# Nanomaterials-Based Electrochemical Sensors for
## 2 In Vitro Detection of Typical Neurotransmitters
At a pH of 7.0 and 30 \({}^{\circ}\)C, the ChO hydrolyzed Ach and generated H\({}_{2}\)O\({}_{2}\), which was indirectly detected using amperometry with an optimal response time at 3 s, with a working linear range from 0.005 to 400 uM and detection limit of 0.005 uM for Ach. This electrode was tested using the blood serum of normal and Alzheimer's patients [94]. The enzyme-coated electrode lost 50% of its initial activity over a period of 3 months. Furthermore,AchE-MWCNT-Fe\({}_{3}\)O\({}_{4}\) nanoparticles-chitosan was demonstrated for the rapid detection of Ach. The fabricated electrode exhibited pH- and temperature-dependent sensing performance with a LOD of 0.6 nM [95]. The detection of ACh was also done using AChE-Cho immobilized polypyrrole-polyvinylsulphonate-based enzymatic electrodes, where the detection limit of the sensor was 5.0 nM. In addition, the storage and operational stability were studied for 18 continuous assays and retained an 83.1% current response. The sensitivity of the developed sensor decreased with the increasing of the film thickness and the increasing of the enzyme concentration, because the film thickness and enzyme concentration were proportional to the conductivity of the modified films [96]. The analysis of neurotransmitters requires that the detection limit be low enough to measure the smallest concentration that might exist in an organism, as well as a linear range that encapsulates the typical concentrations that exist in the organism in question. This means that a lower detection limit and a wide linear range are the most beneficial when comparing different nanomaterials. The disparity between the linear range and the detection limit of different nanomaterials may be due to the rate at which the electron transfer takes place, the accessibility of the neurotransmitter on the electrode surface (i.e., whether or not more molecules can travel into the surface to allow for more oxidation events), or the affinity of the neurotransmitter to the electrode surface. In addition, different applied potentials may be required for different nanomaterials, as seen in Table 1. Overall, ACh detection was performed based on the sensing of enzymatically produced hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). Most of the reported biosensors for ACh involved multiple steps for the immobilization and functionalization of the electrodes, which are time consuming and tedious processes. Moreover, the stability of the biosensors was limited due to the enzyme immobilization, which is sensitive to pH and temperature. Therefore, enzyme-free nanomaterials-based electrochemical sensors need to be explored for the detection of ACh with high sensitivity, high stability, and a stable response. _Purines or adenosine 5\({}^{\prime}\)-triphosphate (ATP)_ is an important neurotransmitter, which is present in peripheral tissues and serves as a neurotransmitter and neuromodulator in the perivascular nerves. It has a role in physiological and pathophysiological factors such as the age, type, species, size, tone, pressure, and temperature of the blood vessels. It also combines with NE to become a predominant neurotransmitter at high blood pressures [97]. The oxidation of ATP follows a simple cleavage of the bond between the phosphorous and oxygen of the phosphate groups. This cleavage releases two electrons to form adenosine 5\({}^{\prime}\)-diphosphate (ADP). A target-responsive electrochemical aptamer switch (TREAS) was functionalized on a gold surface that was dually labeled with 3\({}^{\prime}\)-SH and 5\({}^{\prime}\) ferrocene for the detection of ATP among guanosine triphosphate, cytidine triphosphate, and uridine triphosphate interferents. In SWV, the peak current increased only with the addition of ATP (10-1 mM) at a potential of 0.26 V [98]. The simultaneous quantification of potassium ions and ATP was carried out using a nanochannel-based electrochemical method. A Pt-porous aluminum analog of ion channels detected the concentrations of K\({}^{+}\) and ATP in the ranges of 0.05-10.0 mM and 0.0005-1.0 mM, respectively.