# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 5 Sensing Technologies
### Carbon Nanotubes (CNTs)  
Carbon nanotubes (CNTs) were first observed in 1991, have been investigated for over 20 years as gas sensors, and became the assured materials in terms of sensing characteristics to low gas concentrations at room temperature, lower response times and good corrosion resistance. These transducers can respond physically or chemically to different gases; to O\({}_{2}\) and CO\({}_{2}\) the response is reversible and linear, whereas to other gases, irreversible chemical reactions may happen. It also has higher costs due to the difficulty in fabrication and reproduction. The use of CNTs to sense gases can be set as individual CNTs, or arrays, where multiple units of these transducers make the measurements [28; 132].  
The fabrication process of these devices generate impurities that are inserted into the nanotubes, and must be cleaned, in order to generate precise measurements; the cleaning process is of key importance in making CNTs work properly, what impacts negatively on producing these transducers in large scale [133]. CNTs are constructed from graphene sheets, which are rolled, in order to produce these transducers. The angle and the radius in which the sheets are rolled determine whether the carbon nanotube will be metallic or semiconductor. The rolling process also determine if the CNT will be single-walled or multi-walled [25; 28; 134; 135].  
Single-walled carbon nanotubes (SWCNTs) are constructed from a single graphene sheet. The use of a single SWCNT presents worse sensing characteristics than using a single multi-walled carbon nanotube (MWCNT); although the use of a network of SWCNTs have been reported by [PERSON] et al. [23]to present better sensing characteristics and mechanical resistance, as well as faster response and recovery times [20; 23; 25; 69; 132; 133; 134; 135].  
MWCNTs are composed of two or more graphene sheets, displayed concentrically, and supported by Van der Waals forces. The chemical bonds, similar to the ones found on graphite, provide unique mechanical characteristics to the MWCNTs. The use of more than one graphene sheet provides better sensing characteristics to these transducers [20; 25].  
The sensing mechanisms of CNTs are described as the variation of conductivity or electrical resistance due to direct contact with the target gas; the variations of the electrical characteristics are proportional to the concentration of the gas [134]. As no commercial solutions were found, the developed sensors should be analyzed with known concentrations of the target gas to ensure the correct calibration and analysis of the gases. No reports on the linearity of these transducers were found.  
One of the drawbacks of using CNTs is the recovery time; when the target gas is in contact with the transducer, the gas penetrates the nanotubes, as the material absorbs the gas, the time that is necessary for the CNT recover from this contact is higher than other sensors, which can lead to inaccurate readings, causing malfunction of the systems dependent on the measurements [132]. The combination of CNTs with other sensing techniques have been reported to reduce the recovery time, as well as improve sensing and mechanical characteristics, as in the combination of CNTs and polymers to sense environmental gases [20; 131].