Citation: Chesler, P.; Hornoiu, C. MOX-Based Resistive Gas Sensors with Different Types of Sensitive Materials (Powders, Pellets, Films), Used in Environmental Chemistry. Chemosensors 2023, 11, 95. https://doi.org/10.3390/ chemosensors11020095 Academic Editors: Maria Josè Lo Faro and Salvo Mirabella Received: 25 December 2022 Revised: 20 January 2023 Accepted: 26 January 2023 Published: 29 January 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). chemosensors Review MOX-Based Resistive Gas Sensors with Different Types of Sensitive Materials (Powders, Pellets, Films), Used in Environmental Chemistry Paul Chesler * and Cristian Hornoiu * “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, 202 Splaiul Independentei, 060021 Bucharest, Romania * Correspondence: pchesler@icf.ro (P.C.); chornoiu@icf.ro (C.H.) Abstract: The identification of an unknown gaseous species or the composition of a gaseous mixture can be performed using various experimental techniques such as: mass spectrometry, chromatog- raphy, nuclear magnetic resonance (NMR), infrared (IR), X-Rays, or by combining these analytical techniques (in automated analyzers). Unfortunately, these techniques use highly expensive equip- ment and require the use of qualified personnel. Using gas sensors is a viable and inexpensive alternative. The most commonly used sensors in the field are resistive type chemosensors (chemiresis- tors), due to their simple detection mechanism and low manufacturing costs. The detection principle of these sensors is based on the catalytic reaction between the sensitive material of the sensor and the target gas. This reaction occurs with the release or consumption of electrons, influencing the overall electrical resistance of the sensor. This review describes various MOX-based chemiresistors, which contain different types of sensitive substrates, such as powders, pellets or films, as well as a clear tendency towards sensor miniaturization and the constant improvement of the fabrication techniques towards greener and more cost-effective synthesis routes over time. The goal of this research was to obtain sensors with high 3S parameters (sensitivity, selectivity, and stability), that can be mass-produced and implemented on a wide scale. Keywords: resistive gas sensors; chemiresistors; sensitivity; selectivity; stability; eco-friendly; low-cost; miniaturized; micro-sensors; mass-produced 1. Introduction Breathing air contains three primary gaseous components: nitrogen (N 2 ), oxygen (O 2 ), and argon (Ar). The proportions in which these components are found are N 2 = 78.08%, O 2 = 20.95%, and Ar = 0.93%. These components are vital for life on this planet. In addition to these molecules, the air also contains other more or less harmful species in concentrations below 0.05% such as carbon dioxide (CO 2 ), neon (Ne), helium (He), methane (CH 4 ), and krypton (Kr). Another important component of the planet’s atmosphere is water (H 2 O), present in vapor form, in a concentration of 0–3%. Many substances of natural origin may be present in small amounts, locally and/or according to season variables in the form of aerosols (in a sample of unfiltered air), including mineral and organic dust, pollen, spores, or ash particles (in areas with high volcanic activity). Various pollutants coming from industry may be encountered in the atmosphere as aerosols or vapors, such as chlorine, fluorine compounds, mercury, or sulfur compounds such as hydrogen sulfide (H 2 S) and sulfur dioxide (SO 2 )[1]. Harmful species present in the air can have short, medium, or long effects on human health, and can even cause death in some cases. This is exemplified by the effects produced by various concentrations of an extremely toxic gas that may be present in the air: carbon monoxide (CO). CO is nicknamed “The Silent Killer” because of his ability to kill quickly and quietly, the potential victims not even being aware of the danger they are in. Its detection by humans without specialized equipment is Chemosensors 2023, 11, 95. https://doi.org/10.3390/chemosensors11020095 https://www.mdpi.com/journal/chemosensors