Using Scanning Electrochemical Microscopy (SECM) to Measure the Electron-Transfer Kinetics of Cytochrome c Immobilized on a COOH-Terminated Alkanethiol Monolayer on a Gold Electrode

摘要

Cytochrome c was electrostatically immobilized onto a COOH-terminated alkanethiol self-assembled monolayer (SAM) on a gold electrode at ionic strengths of less than 40 mM.Scanning electrochemical microscopy (SECM) was used to simultaneously measure the electron transfer (ET) kinetics of the bimolecular ET between a solution-based redox mediator and the immobilized protein and the tunneling ET between the protein and the underlying gold electrode.Approach curves were recorded with ferrocyanide as a mediator at different coverages of cytochrome c and at different substrate potentials,allowing the measurement of k_(BI) = 2 x 10~8 mol~(-1) cm~3 s~(-1) for the bimolecular ET and k deg = 15 s~(-1) for the tunneling ET.The kinetics of ET was also found to depend on the immobilization conditions of cytochrome c:covalent attachment gave slightly slower tunneling ET values,and a mixed CH_3/COOH-terminated ML gave faster tunneling ET rates.This is consistent with previous studies and is believed to be related to the degree of mobility of cyt c in its binding configuration and its orientation with respect to the underlying electrode surface.