# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
_Nitric oxide (NO)_ is a gaseous neurotransmitter that is synthesized by three isoforms of nitric oxide synthase (NOS): NOS-I (neuronal NOS), NOS-II (cytokine inducible NOS), or NOS-III (endothelial NOS) [84]. Neuronal NO diffuses through cell membranes and functions as a neuromodulator in the brain. It is involved in various behaviors as it interacts with many signaling pathways [85]. The electrochemical oxidation of NO may result in nitrite (NO\({}_{2}\)\({}^{-}\)) and nitrate (NO\({}_{3}\)\({}^{-}\)). A copper (II) chlorophyllin-zinc oxide screen-printed electrode was used for the detection of NO with a wide linear range of concentration from 200 to 1 mM. The sensor exhibited sensitivity of 96.4 nA\(\cdot\)uM\({}^{-1}\) with a LOD of 100 nM. This electrode also assisted with the detection of NO metabolites, such as NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\), in human serum samples prior to and following the introduction of a beet root supplement [86]. An investigation into the electrochemical oxidation of NO was conducted using platinum-tungsten nanoparticles, rGO, and an ionic liquid (IL), where a sharp peak for the NO oxidation was observed at a potential of 0.78 V versus Ag/AgCl with CV and DPV [87]. As shown in Figure 7, the proposed electrochemical sensor exhibited high stability with a low LOD (0.13 nM) and high sensitivity (3.01 uA\(\cdot\)uM\({}^{-1}\) cm\({}^{2}\)). The sensor was investigated in real samples such as human serum and urine samples and demonstrated exclusive selectivity towards NO [87]. Gold/platinum/reduced graphene oxide was utilized for NO detection, which exhibited a high sensitivity of 7.35 uA\(\cdot\)uM \({}^{-1}\) and a LOD of 2.88 nM with negligible interference from other organic molecules [88]. The noble metal-based nanocomposite was then tested to monitor the release of NO in cardiac cells. Amperometry was employed to exploit the detection of NO in cardiac and cancer cells using a hierarchical nanoporous gold (HNG) electrode, where Figure 8 details NO sensing in live cells [89]. This HNG electrode provided an effective platform for monitoring NO in biological processes and in bioreactors. The nitrite anion may affect the selectivity of the sensors because it is the main interference molecule, and its oxidation occurs close to the NO oxidation potential. In addition, these NO oxidations are typically held at higher over potentials, so the amperometric technique may not be suitable for NO sensing in the presence of other neurotransmitters. However, dialysis and/or electrophoresis coupled with amperometric techniques may be utilized for real sample testing. The miniaturization of these sensors is desirable for the in vivo electrochemical detection of NO. _Acetylcholine (ACh)_ is a well-known neurotransmitter that mediates the chemical transmission of neural signals. ACh signaling in the synapses of the central and peripheral nervous system regulate most biological and physiological functions, such as ligand-gated ion channels and bursting mode neuronal firing. Imbalances of Ach in both the central and peripheral nervous systems are associated with different health issues such as Parkinson's disease, Alzheimer's disease, and myasthenia gravis [90]. Ca\({}^{2+}\)-induced ACh from leukemic cells could be monitored electrochemically, which may lead to the development of detection kits for Alzheimer's and Parkinson's diseases. In addition, variations in the levels of Ach disrupts behavior, learning, and sleep [91; 92]. In the electrochemical oxidation of ACh, two-electron transfer is involved and leads to choline and acetate as byproducts. An Au self-assembled monolayer surface was investigated for the acetylcholinesterase-cholinesterase (AChE) activity assay and its inhibitors using electrochemical oxidation. The generated _o_-Quinones were used as redox probes [93]. AChE and choline oxidase (ChO) enzymes were covalently bound to a chitosan/gold/ferric oxide nanocomposite.