Cytochrome c-gold nanoparticle hybrid system for studying of long-range interfacial electron transfer by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) techniques

A hybrid of cytochrome c (cyt c) and a 4-5 nm gold nanoparticle (AuNP) on Au electrode (AuE) was prepared. In the hybrid system, AuNPs was attached to the AuE via 1,6 hexandithiol (1,6 HDT) monolayer and capped with captopril (capt) molecule then electrostatically conjugated with cyt c. The AuE/AuNP/cyt hybrid system was characterized by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM). CV shows a near ideal voltammetric response of protein with low peak separation in both low and very high scan rates. It is suggested that AuNP curvature plays an important role in controlling the cyt c surface density and resultant low electrostatic repulsion between cyt c molecules during reduction or oxidation. Moreover, enhancement of the interfacial electron transfer (ET) rate of AuE/AuNP/cyt system compared with capt/cyt c was observed. SECM approach curves were recorded at various surface concentrations of cyt-c and different substrate overpotentials to measure both the kinetics of the bimolecular ET between a Fe(CN)6-4 as a redox mediator and immobilized cyt c and the tunneling ET between cyt c and the AuE [1,3]. These studies gave fast rate constants, 1×109cm3mol-1s-1 for bimolecular and 69.6 s-1 for tunneling ET rate constants.