The direct electrochemistry of ferritin compared with the direct electrochemistry of nanoparticulate hydrous ferric oxide

Horse spleen ferritin is a naturally occurring iron storage protein, consisting of a protein shell encapsulating a hydrous ferric oxide core about 8 nm in diameter. It is known from prior work that the protein can be adsorbed onto the surface of tin-doped indium oxide (ITO) electrodes, where it undergoes voltammetric reduction at about –0.6 V vs Ag/AgCl. This is accompanied by dissolution of Fe(II) through channels in the protein shell. In the present work, it is demonstrated that a pre-wave at about –0.4 V vs Ag/AgCl is due to the reduction of FePO4 also present inside the protein shell. In order to prove that the pre-wave was due to the reduction of FePO4, it was first necessary to prepare 8 nm diameter hydrous ferric oxide nanoparticles without protein shells, adsorb them onto ITO electrodes, and then study their electrochemistry. Having achieved that, it was then necessary to establish that their behaviour was analogous to that of ferritin. This was achieved in several ways, but principally by noting that the same electrochemical reduction reactions occurred at negative potentials, accompanied by the dissolution of Fe(II). Finally, by switching to aqueous phosphate buffer, the pre-wave could be unambiguously identified as the reduction of FePO4 present as a thin layer on the hydrous ferric oxide nanoparticle surfaces. Although the bare and protein-coated hydrous ferric oxide nanoparticles were found to behave identically toward electrochemical reduction, they nevertheless reacted very differently towards H2O2. The bare nanoparticles acted as potent electrocatalysts for both the oxidation and the reduction of H2O2, whereas the horse spleen ferritin had a much lesser effect. It seems likely therefore that the protein shell in ferritin blocks the formation of key intermediates in hydrogen peroxide decomposition.