Proton-Conductive Gas Sensor: a New Way to Realize Highly Selective Ammonia Detection for Analysis of Exhaled Human Breath

The analysis of exhaled human breath has great significance for early noninvasive diagnosis. Poor selectivity and strong humidity are two bottlenecks for the application of gas sensors to exhaled breath analysis. In this work, we utilized the adsorption, dissolution, ionization, and migration processes of ammonia in wet nonconjugated hydrophilic polymers to realize effective ammonia detection. The indispensable high-humidity atmosphere of exhaled breath was turned into a favorable condition for ammonia sensing. Nonconjugated polymer sensors can distinguish ammonia from most other gases because of its extremely high solubility and good ionization ability. A sensor based on poly(vinyl pyrrolidone) (PVP) could detect 0.5 ppm ammonia with an extremely high selectivity. The ammonia-sensing mechanism was thoroughly investigated by complex impedance plots (CIPs) and a quartz crystal microbalance (QCM) measurement. Finally, the potential of the PVP sensor for ammonia detection in exhaled breath was evaluated in simulated environments.