# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
_Dopamine (DA)_ is a neurotransmitter of the phenethylamine family that participates in metabolism, attention span, central nerve function, emotions, and neuronal elasticity. It also regulates plasma glucose levels, endocrine function, and neuroimmune regulation [40; 41; 42]. The DA receptor in the peripheral nervous system activates DA, and the activated DA is involved in cross-talk between the immune and neural systems. An abnormal activated DA level may cause neuro-immune system dysfunctions [43; 44; 45]. Molecularly imprinted polymers on a gold electrode were employed for the detection of DA in human blood serum using DPV with the limit of detection of 0.13 \(\upmu\)M [46]. The designed electrode exhibited high selectivity, effectively discriminating DA molecules from AA and UA. Electrochemical detection with microchip electrophoresis is a novel approach for the analysis of neurotransmitters. A palladium microelectrode with microchip capillary electrophoresis was employed for the selective detection of DA with a sensitivity of 2.6 pA \(\upmu\)M and a detection limit of 2.1 \(\upmu\)M. As palladium does not show high catalytic activity for the electrochemical oxidation of DA, the integration of platinum and/or gold with palladium was proposed to improve the performance of the palladium-based sensor [47]. Gold nanowires (Au NW), with different widths in the range of 30-1500 nm, were used for the oxidative detection of DA [48]. The current density of the Au NW electrode (25.87 mA cm\({}^{-2}\) @ NW -614 nm) was much higher than that of an Au thin film (0.043 mA cm\({}^{-2}\)). However, the selectivity of the NW-based sensor needs to be improved for real sample tests. Core-shell and hollow poly(3,4-ethylenedioxythiophene)/poly(\(N\)-methylpyrrole)/poly(3,\(A\)-ethylenedioxythiophene) microspheres, fabricated using chemical polymerization with FeCl\({}_{3}\), had the capacity to detect DA with a concentration range from 0.5 to 2 mM [49]. These polymer-based electrode materials showed efficacy as electrochemical sensors, and this approach may be used to fabricate various multilayered sensor platforms with dopants. As displayed in Figure 3, polymer-based organic electrochemical transistors (OECT) with graphene were demonstrated to have a limit of detection (LOD) of 5 nM for DA [50]. The mechanism of the electrochemical oxidation of dopamine involves a two-electron transfer to form dopamine-o-quinone. A magnetic Fe\({}_{3}\)O\({}_{4}\)/rGO-based sensor has been reported for the simultaneous detection of AA, DA, and UA using DPV [51]. A carbon nanorod array electrode was developed for the sensitive detection of dopamine with a limit of detection of 60 nM and a sensitivity of 7 nA \(\mu\)M\({}^{-1}\)[52]. [PERSON] et al. fabricated a graphene-based electrode to detect DA in the presence of a number of potential interferents such as EP, UA, and AA. Moreover, the DA in rabbit serum was also analyzed with a recovery of 97.8% [53]. A generator-collector approach was established for selective DA detection in the presence of AA using microwell arrays with the concept based on redox cycling. Under such experimental conditions, the interfering species is discriminated diffusionally. The analyte may freely diffuse into the microcavities and be involved in the electrochemical reactions. However, more theoretical and experimental findings are required to understand the detection mechanisms in the presence of other neurochemicals, especially EP and norepinephrine [54]. DA oxidation was conducted on the surface of multilayer graphene nanobelts/glassy carbon electrode using amperometry in the presence of potential interferents. The amperometry response of the graphene nanobelt exhibited a sensitivity of 0.95 \(\mu\)A \(\mu\)M\({}^{-1}\) for the DA oxidation [55].