# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 3 Amine Neurotransmitter Detection via GFETs
### Epinephrine and Norepinephrine
The device's current was primarily driven by hole transport, with a significant density of hole carriers at the channel surface. The PPy p-type semiconductor behaviour contributed to hole-transporting properties when oxygen or water are absorbed by rGO. The sensor exhibited good reproducibility with no change in response after repeated experiments, with a good signal-to-noise ratio of up to 21 days. Nonetheless, the sensor's degradation after the eighth cycle due to repeated channel washes suggested further optimisation for enhanced durability under operational conditions is needed. [PERSON] et al. [168] introduced a high-performance FET H\({}_{2}\)O\({}_{2}\) sensor based on rGO and molybdenum disulfide (MoS\({}_{2}\)) with \(<\)1 s response time. The sensor exhibited n-type doping behaviour, indicating predominant electronic conduction. MoS\({}_{2}\), known to mimic peroxidase enzymes [227], catalyses H\({}_{2}\)O\({}_{2}\) breakdown and creates intermediate reactive enzyme complexes that generate positive charge (H\({}^{+}\)) and hydroxyl radicals [228]. When exposed to H\({}_{2}\)O\({}_{2}\), H\({}^{+}\) were generated on the surface, decreasing carrier concentration and current levels, which was evident in decreased sensitivity with respect to concentration increase. The sensors without MoS\({}_{2}\) showed minimal current fluctuations, indicating enhanced catalytic and sensing capabilities. The sensor response remained similar after 7 measurements, indicating a good potential for repeatability. [PERSON] et al. [169] developed a high-performance GFET-type nanosensor for sensitive (a response time of \(<\)1 s at a LOD of 100 fM) and selective H\({}_{2}\)O\({}_{2}\) recognition. The CVD-grown graphene channel was functionalised with 5-diaminonapthhalene (DAN) and glutaraldehyde-hyde (GA) to reinforce stability in the liquid state. The surface was immobilised with Cytochrome c (Cyt c), a redox-active water-soluble metalloprotein with a heme structure, facilitating electron transport [229]. Changes in Cyt c redox state induced by H\({}_{2}\)O\({}_{2}\) affected dynamic current variations in the Cyt c/underlying GFET platform. The sensing mechanism relies on the oxidised Cyt c\({}^{3+}\) and the oxidation of Cyt c\({}^{2+}\), leading to an increase in current, serving as a detection signal. Hence, positively charged residues in Cyt c enable interaction with negatively charged molecules on the graphene surface [230]. FET performance demonstrated typical p-type semiconductor behaviour; the monolayer graphene platform used in this sensor contributed to improved sensitivity due to enhanced electron transfer rates compared to multilayer graphene or graphite. A recent study by [PERSON] et al. [170] exhibited a novel and cost-effective Cyt c immobilised carboxylated MWCNT. This bioFET, with a quick response time (around 1 s) at a LOD of 9.13 fM, effectively distinguished H\({}_{2}\)O\({}_{2}\) from interferets with a response time of 1 s, highlighting its potential for a wide range of bioapplications.