# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 6 IoT-Based Wireless Gas Sensors
### Gas-Sensing Solutions for IoT and Wireless Sensors and Actuators Networks (WSANs)  
During the past two decades, authors have focused on proposing and developing wireless gas sensors for diverse applications. The primary approaches on wireless gas sensors were made for monitoring large areas with no specific wireless protocol implemented. Sensors were able to transmit the sensed information through satellite networks or the Industrial, Scientific and Medical (ISM) bands through simple modulation techniques [2; 143; 144]. Later, systems to transfer sensed data through other technologies were proposed, using cellular networks for long range [34; 105; 109; 110]. Bluetooth, ZigBee and Wi-Fi have been used to short range communications [104; 107; 108]. Recently, other systems have been proposed to attend the demand of remotely sensing gases in different ambiences, such as industrial, domestic or even outdoor remote areas [109; 110; 111; 34; 105]. As new communication protocols are emerging, they are being used to cover wireless sensors networks, when the characteristics of these networks correspond to those of these protocols. Some authors have proposed sensors with more than one wireless protocol [104; 110], and different cellular networks have been used to perform the transmission, from the GPRS to the LTE [109; 110; 34; 105]. To attend to the demand for gas sensing on smart cities and remote area monitoring, sensors with different characteristics of the sensing elements, data transfer and energy consumption have been proposed, as in these scenarios, energy supply from power lines may not be available and batteries will not keep sensors functioning for long periods. A CO sensor using electrochemical transducers, employing techniques to reduce energy consumption or to grant the necessary power supply by alternative means, as solar panels and aeolic generators, was proposed by [PERSON] et al. [104] to monitor urban areas with ZigBee, although no middleware integration nor online website was reported to provide data exhibition to the users. Based on the second generation of mobile communications, different researchers have proposed gas sensors to attend the remote area-monitoring perspective. GPRS technology was used in [34] and [112]. The first presented a CO\({}_{2}\) sensor for monitoring this gas concentration in remote areas with an optical sensing element and a Global Positioning System (GPS) module; the collected data is stored on a database, as well as on a SD-card and exhibit on a web page. In the second, authors have proposed a VOC, NO\({}_{2}\) and O\({}_{3}\) sensor with MOS transducers to collect information on urban air pollution, where data is stored on an online database. Based on the Global System for Mobile Communication (GSM), [PERSON] et al. [108] proposed and developed a CO and NO\({}_{2}\) sensor with electrochemical elements to monitor air quality on urban areas, collecting data mainly during the 2015 Hong Kong marathon. [PERSON] et al. [103] proposed a natural gas wireless sensor with a MOS transducer, with deployments in different networks, allowing easy adaptation to divergent scenarios, through the GPRS, 3G, 4G, LoRaWAN and Bluetooth networks; although this work was focused on environmental monitoring and automation, the sensor was not integrated on a middleware with a dashboard, to facilitate the visualization of the collected data. Research focused on gas sensors for indoor monitoring has started with the development of new devices with the capacity of sensing at least one target gas with good precision and alert the users by enabling an alarm, which become a commercial product, installed in many houses, apartments and offices. Wireless gas sensors focused on indoor monitoring were proposed in the literature by some authors, with different characteristics. [PERSON] et al. [145] presented a ZigBee-based optical gas sensor to monitor VOCs in enclosed environments, displaying the collected information in real-time in an online dashboard.