In this work, we will focus on the working principle of chemoresistive gas sensors based on nano-crystalline metal-oxides. We addressed the problem of band bending and Schottky barrier formation in metal-oxide sensors and considered the possibility of oxygen in- and out-diffusion in a thermionic-emission conduction model. We specify the model to nanostructured particles to show peculiar behavior of the latter with respect to non-nanostructured materials. Finally, the application of these materials in the field of colorectal cancer prevention has been presented. Since 2013, our team has been active in the study of colorectal cancer biomarkers leading to the establishment of a start-up for the technological transfer of these devices. Here the project, still ongoing, aims to find the difference between the odour of faeces of colorectal cancer patients and of healthy subjects.