# Nanomaterials-Based Electrochemical Sensors for
## 1 Introduction
Among the noble metal-based nanomaterials, gold nanoparticles have been widely exploited for biomedical applications due to their peculiar properties such as chemical, optical, electrical, and catalytic biocompatibility and its ability to conjugate with biomolecules [18]. Silver nanoparticles have also been extensively explored for practical applications due to their highly selective and sensitive performance towards the detection of biomolecules [19; 20; 21]. In addition, various non-noble metal-based metal/metal oxides have been fabricated and tested for the in vitro analysis of neurotransmitters. Figure 1: Schematic representation of the application of various nanoparticles for the in vitro and in vivo detection of neurotransmitters using different electrochemical methods. Furthermore, carbon-based nanomaterials have been extensively explored in the real-time analysis of biomolecules due to their biocompatibility, non-toxic properties, and low costs. Different types of carbon materials, such as single-walled CNTs, multi-walled CNTs, graphene, and reduced graphene oxide (rGO) have been exploited for biosensing and electrochemical sensing applications [22; 23]. CNTs exhibit both metallic and semiconductor properties, depending upon the diameter, chirality, and number of sheets of the tubes. Arc discharge, laser ablation, and chemical vapor deposition are the major methods for the growth of CNTs. Among the carbon-based nanomaterials, CNTs have been extensively explored in the development of electrochemical sensors due to their high surface-to-volume ratio, rapid electrode kinetics, and high conductivity [24; 25]. Another widely studied carbon-based material is graphene, which is a two-dimensional carbon material with its carbon atoms arranged in hexagonal fashion. This novel carbon material exhibits good electrochemical, thermal, mechanical, and optical properties, which are desirable for designing a high-performance sensor platform. Various carbon nanomaterials-based electrochemical sensors and biosensors has been reported for pharmaceutical, environmental, and biomedical applications [26; 27; 28; 29; 30; 31]. The advantages of using a nanomaterials-based approach for electrochemical sensing are the improved selectivity, reduction of cost, potential for miniaturization, and higher sensitivity. Detecting neurotransmitters using a bare electrode could be challenging due to its low sensitivity and selectivity. Nanomaterials-based electrochemical sensors provide distinct advantages over those without nanomaterials. Broad scientific knowledge and modern engineering have led to the design and development of miniaturized and portable electrochemical sensors and biosensors for the in vitro and in vivo detection of neurotransmitters. Nanomaterials-based electrochemical sensors have increasingly appealed to researchers in the disciplines of analytical chemistry due to their augmented electrocatalytic activity, high surface area-to-volume ratios, and possible functionalization enabling them with tailor-made physicochemical properties. Over the last decade, nanomaterials-based electrochemical sensor and biosensors emerged as a promising strategy for the detection of neurotransmitters with improved selectivity and sensitivity [32; 33; 34]. This review will strive to elucidate the recent development of nanomaterials-based electrochemical sensors for the in vitro and in vivo detection of common neurotransmitters, including epinephrine, dopamine, serotonin, norepinephrine, glutamate, acetylcholine, nitric oxide, and purines. Besides the in vitro detection, this review also highlights some recent in vivo monitoring of neurotransmitters in the live tissues of drosophila, rats, chicken, and zebrafish. Some core issues associated with the electrochemical approaches used in the detection and monitoring of neurotransmitters and the future development in this exciting area are addressed.