Building complex two-dimensional structures: Methylene blue on self-assembled monolayer-covered Au(111)

The charge transfer of methylene blue molecules immobilized on different monolayer covered-Au(111) surfaces has been studied by electrochemical techniques complemented by scanning tunneling microscopy. The positively charged methylene blue ions (MB⁺) are effectively immobilized on negatively charged S and I adlayers. The amount of charge related to the (MB⁺/MBH) surface redox couple (q_surf) depends on the surface concentration of the adsorbed S and I atoms and on their geometric arrangement. When methylene blue immobilization is made on COO^--terminated thiols (mercaptopropionic acid, MPA, and mercaptoundecanoic acid, MUA), the value of q_surf is markedly reduced, indicating that the electron transfer through hydrocarbon chains is very difficult, although the MB⁺ is effectively immobilized. Sulfide contamination introduces defects in the MPA and MUA layers and, thus, markedly increases the number of immobilized methylene blue molecules that are active for charge transfer. Electrochemical runs made with methylene blue in solution demonstrate that defects are the main path for electron transfer through COO^--terminated thiol-covered Au(111) surfaces. For lowdefect COO^--terminated thiol adlayers, a diode-like effect related to the blocking of the anion-transport through small pores with negatively charged gates is observed.