Internalization mechanism and accumulation of DMSA-SPION inside cells. (A) Temperature dependence of DMSA-SPION uptake. (a, a’) Control cells. (b) Cells incubated at 4 °C for 3 h with DMSA-SPION. (b’) Cells incubated for 3 h with same nanoparticles at 37 °C. Scale bars 10 μm. (B) Subcellular localization. (a, b) Visualization of control cells and cells incubated with nanoparticles for 24 h by bright field microscopy, respectively. (a’, b’) Lysosomes labeled with LysoTracker Red probe in the same cells, respectively. (a”, b”) Overlay images of control and treated cells, respectively. Scale bar 20 μm.