Internalization of gold nanoparticles in the cell lines A549 and NCIH441. The human ATII-like cell lines A549 and NCIH441 were incubated in the presence of AuS0302-RIT and AuS0302-RIS02 nanoparticles. After 3 hr incubation, cells were fixed and analyzed by transmission electron microscopy (TEM). At 37°C, 0.3 mM AuS0302-RIT (E-F and I-J) and AuS0302-RIS02 (G-H and K-L) were taken up by the cell lines A549 and NCIH441. Gold nanoparticles were observed localized in intracellular membrane-bound vesicles and not freely dispersed in the cytoplasm. In parallel, cells were treated with ice-cold nanoparticle solutions and incubated at 4°C for 3 hr. None of the gold nanoparticles investigated was taken up by the A549 (A and B) and by the NCIH441 (C and D) cells at 4°C, as shown by TEM, suggesting that particles are internalized by active endocytosis. Size bar: 1 μm.