Controlling Response of Polyaniline Towards Humidity by Self-Assembly Fatty Acids

Abstract

Conductive polymers based sensors are potentially a powerful tool for detecting a variety of analytes in gas phase [1-2]. However, the accuracy and reliability of such sensors are threatened by the humidity interference, which limits their use in demanding applications of air-quality monitoring and healthcare [3]. Thus, there is an urgent need to suppress the response of conductive polymers based sensors to humidity [4]. However, it is difficult to fabricate sensors with anti-humidity interference ability.

In this work, a method to control the humidity response of polyaniline (PANI) chemi-resistor based sensors is proposed. The key principle of the method is to self-assemble a fatty acid layer on the surface of PANI to decrease the surface energy and shield the water vapour to adsorb on PANI polymer chains. As a result, the response of PANI sensors towards water vapour is suppressed. To study the relationship between the humidity responses and the chain length of fatty acids, a series of fatty acids with different chain lengths were employed to assemble on PANI surface (Fig.1).

The surface properties of fatty acids modified PANI films were characterized by ATR-FTIR spectra and contact angle test. The results show that the long chain alkyl acid can form more densely packed hydrophilicmolecule layer, which can prevent the water molecular diffuse and adsorb on PANI surface. In addition, the contact angle of fatty acid modified PANI films aredepended on the chain length of surface modified alkyl acid. The fatty acid with a longer chain is more hydrophobic than the shorter ones.

The unmodified and fatty acid modified PANI chemi-resistors were electrically tested under exposure to different concentration of humidity. To further investigate the effect of fatty acid modification on the humidity sensing responses of the PANI gas sensors, the statistical response upon water vapour were summarized in Table 1. The results show that the humidity response suppression effect is depending on the chain length of surface modified alkyl acid. The long chain alkyl acid modified PANI sensors exhibit great suppression of the humidity sensing response; while the short chain modified PANI sensors have weak suppression of the humidity sensing response. Not only the sensor responses at certain water vapourconcentration, but also the concentration depend water vapour response (slopes of linearly fitted lines) were gradually decreased with the surface modified fatty chain length increasing. In specific, the Behenic acid (C22) modified PANI sensors are not sensitive to change of the humidity concentration with a concentration depend water vapour response as low as 0.008 %/ppm, which is only 1/5 of the unmodified PANI sensors.

In summary, we have developed a fatty acids modification based approach to suppress the humidity response on the PANI gas sensors. The effects of chain length of fatty acids on the sensing performance of PANI based gas sensors to a serial concentration of water vapour were studied. The results showed that the sensing response towards water vapour decreased asincreasing the chain length fatty acids. In addition, the concentration depend response, namely the slopes of linearly fitted lines, were gradually decreased with the surface modified fatty chain length increasing. These results indicated that the long chain fatty acid modification on PANI surface can efficiently suppress the humidity response. Furthermore, the surface analysis by contact angle testament showed that the suppression of the humidity response was due to the hydrophobic nature of fatty acid modified PANI surface. This approach to suppress humidity response by hydrophobic surface modification could also be applied on other types of gas sensors.