XPS study of the effect of the conditions of peptide chemisorption to gold and silver coated polymer surfaces

Chemisorption of peptides and active moieties onto gold and silver coated surfaces is an attractive method for studying the effect of fundamental surface properties on biological interactions. In theory, the monolayers formed have a high density of the active group of interest, and the relatively mild conditions associated with chemisorption should allow biological activity to be maintained. While the conditions for chemisorption are widely reported in the literature, significant differences exist between research groups and the effects of changing these conditions on the resultant surface have not been fully examined. Furthermore, comparisons have not been made between gold and silver as potential substrates in these applications. In the current work, glutathione and cysteine were chemisorbed onto gold- and silver-coated polymers. The effect of varying the chemisorption conditions was evaluated by XPS analysis of the resultant surfaces. Factors identified as having potentially significant effects on chemisorption procedure included chemisorption time, peptide concentration, peptide, pH of the chemisorption solution, mixing and material of the incubation container. Factor significance was evaluated using a two level fractional factorial design of experiments (DOE), with sulfur content determined by XPS used as a measure of chemisorption effectiveness. Significant differences were noted between the silver and gold-coated surfaces, with a higher amount of sulfur and hence, by inference, peptide found in general on silver surfaces reacted under identical conditions. On the gold surfaces, peptide concentration, peptide type, and chemisorption time were found to have a significant effect on the composition of the resulting surface. On the silver surfaces, factor specific differences were not as significant but there were a number of two factor interactions. The results provide further evidence of the differences in interactions with thiol between silver and gold and suggest that changes in the chemisorption conditions can dramatically affect the resultant surface composition.