Chemical sensors are crucial in detecting harmful substances in the environment due to their ability to remain stable, their sensitivity, fast response time, and long lifespan. One important aspect of chemical sensors is the electrode material, which is typically a metal oxide semiconductor that is simple to produce and has excellent electrical properties in environmental conditions. ZnO, II–VI (n type) semiconductor with a wide bandgap (3.37 eV), high excitation binding energy and electron mobility as 60 meV and 400 cm2/V s respectively. It also has other desirable properties, including less cost of synthesis, chemical stability, biocompatibility, and environmental friendliness and is recently an excellent research area for detecting harmful and toxic gases. Additionally, transition metal oxides (TMOs) ranging from II to IV possess a wide range of physical properties, such as high-temperature superconductivity, piezoelectricity, ferroelectricity, magneticity, multi-stiffness, and resistivity. Graphene, a unique 2D material exhibits exceptional properties, such as high carrier mobility, electrical conductivity, thermal conductivity and optical transmittance along with large theoretical surface area. The benefits of using graphene for sensors are that its surface area and atomic thickness allow for direct contact between analytes and entire carbon atoms. This section discusses the use of semiconductor-based electrodes, including ZnO, binary metal oxides, and graphene oxide, for detecting toxic chemicals.