Polyaniline is a conducting polymer with incredible promise, but it has had limited use due to poor reaction control and processability associated with conventional morphologies. Polyaniline nanofibers, on the other hand, have demonstrated, through manufacturing techniques discovered during the past decade, increased processability, higher surface area, and improved consistency and stability in aqueous dispersions, which are finally allowing for expanded commercial development of this promising polymer. This review explores some intriguing applications of polyaniline nanofibers, as well as the advantages and remaining challenges in developing better products using polyaniline in this new morphology.

Polyaniline nanofibers show great promise as chemical sensors due to their high surface area and small diameters, which facilitate the diffusion of molecules and dopants into the nanofibers. [1, 2] Recently we reported a facile, practical, bulk synthetic method that yields pure, uniform polyaniline nanofibers with narrow size distributions that can be adjusted from 30 to 120 nm. [3] Other recent synthetic methods for nanostructured polyaniline include: induced self-assembly of nanostructures, [4] the use of nanostructured seeds, [5] electrochemical methods, [6] and electrospinning. [7] We have shown that polyaniline nanofibers respond much better than conventional films to a number of different gases including acids, bases, and reducing agents with large changes in conductivity-over eight orders of magnitude in some cases. [1, 2] However, polyaniline nanofibers do not respond well to certain classes of chemicals, such as hydrogen sulfide. In addition to providing a facile chemical route to polyaniline nanofibers with controlled distributions of diameters, our synthesis approach yields nanofibers that are dispersed in water. Water dispersions of nanofibers have several advantages for sensor applications including: 1) the simple formation of nanofiber films by drop casting, 2) the ability to modify the nanofibers by reaction with water-soluble inorganic or biological chemicals to create composite materials with different properties that can enable new applications, and 3) more environmentally benign processing than with organic solvents. Through the formation of new nanostructured conducting organic/inorganic hybrid materials, we demonstrate the ability to control and enhance the response of polyaniline nanofibers to molecules such as hydrogen sulfide. Specifically we show that nanofiber composites with transition-metal chlorides have a remarkable response to hydrogen sulfide that is four orders of magnitude greater than the corresponding unmodified nanofibers.Hydrogen sulfide is a weak acid that is important to detect because it is colorless, flammable, heavier than air, and dangerous at concentrations above 20 ppm. [8] There is also concern about its potential use in terrorist attacks. [9] Polyaniline gives a robust response to strong acids since they have the ability to fully dope polyaniline, which results in very large changes in conductivity. However, weak acids only partially dope the polymer and the response of polyaniline to hydrogen sulfide is minimal. [10] The approach we use here to enable the detection of hydrogen sulfide is by reaction with metal salts incorporated in a polyaniline nanofiber matrix. It is well known that hydrogen sulfide reacts with many metal salts in solution to form a metal sulfide precipitate and a strong acid as the by-product [11] as shown in Equation (1):where MCl 2 is a metal chloride salt and MS is a metal sulfide. The emeraldine base form of polyaniline can then detect the acid that is generated by becoming doped with a large resultant change in conductivity, as shown ...

A composite system comprised of polyaniline nanofibers bonded with gold nanoparticles is shown to possess a memory effect via a charge transfer mechanism. The charge transfer occurs between the imine nitrogen in the polyaniline and the gold nanoparticles as confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy. This charge transfer enables a bistable electrical conductivity, allowing the material system to be used as a digital memory device. The charge transfer is further confirmed by the elimination of the conductance switching when the fully reduced form of polyaniline, leucoemeraldine, which possesses no imine nitrogens, is used in place of the emeraldine form.

Asymmetric films formed by flash-welding polyaniline nanofiber mats demonstrate rapid reversible actuation in the presence of select aqueous acids and bases. These continuous single component bending/curling actuators have several advantages over conventional dual component, bimorph actuators including ease of synthesis, large degree of bending, patternability and no delamination. The films are made through a controlled, facile, all aqueous process that yields water dispersed polyaniline nanofibers that are readily cast into films. Flash welding photothermally cross-links and melts the top surface of the nanostructured polymer producing an asymmetric film. The resultant cross-linked surface is quite dense and has a reduced number of protonic acid doping sites available. The film surface is therefore less susceptible to the protonic acid doping which expands the underlying high surface area nanofiber layer. Actuation occurs at a comparable or faster rate than bimorph actuators with an unprecedented > 720°bending relative to the initial flat position for a 2.5 cm length film. The collective movement of the individual nanofibers in the asymmetric film creates a large degree of actuation resembling natural muscle. These bending actuators could be developed for use in microtweezers, microvalves, artificial muscles, chemical sensors and/or patterned actuator structures.Polyaniline and other conducting polymers have been of interest for their actuation properties for more than two decades. [1][2][3][4][5][6][7][8] The actuation is due to the unique chemistry of conducting polymers, which generally swell reversibly with the incorporation of dopant ions and their associated solvent molecules. Previous work on polyaniline actuators has involved dispersing conventional polyaniline in highly polar solvents such as N-methyl pyrrolidinone for casting into fibers, [9][10][11] rods, [12][13][14] sheets, [5] layered bimorphs [15,16] and integrally skinned asymmetric membranes. [7,[17][18][19] Elongation or contraction of polyaniline films and fibers has been induced by oxidation state, electrostatic or conformational changes as well as combinations of all of these to create linear or bending movement depending on the initial structure. Typical bending actuators require the use of two or more different materials bound together to produce a bimorph. One material forms the active part that expands or contracts relative to the inactive part upon stimulation, thus inducing bending. Bending of bimorph structures has generally been limited to < 90°and problems with adhesion between the layers often leads to delamination especially with extended use. [20] Alternative bending structures have been proposed such as active dual layers, which expand and contract cooperatively.[21] Wang, et al. [7] made a major advance by developing integrally skinned asymmetric membrane (ISAM) bending polyaniline actuators. ISAMs use a single material processed so that one side of the film has much higher porosity than the other side. Doping induced swelling ...