Low Conductivity Polyaniline–Gold Nanocomposite via Interfacial Polymerization: Efficient Humidity Sensing by Thin Films and Mechanistic Insights through In Situ Experiments

Polyaniline (PANI) and its composites have been studied extensively as humidity sensors; however, direct experimental insights into the underlying mechanistic basis are limited. In this work, polyaniline–gold (PANI–Au) nanocomposite thin films are fabricated using a simple single-step interfacial polymerization protocol, with chloroauric acid used concomitantly as the oxidizing agent, acid dopant, and the gold nanoparticle precursor. Contrary to many earlier reports, the composite shows a higher ambient temperature electrical resistivity (1.3 Ω m) compared to PANI, synthesized using the same protocol with peroxodisulfate as the oxidative polymerization agent (8.0 × 10–3 Ω m), as a result of the greater emeraldine base fraction and the low Au loading. PANI–Au thin film shows a significantly more efficient (responsivity = 79%) and faster (response time = 7.8 s) humidity sensing than PANI (37%, 18.0 s, respectively) in the relative humidity range, 11–85%, thanks to the critical role of the PANI–Au interfaces. In situ Raman spectroscopy coupled with computation of model oligomeric systems provides critical insight into the molecular level impact of humidity on PANI; a fundamentally different mechanistic origin of the sensing response of PANI–Au is revealed through impedance spectroscopy. The insights provided here open up novel design possibilities for nanocomposite based sensors, and the new materials herald the development of low cost polymer–metal thin films with high sensing capability and potential scalability.