USING SURFACE-PLASMON RESONANCE SPECTROSCOPY TO MEASURE THE ASSOCIATION OF DETERGENTS WITH SELF-ASSEMBLED MONOLAYERS OF HEXADECANETHIOLATE ON GOLD

This paper describes the use of surface plasmon resonance (SPR) spectr oscopy to measure the rates and extents of association of four deterge nts-sodium dodecyl sulfate (SDS), beta-octyl glucoside, Triton X-100, and Tween 20-to self-assembled monolayers (SAMs) of alkanethiolates on gold. SAMs presenting hexaethylene glycol groups resisted the adsorpt ion of all four detergents. These same detergents associated with hydr ophobic SAMs presenting methyl groups; the concentration of detergent molecules on the surface was 120-280 pmol/cm(2). The associations of t he detergents with the hydrophobic SAM were described well by the Lang muir adsorption isotherm. The dissociation constants K-d (M) for the d esorption of the detergents from the surface correlated with the criti cal micelle concentration (cmc, M) of the detergents in solution, and followed the relationship cmc approximate to 7 (+/-2)K-d. The efficacy of SDS in removing the protein fibrinogen adsorbed on a hydrophobic S AM depended strongly on the concentration of detergent. SDS at a conce ntration three times greater than the cmc removed (or displaced) the a dsorbed layer of protein in seconds; SDS at a concentration three time s smaller than the cmc did not desorb it even after several minutes. T his paper shows that SPR is a useful analytical technique for characte rizing the interactions of detergents-and other molecules having low m olecular weight-with the well-defined surfaces of SAMs.