Gas sensing properties of a composite composed of electrospun poly(methyl methacrylate) nanofibers and in situ polymerized polyaniline

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Abstract

Poly(methyl methacrylate) (PMMA) nanofibers with different diameters were fabricated by electrospinning and their composites with polyaniline (PANI) were formed by virtue of in situ solution polymerization. The coaxial composite nanofibers so prepared were then transferred to the surface of a gold interdigitated electrode to construct a gas sensor. The structure and morphology of the PANI/PMMA composite fibers were characterized by UV–vis spectroscopy and scanning electron microscopy, which indicated that the coaxial nanofibres of PANI emeraldine salt and PMMA were successfully prepared. The electrical responses of the gas sensor based on the composite nanofibres towards triethylamine (TEA) vapors were investigated at room temperature. It was revealed that the sensor showed a sensing magnitude as high as 77 towards TEA vapor of 500 ppm. In addition, the responses were linear, reversible and reproducible towards TEA vapors ranging from 20 to 500 ppm. The diameters of the electrospun PMMA fibers had an effect on the sensing magnitude of the gas sensor, which is proposed to relate to the difference in the surface-to-volume ratio of the fibers. Furthermore, it was found that the concentration of doping acids only led to changes in resistance of the sensor, but could not affect its sensing characteristics. In contrast, the nature of the doping acids was determinative for the sensing magnitude of the sensor. The gas sensor with toluene sulfonic acid as the doping acid exhibited the highest sensing magnitude, which is explained by taking into account of the sensing mechanism and the interactions of doping acids with TEA vapor.

Introduction

Electrospinning (ES) was a technology developed more than a century ago [1]. It is featured with ease of fabrication of continuous nanofibers, and has received much attention worldwide because of great concerns for nanostructured materials in recent years. A large number of papers have been published on preparation of nanofibres with different morphology, such as porous nanofibers [2], [3], nanotubes [4], [5] and beaded nanofibers [6], by virtue of this simple and versatile technique. In addition, great efforts have been paid to explore the potential applications of the as-prepared nanomaterials in the fields of medicine [7], photonics [6], catalysts [8], sensors [9], etc.

Until now, much progress has been achieved in using nanomaterials prepared by ES in the biomedical field. Many biopolymers and biodegradable polymers were electrospun as non-woven membranes and used in tissue engineering biomaterials, mainly because of their interconnected, three-dimensional porous structure and relatively large surface areas, similar to the morphology of natural extra-cellular matrix [7], [10]. However, the researches on chemical and biosensors based on electrospun nanomaterials are not often reported. Wang et al. [11] used nanofibers electrospun from poly(acrylic acid)-poly(pyrene methanol) and thermally cross-linkable polyurethane latex mixture solutions as fluorescence quenching-based optical sensors, and found that they were highly responsive to metal ions (Fe3+ and Hg2+) and 2,4-dinitrotoluene due to high surface-to-volume ratios of the nanofibres. Patel et al. constructed a potential biosensor by using electrospun silica nanofibres to encapsulate horseradish peroxide enzymes [12]. Yoon et al. found that polymerization of electrospun diacetylene monomers under UV irradiation led to colorimetric sensors for volatile organic compounds [13]. Even less work is done on applying the ES technique in the development of popular and attractive gas sensors whose electrical properties change upon interaction with the analytes. Only Liu et al. [9] prepared a single polyaniline nanowire chemical sensor with a rapid response and reversible resistance change upon exposure to ammonia vapor of even very low concentration. Pinto et al. investigated the electrical responses of isolated electrospun polyaniline nanofibres to the vapors of saturated alcohols [14].

Polyaniline (PANI) is one of the most important intrinsically conducting polymers [15]. Due to the advantages of simple preparation, good chemical stability, high conductivity, etc., PANI and its composites were widely investigated as gas sensing materials for the detection of a number of chemicals, including ammonia, NO2, CO, HCl, CHCl3, N2H4, organic amine, methanol vapor, etc. [9], [14], [16], [17], [18], [19], [20], [21], [22], [23], [24]. It has been known that the morphology and structures of the sensitive materials have great effect on their sensing properties. And PANI nanofibers were reported to exhibit better sensing properties than PANI thin film in terms of sensitivity, response time, etc., which was generally proposed to relate to the high surface-to-volume ratio brought about by the nanostructure [9], [20], [21], [22], [23]. However, the researches on gas sensors based on nanostructured PANI are still quite limited.

In this paper, poly(methyl methacrylate) (PMMA) nanofibers were prepared by electrospinning technique. On the surface of PMMA fibers, polyaniline was grown by in situ polymerization to obtain a composite of coaxial PANI/PMMA nanofibers. The nanostructured composite was successfully transferred to an interdigitated gold electrode to construct a gas sensor. Its electrical responses towards TEA vapor, a typical organic amine existing in food processing and industry production, were measured at room temperature. The effects of diameters of the fibers, electrospinning time, the nature and concentration of doping acids, etc. on the gas sensitive properties of the composite have been investigated.

Section snippets

Chemicals

All the chemicals used were of analytical grade. Aniline was distilled under reduced pressure before use. PMMA (MW: 105) was purified by reprecipitation in methanol from its acetone solution. Other reagents were used as received.

Preparation of coaxial nanofibers composed of PMMA and PANI

The nanofibers of PMMA were first prepared by using electrospinning technique with a device similar to that described in Ref. [25]. Typically, a solution of PMMA in dimethylformamide (DMF) (0.18 g/mL or 0.32 g/mL) was filled in a syringe bearing a hyperdermic needle,

Characterization of PANI/PMMA coaxial nanofibers

In this paper, PANI/PMMA coaxial nanofibers were prepared by a combination of electrospining technique and in situ polymerization method. The UV–vis spectra of PMMA and the composite nanofibers are shown in Fig. 2. It can be seen clearly that no absorption peaks were observed in the spectrum of PMMA nanofibers in the studied wavelength region (350–900 nm). In contrast, two obvious absorption peaks at approximately 400 and 760 nm, were clearly observed in the spectra of the composite nanofibers.

Conclusions

Coaxial PANI/PMMA composite nanofibers were prepared by using electrospinning technique and an in situ polymerization method. The composite fibers exhibited a high sensing magnitude towards TEA vapor in the range of 20–500 ppm. In addition, the responses were linear, reversible and reproducible, suggesting their potential as a sensitive material for the detection of low concentration amine vapor. Both the diameters of nanofibers and doping acids have great effect on the sensing characteristics

Acknowledgements

The work is financially supported by the National Nature Science Foundation of China (Contract no. 50403020) and Zhejiang Province Natural Science Foundation (Grant no. M203093).

Shan-zuo Ji is a postgraduate student in the Department of Polymer Science and Engineering, Zhejiang University, China. His research interests are polymer and composite materials for gas sensors.

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