# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
###### Abstract  
Ambient gas detection and measurement had become essential in diverse fields and applications, from preventing accidents, avoiding equipment malfunction, to air pollution warnings and granting the correct gas mixture to patients in hospitals. Gas leakage can reach large proportions, affecting entire neighborhoods or even cities, causing enormous environmental impacts. This paper elaborates on a deep review of the state of the art on gas-sensing technologies, analyzing the opportunities and main characteristics of the transducers, as well as towards their integration through the Internet of Things (IoT) paradigm. This should ease the information collecting and sharing processes, granting better experiences to users, and avoiding major losses and expenses. The most promising wireless-based solutions for ambient gas monitoring are analyzed and discussed, open research topics are identified, and lessons learned are shared to conclude the study. gas sensors; smart sensors; environmental monitoring; Internet of Things +
Footnote †: journal: Sensors  
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and difficult access, for instance. Moreover, wireless actuators were also placed into networks known as Wireless Sensors and Actuators Networks (WSANs), working in a collaborative way to ensure automatic and intelligent decision making on certain events, reacting with environmental changes to provide the best user experience, without needing users' interference [3; 4; 5; 6; 7]. Currently, control systems have reached complex levels where buildings are being automated to make the best decision for users, self-driving cars and autonomous planes are being experimented on, relying on autonomous decisions based on data provided by numerous sensors placed on vehicles. Hereafter, these data will be provided not only by the embedded sensors on devices being controlled, but also by data collected on other devices and even in other networks. These significant data will be shared among the devices in the same context by the Internet, through the Internet of Things (IoT) paradigm, allowing even more crucial precise decisions [8; 9; 10; 11; 12; 13; 14]. Mechanisms of sensing gases have been studied since the 19 th century when the first method to notice the presence of unwanted gases at underground mines was performed by using canaries and observing their states. The presence of toxic gases is deadly for the birds and the workers would have enough time to leave the place without harm [15]. More recently, the detection of other gases has been studied with the purpose of avoiding incidents and accidents, as fires and explosions involving flammable gas leakages as well as providing better results from industrial processes involving chemical reactions [16; 17; 18; 19]. Much research focuses on the sensing materials, always trying to improve gas measurements in terms of accuracy, precision, and response time. The sensors' miniaturization is also a recurrent topic of research although these topics of study are mostly linked with the electrochemical transducers while techniques that make use of propagation characteristics have repeatability as a main theme of refinement [20; 21; 22; 23; 24; 25; 26; 27; 28]. Several survey papers were published targeting particular characteristics of gas-sensing technologies. [PERSON] et al. [20], [PERSON] et al. [21], and [PERSON] et al. [22] researched metal oxide nanostructures for sensing gases. [PERSON] et al. [20] studied hybrid gas transducers using graphene to sense toxic gases while [PERSON] et al. [21] approached these nanostructures focused on sensing volatile organic compounds (VOC), and [PERSON] et al. [22] studied these nanostructures in general. In [23], the authors covered the detection of gases by carbon nanotubes (CNTs) transducers. Optical fiber transducers to detect oxygen (O\({}_{2}\)) and carbon dioxide (CO\({}_{2}\)) are covered in [24]. [PERSON] [25] has studied nanomaterials as gas sensors.