Cells were grown on 13 mm Thermanox™ coverslips (catalog
number 26028, Thermanox, Nunc, Rochester, NY, USA) and
then treated for 24 hours with a corresponding concentration
of Fe@Au (ie, 10 µg/mL Fe@Au for OECM1 or 50 µg/mL
Fe@Au for CRC). Cells were then fixed with 2% glutaraldehyde
(catalog number G5882, Sigma-Aldrich) for 1 hour
at 4°C. Samples were washed twice in PBS, followed by
secondary fixation in 1% osmium tetroxide in PBS for 1 hour
at 4°C, in the absence of light. Subsequently, samples were
dehydrated in graded ethanol solutions. Cells were soaked in
30%, 50%, 70%, 80%, 90%, and 95% ethanol for 5 minutes