Factors that Influence the Stability of Self-Assembled Organothiols on Gold Under Electrochemical Conditions

by Everett, William Russell; Fritsch, Ingrid

Several factors contribute to the loss and retention of self-assembled monolayers, SAMs, of organothiols at gold electrodes under potential control in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile and methylene chloride. Near 0 V vs. Ag/AgCl (saturated KCl), SAMs of dodecanethiol have maximum stability. Under driest and most oxygen-free conditions at negative potentials, such as -1.8 V, SAMs exhibit a 90% loss in surface coverage in 1 h in methylene chloride and in less than 1 min in acetonitrile. At more positive potentials such as +0.8 V, the rate of SAM loss is slower, and only ca. 33% of surface-confined molecules were removed after 1 h in methylene chloride, but 100% after 10 min in acetonitrile. Although the rates of removal are different for the two solvents, the potential window of greatest stability is similar, centered near 0 V and extending to -1.0 V and +0.5 V. This window is independent of the type of gold substrate: Au/Cr/glass, Au/(3-mercaptopropyl)trimethoxysilane/gl and Au foil. In both solvent systems, water greatly diminishes the stability of the surface-bound species at negative and positive potentials. The addition of oxygen, however, greatly enhances the stability of the surface-bound species. Details of the mechanisms for the effect of oxygen and water are not known at this time. However, positive potentials (+0.7 V) cause formation of gold oxide, which increases with water concentration. At negative potentials (-1.5 V) reduction of residual oxygen is facilitated by the presence of water and may form OH- that interacts with sAMs. stirring under both argon-purged and oxygenated conditions at negative potentials diminishes SAM stability; this is thought to be explained by enhanced flux of either water to the surface or desorbed species away from the surface. Uncompensated resistance and shifts in liquid junction potentials of the reference electrode do not explain differences in potential-dependent SAM stability between the two solvents.

Analytica Chimica Acta
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1873-4324; 0003-2670