Gas sensing has a great influence in many areas across a wide range of applications, including ensure safety via detection of dangerous or flammable gases in environments, ranging from domestic atmosphere to process and petrochemical industries, monitor feedstocks and air pollutants, measuring quality of gases mixture or quantity of key species. Metal oxides is an emerging class of sensing materials that cover the entire range from metals to semiconductors and isolators which sensitivity proprieties in the field of chemical sensing are known for more than five decades. Considering electrochemical devices, initial drawbacks like low selectivity, signal drift, low response and sensitivity to humidity have push researchers to improve strategic characteristics of sensors deeply studying the sensing mechanisms and optimizing crystal structure, morphology, and composition, but operational limits like inability to operate in harsh environments, for instance, high temperature, high pressure, or corrosive ambient is still a challenge. To overcome these operational limits, using materials able to change optical proprieties (i.e. absorbance, transmittance, reflectance, refractive index, etc.), called optical gas sensors, instead of electronical proprieties; will offer higher thermal stability, good poison resistance, no electrical or magnetic fields dependence, in addition to fast response, high flexibility and low-cost devices.