# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
The polymer-incorporated sensing platform assisted to control the accumulation of 5-HT on the electrode surface for high performance. However, a high concentration of glucose significantly affected the 5-HT signal by about 30%. Based on Fe\({}_{3}\)O\({}_{4}\)-MWCNT-poly (bromocresol green) film, the electrochemical detection of 5-HT was also achieved using DPV, and it permitted detection in the concentration range between 0.5 and 100 \(\upmu\)M with a detection limit of 0.08 \(\upmu\)M [63]. The interference of UA oxidation significantly limited the sensing performance of the electrode for 5-HT when the UA concentration increased by 20-fold. A polyethyleneimine CNT fiber microelectrode (PEI-CNT MEs) was employed for the oxidation of 5-HT, and a comparison of its performance with a carbon-fiber microelectrode (CFMEs) is shown in Figure 6. The PEI-CNT MEs showed high selectivity and a stable response compared with CFMEs. The biofouling by serotonin and its metabolite interferant 5-hydroxyindoleacetic acid (5-HIAA) decreased the current response to 50% for CFMEs, whereas no significant current drop was observed with respect to PEI-CNT MEs (retains 100%) [64]. Thus, these PEI-CNT MEs may be beneficial for the real-time analysis of 5-HT. The detection of 5-HT can be improved through the incorporation of a selective ion exchange resin capable of repelling interfering molecules. _Norepinephrine (NE)_ is involved in the elevation of heart rate, blood pressure, pupil dilation, airway dilation, and the constriction of blood vessels. It also participates in glycogen metabolism  
Figure 6: 5-hydroxyindoleacetic acid (5-HIAA) fouling in vitro. One micromolar serotonin was detected at a polyethyleneimine CNT (PEI-CNT) fiber electrode before and 2 h after it was bathed in 10 \(\upmu\)M 5-HIAA, while the waveform was applied. The applied waveform was the serotonin waveform (0.2–1.0 to \(-\)0.1 to 0.2 at 1000 V\(\cdot\)s\({}^{-1}\)). **(A)** The cyclic voltammograms (CVs) are similar before and after 5-HIAA at the PEICNT microelectrodes; **(B)** For carbon-fiber microelectrodes (CFMEs) after 2 h in 10 \(\upmu\)M 5-HIAA, the oxidation peak of serotonin was no longer visible; **(C)** Bar graphs depicting the change in peak oxidative current for serotonin prior to and following exposure to 10 \(\upmu\)M 5-HIAA for 2 h [64]. Copyright 2014, American Chemical Society. and plays a major role in the sleep-wake cycle, attention, focus, learning, and memory [65]. Despite the significant volume of robust mechanistic and clinical trial data supporting the benefits of the renin-angiotensin-aldosterone system and the beta-adrenergic receptor blockade in adults with systolic heart failure, it remains unclear whether these mechanisms are also involved in mediating heart failure in single-ventricle patients [66]. Without such knowledge, further clinical trials of neurohormonal blockades in single-ventricle patients will remain unappealing [67; 68]. The oxidation of norepinephrine is similar to that of epinephrine, as illustrated in Figure 2C, where a two-electron transfer takes place to form norepinephrine quinone. Hydrothermally prepared MoO\({}_{3}\) nanowires deposited on GCEs showed an improved response for the oxidation of NE with a response time of 2 s and detection limit of 0.11 uM [69]. From the CV curves, the heterogeneous electron transfer rate constant was calculated to be 8.03 \(\times\) 10\({}^{-4}\) cm s\({}^{-1}\). The improved electrochemical performance was attributed to the high catalytic activity and one-dimensional (1D) microstructure of the MoO\({}_{3}\) nanowires, which allowed for easier electron transport and increased the overall performance of the sensor. In addition, nanorod-shaped FeMoO\({}_{4}\) was investigated for the electrochemical detection of NE, and the LOD was determined to be 3.7 nM. These 1D non-noble metal-based nanostructures exhibited high sensing performance similar to enzyme- and noble metal-based electrochemical sensors [70].