# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 4 Challenges and Limitations
### Environmental Factors  
Temperature changes, humidity, exposure to air, pH levels, and the non-specific adsorption of biomolecules can all cause variations in the sensor's response, altering its stability, selectivity, and sensitivity. Understanding and reducing the impact of these variables is critical for developing GFET-based biosensors for accurate and repeatable detection in real-world applications.  
Environmental factors such as temperature, humidity, exposure to oxygen (O\({}_{2}\)), and nitrogen (N\({}_{2}\)) significantly affect the performance of GFET biosensors. Under N\({}_{2}\) and increasing temperature, the drain/source resistance (R\({}_{\mathrm{SD}}\)) rises nonlinearly due to reduced carrier mobility [253, 254]. Exposure to O\({}_{2}\) shifts R\({}_{\mathrm{SD}}\) towards a p-type regime, with incomplete recovery at lower temperatures, suggesting that O\({}_{2}\) desorption is slow and chemisorption-like (covalent binding) [253]. Moisture adsorption and desorption on graphene are reversible and physisorption-like, with greater resistance variation at lower temperatures. In humid conditions with N\({}_{2}\), carrier mobility decreases as temperature rises. GFET sensitivity to moisture depends on both temperature and gate voltage, with decreased electron regime sensitivity at higher temperatures. Conversely, under O\({}_{2}\), humidity sensitivity decreases while O\({}_{2}\) sensitivity increases as temperature rises.  
Pristine monolayer graphene does not respond significantly to changes in pH concentration [255] due to its lack of terminal OH groups, sensing instead the electrostatic potential of the solution. This hydrophobic nature limits pristine graphene's application for pH detection. However, defective graphene with surface bonds shows significant shifts in the Dirac point with pH changes. Enzymes can be immobilised near the graphene surface, where pH changes from enzymatic reactions alter the GFET's conductivity [143], serving as the signal transduction mechanism. For example, functionalising GFET channels with PABA enables the detection of enzyme-catalysed hydrolysis in a flow configuration [177]. Depending on the pH, H\({}_{3}\)O\({}^{+}\) (n-type doping) shifts the Dirac point negatively, while OH(p-type doping) shifts it positively [256], affecting conductivity accordingly. GFET biosensor performance will typically be influenced by pH levels due to some inevitable defects in its structure. [PERSON] et al. [257] developed a computational model of a trilayer graphene nanoribbon (TGN)-based ion-sensitive FET (ISFET) to investigate how pH changes impact current-voltage (I-V) characteristics (shown in Figure 5a). The model demonstrates that as pH increases, \(\mathrm{I_{D}}\) decreases due to the protonation/deprotonation of carboxylated TGN and the presence of charged ions in pH buffers, which also negatively affects the threshold voltage. Liquid gating controls the Fermi energy of TGN carriers, allowing for the transition from holes to electrons at a voltage below 0.8 V (shown in Figure 5b), with mobility remaining largely unaffected by pH changes. An increased pH is shown to shift \(\mathrm{V_{G}}\) positively [107], attributed to hydroxide ions acting as electron scavengers [258]. Stringent control on the initial pH level prior to GFET sensing is thus an essential requirement for measurement repeatability.