EFFECT OF SURFACE WETTABILITY ON THE ADSORPTION OF PROTEINS AND DETERGENTS

This report describes the use of surface plasmon spectroscopy to study the effect of surface wettability on the nonspecific adsorption of pr oteins and detergents to self-assembled monolayers (SAMs) of alkanethi olates on gold. The adsorption of both proteins and detergents to unch arged SAMs showed a general dependence on the wettability of the surfa ce as determined by the contact angle of water on the SAM under cycloo ctane (theta(co)). The effect of the wettability of the SAMs on the ad sorption of sodium dodecyl sulfate (SDS) was dependent on whether mice lles were present. Above the critical micelle concentration (cmc), SDS adsorbed only on surfaces that gave contact angles with values of cos theta(co) < 0 (i.e., the transfer of the surface from water to cycloo ctane has a favorable free energy). Below the cmc, the requirement for adsorption was much more stringent: SDS adsorbed only on the surfaces that gave values of cos theta(co) < -0.9. Similarly, the effect of th e wettability of the SAMs on the adsorption of proteins showed a depen dence on the size of the proteins. The smaller proteins tested (ribonu clease A and lysozyme) adsorbed only on the least wettable surfaces te sted (cos theta(co) < -0.83). The larger proteins tested (pyruvate kin ase, fibrinogen, and gamma-globulin) also adsorbed best to the least w ettable surfaces, but adsorbed to some extent on almost all the surfac es; the single exception was a SAM presenting hexa(ethylene glycol) gr oups at the surface, to which no protein adsorbed. Films of adsorbed p roteins were desorbed from the SAMs by treatment with detergent.