Magnetic nanoparticle (MNP) uptake depends on functionalization and Nucant multivalent pseudopeptide (N6L) and doxorubicin (DOX) were slowly released intracellularly. Cell uptake of the different MNP formulations (MF66, MF66-N6L, MF66-DOX or MF66-N6LDOX) in living MDA-MB-231 cells incubated with MNP at 100 μg Fe/ml for 24 h visualized on optical microscopy immediately (0 h) or 48 h post incubation (Olympus BX61 epifluorescence microscope equipped with an Olympus DP50 digital camera (Olympus, Tokyo, Japan), and processed using Photoshop CS2 software (Adobe Systems, San Jose, CA, USA)). a Live cells were imaged under bright field (MNP, brown spots) and fluorescence microcopy (DOX, red emission). i, i’ Untreated control cells. ii, ii’ Cells incubated with MF66 MNP. iii, iii’ Cells incubated with MF66-N6L. iv, iv’ Cells incubated with MF66-DOX. v, v’ Cells incubated with MF66-N6LDOX. b Internalization and uptake of MNP (blue spots) inside MDA-MB-231 cells stained with Prussian blue reaction for iron oxide detection after 0 and 48 h post incubation. i Untreated control cells. ii Cells incubated with MF66 MNP. iii Cells incubated with MF66-N6L. iv Cells incubated with MF66-DOX. v Cells incubated with MF66-N6LDOX. Scale bar = 10 μm.