pH-dependent electrochemical behavior of proteins with different isoelectric points on the nanostructured TiO₂ surface

Nanostructured materials are widely employed for facilitating electron transfer of proteins, and then constructing the third-generation biosensors. For further understanding the mechanism that nanomaterials facilitate electron transfer of proteins, nanostructured titanium dioxide (TiO₂) film was prepared as an example for investigating the interaction between proteins and electrode surfaces. The TiO₂ film was characterized by electrochemical methods, and the point of zero charge (pzc) was determined to be ∼6.27. On the other hand, cytochrome c (cyt. c) and superoxide dismutase (SOD) were raised as model proteins because they have different isoelectric points of 10.5 and 4.9, respectively. Direct electron transfer of cyt. c and SOD was achieved at the nanostructured TiO₂ surfaces at pH 7.0 and the proteins were verified to be stably immobilized onto the TiO₂ films. Furthermore, electrochemical behavior of cyt. c and SOD modified on nanostructured TiO₂ film was investigated in a series of different pH PBS solutions based on cyclic voltammetry. It was found that electron transfer of proteins strongly depended on pH of solutions, i.e., the surface charge of TiO₂ film. This observation could be ascribed to the electrostatic interaction between proteins and surfaces. This is the first report to provide the insight in the relationship between electron transfer of proteins and interaction of surfaces charge.