Green Synthesis of Gold Nano-bioconjugates from Onion Peel Extract and Evaluation of Their Antioxidant, Anti-inflammatory, and Cytotoxic Studies.

Plant secondary metabolites such as flavonoids demonstrate high degrees of antioxidant, anti-inflammatory, and anticancer activities. Among flavonoids, quercetin plays an important role in inflammation by downregulating the level of various cytokines. Thereby, in this work, onion () peel was successfully utilized for the synthesis of gold nano-bioconjugates acting as a natural therapeutic drug. In this process, crude onion peel extract was first divided into different fractionates, namely, ethyl acetate, butanol, methanol, and water, and they were subjected to various preliminary studies of antioxidant activities. The ethyl acetate fractionate shows high antioxidant activities in all the assays. The bioactive components were identified and found to contain a high amount of quercetin as confirmed by liquid chromatography with tandem mass spectrometry and high-performance liquid chromatogrpahy. Three gold nano-bioconjugates were prepared with different concentrations of the ethyl acetate fractionate. Various biochemical anti-inflammatory assays were carried out and compared with the active ethyl acetate fraction of the onion peel drug (OPD). The cytotoxicity of the nano-bioconjugate system and the OPD was checked in the myoblast L6 cell line from skeletal muscle tissues to evaluate the toxicity. All the three nano-bioconjugates A, B, and E demonstrated high percentages of cell viability, viz., 73.07, 72.3, and 69.15%, respectively, at their highest concentration of 200 μg/mL. The OPD also showed 88.56% cell viability with no toxic effects in the myoblast L6 cell line from skeletal muscle tissues. The reactive oxygen species reduction of nano-bioconjugate B showed a marked reduction of 76.77% at a maximum concentration of 200 μg/mL, whereas the OPD showed 68.17%. Hence, through this work, a cheap source of nano-bioconjugates is developed, which can act as a potent antioxidant and anti-inflammatory agent and are more active in comparison to the OPD alone.