ARTICLE pubs.acs.org/ac
Cholesteric Liquid Crystals Doped with Dodecylamine for Detecting Aldehyde Vapors Laura Sutarlie, Jia Yi Lim, and Kun-Lin Yang* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117576 ABSTRACT: In this paper, we report a study of using cholesteric liquid crystals (CLCs) doped with dodecylamine for detecting aldehyde vapors. CLCs doped with dodecylamine show color change from red to yellow-green upon exposure to 300 ppmv pentyl aldehyde within 60 s. In contrast, no colorimetric response is observed when pure CLCs are used. Characterization by using FT-IR shows that the possible mechanism responsible for the colorimetric response is the formation of an imine bond between dodecylamine and pentyl aldehyde. A new CdN peak at 1670 cm1 appears in the spectrum after the exposure to aldehyde vapors. The CLCs doped with dodecylamine show good selectivity for pentyl aldehyde; they do not respond to 200 ppmv pentyl alcohol, pentylamine, acetone, ethanol, and water vapor. This study demonstrates the potential applications of doped CLCs as low-cost and portable gas sensors.
A
ldehydes are widely used in industry as precursors for many chemical compounds and polymer synthesis. However, most aldehydes are hazardous because they are highly flammable and harmful for health and the environment. Therefore, for safety reasons, detecting and monitoring aldehydes present in the environment is necessary. Methods for aldehyde vapor detection have been discussed in some literature reviews.1,2 These methods include derivatization of aldehyde vapor in an impinger charged with (2,4-dinitrophenyl)hydrazine followed by gas/liquid chromatography3,4 and direct aldehyde vapor detection using gas chromatography and infrared spectroscopy. Although they can detect aldehyde vapor at the parts per billion level, they require complex instrumentation and depend on electric power for operation. In the colorimetric methods, aldehydes react with pararosananiline5,6 or 3-methyl-2-benzothiazolinone hydrazone7,8 solution to give imines, but they require delicate aldehyde vapor sampling in the solution and other reagents to develop the color. Hence, portable and low-cost detection methods for aldehyde vapors are still needed. Alternatively, cholesteric liquid crystals (CLCs) show promise for colorimetric vapor detection. In the past, it has been reported that the colors of CLCs are responsive to temperature.913 In fact, CLCs also show colorimetric responses when they are exposed to chemical vapors.1420 This is because the dissolution of chemical vapors in CLCs creates a concentration gradient in CLCs and results in a net torque because of the chirality of CLC molecules.21 The net torque affects the rotation of CLC molecules around their helical axis and subsequently alters the distance for one rotation (the pitch) and the wavelength of the reflected light. Furthermore, the colorimetric responses of CLCs to chemical vapors have the following characteristics. First, the colorimetric responses of CLCs are reversible because the vapor can diffuse in and out of the CLCs. Second, at the same vapor concentration, the colorimetric responses of CLCs to large r 2011 American Chemical Society
molecules are stronger than that to small molecules because the former cause stronger disruption in the CLC molecular orientation.17,20 Third, for chemical vapors with similar molecular sizes, CLCs show selectivity for chemical vapors such as those from primary amines and alcohols that can form hydrogen bonds with CLC molecules.20 On the basis of the above characteristics, CLCs are more suitable for the detection of chemical vapors which have a large molecular size and can form hydrogen bonds with CLCs. In view of this, detecting aldehyde vapor by using CLCs poses a challenge because aldehydes do not form hydrogen bonds with CLCs. In the past, several strategies have been proposed to modify CLC molecules directly to allow chemical reaction between CLCs and target molecules. For example, to detect primary amines, Mohr et al.22 modified cholesteryl molecules with trifluoroacetyl groups because primary amines react with trifluoroacetyl groups and form hemiaminal groups.23 This sensor is able to detect 271 ppmv butylamine, but the response time is more than 15 min. In our present study, instead of modifying CLC molecules with a molecular receptor, we added small amounts of molecular receptors into CLCs as a dopant. The molecular receptor used here for aldehyde vapor detection is dodecylamine because its primary amine group can react with aldehydes to form imines.2426 However, it is unclear whether this reaction can proceed in the CLC phase. Therefore, we also study the formation of imines inside the CLC by using FT-IR spectroscopy. This study provides a better understanding of how doped molecules in CLCs react with target gas molecules to facilitate colorimetric responses of CLCs. Received: March 6, 2011 Accepted: June 1, 2011 Published: June 01, 2011 5253
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’ EXPERIMENTAL SECTION Materials. Cholesteryl benzoate, cholesteryl nonanoate, cholesteryl oleyl carbonate, pentylamine, pentyl alcohol, pentyl aldehyde, butyl aldehyde, acetone, ethyl acetate, butylamine, hexylamine, octylamine, decylamine, dodecylamine, (N,N-dimethyl-N-octadecyl-3aminopropyl)trimethoxysilyl chloride (DMOAP), and trichloro(1H,1H,2H,2H-perfluorooctyl)silane were purchased from SigmaAldrich (Singapore) and used as received. The Sylgard 184 silicone elastomer kit for making poly(dimethylsiloxane) (PDMS) was purchased from Dow Corning (Midland, MI). All glass slides were purchased from Marienfeld (Germany) and cleaned with 5% (v/v) Decon-90 solution prior to use. Preparation of Thin Films of PDMS. PDMS was prepared by mixing PDMS prepolymers with curing agent (in a 10:1 weight ratio), and then the mixture was degassed prior to use. A thin film of PDMS was prepared by pasting the mixture on the surface of a fluorinated slide and then covering it with another fluorinated slide. A 100 μm thick plastic spacer was placed between the two glass slides to maintain a uniform thickness. The film of PDMS was then polymerized at 50 °C for 12 h. Then it was peeled from the glass slides. The fluorinated slides mentioned above were prepared by immersing clean glass slides in heptane containing 0.05 M trichloro(1H,1H,2H,2H-perfluorooctyl)silane. After 30 min of incubation, the glass slides were rinsed with heptane and baked in a 100 °C vacuum oven for 15 min to promote silane cross-linking. Preparation of CLCs and CLCs Doped with Dodecylamine. Pure CLCs were prepared by melting cholesteryl oleyl carbonate, cholesteryl nonanoate, and cholesteryl benzoate in a weight ratio of 0.32:0.58:0.1. Meanwhile, CLCs doped with dodecylamine were prepared by mixing 2 wt % dodecylamine with 98 wt % CLCs. Thin films of CLCs doped with dodecylamine were prepared by pasting the CLC doped with dodecylamine onto clean glass slides. The backside of the glass slides was darkened by using black tape to reduce reflection. In some cases, clean glass slides or DMOAP-coated glass slides27 were placed directly on the CLC thin films as shown in Scheme 1a. In other cases, the CLC thin films were first covered by using PDMS thin films (∼100 μm), and then the glass slides were placed on top of the PDMS thin films as shown in Scheme 1b. Exposure of CLC Thin Films to Vapor Analytes. A drop of liquid was dispensed into a bottle with a fixed volume as shown in Table 1. Then thin films of CLCs or CLCs doped with dodecylamine were placed inside the bottle, and the bottle was heated to 50 °C for 15 min, allowing the liquid to evaporate into vapor. Different vapor concentrations were controlled by varying the volumes of liquid. After the liquid completely evaporated, the bottle was moved to a 34 °C oven, and the colorimetric responses of CLCs or CLCs doped with dodecylamine to vapor analytes were observed. All images were captured by using a digital camera (Sony, Japan). UVVis Spectroscopy. Visible spectra were obtained by using a UVvis spectrometer (Cary 50) manufactured by Varian (Australia). This spectrometer was connected to a temperature controller. Thin films of CLCs (supported on a glass slide and covered with PDMS) were placed inside a glass cuvette and aligned perpendicularly to the incident light. Chemical vapors were generated by adding a few droplets of liquid into the cuvette, which was connected to a big bottle to obtain the desired vapor concentrations. Fourier Transform Infrared (FT-IR) Spectroscopy. All samples were prepared by dissolving 80 mg of CLCs doped with dodecylamine in 500 μL of heptane. The dodecylamine concentration was fixed at 20 wt % to get better signal-to-noise
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Scheme 1. Configurations of CLC-Based Gas Sensorsa
A thin film of CLCs is supported on a clean glass slide with two pieces of plastic spacer placed on the side to control the thickness of the CLC thin film. Half of the CLC thin film is protected by using a cover glass slide to serve as an internal control (not permeable to gas). (a) No PDMS film between the CLC thin film and the cover slide. (b) A PDMS film is used to separate the CLC thin film and the cover slide. a
Table 1. Amount of Liquid Analytes Added inside Bottles and Their Vapor Concentration vapor
volume of
volume of
vapor analyte
concn (ppmv)
liquid analyte (μL)
bottle (L)
pentyl aldehyde
200
0.56
0.63
300
0.83
0.63
2000
5.54
0.63
pentylamine
200
0.68
0.63
pentyl alcohol acetone
200 200
0.63 0.38
0.63 0.63
ethanol
200
0.15
0.63
water
200
0.10
0.63
(S/N) ratios. After the CLCs were completely dissolved in heptane, 6 μL of the CLC solution was evaporated on the window of an FT-IR liquid cell (Pike Technologies, Madison, WI) to form a thin film of CLCs on the window. The liquid cell was then placed inside the FT-IR spectrometer (Shimadzu, Japan). This spectrometer is equipped with a mercurycadmiumtelluride (MCT) detector. FT-IR spectroscopy measurement was conducted in transmission mode. For each spectrum, 200 scans were accumulated and the resolution was maintained at 4 cm1. For vapor exposure experiments, the liquid cell was first removed from the chamber (this is necessary because the exposure of CLCs to infrared for a long period of time often causes the melting of CLCs). Subsequently, 2 μL of liquid to generate the vapor was spread onto a piece of Scotch tape, and the entire window of the liquid cell was covered with the tape, allowing the liquid to evaporate and fill the space between the tape and the CLC thin film. After vapor exposure, the tape was removed, and the liquid cell was placed back into the sample holder for taking IR spectra.
’ RESULTS AND DISCUSSION Effect of Glass on CLCs Doped with Dodecylamine. In this study, we chose dodecylamine as a molecular receptor because the primary amine group of dodecylamine can react with an aldehyde group. Moreover, dodecylamine is solid and has low volatility at room temperature. First, we prepared a thin film of CLCs doped with 2 wt % dodecylamine, and half of the CLC thin film was covered with a clean glass slide (Scheme 1a). In this configuration, the CLC thin film under the glass slide does not 5254
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Figure 1. Images of various CLC thin films with various covers on half of the thin films (top side of each image). CLCs doped with 2 wt % dodecylamine with (a) a clean glass slide cover and (d) a clean glass slide cover on top of a PDMS thin film as prepared in Scheme 1b. Undoped CLCs with (b) a clean glass slide cover and (c) a DMOAP-coated glass slide cover. The surrounding temperature when the image is taken is indicated below each image.
Scheme 2. Arrangement of Layers of CLC Molecules Doped with Dodecylamine near the Boundary with (a) Air and (b) a Clean Glass Slidea
a
The presence of dodecylamine adsorbed on the clean glass slide decreases the tilt angle of CLC layers (θ) and reduces the effective pitch (P0 ) for light reflectance. The effective pitch is a function of the CLC pitch (P) as P0 = P cos θ.
come into contact with the aldehyde vapor. Consequently, this part will not produce any colorimetric responses after exposure to aldehyde vapors, making it suitable to serve as an internal visual control in this system.20 Figure 1a shows the appearance of the CLCs doped with 2 wt % dodecylamine. Interestingly, even before exposure to any vapors, the doped CLCs show different colors on the covered and uncovered parts. In contrast, undoped CLCs show uniform color on both parts (Figure 1b).This result suggests that dodecylamine may adsorb at the cover slide and shorten the effective pitch of the CLCs. One possible reason is that the presence of dodecylamine lowers the surface energy and leads to a lower tilt angle of the CLC layer. Thus, the effective pitch is reduced as shown in Scheme 2. To test this hypothesis, we placed a DMOAP-coated slide on a thin film of undoped CLCs, covering half of its surface area. The DMOAP-coated slide has terminal long hydrocarbon chains,28
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Figure 2. Dodecylamine dopant effect on the optical property of CLCs. (a) Effect of increasing dodecylamine concentration (wt %) on the color uniformity of CLC thin films. (b) CLC thin film doped with 2 wt % dodecylamine kept at 34 °C after 1 week showing color stability without dodecylamine phase separation or evaporation. The surrounding temperature when the image is taken is indicated below each image.
resembling a monolayer of dodecylamine adsorbed on the glass surface. Figure 1c shows that the covered part shows green color whereas the uncovered part shows red color. This result is similar to that of Figure 1a. Thus, this experiment confirms that a hydrocarbon-terminated surface will shorten the effective pitch of CLCs. To prevent the problem of having different colors on the covered and uncovered parts, we first placed a gas-permeable29 PDMS film (∼100 μm) on the entire surface of CLCs doped with dodecylamine and then covered half of the surface with a glass slide as shown in Scheme 1b. In this configuration, the entire CLC thin film is in contact with PDMS only, and only the region not covered by the glass is permeable to gas. In Figure 1d, the entire CLC thin film shows a uniform red color. Therefore, this configuration was used in subsequent experiments. Effect of Dopant Concentrations. Next we studied the effect of the dopant concentration on the colors of CLCs. Figure 2a shows the images of CLCs doped with 210 wt % dodecylamine. The CLCs doped with 2 or 4 wt % dodecylamine show uniform red color. However, when the dodecylamine concentration is 6 wt %, some irregular spots with different colors can be seen. When the dodecylamine concentration is further increased to 10 wt %, the color patterns of CLCs become irregular. This result implies that dodecylamine with concentrations above 6 wt % do not dissolve completely in CLCs. To ensure that dodecylamine dissolves in CLCs completely, we fixed the concentration of dodecylamine at 2 wt %. The stability of the CLCs doped with 2 wt % dodecylamine was also investigated. Any sign of color change is an indication of phase separation or evaporation of dodecylamine from the CLCs. Figure 2b shows the images of the CLCs doped with 2 wt % dodecylamine kept at 34 °C right after and 1 week after sample preparation. No color change after 1 week can be observed. This result shows that the dodecylamine dissolved in CLCs is very stable. There is no phase separation or evaporation of dodecylamine from the CLCs. Therefore, we conclude that the lifetime for the thin film is more than 1 week. However, the lifetime also depends on the storage condition. If the thin film is stored at room temperature, the lifetime of the thin film is 5255
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Figure 3. Colorimetric responses of CLCs doped with 2 wt % dodecylamine at 34 °C after exposure to (a) water, (b) ethanol, (c) acetone, (d) pentylamine, (e) pentyl alcohol, and (f) pentyl aldehyde at the same concentration of 200 ppmv for 15 min.
reduced to 2 days because of crystallization of CLCs. The crystallization can be removed by melting the CLCs and spreading the CLCs again to form the thin film. Colorimetric Responses of CLCs to Pentyl Aldehyde Vapor. To investigate the colorimetric response of CLCs to pentyl aldehyde vapor, we exposed a thin film of CLCs (doped with 2 wt % dodecylamine) to 200 ppmv pentyl aldehyde. Here we chose pentyl aldehyde because it is used in resin production and food additives and it appears as an intermediate in various chemical industries. Figure 3 shows that, after 15 min, the gas-permeable region of the CLC thin film turns to green color whereas the nonpermeable region with the glass cover is still in red color. In contrast, when undoped CLCs were exposed to pentyl aldehyde or when dodecylamine-doped CLCs were exposed to water, ethanol, acetone, pentylamine, and pentyl alcohol, no color change was observed. This result implies that the colorimetric response is triggered by a chemical reaction between dodecylamine and pentyl aldehyde. It is well-known that primary amines can react with aldehydes to form imines in organic solvents, but it is still unclear whether this reaction can occur in CLCs. Figure 4a shows an IR spectrum of CLCs doped with dodecylamine (20 wt %). Peaks at 3334 and 1652 cm1 can be assigned to NH stretching and bending of the amine group, respectively. Figure 4b shows a spectrum of the same sample after exposure to saturated pentyl aldehyde vapor for 1.5 min. In this spectrum, both peaks at 3334 and 1652 cm1 disappear and a new peak at 1670 cm1, which is attributed to CdN,30 can be seen. This result suggests that dodecylamine and pentyl aldehyde can react and form imine inside CLCs. We also exposed the CLCs doped with dodecylamine (20 wt %) to saturated pentyl alcohol vapor (Figure 4c) and pentylamine vapor (Figure 4d). In both cases, no changes in the NH stretching (3334 cm1) and bending (1652 cm1) peaks can be observed after 5 min. Response Time and Reversibility. To determine the response time of CLCs to pentyl aldehyde, we exposed a thin film of CLCs doped with 2 wt % dodecylamine to 300 ppmv pentyl aldehyde. As shown in Figure 5a, the gas-permeable region of the thin film first changes its color to yellow-green after 60 s. The color finally becomes green after 120 s, and no further color changes can be observed. As the first color change can be observed within 60 s, the response time to 300 ppmv pentyl aldehyde is estimated to be 60 s. This response time is relatively slow and may be caused by the presence of the PDMS film on top of the CLC thin film. When the PDMS film is removed, the response time (from red to green) to 300 ppmv pentyl aldehyde is only 15 s (data not shown). This result confirms that the PDMS forms a diffusion barrier for gas in this system. Next we studied the reversibility of the colorimetric response of CLCs doped with 2 wt % dodecylamine by exposing the sample which was previously exposed to 300 ppmv pentyl aldehyde to air.
Figure 4. FT-IR spectra showing the change of the NH stretching peak at 3334 cm1 and NH bending peak at 1652 cm1 and formation of the CdN peak at 1670 cm1 of CLCs with dodecylamine dopant in the following conditions: (a) initial, (b) after 1.5 min of exposure to saturated pentyl aldehyde vapor, (c) after 5 min of exposure to pentyl alcohol vapor, and (d) after 5 min of exposure to pentylamine vapor.
Figure 5a shows that the sample still shows yellow-green color after 5 min, but after 30 min, it returns to its original red color. This result implies that the colorimetric response is reversible. The long reversible time is mainly attributed to the slow dissociation rate of the imines formed between dodecylamine and pentyl aldehyde inside the CLCs. Since the colorimetric response is reversible, we study its reusability by exposing the same sample to 2000 ppmv pentyl aldehyde vapor. Figure 5b shows that it turns to yellow-green after 20 s, green after 30 s, dark green after 60 s, and blue after 120 s. No further color changes can be observed afterward. This result shows that the thin film is reusable. From Figure 5b, we also note that the response time to 2000 ppmv pentyl aldehyde vapor is only 20 s, probably because the reaction rate and diffusion rate of pentyl aldehyde increase with increasing concentration. However, the reversibility time requires 90 min because more imines are formed inside the CLCs. These results show that the thin film is reusable as the colorimetric response is reversible, but its response time and reversibility time depend on the pentyl aldehyde concentration. Furthermore, to test the reusability of the thin film for multiple exposures to pentyl aldehyde, a thin film of CLCs doped with 2 wt % dodecylamine was subjected to four cycles of exposure to pentyl aldehyde (each cycle consists of exposure to 2000 ppmv pentyl aldehyde vapor for 2 min followed by air for 90 min). 5256
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Figure 5. Dynamic colorimetric response and reversibility of a thin film of CLCs doped with 2 wt % dodecylamine after exposure to (a) 300 ppmv pentyl aldehyde vapor and (b) 2000 ppmv pentyl aldehyde vapor at 34 °C. (c) Colorimetric responses of a thin film of CLCs doped with 2 wt % dodecylamine subjected to four cycles of exposure to 2000 ppmv pentyl aldehyde for 2 min followed by air for 90 min.
Figure 5c shows that the thin film turns to a blue color after exposure to 2000 ppmv pentyl aldehyde and returns to a red color in the air in each cycle. These results affirm the reusability of the thin film for multiple exposures. Detection Limit and Sensitivity. Finally, we studied the detection limit and sensitivity of the colorimetric responses of CLCs doped with 2 wt % dodecylamine to various aldehydes by employing visible spectroscopy. The CLCs doped with 2 wt % dodecylamine have one visible spectral peak at 670 nm which shifts to a shorter wavelength upon exposure to various aldehydes. For comparison, shifts in the peak wavelength caused by pentyl aldehyde, butyl aldehyde, and methyl aldehyde with concentration within 1000 ppmv are shown in parts a (01000 ppmv) and b (010 ppmv) of Figure 6. From Figure 6, we estimated that the detection limits for pentyl aldehyde, butyl aldehyde, and methyl aldehyde vapor are 2.77, 4.99, and 85.02 ppmv, respectively. Meanwhile, the sensitivities of CLCs doped with 2 wt % dodecylamine to pentyl aldehyde, butyl aldehyde, and methyl aldehyde are 0.185, 0.1075, and 0.0412 nm/ppmv, respectively. On the basis of these results, the lowest detection limit and the highest sensitivity can be achieved when the CLCs are exposed to pentyl aldehyde vapor. This is caused by the molecular size effect which has been described in past studies.17,20 On the basis of the molecular size, pentyl aldehyde is larger than butyl aldehyde and methyl aldehyde such that it produces stronger disruption on the molecular orientation of CLC molecules.
Figure 6. Shifts in the peak wavelength of CLCs doped with 2 wt % dodecylamine after exposure to pentyl aldehyde (solid triangles), butyl aldehyde (solid squares), and methyl aldehyde (solid circles) and undoped CLCs after exposure to pentyl aldehyde (hollow triangles) and butyl aldehyde (hollow squares) (a) for 01000 ppmv aldehyde vapor (the dashed square region is enlarged in part b) and (b) for 010 ppmv aldehyde vapor.
’ CONCLUSIONS In this paper, we report the utilization of CLCs doped with dodecylamine for the first time to detect aldehydes. The dodecylamine dopant improves the selectivity to aldehyde vapor over amine and alcohol vapors because of imine formation between dodecylamine and aldehyde dissolved in CLCs as demonstrated in our FT-IR spectrum. The thin films of CLCs doped with dodecylamine give colorimetric responses within 60 s, and they 5257
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are reusable for multiple exposures to aldehydes. These thin films of CLCs doped with dodecylamine can be directly used for aldehyde detection without any complex instrumentation or sample pretreatment. Therefore, they can be used as low-cost and portable aldehyde sensors with high selectivity and sensitivity, fast response time, and reusability.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (þ65) 65166614. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Ministry of Education’s Academic Research Fund Tier I under Grant RG-279-001-036 and graduate student research scholarships from the National University of Singapore with ASEAN University Network/ Southeast Asia Engineering Education Development Network support. ’ REFERENCES (1) Otson, R.; Fellin, P. Sci. Total Environ. 1988, 77, 95–131. (2) Vairavamurthy, A.; Roberts, J. M.; Newman, L. Atmos. Environ., A 1992, 26, 1965–1993. (3) Arnts, R. R.; Tejada, S. B. Environ. Sci. Technol. 1989, 23, 1428–1430. (4) Aiello, M.; McLaren, R. Environ. Sci. Technol. 2009, 43, 8901–8907. (5) Miksch, R. R.; Anthon, D. W.; Fanning, L. Z.; Hollowell, C. D.; Revzan, K.; Glanville, J. Anal. Chem. 1981, 53, 2118–2123. (6) Georghiou, P. E.; Harlick, L.; Winsor, L.; Snow, D. Anal. Chem. 1983, 55, 567–570. (7) Nebel, G. J. Anal. Chem. 1981, 53, 1708–1709. (8) Hauser, T. R.; Cummins, R. L. Anal. Chem. 1964, 36, 679–681. (9) Collings, P. J. Liquid Crystals: Nature’s Delicate Phase of Matter, 2nd ed.; Princeton University Press: Princeton, NJ, 2002. (10) Fergason, J. L. Mol. Cryst. 1966, 1, 293–307. (11) Oswald, P.; Pieranski, P. Nematic and Cholesteric Liquid Crystals; Taylor and Francis: Levittown, PA, 2005. (12) DeGennes, P. G. The Physics of Liquid Crystals; Oxford University Press: Oxford, U.K., 1974. (13) Kelker, H., Hatz, R. Handbook of Liquid Crystals; Verlag Chemie GmbH: Weinheim, Germany, 1980. (14) Fergason, J. L. Sci. Am. 1964, 211, 77–85. (15) Novak, T. J.; Poziomek, E. J.; Mackay, R. A. Anal. Lett. 1972, 5, 187–192. (16) Poziomek, E. J.; Novak, T. J.; Mackay, R. A. Mol. Cryst. Liq. Cryst. 1973, 22, 175–185. (17) Dickert, F. L.; Haunschild, A.; Hofmann, P. Sens. Actuators, B 1992, 6, 25–28. (18) Dickert, F. L.; Haunschild, A.; Hofmann, P. Fresenius' J. Anal. Chem. 1994, 350, 577–581. (19) Winterbottom, D. A.; Narayanaswamy, R.; Raimundo, I. M. Sens. Actuators, B 2003, 90, 52–57. (20) Sutarlie, L.; Qin, H.; Yang, K. L. Analyst 2010, 135, 1691–1696. (21) Rey, A. D. Mol. Cryst. Liq. Cryst. 1997, 293, 87–109. (22) Kirchner, N.; Zedler, L.; Mayerhofer, T. G.; Mohr, G. J. Chem. Commun. 2006, 1512–1514. (23) Mohr, G. J. Anal. Bioanal. Chem. 2006, 386, 1201–1214. (24) Layer, R. W. Chem. Rev. 1963, 63, 489–510. (25) Bi, X. Y.; Yang, K. L. J. Phys. Chem. C 2008, 112, 1748–1750. (26) Bi, X. Y.; Yang, K. L. Sens. Actuators, B 2008, 134, 432–437. (27) Xue, C. Y.; Yang, K. L. Langmuir 2008, 24, 563–567. (28) Kahn, F. J. Appl. Phys. Lett. 1973, 22, 386–388. (29) Sutarlie, L.; Yang, K. L. Sens. Actuators, B 2008, 134, 1000–1004. (30) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman and Hall: New York, 1975. 5258
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