# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 3 Amine Neurotransmitter Detection via GFETs
### Epinephrine and Norepinephrine  
[PERSON] et al. [156] pioneered EP detection using GFETs, demonstrating that large-scale micropatterns of continuously conducting rGO films on a PET substrate increased conductance as EP concentrations rose from 1 to 8 mM. The devices exhibited a p-type response with limited interference effects, albeit at reduced signal amplitude. The rGO-PET device demonstrated higher sensitivity compared to rGO-quartz devices. [PERSON] et al. [164] explored highly sensitive EP sensors using poly(styrene sulfonate) (PEDOT: PSS)-based OECTs. EP detection involved oxidising it to EP-quinone at the Pt gate electrode covered with Nafion [160]. The sensor with bare Pt electrodes showed a LOD of 10 nM, while coating the electrodes with graphene decreased its LOD to 1 nM due to the enhanced electrocatalytic activity, with approximately 1200 s response time. NEP sensing can be performed through hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) detection. NEP is released from synaptic vesicles in neurons, attached to adrenergic receptors on target cells [219; 220; 221], and absorbed into the presynaptic neurons or glial cells after exerting its impact. The enzyme monoamine oxidase (MAO) breaks down NEP inside these cells and eliminates NEP's amine group, forming an aldehyde (also known as 3,4-dihydroxyphenylglycolaldehyde (DHPG) or aldehyde (DOPGAL)). This is then converted back to flavin adenine dinucleotide (FAD), producing hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) as a byproduct. For this reason, H\({}_{2}\)O\({}_{2}\) often serves as a useful marker for NEP evaluation during stroke episodes [222; 223]. Several studies have demonstrated flexible sensors using CVD-grown GFET to monitor H\({}_{2}\)O\({}_{2}\). [PERSON] et al. [165] functionalised graphene surface using a 1-pyrenebutanoic acid succinimidyl ester (PSE). As H\({}_{2}\)O\({}_{2}\) concentration increased, the Dirac point moved towards a lower bias gate, attributed to the n-doping effect, with H\({}_{2}\)O\({}_{2}\) molecules acting as scattering potentials [224]. V\({}_{\text{G}}\) modulates graphene channel conductance in a solution-gated setup, and in this case, V\({}_{\text{G}}\) acts effectively across a double layer created at the graphene/solution interface, making the electrochemical gate more effective than the back-gate potential [225]. A study by [PERSON] et al. demonstrated high-performance FET-type H\({}_{2}\)O\({}_{2}\) sensors using rGO transducers integrated with polypyrrole (PPy) nanotube composites with a rapid response time (<1 s) at a LOD of 100 pM [166]. The composites showed superior conductivity compared to separate rGO and PPy nanotube interfaces because of the effective electron transport between the two. However, the conductivity was significantly anisotropic due to inactive interlayer electron transport. The FET's transfer curve during solution gating revealed a concentration-dependent current, suggesting the accumulation of p-type charge carriers at the surface of rGO/PPy nanotube composites. In other words, H\({}_{2}\)O\({}_{2}\) can modify the electrical signal in PPy nanotubes more effectively than rGO, primarily through electrochemical oxidation, resulting in p-doping effects [226]. The robust interactions and increased surface area between rGO sheet and PPy nanotubes contributed to the sensor's effective response, ensuring stable performance even after repeated experiments and exposure to air over a month. These sensors demonstrated ten times enhanced sensitivity and instantaneous response time to changes in H\({}_{2}\)O\({}_{2}\) concentrations compared to glassy carbon electrodes [83]. The sensor also exhibited good reproducibility after 10 repeated experiments. A subsequent study by [PERSON] et al. [167] employed a similar methodology and developed a high-performance sensor using the nanocomposite SiO\({}_{2}\) trench-embedded FET for H\({}_{2}\)O\({}_{2}\) detection with a response time of 5 s at a LOD of 10 pM. The active gate channel material was composed of rGO and PPy.