Background

The conjugated polymer polyaniline is a promising material for sensors, since its conductivity is highly sensitive to chemical vapors. Nanofibers of polyaniline are found to have superior performance relative to materials without nanoscale morphology due to their much greater exposed surface area. A template-free chemical synthesis produces uniform polyaniline nanofibers with diameters below 100 nm, see Figure 1. The process of polymerization can be readily scaled to make kilogram quantities.

Figure 1. Polyaniline nanofiber synthesis.   A. Interfacial polymerization occurs through an interfacial region (oil/water) yielding anisotropic polymer nanoparticles; this procedure is carried out a ambient conditions. The intrinsic morphology of the polymer is nanofibrillar due to homogeneously nucleated growth, the nanofibers form within seconds and can be purified by centrifugation.   B. Transmission electron micrograph of polyaniline nanofibers (J. Huang and R.B. Kaner, Angew. Chem. Inter. Ed. 2004, 43, 5817).

Polyaniline is a conducting polymer that has been widely studied for electronic and optical applications because it possesses a simple and reversible acid/base doping/dedoping chemistry (Figure 2) enabling control over properties such as free volume, solubility, electrical conductivity, and optical activity. ¹ Doping protonates the molecular chains that make up nanofibers and vapors of acids can serve to this purpose. A multimeter can be used to measure the sheet resistance of a film of nanofibers in the presence of vapors of acids or bases demonstrating the reversible doping process. Figure 2 also shows the conversion of the insulating emeraldine base form of polyaniline to the conducting emeraldine salt form upon exposure to HCl. This process occurs instantaneously with films in solution and within minutes in the gas phase and is fully reversible upon exposure of the films to a strong base. Acid doping and base dedoping can be cycled as many times as desired. The change in resistance from the dedoped to the doped form is large and easily measured—dedoped polyaniline is an excellent sensor for acids and doped polyaniline an excellent sensor for bases with orders of magnitude changes in resistance upon exposure to these analytes. Polyaniline can also be used to selectively distinguish between strong and weak acids. Weaker acids will not dope polyaniline as well or as quickly as strong acids, resulting in a change in response time from seconds or minutes extending to hours or days, accompanied by much smaller response levels.²

Figure 2. Reversible doping mechanism in polyaniline.   Repeat unit of the emeraldine oxidation state of polyaniline in the undoped, base form (top), and the fully doped, acid form (bottom). HX represents any protonic acid. (J. Huang and R.B. Kaner Macromolecules. 2005, (38)2, 317).

Figure 3. Schematic Diagram of a Chemiresistor. Acid vapors lower the electrical resistance while base vapors increase it. The active sensing layer is polyaniline nanofibers deposited over brown (unbleached) paper and the electrodes are made of copper.