Objective Polyaniline (PANI) has been intensively investigated owing to its low raw-material cost, high electrical conductivity, good environmental stability under ambient conditions, promising chemical, electrical, and optical properties, as well as its unusual doping characteristics. Therefore, PANI has been found to have a wide range of practical applications in many fields, such as supercapacitors, chemical/biological sensor devices, electromechanical actuators, anticorrosion coatings, separation membranes, and battery electrodes. The application basis of PANI is its synthesis. At present, PANI can be synthesized through various chemical/electrochemical approaches; however, the nanostructure uniformity of PANI at the large scale is poor, and the controlled growth of PANI microstructures is difficult in these preparation methods. Furthermore, it is unfavorable to realize the integration and miniaturization of devices. Hence, a synthetic method that is capable of developing regular, controllable, and uniform PANI nanostructures at a micro/nanoscale is required.

Two-photon polymerization (TPP) is a photopolymerization method based on the two-photon absorption effect and is an extremely powerful method to achieve real three-dimensional (3D) microdevices. Using femtosecond lasers, which exhibit the characteristics of ultrashort pulse width, ultrahigh precision, and low heat input closely focused into a volume of polymer material, TPP has been employed in the fabrication of diverse micro-objects, such as biochips, micro/nanofluidic devices, and micro/nanoelectromechanical systems. Currently, two-photon polymeric materials are primarily commercial photoresists and hydrogels. These two-photon polymer materials can exhibit strong 3D processing capabilities and better biocompatibility; however, they do not exhibit electrical conductivity. Therefore, we attempt to propose the TPP method to prepare fine and controllable PANI structures and aim to provide new ideas for the preparation of conductive polymers and their wide applicability in sensors, microdetectors, and other micro/nano devices.

Methods PANI micro/nanostructures with diverse morphologies were fabricated using the TPP method based on femtosecond lasers. First, using aniline as the monomer and nitric acid as the oxidant, aniline mixed solutions with different molar ratios were prepared. Then, a drop of aniline mixed solution was fixed on the substrate, and the fabricated substrate was placed on the 3D moving stage for TPP processing. After that, a PANI microstructure attached to the glass substrate could be obtained. In addition, the morphology of the PANI microstructure was analyzed through scanning electron microscopy and atomic force microscopy, and the chemical composition of the PANI sample was characterized via Fourier transform infrared spectrometry (FT-IR). Current-voltage curves and resistance values of a single PANI line were tested using a micromanipulated cryogenic probe station-semiconductor characteristic parameter analyzer in a nitrogen atmosphere.

Results and Discussions PANI micro/nanowires with different morphologies can be prepared by adjusting the molar ratio of aniline to nitric acid (Fig. 3). The most prominent performance is whether the PANI lines are connected by convex hulls. Under the same laser power and scanning speed, when the molar ratio of aniline to nitric acid is relatively low (samples 1 and 4), it is easier to yield PANI lines with a relatively flat structure. As the concentration of the aniline monomer increased, the PANI lines became denser and thicker. Then, we illustrated the polymerization mechanism of aniline and explained the influence of the aniline concentration on the morphology of PANI lines (Fig. 4). The water-insoluble aniline polymer was synthesized at the water interface. The concentrations of aniline and nitric acid are closely related to the distribution of water-soluble aniline oligomers. When the concentration of aniline was low, the PANI lines with uniform and thin shapes were prepared because all of the aniline monomers in the laser scanning path at the laser focus were converted into aniline oligomers. When the number of aniline monomers in the laser scanning path was extremely large, PANI lines with a convex structure were prepared, which was attributed to the effect on the migration distance of the aniline oligomer with 3D Gaussian distributions. Although samples 1--4 can produce PANI lines, their performances are easily affected by the environment and the stability of the mobile station. To generate PANI with better conductivity and repeatability, we optimized the aniline mixed solution. The performance of the TPP of sample 5 was better than that of other samples (Fig. 5). In addition to the molar ratio of aniline to nitric acid, femtosecond laser power also affected the morphology of PANI lines. Under the high laser power, the PANI lines appear as a more discrete convex hull structure (Fig. 6). With the increasing laser scanning speed, the morphology of the PANI lines became looser, the intermittent situation was intensified, and the width of PANI lines reduced slightly (Fig. 7). Furthermore, FT-IR spectra of PANI were analyzed, which proved that PANI could be successfully prepared by TPP (Fig. 8). The electrical conductivities of the PANI lines were characterized and shown as 5.79×10 ^-6 S·cm ^-1 (Fig. 10).

Conclusions To directly prepare microstructures of small-scale conductive polymers with controllable shape at one time, the TPP method based on femtosecond laser is proposed, which can realize the precise and controllable preparation of micro/nano-sized PANI. When the ratio of aniline to nitric acid was 1.14∶1, the concentration of aniline was 0.69mol·L ^-1, the laser power was 14.1mW, and the laser scanning speed was 6μm·s ^-1. We could obtain the best performing of PANI lines with continuous structure, compact and smooth surface, and good stability. In addition, the FT-IR spectrum characterization of PANI lines demonstrates that PANI is successfully achieved using the TPP method. The electrical conductivity test of PANI shows that PANI is conductive, and its electrical conductivity is 5.79×10 ^-6 S·cm ^-1. This study provides a feasible solution for the controllable preparation of conductive polymer microstructures, and the controllable preparation of PANI micro/nanostructures can provide new ideas for the development of conductive polymers in integrated devices.