This work demonstrated that ultrasmall
gold nanoparticles (AuNPs) smaller than 10 nm display unique advantages over nanoparticles larger than 10 nm in terms of localization to, and penetration of,
breast cancer cells, multicellular
tumor spheroids, and
tumors in mice. Au@
tiopronin nanoparticles that have tunable sizes from 2 to 15 nm with identical surface coatings of
tiopronin and charge were successfully prepared. For monolayer cells, the smaller the Au@
tiopronin NPs, the more AuNPs found in each cell. In addition, the accumulation of Au NPs in the ex vivo
tumor model was size-dependent: smaller AuNPs were able to penetrate deeply into
tumor spheroids, whereas 15 nm nanoparticles were not. Owing to their ultrasmall nanostructure, 2 and 6 nm nanoparticles showed high levels of accumulation in
tumor tissue in mice after a single
intravenous injection. Surprisingly, both 2 and 6 nm Au@
tiopronin nanoparticles were distributed throughout the cytoplasm and nucleus of
cancer cells in vitro and in vivo, whereas 15 nm Au@
tiopronin nanoparticles were found only in the cytoplasm, where they formed aggregates. The ex vivo multicellular spheroid proved to be a good model to simulate in vivo
tumor tissue and evaluate nanoparticle penetration behavior. This work gives important insights into the design and functionalization of nanoparticles to achieve high levels of accumulation in
tumors.