### Research Background on Gas Sensors  
The first demonstration that a semiconductor (germanium, Ge) modifies its resistance depending on the surrounding atmosphere was made by [PERSON] and [PERSON] in 1953 [PERSON] and [PERSON] (1953). Their experimental schematics and monitoring example with oxygen are represented in Figure 2a-i,a-ii. In 1954, [PERSON] (1954) described that metal oxides such as zinc oxide (ZnO) modify their semiconducting properties with a change in the partial pressure of oxygen in the surrounding atmosphere. The first gas sensing system was reported in 1964 by [PERSON] (1965), which electronically mimicked the olfactory process (e-nose) using oxidation-reduction reactions of several odorants on distinct microelectrodes Figure 2b. In 1971, [PERSON] patented the first gas sensing device, using tin dioxide (SnO\({}_{2}\)) as the sensitive material ([PERSON] et al., 2018). \begin{table}
\begin{tabular}{l l} \hline \hline \multicolumn{1}{c}{**Olfactory System**} & \multicolumn{1}{c}{**Electronic Nose System**} \\ \hline i. Nostril & i. Sampler \\ ii. Primary neurons & ii. Sensor array \\ iii. Secondary neurons & iii. Signal conditioning and processing \\ iv. Brain & iv. Pattern recognition \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Odour molecules’ ordered pathway comparison in the olfactory and electronic nose systems. Figure 1: Simplified schematics of the human olfactory system (adapted from [PERSON] and [PERSON] (2009), copyright 2009 Elsevier). In the 1980s, the field of semiconductor sensors underwent a significant expansion [7] that led to the appearance of dedicated international peer reviewed journals such as _Sensor Review_, in 1981 (Emerald Group Publishing Ltd.) and _Sensors and Actuators_, in 1989 (Elsevier). However, the study of portable gas sensing systems only made its rapid progress since the publication in 1994 of a review on the e-nose, by [PERSON] and [PERSON] [8]. Molecules interact with thin film sensors by adsorption (physisorption or chemisorption) or chemical reaction. These processes lead to a physical change in the film (sensor) that can be detected by variations in conductivity [8; 9], mass [10], work function [11], or an optical property [12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. Most of the researched and established sensors use the conductivity change (Figure 3) of a given material as a signal, but they have low specificity, are susceptible to electrical noise, and the wiring needed complicates their implementation in some harsh environments. With the development of new technologies for the production of high quality optoelectronic components, optical sensors have undergone a large development over the past decades [22]. Optical sensors show excellent characteristics of sensitivity, selectivity, long lifetime, and fast response [6]. Therefore, they have attracted a great deal of attention for analytical applications, such as environment, industrial, and health monitoring. Furthermore, they have a straightforward working principle and simple hardware configuration, consisting of the sensing platform, a light source, optical fibres, a light detector, a processing unit, and pattern recognition software [22; 23], following the same scheme of Figure 1b. The sensing mechanism is based on detecting a change in the light beam (frequency or wavelength, polarization, phase, or intensity) when the gas molecules interact with the sensing platform. Figure 2: (**a-i**) Schematics of the experimental circuit used by [PERSON] and [PERSON] in 1953 that used germanium and a platinum reference electrode to demonstrate a resistance-dependence on the surrounding atmosphere, and (**a-ii**) contact potential measured in a monitoring cycle when germanium is exposed to wet O\({}_{2}\) and dry O\({}_{2}\) (reused with permission of Nokia Corporation and AT&T Archives, from [PERSON] and [PERSON] [3]).