What Is the Dopaminergic System?

Dopamine is the most abundant catecholamine neurotransmitter in the brain. Dopamine as a neurotransmitter regulates many physiological functions of the central nervous system. Dopamine system dysregulation involves the occurrence of Parkinson's disease, schizophrenia, Tourette syndrome, attention deficit hyperactivity syndrome, and pituitary tumors. ^[1]

Dopamine (DA, or 3-hydroxytyramine, 3,4-dihydroxyphenethylamine) is an endogenous nitrogen-containing organic compound, which is a tyrosine (aromatic amino acid) that undergoes dihydroxyphenylalanine during metabolism. The intermediate produced. ^[3]

When the dopaminergic pathway is low, it causes a lack of pleasure (cannot experience pleasure), emotional indifference (lack of motivation), and poor mood (feeling dissatisfied, unhappy, painful, irritable, and nervous). ^[7]

Dopamine preparation method

Dopamine is used as the reaction monomer. Under basic conditions (pH> 7.5), oxygen is used as the oxidant. The reaction process does not require complicated experimental equipment and steps. When dopamine (usually dopamine hydrochloride is used) is protected from light and aerobic alkaline conditions. Then, polymerization can occur. During the polymerization, the color of the solution will gradually change over time, and the color changes from colorless to brown and finally to black. Under basic conditions, dopamine monomer catechol can be deprotonated to form dopamine quinone. However, the structure of dopamine quinone is unstable and will continue to be oxidized. Intramolecular rearrangement and cross-linking will occur to form dark brown polydopamine. If polydopamine is coated on a different substrate surface, simply place the substrate in a reaction vessel and shake gently. After a certain reaction time, a uniform and dense polydopamine coating can be formed on the substrate surface. The concentration of dopamine monomer is controlled above 2g / L. The thickness of the formed polydopamine film can be determined by the length of the reaction time and the concentration of the dopamine monomer. Studies have shown that the maximum thickness of the polydopamine film is 50 nm, and the concentration of the dopamine monomer and the reaction time will not affect the thickness of the film after exceeding a certain threshold. At present, in addition to the solution oxidation method, there is also an enzyme oxidation process, which uses enzymes to catalyze polymer synthesis. ^[8]

Application of dopamine polydopamine

(1) Structural characteristics of polydopamine

Under different reaction conditions, the microstructure of polydopamine is also different. Although they are all melanin materials, through microscopic observation of electron microscopy, polydopamine (PDA) can have different shapes and different structures such as nanobelts and nanospheres. Polydopamine materials have different adsorption capacities for pollutants with different specific surface areas. This requires reasonable material selection and preparation according to actual wastewater treatment standards. Polydopamine has good stability, strong water dispersion, and excellent biocompatibility. Can be generated at room temperature. Polydopamine micro / nanospheres with different structural characteristics can be prepared by controlling the oxidative self-polymerization reaction conditions such as the initial concentration of dopamine, the type of buffer solution, and the reaction time. Polydopamine micro / nanospheres contain a large number of amino and phenolic hydroxyl groups. Because amino and phenolic hydroxyl groups can be combined with metal ions, polydopamine micro / nanospheres can also be used to treat heavy metal ions in water pollution. In addition, polydopamine micro / nanospheres can be used as reducing agents for metal ions to prepare metal nanomaterials. ^[8]

(2) Modification of membrane by polydopamine

Developing a separation membrane with high surface energy and hydrophilicity to treat pollutants in wastewater is one of the directions that scientists are committed to in the field of membrane technology. Experiments show that dopamine can undergo oxidative polymerization on the surface of various substrates under basic aerobic and dark conditions, forming a dense composite film with strong adhesion, and grafting each surface of the polydopamine-coated film. A modified molecule or polymer that is conducive to the specific adsorption of target pollutants can realize the functionalization of the composite membrane. ^[8]

This thin-layer composite nanofiltration membrane has good application prospects in seawater desalination. The use of membranes to adsorb pollutants in the water environment is also an adsorption method. The preparation of various composite membranes to treat wastewater is also one of the research directions of environmental scientists. Polydopamine can be used not only as a graft directly, but also as a biological Glue to connect chemicals that do not easily adhere to the membrane. ^[8]

(3) Surface modification of nanomaterials by polydopamine

In addition to modifying various solid substrate materials and membrane materials, polydopamine can also modify nanomaterials. For nanomaterials, dopamine can be used for ordinary coating or secondary functional modification. Nanomaterials, as a new type of adsorbent in the environmental field, have many advantages. Coating polydopamine on the surface of nanomaterials can realize the greater application ability of such adsorbents in environmental governance. ^[8]

Polydopamine (PDA) can be coated on a variety of substrates by auto-oxidative polymerization of dopamine (DA). Because dopamine has adhesion, reduction, and biocompatibility, dopamine and its derived composite materials are gradually used in various fields. Among them, in the environmental field, dopamine can be used in membrane technology and pollutant adsorption. ^[8]

Traditional membrane technology and adsorption have higher requirements for the substrate to be modified, and may cause secondary pollution to the environment during the modification process. However, the functionalization of dopamine will avoid this situation and the entire preparation process will also It is a green, environmentally friendly, easy to implement, and stable function. By modifying the surface of substrates such as solids, membranes, or nanomaterials, a new multifunctional material is prepared and used in water treatment, which greatly improves the adsorption performance of the adsorbent. There are many successful examples today to prove the importance of this material in environmental adsorption, and people are still exploring its innovative applications. ^[8]

Many domestic research achievements have been made on the application of dopamine in water environment treatment in China, but it is only staying in the basic research stage, and the application of dopamine in actual wastewater treatment and its industrial production is not very extensive. Dopamine can have unexpected effects on humans in the environmental field. At home and abroad, although dopamine has been widely used in the functionalization of various materials, its polymerization mechanism cannot be accurately grasped, and it remains at the exploratory stage. Further studies on its reaction kinetics and structure of intermediates are needed to expand Its application fields. ^[8]