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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 (H 2 O 2 ) at 50 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 Co 3 O 4 , 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 Co 3 O 4 nanoparticles.Nanoparticles (NPs) are attracting researchers' interest as the key material of nanotechnology. NPs are already used or expected to be used in many fields, such as magnetic recording materials, catalytic materials, fluorescent markers, drug delivery systems, and quantum electronics. Especially, conductive or semiconductor NPs are anticipated to be good quantum electronics device components. For this application, the size of the NP is critical, because the electron energy levels are strongly dependent on the NP size. Consequently, homogeneous NPs are required to satisfy this requirement. It has thus been proposed that the cavity of the cage-shaped protein, apoferritin, should be used for the NP synthesis. Since the protein molecules have an atomically identical structure, the NP synthesized in the cavity should have the same diameter.Apoferritin is an iron storage cage-shaped protein which is present in many biological species. When iron ions are sufficient, iron atoms are stored in the cavity as an iron hydroxide core in vivo. 1,2 Figure 1 is a schematic drawing of the apoferritin molecule. The outer and inner diameters are approximately 12 and 7 nm, respectively. There are narrow channels along the three-fold axis connecting the cavity and outside, through which iron ions penetrate the cavity. The protein shell is composed of 24 subunits and there are two types of subunits, called L-subunit and H-subunit. Only H-subunit has the ferroxidase site, which oxidizes the iron(II) ion and produces the iron(III) hydroxide cores. 1,2 Meanwhile, L-subunits play an important role for the ferritin molecule to self-assemble into a two-dimensional crystal at the air-water interface with a salt-bridge interaction. 3,4 Making use of the same size nanodot core of the apoferritin and the self-assemble ability of the protein shell to make a two-dimensional crystal, together with the selective elimination of the labile protein moiety, we fabricated a nanodot array for a floating ga...