Cell proliferation was determined according to a previously described method31 (link) and per manufacturer’s instructions (Roche, Basel, Switzerland). Cells were incubated with AgNPs for 24 h; the BrdU labeling solution was added to the culture medium 2 h before the end of the incubation. Cells were fixed and the level of incorporated BrdU was determined using the Cell Proliferation ELISA BrdU assay kit (Roche) following the manufacturer’s instructions. Proliferation activity of the untreated cells at the time point of 0 h was considered as 100%.