Article pubs.acs.org/Langmuir
Investigation of the Conformational Changes of a Conducting Polymer in Gas Sensor Active Layers by Means of PolarizationModulation Infrared Reflection Absorption Spectroscopy (PM-IRRAS). Luciano Caseli,*,† Jonas Gruber,‡ Rosamaria W. C. Li,§ and Laura O. Péres† †
Laboratory of Hybrid Materials, Federal University of São Paulo, Diadema, SP, Brazil Institute of Chemistry, University of São Paulo, São Paulo, SP, Brazil § Universitary Center Estácio UniradialCampus Vila dos Remédios, São Paulo, SP, Brazil ‡
S Supporting Information *
ABSTRACT: Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PMIRRAS) was employed to observe the changes in the molecular conformation of poly(2phenyl-1,4-xylylene) (PPPX) films that occurred after exposure to organic solvent vapors. The PPPX films were supported on solid matrixes by casting, spin-coating, and Langmuir−Blodgett (LB) techniques. The results show that the polymer is sensitive to the solvent vapors, which affect some of the vibration dipole moments, as detected by PM-IRRAS. The sensitivity depends on the method employed to immobilize the polymer, with more significant changes in films formed using techniques that result in a less systematically organized conformation. This feature enables the use of surface vibration spectroscopy to detect organic solvent vapors and may be applied in an artificial nose. In this work, poly(2-phenyl-1,4-xylylene) (PPPX)4 films were produced by casting, spin-coating, and Langmuir− Blodgett techniques. Conductance measurements of casting films of conducting polymers can be sensitive to ethanol present in fuels,6 but molecular understanding of the polymer conformation upon vapor contact has not been clear until the present. Changes in the conformation of the polymer immobilized on the films when exposed to organic solvents can be investigated in detail using PM-IRRAS. For this reason, in this work PM-IRRAS was employed to investigate at the molecular level the conformation of PPPX immobilized on the films, with different levels of organization and thickness, when exposed to organic solvents with different chemical natures.
1. INTRODUCTION The human nose is able to detect approximately 10 000 different odors, including toxic and unpleasant vapors that can affect the immune system.1 Artificial noses, although limited when compared to the human nose, are devices that can detect vapor, generating a signal, which can be electric, optical or gravimetric. These noses can be used to detect contaminated food, beverages, and other systems whose vapors are toxic to human beings.2,3 Electronic noses with conducting polymers as active layers have been reported as useful tools in environmental control4,5 and for automotive fuel analysis.6 However, the detection mechanism of polymeric sensors that are exposed to volatile compounds has not been completely elucidated, which is an important step for the development of better sensors. Studies concerning the literature on organic polymer sensors and their response mechanism are scarce. Published papers focus on the characteristics of the films that form the sensors. For instance, Mehne et al.7 employed Polarization-Modulation Infrared Reflection−Absorption Spectroscopy (PM-IRRAS) to elucidate the morphology of the polyethylene glycol (PEG) used in the sensor. Infrared spectroscopy has also been used to discriminate different red wines.8 In this sense, PM-IRRAS is used to distinguish molecular changes of molecules present in surfaces. This technique is specific for surfaces, is supported on either liquids or solids,9 and has been employed to identify molecules present in monolayers supported at liquid−air interfaces 10−12 and ultrathin films supported on solid matrixes.13,14 However, nothing has been reported, to the best of our knowledge, on changes in the molecular structures of ultrathin films when exposed to organic solvent vapors. © 2013 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Preparation of Polymer. Poly(2-phenyl-1,4-xylylene) (PPPX) (Figure 1) was synthesized via cathodic reduction of 2,5bis(bromomethyl)biphenyl as described in the literature.4 2.2. Deposition of the Films. The films were formed on glass slides by drop-casting, spin-coating and Langmuir−Blodgett methods. PPPX was deposited by casting from a solution containing 4.5 mg of
Figure 1. Structure of PPPX. Received: October 25, 2012 Revised: January 29, 2013 Published: February 1, 2013 2640
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
Langmuir
Article
polymer and 5.0 mL of chloroform. A thin (10−30 μm) uniform layer of PPPX was deposited on a sensor substrate by spin-coating a solution containing 4.5 mg of PPPX and 5.0 mL of chloroform (300 rpm). For LB films, the concentration used before spreading was 0.5 mg mL−1. The quartz slides used to deposit the films were cleaned by treating with a 5% KOH in ethanol solution in an ultrasonic bath for 5 min. For the LB film transfer, the polymer solution was first spread onto the surface of water contained in a Langmuir trough (mini trough KSV instruments, Finland), compressed to the desired surface pressure and then transferred to solid supports. The transfer was carried out with a dipping rate of 1 mm min−1 at a constant surface pressure (30.0 ± 0.2 mN/m) so that the layer was obtained by raising the substrate from the aqueous subphase. Then, the transfer was done by the vertical dipping method. To test the stability of the monolayer before the transfer, the film was compressed until 30 mN/m and the barriers then stopped. In the sequence, the surface pressure decrease was followed. In 30 min, only a decrease of less than 1.0 mN/m was observed, which confirms the monolayer stability. Before transferring the film, the monolayer was kept at a constant surface pressure, and negligible variation in surface area was observed. The transfer ratio in the deposition (1.05) was employed as an indicator of the quality of the LB deposition, and only the regularly built-up LB films were considered for further characterization. All LB films built were carried out as a single monolayer deposition. All films were produced at a constant temperature of 25 ± 1 °C. Before exposing the films to organic vapors, the thickness of the films were estimated using AFMNanoscope IIIA instrument (hold in “Laboratório de Filmes Finos from IFUSP”), with a silicon tip, in the taping mode. 2.3. Infrared Measurements. Polarization-Modulation Infrared Reflection Absorption Spectroscopy measurements were taken with a KSV PMI 550 instrument (KSV instrument Ltd., Helsinki, Finland). The polymer film supported on the solid substrate was set up so that the light beam reached the monolayer at a fixed incidence angle of 75°, which provides the best signal-to-noise ratio. The incoming light was continuously modulated between s- and p- polarization at a high frequency, which allowed the simultaneous measurement of the spectra for the two polarizations. The difference between the spectra provided surface-specific information, and the sum provided the reference spectrum. With simultaneous measurements, the effects of water vapor and carbon dioxide are largely reduced. Then, PM-IRRAS signal was calculated based on the equation (Rp + Rs)/(Rp − Rs), where Rp and Rs are the reflectivities obtained for the p-polarized light and s-polarized light, respectively. Normalized signal were obtained using the expression (S(d) − S(0))/S(0), where S(d) and S(0) are the signals for the covered and uncovered solid substrate, respectively. Therefore, spectra obtained for the solid substrate without the polymer were considered as background spectra. The signal-to-noise is improved with the number of PM-IRRAS spectra obtained. More theoretical aspects of the technique are described elsewhere.9 First, the plates were permitted to dry at room temperature over a period longer than 1 day. The PM-IRRAS spectra obtained for the films confirmed the absence of any solvent before the exposure to the vapors. In the sequence, after the PM-IRRAS spectra of the pristine material, the film was exposed to an environment containing organic solvent vapor (chloroform, ethanol, hexane, or benzene, Merck), and a new spectra was collected to determine the influence of the solvent vapor. For that, the solvents were used in pure state and no dilution was carried out, in order to provide a saturated environment in a closed chamber. The solvent was removed, and a new spectrum was collected after at least 2 h permitted to the completed removal of the vapor. The solvents chloroform, ethanol, hexane, and benzene were chosen because they are frequently employed as organic solvents, and because they have different polarity natures.
Table 1. Estimative of the Thickness of PPPX Thin Films film
thickness (nm)
casting spin-coating LB films (1 layer)
103.4 63.4 5.4
and the casting films are the thickest. This result is important considering the different degree of organization of these films, which is close related to their thickness. The films were exposed to the vapor of organic solvents to understand their influence on the polymer conformation. It was noticed that for times longer than 15−20 min, there is no significant alteration in the infrared spectra, and for this reason, an exposure time of 30 min was chosen before obtaining the infrared spectra for all cases. Although a few seconds are needed to detect changes in certain properties of polymeric films, as reported in conductance measurements,6 this longer time was necessary for a reliable comparison of the effects between the different films employed. Also, it is important to mention that the original solvent must influence the initial polymer conformation. To minimize this effect, the films were permitted to dry, and PM-IRRAS spectra were followed until no solvent signal has been detected. For casting films (Figure 2), it is possible to observe the polymer bands as described in the literature.4 The aromatic ring
Figure 2. PM-IRRAS spectra for PPPX casting film exposed to the solvents, as indicated in the inset, for 30 min. The curve without any solvent (black) is also shown for comparison. (A.U. = arbitrary unity).
stretching modes are present, and the most intense band at 1483 cm−1 is negative to the baseline, suggesting that the aromatic rings were predominantly perpendicular to the substrate surface.15,16 When the polymer was exposed to the organic solvents chloroform, ethanol and hexane, the band at 1483 cm−1 became positive to the baseline, which indicates that the aromatic rings were mostly parallel to the matrix surface. Additionally, the band at 1603 cm−1 indicates the presence of the solvents. The results for the CH aliphatic bending modes were different. Originally, the two bands (1437 and 1324 cm−1) were positive, indicating that these C−H bonds were predominantly parallel to the matrix surface. When exposed to the solvent, the band at 1437 cm−1 remained positive but thinner, which indicates a higher organization of the film structuring, and the band at 1324 cm−1 became negative, indicating a twist of the vibrational moment. Only for benzene vapors, these bands maintained in the same position as for the original one, revealing no significant alteration in the
3. RESULTS AND DISCUSSION First of all, the thicknesses of the films were measured and reported in Table 1. It is possible to observe, as expected, that the LB films are the thinnest, with monomolecular dimensions, 2641
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
Langmuir
Article
result suggests permanent structural changes. An orientation change occurs in order to readapt the chemical groups of the polymer when interacting to the solvent exposed in the gas phase. Groups present in the polymer, such as benzene ring, reorient to enhance interactions with chemical groups present in the solvent vapor which has a higher affinity. When the polymer was deposited by spin-coating, the results were different. In Figure 4, we show the bands of the polymer exposed to the solvents. For the pristine material, the aromatic ring stretching (1608 and 1483 cm−1) shows a wide and positive band, indicating that the aromatic rings are parallel to the plane. When the polymer was exposed to the ethanol, a shift to 1473 cm−1 (Figure 4A) of the band centered at 1476 cm−1 was observed. For the other solvents, also shifts to lower energies were observed. The other two bands observed in the casting film for CH aliphatic bending modes were also shifted, and the band at 1523 cm−1 changed from negative to positive, showing that the carbon−carbon stretching within the ring became parallel to the plane after the exposure to the solvents. The band in 1608 cm−1 became wider as a consequence of the loss of organization of the chains when exposed to the solvents. Removing and replacing the solvents does not change the general profile of the spectra. Even so, it is possible to observe some changes. For instance, the relative intensity between the bands at 1640 cm−1 and at 1523 cm−1 increased, as did the 1476/1523 cm−1 relative intensity. This finding is indicative that for spin-coating films, there are some slight changes in the polymer structure when solvents are removed and replaced
configuration of the polymer adsorbed. This can be attributed to the affinity between aromatic rings of the polymer and the solvent vapor. Figure 3 demonstrates both conclusions
Figure 3. Schematic drawing of a PPPX monomer unit deposited onto the surface before (A) and after exposure to the solvents chloroform, ethanol, and hexane (B).
described above. The carbon−carbon stretching within the ring (1521 cm−1) could be seen only in the pristine material. After exposure to the solvents, the band disappeared, which suggests that the plane of the C−C stretching is more disordered when in contact with the solvents. The presence of different types of solvents, both polar and nonpolar, did not cause different results in the PM-IRRAS spectra. Moreover, when the polymer was once more exposed to air (removing the organic solvent vapor environment), the infrared spectrum did not return to the profile observed in the pristine spectra. This
Figure 4. PM-IRRAS of spin-coating PPPX exposed to ethanol (A), and hexane (B). The sequence of spectra from bottom (A and B) is as follows: (i) pristine material before exposure to solvent vapors; (ii) after first exposure; after removal of the solvent; and (iii) after second exposure. Comparative spectra for all solvent vapors (chloroform, ethanol, hexane, and benzene) in the first exposure to PPPX spin-coasting films (C). 2642
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
Langmuir
Article
Figure 5. Surface Pressure−Area (left) isotherms and PM-IRRAS spectra (right) for PPPX Langmuir monolayers formed at the air−water interface.
(Figure 4). However, the original spectrum of the film before exposure to the solvents was not obtained, which again suggests a permanent change in the structure of the polymer. Also, for ethanol, the band at 1523 cm−1 became more evident in the same way as for the other solvents. However, after removing the solvent, the band at 1456 cm−1 became more apparent in relation to the other neighboring bands. After replacing the solvent, the spectrum did not revert to the previous spectrum: three bands in the region of 1443−1476 cm−1 appeared with similar intensities. This indicates that the contact with ethanol vapor, and subsequent removal and replacing of the solvent vapor, caused progressive changes in the polymer structures, which are not reversible. For hexane, a nonpolar solvent (Figure 4B), the spectra show wider bands, with possible overlapping. The band at 1608 cm−1 disappeared and two noticeable bands appeared at 1518 and at 1422 cm−1. After removing the solvent, a band at 1471 cm−1 appeared, but the replacement of the solvent vapor caused the spectrum to be similar to the previous spectrum. In this case, it seems that there is some reversibility when removing and replacing the solvent vapor. For chloroform, similar effects were observed in the spectra. Also, it is important to mention that for chloroform and benzene vapors, although some changes were observed when first exposed to the film (Figure 4C), after removal and replacement of the vapor, no change was observed, indicating permanent conformation of the polymer on the film. Also, for benzene (Figure 4C), the band observed in 1473 cm−1 for the other solvents is shifted to 1465 cm−1, with a broader band that may be overlapping other ones at higher energies (1460−1400 cm−1). As observed for casting films, benzene vapors may have specific molecular affinity with the polymer adsorbed on the solid matrix. The results so far indicate discrimination between the effects from the three types of solvents: one highly polar (ethanol), one nonpolar (hexane), and one with intermediate polarity (chloroform). Additionally, the effects of reversibility are distinct when the results from each solvent vapor are compared. However, for all of the solvents, the original spectra related to the films before the exposure to the vapors do not return at any moment during the procedure of removing and replacing the solvent vapors. Comparing the spectra obtained for drop-casting and for spin-coating, the band in ∼1320−1360 cm−1 is not inverted for spin-coating as it is for drop-casting. This is a first indicator that the thickness and level of organization of the film may affect the
arrangement of the polymer on the solid matrix and the way by which this arrangement is influenced by organic vapors. To investigate how more organized films are influenced by solvent vapors, Langmuir−Blodgett films of PPPX were produced. Figure 5 shows the surface pressure−area isotherm for Langmuir monolayers of PPPX formed on the air−water interface. The isotherm is typical of fluid films, which was expected considering the flexibility of the macromolecule. The relatively high surface pressure (∼40 mN/m) attained at the end of the compression shows that the monolayer is resistant to compression, with no defined collapse until the minimum area has been reached. Figure 5B shows the PM-IRRAS spectra for the PPPX Langmuir monolayer. A noticeable negative band at ∼1640−1650 cm−1 is observed as consequence of the shift of the interfacial water and changes in its structure and spectral properties in the presence of the polymer at the interface.17 This result could be a consequence of the repulsive forces between the polymer and the aqueous subphase at the interface, which minimize the amount of water molecules at the interface. The bands in 1523 and 1480 cm−1 are ascribed to the vibrations of the aromatic rings, and the bands at 1437, 1376, and 1344 cm−1 are due to aliphatic C−H bends. Only one layer could be deposited as a Langmuir−Blodgett film at a surface pressure of 30 mN/m, and successive layers could not be deposited because of the high affinity of the polymer for the air−water interface. The 1-layer LB film had a high-quality transfer ratio (1.05), which means that it was almost perfectly organized at the solid substrate. Lower surface pressure values (20 and 10 mN/m) were used to transfer the films, but no high-quality transfer ratios were obtained. It is important to emphasize that the monolayer is not collapsed at 30 mN/m, since no element in the shape of the isotherms that characterize the monolayer collapse is observed. Also regular and reproducible transfer ratios were obtained, providing films with thickness close to molecular dimensions of the polymers. These facts are additional indicators that the film was transferred as a noncollapsed monolayer. Figure 6 shows that little influence was observed by PMIRRAS when the LB films were exposed to the solvents, i.e., there were no shifts or orientation changes of the bands. This finding proves that the polymer needs to be thick enough for the solvent vapor to influence the vibration dipoles of the polymer. The high organization of the polymer in the LB film and the high adherence to the substrate did not allow significant changes in the polymer structure when subjected to vapor penetrating the polymer chain. It is likely, therefore, 2643
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
Langmuir
Article
fundamental changes of macromolecules adsorbed on solid matrixes.
■
CONCLUSIONS Thin films of poly(2-phenyl-1,4-xylylene) adsorbed on solid matrixes can be employed to qualitatively detect vapors of ethanol, hexane, and chloroform. The detection was more sensitive when casted or spin-coated films were employed when compared with LB films. Successive steps including the removal of solvent and further solvent replacement reveal the distinct ability to return to the original conformation, or not. This ability could be related to changes in the orientation of the chemical groups of the polymer, which can be identified with PM-IRRAS. This work reveals the possibility of using infrared spectroscopy to detect vapors of organic compounds in the environment by gas sensors formed using poly(p-xylylene) derivatives as active layers.
Figure 6. PM-IRRAS for PPPX Langmuir−Blodgett films.
■
that the interchain interactions that exist in thicker films are responsible for the sensitivity of the films to solvent vapors. These thicker films are promising for future vapor sensors of organic solvents. In attempt to understand if the changes observed in the PMIRRAS spectra of PPPX films obtained by casting and spincoating were reversible when the solvent was removed, a new spectra was registered after 1 week, as seen in Figure 7. We noted that the spectra had not returned to the initial fingerprint before vapor exposure, proving that the process is irreversible. Even for spin-coating, the procedure was carried out to remove the solvent, and we observed that it had not returned to the original spectrum. When hexane was replaced, the excess of solvent caused the return to the spectrum taken when the solvent was first exposed. After subsequent removal of the solvent, and after one week, the spectrum changed completely from its original profile. These results prove that the vapor sensor can be reusable when excess of vapor is employed, but the original structures cannot be replaced. In fact, the main idea of this work was to exploit in a proofof-concept experiment the action of solvent vapors on polymers immobilized on solid matrixes. Of particular relevance was the possibility to explain why some film architecture performed better than the other in terms of sensitivity to vapors. It is also clear the indication that improved organic devices, such as sensors, can be achieved if fundamental studies on interfaces are carried out. Also, we may envisage the employment of this idea as a suitable methodology with the capacity of observing
ASSOCIATED CONTENT
S Supporting Information *
Description of the material. This material is available free of charge via the Internet at http://pubs.acs.org
■
AUTHOR INFORMATION
Corresponding Author
*Fax: +55-11-40436428. E-mail:
[email protected] . Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS FAPESP, CAPES-INCT (Eletrô nica Orgânica), CAPESnBioNet (Films and Sensors) are acknowledged for the financial support.
■
REFERENCES
(1) Baldwin, E. A.; Bai, J.; Ploto, A.; Dea, S. Electronic noses and tongues: Applications for the food and pharmaceutical industries. Sensors 2011, 11, 4744−4776. (2) Péres, L. O.; Li, R. W. C.; Yamauchi, E. Y.; Lippi, R.; Gruber, J. Conductive polymer gas sensor for quantitative detection of methanol in Brazilian sugar-cane spirit. Food Chem. 2012, 130, 1105−1107. (3) Sinelli, N.; Benedetti, S.; Bottega, G.; Riva, M.; Buratti, S. Evaluation of the optimal cooking time of rice by using FT-NIR spectroscopy and an electronic nose. J. Cereal Sci. 2006, 44, 137−143.
Figure 7. PM-IRRAS of PPPX exposed to hexane by casting (left) and spin-coating (right). 2644
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
Langmuir
Article
(4) Li, R. W. C.; Carvalho, L. R. F.; Ventura, L.; Gruber, J. Low cost selective sensor for carbonyl compounds in air based on a novel conductive poly(p-xylylene) derivative. Mater. Sci. Eng., C 2009, 29, 426−429. (5) Li, R. W. C.; Ventura, L.; Gruber, J.; Kawano, Y.; Carvalho, L. R. F. A selective conductive polymer-based sensor for volatile halogenated organic compounds (VHOC). Sens. Actuators, B 2008, 131, 646−651. (6) Benvenho, A. R. V.; Li, R. W. C.; Gruber, J. Polymeric electronic gas sensor for determining alcohol content in automotive fuels. Sens. Actuators B 2009, 136, 173−176. (7) Mehne, J.; Markovic, G.; Pröll, F.; Schweizer, N.; Zorn, S.; Schreiber, F.; Gauglitz, G. Characterisation of morphology of selfassembled PEG monolayers: a comparison of mixed and pure coatings optimized for biosensor applications. Anal. Bioanal. Chem. 2008, 391, 1783−1791. (8) Zhang, Y.; Chen, J.; Lei, Y.; Zhou, Q.; Sun, S.; Noda, I. Discrimination of different red wine by Fourier-transform infrared and two-dimensional infrared correlation spectroscopy. J. Mol. Struct. 2010, 974, 144−150. (9) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet, M.; Turlet, J. M. Polarization modulation ftir spectroscopy at the air-water-interface. Thin Solid Films 1994, 242, 146−150. (10) Caseli, L.; Masui, D. C.; Furriel, R. P. M.; Leone, F. A.; Zaniquelli, M. E. D.; Orbulescu, J.; Leblanc, R. M. Rat osseous plate alkaline phosphatase as Langmuir monolayerAn infrared study at the air−water interface. J. Colloid Interface Sci. 2008, 482, 476−482. (11) Pavinatto, F. J.; Pacholatti, C. P.; Montanha, E. A.; Caseli, L.; Silva, H. S.; Miranda, P. B.; Viitala, T.; Oliveira, O. N. Cholesterol mediates chitosan activity on phospholipid monolayers and LangmuirBlodgett films. Langmuir 2009, 25, 10051−10061. (12) Nasir, M. N.; Besson, F. Interactions of the antifungal mycosubtilin with ergosterol-containing interfacial monolayers. Biochim. Biophys. Acta-Biomembr. 2012, 1818, 1302−1308. (13) Brosseau, C. L.; Leitch, J.; Bin, X.; Chen, M.; Roscoe, S. G.; Lipkowski, J. Electrochemical and PM-IRRAS a glycolipid-containing biomimetic membrane prepared using Langmuir-Blodgett/LangmuirSchaefer deposition. Langmuir 2008, 24, 13058−13067. (14) Ramin, M. A.; Le Bourdon, G.; Daugey, N.; Bennetau, B.; Vellutini, L.; Buffeteau, T. PM-IRRAS investigation of self-assembled monolayers grafted onto SiO2/Au substrates. Langmuir 2001, 6076− 6084. (15) Buffeteau, T.; Desbat, B.; Turlet, J. M. Polarization modulation FT-IR spectroscopy of surfaces and ultra-thin films: Experimental procedure and quantitative analysis. Appl. Spectrosc. 1991, 45, 380− 389. (16) Frey, B. L.; Corn, R. M.; Weibel, S. C. Polarization-Modulation Approaches to Reflection-Absorption Spectroscopy. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffith, P. R., Eds.; John Wiley and Sons, Ltd.: Chichester, UK, 2001; Vol. 2, p 1042. (17) Saccani, J.; Castano, S.; Beaurain, F.; Laguerre, M.; Desbat, B. Stabilization of phospholipid multilayers at the air−water interface by compression beyond the collapse: A BAM, PM-IRRAS, and molecular dynamics study. Langmuir 2004, 20, 9190−9197.
2645
dx.doi.org/10.1021/la3050797 | Langmuir 2013, 29, 2640−2645
