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Sensors and Actuators B 195 (2014) 156–164

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

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

reparation and gas sensing properties of Langmuir–Blodgett thinlms of calix[n]arenes: Investigation of cavity effect

ustafa Ozmena,∗, Zikriye Ozbekb,∗∗, Mevlut Bayrakci c, Seref Ertula,ustafa Ersoza, Rifat Capand

Department of Chemistry, University of Selcuk, Konya 42075, TurkeyDepartment of Bioengineering, University of Canakkale Onsekiz Mart, 17100 Canakkale, TurkeyUlukisla Vocational School, University of Nigde, Nigde 51100, TurkeyDepartment of Physics, University of Balikesir, Balikesir 10145, Turkey

r t i c l e i n f o

rticle history:eceived 10 June 2013eceived in revised form0 December 2013ccepted 12 January 2014vailable online 20 January 2014

eywords:

a b s t r a c t

Characterization and organic vapor sensing properties of Langmuir–Blodgett (LB) thin films ofcalix[n]arene (n = 4, 6, 8) derivatives are reported in this work. Surface pressure–area isotherm graphshows that very stable monolayers are formed at the water surface. The results indicate that good qual-ity, uniform LB films can be prepared with a transfer ratio of over 0.95. Calix[n]arene LB films have beencharacterized by contact angle measurements, quartz crystal microbalance (QCM), scanning electronmicroscopy (SEM) and atomic force microscopy (AFM). LB film of calix[8]arene which has the largestcavity yields a gradient with a mass value of 773 ng per layer according to the QCM results. AFM and

alix[n]areneapor sensingangmuir–BlodgettCM

SEM images showed a dense surface morphology obtained for all samples. QCM system was used for themeasurement of sensor response against chloroform, benzene, toluene and ethanol vapors. These LB filmsamples yield a response to all vapors with a large, fast, and reproducible due to the adsorption of vaporsinto the LB film structures. Among them, calix[8]arene LB film has higher sensitivity toward the organicvapors because of a large cavity size. This study can be concluded that the cavity size of calix[n]arenemolecule could have an important role in the research area of room temperature vapor sensing devices.

. Introduction

Langmuir–Blodgett (LB) thin film technique is an elegantethod to fabricate ultra thin films with a controlled thickness at

molecular level and with different molecular orientations [1]. Inhe last decade LB technique was used for several calix[n]arenesue to their different functional groups, structural characteristicsnd stability [2–4]. Applications of calix[n]arenes macrocyclic hostompounds in material sciences are becoming widespread andnclude mass [5], ion [6] and optical [7] sensor, non-linear optics,

olecular tectons [8] in crystal engineering and LB films for gas sep-ration [9]. The monitoring of volatile organic compounds (VOCs)s a very important task due to stringent environmental standardsnd regulations on VOCs in many countries and their natural tox-

city is dangerous for the environmental and human being [10].OCs are extensively used in our home or office environmentsuch as laser printers, cleaning solvents, paints, wood preservatives,

∗ Corresponding author. Tel.: +90 332 223 38 93; fax: +90 332 241 24 99.∗∗ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 41.

E-mail addresses: musozmen@gmail.com (M. Ozmen), zikriye@comu.edu.trZ. Ozbek).

925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2014.01.041

© 2014 Elsevier B.V. All rights reserved.

carpet backing, plastics, and cosmetics. They are highly danger-ous and can cause many diseases for example acute, chronic orlong-term effects such as affecting the nose, throat and lungs thatasthma-like reactions, eye irritation and cancer [11]. The funda-mental shape of calixarenes is that of a cup with a defined upperand lower rim and a central annulus, enabling calixarenes to act ashost molecules as a consequence of their preformed cavities. It iseasy to modify either the upper and/or lower rims to prepare var-ious derivatives with differing selectivities for various guest ionsand small molecules [12,13].

In this study, the preparation of LB films of calix[n]arene (n = 4,6, 8), derivatives was evaluated at the water surface using isothermgraphs. Investigation of composition and structural organizationof films on glass substrate was performed by contact angle (CA),atomic force microscopy (AFM) and scanning electron microscopy(SEM) results. Quartz Crystal Microbalance system (QCM) wasused to demonstrate the thin film deposition on a quartz crystalsubstrate. Furthermore we used these films to investigate thecavity effect of calix[n]arenes on organic vapors such as benzene,

chloroform, toluene and ethanol. The cavity effect is cavity diam-eter of the calix[n]arene compounds having different cavity size.Cavity sizes of calix[4]arene, calix[6]arene, and calix[8]arene are3.0, 7.6, and 11.7 A, respectively. With increasing cavity sizes,

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M. Ozmen et al. / Sensors and

exibility of calixarene skeleton increases and this situation affectshe host-guest interaction behavior of calixarenes. Therefore, thisffect was discussed in the manuscript and supported by someublished literature results.

. Experimental details

.1. Materials

In this study, however p-tert-butyl calix[4]arene (1), p-tert-butylalix[6]arene (2) and p-tert-butyl calix[8]arene (3) given in Fig. 1 areommercially available, we synthesized them in our laboratory tobtain sample of higher purity as described before [14–16]. All calixolecule structures have been characterized through 1H NMR, FTIR

ATR) and Elemental analysis and they were in cone conformationn solution as proved by the appearance of ArCH2Ar which displays

typical AB type proton signal at 3.20–4.20 ppm (J = 13.1–13.3 Hz),espectively.

Glass substrates from Fisher Scientific were cleaned prior tourface activation, first by shaking with detergent for 30 mint 70 ◦C, then in sonication bath (Bandelin Sonorex RK 255 H)ith acetone (10 min), dichloromethane (10 min) and ultra pureater (18.2 M� cm) (2 min, several times), respectively. These sub-

trates were placed in an individual vial in which RCA1 mixture of2O:H2O2:NH4OH (5:1:1, v/v) and subsequently RCA2 mixture of2O:H2O2:HCl (5:1:1, v/v) was added and sonicated for 30 min at0 ◦C in order to activate the surface further by creating a greaterensity of hydroxyl groups. The substrates were carefully washedith copious amounts of ultra pure water, then dried in a flow ofitrogen and annealed at 100 ◦C under vacuum for 4 h.

.2. LB film preparation and characterization

A computer control NIMA 622 alternate LB trough was employedo study the molecular behavior of our molecules at the water sur-ace and to produce LB films onto glass and quartz substrates. Beforeach experiment, barriers and the Teflon trough of the LB film

Fig. 1. Chemical structures of used calix[n]arene molecules for LB films.

ators B 195 (2014) 156–164 157

system were rinsed with ultrapure water after being cleaned withchloroform. The surface pressure was measured by using a Wil-helmy balance, equipped with a strip of chromatography papersuspending at the air–water interface at the room temperature(20 ◦C) which was controlled using Lauda Ecoline RE204 modeltemperature control unit. Calix[n]arene molecules were dissolvedin chloroform with a concentration of 1 mg mL−1 and were subse-quently spread onto ultrapure water subphase at pH 6. Solutionswere spread by a Hamilton microliter syringe onto the subphasesolution by distributing the droplets over the entire trough area.A time period of 15 min was allowed for the solvent to evapo-rate before the area enclosed by the barriers was reduced. Thepressure–area (�–A) isotherm graph given in Section 3.1 wasdetermined with the accuracy of 0.1 mN m−1. (�–A) graphs wererecorded as a function of surface area using the compression speedof barriers at a value of 172 mm2 min−1.

LB films were characterized by using contact angle measure-ments, AFM and SEM techniques. Surface wettability measure-ments of the obtained LB films were performed by measuring thecontact angle of 5 �L sessile water droplets on their surface usingKSV CAM 200 (Finland) goniometer. Contact angle measurementswere taken at least five times at different locations on the surface.The average values were used in contact angle analysis. Measure-ments showed a standard deviation of the contact angle of less than3◦. The surface morphology of bare glass and calix[n]arene LB filmswere characterized using a Solver Pro AFM from NT-MDT (Russia).Tapping mode of AFM in air was used to investigate the surface mor-phology of films. The morphology of films was also characterizedby a Zeiss EVO LS-10 field emission SEM instrument equipped withan Inca Energy 350 X-Max (Oxford Instruments, UK) spectrometerwas used to obtain SEM images. Samples were sputter-coated withAu (60%) and Pd (40%) alloy using a Q150R (Quorum Technologies)instrument. Images were obtained at 3 × 10−4 Pa working pressureand 15 keV accelerating voltage using InLens detection mode (2 mmworking distance).

2.3. QCM measurements

In order to study the sensing properties of these calix[n]areneLB films, a thinly cut wafer of raw quartz sandwiched betweentwo electrodes in an overlapping keyhole design was used for theQCM measurement. A block diagram of our home made QCM mea-surement system is shown in Fig. 2. Standard QCM crystals witha nominal resonance frequency of 3.5 MHz were commercializedfrom GTE SYLVANIA Company. All experiments were carried outat room temperature (20 ◦C) using an oscillating circuit designedby us. The quartz crystal was inserted into the electronic controlunit, and the frequency of oscillation was monitored as a functionof time using dedicated software. The values of frequency changes,which indicate the degree of response, are measured with an accu-racy of 1 Hz. After each deposition cycle, the LB film sample wasdried for half an hour and the mass change was monitored usingthis computer controlled QCM measurement system. This systemwas used for the confirmation of the reproducibility of LB film mul-tilayers using the relationship between the QCM frequency changesagainst the deposited mass, which should depend on the numberof layers in the LB film.

A special gas cell was constructed to study the kinetic responseof calix LB films on exposure to organic vapors by measuring thefrequency changes. These measurements were performed with asyringe. The response was recorded as a function of time when thesample was periodically exposed to the organic vapors for at least

2 min and was then allowed to recover after injection of dry air.This procedure was carried out during several cycles to study theconcentration changes and to observe the reproducibility of the LBfilm sensing element. All organic vapor measurements were taken

158 M. Ozmen et al. / Sensors and Actuators B 195 (2014) 156–164

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Fig. 2. Schematic diagram of the

n dry air condition in a small gas cell which could eliminate theffect of water vapor on the response properties of calix[4]areneB films [17].

. Results and discussions

.1. Isotherm results

The pressure–area (�–A) isotherm graphs given in Fig. 3ere determined with the accuracy of 0.1 mN m−1. Calix[4]arene

Fig. 3. �–A isotherm graph of calix[4,6,8]arene monolayer.

made QCM measurement setup.

monolayer shows the smallest area per molecule because it hasthe smallest molecular weight and area. After the gas phase,this molecule shows a very sharp increase until surface pressureof 24 mN m−1 and then monolayer at the air–water interfacestarts to collapse. When the molecular weight is increased, phasetransitions and collapse point are also increased.

In studies, calix[n]arene derivatives having different cavity sizeon the upper and lower rim is a common factor [18,19]. So the areaper molecule depends on molecular cavity size of calix[4,6,8]arene.Fig. 3 demonstrates that increasing the size of the cavity diameteron the lower rim leads to the shift of the isotherm to higher area dueto the stabilization of the molecule in its cone conformation. Thispartial monolayer collapse at around 24 mN m−1 may be inducedby conformational changes in the calix[4]arene, calix[6]arene andcalix[8]arene LB films. For the calix[6]arene, Ishikawa et al. [20]also observed a weak flexure in the isotherm at a surface pres-sure around 25 mN m−1, more similar to that observed by us forthe calix[6]arene, and a great flexure for the calix[8]arene at a sur-face pressure 18 mN m−1. Davis et al. [21] also observed a flexurein the isotherm of a calix[8]arene ethyl ester at a surface pres-sure of around 30 mN m−1, more similar to that observed by usfor the calix[8]arene. In the authors’ opinion, the conformationalchanges favor the slip out of some molecules from the monolayerbecause of the increasing surface pressure, leading to a flexure inthe isotherm. As the conformational changes are molecular cavitysize dependent, collapse point are proportionally increased withthe cavity size, which is observed experimentally. Our isotherm

graph has a good agreement with the isotherm graph of thecalix[n]arene monolayer given in the literature [22]. Monolayersof calix[n]arene molecules at the water surface were found tobe stable and surface pressures of 20 mN m−1 were selected for

M. Ozmen et al. / Sensors and Actuators B 195 (2014) 156–164 159

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Table 1Gradient of QCM results and deposited mass values.

LB film Gradient (Hz/layer) �m (ng)

Calix[4]arene 29.4 795

ig. 4. Contact angle measurements of Langmuir–Blodgett films substrates: (a) barelass surface, (b) calix[4]arene LB film, (c) calix[6]arene LB film, (d) calix[8]arene LBlm.

ll LB film depositions on the quartz crystal substrate for QCMeasurements.

.2. Wettability measurements

We measured the contact angle as an indirect confirmation ofhe coating of the calix[n]arene on the glass substrate. The con-act angle is very sensitive quantitative indicator of wettabilityf the activated glass surface upon coating of the calix[n]arene.ontact angle measurements were carried out using the sessilerop method to investigate the wetting properties of the formedB films on glass (Fig. 4). The change in the contact angle revealshanges in the hydrophilic character of the surface, which in turnan be related to the formation of the calix[n]arene layers on glass.he water drop contact angle on bare glass surface depends sig-ificantly on the surface pretreatment and can vary between 3◦

nd 15◦. For our glass surfaces, the equilibrium contact angle ofilli-Q water on activated glass surface was measured as 3◦ ± 1◦

hile for the glass coated with the calix[4]arene, calix[6]arenend calix[8]arene molecules the measured contact angles of waterrop were 80.18◦ ± 1.3◦, 84.65◦ ± 1.5◦ and 89.9◦ ± 0.8◦, respectively.e envisage that these increases attributed to the ascending alkyl

roups of calix[n]arenes [23].

.3. AFM and SEM analysis

Fig. 5 shows AFM images for the calix[n]arene with the char-cteristics of LB films transferred from water subphase, revealinghe nanometric structure of the films. Films are quite homogeneousnd smooth, however the image for calix[6]arene shows very bigslands increasing in height in comparison with others (Fig. 5c).he LB films for calix[4]arene and calix[8]arene present condensedslands but comparing the AFM images, it seems that the film forhe calix[8]arene is slightly more uniform since the number of

slands is much more (Fig. 5b and d). Analyzing the image of Fig. 5t is obtained on a 5 �m × 5 �m scale that the area roughness andoot-mean-square (rms) are, respectively, 2.00 nm and 2.95 nm forare glass surface, 13.02 nm and 16.59 nm for calix[4]arene LB film,

Calix[6]arene 15.3 413Calix[8]arene 28.6 773

18.91 nm and 24.20 nm for calix[6]arene LB film and 3.04 nm and3.81 nm for calix[8]arene LB film.

Comparison of the images by scanning electron microscopy ofthe activated glass substrate (Fig. 6a) with that of the calix[n]areneLB films (Fig. 6b–d) suggests that a matrix of organic calix[n]arenemolecules have been formed. The surface morphology shows thatthe growths of the compounds are uniform, continuous and com-pact. However there are regions of overgrowth as is evident inFig. 6b.

3.4. QCM measurements

QCM measurements can be basically described a mass per unitarea by measuring the change in frequency on quartz crystal res-onator in real-time. The resonance frequency is disturbed by theaddition or removal of a small mass due to film deposition at thesurface of the acoustic quartz crystal substrate. QCM measurementsare easily made to high precision; hence, it is capable of measuringmass changes as small as a fraction of a monolayer or single layer ofatoms. This high sensitivity and the real-time monitoring of masschanges on the sensor crystal make QCM a very attractive tech-nique for an LB film research area. Especially, it is widely applied tomonitor the deposition quality of thin films on a quartz crystal sub-strate because the resonant frequency, �f, is extremely sensitive toa small mass change given by [24]:

�f = −2f 20 �m

�1/2q �1/2A

(1)

where f0 is the initial frequency of the crystal (Hz), �m is the masschange (g), A is the piezo-electrically active area (0.785 cm2), �q

is the density of quartz (2.648 g cm−3), �q is the shear modulus ofquartz (2.947 × 1011 g cm−1 s−2).

Fig. 7 gives the relationship between the deposited mass ofcalix[n]arene LB film monolayer and frequency shift. A system-atic change in the frequency with an increase in the number ofmonolayer is clearly observed. The change of frequency as a func-tion of the number of monolayer is closely associated with the LBlayer mass change, and the process was shown to be reproducible.This linear relationship suggests that an equal mass per unit area isdeposited onto the quartz crystal and a uniform LB film structureis fabricated in this work. The mass deposited on the quartz crys-tal per layer is given in Table 1. Similar results are described thatthe increasing surface pressure increases the amount of depositedmass [25]. If an extrapolation of each curve to the intersection withY-axe is taken, it is obvious that only curve of calix[4]arene goesnear point of origin (0,0). The extrapolations of calix[6]arene andcalix[8]arene curves do not show the same behavior. For exam-ple, the intersection of calix[6]arene and calix[8]arene curves are277 Hz and 77 Hz, respectively. This can be explain that more thanhalf of total mass of LB film was transferred on the first cycle whileothers second, third and fourth steps together added less mass thanthe first one. This decrease of deposition mass onto the quartz crys-tal substrate could be due to a change of the surface morphology

of deposited layers. Similar results are given in the literature usingother calix[8]arene LB films and it was concluded that the deposi-tion process is strongly depend on the surface interaction betweensubstrate and monolayer at the water–air interface [3].

160 M. Ozmen et al. / Sensors and Actuators B 195 (2014) 156–164

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Fig. 5. AFM images of Langmuir–Blodgett films: (a) bare gl

It can be seen from Table 1 that the deposition of LB film layer

s highly depending on the cavity diameter [26–28]. Althoughalix[8]arene has the largest ring cavity diameter, calix[4]areneas the highest gradient due to the number of tert-butyl groups.he difference in the number of tert-butly groups and the cavity

rface, (b) calix[4]arene, (c) calix[6]arene, (d) calix[8]arene.

diameter of these two compounds may cause variation in their

coating properties [29–31]. We proposed that these two materialsare much better organized than calix[6]arene since it has thelowest gradient. Therefore, the OH groups of calix[6]arene mayeasily projected outside the cavity [32].

M. Ozmen et al. / Sensors and Actuators B 195 (2014) 156–164 161

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the normalized response as a function of time when the samplewas periodically exposed to chloroform vapor with a concentration

Fig. 6. SEM images of Langmuir–Blodgett films: (a) bare gl

The VOCs sensitive property of calix[n]arene LB film was inves-igated by using QCM sensing measurement system. The VOCsensing mechanism in LB thin films is usually given by three wayss surface adsorption effect, bulk diffusion and desorption process.n the initial process, the measurements were based on a sharplyrequency shifts (Hz) due to the surface adsorption effect betweenB film and VOCs [33]. After this interaction, the bulk diffusionrocess causes an increase in effective mass, which reduces theesonant frequency of the crystal, in direct proportion to the VOCsressure. The frequency change is directly proportional to the num-er of adsorbed VOC molecules. After the fresh air is injected byyringe into the sensing chamber, the gas molecules adsorbed tohe surface of the thin film are quickly separated from the surface.herefore, a rapid decrease on sensor response occurs while thedsorption continued. When the numbers of adsorbed and des-rbed molecules are equal, the frequency shift achieves a stable

alue until the fresh air flushed into gas cell. If these effects are fullyeversible with no long-term drift effect, this can be giving a highlyeliable and repeatable VOCs sensing thin film. The relevant detec-ion parameters like sensitivity, reversibility and reproducibility

Fig. 7. Frequency shift as a number of layers.

rface, (b) calix[4]arene, (c) calix[6]arene, (d) calix[8]arene.

were also evaluated for various VOCs using the real time kineticmeasurements. In an attempt to investigate the kinetic response ofcalix[n]arene molecules with different cavities, all LB films exposedto chloroform vapor. Using QCM measurement technique the reso-nance frequency was recorded as a function of time and is presentedin Fig. 8. The normalized response described in Eq. (2) is calculatedas the difference between the observed frequency response (f) andthe baseline frequency response (f0). The resultant quantity is thendivided by the baseline frequency response.

Normalized response = f − f0f0

(2)

The values of �f [�f = (f − f0)], which indicate the degree ofresponse, are measured with an accuracy of 1 Hz. Fig. 8 shows

value of 173.91 × 103 ppm for 2 min and followed with an injectionof dry air for another 2 min period. The concentration values of

Fig. 8. The response of chloroform vapor to all calix[n]arene LB films.

162 M. Ozmen et al. / Sensors and Actuators B 195 (2014) 156–164

Table 2The concentration values of organic vapors.

Organic vapors � (g/cm3) M (g/mol) c (20%) ×103 ppm c (40%) ×103 ppm c (60%) ×103 ppm c (80%) ×103 ppm c (100%) ×103 ppm

Chloroform 1.483 119.38 34.78 69.56 104.34 139.13 173.91Benzene 0.876 78.11 31.41 62.83 94.25 125.67 157.09

7 79.31 105.75 132.190 143.85 191.81 239.76

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Toluene 0.870 92.14 26.43 52.8Ethanol 0.789 46.11 47.95 95.9

rganic vapor (see Table 2) in ppm are calculated by the formula asollows [17]:

= �V(22.4 L mol−1) 106

M V0(3)

here c (ppm) is the concentration of vapor, � (g/mL) is the densityf vapor, V (mL) is the volume of vapor which is injected into theas chamber, M (g/mol) is the vapor molecular weight, and V0 is theolume of the gas chamber (∼0.002 L). The vapor volume values aresed in this study in the following order: 20% for V = 2 mL, 40% for

= 4 mL, 60% for V = 6 mL, 80% for V = 8 mL, and 100% for V = 10 mL.It is very clear that all calix[n]arene molecules yield a fast and

eversible response to chloroform vapor. The highest responsesere yielded with calix[6]arene and calix[8]arene molecules. It

ould be that the cavity plays a role for the adsorption of vaporolecules. In literature, it is well-known that possible solid inclu-

ion complexation is obtained between chloroform molecule andalix[4]arene compound by hydrogen binding and/or anotherolecular interactions such as van der Walls interaction and

ipole–dipole attraction in cavity of calixarene [34]. Also, simi-ar interaction was observed in this manuscript for calix[6] andalix[8]arene molecules. But this interaction was increased withncreasing cavity diameter of calix[n]arenes (from 4 to 8). Thistructural difference has an important effect on the complexa-ion properties of these calixarenes, since the accessible basket of-tert-butylcalix[8]arene is effectively larger than that of p-tert-utylcalix[4 and 6]arene. Comparing the inner cavity size of thealix[n]arenes, it is expected that the larger calix[8]arene cavityould geometrically be more suited for a closer and stronger inter-

ction with chloroform than the smaller calix[4 and 6]arene cavity.lso, this situation is accordance with published literature resultsf our groups toward neutral organic molecules [14,16]. During theesorption process the vapor molecules were taken away from theas cell, thus the frequency change decreases rapidly.

Fig. 9 displays the kinetic measurement where �f has beenlotted as a function of time when calix[n]arene LB thin filmsere exposed to different VOCs (chloroform, benzene, toluene and

thanol) in air for 2 min followed by injection of dry air for another min. The concentration of VOCs was increased from the first tohe last injections from 20% to 100%. The response of calix[n]areneB thin films to organic vapor is reversible when the gas cell isushed with dry air. This kinetic measurement was carried out foreveral cycles to observe reproducibility of sensor material. Whenhe sensor was exposed to chloroform vapor of the same (constant)oncentration, a plot of this response is shown in Fig. 10. These threeycles of kinetic measurement indicated that the sensor responses reproducible.

The kinetic responses of the calix[n]arene LB films in the form ofrequency change to all vapors are almost reversible with responsend recovery times given in Table 3 in the order of a few secondshen the gas cell is flushed with dry air. The first stage in VOC

nalysis is to flush a reference gas (dry air) through the sensor tobtain a baseline. The sensor is exposed to the vapor, which causes

hanges in its output signal until the sensor reaches steady-state.he vapor is finally flushed out of the sensor using the dry air and theensor response returns back to its baseline. The time during whichhe sensor is exposed to the vapor is called to as the response time

Fig. 9. Kinetic measurements of calix[n]arene LB films as a function of time: (a)calix[4]arene, (b) calix[6]arene, (c) calix[8]arene.

M. Ozmen et al. / Sensors and Actu

Fig. 10. Kinetic measurement of a constant concentration for chloroform.

Table 3Response and recovery times of calix[n]arene LB films.

LB Film Vapor Response time(s)

Recovery time(s)

Calix[4]arene Chloroform 6 10Benzene 4 8Toluene 4 11Ethanol 4 10

Calix[6]arene Chloroform 5 11Benzene 6 10Toluene 5 10Ethanol 5 9

Calix[8]arene Chloroform 4 7

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Benzene 3 8Toluene 4 10Ethanol 4 8

hile the time it takes the sensor to return to its baseline resistances named the recovery time.

The calix[6]arene derivative provided a high affinity towardhloroform. The affinity of the calix[8]arene derivative was almostquivalent to that of calix[6]arene. However, since the interactionetween chloroform and the calix[4]arene was weak, the chloro-orm binding was lower. These results indicate that the cavity sizef calixarenes is one of the most important factors for chloroforminding. It is expected that the larger cavities would geometri-ally be more suited for a stronger interaction with neutral organicolecules [14,35]. However, calix[8]arene have less effect than

alix[6]arene. This situation might be attributable to the dimen-ion of calix[6]arene which is most optimal for the chloroformolecule, because “host-size selectivity” does exist in host-guest

ype complexation with calixarenes [36,37]. Hydrogen bonding andeak interaction forces as van der Waals and dipole–dipole are

he most important forces responsible for the binding of chloro-orm. A large number of articles have been published to elucidatehe inclusion phenomena by calixarene derivatives [38–40]. Ineneral, calix[6]arene or calix[8]arene, having larger cavity thanalix[4]arene, are thought to be suitable for the inclusion of organicolecules. Furthermore, calix[6,8]arenes have flexible structure

wing to the large cavity of calixarene skeleton. However, aftermmobilization of calixarene derivatives on the surface, this flexi-ility can be converted to rigid structures. Thus, this rigidificationf the calixarenes leads to the stable and facile preorganisationetween the calix[6,8]arenes arms and organic molecules.

. Conclusion

The characterization and organic vapor sensing propertiesf LB thin films of calix[n]arene derivatives having different

[

[

ators B 195 (2014) 156–164 163

cavity diameter were investigated. They are very well orderedat the air–water interface as an LB monolayer which is trans-ferred at several substrates with a transfer ratio of over 0.95. Usingwettability measurements, the contact angles have determinedas 80.18◦ ± 1.3◦, 84.65◦ ± 1.5◦ and 89.9◦ ± 0.8◦ for calix[4]arene,calix[6]arene and calix[8]arene, respectively. AFM and SEM imagesalso showed us that the morphology of LB films were different thanbare glass substrate. It is concluded that the calix[n]arene filmson the glass surface are uniform, dense, and homogeneous withsome surface aggregates. The resonance frequency is recorded as afunction of time to investigate the kinetic response of calix[n]arenehaving different cavity sizes as an LB film toward chloroform, ben-zene, toluene and ethanol vapors. We yielded the highest responseto chloroform vapor with a fast and almost fully reversible responsein the order of a few seconds using calix[8]arene LB film. Expos-ing to other vapors calix[n]arene LB films were more selective tochloroform than other vapors with a large, fast, and reproducibleresponse. It could be concluded that the complexation of neutralmolecules depends on the structural properties of the calix[n]arenesuch as hydrophobic cavity diameters, hydrogen binding ability,stability or rigidity, and also depends on dipole–dipole interactionbetween calix[n]arene and neutral molecules.

Acknowledgement

Financial support for this work is provided by the ResearchFoundation of Selcuk University (BAP) and is gratefully acknowl-edged.

References

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Biographies

Mustafa Ozmen received the MSc, and PhD degrees in chemistry from Selcuk Uni-versity, Konya, Turkey, in 2006, and 2011, respectively. He was appointed as aresearch assistant from 2005 to 2011 at the Chemistry Department of Selcuk Uni-versity in Turkey, and since then he was as a research assistant doctor at the samedepartment. His research interests include self-assembly, micro/nano patterningtechniques, synthesis of nanoparticles (magnetic, gold, silver and TiO2) and theirfunctionalization, organic thin film deposition via Langmuir–Blodgett technique andthe spectroscopic and optical characterizations of organic thin film materials andtheir applications as biosensor.

Zikriye Özbek received her MSc, and PhD degrees in physics from the University ofBalikesir, Turkey in 2007, and 2012, respectively. She has appointed as an assistantprofessor from 2013 at the Bioengineering Department of Canakkale Onsekiz MartUniversity in Turkey. Her research area is fabrication of Langmuir–Blodgett thinfilms.

Mevlut Bayrakci received his BSc degree in Department of Chemistry from NigdeUniversity in 2004 and MSc and PhD degrees in chemistry from Selcuk Univer-sity, Konya, Turkey, in 2007, and 2012, respectively, under the supervision of Dr.Ertul and Prof. Yilmaz. His research interests are in the design and synthesis ofmacrocyclic compounds such as calixarene and crown ether and their use as drugsolubilizing agents as well as their metal complexes. Currently, he has been work-ing as an assistant professor at the Ulukisla Vocational School of Nigde University inTurkey.

Seref Ertul received his BSc in Department of Chemistry from Ataturk University,Erzurum, Turkey in 1989 and MSc degree in Department of Chemistry from Sel-cuk University Konya, Turkey in 1991 and PhD degree in Department of Chemistryfrom Selcuk University Konya, Turkey in 1997. He has been working as an asso-ciate professor at the Chemistry Department of Selcuk University in Turkey. Hisresearch interests include design and synthesis of supramolecular structures basedcalixarene, crown ether and/or Schiff bases and their use as sensors toward metalcations and toxic anions.

Mustafa Ersoz has received his MSc degree at University of Selcuk (Konya, Turkey) in1985. He obtained his PhD at University of Glasgow in 1994. Following postdoctoralexperience at GKSS Research Center, Germany, he then spent 1 year within thesurfactant and colloid research group at University of Hull, United Kingdom. Muchof his research is focused toward the incorporation of self-assembled monolayerswithin ultra-thin films and their applications, as well as patterning techniques suchas microcontact printing. He is the author of in excess of 120 published papers. Heis a member of the Turkish Academy of Sciences (TUBA).

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 in Turkey.

He had a PhD scholarship from Turkish High Education Council between 1993 and1998 and had Oversea’s Research Student Award (UK) from 1995 to 1998. His maininterests are pyroelectric heat sensor, gas sensor for environment applications, theelectrical and optical properties of organic thin film materials. He has been workingas a professor since 2007 at the University of Balikesir.