Studies of electrochemically transformed ferritin adsorbed at tin-doped indium oxide electrodes using X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to (1) confirm the presence of ferritin on tin-doped indium. oxide (ITO) as suggested by voltammetry, (2) determine whether ferritin desorbs upon reduction, (3) verify whether iron is released when ferritin is reduced in the presence of an iron-chelating agent, and (4) ascertain whether the protein-iron complex could be electrochemically reconstituted. The N 1s signal increases dramatically after the ITO electrode is exposed to ferritin. Substantial decreases in the Fe/N peak ratio support the hypothesis that iron release can be electrochemically induced by reducing ferritin in the presence of an iron-chelating agent. These data reveal that ferritin adsorbs onto ITO from solution at controlled electrode potential and that ferritin can be electrochemically induced to release iron without the need for homogeneous reducing agents. However, following the exposure of an ITO/ferritin ("emptied") to ferrous ion at positive electrode potential, the XPS spectrum reveals a substantial increase in the iron signal and a decrease in substrate signals. These results indicate that iron deposits directly onto the ITO substrate, casting doubt on the previous conclusion that adsorbed ferritin can be electrochemically reconstituted.