A voltammetric study of monolayers and bilayers self-assembled on metal electrodes

Three types of aliphatic thiols, n-dodecanethiol, n-hexadecanethiol and n-o ctadecanethiol, differing in their physical state in self-assembled monolay ers at ambient temperature were tested from the point of view of their inte grity and usefulness as the sub-layer for the second, adjacent layer of pho spholipids. These self-assembled monolayers were formed on mercury and poly crystalline gold electrodes in order to assess their electrochemical behavi or as monolayers passivating the electrodes against various redox probes pr esent in aqueous phase, such as hexaamineruthenium(III) chloride and benzoq uinone, differing in their heterogeneous rate constants and hydrophobic-hyd rophilic character. Subsequently, as such a type of hydrophobic alkanethiol monolayer is frequently used as a part of alkanethiol/phospholipid asymmet ric bilayer membranes, chemically bound to the metallic surface, the monola yer was covered with the adjacent phosphatidylcholine monolayer, and the el ectrochemical behavior of such an asymmetric system was again tested. The r esults obtained show that the liquid-like monolayers, particularly those fo rmed on mercury, possess better passivating properties (much smaller number of defects) as compared to crystalline thiols, also providing a better dri ving force for the attachment of the second liquid-crystalline phosphatidyl choline monolayer. An interesting observation was also noted that the outer phospholipid monolayer imposes a larger energy barrier to the penetration of benzoquinone that hexaamineruthenium(III) cations. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.