Cellular Uptake, Stability, and Safety of Hollow Carbon Sphere-Protected Fe₃O₄ Nanoparticles

Magnetic iron oxide (Fe₃O₄) nanoparticles (NPs) have attracted extensive attentions in biomedical fields such as magnetic resonance imaging (MRI). However, the instability and unfavorable dispersity of bare Fe₃O₄ NPs is a challenge for biomedical applications. Herein, we proposed a strategy using hollow carbon sphere (HCS) as a shell structure to endow Fe₃O₄ NPs better stability, dispersity, as well as biocompatibility. To verify intracellular behaviors and biosafety of HCSdecorated Fe₃O₄ nanoparticles (Fe₃O₄@HCS NPs), the assessment of cellular effects of these NPs based on synchrotron radiation-based techniques were done to explore detailed interaction between Fe₃O₄@HCS NPs and liver cells, HepG2. We found that a large number of NPs were internalized by cells in a time-dependent manner determined by inductively coupled plasma mass spectrometry (ICP-MS), which was further supported by intracellular accumulation of iron via X-ray fluorescence (XRF) imaging. Moreover, confocal imaging showed that these NPs mainly located in the lysosomes where they remained stable and undissolved within 72 hours, which was verified by chemical form characterization of iron via Fe K-edge X-ray adsorption near edge structure (XANES). With the coating shell of HCS, the release of iron ions was prevented even in acidic lysosome and the integrity of lysosomal membrane remained unchanged during the storage of NPs. As a result, Fe₃O₄@HCS NPs exhibited low level of oxidative stress and induced negligible cytotoxicity towards HepG2 cells. Based on the powerful techniques, we demonstrated that the carbon outer layer provides a physical barrier that helps remain excellent properties of magnetic Fe₃O₄ NPs and good dispersity, chemical stability, as well as biocompatibility for potential applications in biomedical fields.