72 mixture was determined at 280 nm by Ultraviolet Visible Spectrophotometer (UV–vis). 1 mL of 5% solution of K4[Fe(CN)6] in H2O was added, and the absorbance of samples was read after 10 min at 690 nm. A standard curve of an aqueous FeCl3 6H2O solution was treated in the same conditions to quantify the amount of cellular uptake of MNPs. The cellular uptake of MNPs was expressed at the amount of Fe2O3 (weight, lg) per milligram of protein. TEM Analysis The SMC incubated with DMSA-MNPs for 24 h were washed with PBS and ﬁxed in 4% glutaraldehyde solution Nanoscale Res Lett (2009) 4:70–77 for 1 h at 4 C. The cells were postﬁxed in 1% osmium tetroxide for 1 h at room temperature and washed. Then, cells were scraped and concentrated in 2.5% agar in 0.05 M cacodylate buffer. The obtained samples were then treated with 2% uranyl acetate solution for 1 h and sub- sequently dehydrated by means of ethanol/water solutions, with increasing ethanol content and embedded in epoxy resin [27]. The samples were cut at 70 nm (ultrathin sec- tions) with an ultramicrotome. Ultrathin sections were transferred to the 300 mesh copper grid and stained with 5% uranyl acetate. The copper grid was observed on a transmission electron microscope (TEM, HITACHIH-600) at 80 kV. Fig. 1 The TEM image and size distribution of nanoparticles. a DMSA-MNPs, b APTS-MNPs, and c GLU- MNPs 123