7.3.4. Neurotransmitters

Neurotransmitters play a key role in acting as mediators of the autonomic system in the human body. Detection of biological abnormalities related to neurotransmitters (their concentration or metabolites) in the biological fluid is of fundamental importance in medical diagnostics. Differences in the level of neurotransmitters may be related to the occurrence of various diseases substrates such as schizophrenia, Parkinson's disease, Alzheimer's disease, Huntington's chorea, adrenocortical cancer and other cancers and depression. Monitoring the concentration and products of the neurotransmitters synthesis pathway is a promising strategy for early detection and thus preventing the development of these diseases. Table 6 shows an exemplary strategies based on QDs for neurotransmitters determination [88–92].


**Table 6.** QDs-based sensors for neurotransmitters determination.

Wang et al. [89] described a fluorescence assay for the fluorometric determination of dopamine (DA). It is based on the use of silica-coated CdTe quantum dots (QD@SiO2). When dopamine is added to a solution of the QD@SiO2 and then oxidized by oxygen under the catalytic action of tyrosinase to form dopamine quinone, the fluorescence of QD@SiO2 decreases, due to an electron transfer quenching process. Linear relationship over the range from 0.05 to 30 μM DA and high LOD of 12.5 nM. This suggested that the novel assay provided a promising possibility for further utilizing as an efficient platform for measuring DA in biological and environmental applications.

Another example of dopamine detection prepared by Hun et al. described photoelectrochemical sensor for dopamine which yields a signal upon irradiation with visible light. The electrons of SnSe QDs were excited under irradiation with visible light and transformed from valence band to conduction band. Dopamine, as an electron donor, provided the electrons to SnSe QDs. As a result, the enhanced photocurrent was obtained. This sensing system responds linearly to DA in the 0.01 μM to 10 μM concentration range and with a 3 nM LOD [90].

A novel molecular imprinted sensor based on CdTe@SiO2 QDs has been developed by Wei et al., for norepinephrine (NE) recognition. The synthesized nanosensor had a distinguished selectivity and high binding affinity to NE. Under optimal conditions, the relative fluorescence intensity of CdTe@SiO2@Molecular-imprinted-polymer linearly decreased with an increase in the concentration of NE in the range of 0.04–10 μM. The LOD was calculated as 8 nM [91].

A very promising alternative for construction of new micro-devices is LTCC (Low Temperature Co-fired Ceramic [93]) technology, consisting in the creation of three-dimensional structures of electronic systems based on pressed and co-poured ceramic foils with printed functional layers [94,95]. This method was used by Baluta et al. They proposed a convenient fluorescence dopamine-sensing strategy based on polydopamine formed on the surface of graphene quantum dots (GQDs). This sensing system utilized the catalytic oxidation of DA to dopamine-o-quinone (DOQ), and then to poly(DA), which can selectively quench the strong luminescence of GQDs due to FRET. Such constructed biosensor exhibited a broad linear range from 1 μM up to 200 μM with LOD 80 nM [96].
