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.