E-noses: an introduction

In the last few years, intensive research has led to the widespread investigation of nanomaterials oriented toward the fabrication of solid-state gas sensors based mainly on metal oxide nanomaterials. SnO2 and WO3 have been widely explored in gas-sensor applications owing to their extraordinary physical and chemical properties. The development of these gas sensors has provided an efficient means of detecting volatile organic compounds (VOCs) in exhaled breath who remains critical for the early diagnosis of diseases. VOCs emanating from different bodily fluids have long been investigated as noninvasive indicators of the status of many health parameters. This concept gave rise to the biotechnology discipline where the determination of VOCs using multisensory systems is related to the presence of certain diseases. Herein, we developed an electronic nose (e-nose) which consists of six MQ sensors based on SnO2 and six interdigitated sensors based on metal functionalized WO3 nanowires. For emerging applications, this chapter aims to investigate whether the possibility of using an e-nose system will be able to monitor the exhaled VOCs as a noninvasive method to discriminate between patients with chronic kidney diseases, diabetes mellitus, and lung cancer. Advanced multivariate statistical approaches, such as principal components analysis, hierarchical cluster analysis, or support vector machines, were used for data processing. The application of these chemometrics on the database gathered from e-nose provided significant findings in terms of classification and identification of subjects depending on their health states. We can conclude, through these studies that the e-sensing performance of e-nose based on WO3 can be enhanced by UV illumination and the selectivity of these sensors can be changed using different noble metals or by changing the operating temperature.