15 Pure Appl. Chem., Vol. 78, No. 1, pp. 15–27, 2006. doi:10.1351/pac200678010015 © 2006 IUPAC Syntheses and applications of conducting polymer polyaniline nanofibers* Jiaxing Huang ‡ Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, CA 90095-1569, USA Abstract: Nanofibers with diameters of tens of nanometers appear to be an intrinsic morpho- logical unit that was found to “naturally” form in the early stage of the chemical oxidative polymerization of aniline. In conventional polymerization, nanofibers are subject to second- ary growth of irregularly shaped particles, which leads to the final granular agglomerates. The key to producing pure nanofibers is to suppress secondary growth. Based on this, two methods—interfacial polymerization and rapidly mixed reactions—have been developed that can readily produce pure nanofibers by slightly modifying the conventional chemical syn- thesis of polyaniline without the need for any template or structural directing material. With this nanofiber morphology, the dispersibility and processibility of polyaniline are now much improved. The nanofibers show dramatically enhanced performance over conventional polyaniline applications such as in chemical sensors. They can also serve as a template to grow inorganic/polyaniline nanocomposites that lead to exciting properties such as electrical bistability that can be used for nonvolatile memory devices. Additionally, a novel flash weld- ing technique for the nanofibers has been developed that can be used to make asymmetric polymer membranes, form patterned nanofiber films, and create polymer-based nanocom- posites based on an enhanced photothermal effect observed in these highly conjugated poly- meric nanofibers. Keywords: conducting polymers; welding; memory device; nanocomposites; sensors; polyaniline; nanofibers. INTRODUCTION A tremendous amount of research has been carried out in the field of conducting polymers since 1977 when the conjugated polymer polyacetylene was discovered to conduct electricity through halogen dop- ing [1–3]. The 2000 Nobel Prize in Chemistry recognized the discovery of conducting polymers and over 25 years of progress in this field [4,5]. In recent years, there has been growing interest in research on conducting polymer nanostructures (i.e., nano-rods, -tubes, -wires, and -fibers) since they combine the advantages of organic conductors with low-dimensional systems and therefore create interesting physicochemical properties and potentially useful applications [6–11]. Traditionally, an advantage of polymeric materials is that they can be synthesized and processed on a large scale at relatively low cost. And many of the applications (sensors, functional coatings, catalysts, etc.) of conducting polymers in- deed need bulk quantity materials. Therefore, developing bulk syntheses for conducting polymers *Pure Appl. Chem. 78, 1–64 (2006). A collection of invited, peer-reviewed articles by the winners of the 2005 IUPAC Prize for Young Chemists. ‡ Current address: Department of Chemistry and Miller Institute for Basic Research in Science, University of California, Berkeley, Berkeley, CA 94720-1460, USA; Tel.: (510)-642-2867; Fax: (510)-642-7301; E-mail: jxhuang@berkeley.edu