Concentration dependence of aggregate formation upon adsorption of 5-(octyldithio)-2-nitrobenzoic acid on gold electrodes

The adsorption of 5-(octyldithio)-2-nitrobenzoic (O-DTNB) onto gold electro de surfaces has been investigated with a quartz crystal microbalance, and t he process appears to follow the Freundlich adsorption model. Atomic force microscopy images of adsorbed films obtained from 20 muM and 10.0 mM ethano lic solutions of O-DTNB exhibited dramatically different morphologies which were ascribed to the formation of multilayer equivalent aggregates. Simila r to adsorbed films derived from 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) , cycling the potential past -0.55 V for adsorbed films of O-DTNB resulted in the generation of a reversible, pH-dependent, redox couple ascribed to f ormation of the hydroxylamine from the nitro group. For films obtained from a high concentration (10.0 mM) of O-DTNB, generation of the reversible red ox couple did not appear to result in desorption of the additional adsorbed material as ascertained from EQCM data. The cyclic voltammetric profiles o f [Fe(CN)(6)](3-), hydroxymethylferrocene, and [Co(phen)(3)](2+) at gold el ectrodes modified with O-DTNB was pH dependent, and this behavior was ascri bed to the acid/ base behavior of the carboxylic acid substituent.