Electrochemical separation of the inner monolayer in methylene blue/leucomethylene blue conductive films induced by a sulphur-modified gold surface

Abstract The effect of the first adsorbed monolayer of organic molecules (inner monolayer) on the growth of thick films has been investigated using a regular polyoriented gold electrode covered with sulphur and the methylene blue (MB + leucomethylene blue (LMB) redox system as a model. The sulphur-modified gold electrode enables clear distinction of two regions in the film, simplifying the understanding of its growth mechanism. Each region of the film is identified unambiguously by voltammetry. The voltammetric sharp peak profiles reported here indicate that no ohmic limitation to the electrochemical reaction is introduced by the film. This high efficiency for electric charge transport through the film is interpreted in terms of the molecular order in the thick film induced by the molecular arrangement in the inner monolayer. The exceptional stability of the first MB + /LMB monolayer is explained by (i) a strong tendency towards the formation of a sulphur heteroatom-sulphur adatom bond and (ii) the fact that the matrix of the electrode surface supports a crystalline-like stacking which accommodates a methylene blue cation and fluoride amons. It has been found experimentally that the maximum number of MB + molecules in the inner layer is 1.53 × 10 14 cm −2 , while the model proposed for a regular polyoriented surface predicted a value of 1.47 × 10 14 cm −2 .