In situ STM study of self-assembled mercaptopropionic acid monolayers for electrochemical detection of dopamine

An Au(111) electrode coated with a self-assembled monolayer of 3-mercaptopropionic acid (MPA) has been investigated by square-wave voltammetry for electrochemical detection of dopamine in the presence of ascorbic acid and characterized by in situ STM. The electrode is found to shift selectively the unwanted oxidation of ascorbic acid to higher potentials while leaving the oxidation of dopamine unchanged. The selectivity is lost below approx. -0.5 V (Ag | AgCl | 3 M KCl) due to a reductive desorption of MPA, and also above approx. 1.2 V where the Au electrode begins to oxidize. STM shows that even a brief oxidation results in large change in the molecular packing structure of the MPA monolayer. Before the oxidation, the MPA molecules in the monolayer pack into several phases. The most dominant phase is an incommensurate p × √3 structure, while commensurate phases, 5 × √3, 6 × √3, 8 × √3 and 10 × √3 have also been observed frequently. In some areas, the molecules pack into a superlattice 3 × 4√3 that can be attributed to a periodic mixing of two discrete conformations of MPA. After cycling the potential to the oxidation potentials, the ordered structures transform into a less compact disordered structure. The exposed bare Au electrode in the disordered structure is believed to be responsible for the loss of the electrode selectivity.