Electrochemistry of surface-grafted stimulus-responsive monolayers of poly(ferrocenyldimethylsilane) on gold

Poly(ferrocenyldimethylsilane)s with various degrees of polymerization and featuring a thiol end group were chemically end-grafted onto gold substrates by self-assembly, forming redox-active monolayers. The monolayers were characterized by contact angle measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Layer thickness values were determined by surface plasmon resonance spectroscopy and ellipsometry. The electrochemical properties of these films in aqueous NaClO(4) were studied using cyclic voltammetry (CV), differential pulse voltammetry, chronoamperometry, and chronocoulometry. Cyclic voltammograms showed two reversible redox peaks, indicating a stepwise oxidation of the electroactive sites. The first oxidation step showed reversible behavior at low scan rates and quasi-reversible behavior at higher scan rates. Peak currents (i(p)) plotted against the square root of scan rates (v(1/2)) for the first oxidation peak and for the corresponding reduction peak exhibited a linear dependence, indicating that the oxidation process in the first step is controlled by the diffusion of counterions into the polymer film. For the second oxidation peak and the corresponding reduction peak, i(p) varied linearly with v. This redox behavior is characteristic of surface-immobilized electroactive layers. The higher reversibility of the second oxidation and reduction waves in the CV experiments was explained from the solvation of the surface-grafted poly(ferrocenylsilane) (PFS) chains, which depends on the degree of oxidation. Oxidized PFS films are swollen in the aqueous electrolyte solutions, leading to a higher segmental mobility of the polymer chains and a much increased counterion mobility within the film. Kinetic parameters for the redox processes were obtained from chronocoulometry experiments.