# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
Moreover, the electrochemical oxidation of NE was demonstrated on carbon paste electrodes with carbon nanotubes (CNT-CPE) and a molybdenum (vi) complex, which facilitated the electron transfer of NE at the surface of the electrode, which led to the non-interferent detection of NE with a LOD of 0.039 uA\(\cdot\)uM\({}^{-1}\)[71]. The simultaneous detection of NE and acetaminophen was proposed using MnO\({}_{2}\)-decorated rGO. The proposed MoO\({}_{2}\)-rGO simultaneously detected the NE and acetaminophen with detection limits of 2.0 nM and 0.4 uM, respectively [72]. Screen-printed mesoporous carbon was utilized for the oxidation of NE, and the LOD was determined to be 0.589 nM using the amperometry technique [73]. The performance of the sensor is directly related to the -surface area of the mesoporous carbon. Further, various metal and/or metal oxide-decorated carbon-based nanocomposites have been utilized for the detection of NE in the presence of other neurochemicals [74; 75]. However, the selective detection of NE is still a challenging task due to the mutual interference of other neurotransmitters, especially in the presence of EP and DA. Again, the use of a selective resin, ion channel, or functional groups could be a viable strategy to achieve higher selectivity for the detection of NE, as well as EP and DA. _l-Glutamate (l-Glu)_ is an excitatory transmitter in the vertebrate brain that is expressed in glial cells, behaves as potential neurotoxin by its overstimulation of its membrane receptors, and participates in the synthesis of the glutathione antioxidant [76]. Signal processing in the retina is controlled via the integration of excitatory and inhibitory inputs of retinal neurons [77; 78]. The distribution of glutamate receptor \"subtype\" retinal neurons has been characterized by immunohistochemical techniques [79]. The oxidation of l-Glu involves an oxidative deamination in which the amine group on the second carbon is converted to a carbonyl group. This two-electron transfer forms \(\alpha\)-ketoglutarate as the product. A multisite array of two microelectrodes on a ceramic base was fabricated to detect l-Glu using glutamate oxidase, where the low detection limits (0.98 uM) and excellent linear range from 0 to 200 uM were observed for the in vitro analysis of l-Glu [80]. This approach was also useful for glucose detection. A ferrocene-incorporated single wall carbon nanotube (SWCNT) was investigated for the oxidation of l-Glu using CV and amperometry techniques. The ferrocene/SWCNT exhibited a fast response to attain dynamic equilibrium for each addition of l-Glu from 1 to 7 uM and generated a steady-state current in amperometry [81]. [PERSON] et al. conducted l-Glu oxidation using a carbon-containing electrode modified with gold (CCEMG) and showed an analytical signal for l-Glu with linear behavior in the range from 3.2 \(\times\) 10\({}^{-4}\) to 2.5 \(\times\) 10\({}^{-3}\) M. This enzyme-free detection method revealed that the potential of the l-Glu oxidation peak was dependent upon the pH [82]. The enzymatic electrochemical detection of l-Glu was performed through the immobilization of glutamate oxidase (Gluox) onto a carboxylated MWCNT/gold nanoparticle/chitosan complex film modified on a gold electrode. The optimal response time of the fabricated biosensors was 2 s with a high sensitivity (155 nA\(\cdot\)uM\({}^{-1}\cdot\)cm\({}^{-2}\)) and low detection limit (1.6 uM). Further, this biosensorshowed no potential interference with other biomolecules that were present in blood serum under physiological conditions [83]. However, these enzymatic biosensors often suffered poor stability, low conductivity, and the blockage of catalytic sites. To overcome this issue, the design of functional nanomaterials and electrochemical biosensors needs to be further improved. For instance, core-shell alloys and multi-dimensional nanocomposites may be designed to enhance the electron transfer rate and durability of the biosensors.