Owing to their chemical tunability and extraordinary electronic, magnetic, and optical properties, functional oxide nanoparticles have been widely proposed for biomedical applications, such as drug deliveries, brain stimulation and magnetic hyperthermia therapy. Besides their functionalities, however, understanding the behavior of the oxide nanoparticles after injected into body is utmost important as it is directly associated with health issues. Especially, as the particles degrade over time, the interactions between the degradation products and cells and proteins need to be clarified. It has been proven that the iron and cobalt containing nanoparticles, such as iron oxide or cobalt ferrite, can be degraded and captured intracellularly by the endogenous protein, i.e. ferritin. Hitherto, only the local technique, Transmission Electron Microscopy (TEM) analysis, has been employed for the direct observation of Fe, Co ion uptake by ferritin [1,2]. Here, we have performed Electrochemical Impedance Spectroscopy (EIS) measurements on iron and cobalt containing nanoparticles (CoFe2O4 and BiFeO3-CoFe2O4 core-shell nanoparticles) under different biological environments to observe the degradation behavior of the nanoparticles. The impedance measurement showed that the resistance of the nanoparticles gradually reduces as the ferritin captures iron and cobalt ions.
