Internalization of gold nanoparticles in HDMEC and hCMEC/D3 analyzed by transmission electron microscopy. HDMEC (A-C) and hCMEC/D3 (D-F) were incubated with 300 μM gold nanoparticles for 24 hours. After exposure, cells were extensively washed, fixed with paraformaldehyde and examined by transmission electron microscopy (TEM). AuS0302-RIT, AuS0302-RIS02 and AuS0302-RIS04 were found in intracellular vesicles which were mostly located in the perinuclear region. The arrow heads indicate the gold nanoparticles within the vesicles. Scale bar: 1 μm.