Self-assembly and electrochemical desorption of thioctic acid monolayers on gold surfaces

Self-assembled monolayers of thioctic acid (1,2-dithiolane-3-pentanoic acid) on Au surfaces were studied by electrochemical desorption experiments and X-ray photoelectron spectroscopy. The cathodic stripping of thioctic acid monolayers from Au electrodes in 0.5 M KOH electrolyte is characterized by two desorption features. The first, centered at -0.93 V (vs Ag/AgCl), is in good agreement with earlier studies of thioctic acid monolayers on Au. The second peak occurs at about -1.27 V and, to our knowledge, has not previously been reported. Kinetic studies, in which electrochemical desorption curves and XPS spectra were recorded as the total self-assembly time was varied from 1 min to 2 weeks, reveal two distinct adsorption states of thioctic acid on gold surfaces. The first voltammetric wave (-0.93 V) is associated with the desorption of thioctic acid itself, and the second is due to the desorption of oxidized thioctic acid (present as the both the disulfinate and the disulfonate). Finally, oxidized TA was prepared by two synthetic routes and self-assembled monolayers of oxidized TA were allowed to form. Electrochemical desorption experiments showed a single desorption wave at -1.27 V, confirming our assignment.