Fabrication of Thin Films of Phosphonated
Calix[4]Arene Bearing Crown Ether
and Their Gas Sensing Properties
Inci Capan, Mevlut Bayrakci, Matem Erdogan, and Mustafa Ozmen
Abstract— Calix[4]arenes bearing phosphate and crown ether groups are shown to hold considerable promise as the active layer in volatile organic compound (VOC) vapor sensor. Its chemical sensing selectivity and sensitivity can be altered by different alkyl side chains. These organic architectures were successfully deposited by the spin coating method on gold-coated glass surfaces. Surface plasmon resonance technique was used for VOCs detection to evaluate the chemical-sensing properties of these calix[4]arene derivatives. Spun thin films of calix[4]arene derivatives were exposed to a variety of VOC vapors, and the resonance angles changes of these films were recorded for the specific analyte vapors, such as chloroform, carbon tetrachloride, dichloromethane, ethanol, benzene, and toluene. Measurements were made at room temperature, and the responses were found to be fast and appeared to be completely reversible. The sensing results showed similar response patterns, and our data strongly indicate that response of used thin films of calix[4]arene derivatives to chloroform is much higher than those of any other VOCs used with the values of 0.47 × 10−3, 1.042 × 10−3, and 1.952 × 10−3 (% / ppm) for the compounds 1, 2, and 3, respectively. Furthermore, gas sensing interaction mechanisms of the thin films were evaluated in terms of the molar volumes, dipole moments, and refractive indexes of the analyzed gas molecules.
Index Terms— Calixarene, gas sensing, SPR, thin film.
I. INTRODUCTION
I
T IS crucial to determine the concentrations of volatile organic compounds (VOCs) that is a very important prob-lem in terms of health and safety [1]. It is known that volatile organic compounds cause various diseases such as asthma, dizziness and headache during short-term exposure, while various cancer diseases increase risk in the long term [2]. Exposure to even low concentrations of VOCs, especially chlo-roform [3], [4], carbon tetrachloride [5], dichloromethane [6], ethanol [7], benzene [8], and toluene [9], in indoor environ-Manuscript received October 8, 2018; accepted October 28, 2018. Date of publication October 31, 2018; date of current version January 11, 2019. This work was supported in part by the Research Foundation of Selçuk University under Grant 16611114 and in part by the Research Foundation of Karamano˘glu Mehmetbey University under Grant 25-M-15. The associate editor coordinating the review of this paper and approving it for publication was Dr. Camilla Baratto. (Corresponding author: Mustafa Ozmen.)I. Capan and M. Erdogan are with the Department of Physics, Fac-ulty of Science, University of Balikesir, 10145 Balikesir, Turkey (e-mail: inci.capan@gmail.com; merdogan@balikesir.edu.tr).
M. Bayrakci is with the Department of Bioengineering, Faculty of Engineer-ing, Karamano˘glu Mehmetbey University, 70200 Karaman, Turkey (e-mail: mevlutbayrakci@gmail.com).
M. Ozmen is with the Department of Chemistry, Faculty of Science, Selçuk University, 42250 Konya, Turkey (e-mail: musozmen@gmail.com).
Digital Object Identifier 10.1109/JSEN.2018.2878840
ment is dangerous and carcinogenic. Therefore, effective meth-ods such as spectrophotometry, gas chromatography and high performance liquid chromatography have been used to monitor VOCs in an accurate quantification. However, these techniques generally have some disadvantages like relatively expensive, non-portable equipment, time consuming pre-treatment steps and trained operators [10]. However, Surface Plasmon Res-onance (SPR) measurement system can be used to detect VOCs at even low concentrations. In addition, this technique is more convenient than other methods due to its simplicity, fast response, not being affected from outside, etc [11]–[13].
Calixarenes, which have almost unlimited derivatization potential possess many possible applications in some branches of technology such as biotechnology [14], medical and phar-maceutical [15], [16], also became promising molecules for sensor applications due to their sensing and selectivity abilities as host-guest chemistry [17]–[19]. In recent years, the studies with calixarene thin films showed their unique properties of fast and fully reversible adsorption of VOCs in high concentra-tions close to the saturated vapor pressures [20]. A wide used parameter for comparison of measurement methods is sensi-tivity, which gives a linear relation between the measurement signal (frequency and angle for QCM and SPR, respectively) and physical parameters for sensing layer (mass density for QCM or reflective index for SPR). A number of gas sensing studies were carried out by using different techniques. Mass sensitive technique QCM [21]–[23] is widely used for the detection of the gas molecules as well as optical [24], [25] and electrical [26], [27] techniques.
As the best of our knowledge, there are no reports on study of VOCs detection using phosphonated calix[4]arene bearing crown ether. This work is therefore dedicated to the preparation of spun thin films calix[4]arene bearing phosphate and crown ether and their testing to a range of VOCs.
II. EXPERIMENTAL
A. Reagents
The following amphiphilic calixarene molecules (Scheme 1) were used in this work: a range of phosphonated calix[4]arenes having different alkyl chains length of crown ether group on the upper rim, named as compound 1{5,17-bis(dimethoxy-ph-osphonyl)methyl-calix[4]arene(ethyleneamido)crown}, com-pound 2{5,17-bis(dimethoxy-phosphonyl)methyl-calix[4] arene(propyleneamido)crown}, and compound 3{5,17-bis 1558-1748 © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
Fig. 1. Schematic illustration of the experimental process used to fabricate the spun thin films and the SPR gas cell used for the gas sensing measurements.
(dimethoxy - phosphonyl)methyl-calix[4]arene(butyleneamido) crown}, respectively. Their synthesis process and characterization were described previously [28]. Chloroform, carbon tetrachloride, dichloromethane, ethanol, benzene and toluene which were used during gas sensing experiments are purchased from Sigma Aldrich and used without further purification.
B. Thin Film Fabrication
The spin coating thin film fabrication technique is a very convenient way to produce uniform thin films. The substrate is rotated at high speeds to allow the solution spread onto the substrate to form thin film with the help of the centrifugal force. The solutions of the calix[4]arene derivatives have been prepared by using chloroform and DMSO solvents with a volume ratio of 9:1 respectively. Solutions with a concentration of 1 mgmL−1 have been prepared for each calix[4]arene derivative. Spun thin films of the calix[4]arene derivatives have been fabricated using a manually controlled Spincoat G3P-8 model spin coater. A schematic representation of the fabrication of thin films has been presented in Fig. 1. The ultrasonically cleaned substrate was fixed onto the substrate
films fabricated onto quartz glass substrates. AFM images were obtained by using NT-MDT AFM NTEGRA Solaris model instrument. Semi-contact mode of AFM in air was used to investigate the thin film surface morphology. SEM observations were made in a Zeiss EVO LS10 model scanning electron microscope operated at 20 kV. Surface wettability measurements were performed using Krüss FM40Mk2 Easy-Drop contact angle instrument. Contact angle measurements were taken at least five times at different locations on the sur-face. The average values were used in contact angle analysis.
D. Gas Sensing Measurements
SPR system consists of a surface which is prepared to facilitate the resonance of the surface plasmons. For this purpose, a monochromatic light beam hits the thin film at a critical angle that is called SPR angle. A schematic illustration of the prepared surface (thin film fabricated onto a 50 nm thick gold layer mounted onto the glass prism using the index matching fluid) with the gas cell is presented in Fig. 1. SPR curves are recorded where the change in reflected light inten-sity is monitored with the change in the angle of incidence. This experiment is also performed during the injection of the saturated gas into the gas cell, which results with a shift in the angle of resonance. The experimental SPR curve data were fitted using the Winspall software (written by Wolfgang Knoll, developed at the Max-Plank Institute for Polymer Research, Germany) in order to evaluate the film thicknesses (d) and their refractive indexes (n). Kinetic graphs are recorded via the change in the reflected light intensity during injection of the saturated gas into the gas cell for 2 min. followed by injection of dry air into the gas cell for another 2 min. In this work, these measurements are recorded for increasing content of saturated gas in 5 steps changing between 20% and 100% of the saturated gas amount. All gas tests were performed at room temperature and the humidity of the environment was 32%.
III. RESULTS ANDDISCUSSION
A. UV-Visible Measurements
UV-Visible spectra of the solutions of calix[4]arene deriv-atives in chloroform/DMSO solution between the wavelength range of 230 and 350 nm was given in Fig. 2. The maximum absorbance values of the solution spectra of compound 1 and 2 are observed around 265 nm and for the solution spectra of
Fig. 2. (a) UV-Visible spectra of the solutions of calix[4]arene derivatives in chloroform/DMSO solutions between the wavelength range of 230 and 350 nm, (b) the UV-Visible spectra of the spun thin films of calix[4]arene derivatives using chloroform/DMSO solution.
compound 3 is observed at 260 nm. The UV-Visible spectra of the spun thin films prepared using the solutions are given in the inset of Fig. 2. Each spun thin films of calix[4]arene derivatives were characterized with a single absorption peak at 285 nm with a red shift of around 20 nm. This may be a result of the molecular aggregation during the film formation on the solid substrate. This has been reported previously for thin films of other calixarene molecules [29].
B. SPR Measurements
SPR curves which show the variation of the reflected light intensity with the angle of incidence are recorded to specify the appropriate angle of incidence –namely theθSPR- in which the minimum reflected light intensity occur and expressed as [30]
θS P R= sin−1 ε M εP(εM+ 1) 1/2 (1) where εM is the dielectric constant of the metal film sandwiched between a glass prism of dielectric constant εP and air. A shift in the resonance angleθSPRoccurs when a thin film with a thickness d and dielectric constant ε is deposited onto the metal film. This shift (θSPR) is given by [31]:
θS P R= (2π/λ) (|εM|) 3/2 d √ε Pcosθ (|εM− 1|)2ε (ε − 1) (2) where |εM| is the modulus of the real part of the dielectric constant of the metal film and ε is the dielectric constant of the film. The shift in the resonance angle is dependent on the thickness and the dielectric constant of the film deposited onto the gold layer.
Fig. 3. SPR curves of the uncoated gold layer and spun thin films on gold-coated substrate using the compound 1, compound 2 and compound
3 molecules.
In Fig. 3, SPR curve of the uncoated gold layer is presented with a θSPR value of 43.83°. In the same graph the SPR curves of the spun thin films on the gold coated substrate are also presented. A shift in the angle of incidence (θSPR) is observed for all spun thin films with theθSPRvalues of 45.35°, 45.28° and 45.69° for the compound 1, compound 2 and compound 3 thin films, respectively. The shift in the angle of incidence indicates the successful deposition of the spun thin films on the gold layer with theθSPR values of 1.52°, 1.45° and 1.86°, respectively [32].
Any application, which change the thin films’ thickness and/or optical parameters, may cause a shift in the SPR curve of the thin films. An example of this kind of an application is the exposure of the toxic gas molecules into the gas cell in order to provide a change in the physical parameters of the thin film via the interaction of the gas molecules with the thin film for gas sensing detection. In Fig. 4a, the SPR curves of the uncoated gold layer and spun compound 3 thin film before, after and during exposure to saturated chloroform vapor are presented. A shift in the resonance angle was observed in exposure to the chloroform vapor to spun compound 3 thin film with a value of 0.21° and when dry air is injected into the gas cell, it is observed that the SPR curve and θSPR is reversible. Similar SPR curves are obtained during the expo-sure of the saturated carbon tetrachloride, dichloromethane, ethanol, benzene and toluene vapors. θSPR values due to exposure of the gas molecules into the gas cell are listed in Table I. According to the Eq. 2, this shift in θSPR may be a result of the change in thickness of the thin film and/or the change in the optical parameters of the thin film such as refractive index, both real and imaginary parts.
In order to quantify the change in the thin film parameters the experimental SPR curve data were fitted using the Winspall Software. In Fig. 4b, the experimental and fitted data of the SPR curves are presented. The unexposed thicknesses and refractive indexes of the calix[4]arene thin films together with the changes in the thickness (d) and the refractive indexes (n) of the thin films due to the exposure of the
Fig. 4. SPR curves of (a) the gold layer without thin film, spun compound 3 thin film before, during and after exposure to saturated chlo-roform vapor, (b) the experimental and fitted data of the spun compound 3 thin film before, during and after exposure to saturated chloroform vapor. θ shows the shift in angle of incidence caused by the exposure of the gas molecules into the gas cell.
TABLE I
THESHIFT INθSPRIN THESPR CURVES FOR THETHINFILMSDURING
EXPOSURE TOVOLATILEORGANICGASES
analyte gas molecules were presented in Table II. During the fitting process it was assumed that k = 0 for thin films, since they are transparent at λ = 633 nm [32], [33]. The thicknesses of the unexposed spun thin films were found between the range 94 Å and 104 Å with the refractive index values changing between 1.44 and 1.49, which is consistent with the literature [32]–[34]. According to the Table II the exposure of the gas molecules results with a change in both thickness and refractive index with the values up to 7.5 Å for thickness and 0.039 for refractive index. Similar behavior of the calixarene thin films was observed in the literature [35], which implies the swelling of the thin film during adsorption.
Fig. 5. AFM (A and B) and SEM (C and D) images of bare and compound
3 coated glass surfaces, respectively. Water Contact Angles of bare (inset of
image C) and compound 3 coated glass (inset of image D) surfaces.
The biggest change has been observed during the chloroform gas exposure to the spun thin film of the compound 3.
C. AFM, SEM and Contact Angle Measurements
The morphological features of the spun compound 3 thin film were examined by Atomic Force Micros-cope (AFM), Scanning Electron Microscopy (SEM) and Con-tact Angle (CA) Analysis. AFM image shows that the bare glass surface is quite smooth, since it only exhibits features at a scale of about 16 nm (Fig. 5A). Moreover, the root mean square (rms) roughness value of bare glass surface is quite small (0.41 nm). On the contrary, AFM image of compound 3 coated glass surface (Fig. 5B) is rougher with features as large as 90 nm. As a result, the rms roughness value increases from 0.41 nm to 16.13 nm. The increase in rms leads to the increase in the specific surface area. The enhanced sensor
Fig. 6. Kinetic graph for the films of calix[4]arene derivatives recorded during exposure to chloroform vapor.
surface area can then accommodate more adsorption sites, which justifies the enhanced adsorptive properties of spun thin film fabricated with compound 3 [36]. Representative SEM images of bare glass and compound 3 coated film on glass are shown in Fig. 5C,D. Compared to bare glass surface, compound 3 film clearly showed very rough surface that the macromolecule irregularly distributed throughout the surface. Insets of Fig. 5C and D show representative contact angle (CA) measurements of bare glass and compound 3 coated on glass surface. The equilibrium contact angle of bare glass surface was measured as 5°± 0.8° while for the coated film with the compound 3 molecules the measured contact angle of water drop was 68.2° ± 1.5°. This increased CA was attributed to the presence of compound 3 molecules.
D. Sensing Properties of the Thin Films
The kinetic graphs recorded during the successive exposure of the increasing content of the saturated chloroform vapors among 20% and 100% were presented in Fig. 6. Normalized response which is defined with Eq. 3 has been calculated as the difference between the reflected light intensity of the baseline (I0) and the observed reflected light intensity during the cycles (I) and the resultant quantity is divided by the baseline intensity and multiplied by 100.
N or mali sed Res ponse(%) = I − I0 I0 × 100
(3) The time allowed to the thin films of calix[4]arene deriva-tives for their interaction with the gas molecules was 2 min. which was followed by another 2 min. for the dry air injection. The similar kinetic graphs have been recorded for all tested VOCs but they are not presented here. However the calibration curves which give information on the normalized response of the thin films versus the concentration of these gases are presented in Fig. 7. Limit of detection values for the analyzed VOCs were calculated via the calibration curves were found to be down to 10.23×103ppm for chloroform, 5.07×103ppm for carbon tetrachloride and 17.01×103ppm for dichloromethane. Higher values compared with the chlorinated gases were calculated as 39.02 × 103 ppm for ethanol, 23.1 × 103 ppm
Fig. 7. Calibration graphs obtained during exposure to the chlorinated VOCs for compound 1 (a), compound 2 (c) and compound 3 (e) thin films and during exposure to ethanol, benzene and toluene vapours for compound 1 (b), compound 2 (d) and compound 3 (f) thin films.
TABLE III
THERESPONSE OF THETHINFILMSPER PPM FOREACHANALYTE
GASMOLECULESTOGETHERWITHSOMEPHYSICAL
PROPERTIES OF THEGASMOLECULES
for benzene and 25.02 × 103 ppm for toluene. The sensitivity of the thin films can be estimated by using the slope of the calibration curves in terms of the normalized response value per ppm of each gas molecules. The obtained values are listed in Table III with some physical properties of the gas molecules such as dipole moments, molar volumes and the refractive indexes.
It is observed that the responses per ppm are much more higher for all thin films in case of the exposure of the chlorinated solvents compared with the other anayte gases. Compared with the gas sensing performance of compound 1, compound 2 and compound 3 thin films, the best results were observed for compound 3 especially for chloroform vapor. The high dipole moment and small molar volume of the chloroform and dichloromethane gases are believed to
When the response of the individual gases are investigated it is seen that the higher response is detected using the compound 3 thin film to chloroform vapor. This situation is probably due to the nature of chloroform molecules. Because, in literature, it is well-known that the lack of more polar acidic protons in both molecules prevents the possible intramolecular interaction which is support the effective binding between compound 3 and VOCs molecules [41]. Furthermore, high selectivity of compound 3 thin film to chloroform vapor can be attributed to the electron rich two-amide groups of crown ring that selectively interacts with electron deficient Cl atom of chloroform molecule [42]. It might be possible that interaction of the Cl atom of chloroform with the π-electrons of the amide groups provides a preorganization of the macromole-cule. In addition, although all used calixarene derivatives have same chemical structure, crown rings of compound 1 and compound 2 have not suitable cavity sufficiently for the other VOCs molecules with respect to the compound 3 and this situation causes steric interaction with the VOCs molecule. Therefore, chloroform is believed to bind to the crown ether ring only from the least hindered side, possibly using crown moiety consisted of 1,4-diamidobutane [43].
The effect of compound 3 as a sensor in detection of chloroform afforded by the calixarenes was most probably due to the incorporation of the non-polar portions of chloroform molecule into the non-polar cavity of the calixarene skeleton similar to chloroform:calixarene complexes [23]. Furthermore, compared the compounds 1-3 bearing ethyl, propyl and butyl chains between two amide functions of the crown skele-ton, compound 3 have more flexible, and low polar crown moiety with respect the other compounds 1 and 2. These flexible and non-polar properties have also enhanced the possible molecular complex formation between compound 3 and chloroform by weak non-polar inter-molecular forces as van der Waals and thus proving the crucial role of crown moiety in the recognition process. In addition, the increase of molecular size or of the number of carbon atoms in aliphatic crown linkages (from ethyl to butyl) increased the chloroform binding capability of compound 3. Owing to these excellent properties of crown moieties, modification of calixarene skeleton with crown moieties has been studied intensively and these derivatives have been extensively used in the chemical applications such as cations, anions and neutral recognitions [44], [45].
work and future studies will illuminate new design dimensions for sensors based phosphonated calix[4]arene bearing crown ether sensing materials.
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Inci Capan was born in Balikesir, Turkey, in 1977.
She received the M.Sc. and Ph.D. degrees in physics from the University of Balikesir, in 2003 and 2008, respectively. Since 2018, she has been a Professor at the Department of Physics, University of Balikesir. Her research interests include organic thin film depo-sition and their use as gas sensors in environmental applications.
Mevlut Bayrakci received the B.Sc. degree from the
Department of Chemistry, Nigde University, in 2004, and the M.Sc. and Ph.D. degrees in chemistry from Selçuk University, Konya, Turkey, in 2007, and 2012, respectively. He has been an Associate Profes-sor at Karamano˘glu Mehmetbey University, Turkey. His research interests are in the design and synthesis of macrocyclic compounds, such as calixarene and crown ether and their use as drug solubilizing agents and their metal complexes.
