Impedance spectroscopy and zeta potential titration of dopa-melanin films produced by oxidation of dopamine

The deposition of dopa-melanin films from dopamine solutions in oxidative conditions may offer new opportunities in surface coating methods. After having demonstrated that such films become impermeable to ferrocyanide anions after having reached a thickness of a few nanometers (F. Bernsmann, A. Ponche, C. Ringwald, J. Hemmerle, J. Raya, B. Bechinger, J.C. Voegel, P. Schaaf, V. Ball, Characterization of Dopamine-Melanin Growth on Silicon Oxide, Journal of Physical Chemistry C 113 (2009) 8234–8242) and after the determination of their zeta potential in the same pH conditions as those used for the film deposition we wish now to investigate the impedance spectra and zeta potential titration curve of such thin films. These properties are important to understand the electrical properties and surface chemistry of the melanin films, respectively. Such an investigation will hence allow to improve future applications of dopa-melanin films. The impedance spectra show that the films charge transfer and mass transfer resistance increase rapidly as the film deposits in parallel to a decrease of their permeability to ferrocyanide anions. The zeta potential versus pH titration curves display two inflexion points at pH values close to 6 and 9 which can be related to the titration of quinone-imine and catechol groups respectively. For the first time we show that dopa-melanin prepared in these conditions has an isoelectric point close to 4. The relevance of this finding is discussed.