Citation: Kumar, P.; Soni, I.; Jayaprakash, G.K.; Flores-Moreno, R. Studies of Monoamine Neurotransmitters at Nanomolar Levels Using Carbon Material Electrodes: A Review. Materials 2022, 15, 5782. https://doi.org/10.3390/ ma15165782 Academic Editors: Bengi Uslu and J. G. Manjunatha Received: 30 June 2022 Accepted: 16 August 2022 Published: 22 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Studies of Monoamine Neurotransmitters at Nanomolar Levels Using Carbon Material Electrodes: A Review Pankaj Kumar 1 , Isha Soni 1 , Gururaj Kudur Jayaprakash 1,2, * and Roberto Flores-Moreno 3 1 Laboratory of Quantum Electrochemistry, School of Advanced Chemical Sciences, Shoolini University, Bajhol, Solan 173229, India 2 Department of Chemistry, Nitte Meenakshi Institute of Technology, Bangalore 560064, India 3 Departamento de Química, Universidad de Guadalajara, Blvd. Marcelino García Barragán 1421, Col. Olímpica, Guadalajara 44430, Mexico * Correspondence: gururaj@shooliniuniversity.com Abstract: Neurotransmitters (NTs) with hydroxyl groups can now be identified electrochemically, utilizing a variety of electrodes and voltammetric techniques. In particular, in monoamine, the position of the hydroxyl groups might alter the sensing properties of a certain neurotransmitter. Numerous research studies using electrodes modified on their surfaces to better detect specific neuro- transmitters when other interfering factors are present are reviewed to improve the precision of these measures. An investigation of the monoamine neurotransmitters at nanoscale using electrochemical methods is the primary goal of this review article. It will be used to determine which sort of electrode is ideal for this purpose. The use of carbon materials, such as graphite carbon fiber, carbon fiber micro-electrodes, glassy carbon, and 3D printed electrodes are only some of the electrodes with surface modifications that can be utilized for this purpose. Electrochemical methods for real-time detection and quantification of monoamine neurotransmitters in real samples at the nanomolar level are summarized in this paper. Keywords: carbon materials; neurotransmitters; voltammetric techniques; oxidation–reduction; surface-modified electrodes; electrochemical detection 1. Introduction The chemical messengers called neurotransmitters influence a large number of physi- ological and psychological human bodily functions. Because they are directly related to the central nervous system (CNS), they directly handle neurophysiological exercises, such as emotions, sleep, memory, and other cognitive functions by amplifying, transferring, and converting signals through a synapse to a target cell. Defective NTs in the CNS cause a slew of diseases, including Huntington’s syndrome, Parkinson’s, Alzheimer’s disease, autism, and epilepsy. Additionally, it can cause psychotic diseases, such as dementia, attention deficit hyperactivity disorder, schizophrenia, and depression, as well as dejec- tion, anguish, congestive heart failure, glaucoma, arrhythmias, and sudden infant death syndrome. In 1921, German pharmacologist Otto Loewi identified the first extensively detectable neurotransmitter, acetylcholine (Ach). More than 500 NTs were found from 1921, but the actual number of these distinct NTs in humans has yet to be determined. NTs are stored in synaptic vesicles and released into the synaptic cleft and diffused across it, where they get attached to specific receptors on the post-synaptic neuron’s membrane. This bonding perhaps exerts excitation (depolarization) or inhibitory (repolarization) control over the post-synaptic neuron. Immunocytochemical approaches that identify the site of the transmitter material are commonly used to determine the anatomical localization of NTs. Many transmitters, primarily neuropeptides, are co-localized, indicating that a neuron can release multiple transmitters from its synaptic terminal, according to immunocytochemical techniques. Neurotransmitters can be classified in a variety of ways. It is sufficient to divide Materials 2022, 15, 5782. https://doi.org/10.3390/ma15165782 https://www.mdpi.com/journal/materials