Metal oxide -based electrical/electrochemical sensors for health monitoring systems

There is a growing demand to develop sensors for health monitoring applications such as wearable systems for early detection of chronic diseases signs, water and food quality monitoring, and industrial production equipment monitoring. Among the many non-invasive health-monitoring sensors, ammonia, pH, and temperature sensors are particularly important as they have a critical effect on health of living organisms by changing chemical and biological reactions. This article provides a comprehensive examination of the attributes found in electrolytic solutions and the impact of electrolyte interface surface in pH sensors, along with the sensing mechanism in each sensor. The paper also includes a discussion on a variety of metal oxide materials that have been employed to develop electrical/electrochemical pH, temperature, and ammonia gas sensors using different sensing mechanisms. Finally, several challenges and future perspectives related to these sensors are discussed. The main outcome of this review is to offer a detailed understanding of the sensing mechanisms and recent developments specific to each sensor type. The outlook of this article highlights the potential for integration of these sensors into affordable, flexible, and accurate wireless health monitoring systems with a focus on wearable applications.