REDUCTIVE DESORPTION OF ALKANETHIOLATE MONOLAYERS AT GOLD - A MEASURE OF SURFACE COVERAGE

An approach for the determination of the surface coverages of monolayer films formed by the chemisorption of thiol compounds at evaporated gold electrodes is described. The approach is based on the voltammetric measurement of the charge passed for the one-electron reductive desorption of the resulting gold-bound thiolate layer (Au-SR) in alkaline solution (pH > 11): Au-SR + e- --> Au(0) + -SR. The applicability of this approach is evaluated using monolayers formed from a methyl-terminated thiol (dodecanethiol) and two ferrocene-terminated thiols of differing polymethylene chain length. Coverages for the ferrocene-terminated layers are assessed by the reductive desorption process, the ferrocene-ferricenium redox transformation, and the loss of the adsorbate precursor from the film-formation solution. The coverages determined by all three methods are found to be comparable. For the methyl-terminated structure, the coverage from the reductive desorption measurement agrees with that expected for the (square-root 3BAR x square-root 3BAR) R30-degrees overlayer at Au(111) (the predominant crystal face of evaporated gold films) after accounting for surface roughness. In addition, general structural and reactivity characteristics of the ferrocene-terminated layers, as gleaned from the electrochemical measurements and from characterizations using infrared reflection absorption spectroscopy, are presented and compared with those of the methyl-terminated layer. Taken together, these results verify the utility of the reductive desorption measurement as a quantitative tool for determining the surface coverage of thiolate monolayers at gold.