Metal-oxide gas sensors for exhaled-breath analysis: a review

The analysis of exhaled breath is important for health vs. disease diagnostics. Diagnosis and monitoring methods based on metal-oxide gas sensors are highly attractive, owing to their portability and ease of operation. These methods are non-invasive and affordable, and allow early disease diagnosis. Metal-oxide gas sensors are heat-resistant and can easily be combined with various nanostructures. We review concurrent research into metal-oxide gas sensors used to detect various biomarkers (e.g., acetone, nitric oxide, hydrogen sulfide, and ammonia) in the exhaled breath of humans. We also review efforts to improve the properties of metal-oxide gas sensors, especially structures, morphologies, the control of impurities in semiconductor materials, and changes in the specific surface area of metal-oxide materials that have been used in gas sensors to analyze exhaled breath. Various approaches to the humidity resistance characteristics and nonlinear responses of metal-oxide semiconductor gas sensors are addressed and should be applicable in real life.