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) spectroscopy to measure the rates and extents of association of four detergents - sodium dodecyl sulfate (SDS), β-octyl glucoside, Triton X-100, and Tween 20 - to self-assembled monolayers (SAMs) of alkanethiolates on gold. SAMs presenting hexaethylene glycol groups resisted the adsorption of all four detergents. These same detergents associated with hydrophobic SAMs presenting methyl groups; the concentration of detergent molecules on the surface was 120-280 pmol/cm². The associations of the detergents with the hydrophobic SAM were described well by the Langmuir adsorption isotherm. The dissociation constants K_d (M) for the desorption of the detergents from the surface correlated with the critical micelle concentration (cmc, M) of the detergents in solution, and followed the relationship cmc ≈ 7 (±2)K_d. The efficacy of SDS in removing the protein fibrinogen adsorbed on a hydrophobic SAM depended strongly on the concentration of detergent. SDS at a concentration three times greater than the cmc removed (or displaced) the adsorbed layer of protein in seconds; SDS at a concentration three times smaller than the cmc did not desorb it even after several minutes. This paper shows that SPR is a useful analytical technique for characterizing the interactions of detergents - and other molecules having low molecular weight - with the well-defined surfaces of SAMs.