Different synthesis methods allow to tune the permeability and permselectivity of dopamine-melanin films to electrochemical probes

Different methods to prepare melanin films by dopamine oxidation are compared with respect to their permeability to electrochemical probes being either neutral or carrying a positive or a negative charge. To examine charge-dependence of the permeability differently charged probes of similar size (hexaamineruthenium, ferrocenemethanol and hexacyanoferrate) are employed. We deliberately not investigated the permeability of metal cations through the melanin films owing to their possible complexation by the catechol groups of melanin. The films prepared by solution oxydation methods (O-2 or Cu2+ as oxidants) are impermeable to hexacyanoferrate as soon as their thickness reaches 5-10 nm but remain permeable to hexaamineruthenium and ferrocenemethanol up to thickness reaching 25-40 nm before becoming also impermeable. The melanin films prepared by electrochemical deposition display a marked difference: they remain permeable to hexacyanoferrate up to 35 nm in thickness and to hexaamineruthenium and ferrocenemethanol up to the maximal thickness that can be reached by this deposition method. When the maximal film thickness of 45 nm is reached the film becomes also impermeable to these two redox probes. These results are explained on the basis of structural differences of the melanin films at the molecular level. The permselectivity of the melanin coatings is related to their negative surface charge density at the working pH of 7.5. (C) 2011 Elsevier Ltd. All rights reserved.