# Nanomaterials-Based Electrochemical Sensors for
## 1 Introduction  
In multicellular organisms, the nervous system transmits information generated in the brain to the entire body [1]. In this process, nerve cells transfer signals from neurons to targeted neuronal chemical synapses. This signal transfer occurs electrically and chemically between nerve cells. In brief, firstly, electrical signals are transmitted along the cell membrane within cells. Secondly, the electrical signals are converted into chemical signals for communication between cells. These chemical signals are conveyed by small messenger molecules, called neurotransmitters, which drive the behavioral, cognitive, and emotional states of an organism. They also have impact on muscle movement and heart rate, while regulating sleep, memory, and learning. There are different types of neurotransmitters that are described as amino acids, peptides, monoamines, gas transmitters, trace amines, and purines. In current analytical research, neurochemical quantification has led to many enhancements toward the understanding of the biochemistry of the nervous system. Abnormal neurotransmitter levels, such as-degenerative diseases, mental disorders, depression, and many other health issues, may significantly affect physiological and behavioral processes. The quantitative detection of neurotransmitters has the strong potential to aid in the development of medical diagnostics and therapeutics [2]. Spectroscopic, chromatographic, and electrochemical strategies are the most commonly used analytical methods for the detection of neurotransmitters. For instance, optical spectroscopy, liquid chromatography, capillary electrophoresis separations, mass spectrometry, microelectrodes, electrochemical sensors, and biosensors are widely employed techniques for the detection of neurochemicals [3]. To utilize spectroscopic and chromatographic techniques, sample separation and purification, which are tedious and time consuming, are required prior to measurements. Moreover, in the optical-based techniques, sample preparation is an additional step and is involved with other chemical species, such as fluorescent dyes or metal complexes, for the selective and sensitive detection of neurotransmitters [4]. The applicability of these technologies are further limited in in vivo measurements due to complex instrumentation, difficulties in miniaturization, and high cost. In contrast, electrochemical-based approaches with microelectrodes might be employed either directly or following sample purification via ultra-dialysis, centrifugation, or perfusion methods [5]. The electrochemical analysis of neurotransmitters involves the measurement of changes in currents when an electrode potential is applied. In principle, if a neurotransmitter can be oxidized electrochemically, it should be detected and monitored through in vivo or in vitro electrochemical measurements. In vitro studies, conducted outside of a living organism in a controlled environment, are commonly used for electrochemical measurements. These studies prove the ability of a certain nanomaterial to detect a given neurotransmitter but are not reflective of the complexity of a true living organism. For this reason, in vitro studies may not be a truly accurate description of the nanomaterials' performance within a living organism. In vivo studies are conducted within living tissue and are typically more complicated than in vitro measurements. In vivo measurements are more reflective of the true performance of the nanomaterial being studied. Short-term analysis may confirm the feasibility of a proposed nanomaterials-based electrochemical sensor, but long-term measurements should be performed to ensure the stability and toxicity of a nanomaterial in a living organism. The integration of both in vivo and in vitro measurements would provide important information regarding the ability of a nanomaterial to detect neurotransmitters.