# IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities
## 5 Sensing Technologies
### Electrochemical  
Electrochemical gas sensors were the first gas sensors to be created, starting with the Clark Cell in 1956, to sense levels of oxygen. In 1976, the Lambda probe was developed to sense oxygen in combustion engines, constructed with a solid electrolyte Yttria-Stabilized Zirconia, requiring high temperatures to function properly; the working temperature is variable, and depends on the chosen electrolyte, which can be liquid or solid, the latter allowing the miniaturization of the sensors [113; 114].  
These sensors benefit from chemical reactions to sense a target gas where the product of the reaction and its speed is proportional to the gas target concentration. The materials used to compose these sensors have different target gases, depending on the temperature of the reaction. These sensors are basically composed of a membrane that separates the ambient gases from the electrolyte solution, the electrolyte, which is mainly composed by a liquid compound of acids or bases, and the electrodes that are consumed by the electrochemical reaction. The polarization of the electrodes and measurement of the variation of the output parameter is dependent on the sensing approaches, which could be potentiometric, voltametric, conductimetric, or amperometric. A filter, between the membrane and the environment gases, can be used to limit contact with unwanted gases, improving accuracy and sensitivity on these devices [90; 92; 115; 116; 117; 31]. The representation of an amperometric electrochemical gas sensor is shown in Figure 2.  
Electrochemical transducers are easily miniaturized, being able to reach the order of a few millimeters, which is beneficial to embed the sensors in printed circuit boards (PCBs). All sub types of electrochemical sensors rely on the reduction of the cathode to hydroxyl ions; when the anode material is completely oxidized, the sensor has reached the end of its lifetime and must be changed [31, 32]. Electroanalytical measurements can be executed with four different sensing approaches, in where each sensing technique will better fit a different necessity of sensing gases: Potentiometric, Voltametric, Conductimetric and Amperometric [90, 92, 117].  
**Potentiometric Sensors:** they have as an output the electrical tension proportional to the equilibrium potential of an indicator electrode, where the absence of the target gas results on the output of 0 Volts (V). This measurement method results in high selective transducers, which are able to measure only low concentrations of the target gases [90, 92, 117].  
**Volumetric Sensors:** they have output current as a function of the concentration of the target gas and the applied potential, generating measurements of low gas concentration; this allows the measurement of more than one gas, with the variation of the input electrical potential. The applied potential, as well as the choice of the electrodes, can result in a highly selective transducer [90, 92, 117].  
**Conductimetric Sensors:** they are mostly used by detecting the presence of the target gas after a determined threshold; this relies on the measurement of the resistance of the solution, which means that these sensors are not selective and more than one gas can change its resistance [90, 92, 117].  
**Amperometric Sensors:** these have the variation of the electrical current as a sensing parameter, where the reduction of the cathode to hydroxyl ions generate a current, proportional to the concentration of the target gas. The cathode and anode must be linked with a resistor, in where the variation of the current on the load resistor can be measured as a tension value [32, 93, 118, 119].