Permanganate-based synthesis of manganese oxide nanoparticles in ferritin

This paper investigates the comproportionation reaction of Mn^II with ${{MnO}_4}^{-}$ as a route for manganese oxide nanoparticle synthesis in the protein ferritin. We report that ${{MnO}_4}^{-}$ serves as the electron acceptor and reacts with Mn^II in the presence of apoferritin to form manganese oxide cores inside the protein shell. Manganese loading into ferritin was studied under acidic, neutral, and basic conditions and the ratios of Mn^II and permanganate were varied at each pH. The manganese-containing ferritin samples were characterized by transmission electron microscopy, UV/Vis absorption, and by measuring the band gap energies for each sample. Manganese cores were deposited inside ferritin under both the acidic and basic conditions. All resulting manganese ferritin samples were found to be indirect band gap materials with band gap energies ranging from 1.01 to 1.34 eV. An increased UV/Vis absorption around 370 nm was observed for samples formed under acidic conditions, suggestive of MnO₂ formation inside ferritin. (paper)