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Molecules 2020, 25, 3191 12 of 24 with that of prepared silver nanoparticles. In general, contradictory results can be found in literature. For example, Ahn et al. [123] determined antioxidant capacity of thirty plant extracts and AgNPs prepared by using them with the conclusion that the scavenging activity using DPPH assay increased with higher amounts of either extract or AgNPs. In general, the extracts showed better scavenging activity than the AgNPs (Figure 2a). Authors explained this phenomenon by the role of phytochemicals. An extract with high scavenging activity leads to the rapid formation of small AgNPs seeds, which grow into larger nanoparticles with assistance of phytochemicals presented in the matrix [123]. A reduced antioxidant capacity of AgNPs in comparison with extract was also observed by Demirbas [125]. They prepared silver nanoparticles by biological method using extract of red cabbage (Brassica oleracea var. capitate f. rubra). The authors proposed that AgNPs were synthesized using antioxidant power of red cabbage extract to reduce Ag+ ions, so AgNPs may promote superoxide radicals which would consume antioxidant capacity of red cabbage [125]. Lower values of silver nanoparticles (prepared by Aesculus hippocastanum) antioxidant capacity was also obtained by Küp [126] who used three different method of scavenging activity determination—DPPH, total reducing power, and superoxide anion radical scavenging assay. The results of DPPH and superoxide anion radical scavenging assays lead to reduced antioxidant capacity of prepared nanoparticles in comparison to plant extract, but the total reducing power of AgNPs indicated more reducing activity than plant extract. The relevant explanation is missing, but authors rely on the fact that reducing power may serve as a major sign of potential antioxidant activity because of the ability of reduction of the Fe3+ ferricyanide complex [126]. On the other hand, nanoparticles prepared by Zarrabi et al. [124] showed higher scavenging activity of free radicals by DPPH method than walnut (Juglans regia) extract (Figure 2b). They speculated that improved antioxidant properties of AgNPs are due to the simultaneous activity of polyphenols as antioxidant agents and AgNPs as a catalyst [124]. Similar results were noticed by study Elemike et al. [127], who prepared Ag nanoparticles by Costus afer. AgNPs showed greater antioxidant capacity than the leaf extract and their activity was comparable to that of ascorbic acid [127]. The present phytochemicals (flavonoids) and silver ions could serve as antioxidants through single electron and hydrogen atom transfer [127]. Other theory was presented by Vijayan et al. [128], who indicate that the increased antioxidant properties of nanoparticles compared to extract can be attributed to the adsorption of bioactive compounds of leaf extract over spherically shaped nanoparticles. Analogically, AgNPs prepared by extract of Melia azedarach exhibited higher antioxidant capacity according to extract [129]. According to the authors, the antioxidant ability of AgNPs is caused by the presence of phenolic compounds, terpenoids, and flavonoids in plants which allow nanoparticles to act as singlet oxygen quenchers, hydrogen donors, and reducing agents [129]. An interesting study was published by Das and his co-workers, who studied the antioxidant properties of two types of biosynthesized AgNPs by sweet potatoes Ipomoea batatas (L.) [138]. They compared three assays—ABTS, DPPH, and NOx (nitrite/nitrate oxide) at variety of concentrations. The ABTS scavenging activity proceeded in the range of 3.98–12.28%, for DPPH it was 26.30–46.53%, and for NOx the values were 4.30–12.94%. The moderate scavenging effect might arise from the interference of several functional groups presented in extract, which play an important role in the capping and stabilizing on AgNPs. The scavenging activity against DPPH radical is higher than that against ABTS and NOx. Lower values of antioxidant capacity against NOx were explained by the difference in the reaction mechanism [138]. Comparison of ABTS and DPPH assay was also published by Otunola [139]. In that study, AgNPs were prepared by garlic (Allium sativum), ginger (Zingiber officinale), and cayenne pepper (Capsicum frutescens) extracts. The AgNPs exhibited potential antioxidant properties against both radicals expressed as IC50 values. The highest activity against ABTS was evidenced for cayenne pepper (IC50 was 31.25 μg/mL), whereas the highest value of IC50 (<3 1.25 μg/mL) against DPPH radicals was observed for garlic [139].PDF Image | Antioxidant Activity Determination of Silver Nanoparticles
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