Identification of Ferrous-Ferric Fe₃O₄ Nanoparticles in Recombinant Human Ferritin Cages

Abstract Recombinant ferritin is an excellent template for the synthesis of magnetic nanoparticles. This paper describes carefully performed experiments both to identify ironoxides within nanoparticles and to measure the number of iron atoms in the cores of recombinant human H-chain ferritin (HFn), based on spectroscopy techniques. Using electron energy-loss spectroscopy (EELS) analysis, magnetite (Fe₃O₄) has been unequivocally identified as the ironoxide formed within HFn cores under special preparation conditions. Atom counting analysis by EELS and high-angle annular dark-field imaging further allowed the correlation of the particle sizes to the real Fe atom numbers in a quantitative manner. These results help clarify some structural confusion between magnetite and maghemite (γ-Fe₂O₃), and also provide standard data for the number of Fe atoms within Fe₃O₄ particles of a given size, whose use is not limited to cases of magnetite synthesized in the cores of recombinant human ferritin.