# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 7 Discussion and Open Issues
### Sensing Technologies  
Table 1 shows the main characteristics of the sensing technologies reviewed in this survey paper. To the best of the authors' knowledge, until the date of this publication, no authors have reported the approximately lifetime of carbon nanotubes transducers for sensing gases.  
The studied solutions for gas monitoring in connected environments mostly apply the most unique technologies to sense gases, as the electrochemical, MOS and catalytic, as these technologies are the most available on the market. These technologies also consume less energy than acoustic and optical gas sensors, which is not the best option for battery-based sensors. Polymers have a short lifetime and carbon nanotubes were not found in commercial solutions, being unviable for IoT-based solutions.  
Despite the possibility of interference from other gases and environmental factors, such as temperature and humidity, sensors based on chemical reactions can have their precision and accuracy increased with the employment of recalibration processes along with filters for other gases.  
Some authors have employed the combination of two sensing technologies like acoustic and optic methods, resulting in a better detection capacity without environmental interference and with no need for periodic calibration, reducing the complexity of the proposed solutions [148; 149]. Moreover, these technologies do not need a temperature compensation and do not require the employment of heaters and a pre-heating time, which also reduce the energy consumption of these solutions.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Technology** & **Description** & **Lifetime** & **Strengths** & **Weaknesseses** \\ \hline
**Electrochemical** & \begin{tabular}{c} Composed of a \\ membrane, an \\ electrolyte, and \\ electrodes. Reacts \\ with the target gases, \\ generating a \\ variation on the \\ output signal. \\ \end{tabular} & \begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} & \
\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} &
\begin{tabular}{c} Interference from other gases \\ and environmental factors \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} \\
**Electrochemical** & \begin{tabular}{c} Composed of a \\ membrane, an \\ electrolyte, and \\ electrodes. Reacts \\ with the target gases, \\ generating a \\ variation on the \\ output signal. \\ \end{tabular} & \begin{tabular}{c} 2-5 years \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} & \
\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} &
\begin{tabular}{c} Low sensitivity \\ and selectivity \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} \\
**Metal Oxide** & \begin{tabular}{c} Comsists of a metal \\ oxide semiconductor \\ connected through a \\ wire, involved in a \\ ceramic structure, \\ where it is heated. \\ \end{tabular} & \
\begin{tabular}{c} \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\ \end{tabular} &
\begin{tabular}{c} Low cost \\ \(\bullet\) \\ \(\bullet\) \\ \(\bullet\) \\