# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 3 Amine Neurotransmitter Detection via GFETs
### Dopamine  
DA is a redox active molecule and interacts with graphene's surface as a reducing agent. The DA's electron donation to graphene leads to DA oxidation and produces DA-o-quinone. This electron transfer changes the carrier density and graphene's conductivity, leading to an upward shifting of the Fermi level in graphene, instigating n-type doping [181]. Correspondingly, the Dirac point shifts towards more negative gate voltage as DA concentration and electron donation rise [158]. Conversely, if a suitable reducing environment or a cathodic potential is applied, DA-o-quinone can endure a reduction process, which then leads to receiving electrons and transforming back to DA. This redox cycling is important in real-time sensing, allowing for continuous DA level monitoring. It is important to recognise interfering analytes when detecting DA within bodily fluids, as this impedes precise detection and impairs the sensor selectivity. [PERSON] et al. [156] pioneered GFET application in DA sensing by designing large-scale micropatterns of continuously conducting reduced graphene oxide (rGO) films on quartz and Poly(ethylene terephthalate) (PET) substrates for front-gated (solution-gated) sensing. The rGO-PET's conductance improved as the DA concentrations were increased from 1 to 8 mM due to the doping effect. DA can interact with rGO through the strong \(\pi-\pi\) interaction, and its exposure to the rGO-PET platform causes the right shift of the Dirac point. This indicates that the aromatic DA molecules induce p-doping (or increase of hole concentration) in the rGO film [182]. However, these sensors have a degraded LOD compared to other transistor-based DA sensors [183]. A study by [PERSON] et al. [105] demonstrated a solution-gated FET in unmodified graphene biosensors, which displayed a shift towards smaller V\({}_{\text{G}}\) in the presence of a 1 uM DA/PBS solution. The graphene gate's transfer curve was considered anodeous, and DA was electro-oxidised to O-dopamine quinone. This chemical reaction produces a faradic current [184, 185, 160, 183], which lowers the potential drop at the electrolyte/gate interface and shifts the transfer curve to a lower V\({}_{\text{G}}\). To enhance the selectivity of these sensors, they modified the channel by Nafion. It has been reported that Nafion reduces interference from negatively charged chemicals by electrostatic contact [186, 187, 188]. Consequently, the devices showed enhanced selectivity to DA with an LOD of 1 nM, making them suitable for clinical applications. A subsequent study by [PERSON] et al. [158] found similar results as the abovementioned research. A GFET biosensor with an unmodified CVD graphene channel showed low selectivity due to high oxidation potentials of interferences of DA. Hence, the channel surface was modified with a cation-selective Nafion membrane for DA charge separation, which prevented anions from reaching the electrode surface, allowing for cations to flow through readily [190]. The GFET exhibited improved selectivity with an LOD of 0.11 uM. Since