STM investigations of Au(1 1 1) electrodes coated with vitamin B12 derivatives

Vitamin B12 derivatives immobilized at flame-annealed Au(1 1 1) electrode surfaces have been investigated in close correlation with their structural properties and spatial arrangement at the electrode substrate by scanning tunneling microscopy (STM) in air and in aqueous 0.1 M NaClO4 solution. The investigated compounds were symmetrical (B12C10S–SC10B12) and nonsymmetrical (B12C10S–SC10) dialkyl disulfide derivatives of vitamin B12, attached to the electrode surfaces by the S–Au bond. The ex situ and in situ STM experiments show the formation of a surface layer, whose packing density and structure is presumably controlled by the spatial arrangement of the large cobyrinate head groups. In presence of the symmetrical B12 compound, a disordered surface layer is observed. Voltammetric investigations show that, in 0.1 M NaClO4, this layer becomes unstable at potentials approximately ⩽ −1000 mV vs. MSE and is almost completely removed at more negative potentials. The STM imaging properties of the nonsymmetrical B12 surface layer show a significant dependence on the tunneling distance. In particular, at small tunneling distances, a highly regular hexagonal surface pattern is observed that suggests strongly the presence of an ordered surface assembly. Modeling of the B12 head group has been performed to provide information for a structure-related interpretation of the high-resolution STM images. The investigations are first STM results obtained at such B12 modified electrodes.