# Research Progress of Gas Sensor Based on Graphene and Its Derivatives: A Review
## 4 Graphene Gas Sensor
### Gas Sensors Based on Pristine Graphene
The sensor worked without electricity; was soft, wearable, and small; and performed well, thus having broad application prospects. In 2017, [PERSON] from Delft University of Technology prepared the graphene sensing layer by chemical vapor deposition on pre-patterned catalyst and then it was eased onto the underlying SiO\({}_{2}\) through a completely transfer-free process [63]. The gas sensing materials had different line width: 5 and 10 \(\upmu\)m. The latter one, having a sensing area reduced by half with respect to the former one, showed a higher sensitivity upon exposure towards both gases, indicating that the sensitivity can be modulated by varying the geometry of the device exposure area. [PERSON] et al. from Jiangsu University proposed a vertical responsive gas sensor based on three-dimensional porous graphene  
Figure 5: (**a**) Comparison of the response of monolayer, double and triple layer graphene to NH\({}_{3}\) with a flow rate of 200 mL/min; and (**b**) The change in response due to L/w ratio and surface area of graphene with NH\({}_{3}\) flowing at 200 mL/min [56]. ultrathin film, as shown in Figure 7[64]. Different from the flat transportation response, the current in the structure flowed in a direction perpendicular to the graphene film, which avoided the impediment to carriers due to the graphene's adhesion to gas molecules in the plane. The response of the gas sensor to 50 ppm dimethyl methylphosphonate (DMMP) molecule can be over 10%. In 2017, [PERSON] et al. from Key Laboratory of Optoelectronic Technology & Systems, Chongqing University proposed a graphene-based long-period fiber grating surface plasmon resonance (LPFG SPR) sensor for high-sensitivity gas sensing and its preparation process, as shown in Figure 8[65]. A monolayer of graphene was coated onto the Ag film surface of the LPFG-SPR sensor, which increased the intensity of the evanescent field on the surface of the fiber, thereby enhancing the interaction between the SPR wave and molecules. Such features significantly improved the sensitivity of the sensor. The experimental results demonstrated that the sensitivity of the graphene-based LPFG SPR sensor can reach 0.344 nm\(\%^{-1}\) for methane, which was improved 2.96 and 1.31 times with respect to the traditional LPFG sensor and Ag-coated LPFG SPR sensor, respectively. Figure 8: Schematic of the graphene-based LPFG SPR sensor [65]. Figure 6: (**a**) Design concept of biaxially stretchable patterned-graphene sensor integrated with a MSC array; and (**b**) Current flow, induced by periodic exposure of 200 ppm NO\({}_{2}\), through the graphene sensor operated by an external power supply of 1 V [62]. Figure 7: Vertical responsive gas sensor based on 3D porous graphene ultrathin film [64]. In 2017, [PERSON] et al. from University of Utah presented NH\({}_{3}\) sensing with ultra-low energy consumption for fast recovery and a graphene sheet based on a suspended micro-heater, as shown in Figure 9[66]. The silicon micro-heater periodically heated the sensor at a frequency of 1 Hz, which increased the sensor's response time, recovery time, and sensitivity. The sensor showed rapid recovery through heating of the silicon heater. As the temperature increased, the desorption gradually increased by reducing hydrogen bonding. The new oscillation method of heating improved the sensitivity to 0.049 (\(\Delta\)R/R\({}_{0}\)) measured at a flow rate of 18.8 sccm NH\({}_{3}\) (g) for 70 s. The experimental tests showed that a 60% duty cycle does not sacrifice sensitivity or recovery, while dropping the total power consumption from 1.76 mW to 1.05 mW. It is observed that the pristine graphene has rapid response and low detection limit on NO\({}_{2}\), NH\({}_{3}\) and some organic gas, and even single atom changes can be detected. However, the sensitivity is not high, and it must be accurately detected by the instrument to have obvious changes.