Uptake of 2, 6, and 15 nm AuNPs by MCF-7 monolayer cells. (a) Bright field images of cells after treatment with 1 nM gold nanoparticles for 24 h. (b) Quantitative ICP-MS measurement of AuNP uptake by cells treated as in (a). (c) Percentage of gold nanoparticles localized in the nucleus compared to the whole cell after treatment with 2 and 6 nm Au nanoparticles at 1 nM. (d) TEM images of cells treated with 1 nM nanoparticles for 24 h. Red arrows indicate the gold nanoparticles. Boxed regions are enlarged in the adjacent panels.