Electrosynthesis of Biocompatible Free-Standing PEDOT Thin Films at a Polarised Liquid|Liquid Interface

Conducting polymers (CPs) find applications in energy conversion and storage, sensor, and biomedical technologies, once processed into thin films. Hydrophobic CPs, like poly(3,4-ethylenedioxythiophene) (PEDOT), typically require surfactant additives, such as poly(styrenesulfonate) (PSS), to aid their aqueous processability as thin films. However, excess PSS diminishes CP electrochemical performance, biocompatibility, and device stability. Here, we report the electrosynthesis of PEDOT thin films at a polarised liquid|liquid interface, a method non-reliant on conductive solid substrates that produces free-standing, additive-free, biocompatible, easily transferrable, and scalable 2D PEDOT thin films of any shape or size in a single-step at ambient conditions. Electrochemical control of thin film nucleation and growth at the polarised liquid|liquid interface allows control over the morphology, transitioning from 2D (flat on both sides with thickness <50 nm) to “Janus” 3D (with a flat and rough side, each showing distinct physical properties, and thickness >850 nm) films. The PEDOT thin films were p-doped (approaching the theoretical limit), showed high π-π conjugation, were processed directly as thin films without insulating PSS, and were thus highly conductive without post-processing. This work demonstrates that interfacial electrosynthesis directly produces PEDOT thin films with distinctive molecular architectures inaccessible in bulk solution or at solid electrode-electrolyte interfaces and emergent properties that facilitate technological advances. In this regard, we demonstrate the PEDOT thin film’s superior biocompatibility as scaffolds for cellular growth, opening immediate applications in organic electrochemical transistor (OECT) devices for monitoring cell behaviour over extended time periods, bio-scaffolds and medical devices, without needing physiologically unstable and poorly biocompatible PSS.