Size-dependent localization and quantitative evaluation of the intracellular migration of silica nanoparticles in Caco-2 cells

Localization and intracellular migration of 32 and 83 nm SiO2nanoparticles in relation to the nucleus was evaluated in vitro on undifferentiated human colon carcinoma (Caco-2) cells. The fluorescence dye Atto647N was incorporated into the particles, which enabled detection by high resolution, nondiffraction limited stimulated emission depletion (STED) microscopy. The distribution and agglomeration of nanoparticles was measured with STED microscopy after 5, 24, 48, and 72 h. Analyses revealed that only 32 nm silica particles penetrated into the nucleus of Caco-2 cells upon exposure for 24 h. Here, they formed agglomerates up to 300 nm after 72 h of incubation. Quantitative evaluation of the migration of 32 nm compared to 83 nm particles demonstrated that 32 nm particles obviously migrated faster into and through the cell in the beginning (5 h time point). The presence and agglomeration inside the cells and the penetration into the nucleus were considered to potentially activate cytotoxic responses. Therefore, the cytotoxic (WST-1 assay) and genotoxic (comet assay) effects of the silica nanoparticles were evaluated. Even though 32 nm silica particles are penetrating into the nucleus, neither cytotoxic nor genotoxic effects were detected for either particle size.