# Nanomaterials-Based Electrochemical Sensors for
## 3 In Vivo Analysis of Neurotransmitters
The in vivo detection of L-Glu and ascorbate was carried out simultaneously in the striatum of the rats using microsensors [139]. Following the detection of L-Glu after a few minutes, the current response was decreased due to the uptake of L-Glu from the extracellular space. The above statement was well analyzed by the addition of glutamate oxidase, which assists brain cells with the consumption of L-Glu. In such cases, the neurotransmitter may have been consumed and created deficiencies in localized regimes of the living organism. The in vivo measurements of Glu in the striatum and prefrontal cortex of freely moving mice proceeded using a ceramic-based enzyme-coated Pt microelectrode. A microelectrode array (MEA) was surgically fixed to the central nervous system (CNS) of live rodents 50-100 \(\upmu\)m below the surface of the brain. The baseline correction was done, and the electrode was employed to analyze the injected L-Glu and its effects at 3, 5, and 7 days post-implantation [140]. The enzymatic MEA exhibited fast temporal resolution (less than 1 s), excellent spatial resolution (micron-scale), and low detection limits (\(\leq\)200 nM). The dendrimer-attached Pt/MWCNTs demonstrated selective detection of L-Glu [141]. In the L-Glu analysis, aside from mutual interference of other neurochemicals, peroxide formation was the most problematic issue that affected the selectivity in the L-Glu biosensors [142]. A Pt/Ir wire was used to detect NO in tumor-bearing mice to predict the tumor characteristics [143]. Intratumorally, CV and chronoamperometry were measured for various injected volumes of NO, which initially exhibited an increase in peak intensity, followed by a gradually decrease. This is due to the measurement of exogenous NO and its consumption by the tumoral tissue. Hemin/MWCNT/CFMEs were engineered as a biomimetic sensor to investigate the measurement of NO in the brain. The developed microsensor exhibited an average sensitivity of 1.72 nA \(\upmu\)M\({}^{-1}\) with a LOD of 25 nM [144]. These microsensors allowed the detection of exogenous NO and did not show any response for AA, DA, and nitrite. This may be due to the low concentration of interference molecules. In another study, the distribution, release, and modulation of NO in zebrafish embryo intestines were quantified using CFMEs. The distribution of NO was monitored by DPV at different sites of the zebrafish, demarcated as (1) anterior, (2) middle, and (3) posterior, where a high level of NO was detected in the middle segment (3.78 \(\pm\) 0.64 \(\upmu\)M) in comparison with the anterior (1.08 \(\pm\) 0.41 \(\upmu\)M) and posterior (1.00 \(\pm\) 0.41 \(\upmu\)M). This study revealed the feasibility of the NO detection in the embryos [145]. Figure 12: In vivo monitoring of 5-HT. **(A)** a photograph of the real-time and in vivo experiment after the rat was intraperitoneally anesthetized; **(B)** A zoomed picture; **(C)** the real-time recording of the inserted PEDOT/CNT/acupuncture needle in ST 36 upon the repeated injection of 10 \(\upmu\)L, 10\({}^{-5}\) M of 5-HT. Operating potential, 0.5 V vs. Ag/AgCl [137]. Copyright 2016, Nature. Thus, in vivo measurements have been demonstrated for a number of neurotransmitters using various nanomaterials, showing that (i) monitoring of chemical changes within living tissue can be achieved; (ii) this may be useful for the diagnosis of diseases; and (iii) there is potential for control and manipulation of chemical changes within living tissue. The effective in vivo analysis of neurotransmitters will have significant applications, not only for medical diagnostics and therapeutics, but also for the investigation of the specific effects of drug molecules in animal models.