Synthesizing CdSe nanoparticles by using a low concentration of cadmium ions and the apoferritin protein cage of marine pennate diatoms

•CdSe nanoparticles were synthesized in the apoferritin from a pennate diatom.•The core formation ratio of CdSe NPs is 20% by using 0.02 mM Cd ions.•The diameter of synthesized CdSe NPs is 7 nm with a narrow size distribution.•The cysteine residues of the apoferritin disturbed the synthesis of CdSe NPs.•The mechanism of CdSe NPs synthesis in the apoferritin is the all-or-nothing type.

Cadmium selenide (CdSe) nanoparticles (NPs) were first synthesized in the cavity of recombinant cage-shaped protein from marine pennate diatoms. Our constructed recombinant apoferritin (FerA-dCys) from the diatoms revealed that the removal of cysteine residues on the surface of the recombinant apoferritin enhanced the CdSe NPs synthesis in the apoferritin cavity. The CdSe NPs were synthesized in the FerA-dCys with a very low concentration of cadmium ions (0.02 mM). The core formation ratio of CdSe NPs showed more than 20%. The average diameter of synthesized CdSe NPs was 7 nm with a small size distribution. The synthesized NPs were analyzed by energy-dispersive X-ray spectroscopy (EDX) and high-resolution transmission electron microscopy (HR-TEM), and it was confirmed that the synthesized NPs in the FerA-dCys were consistent with Cd and Se ions.