A Facile Way to Simultaneously Improve Humidity-Immunity and Gas Response in Semiconductor Metal Oxide Sensors

The metal-oxide-based gas sensors show great potential in exhaled breath analysis owing to their simple, fast, and noninvasive characteristics. However, the exhaled breath contains moisture, and the surface-active sites of metal oxides are easily poisoned by water molecules, leading to degradation of the sensor performance, particularly the gas response and selectivity. Therefore, it is essential to develop oxide sensors that can reliably sense target gases over a wide humidity range without sacrificing the gas response. In this study, a facile strategy was proposed to incorporate hydrophobic La into an oxide sensor to simultaneously improve the humidity-stability and sensitivity of NH₃ detection for early prediction of kidney failure. WO₃ sensors doped with various concentrations of La were successfully synthesized, and their gas-sensing performances under various humid conditions were systematically investigated. Interestingly, a small amount of La doping (1 at. %) effectively prevented water poisoning and improved the gas response simultaneously. This sensor was able to selectively detect NH₃ up to 200 ppb with a limit of detection (LOD) of ∼780 ppt over a wide range of humidity. The concurrent enhancement in gas response and humidity-immunity was attributed to the surface hydrophobicity and increased specific surface area caused by the incorporation of La.