Synthesis of Co3O4 nanoparticles using the cage-shaped protein, apoferritin

The one-pot synthesis of cobalt (Co) oxide nanoparticles in the apoferritin cavities was studied. A detailed survey revealed that the optimum conditions are mixing 3 mM Co(II) ion with 0.5 mg/mL apoferritin in 100 mM HEPES pH 8.3 buffer solution followed by the oxidation of Co(II) ion by the addition of hydrogen peroxide (H2O2) at 50 degrees C. Under these optimum conditions, Co oxide cores were formed in almost all the apoferritin cavities in a spatially selected manner. The selection of the buffer reagent was critical to synthesizing the Co oxide cores and the elevated temperature was effective for the fast synthesis. X-ray photoemission spectroscopy (XPS) and electron energy-loss spectroscopy (EELS) proved that the cores contain cobalt atoms. X-ray powder diffraction (XRD) structure study revealed the core structure as Co3O4, which was consistent with the lattice images of the cores observed by high resolution TEM. The biological process presented in this paper provides a simple and mass-producible method for producing homogenous Co3O4 nanoparticles.