Aqueous microdroplet chemistry enables sustainable, scalable synthesis of polyaniline

Polyaniline (PANI) is a widely studied conductive polymer with applications from antifouling coatings to flexible electronics. Its conventional synthesis typically relies on strong oxidants, elevated temperatures, and prolonged reaction times. Here, we introduce an innovative and sustainable method for synthesising PANI that exploits the unique chemistry of aqueous microdroplets. At the air–water interface of these microdroplets, spontaneous proton-coupled electron-transfer reactions occur in a naturally acidic, oxidative environment, converting aniline monomers into reactive anilinium cations and radicals. These transient intermediates rapidly initiate oxidative polymerisation, yielding both the emeraldine salt and base forms of PANI. To further enhance polymer yield and support scale-up, ferrous ions (Fe²⁺) are incorporated to catalyze the decomposition of hydrogen peroxide into hydroxyl radicals (OH•), thereby accelerating chain propagation. This ambient, interfacial approach enables complete conversion of a 50 mL, 0.4 M or 0.6 M aniline solution into gram-scale quantities of PANI within just 30 min. The resulting polymer can be directly deposited onto diverse substrates. As demonstrations of utility, PANI-coated alumina membranes achieved 95% oil rejection in oil–water separation, and PANI-modified microelectrodes exhibited ideal electrochemical behaviour. This work establishes microdroplet interfacial chemistry as a robust and generalisable strategy for green, scalable conductive polymer synthesis. A conductive polymer (polyaniline) can be conveniently synthesised at the gram scale by taking advantage of interfacial redox potential of spraying water microdroplets under ambient conditions. This microdroplet polymerisation method shows promising potential in the on-demand coating of hydrophobic materials on any object surface.