Oxidative desorption of thiols as a route to controlled formation of binary self assembled monolayer surfaces

Abstract

Binary self-assembled monolayers have been prepared by an alternative route to the conventional approach where a surface is exposed to a solution of both thiols at open circuit. A particular composition can be prepared with relative ease and the method is particularly useful for binary monolayers comprising two thiols with very different chain lengths since the problem of the longer chain thiol displacing the shorter over time is avoided. This is achieved by first preparing a full monolayer of the longer thiol, then oxidative desorption is carried out using cyclic voltammetry to remove a known fraction (as measured from the charge associated with gold oxide reduction) of the original SAM. Experiments with several thiols of chain lengths from 9 to 16 carbon atoms show that a considerable degree of control may be exerted over the rate at which the thiol is removed, using factors such as the potential limit, scan rate, pH and electrolyte polarity. The amount of thiol removed can thus also be controlled in a similar fashion. Once the oxidative desorption has taken place to the required degree, the vacancies created can be filled by exposure to a solution of a second thiol, differing in either chain length, termination or both. Deposition of thiols was carried out both under potential control (for longer chains) and by simple adsorption (for shorter chains). Several binary SAMs were prepared using this method and characterised using simple cyclic voltammetry with redox probes and selective reductive desorption. This method of preparation of binary SAMS may provide an alternative to both conventional methods using solution exposure and to methods that employ reductive desorption to expose selected facets on a polycrystalline surface. It may also be used to create partially blocked electrodes.