Human breath analysis; Clinical application and measurement: An overview

Human breath has been recognized as a complex yet predictive mixture of volatile organic compounds (VOCs) and inorganic gas species that can be utilized to non-invasively diagnose common diseases. Current laboratory techniques such as gas chromatography/mass spectrometry (GC/MS) and high-performance liquid chromatography (HPLC), are capable of VOC detection down to ppm concentrations. However, these methods are expensive, non-portable, require pre-processing of the exhaled VOCs, and expert operators, making them unsuitable for wide-spread use. Portable gas sensors have various advantages over other methods used in gas analysis, including ease of transportation, reduced treatment costs, fast results, and improved patient experience. Recent advancements in gas sensing technologies have enabled such devices to be used to diagnose, predict, and monitor a wide range of diseases and conditions, however, many challenges need still need to be addressed (i.e., sensitivity and selectivity) before they can be employed for such applications. Although nanotechnology has greatly improved the performance of gas sensor materials and their capacity to detect VOCs in human breath, issues around repeatability and accuracy remain, as well as adequateness due to the close proximity of the human body and the sensor device. This review focuses on how recent advancements in nanotechnology and solid-state materials have enabled VOC gas sensors to evolve into miniaturized, sensitive and selective devices for monitoring human breath in clinical applications. An introduction to the key aspects of breath analysis, including sources of VOCs in human breath and their role in disease diagnosis, is discussed. Furthermore, the current limitations and future prospects of such gas sensors for breath monitoring applications are also discussed in detail.