# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 4 Evolution of Gas-Sensing Technologies  
The attempt of sensing gases had become necessary when several pitmen lost their lives during underground mine explorations due to the lack of O\({}_{2}\) or to the leakage and accumulation of other colorless, odorless gases, mainly CO\({}_{2}\), CO and CH\({}_{4}\). During the 19 th century explorations, miners used to have canaries with them while working in the mines to signalize the presence of unwanted gases: while the birds were in their pits, there was enough oxygen for them to breath; in contrast, if the canaries had succumbed or passed out, the quality of the air was not good enough for the people inside to breath, and they should leave the mine immediately [15; 85; 92]. Much has evolved in the last century, and gas sensing has become a key feature in numerous activities, such as medicine, sports, the industrial field, environmental monitoring and pollution control [28; 31; 32; 34; 35; 93]. Oxygen sensors were the first to be developed: in 1956, [PERSON] developed the first electrochemical oxygen sensor, known as the Clark Cell [31; 94; 95]; in 1961, [PERSON] and [PERSON] developed the Lambda probe to perform oxygen measurements in vehicle engines, helping with the admission control and fuel mixture to achieve the best performance, in terms of fuel consumption or in terms of power [31; 96], and it has been produced by Bosch since 1976. Both developed sensors are consumable, reacting with oxygen, in order to provide an output value representing the gas concentration in the environment [97; 98]. After the development of these expensive, and not so accurate sensors, the research for new technologies capable of granting more accurate measurements and more durable devices to this field have been taking advantage on many characteristics of the sensed gases; as an example, oxygen has magnetic characteristics, consequently, it can be measured by the attraction to a magnetic field. Other gases can be measured taking advantage on their ultrasonic properties or by optical spectrometry [28; 31; 96]. Gas sensors rely on a physical or chemical reaction with the gas that is trying to be sensed in order to generate a response proportional to the concentration of the gas, thus the speed of the reaction. Some of the sensors have a reversible reaction, while other have an irreversible one, the latter being expendable with 2 to 5 years of lifetime. The response generated by the presence of the target gas is either linear or non-linear, depending on the materials and target gases [28; 97; 98]. It is fundamental to grant the quality of the air patients are breathing at a hospital, as well as to provide the correct mixture to divers, in particular to deep diving, where the gas mixture the diver must breathe is different according to the time and depth of the activity. Not only to grant the quality of air, but also to avoid gas leakage and its consequences such as poisoning or explosions, sensing gases is of great importance at the industry [17; 32; 36; 37]. Figure 1: Illustration of Internet of Things (IoT) verticals and market opportunities for multi-gas smart sensors: smart homes, agriculture, smart cities, industry, healthcare, and smart grid. Over time, other sensing methods were developed, exploring the propagation characteristics of the gases to perform measurements, and comparing with a reference to determine which gas is being sensed. The analyzed characteristics vary from signal attenuation, frequency shifting, propagation time, among others, and are performed by acoustic or optical sensors [94; 97; 28; 98; 99; 100; 101; 102]. Moreover, the miniaturization of gas transducers has taken place in the research topics [20; 21; 22; 22], as well as the development of wireless gas sensors, to monitor remote areas and easily collect and analyze data from the environment [34; 35; 95; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114].