Nitrogen oxides (NOx) are hazardous air pollutants that require real-time monitoring and immediate feedback [1]. Conventional chemiresistive metal-oxide gas sensors mainly provide electrical signals, which are not directly perceivable by users. Here, we propose an integrated gas-sensing platform that couples a chemiresistive sensor with a viologen-based electrochromic (EC) device to enable naked-eye NO_x detection through visible color changes.
Cu-doped In₂O₃ was employed as the sensing material because of the high reactivity of In₂O₃ toward NOx at room temperature, while Cu doping was introduced to improve response and recovery speeds [2]. The sensing films were fabricated using a nanoparticle deposition system (NPDS), a dry deposition technique that enables film formation under low-vacuum and ambient-temperature conditions without solvents or toxic chemical precursors.
The Cu-doped In₂O₃ sensor was electrically integrated with the viologen-based EC device, where changes in sensor resistance were converted into variations in the voltage applied to the EC device. This electrical response was subsequently visualized as an electrochromic color change. The proposed system demonstrates a simple proof-of-concept approach for converting invisible gas-sensing signals into intuitive visual information for room-temperature NOx monitoring.
 

Electrochromic,Chemiresistive gas sensor,Doping,Visualization system,Room-temperature gas sensor