Physical vapor deposited zinc oxide nanoparticles for direct electron transfer of superoxide dismutase

This communication describes a novel and facile approach to direct electron transfer of copper, zinc-superoxide dismutase (Cu, Zn-SOD) at a physical vapor deposited zinc oxide (ZnO) nanoparticles surface. SOD exhibits quasi-reversible electrochemical behavior in phosphate buffer solution (PBS, pH 7.25), with apparent formal potential of 195.2 ± 4.6 mV vs. Ag|AgCl and high heterogeneous electron rate constant of 10.4 ± 1.8 s^-1. Electrochemical results indicate that SOD stably immobilizes on the nanostructured ZnO film and processes its intrinsic enzymatic activity after adsorbed on ZnO nanoparticles. An electrostatic interaction between SOD with low isoelectric point (pI = 4.9) and ZnO nanoparticles with surface pK_a of 9.35 is suggested to be the main force for the adsorption of SOD molecules on the nanostructured ZnO surface, and as a consequence, ZnO nanoparticles surface is favor for direct electron transfer of SOD. The enhanced electron transfer of SOD, together with intrinsic bidirectional catalytic activity of SOD toward superoxide anion (O₂^-) dismutation paves a way for the development of a third-generation O₂^- biosensor.