Sensing a self-assembled protein structure through its permeation by a ferrocene labeled poly(ethylene glycol) chain

Cyclic voltammetry was used for systematic analysis of the diffusion of a bulky mono ferrocene poly(ethylene glycol) ((PEG-Fc, MW ca, 3400 g/mol) redox probe through a film made of N successively self-assembled monolayers of immunoglobulins immobilized at the electrode surface. A membrane-like model of the film is considered. A theoretical approach is developed showing that, in the general case, five parameters are involved in the control of the process. The influence of each parameter and the possible limiting situations are discussed. Quantitative information is derived from the changes appearing in the experimental cyclic voltammograms when N increases and/or when the potential scan rate is varied over 3 orders of magnitude. The PEG-Fc probe is bully enough to sense the presence of the IgG self-assembly. Its diffusion coefficient through the membrane-like film is roughly 2 orders of magnitude smaller than in solution. The diameter of the channels which exist between the self-assembled IgG layers is not much larger than the 4.2 nm hydrodynamic diameter of the PEG coil.