Electrodeposition of bilayers of dithiols: A new route to complex organic interfaces

Abstract

Organized organic films form a new class of nanomaterials, having potential applications in biosensing and molecular electronics. The development of such devices is impeded by the lack of simple and reliable methods for depositing molecular assemblies. An original electrodeposition scheme that allows for the formation of functionalized organic monolayers has been developed. With this method, it is possible to create organic interfaces that can be made to selectively react through application of an electric field. This electrochemical approach has been used to form reproducible monolayers and bilayers of aromatic and alkyl dithiols. The mechanism guiding the adsorption of dithiols at the gold surface (from basic aqueous electrolyte solution) proceeds in two steps. First, a monolayer of vertically oriented dithiols (i.e. attached with a single S-Au bond) is formed via a nucleation and growth process. Next, the unreacted sulfur end groups dimerize with dithiolates in solution to form a bilayer. More specifically, bilayer formation occurs through the nucleophilic attack of an activated thiolate end group by a dithiolate from solution and follows a Langmuir process. The extent of bilayer formation depends on the accessibility of the unbound thiolate and its distance from the electrode surface. The interaction between the negatively charged thiolate and the aqueous electrolyte plays a major role in the assembly of these compounds. In addition, intermolecular interactions are influential in the packing of rigid (i.e. aromatic) and flexible (i.e. alkyl) dithiol films.