Electrostatic modification of ferritin onto polypeptide-functionalized indium oxide electrode surfaces: Electrochemical and AFM studies

We demonstrated the direct electron transfer reaction of ferritin immobilized onto polypeptide-functionalized indium oxide electrodes using electrostatic interactions between polypeptides and ferritin. Polypeptides such as poly(l-lysine) and poly(l-arginine) were strongly adsorbed onto indium oxide electrode surfaces by electrostatic interactions. The modification of electrode surfaces with poly(l-lysine) was achieved by immersing indium oxide electrode into 1 mg ml−1 poly(l-lysine) (molecular weight: 80,000 and 84,000 Da) for approximating 10 min. Ferritin molecules were fully immobilized onto poly(l-lysine)-functionalized electrodes at immersion times approximating 30 min. After accounting for the roughness for the electrode surface, the surface coverage of ferritin on the functionalized-indium oxide electrode was evaluated to be 9–13 × 1011 molecules cm−2, which indicates that ferritin molecules were densely packed like a full monolayer. Ferritin immobilized onto functionalized-electrodes showed the direct electron transfer reaction with the electrode. Potential value dependence of redox peaks on ferritin immersion times was not observed. Poly(l-arginine) (molecular weight: 94,000 Da) also acted as a modifier for immobilization of ferritin by electrostatic interactions. The electrochemical behavior of ferritin immobilized onto poly(l-ariginine)-functionalized electrodes was similar to that observed in poly(l-lysine)-functionalized systems. We obtained direct evidence for electrostatic interactions between ferritin molecules and poly(l-lysine) by tapping-mode AFM measurements; molecular ferritin binding to poly(l-lysine) molecular wires was observed.