https://phys.org/news/2018-05-electronic-nose-variety-scents.html

The electronic nose only is a few centimeters in size. It contains the electronics needed, including the technology to evaluate the gases. The "nose" consists of a sensor chip equipped with nanowires made of tin dioxide on many individual sensors. The chip calculates specific signal patterns from the resistance changes of the individual sensors. These depend on the molecules in ambient air, differ for the different scents and, hence, are characteristic and recognizable. If a specific patterns has been taught to the chip before, the sensor can identify the scent within seconds.

To start the process, the researchers use a light-emitting diode that is integrated in the sensor housing and irradiates the nanowires with UV light. As a result, the initially very high electrical resistance of tin dioxide decreases, such that changes of resistance caused by molecules responsible for the smell and attached to the tin dioxide surface can be detected. "When the sensor perceives a smell, the resistance decreases further. If a smell disappears, the electrical resistance increases again to the initial level and the "nose" is ready for other measurements," Sommer says.