# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## Gas Sensor  
### Key Parameters of Gas Sensor  
Gas sensors are the crucial components to detect the type and concentration of gas. The components can transform gas composition, gas concentration and other information from non-electricity to electricity to achieve the measurement of gas [10].  
The key parameters of gas sensor measuring performance include the following aspects [11]:  
1. Sensitivity  
The sensitivity represents the change degree in response [12]. Sensitivity is usually expressed as the ratio of the resistance of the gas sensor in the gas under test to the resistance in normal air (or N\({}_{2}\)) and is denoted by S (S = R\({}_{\rm g}\)/R\({}_{\rm a}\), where R\({}_{\rm g}\) means the resistance of gas sensing materials at a certain concentration of target gas, while R\({}_{\rm a}\) means the resistance of gas sensing materials at ambient air atmosphere).  
2. Selectivity  
Selectivity is the ability of gas sensors to differentiate gas species in the presence of multiple gases. The sensitivity of a good sensor to target gas should be higher than that of the interfering gas Response time (t\({}_{\rm res}\))/Recovery Time (t\({}_{\rm res}\)).  
The response time and recovery time reflect the response and desorb speed of the gas sensor to the detected gas. The faster is the speed, the better is the performance. When the gas-sensitive material is in contact with the gas to be measured or the gas concentration changes, it takes a certain time for the material resistance to change and reach a steady state. We define the response time, expressed as t\({}_{\rm res}\), as the time it takes for the resistance to reach 90% change at this concentration when the sensor contacts the detected gas. Similarly, the recovery time is defined and expressed as t\({}_{\rm rec}\).  
3. Stability  
Stability is the ability of gas sensors to maintain the same output characteristics over a specified period when the measured gas concentration remains unchanged but other conditions change. If the fluctuation is fierce, the accurate information of the detected gas cannot be reflected.  
4. Repeatability  
Repeatability is the degree to which test results are consistent when gas sensors are continuously tested in the same test environment. Repeatability can affect the working life of the sensor.  
Limit of detection (LOD)  
The LOD is the minimum gas concentration that gas sensors can detect. The concentration requirements of the detection limit will be different for different application areas. LOD is usually defined on the basis of signal to noise ratio (S/N), that is, the signal measured by the low level sample is compared with the signal measured by the blank sample to calculate the minimum concentration that can be reliably detected. The detection limit is generally determined by a S/N of 3:1.  
6. Working temperature  
The working temperature is the temperature that can gives the gas sensor its highest sensitivity. The adsorption and desorption rate of gas depends on the reaction temperature, and different sensing characteristics are obtained at different operating temperatures.