## 3 Results and Discussion
### Sensor Performance
#### 3.5.2 OTFT Chloride Ion Sensor  
As previously described, the chloride sensor is composed of the C\({}_{60}\) OTFT in which the gate conductive layer was functionalized with hydrogenated graphene. The sensing performance of the fabricated OTFT chloride sensor against a broad range of chloride ion concentrations is evaluated and compared to the core OTFT device comprising Gr: PEDOT:PSS as the gate conductive layer. Figure 8a displays the output characteristic of the chloride sensor before and after exposure to different Cl\({}^{-}\) concentrations (3 ppm \(-\)75 ppm) at a constant gate bias V\({}_{\text{GS}}\) = 1 V. As evident, the sensor undergoes a decrease in the saturation current associated with the decrease in conductivity resulting from hydrogenated graphene bonding with chloride ions. The output current was also measured at V\({}_{\text{DS}}\) = 13 V, during which the sensor operates in the linear regime. Transistors with varying channel lengths and aspect ratios were exposed to Cl\({}^{-}\) solution and their output current was measured. For all OTFT Cl\({}^{-}\) sensors, the output current density exhibits a logarithmic dependence over concentration ranging between 3 ppm and 75 ppm, as shown in Figure 8b. Transistor dimension variations do not affect responses to seen analytes, where changes in I\({}_{\text{DS}}\) remain consistent regardless of the sensor's surface area. The sensor response experiences slight diversion after recurring measurements, often observed on organic electronics, but remains relatively uniform with respect to analyte concentration. Figure 7: **(a)** Transient response from OTFT humidity sensor before, during and after exposure to surface wetness with device architecture: L = 15 μm and W = 300 μm. (b) Humidity sensor response illustrating the current transient, response time, and recovery time of the sensor. The sensitivity of the sensor was estimated at 4%/ppm and was calculated using Equation (3), where \(\Delta\)I\({}_{\text{resp}}\) is the change in the output current after exposure to Cl\({}^{-}\), I\({}_{\text{ref}}\) is the baseline output current, and C is the Cl\({}^{-}\) concentration. \[S\ =\ \frac{\Delta\text{I}_{\text{resp}}}{\text{C}\text{I}_{\text{ref}}}\times 100 \tag{3}\]  
For practical application of environmental sensors, they typically need to operate in environments where the temperature may vary significantly. The temperature dependence of the OTFT sensor was investigated by placing the OTFT on a Peltier cooler-regulated thermal chuck and the drain current versus the gate voltage dependence was plotted at a constant drain-source voltage of 200 mV. Sensor response to increasing temperature is depicted in Figure 8c, which shows the expected increase in the drain current attributed to thermal effects. The thermal effects' influence on the sensor operation appears as a decrease in the threshold voltage (0.2 V to 0.05 V), although not significant, it should be taken into consideration. The mobility, on the other hand, undergoes an increase of ~70% as the temperature is increased from 25 \({}^{\circ}\)C to 45 \({}^{\circ}\)C.  
It was demonstrated that the in-house synthesized hydrogenated graphene displays chemical affinity toward Cl\({}^{-}\) ions which results in a decrease in electrical conductivity. This was achieved using C\({}_{60}\) OTFT as a transducer which experiences a decrease in output current as hydrogenated graphene is exposed to Cl\({}^{-}\). Our Cl\({}^{-}\) ions sensor is able to detect small concentrations far below reported corrosion initiation thresholds of 100 ppm to 3500 ppm [5,6]. Investigation of device scalability demonstrates the achievability of a miniaturized Cl\({}^{-}\) sensor without compromising the reliability of the measurements.