Electrochemical self-assembly of monolayers from alkylthiosulfates on gold

Selective preparation of self-assembled monolayers (SAMs) on gold was accomplished by electrolysis of alkylthiosulfates (Bunte salts) in tetrahydrofuran in the presence of tetrabutylammonium tetrafluoroborate as the supporting electrolyte. Tetrafluoroborate ions inhibited the spontaneous chemisorption of alkylthiosulfates to form monolayers, so that monolayers formed only on gold electrodes at a sufficiently oxidative potential. This oxidative potential was applied using square-wave pulses, and as expected, the degree of completeness of the monolayers depended upon both the voltage and the period during which it was applied. Using this method, monolayers could be formed quickly, in only twenty 5-s pulses. X-ray photoelectron spectroscopy and infrared spectroscopy supported our hypothesis that this process involves formation of stable gold-thiolate bonds at the gold surface and that (bi)sulfate is a side-product under our experimental conditions. Unlike the preparation of SAMs from alkanethiols, clean gold is not required for forming complete monolayers from alkylthiosulfates. Preliminary results indicate that monolayers of alkanethiolates can also be formed by reductive electrolysis of corresponding alkylthiosulfates in aqueous solution.