Electrochemistry: Adjustment of monolayer charge with pH.

摘要

Using self-assembled monolayers of molecules that contain ionizable functional groups, adjusting the pH of the adjacent solution allows the control of the interactions of the modified electrode and the solution. In this work, self-assembly of 3-mercaptopropionic acid (MPA) on gold electrodes allowed evaluation of the electrode behavior at different pH's using cyclic-voltammetry and chronocoulometry. Cyclic voltametry was used to determine the surface coverage on the gold electrode of the 3-mercaptopropionic acids was 2.3(+/-0.4) x10-9 mol/cm2. The intercept of chronocoulometry plots for the 3-mercaptopropionic acid modified electrodes at pH7 was found to be 7.4x10-7; and, was found to be 2.3x10-6 at pH 3. When the pH of the adjacent solution was adjusted to pH 7, the carboxylic acid group of the MPA deprotonates causing an electrostatic repulsion with the anionic ferrocyanide species. This prevents the anionic reactant from approaching the surface and creates a local concentration deficit. With a n-butanethiol modified electrode, the chronocoulometry intercept was found to be independent of pH.