- Metal oxide semiconductor (MOSFET), where the molecules entering sensor areas will be charged positively or negatively, will directly affect the electric field inside MOSFET.
- Metal oxide sensors absorb gas molecules to provoke a change in conductivity, and the change is measured for the number of volatile compounds and adsorbed.
- Piezoelectric sensors adsorb gas molecules on the surface of the polymer that leads to the change in the mass on the sensor surface, which in turn produces a change in the resonant frequency of the crystal.
Commonly Used Sensors Designed for Electronic Noses
Disruptive technological advancement waves in the areas of cloud, artificial intelligence (AI), and the Internet of things (IoT) created a positive impact and the demand for the technology used for electronic noses. These e-noses provide external benefits to various commercial industries, agriculture, biomedical, cosmetics, environment, food & beverage, and various research & scientific fields. Electronic noses are instruments that attempt to mimic the human olfactory system with an array of chemical sensors designed to detect either gases or vapors. Due to continuing improvements in gas sensor technologies and falling manufacturing costs, the e-nose market is expected to reach over $17.9M by 2025.  Working Principle The instrument essentially consists of sensor arrays, pattern recognition modules, and a headspace sampling unit to generate signal patterns used to characterize smells. Therefore, it can be classified into three major parts: detecting system, computing system, and sample delivery system. The air sample is pulled by a vacuum pump and led through a tube into a smaller chamber consisting of an electronic sensor array. Then the sample delivery system first enables the generation of the headspace of samples or volatile compounds that are fractionally analyzed there and sends the samples to the detection system. The detection system consists of a group of sensors that contact the volatile compound samples and reacts with the sensors causing electrical changes. These electrical changes are digitally transmitted into the computing system for data analysis. Commonly Used Sensors The digital output generated by electronic nose sensors is analyzed and interpreted, and the values are shown in the display based on the results. The most important component in the construction of e-noses is the sensor, and the commonly used sensors include: