# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
As shown in Figure 4, an electrochemically formed nickel oxide nanoparticle/multiwalled carbon nanotube (MWCNT) was tested for the simultaneous detection of neurochemicals, and the sensitivity of DA and EP were calculated to be 3.8 and 3.9 \(\mu\)A \(\mu\)M\({}^{-1}\), respectively [56]. Further, the detection was attempted in real samples such as human cerebrospinal fluid, human serum, and lung fluid spiked with DA and EP. The experimental measurements revealed that the recovery percentage was in the range between \(\sim\)97-106%. Figure 2: **(A)** Differential pulse voltammetry; **(B)** amperometry response of the nanoporous sponge-like Au–Ag electrode in 0.1 M PBS containing different concentrations of epinephrine (EP); **(C)** The electron transfer–chemical reaction–electron transfer reaction (ECE) mechanism of epinephrine electrochemical oxidation [36]. Copyright 2016, Springer. An organic field-effect transistor type (OFET) was developed using platinum nanoparticle-decorated rGO for the selective detection of dopamine in the presence of non-target molecules such as AA and UA, as well as tyrosine and phenethylamine [57]. Aside from nanomaterials-based sensors, an RNA-modified aptamer electrode was explored for the detection of DA in blood serum, as shown in Fig. 4.  
in Figure 5. The sensitivity and LOD were 67 \(\pm\) 1 nA \(\upmu\)M\({}^{-1}\) cm\({}^{-2}\) and 62 nM, respectively [58]. The positively charged cysteine self-assembled monolayer (SAM) discriminated DA from the negatively charged interference molecules (e.g., AA) on the aptamer, which provided the selectivity to the fabricated sensors. In the DA detection, the potential interference of AA and UA may be avoided by the careful design of the electrode and the selection of materials [59]. However, interference from structurally related neurochemicals (e.g., DA and EP) is challenging to avoid. In such a case, receptor-incorporated nanomaterials-based sensing strategies need to be developed for the selective detection of DA. _Serotonin or 5-hydroxytryptamine (5-HT)_ is involved in many biological processes, including behavior and sensory perception. The concentration of 5-HT in the human brain is 10 mg, where 2% is present in the central nervous system. Changes in 5-HT concentrations create different neurological and psychiatric issues. Further, 5-HT levels are directly related to gastrointestinal illness [60]. For all the above reasons, the detection of 5-HT has great significance in the diagnosis of various diseases and in elucidating its role in several neurological disorders. As low 5-HT levels in the brain may be an indicator of depression, its presence in the blood, serum, plasma, and platelets may be considered as a peripheral biochemical marker for depression [60]. The oxidation of 5-HT involves the transfer of two electrons to form serotonin quinoneimine. [PERSON] et al. fabricated an electrode with platinum nanoparticles/polypyrrole/MWCNT/silver nanoparticles for the detection of 5-HT and achieved a detection limit of 0.15 \(\upmu\)M using DPV [61]. The synergistic effect of the nanohybrid was responsible for the sensitive detection of 5-HT. As in the abovementioned work, gold/polypyrrole nanocomposites were developed for the detection of 5-HT and revealed high sensitivity (0.3316 \(\upmu\)A\(\upmu\)M\({}^{-1}\)) and a low  
Figure 5: Schematic representation (left panel) of the flow-through wall-jet electrochemical cell (with a working electrode (WE), a counter electrode (CE), and an Ag/AgCl (0.1 M KCl) reference electrode (RE)) used for dopamine electroanalysis and the aptamer-modified gold electrode surface. The right panel image shows the chronoamperometric response of the aptasensor to dopamine, selectively trapped from the flow of neurotransmitters over the sensor surface. Following each measurement, the electrode surface was regenerated in a flow of a blank buffer solution to remove the oxidized analytes [58]. Copyright 2016, American Chemical Society. detection limit of 33.22 nM [62].