Detection of Toxic Chemicals for Homeland Security Using Polyaniline Nanofibers

Abstract

The electrical properties of the conducting polymer polyaniline change greatly upon exposure to various chemicals. Specifically, polyaniline undergoes doping and dedoping chemistry with acids and bases that result in conductivity changes of over eight orders of magnitude. This large range in conductivity can be utilized to make polyaniline chemical sensors. Polyaniline nanofibers are chemically synthesized using a simple, template-free method that produces nanofibers with narrow size distributions. They are easily cast on microelectrode arrays and are shown to respond significantly better than conventional films to a number of different gases, such as acids, bases, hydrazine, and organic vapors. This is explained through their high surface area, small diameter, and porous nature of the nanofiber films that appear to allow better diffusion of vapors into the films. Polyaniline nanofibers disperse well in water and, as a result, have been used to make new composite materials with water soluble compounds, such as metal salts. These composite films can then be used to enhance sensing to gases that unmodified polyaniline would otherwise not be able to detect. For example, metal salt/polyaniline nanofiber composite films are used to enhance polyaniline's ability to sense hydrogen sulfide due to a reaction of the metal salt with hydrogen sulfide that releases a strong acid that then dopes the polyaniline, resulting in a significant increase in conductivity. The wide range in gas detection capabilities and the use of composite films makes polyaniline nanofibers versatile chemical sensor materials that have good potential for many chemical detection applications, including homeland security.