# Advances in Graphene Field Effect Transistors (FETs) for Annine Neurotransmitter Sensing
## 1 Introduction  
Neurotransmitters (NTs) are endogenous chemicals that facilitate signal transmission between neurons [1] and enable brain-body integration, where neurons can function as excitatory or inhibitory. They regulate heart rate and muscle tone, learning, sleeping, memory, consciousness, mood, and appetite [1; 2]. NTs are also classified by molecular structure (amino acids, soluble gaseous, and biogenic amines) and function speed [2; 3]. A spectrum of neurological disorders is linked to dysregulation of NT levels because of their significance in the central nervous system. As a result, measuring and evaluating NT levels are valuable when diagnosing NT-related disorders.  
A biosensor is a device that detects biological chemicals and converts concentration signals into an electronically readable output. It typically consists of biologically sensitive probes that identify biological elements (such as enzymes, antibodies, antigens, and nucleic acids), appropriate physical and chemical transducers, and an analysis system that includes a signal amplification device [4]. Optical, resistive, and semiconductor biosensors are all common types of biosensors. Field effect transistors (FETs) have shown great technical potential when deployed as bio-detection platforms due to their simple operation, high sensitivity, fast response time, real-time signal amplification, easy miniaturisation, and integration for high-throughput screening [5; 6; 7; 8]. In FET biosensors (bioFETs), interaction with biological analytes is represented by a change in the sensor's electrical conductance. The device consists of three terminals: a source, a gate that is coupled to the biosensing element, and a drain. The applied electric field at the gate regulates the conductivity of the channel linking the source and drain.  
FET-based biosensors detect biological materials by attaching a bio-sensitive probe to the target analyte, causing charged ions to be released [9]. This changes the carriers in the gate channel material, influencing electrical properties such as source/drain currents, which thus provide the electrical output for this bio-detection. The accomplishment of high-efficiency signal transduction in FET biosensors is strongly dependent on the development of semiconductor materials as well as the optimisation of device geometry and functionalisation methods. Since the exfoliation of graphene sheets from graphite [10],graphene has garnered considerable attention for its remarkable mechanical, thermal, and optoelectronic properties [11]. Graphene has been shown to have extraordinarily high charge carrier mobility as well as high sensitivity to electrostatic changes in its surrounding environment [12; 13], making it a viable material for sensing applications. In GFETs, graphene acts as the channel in the FET structure, and the target analyte immobilises on the graphene surface, changing its conductivity, which is then detected at the output [14].  
The primary purpose of this review article is to provide an overview of the most recent advances in graphene-based field effect transistors (GFETs), which have been shown to be an effective approach for the electrochemical sensing of amine NTs, with high selectivity and sensitivity. First, this review will briefly discuss the background of biogenic amine NTs and FETs in biosensing. Of the various nanomaterials used for the FET biosensor channel, this paper will concentrate on graphene-based nanomaterials, which have a demonstrated track record of improving the FET channel efficacy. Next, the GFET developments in amine NT sensing are extensively highlighted. This review considers the bulk of published studies on this topic to explain the key techniques used in developing operational GFETs for amine NT sensing. Challenges in utilising GFETs for NT detection are then discussed, with an outlook for this research field concluding this review.