Electrochemically tunable PEDOT:PSS film for surface acoustic wave-based humidity sensing

The increasing demand for humidity detection in both commercial and industrial applications has driven the rapid advancement of humidity sensors built on various technologies. Among these, surface acoustic wave (SAW) technology has emerged as a promising solution, thanks to its inherent advantages, including compact size, high sensitivity, and straightforward operating principle. In this study, we present a surface acoustic wave (SAW)-based humidity sensor featuring a functional polymer film as the active sensing layer. The film was deposited through an electrochemical bottom-up approach using the monomer 3,4-ethylenedioxythiophene (EDOT) and the polyelectrolyte Poly(sodium 4-styrenesulfonate) (PSS) in deionized water. Cyclic voltammetry was employed to optimize the deposition process, ensuring controlled film growth and a preserved radiofrequency response of the sensor. The resulting PEDOT:PSS film was extensively characterized morphologically via Optical Microscopy and Atomic Force Microscopy (AFM). Sensor's electrical properties were assessed using a Vector Network Analyzer (VNA) under controlled environment. The sensor exhibits a maximum sensitivity of 16.8 ppm/%RH and a detection limit (LOD) of 0.3 %RH. The study further explores the enhancement of humidity sensitivity through the active layer surface control, aiming to improve sorption interactions and selectivity. These findings contribute to the development of high-performance SAW humidity sensors with potential for integration into compact, low-power monitoring systems.