# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 2 Problem Outline
### FET Technology in Biosensing  
The interaction of FET sensors with biological analytes is translated into a change in the sensor's electrical conductance. FETs were first used for biological purposes in 1980, where ion-sensitive FETs were employed for pH sensing; in this case, a penicillin-sensitive biological FET (bioFET) was constructed by immobilising Penicillin G enzymes on the surface of an ISFET [92]. The low surface-to-bulk ratio of the early FET sensors, which were fabricated using conventional semiconductors (e.g., Si) and oxides (e.g., Ta\({}_{2}\)O\({}_{5}\) or Al\({}_{2}\)O\({}_{3}\)), frequently resulted in their low sensitivity. The development of several susceptible FET sensors for ion and molecule detection was spurred by the discovery of low-dimensional semiconductors with extraordinarily high surface-to-bulk ratios [93; 94]. The most well-established bioanalytical sensors among them are silicon nanowire FETs (Si-NWFETs) and carbon nanotube FETs (CNTFETs), which have both been extensively demonstrated [95; 96] and miniaturised into single-molecule FETs containing biomolecules [97; 98; 99]. However, the synthesis, manipulation, and scalable integration of 1D nanomaterials provided practical hurdles, which impeded the development of 1D FETs despite their good performance. Carbon nanotubes (CNTs) [100] or MoS\({}_{2}\)[101; 102; 103; 104] have demonstrated exceptional sensitivity and selectivity in biomolecular detection at pM concentrations in FET design. The threshold values of FETs have been increased towards the fM level through contemporary amplification techniques. However, these materials raise concerns about biocompatibility, biodegradability, and recovery following transducer application--particularly when considering in vivo applications.  
Since the discovery of graphene, numerous studies have been exploiting graphene-based field-effect transistor (GFET) for monitoring pH variation, cellular metabolisms, the hybridisation of deoxyribonucleic acid (DNA), antigen-antibody bindings, and enzymatic reactions [105; 106; 107; 108]. [PERSON] et al. [10] studied the first GFET, highlighting its high carrier mobility because of its extraordinary surface-to-bulk ratio, and remarkable chemical and mechanical stability [11]. Graphene exhibits noteworthy sensitivity to electrostatic changes in its near environment [12; 13], making it a promising material for sensing applications. GFET offers simple operation, quick response times, label-free real-time monitoring, access to a larger variety of controlled surface chemistry techniques for anchoring bioreceptors, multiplexing capacity, and potential microfluid integration. Although graphene's zero-band gap prevents it from being utilised in digital circuits because of its modest current on/off ratio, it has become a popular gate material for bioFET and point-of-care biomedical devices [109]. The pioneering work of [PERSON] et al. [110] using GFET for detecting hybridisation between single-stranded tethered DNA and its complementary sequence generated a greater interest in integrating graphene in bioFETs.