Fabrication and characterization of polyaniline-based gas sensor by ultra-thin film technology

Sensors and Actuators B 81 (2002) 158±164

Fabrication and characterization of polyaniline-based gas sensor by ultra-thin ®lm technology Dan Xiea,*, Yadong Jiangb, Wei Pana, Dan Lib, Zhiming Wub, Yanrong Lib a

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China Department of Materials Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, PR China

b

Received 2 February 2001; received in revised form 19 February 2001; accepted 24 August 2001

Abstract Pure polyaniline (PAN) ®lm, polyaniline and acetic acid (AA) mixed ®lm, as well as PAN and polystyrenesulfonic acid (PSSA) composite ®lm with various number of layers were prepared by Langmuir±Blodgett (LB) and self-assembly (SA) techniques. These ultrathin ®lms were characterized by ultraviolet±visible (UV±VIS) spectroscopy and ellipsometry. It is found that the thickness of PAN-based ultra-thin ®lms increases linearly with the increase of the number of ®lm layers. The gas-sensitivity of these ultra-thin ®lms with various layers to NO2 was studied. It is found that pure polyaniline ®lms prepared by LB technique had good sensitivity to NO2, while SA ®lms exhibited faster recovery property. The response time to NO2 and the relative change of resistance of ultra-thin ®lms increased with the increase of the number of ®lm layers. The response time of three-layer PAN ®lm prepared by LB technique to 20 ppm NO2 was about 10 s, two-layer SA ®lm was about 8 s. The mechanism of sensitivity to NO2 of PAN-based ultra-thin ®lms was also discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: LB ®lms; SA ®lms; Polyaniline; Gas sensor; Nitrogen dioxide

1. Introduction Conducting polymers are an important and interesting class of organic conductors. Among the members of this family, polyaniline (PAN) is promising due to its ease of preparation, high environment stability, stable electrical conduction mechanism and its special properties, etc. [1±3]. It is of interest to study the properties of ordered multilayers of polyaniline particularly from the point of view of its possible application in molecular electronic devices and chemical sensitive devices [4,5]. In this regard, Langmuir±Blodgett (LB) and self-assembling (SA) techniques have been extensively used to fabricate various ultra-thin ®lms, as they result in ultra thin ®lms with known thickness and molecular orientation [6,7]. The sensitive element fabricated by LB technology has higher sensitivity and faster response time, which can work in room temperature [8,9]. Conducting polyaniline may also be used as sensitive layers in chemical microsensor, and the effect of the substantial change in chemical and biological sensing on the electronic properties of polyaniline has been studying [10,11]. At the same time, because the living and working * Corresponding author. E-mail address: [email protected] (D. Xie).

environment exposes humans to a variety of toxic gases and vapors such as nitrogen dioxide (NO2), this has motivated the development of sensitive and speci®c personal monitoring sensors, which are capable of detecting such toxic gas [12]. There are some reports about PAN ®lms prepared by LB technique used as gas sensors. But compared with it, there has fewer reports about PAN-based composite SA ®lm gas sensors. In the present work, we studied the fabrication of PAN-based ®lms prepared by LB technique and composite SA ®lms, and compared their gas-sensing properties to NO2. 2. Experimental 2.1. PAN-based films prepared by LB technique Polyaniline (emeraldine base (EB) form) was prepared chemically employing the method reported by MacDiarmid et al. [2]. N-Methylpyrrolidone (NMP) was used as the processing solvent. One of the spreading solution was obtained by dissolving a mixture of PAN and acetic acid (AA) in NMP in an ultrasonic bath for 1 h (in order to measure the resistance of PAN ®lms by LB technique easily, AA was used to improve the conductivity of PAN by its

0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 0 9 4 6 - 7

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Fig. 1. The schematic diagram of layer-by-layer SA of PAN±PSSA films.

protonating effect) [13±14]. At the same time, EB powder was treated with acetic acid in order to decrease the rigidity of the polymer chains, facilitating easier transfer to the substrate during the dipping procedure [15]. Another was obtained by dissolving pure PAN in NMP. The typical concentration was about 0.1 mg/ml. Deionized water was used for making the pH value of subphase equal to 6.0 and 4.0, respectively, for pure PAN and the mixture of PAN and AA [16]. Monolayer studies and multilayer LB ®lms deposition were carried out with model MC-1 LB instrument. 2.2. PAN composite SA films Polystyrenesulfonic acid (PSSA) is a typical polymeric strong acid, and PAN base can be doped by such polymeric acid. Based on doping-induced deposition effect, PAN± PSSA composite SA ®lms were fabricated with small molecule dopants freely utilizing the doping reaction of PAN and PSSA. The solution of PAN base was prepared by dissolving the powder in (NMP), vigorously stirred, and PAN ®ltered before use. A 0.1% PAN solution was used in our experiment unless specially stated, which could be used all the time as PAN base has relatively good solubility in NMP and its dilute solution is stable at room temperature. Poly(diallyldimethylammoniumchloride) (P‡) with high molecular weight was used as received from Aldrich. Quartz and silicon were used as substrates for optical absorption and electrical conductivity measurement, respectively. These substrates were ®rstly treated by a hot H2SO4:H2O2 (7:3) bath for 1 h, sonicated in ultra-pure water for 10 min, and thoroughly rinsed with water. The positively charged substrates were obtained by immersing the substrates in 1% P‡ aqueous solution for 20 min and rinsed by water [17]. After dried by compressed air, these substrates were immersed into 1% PSSA aqueous solution for 10 min, and then a monolayer of PSSA was absorbed on the surface via electrostatic interaction. After the substrates were rinsed for 30 s with deionized water and dried with nitrogen gas, they were immersed into PAN NMP solution for 10 min, then rinsed with N,N-dimethylformamide (DMF), dried with compress air. After drying, the substrates were immersed in PSSA solution again for 30 min, rinsed with deionized water and dried. Repeating the process mentioned above,

PAN±PSSA composite SA multilayered ®lms were obtained. The schematic diagram of the process is shown in Fig. 1. 2.3. Relative measurements Quartz and silicon were used as substrates for optical absorption and electrical conductivity measurement. In order to measure electronic properties of PAN-based ultra-thin ®lms, the gold inter-digitated electrode was used in this work. It consists of gold electrodes patterned onto the surface of the silicon plate; the overlap electrode length was 20 mm and the inter-electrode spacing was 20 mm too. Chemiresistors were fabricated by coating these electrodes with the PAN ultra-thin ®lms. Ultraviolet±visible (UV±VIS) spectra were obtained with UV1100 spectrophotometer. The thickness of PAN-based monolayer was measured by L116C ellipsometer. The gas sensing property was studied by placing the samples in a chamber through which gas could passed. NO2 gas was diluted with high purity level N2 (99.99%) passing through the test chamber at a ¯ow rate of 500 ml/min, controlled by National Standards Research Center MF-2 model gas blender. Gas entering the chamber passed directly over the sensor surface and the desorption cycle was performed in air. The concentration we used in the experiment was ranged from 1 to 200 ppm. The sensitivity and response of PAN-based ultra-thin ®lms to NO2 were obtained by measuring the relative change of resistance at room temperature. The value of resistance of ultra-thin ®lms was measured using the Changzhou Tonghui TH2682 resistance meter [18]. 3. Results and discussion 3.1. Surface pressure±area (p±A) isotherm of PAN-based films prepared by LB technique A typical surface pressure±mean molecular area (p±A) isotherm of PAN-based single-layer ®lms prepared by LB technique at compression speed of 0.23 mm/min are shown in Fig. 2. By extrapolating the compression isotherm to 0 mN/m, the value for the average area Am per molecule can be obtained. From Fig. 1, we can see that the limiting mean

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D. Xie et al. / Sensors and Actuators B 81 (2002) 158±164

Fig. 2. p±A isotherm of PAN-based single-layer film at subphase temperature of 20 8C and compression speed 0.23 mm/min. (a) Pure PAN film at subphase pH of 6.0; (b) PAN±AA mixed film at subphase pH of 4.0.

Ê 2, which molecular area for PAN single-layer ®lm was 7.7 A 2 Ê is in contrast with the relatively larger area of 25 A for PAN and AA mixture ®lm. Considering molecular areas of alkyl Ê 2, respectively, the molechain and aniline are 19 and 24 A 2 Ê cular area of PAN (7.7 A ) is too small to form a close packed monolayer. It indicates that PAN does not form true monolayer on the water surface as it yields a lower Am than expected, which may be explained by postulating that the PAN is not distributed as single molecules, but rather as aggregates in the subphase. In order to prove it, the thickness per layer of PAN ®lm formed during the LB process was Ê determined by ellipsometry. While the mean thick61.5 A ness of PAN±AA mixed single-layer ®lm prepared by LB Ê , which was close to that of PAN technique was about 58.5 A ®lm.

Fig. 3. The UV±VIS spectra of PAN multilayered films prepared by LB technique with various number of layers. The inset shows increases in the absorbency at 320.5 nm with the number of layers.

The transfer pressures of PAN single-layer ®lm (18.5 mN/ m) is close to that of PAN±AA ®lm (20 mN/m). The singlelayer ®lms were transferred onto different substrates at a constant surface pressure referred to above by the vertical dipping method (dipping speed ˆ 1 mm/min) at subphase pH of 6.0 and 4.0, respectively, for PAN and PAN±AA mixture; temperature of 20 8C. Deposition was mostly of Ztype. Uniform transfer has also been inferred from the linear increase of absorption in UV±VIS spectra) with the increase of number of ultra-thin ®lms prepared by LB technique. As showed in Figs. 3 and 4, the absorbency of ultra-thin ®lm increases linearly with the increase of the number of

Fig. 4. The UV±VIS spectra of PAN±AA multilayered films prepared by LB technique with various number of layers. The inset shows increases in the absorbency at 346.5 nm with the number of layers.

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Fig. 5. The UV±VIS spectra of PAN±PSSA multilayered SA films with various number of layers. The inset shows increases in the absorbency at 320 nm with the number of layers.

PAN-based ®lm layers, indicating that the absorbed materials in each cycle are equivalent [5,19]. 3.2. Assembly of PAN±PSSA films As shown in Fig. 1, when the substrate with sulfonic acid group was immersed in PAN solution, a monolayer of PAN base was absorbed onto the surface as the result of the acid± base reaction between PAN base and sulfonic acid. The asprepared PAN monolayers could be doped by PSSA if the ®lm was subsequently immersed into PSSA solution. Its UV±VIS spectra after being treated by PSSA are shown in Fig. 5, which are in agreement with that of emeraldine salt. The result of the doping reaction did not only lead to the increase of conductivity of PAN, but also induced the deposition of another monolayer of PSSA molecule and made the surface become acid again (see Fig. 1). Such surface can absorb another monolayer of PAN if the substrate was immersed in PAN solution again. Alternately, immersed the substrate in PAN and PSSA solutions, a multilayered ®lm of PAN±PSSA can be obtained. The thickness of the ®lms can be controlled by assembling the times and the PAN solution concentration. As shown in Fig. 5, the absorbency of the ®lm increased linearly with the increase of number of PAN±PSSA. The mean thickness of PAN±PSSA SA monolayer, determined by ellipsometry was Ê . It indicates that the thickness of PAN-based about 45 A ultra-thin ®lms can be controlled at nanometer scale. It is found that the absorbency of the substrate only treated by P‡ did not increase after being immersed in PAN solution and rinsed by DMF, indicating that the positively charged surface has very poor ability to absorb PAN from its solution and the acid of the surface is essential to the absorption

process. Therefore, We may conclude that the acid±base reaction of sulfonic acid with amine and imine groups in PAN is the driving force for the process. It is the acid±base reaction between PAN and PSSA that drove the assembling process. The as-assembled ®lm has good adhesion to the substrate. The PAN ®lm prepared by the casting method can be peeled off by immersing the ®lm in water for several minutes [20]. However, the ®lm fabricated by the SA technique can adhere so strongly to the substrate that it can resist thorough rinsing by water and DMF. The strong adhesion results from the chemical bonding between the ®lm and the surface of the substrate. In addition, the polymeric acid, PSSA, does not only play a role as a dopant to PAN, but also as a ``glue'' between PAN layers to make the ®lm very solid. Good adhesion is very advantageous to applications in electronic devices. 3.3. Gas-sensing property of ultra-thin films The original resistance (R0) of PAN-based ®lms prepared by LB technique and PAN±PSSA SA ®lm in air were 10±20 and 1±2 MO, respectively. The R0 value is different with the change of the number of ®lm layers. So, only the resistance range was given. The sensitivity and responsivity of PANbased ultra-thin ®lms to NO2 are in close relationship with the number of ®lm layers. The response time to NO2 and the relative change of resistance of ultra-thin ®lms increase with the increase of the number of ®lm layers, which can be seen from Figs. 6 and 7. The thinner the ®lms, the more the ®lm contacts with NO2, the higher the degree of oxidation and the shorter the response time become. The response time of PAN and PAN±AA ultra-thin ®lms prepared by LB technique as well as PAN±PSSA SA ®lms with different number of layers

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Fig. 6. Plots of the relative change of resistance ((R0 R)/R0) of polyaniline-based films prepared by LB technique with various number of layers vs. response time at 20 ppm NO2 (22 8C). (a) PAN; (b) PAN±AA.

Fig. 7. Plots of NO2 gas-sensing property and response property of PAN±PSSA SA films with various number of layers. (a) The relative change of resistance ((R R0 )/R0) vs. response time; (b) the relative change of resistance ((R R0 )/R0) vs. NO2 concentrations.

to 20 ppm NO2 gas are shown in Tables 1 and 2, respectively. From Tables 1 and 2, it is found that the response of PAN ®lm is faster than that of PAN±AA mixed ®lms, while PAN±PSSA SA ®lms show even better responsivity to NO2 gas. The thinner the ®lms, the easier NO2 gas molecules adsorb on the surface of ®lms and diffuse into the ®lms, the faster the response becomes. The ultra-thin ®lms prepared by SA technique is thinner than that of being prepared by LB technique. Moreover, the forming and working mechanism of ultra-thin ®lms prepared by SA technique are very different from that of being prepared Table 1 The response time of PAN-based ultra-thin films prepared by LB technique with different number of layers to 20 ppm NO2 gas at 22 8C Number of film layers

3

9

15

21

27

Response time of PAN film (s) Response time of PAN±AA film (s)

10 15

20 30

30 50

60 90

90 150

by LB technique, the polymeric acidÐPSSA plays a much more important role in the SA process, therefore, their responsivity to NO2 gas is different, SA ®lm has faster response to NO2 gas. Table 3 shows the response time of 15-layer PAN and PAN±AA ®lms to different concentration of NO2 gas. From it, we can see that with the increase of NO2 gas concentration, it takes a longer period of time for the resistance to reach a stable value, so the response time increases. Fig. 8 shows the recovery property of PAN±PSSA SA ®lm and PAN ®lm prepared by LB technique. It is found that PAN±PSSA SA ®lm has faster recovery property: the recovery time of 14-layer SA ®lm and 15-layer PAN ®lm prepared by LB technique to 20 ppm NO2 gas is close to 2 and 4 min, respectively. The reversibility lacked in PAN±AA mixed ®lm. The SA ®lm is thinner than ®lm prepared by LB technique, NO2 gas molecules are easier to desorb

Table 3 The response time of 15-layer PAN-based ultra-thin films prepared by LB technique to different concentration of NO2 gas at 22 8C

Table 2 The response time of PAN±PSSA SA films with different number of layers to 20 ppm NO2 at 22 8C

NO2 gas concentration (ppm)

20

40

60

80

100

Number of SA film layers Response time of PAN±PSSA SA film (s)

Response time of PAN film (s) Response time of PAN±AA film (s)

30 50

50 70

90 100

120 150

150 180

2 8

8 15

14 25

20 35

26 50

D. Xie et al. / Sensors and Actuators B 81 (2002) 158±164

Fig. 8. Response recovery property of PAN ultra-thin films to 20 ppm NO2 at 22 8C. (a) 14-layer PAN±PSSA SA film; (b) 15-layer PAN film prepared by LB technique.

from the surface of ultra-thin ®lm, which will lead to faster recovery. PAN is different from common conducting polymer, the conductance of PAN mainly depends on two factors: the degree of protonation; and the degree of oxidation. In order to improve conductivity of PAN, we can use not only protonation-doping but also oxidation-doping, which both can make PAN conductive. Because NO2 is a well-known oxidizing gas which, on contact with the p-electron network of polyaniline, is likely to result in the transfer of an electron from the polymer to the gas. When this occurs, the polymer becomes positively charged. The charge carriers thus created

Fig. 9. Plots of the relative change of resistance ((R0 concentrations (22 8C). (a) PAN; (b) PAN±AA.

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give rise to the increased conductivity of the ®lms. So, the resistance decreases greatly ®rst. That is to say, PAN is oxidized by NO2 when PAN ®lms contacted with NO2 gas. The increase of the degree of oxidation will lead to the improvement of conductivity. However, with the continuous increase of NO2 concentration, the conductivity decreases at this time. It suggests that with the increase of oxidation time, the degree of oxidation improves much more, which will change the semi-oxidation and -reduction state of PAN to a higher oxidation state resulting in the decrease of conductivity. It is also found that PAN±AA mixed ®lms show reduced sensitivity, this may be due to the fact that acetic acid molecules have occupied and chemically blocked sensitive sites responsive to NO2 [21,22]. These results can be seen from Fig. 9. However, to PAN±PSSA SA ®lms, the resistance increases constantly with the increase of NO2 concentration (Fig. 7). The reason is that after being exposed to NO2 gas, oxidation degree improves, which make PAN change to a dedoping state. In order to con®rm the explanation, PAN±PSSA SA ®lm was treated by an oxidantÐ(NH4)2S2O8. The PSSA solution with 0.03% (NH4)2S2O8 was used in our experiment. If PAN base ®lm was only being treated by (NH4)2S2O8 solution, another monolayer cannot be assembled onto the surface in the following step. However, the process can be continued if PSSA was included in (NH4)2S2O8 solution or the ®lm was subsequently treated by PSSA solution. It is implied that the PSSA has been absorbed onto the PAN ®lm. From it, we inferred that the acid±base reaction between PAN and PSSA has taken place, which was very important to drive the assembling process in the PSSA/(NH4)2S2O8 system. After being treated by (NH4)2S2O8, PAN±PSSA SA ®lm will be oxidized to a higher oxidation state even (NH4)2S2O8 concentration was very small. From the UV± VIS spectra (Fig. 10), we can see that the absorption peak at 850 nm is dramatically blue-shifted and the absorbency decreases at 430 nm after being treated by (NH4)2S2O8. The change of absorption peak after being treated by NO2 gas is the same as that of (NH4)2S2O8, which proves that the effect of dedoping behavior of NO2 on PAN±PSSA SA ®lm results in the increase of resistance.

R)/R0) of polyaniline-based films prepared by LB technique with various number of layers vs. NO2

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Fig. 10. The UV±VIS spectra of PAN±PSSA SA film before and after being treated by NO2 and (NH4)2S2O8.

4. Conclusion Pure PAN and PAN±AA mixed ultra-thin ®lms prepared by LB technique and PAN±PSSA composite SA ®lms basing on the acid±base reaction between PAN and PSSA with various layers have been fabricated. The thickness of these ultra-thin ®lms can be controlled at nanometer scale and the transferred materials in the spreading solution in each deposition are equivalent. The ®lm thickness was approximately 6.2 and 5.9 nm per layer for pure PAN ®lm and PAN± AA mixed ®lm, respectively; and 4.5 nm for PAN±PSSA SA monolayer. Pure PAN ®lms prepared by LB technique have higher sensitivity and responsivity to NO2 than that of PAN±AA mixed ®lms, while SA ®lms have faster recovery property. The mechanism of the sensitivity of PAN-based ultra-thin ®lms can be explained as the oxidation doping of NO2, which leads to the change of conductivity of PAN-based ultrathin ®lms. The sensitivity and responsivity to NO2 are affected by the thickness of PAN-based ®lms. The thinner the ®lms, the higher the sensitivity gets and the faster the response becomes. The response time of three-layer PAN ®lm prepared by LB technique to 20 ppm NO2 is about 10 s, 15layer is about 30 s and its recovery time is about 4 min. The response time of two-layer PAN±PSSA SA ®lm is about 8 s, 14-layer is about 25 s and its recovery time is close to 2 min. The studies of sensitivity of PAN-based ultra-thin ®lms to NO2 indicate that the PAN-based ultra-thin ®lms can be used for sensitive element of gas sensors, which shows promising application especially for microsensor. References [1] S.K. Dhawan, D. Kumar, M.K. Ram, S .Chandra, D.C. Trivedi, Application of conducting polyaniline as sensor material for ammonia, Sens. Actuators B 40 (1997) 99±103.

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