The cellular TEM images were obtained using a JEOL 100S electron microscope, and the samples were prepared according to the previous report.[11 (link)] Briefly, MCF-7 cells were seeded and incubated on 15-mm-diameter Theramanox coverslips (Nalge Nunc International), and placed in 24-well plates (100,000 cells per well) with 1 ml of serum containing media for 24 h before the experiment. The media was replaced by NPs in serum containing media and incubated for 3 h. The media was exchanged with new media, and the MCF7 cells were washed three times with PBS buffer. The cells were then fixed in 2 % glutaraldehyde with 3.75% sucrose in 0.1 M sodium phosphate buffer (pH 7.0) for 30 min and washed with 0.1 M PBS containing 3.75% sucrose, three times over 30 min. The fixed cells were postfixed in 1% osmium tetroxide with 5% sucrose in 0.05 M sodium phosphate buffer solution (pH 7.0) for 1 hour and then washed with DI water three times. They were dehydrated in a graded series of acetone (10% steps) and embedded in epoxy resin. The resin was polymerized at 70 °C for 12 h. Ultrathin sections (70 nm) were obtained with a Reichert Ultracut E Ultramicrotome then imaged under a JEOL 100S electron microscope.