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Molecules 2020, 25, 3191 15 of 24 activity of green synthesized silver NPs increased with higher concentration, while that of chemically synthesized nanoparticles did not show a significant antioxidant activity. The antioxidant capacity of green-synthesized AgNPs was similar to that of plant extract. The authors explained these results by a suggestion that the presence of bioactive compounds on the surface of AgNPs is responsible for the majority of antioxidant capacity and silver nanoparticles are not contributing much [163]. DPPH method was used to determine free radical scavenging activity of silver nanoparticles prepared chemically (using sodium citrate served as reducing agent) and biologically (using extract of Datura stramonium plant as a reducing agent) in [149]. The DPPH radical scavenging ability of green and chemically prepared AgNPs was compared with that of plant extract. Better results were observed for green-synthesized silver nanoparticles [149]. Mittal [157] and his co-workers published a study dealing with the synthesis of AgNPs using Syzygium cumini extract. They observed that the reduction of silver ions and stabilization of prepared AgNPs were mainly because of the action of flavonoids. The authors evaluated the antioxidant properties by Folin-Ciocalteau’s assay for total phenolic and flavonoid content, DPPH, ABTS, and MTT methods. The total phenolic content was lower (8.1 ± 0.1%) than the total flavonoid content (81.2 ± 0.27%), so they concluded that the amount of flavonoids present in silver nanoparticles was ten times higher which was in good agreement with the isolated flavonoids from extract. The DPPH assay showed antioxidant capacity of 59% (in comparison to 87% detected by Trolox at the same concentration). On the other hand, upon using ABTS method, they observed 63% scavenging activity and when using MTT test, 61% scavenging activity at the same concentration (50 μg·mL−1) was observed [157]. Various phenolic compounds including flavonoids (quercetin, rutin, hesperidin), benzoic acids (gallic acid, protocatechuic acid sylicylic acid, benzoic acid), and cinnamic acids (caffeic acid, ferulic acid, p-coumaric acid and trans-cinnamic acid) were used for silver nanoparticles synthesis in [161]. The antioxidant capacities of structurally different phenolic compounds were evaluated. The hydroxylation of aromatic ring played an important role in the reactivity to form silver NPs. The higher degree of hydroxylation in chemical structures of phenolic compounds demonstrated the higher radical scavenging capacity and higher tendency to reduce Ag+ to AgNPs [161]. (PABA-PVA) AgNPs were subjected to the ABTS, DPPH, and H2O2 radical scavenging activities measurements in [160]. All tested nanoparticles have exhibited lower scavenging activity than ascorbic acid standard. The highest free radical scavenging activity was determined against DPPH radical followed by H2O2 and ABTS methods [160]. The expression of antioxidant activity only as a percentage of inhibition has little significance, as antioxidant activity is strongly dependent on conditions, such as solvent and concentration of radicals. In any case, it is better to express or compare the activity with a standard (ascorbic acid, trolox, gallic acid, quercetin, etc.). When comparing the activity with the extract, the choice of concentration seems to be a key factor influencing the result, as the antioxidant activity is concentration-dependent. The resulting antioxidant activity of the nanoparticles significantly depends mainly on the reducing substances in the extract bounded/capped to the surface of the nanoparticles. For the basic study of antioxidant activity, the use of various in vitro methods is a suitable choice, either with a mechanism of action such as hydrogen transfer or electron transfer. In these methods, the mechanism of antioxidant action of silver nanoparticles can be ascribed to the fact that silver can exist in two oxidation states (Ag+ and Ag2+) depending on the reaction conditions and the produced AgNPs may be able to quench free radicals by donating or accepting electrons [164]. Nanoparticles have been shown to have a more complicated mechanism of action in biological systems using in vivo activity-based methods. 5. Conclusions This brief review provided an overview on the antioxidant activity of silver nanoparticles and the methods of its measurement. Generally, there are three methods for AgNPs synthesis—biological, physical,PDF Image | Antioxidant Activity Determination of Silver Nanoparticles
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