# Antioxidant Properties of Selenium Nanoparticles Synthesized Using Tea and Herb Water Extracts
## 3 Results and Discussion
### Antioxidant Activity of Obtained SeNPs
At the same time, a drastic decrease in the content of polyphenols in the samples was observed after SeNPs synthesis. This suggests a significant participation of these compounds in the synthesis reaction. With the increase in the concentration of the extract used, the content of polyphenolic compounds in the reaction mixture increases, which is in line with expectations, as these compounds come from extracts. However, with an increase in the concentration of selenium salts used for the synthesis and constant extract concentration, no clear downward trend is observed. In the case of herb extracts used for synthesis, increasing Se concentration leads to the formation of SeNPs with increasing reductive properties, while this trend is reversed with tea infusions. Polyphenolic compounds contain various functional groups able to perform nanoparticle formation. It has been postulated that the tautomeric transformation of flavonoids from the enol to the keto-form may release a reactive hydrogen atom that can reduce metal ions to form NPs [53]. Moreover, some flavonoids have the ability to chelate metal ions with their carbonyl groups or \(\pi\)-electrons. One of the flavonoids with very strong chelating activity is quercetin, which was detected in green tea infusion. These mechanisms may explain the ability of flavonoids to be adsorbed onto the NPs surface. It is postulated that they are involved, in addition to the reduction stage, also in the initiation stage of NPs formation and their further aggregation. The plant extract that is used in the green synthesis of SeNPs is a mixture of various compounds of plant origin, not only polyphenolic compounds. Thus, the obtained SeNPs are the result of the activity of various types of compounds. The example is ascorbic acid, which was detected in all used infusions, but after the synthesis reaction, its concentration in the post-reaction mixture was below LOD (0.01 mg L\({}^{-1}\)). Ascorbic acid is widely used in chemical synthesis of SeNPs as selenium reductor; therefore, it can be assumed that it fulfills the same role in this case. The concentration of all polyphenolic compounds determined in the extracts decreased significantly. The concentrations of some of them (mainly polyphenolic acids) were below their limits of detection. In this case, it can also be suspected that they participated in the reaction of selenium reduction to the form of nanoparticles. It is easier to control reaction conditions in classical chemical synthesis of SeNPs because the type and concentration of reagents are selected at the experiment planning stage. When extracts are used, a specific plant, the weight of the material to be extracted, and the conditions of the extraction process are selected. However, the chemical composition of the infusion depends on the plant itself, its species, its origin, and the conditions of its cultivation. The use of a plant extract in the synthesis of SeNPs makes it difficult to compare the obtained results with those described in the literature. A different polyphenolic profile of plants, even of the same species but from different crops, may have a key impact on the properties of the obtained SeNPs. The authors often do not specify what caused the selection of such and not another plant. In the paper, we described the use of four plant extracts, but the synthesis was carried out using different ratios of reagents. All obtained SeNPs were characterized by a higher ability to neutralize hydroxyl radicals, but not all of them had dimensions below 100 nm, which significantly hinders their biomedical application. It this study, we used simple Lipton teas which resulted in obtaining of SeNPs, which were bigger than 100 nm. In our previous study we used much more exclusive, leaf tea [42]. In that case, we managed to obtain significantly smaller nanoparticles (the largestfrom the synthesis using green tea was 12 nm). This shows how difficult it can be to relate the results described in the literature to your own.