POTENTIAL-DEPENDENT STABILITY OF SELF-ASSEMBLED ORGANOTHIOLS ON GOLD ELECTRODES IN METHYLENE-CHLORIDE

An electrochemical method is reported that allows the determination of the potential-dependent instability of self-assembled monolayers (SAMs) of dodecanethiol (I) or 10-(ferrocenylcarbonyl)decanethiol (II) on gold electrodes in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF(6)) in methylene chloride (CH2Cl2). ''Instability'' is a measure of the loss in surface coverage of the SAMs. The experiments involve two steps: (1) conditioning a SAM-modified electrode for a given potential and time in 0.1 M TBAPF(6)/CH2Cl2 and (2) quantifying the surface-attached molecules that survive this treatment by cyclic voltammetry of the ferrocenyl moiety in 0.1 M TBAPF(6)/CH2Cl2 or by reductive desorption of the thiolate in KOH/ethanol. Potential-dependent ''exchange'' of SAMs with organothiols in solution is consistent with the potential-dependent loss of SAMs in pure electrolyte. SAMs of I and II in 0.1 M TBAPF(6)/CH2Cl2 are most stable between approximately +0.6 and -0.8 V vs Ag/AgCl (saturated KCl). At +0.8 and -1.8 V, surface-confined molecules are almost completely gone after 1 h. This research is the first to directly yield plots of a slow, time-dependent loss in coverage of SAMs at different potentials. This behavior and methodology will be useful in (1) determining the mechanisms responsible for the stability of organothiolates on metal surfaces and (2) defining conditions for successful use of SAMs in electrochemical applications and in potential-directed modification of individual elements of arrays of microelectrodes.