There are various types of sensors, but gas sensors

There are various types of sensors, but gas sensors can be broadly divided into three main types: electrical, optical, and mass-sensitive. [61]Gas sensors are usually classified as chemical sensors or detectors. [77]The purpose of the chemical assay is to detect subtly changes in a chemical medium by converting the chemical or physical properties of molecular or ionic species (e. g. analytes) into an analytically useful output. Prototype of a chemical assay system The sense of smell is animal, even for humans who generally have more vision than smell, the sense of smell is one of our most basic abilities, and we are able to identify and discriminate more than ten thousand individual scents and maybe even billions (10⁴ −10¹²) To be [78]. The chemical specificity of the olfactory system is not due to the specificity of the receptors for specific analytes (such as conventional lock-and-key models), on the contrary the olfactory system uses hundreds of receptors by identifying a combination response pattern [79]. Despite the complex and conjectural structures of membrane olfactory receptors, it is believed that a large proportion of them are metalloproteins that react highly to aromatic molecules through analyte coordination to the metal-binding site [80]. The sensor array pattern, based on the interaction of individual sensors, makes molecular detection is possible by comparing it to a predetermined response library [79]. Molecular or chemical measurement, exploration, and imaging with markers, chromophores, luminescent, or reporters, i. e. chemical sensors are one of the most essential topics in current chemistry in terms of analytical and supramolecular chemistry due to the fact a wide range of target analytes must be analyzed[81, 82], which includes chemicals, hazardous substances [83, 84], biological macromolecules [85], manipulations [86] and apoptosis imaging for cancer treatments [87, 88].