By modifying the conventional chemical oxidation polymerization of aniline, we have developed two simple approaches to synthesize polyaniline nanofibers with diameters tunable from 30 to 120 nm. The key to producing pure nanofibers is to suppress secondary growth that results in agglomerated particles. Both interfacial polymerization and rapidly mixing aniline, oxidant and acid produce pure nanofibers. These processes are template-free and readily scalable. Stable and processable colloids are now available. Polyaniline nanofibers exhibit an exceptional photothermal effect in which they instantaneously melt and cross-link upon exposure to a camera flash. This novel flash welding technique can be used to form patterned nanofiber films, create polymer based nanocomposites and make asymmetric polymer membranes. These asymmetric structures can act as mechanical actuators (artificial muscles) when exposed to strong acids. Polyaniline nanofibers are useful in many applications such as resistive-type sensors where their high surface area enable very rapid response times often less than two seconds. Polyaniline nanofibers can be modified to respond to many different vapors including toxic agents such as hydrogen sulfide. The key is using a metal salt such as CuCl2 which reacts with H2S to produce CuS and HCl. This process converts a weak acid (H2S) into a strong acid (HCl), which in turn can be readily detected at very low concentrations. Polyaniline nanofibers can be decorated with metal nanoparticles which not only can enhance sensor response but also leads to molecular memory devices.