Invited: Atmospheric Pressure Plasma Modified Polyaniline Crystals for Detection of Volatile Organic Compounds

Among the family of conducting polymers, polyaniline (PANI) is the most studied due to an easy and low-cost synthesis, good environmental stability, and simple doping/dedoping process based on acid/base reactions.^1,2 PANI polymer can be used for different applications including in solar cells, artificial muscles, gas sensors, capacitors, acid-base indicators, etc.³ The nature of PANI conductivity is explained by its ability to form polarons and cation radicals.^3,4 Moreover PANI shows significant variations in conductivity when exposed to various volatile organic compounds (VOC).⁴ This property arises from to the doping-dedoping of conducting PANI layer, which makes it suitable for detection of wide variety of gas molecules.

Changes in the chemical states can alter the sensitivity and selectivity of PANI layer. Chemical modifications by doping of PANI with various acid dopants, co-polymerisation with various thermoplastics, co-polymerisation with aniline derivatives or enzymatic polymerisation of aniline has been used to improve the processability and solubility of PANI as reviewed by Jaymand et.al.⁵. Another techique is low pressure plasma treatment which has been reported to be an efficient technique for the carbon functionalization as well as changing the chemical states of the PANI surface.^6,7 The variations of the chemical changes induced by low pressure oxygen plasma treatment efficiently enhanced the selectivity and sensitivity towards various gases.⁸ PANI exposed to various plasmas has demonstrated also increased resistivity.

Urea is widely used in the manufacture of explosives and  nitrogen fertilisers in industries. Eventhough expoure of urea vapor is harmless at lower concentrations, prolonged exposure of urea can cause irritations to eyes, skin or respiratory systems. We demonstrated a reusable sensor based on emeraldine polyaniline crystals coated on polyethylene terephthalate substrate at room temperatue. To enhance the signal response, we used an atmospheric pressure He plasma treatment which showed pronounced and steady increase in the signal response for six consecutive cycles. The surface changes as well as mechanism of detection after plasma treatement were studied by various analysis techniques including FT-IR, scanning electron microscopy (SEM), and X-ray photoelectron microscopy (XPS).

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