface pressure (10–20 mN m−1)btained for similar polymer materials [32–37]. The thin films wereroduced onto quartz and gold-coated glass substrates for differentpplications such as UV–vis, AFM and SPR measurements.

.2. Absorption spectra

UV–vis is one of the techniques used to obtain informationbout of LB films characterization on the quartz glass slides sur-ace coated with PS. The known technique to control multilayer LBlm producibility is to monitor the change of UV–vis absorbances a function of layer [38,39]. This correlation keep informed abouthe transfer quality and deposition of the monolayers.

First of all, UV–vis was obtained of PS solution dissolved in chlo-oform as seen in the inset of Fig. 3. A large peak was observedt 340 nm in the PS solution spectrum. Absorption of UV–vis inrganic molecules (i.e. PS) is limited to specific functional groupschromophores) that including valence electrons of low excitationnergy. The spectrum of a molecule containing these chromophoress complicated. This is because the superposition of rotational andibrational transitions on the electronic transitions gives a combi-

ation of coming lines. This seems as a sustained absorption band.he UV spectrum of PS has one maxima absorption peak 340 nmhich could be from �–�* transitions of the conjugated �–electron

ystem. This peak is effect of pyrene material in the molecule and

tors B 196 (2014) 328–335

�–�* transitions are related on because of C C double bond of thebenzene ring present in the substance [40]. Subsequently, UV–visspectra was obtained relation changes number of layers. Fig. 3shows the electronic absorption spectra for PS LB films with differ-ent number of layers on quartz slides. They are observed similarspectral in literature and that present the LB film producibility[41–44]. They are blue shift in the short wavelength effected about16 nm after transfer process on the substrate. This shift explainsthe molecular cluster while ranked regularly in the form solidstate [45–49]. During thin film production require more energyfor molecular regulation onto the water surface. With increase ofenergy occurs a shorter wavelength shift (hypsochormic shift). Inliterature, UV–visible spectrums in pyrene derivative studies wereobtained between 327 and 330 nm wavelength [41–43]. As seenin Fig. 3, while the number of layers increased, absorption inten-sity also increased. The top right inset of Fig. 3 shows the plotof absorbance at 324 nm vs. number of layers. It is clear that theabsorption caused by the first two layers is higher than the follow-ing layers. These results are in good agreement with UV–vis spectraresults which were obtained in the pyrene derived studies [41–44].

3.3. Morphology of PS LB film

The surface morphology and the surface root mean squareroughness (rms) of the film deposited on glass slide were inves-tigated by a tapping-mode AFM. A cross section of each AFM imagewas analyzed to determine the mean surface roughness.

Fig. 4 is the AFM images and layer analysis of PS thin filmsdeposited on a flat glass slides. The AFM analysis indicate that 4,12 and 20 layers PS thin films are not homogeneously and regu-larly with rms of 63.33 nm, rms of 60.45 nm, and rms of 118.00 nm,respectively. Moreover, the images show a disordered array of“island aggregation” of PS distributed over the whole surface. Thereason which found to high of rms value is based on molecular clus-ters with long chains of polymer molecules during the formationof thin films and as result formations high hills. Rms values of PSmolecules are observed directly proportional with the number oflayers. These results are in good agreement and consistent withthe literature values [50]. On the other hand, this morphology hasproved to be very useful for gas sensing, in order to promote ingressand egress of the VOCs into and from the thin film. Moreover, thisfeature enhances the surface area of the porous sensing film, whichrepresents an advantage for sensing purposes since, as known, gassensing of thin films is primarily characterized by a fast surfaceadsorption process.

3.4. Gas sensing properties of PS LB film

The Biosuplar 6 is an ideal tool for developing SPR and wave-guide structures for use as gas, bio- or other sensors. Such sensordevices usually related on measuring the angular shift of an SPRresonance or waveguide propagation mode caused by the adsorp-tion of thin layers of target materials on the surface of the sensor.A typical SPR sensor structure consists of a glass slide covered onone side with a thin layer of gold or silver. In this method, a smalldrop of index matching oil is placed on the surface of the Biosu-plar 6 measuring prism to make contact to the uncoated side of theSPR sensor device. Fig. 5 shows the experimental optical set-up forSPR measurements based on Kretschmann configuration [51] anda typical SPR configuration used in SPR-PS gas sensing systems.

This device has been successfully used to study the absorption

and desorption of vapors at different values. The Biosuplar 6 SPR’scomputer-driven rotary table varies the incident angle, and a sharpdip in the light reflected from the glass/gold film interface, causedby the SPR resonance, occurs. The SPR resonance angle depends on

Z. Ozbek et al. / Sensors and Actuators B 196 (2014) 328–335 331

F p betw

tattoa

sbiswg±oetsFtrs

dvisritictharhB

tVf

ig. 3. UV–vis absorption spectra of PS LB films, The inset: (right) linear relationshi

he laser wavelength and the gold film thickness and type, but thengle is also strongly affected by the presence of even extremelyhin layers present on the gold surface. This extreme sensitivity ofhe resonance angle to both the thickness and the refractive indexf thin adsorbed layers make the SPR structure an ideal sensor typest the nanolayer level.

A p-polarized monochromatic (� = 632.8 nm) He–Ne laser lightource was used to excite surface plasmons. An optical contactetween these samples and a semi-cylindrical prism (of refractive

ndex np = 1.515) was made using an index matching liquid (ethyl-alicylate from Aldrich). The shift in the position of SPR resonanceas employed to measure the deposition of PS LB film layer onto the

old-coated slide substrate. Thickness of gold at gold-coated slide is50 nm. Changes in the intensity of reflected light (Irf) as a functionf internal angle were examined for PS LB films at the three differ-nt thicknesses. The SPR curves are shifted to larger angles whenhe number of layers is increased. As seen by the linear increase inhift of SPR angles with the number of layers shown in the inset ofig. 6, it suggests that equal mass per unit area is deposited ontohe gold-coated slide during the coating of PS LB film layers. Similaresults were found for LB thin films studied by a SPR measurementystem [51–53].

In our SPR system, we have done an experiment in order toetermine the intensity of reflected light change of the organicapors, the response of with VOCs and dry air without VOCs. Typ-cal kinetic surface plasmon resonance curve is performed in fourteps: (i) initial baseline, (ii) binding to the surface until equilib-ium is reached, (iii) disassociation from the surface as a dry air isntroduced, and (iv) reestablishment of the baseline value signal vs.ime. Fig. 7 shows in this work for the calibration of SPR system, thenitial sharp increase corresponds to the total of gas cell pressurehange when vapor molecules injected into the gas cell. It is obvioushat the response of dry air without VOCs is smaller than with VOCsowever a very small jump of the intensity of reflected light in dryir is occurred when it is compared with VOCs. If it was completelyelated with injection pressure, maximum responses for VOCs mustave the same level or same the intensity of reflected light change.ut there is a reasonable difference between all responses.

Fig. 7 displays the kinetic measurement where Irf has been plot-ed as a function of time. PS LB thin film was exposed to differentOCs (chloroform, benzene, toluene and ethanol) in air for 5 min

ollowed by injection of clean air for another 5 min. The response of

een absorbance and the number of bilayers, (left) UV–vis spectra solution of PS.

PS LB film to organic vapor is reversible when the gas cell is flushedwith dry air. The response against VOCs is very fast, reversible andreproducible. The response is quickly increased for the few secondsafter injection of organic vapor and then observed to decrease expo-nentially.

All organic vapor measurements were taken in dry air situationin gas cell which could eliminate the effect of water vapor on theresponse properties of PS LB film. In the literature the adsorptionof PS molecules were compared to between the water and VOCs.The results showed that polymeric adsorbent with poly (styrenedivinyl benzene) matrix had minimal uptake of water, which is anindication of an extremely hydrophobic surface. It is clearly shownthat the influence of water vapor on the very hydrophobic VOCs,i.e., benzene adsorption onto poly (styrene divinyl benzene) wasthe weakest [1]. As a result of these we believed that the watervapor effect in our experimental conditions is minimized and canbe negligible. Therefore the effect of water vapor on the responseproperties is not measured or studied in this work.

When Fick’s second law of diffusion is applied to a plane sheetand solved by assuming a constant diffusion coefficient, the follow-ing equation is obtained for concentration changes in time [53]:

C

C0= x

a0+ 2

�

∞∑n=1

cos n�

nsin

n�

a0exp

(−D2

n�2

a20

t

)(1)

where a0 is the thickness of the slab, D is the diffusion coefficient,and C0 and C are the concentration of the diffusant at time zero andt, respectively. x corresponds to the distance at which C is measured.We can replace the concentration terms directly with the amountof diffusant by using:

M =∫

V

CdV (2)

When Eq. (1) is considered for a plane volume element andsubstituted in Eq. (2), the following solution is obtained [30].

Mt

M∞= 1 − 8

�2

∞∑n=0

1

(2n + 1)2exp

(− (2n + 1)2D�2

a20

t

)(3)

332 Z. Ozbek et al. / Sensors and Actuators B 196 (2014) 328–335

Fig. 4. 3D AFM images of PS (a) bare substrate (b) 4 layers (c) 12 layers and (d) 20layers thin films at a scale of 10 �m × 10 �m fabricated at a deposition pressure of16 mN m−1.

Fig. 5. A schematic diagram showing Kretschmann configuration for a kinetic mea-surement system of prism–gold layer–LB thin film layer of PS molecules–air.

Fig. 6. SPR curves of PS LB film as a function of internal angle (inset: angle changeas a function of number of layers).

Fig. 7. The kinetic measurements for the first cycle of organic vapor adsorption(inset: repoducibility of the film for chloroform).

where Mt and M∞ represent the amount of diffusant entering theplane sheet at time t and infinity, respectively. This equation can

be reduced to a simplified form:

Mt

M∞= 4

√D

�a20

t1/2s (4)

Z. Ozbek et al. / Sensors and Actuators B 196 (2014) 328–335 333

Fo

wet

tmcsetcbpvdrmep

(

wattffiv

tocT

TT

Fig. 9. Plot of intensity of reflected light against square root of swelling time, ts. Thesolid line represents the fit of the data to Eq. (6).

ig. 8. Normalized intensity of reflected light against swelling time, ts for variousrganic vapors.

here ts represent the swelling time, which is called early-timequation and this square root relation can be used to interpret forhe swelling data [53].

To measure the kinetic data given in Fig. 7 it is required to takehe PS LB film parameters due to swelling. Fig. 8 represents the nor-

alized intensity of reflected light against swelling time where theonsolidation process involves setting starting times to t = 0 for eachwelling cycles. As seen in Fig. 8, Irf decreased as the time of vaporxposure is increased. It is also seen that changes in Irf against theime of vapor exposure decreased very fast as the chloroform per-entage concentration injected into the gas cell is increased. Theseehaviors can be declared with the chain interdiffusion betweenolymer chains during vapor exposure. As the saturated chloroformapors penetrate into polymeric film, the polymer chains inter-iffuse and transparency of the polymeric film increases, whichesults in the decrease of intensity of light reflected from the poly-eric film. These results can be related to the amounts of diffusant

ntering the polymeric film Mt; that is, Irf should be inversely pro-ortional to Mt [53,54]. Equation (4) now can be written as:

Mt

M∞

)∼(

Irf(t)

Irf(∞)

)−1

= 4

√D

�a20

t1/2s (5)

here Irf(t) and Irf(∞) are intensities of reflected light at any time, tnd saturation point in Irf, respectively. The normalized Irf intensi-ies [Irf(∞)/Irf(t)] are plotted in Fig. 9 for the square root of swellingime according to Eq. (5). The slopes of the linear relations in Fig. 9ound the diffusion coefficients, Ds for the swelling of polymericlm and those values are plotted in Fig. 10 vs. saturated organicapors content.

Gas molecules enter into the polymer chains and expanding

he volume of chain that is caused to swelling. The interactionf produced PS thin films with gas molecules depend on physi-al properties of organic vapors. The physical properties given inable 2 of the VOCs used in this work. According to this table,

able 2he physical properties of the VOCs.

Molar volume(cm3 mol−1)

Solubility parameter(MPa)1/2

Viscosity (cSt)

Chloroform 80.70 18.80 0.380Benzene 86.36 18.70 0.744Toluene 107.00 18.30 0.680Ethanol 58.50 19.40 1.200

Fig. 10. The dependence of the swelling diffusion coefficients, Ds vs. saturated dif-ferent organic vapors content.

ethanol vapor has the lowest molar volume. Gases which have smallmolar volume more penetrates into thin film. But the viscosity valueof ethanol vapor is higher than other vapors, so diffusion slowlyactualizes into thin film. Another factor, the solubility parameterof thin film material and these gases, since it can be expected thatthe gas is selectively adsorbed on the thin film when the solubilityparameter of the gas coincides with that of the thin film mate-rial. Consequently, it was found that of PS solubility parameter (ı):about 19.0 (MPa)1/2 [31], almost coincides with that of chloroformsolubility parameter: 18.80 (MPa)1/2. As a result, it was obtain thatthe PS-film-coated sensor exhibited a high sensitivity and an excel-lent selectivity for chloroform gas. Fig. 7 shows the typical transientresponses of PS-coated sensor for various gases, each with a sameconcentration. It can be seen that the sensor has an excellent selec-tivity and high sensitivity for chloroform although the sensor alsoresponds to benzene, toluene and ethanol. Exposure to these vaporswas found to have a larger effect on the solubility parameter of thisfilm and showed that the film diffusion coefficient increased as aresult of film swelling.

Additionally, for all vapors interacting with PS thin film; inten-

sity of light reflected enlarge with increasing concentrations ofvapor. When was transferred from dry air, the intensity of lightreflected return to the original value rapidly. As seen from Fig. 7,

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34 Z. Ozbek et al. / Sensors and

hloroform, benzene, toluene and ethanol vapors are interactedith PS thin film the fastest response and the maximum inten-

ity of light reflected change takes place under the influence of Clapor. The reason is that, atoms of chlorine which is one of theighest tendency to attract electrons are found in chloroform. Theeak hydrogen band interaction is between chloroform vapor andolymer.

. Conclusion

In this study, we concentrated on finding the gas sensor thatossesses an excellent selectivity for harmful vapors such as chlo-oform, benzene, toluene and ethanol gases. The response time,eversibility, sensitivity and selectivity of the PS LB layers are inves-igated and discussed. According to the experimental results, highlyensitive sensors for each of the four analytics are suggested. Its shown that the penetration of organic vapors into PS LB film isuick enough and diffusion coefficients are connected on the VOCssed. This result can be related in terms of solubility parameters,olar volume and viscosity parameter of swelling items. Diffusion

oefficients for chloroform are higher than other organic vaporsecause chloroform has lowest molar volume and relatively higholubility parameter which indicates that chloroform molecules areore mobile than the other organic molecules and diffuse easily

nto the PS LB film. Otherwise, benzene has a larger viscosity param-ter and also a larger molar volume which means these moleculesre slow to diffuse into the PS LB film, indicating lower diffu-ion coefficients than chloroform. Diffusion coefficients found to beetween 0.66–14.06 × 10−17, 0.85–9.26 × 10−17, 0.35–5.47 × 10−17

nd 0.04–0.42 × 10−17 cm2 s−1 for chloroform, benzene, toluenend ethanol vapors, respectively. Ethanol also has larger solubil-ty parameter and viscosity parameters which explain its loweriffusion coefficients than the other vapors. Also gas interactiontudies in literature [55], diffusion coefficient values were found to.05 × 10−12 and 7 × 10−14 cm2 s−1. Our diffusion coefficient val-es have a compatible with the diffusion coefficient value of theolystyrene molecules given in the literature [55]. As a result,olymer–solvent interaction plays a dominant role during swellingrocesses. The PS LB film may find potential applications in theevelopment of room temperature vapor sensors for chloroformapor. Therefore, gas sensors based on PS are very promising fornvironmental and industrial applications.

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Biographies

Zikriye Özbek received her MSc, and PhD degrees in physics from the Universityof Balikesir, Turkey in 2007, and 2012, respectively. She was appointed as an assis-tant professor from 2013 at the Bioengineering Department of Canakkale OnsekizMart University in Turkey. Her research area is fabrication of Langmuir–Blodgettthin films and the modelling of diffusion process in thin films during gas sensorapplication.

Matem Erdogan graduated from Cumhuriyet University, Sivas-TURKEY in 1990 andreceived his MSc degree at Illinois Institute of Technology, Chicago – US in 1996and his PhD degree at Istanbul Technical University, Istanbul – Turkey in 2003. Hejoined Physics Department, Balikesir University in 2004. He is deeply involved inswelling, drying, shrinking, aging and slow release processes in polymeric gels byusing steady-state and time resolved fluorescence spectrofluorometric techniques.He is also working on the modelling of diffusion process in polymeric thin filmsduring gas sensor application.

Rifat Capan received MSc degree at Hacettepe University, Physics EngineeringDepartment in 1991, Ankara – Turkey and his PhD at the University of Sheffield(UK) in 1998. He established first Langmuir–Blodgett Thin Film Research Group inTurkey. He had a PhD scholarship from Turkish High Education Council between1993 and 1998 and had Overseas Research Student Award (UK) from 1995 to 1998.His main interests are pyroelectric heat sensor, gas sensor for environment applica-tions, the electrical and optical properties of organic thin film materials. Dr. Capanwas appointed as assistant professor between 1999 and 2002 and associate professor

of physics department in 2001 and was the head of physics department between2003 and 2006. He has been working as a professor and the head of Departmentat the University of Balikesir since 2007. He is a member of American ChemicalSociety.