Grafting aryl diazonium cations to polycrystalline gold: Insights into film structure using gold oxide reduction redox probe electrochemistry and contact angle behavior

Abstract

The structure and properties of thin organic films electrografted to conducting surfaces by reduction of the corresponding diazonium salts are not well understood. In this work we used electrochemistry and contact angle measurements to characterize multilayer carboxyphenyl and methylphenyl films grafted to Au surfaces. Freshly grafted films contain material that can be readily removed during potential cycling and sonication. Observation of Au oxide surface electrochemistry confirms a porous bulk film structure as previously found for films electrografted to carbon surfaces. The charge associated with Au oxide reduction was used to estimate the upper limit for surface concentration of modifiers directly attached to the surface after careful preparation of the Au surface prior to and after grafting. Values for the surface concentration of the modifier in the range of 3-4 × 10⁻¹⁰ mol cm⁻² indicate that the first layer of the films is loosely packed. The close correspondence of surface concentration with that previously found for a "monolayer equivalent" of nitrophenyl film covalently grafted to smooth carbon surfaces ((2.5 ± 0.5) × 10⁻¹⁰ mol cm⁻²) supports a similar film formation mechanism, that is, formation of a Au-C bond. We also show that sonication of the Au surface considerably alters the Au oxide reduction charge of an initially well-defined bare Au surface. Hence, Au oxide electrochemistry cannot be used to monitor film changes after sonication. Examination of the voltammetric response of Fe(CN) ₆³⁻/⁴⁻ and measurements of water contact angles at the grafted electrodes give insight into the film-solution interface. Sonication of films in solvents of different polarities leads to different interfacial electrochemistry and hydrophilicity, consistent with a dynamic film structure that can reorganize in response to the environment. The reliable interpretation of changes in redox probe responses thus requires careful consideration of the dynamic mechanical properties of these loosely packed films.