# Nanomaterials-Based Electrochemical Sensors for
## 1 Introduction
To detect and monitor neurotransmitters, various electroanalytical methods have been employed such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), squarewave voltammetry (SWV), amperometry, and so forth [6; 7]. CV is the most extensively used technique for initial electrochemical studies, in which potential is applied to the electrode in a triangular waveform and the resulting current is measured. Based on cyclic voltammograms, qualitative and quantitative information of analytes can be provided by means of measuring the redox peak potential and peak current. CV is also employed to obtain thermodynamic and mechanistic information related to electrochemical reactions. Akin to CV, linear sweep voltammetry (LSV) is also a very useful tool to elucidate reaction mechanisms and electrode processes. These techniques are practically useful for studying more complex reactions of various biologically important molecules, including, but not limited to, neurotransmitters. However, in these techniques, the inherent non-faradaic process contribution is unavoidable, and it may significantly limit the detection of analytes. Thus, in order to measure the small faradaic current, the non-faradic current should be subtracted. Moreover, to minimize the non-faradic contribution toward lowering the limit of detection, differential pulse voltammetry (DPV) and squarewave voltammetry (SWV) have been employed in electrochemical sensing and biosensing of disease-related biomarkers and chemical contaminants [8]. Scanning electrochemical microscopy (SECM) is also another important technique that has been used to detect and monitor neurotransmitters. For instance, dopamine released from single cells was successfully monitored using SECM [9]. currents may be converted into concentrations using calibration factors obtained from standards of known concentrations. This technique might detect neurotransmitters at the sub-nanomolar concentration range. As some biomolecules (e.g., proteins) may not be electrochemically oxidized/ reduced directly, mediators may be utilized for the electrochemical detection of the targeted molecules. However, these techniques may be subject to some complications, such as mutual interference due to the oxidation of multiple biomolecules at the same potential, inherent low concentration, and possible fouling. To overcome these challenges, considerable efforts have been invested to develop nanomaterials-based electrochemical sensors for the sensitive detection of biologically significant molecules for medical applications [4; 8]. In this review article, we mainly focus on the development of various functional nanomaterials-based electrochemical sensors for the detection of neurotransmitters. A variety of nanomaterials have been explored for the electrochemical detection of neurochemicals, as illustrated in Figure 1. For instance, noble metals (e.g., Au, Ag, Pt, and Pd), non-noble metal oxides (e.g., Fe\({}_{2}\)O\({}_{3}\), FeMoO\({}_{4}\), NiO, and Co\({}_{3}\)O\({}_{4}\)), polymers (e.g., polypyrrole, Nafion, and poly(3,4-ethylenedioxythiophene) (PEDOT)), carbon-based nanomaterials (carbon nanotubes (CNT), graphene, and carbon fiber), and the combination of these materials have been used for in vitro and/or in vivo detection of neurotransmitters [9; 10; 11; 12]. The size, shape, and synthetic procedure of the functional nanomaterials may considerably influence the sensing performance. The advancement of synthetic chemistry and the evolution of nanoscience and technology have greatly assisted in the fabrication of sensitive and selective sensing platforms. Numerous methods have been reported for the synthesis of noble metal-based nanoparticles, such as chemical reduction, hydrothermal, solvothermal, and thermal annealing methods, with controlled size and shape [13; 14; 15; 16; 17].