Interfacial regulation of Nafion-decorated TiO₂ composite films for high-response impedance humidity sensor

Achieving high sensitivity, low hysteresis, and long-term stability in humidity sensors remains challenging for intelligent sensing systems. Herein, we report a high-response impedance humidity sensor based on Nafion-decorated TiO₂ composite films prepared through a simple slurry-casting, thermal-annealing, and post-surface-functionalization process. In this design, TiO₂ provides a stable inorganic framework and accessible interfacial adsorption sites, while the Nafion decoration layer introduces sulfonic-acid-based hydrophilic domains and proton-conductive pathways. By regulating the Nafion content, the interfacial water adsorption and charge-transport behavior of TiO₂ films can be effectively optimized. The 1 wt% Nafion/TiO₂ sensor delivers an outstanding response value of 12496.93 at 90% relative humidity, accompanied by a small humidity hysteresis of 2.17%, favorable repeatability, and long-term stability over a 30-day continuous test. Impedance analysis reveals that the enhanced response originates from reduced interfacial resistance, promoted water-assisted proton transport, and diffusion-related ionic conduction in the hydrated Nafion/TiO₂ interfacial layer. Furthermore, the sensor is integrated into a compact wireless platform capable of real-time respiratory signal acquisition and abnormal breathing detection. This work presents a surface-engineering strategy enabling high-performance humidity sensing for practical wearable health-monitoring applications.