# Advancements and Prospects of Electronic Nose in Various Applications: A Comprehensive Review
## 2 E-Nose System
### Gas Sensing Mechanism  
Gas sensors based on MOX operate on the fundamental principle of resistance changes in semiconductor sensor elements when exposed to oxidative or reductive gases at elevated temperatures [55; 56]. These sensors exhibit remarkable energy conversion capabilities, effectively transforming various input signals, whether physical, chemical, or biological in nature, into electrical output signals. For resistive gas sensors, two crucial aspects play a vital role: the recognition of target gas molecules and the subsequent transduction of their signals. At temperatures ranging from 100 to 500 \({}^{\circ}\)C, MOX exhibits a fascinating interaction with atmospheric oxygen, known as ionosorption, which involves the adsorption of oxygen ions in various forms, including molecular O\({}^{2-}\), O\({}^{-}\), and O\({}^{2-}\)[57]. This intriguing adsorption of oxygen ions on the surface of MOX forms the basis of its gas sensing mechanism [57]. The excitation of MOX electrons occurs when the material is exposed to thermal or light energy, causing them to transition from the top of the valence band to the bottom of the conduction band [58]. This process of electron excitation plays a pivotal role in facilitating the subsequent ionosorption of oxygen on the MOX surface. Consequently, various forms of oxygen ions, such as O\({}^{-}\), O\({}^{2-}\), and O\({}^{2-}\), are formed on the MOX surface due to the extraction of electrons from its conduction band when exposed to atmospheric air during the sensing process, as shown in Figure 2a,b [59,60]. The specific types of oxygen ions formed depend on the working temperature of the sensor. At relatively lower working temperatures, typically below 200 \({}^{\circ}\)C, the dominant process involves electrons attaching to oxygen molecules (O\({}_{2}^{-}\)) on the MOX surface, leading to the formation of O\({}^{2-}\) ions as indicated by Equation (1) [61]. However, as the working temperature increases beyond 250 \({}^{\circ}\)C, a more intricate dissociation process takes place. The higher temperature facilitates the breaking of oxygen molecules, resulting in the formation of oxygen ions, including O\({}^{-}\) and O\({}^{2-}\), along with the extracted electrons from the MOX conduction band [61]. The temperature-dependent formation of oxygen ions is described by the following equations [61]:  
\[O_{2(gas)}\leftrightarrow\ O_{2(ads)} \tag{1}\]  
\[O_{2(gas)}+e^{-}_{(front\ MO_{x})}\leftrightarrow\ O^{-}_{2(ads)}\ \ \ \ \ (<200\ ^{\circ}\text{C}) \tag{2}\]  
\[\frac{1}{2}O_{2(gas)}+e^{-}_{(front\ MO_{x})}\leftrightarrow\ O^{-}_{(ads)} \ \ \ \ (<250\ ^{\circ}\text{C}) \tag{3}\]  
\[\frac{1}{2}O_{2(gas)}+2e^{-}_{(front\ MO_{x})}\leftrightarrow\ O^{2-}_{(ads)} \ \ \ \ (>250\ ^{\circ}\text{C}) \tag{4}\]  
Figure 2: Generation of electronic core–shell configurations in (**a,b**) [59]. These equations represent the dynamic processes of oxygen ions' adsorption and formation on the surface of MOX at different temperatures during the gas sensing operation. At temperatures below 200 \({}^{\circ}\)C (Equation (2)), electrons combine with oxygen molecules to form O\({}_{2}\)\({}^{-}\) ions. As the temperature increases beyond 250 \({}^{\circ}\)C, more complex dissociation processes take place. At temperatures below 250 \({}^{\circ}\)C (Equation (3)), 1/2 O\({}_{2}\) molecules combine with electrons to form O\({}^{-}\) ions. However, at temperatures above 250 \({}^{\circ}\)C (Equation (4)), 1/2 O\({}_{2}\) molecules combine with two electrons to form O\({}^{2-}\) ions. These temperature-dependent processes significantly influence the sensitivity and response of the MOX gas sensor to various gases. Metal oxide semiconductors, which play a crucial role in gas sensing applications, are divided into two distinct groups based on their majority carriers.