Reversible Conversion of Conducting Polymer Films from Superhydrophobic to SuperhydrophilicThis work was supported by a US NSF grant (CCF0330451) and a US DoD/DARPA/DMEA grant. We thank A. Yu for help on the conductivity measurement of the samples

Controlling the wettability of a solid surface is important for myriad applications, ranging from self-cleaning surfaces to microfluidics to biomedicine. Recently, a variety of smart surfaces with reversibly switchable wettability have been developed. The reversible switching is realized through the adjustment of electrical potential, temperature, 8] and light illumination, adsorption of biopolymer, and treatment of selective solvents. Among these approaches, the switch of the electrical potential receives special attention because it is simple and conveniently controlled by electricity. Moreover, the switching is readily individually addressable when an array of small surfaces is involved. Lahann et al. constructed a reversibly switching surface by depositing a low-density carboxylate-terminated self-assembled monolayer on a gold surface. Electrical potential was used to trigger the conformational transition of the monolayer, resulting in switching of the surface wettability. However, the change in surface wettability is small (20 to 308 water contact angle), which is likely to limit its practical applications. It is reported that ZnO films and poly(N-isopropylacrylamide)-modified patterned surfaces can undergo reversible wettability switching between two extremes, superhydrophobicity (water contact angle> 1508) and superhydrophilicity (water contact angle< 58) through the use of ultraviolet light and a temperature change, respectively. The photoswitching requires several days to achieve the hydrophilic-to-hydrophobic conversion, and both the photoswitching and thermal switching are difficult to implement to individually address an array of small surfaces. Herein we report a simple electrochemical process to fabricate superhydrophobic conducting polypyrrole (PPy) films and demonstrate that their properties can be switched conveniently from superhydrophobic to superhydrophilic by changing the electrical potential. Conducting polymers (also called conjugated polymers or synthetic metals) such as polypyrrole (PPy) have been studied in great detail because their unique optical, electrical, and mechanical properties offer many new possibilities for device fabrication. Interest has recently developed in their surface properties such as wettability because of potential applications in corrosion protection, conductive textiles, and antistatic coatings, and in the immobilization of biopolymers and growth control of living cells. Usually the conducting polymers contain a positively charged conjugated backbone and negatively charged counterions (dopants). The wettability of conducting polymers depends greatly on the types of dopants used. For example, a PPy film containing a perfluorinated dopant exhibited hydrophobicity (water contact angle> 908), while ClO4 -doped PPy was hydrophilic. Furthermore, the doping level can be controlled by changing the electrical potential, resulting in reversibly switchable surface wettability. Scheme 1