## 2 Chemical Sensors Overview  
Since chemical hazards pose threats to human health, safety, and the environment, instruments for reducing risks related to chemical substance release are developed. The main objective of chemical detectors is to alarm the user to the emergence of a threat as early as possible. Their most essential properties may vary from their capability to detect agents (i.e., selectivity, sensitivity, response time, and false alarm rate) to performance (warm-up time, calibration, mobility, power supply, as well as training and maintenance costs) [18].  
Currently available sensors are based on technologies stemming from analytical chemistry. The most common include:  
1. Ion mobility spectrometry (IMS): A sensor draws an air sample into the reaction area, where it is ionized. A detector plate registers the time at which the ions cross the tube. The plot of the ion current that is generated over a given period indicates the concentration of the agent [19]. IMS was infrequent until the first IM-MS (ion mobility-mass spectrometry) instruments were launched onto the market [20];
2. Flame photometry: This kind of sensor burns an air sample, which emits light of a specific wavelength, which is extracted by an optical filter. Most known elements produce a wavelength that is sufficiently characteristic to allow for an accurate identification [21];
3. Infrared spectroscopy (IR spectroscopy): A gas is exposed to radiation in the range of 2.5-25 micrometres, which passes through it and is then absorbed in wavelengths characteristic for certain elements. IR spectroscopy is used in sensors detecting blistersand nerve agent vapours. IR-based sensors used in field conditions evaluate whether the sample contains a given chemical, rather than providing full identification. A rapidly emerging technique is atomic force microscopy-based infrared spectroscopy (AFM-IR) [22];
4. Surface acoustic wave: A detector detects slight changes in the acoustic waves that interact with the matter attached to the surface of a piezoelectric material [23; 24]. A characteristic pattern of these changes can identify particular blister and nerve agent classes [19];
5. Colorimetric-chemical reactions between various solutions and substrates, and agents, result in colour change [25; 26]. This is one of the oldest and most cost-efficient detection techniques used to detect and identify nerve, blister, and blood agents [27];
6. Raman spectroscopy: A light-scattering technique that uses radiation which is passed through a transparent medium and scattered in various directions depending on the present substances. It is very effective and used for the quick detection of narcotics, explosives, and chemicals [27; 28].  
Other detection technologies rely on photoacoustic IR spectroscopy, gas chromatography (GC), Fourier transform infrared (FTIR) spectrometry and thermoelectric conductivity. From this overview, it can be noticed that there is a vast choice of sensor technologies. However, none of them can meet every need and performance requirement voiced by the users. It is an additional challenge to researchers and developers to create a solution that would offer the possibility of using multiple sensors based on various technologies for threat detection and identification at the same time, under one system or network of sensors.