# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 2 Problem Outline  
### Biogenic Amine Neurotransmitters  
Biogenic amine NTs are categorised as dopamine (DA), epinephrine (EP), norepinephrine (NEP), serotonin (SER), acetylcholine (ACh), and histamine [1]. Catecholamines NTs (DA, EP, and NEP) with an amine in their benzene ring structure have similar molecular structures and physicochemical properties, making it challenging to differentiate them in simultaneous detection. DA is a neuromodulator that uses volume transmission to alter the function of other NTs [15]. It is essential for regulating the central nervous system, endocrine system, neurom Immune system, plasma glucose levels, neuronal flexibility, and metabolism. Abnormal DA concentration may lead to neuro-immune system dysfunctions [16]. EP (or adrenaline) is an excitatory NT [17] and is referred to as a \"fight or flight hormone\" [18]. It is necessary for the effective functioning of the reproductive, cardiovascular, hormonal, and central nervous systems in both humans and animals to control the immune system, blood pressure, heart rate, lipolysis, and glycogen metabolism. EPs are released in reaction to stressful, thrilling, dangerous, or threatening circumstances. Additionally, because it is a cardiac stimulant, it serves as a medication to treat allergic reactions, heart attacks, bronchial asthma, and bronchodilators [18; 19; 20; 21]. In contrast, NEP is a metabotropic natural hormone NT crucial for controlling stress responses, arousal, attention, and cognitive function [22]. Numerous illnesses, including neuroblastoma, ganglion neuronal, coronary heart disease, paraganglioma, Parkinson's disease, and multiple sclerosis, can be brought on by abnormal NEP. Similar to EP, adding NEP to blood within the appropriate concentration can boost blood pressure, heart rate, and blood sugar levels [23] and be used to aid in treating bronchial asthma, myocardial infarction hypertension, and organic heart disease [24]. EP, in combination with DA and NEP, is wrongly and illegally used by some athletes as energy stimulators, increasing the concentration of DA and EP in urine [25]. Consequently, it is abundantly clear how crucial it is to track variations in EP concentration levels, not only to enable the identification of relevant diseases but also to ensure athlete doping compliance [25]. Indoleamine SER is a biogenic monoamine NT and neuromodulator in smooth muscles, platelets, and the central and peripheral nervous systems [26]. Its functions span emotions, hunger, and autonomic processes [26; 27] and sleep regulation, depression, eating disorders, autism, anxiety, psychosis, alcoholism, irritable bowel syndrome, carcinoid tumours, diabetes, and obsessive-compulsive disorders [28; 29]. ACh is a pivotal NT in the cholinergic system [30; 31], regulating neuronal proliferation, differentiation, apoptosis [32], and muscle contractions [33]. Dysregulation in its levels is associated with various neurological disorders [34; 35; 36; 37], thus making precise measurements of ACh concentrations essential for diagnostics and therapeutic purposes. Histamine was first recognised as the smooth muscle contractions of the gut and vasodilatation intermediate [38]. It is synthesised and metabolised in neurons [39] and was then identified in the brain by [PERSON] et al. [40]. Histamine was encountered as an NT after discovering histaminergic cells and their axonal distribution within the brain [41]. This NT is involved in brain alertness, thermoregulation, sleep-wake regulation, pain perception, eating, mood and higher cognitive functioning, and reducing seizure activity [42; 43; 44; 45; 46]. Hence, monitoring histamine is essential for managing and preventing symptoms associated with related illnesses [47; 48; 46]. The brain can only operate effectively when neurons are operating continuously and in harmony. Various NTs influence specific brain activities, and diseases may arise from their dysregulation.