Determination of Real Composition of 3-Mercaptopropionic Acid and 1-Octadecanethiol Mixed Self-Assembled Monolayers by Using Electrochemical and Electrochemical Quartz Crystal Microbalance Measurements

Abstract

In order to clarify the electrochemical response of cytochrome c at the gold electrode modified with mixed monolayers of carboxylic acid terminated alkanethiol (promoter molecule) and n-alkanethiol (barrier molecule), electrochemical and electrochemical quartz crystal microbalance (EQCM) measurements were carried out during the reductive desorption of the mixed monolayer of 3-mercaptopropionic acid and 1-octadecanethiol (C18SH) in alkaline solution. The ratio of mercaptopropionic acid in the mixed monolayer was found to be smaller than that in the modifying solution, showing that C18SH adsorbed more preferentially than mercaptopropionic acid. The peak potentials for the reductive desorption of mercaptopropionic acid became more negative upon increasing the C18SH content in the monolayer. This means mercaptopropionic acid and C18SH did not form patched domains but were mixed well with each other. The mercaptopropionic acid molecules in a well-mixed monolayer should be stabilized through the affinity between the mercaptopropionic acid and C18SH and becomes difficult to be desorbed from the electrode.