Surface charge effects on the redox switching of LbL self-assembled redox polyelectrolyte multilayers

The effect of surface charge on the electron transfer and charge propagation in layer by layer (LbL) electrostatically self-assembled redox poly-electrolyte multilayers (PEMs) has been studied. Thin films comprised of sequential layers of cationic poly(allylamine) covalently modified by osmium bipyridyl pyridine complexes (PAH-Os) and anionic poly(vinylsulfonate) (PVS) have been studied using electro-chemical impedance spectroscopy (EIS) and ellipsometry. For positively charged topmost layers a transition from low frequency redox pseudocapacitance to diffusion due to electron hopping between adjacent redox sites has been observed. This behavior could be modelled by a bound-diffusion modified Randles electrical equivalent circuit. Negatively charged surfaces terminated in PVS, on the other hand, presented a kinetic hindrance for the osmium redox switching process. This may be ascribed to a limitation in the flux of mobile anions compensating charge due to a skin Donnan potential at the film/electrolyte interface or effects on the local electrostatic environment of the osmium sites. Further adsorption of polycations or charge screening by increasing the electrolyte ionic strength restores the redox charge seen when the polycation was the topmost layer.