Electrospun camphorsulfonic acid doped poly(o-toluidine)–polystyrene composite fibers: Chemical vapor sensing

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Synthetic Metals 158 (2008) 259–263

Electrospun camphorsulfonic acid doped poly(o-toluidine)–polystyrene composite fibers: Chemical vapor sensing D. Aussawasathien ∗ , S. Sahasithiwat, L. Menbangpung National Metal and Materials Technology Center, 114 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand Received 5 July 2007; received in revised form 18 December 2007; accepted 15 January 2008 Available online 22 April 2008

Abstract The electrospinning technique was utilized to produce camphorsulfonic acid (HCSA) doped poly(o-toluidine) (POT)–polystyrene (PS) composite fibers in the non-woven mat form. HCSA doped POT–PS composite fibers were fabricated on an interdigited gold (Au) substrate for use as a chemical vapor sensor. The composite fiber sensor responded to volatile chemicals in different ways, depending on the polarity of sensing chemicals. The surface morphology of the non-woven composite fiber mat after chemical vapor sensing was unchanged. This study highlights that composite fibers comprised of polyaniline derivative and a spinnable polymer do have potential for use as chemical sensors due to their good solubility in common solvents and detectable electrical changes at low fiber contents. © 2008 Elsevier B.V. All rights reserved. Keywords: Electrospinning; Camphorsulfonic acid doped poly(o-toluidine)–polystyrene; Composite fibers; Sensors; Chemical vapors

1. Introduction Various kinds of conducting materials such as polypyrrole (PPy), polythiophene (PT), polyaniline (PANI), etc. have been synthesized and studied in many fundamental fields and potential applications [1]. Among these polymers, PANI is one of the most attractive materials for sensing applications due to its relatively high environmental stability, excellent electrical properties, together with the advantage that it can be made from inexpensive raw materials [2–4]. However, it has the drawback of poor solubility in common solvents. In order to increase its solubility in a variety of solvents, derivatives of PANI have been synthesized in the oxidized conducting form [5–8]. Poly(N-alkylanilines) are soluble in common organic solvents owing to the introduction of bulk alkyl groups into the polymer backbone and they are therefore easily processable. The electrospinning technique is a well-known process for making continuous sub-micron to nano-size fibers in the nonwoven mat form. In this process, a high voltage is applied to

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0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.01.007

the anode, immersed in the spinning solution. When the electrical force is higher than the surface tension of the solution, a charged jet of fluid is produced [9–11]. Electrospun nanofibers have a variety of potential uses as sensors [12], tissue engineering scaffolds [13], wound dressings [14], filters [15], and nano-filler materials [16,17] due to their large specific surface area, high fiber aspect ratio and high degree of interconnection. The high specific surface area of electrospun fibers has made electrospun conducting materials very interesting as transducer-active materials for sensing applications. A new fabrication approach to highly sensitive optical sensors has been developed by combining the techniques of electrospinning and electrostatic layer-by-layer adsorption [18]. Liu et al. [19] demonstrated a scanned-tip electrospinning method for depositing isolated and oriented polymeric nanowires. They created individual PANI–PEO nanowire sensors for detecting ammonia (NH3 ) gas at concentration as low as 0.5 ppm with rapid response and recovery time. Aussawasathien et al. [12] prepared lithium perchlorate doped polyethylene oxide electrospun nanofibers for humidity sensing and camphorsulfonic acid (HCSA) doped PANI–PS electrospun nanofibers for hydrogen peroxide and glucose sensing. Owing to the large surface area and good electrical properties intrinsically associated with these nanoscale functional polymer fibers, they achieved significantly

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enhanced sensitivity for the nanofiber sensors compared to their corresponding film-type counterparts. In the present work, electrospun composite fibers of poly(otoluidine) (POT), one of the derivatives of PANI, doped with HCSA were prepared. Polystyrene (PS) was added to control the solution viscosity for the electrospinning process using chloroform as a solvent. The as-spun composite fibers were fabricated on an interdigited Au substrate as a sensing electrode. The as-spun fibers were characterized by SEM before and after chemical vapor sensing. The resistance changes of the sensing material corresponding to the exposure and recovery times were recorded. The effect of the chemical polarity on the sensing measurement of the composite fiber sensor was also determined. This work opens up the possibility of using derivatives of polyaniline as a component in composite fibers for chemical sensing applications by taking advantages of its good solubility in common solvents as well as detectable electrical changes for small amount of fibers in the composite. 2. Experimental procedure 2.1. Materials Poly(o-toluidine) was in-house synthesized [20] with a weight average molecular weight of 7446. Polystyrene (Styron 685 D, weight average molecular weight of 2.99 × 105 g/mol) was supplied from Dow Chemical. 10-camphorsulfonic acid (98%), chloroform, ethanol, and hexane were purchased from Aldrich. 2.2. Electrospinning A mixture of 2 wt% POT, 4 wt% HCSA, and 7.5 wt% PS in chloroform was selected as a composite solution because it generates small fiber diameter with low content of beaded-fibers and ease of spinning compared with other compositions. The aforementioned solution was prepared and magnetically stirred overnight to produce homogeneous solutions for electrospinning at room temperature in air. A voltage of 20 kV was applied to the spinning solution filled in a glass pipette, containing copper wire as an anode. A fiber sample was collected by using a roller,

rotating at a speed of 100 rpm. The gap distance between the tip of the glass pipette and the roller covered with aluminum (Al) foil was 15 cm and the roller was grounded. The diameter of the stretched pipette tip was approximately 0.1 mm. 2.3. Fiber fabrication The electrospun composite fibers were collected on an interdigited electrode (IDE) attached to the roller. The IDE was made by thermal evaporation of chromium (Cr, 20 nm) and Au (200 nm) onto a glass slide. The electrode structure contains nine pairs of electrode arms having a width of 200 ␮m, space 200 ␮m from the adjacent electrode arm. Each arm has a length of 4950 ␮m. The weight of composite fibers on the IDE was approximately 8 × 10−4 g. The composite fibers deposited on the IDE were used to detect the vapors of distilled water, ethanol, and hexane. 2.4. Volatile chemical sensing measurement An IDE covered with HCSA doped POT–PS composite fibers was placed in a test chamber prior to the sensing measurement (see Fig. 1). The measurement system consisted of a gas flow system, a test chamber, a digital multimeter (Keithley 2700), and a computer for data collection and calculation. N2 gas (99.99%) was employed as a carrier gas. The testing procedure started by allowing pure N2 gas flow into the test chamber at 1000 ml/min, background running (BG), for 1 min. Then the two solenoid valves redirected N2 gas to a closed 100 ml Duran® bottle filled with 50 ml solvent in order to carry a saturated volatile chemical (100% of the vapor pressure) in the bottle to the test chamber for 3 min, followed by the recovery step in which only pure nitrogen was introduced to the test chamber for 10 min. The whole process was repeated four times. The changes in resistance value corresponding to sniffing and recovery times were recorded. The average response values were calculated from the sensing curves (four cycles) as the following. response =

dR Rb

Fig. 1. (a) An instrumental set-up of the chemical sensing measurement and (b) a sensing device.

(1)

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where, dR, the difference of resistance values at the peak position and the starting point of the sensing curve, Rb , the resistance value at the starting point of the sensing curve.

Table 1 The response values of the HCSA doped POT–PS composite fiber sensor exposed to different volatile chemicals Volatile chemical

Boiling point (◦ C)

Dielectric constant

Response

Ethanol Distilled water Hexane

78.3 100.0 68.7

24.55 78.39 1.88

−0.360 ± 0.063 −0.534 ± 0.059 0.015 ± 0.014

2.5. Fiber characterization The surface morphologies of HCSA doped POT–PS composite fibers were characterized using SEM (JEOL, JSM-5410 Scann). Prior to characterization, the sample on Al foil was attached onto an SEM stub using conducting tape and then sputter coated with a thin layer of Au using JEOL JEC-1200 Fine Coater at 15 mA for 120 s. The surface morphology of HCSA doped POT–PS fibers before and after chemical vapor sensing was also characterized. 3. Results and discussion As-spun HCSA doped POT–PS composite fibers were dark green in color with fiber diameters in the ranges of ca. 240– 1900 nm. Some beaded-fibers were obtained with varied sizes and amounts of beads existing on fiber threads (see Fig. 2(a)). HCSA doped POT–PS composite fibers were used as sensing materials for saturated vapor of ethanol, distilled water, and hexane. The sensing was carried out for several cycles by repetitive exposure of the sensing device to saturated chemical vapor and N2 gas alternately. Since the sensing device composed of composite fibers on top of a pair of electrodes as shown in Fig. 1(b), it is obviously seen that the observed resistance change comes from a change in the resistivity of composite fibers together with a change in the contact resistance between fibers. It should be also noted that the present method measures the volume resistance rather than the conductivity of an individual fiber. Fig. 3 depicts the typical response curves of the composite fiber sensor subjected to saturated chemical vapors of ethanol, distilled water and hexane. Though the exposure time was only 1 min, the time for complete desorption could be much longer. For instance, when exposed to ethanol for 1 min followed with pure nitrogen for 10 min, the sensor resistance was still not com-

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pletely stabilized. This contributed to the different starting points of the sensor resistance for each exposure round. The responses of the composite fiber sensor when exposed to ethanol, distilled water, and hexane (see Table 1) were −0.36, −0.53, and 0.015, respectively. The minus (−) sign indicated a reduction of resistance values during chemical vapor sensing measurement. Judging from their dielectric constants as presented in Table 1, water and ethanol are high polar solvents while hexane is considered as a low polar solvent. Therefore, it is presumed that high polar solvent has a tendency to decrease the resistance of composite fibers whereas low polar solvent exhibits a tendency to cause an increase in composite fiber resistance. For a conventional and conductive polymer composite, the change in its resistivity upon adsorption of a solvent derives from two major phenomena, i.e. a volume expansion of the polymer and an induction in conformational and electronic changes in a conducting polymer. The expansion of polymers results in an increase in the composite resistivity since some of electrical conduction paths are destroyed by this behavior. The induction in conformational and electronic changes in a conducting polymer causes a decrease in the composite resistivity when the adsorbed chemical is a high polar solvent [21]. It was found that a decrease in resistance values of the composite fibers occurred when they were exposed to a high polar solvent, i.e. distilled water and ethanol, resulted from an ability of the solvent to mobilize the dopant ion, i.e. HCSA ion [21]. When the composite fibers were exposed to a low polar solvent such as hexane, the resistance values slightly increased due to the longer path of the electron and proton transition as a result of

Fig. 2. SEM photographs of HCSA doped POT–PS fibers: (a) before sensing measurement and (b) after ethanol, distilled water, and hexane vapor sensing measurement.

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Fig. 3. The chemical vapor sensing curves (four cycles) of different solvents: (a) ethanol, (b) distilled water, and (c) hexane.

the expansion of polystyrene and POT after hexane absorption. Nevertheless, this effect is overcome by the ability of solvent to mobilize the dopant ion in the composite in case of high polar solvent absorption. The sensing electrode exhibited repeatable sensing measurement when it was subjected to chemical vapors for several times. There was no observable deformation for the HCSA doped POT–PS fiber sensor after ethanol, distilled water, and hexane vapor sensing measurements (see Fig. 2(b)). The fiber diameters remained unchanged, suggesting that this kind of fiber sensor may be reused several times.

to the sensing materials. When used in combination with other types of chemical sensors, there is the promise of using such composite fibers in electronic nose systems for the detection of chemical mixtures. Acknowledgements The authors thank Dr. Piyawit Koombhongse for his support with the work on the electrospinning apparatus. We would also like to acknowledge National Metal and Materials Technology Center for their financial support through the MTEC platform technology program (MT-S-49-POL-07-387-I).

4. Conclusions References HCSA doped POT–PS composite fibers could be generated by the electrospinning process. HCSA doped POT–PS composite fibers may have potential use as a sensing material for chemical vapor detection. The electrical response with chemical vapor changes is still observable even at low amount of fibers. This can be attributed to large specific surface area, high fiber aspect ratio and high interconnecting network of composite fibers [12]. The sensing device composed of HCSA doped POT–PS composite fibers responded to chemical vapors in different ways, depending on the type of sensing chemicals. The resistance of the sensing device had a tendency to decrease, when exposed to a high polar solvent. In contrast, the resistance of the sensing device had a tendency to increase, when subjected to a low polar solvent. The sensing electrode could be reused several times without any change in sensing behavior and/or damage

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