This report describes the use of surface plasmon spectroscopy to study the effect of surface wettability on the nonspecific adsorption of proteins and detergents to self-assembled monolayers (SAMs) of alkanethiolates on gold. The adsorption of both proteins and detergents to uncharged SAMs showed a general dependence on the wettability of the surface as determined by the contact angle of water on the SAM under cyclooctane (θco). The effect of the wettability of the SAMs on the adsorption of sodium dodecyl sulfate (SDS) was dependent on whether micelles were present. Above the critical micelle concentration (cmc), SDS adsorbed only on surfaces that gave contact angles with values of cos θco < 0 (i.e., the transfer of the surface from water to cyclooctane 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 θco < −0.9. Similarly, the effect of the wettability of the SAMs on the adsorption of proteins showed a dependence on the size of the proteins. The smaller proteins tested (ribonuclease A and lysozyme) adsorbed only on the least wettable surfaces tested (cos θco < −0.83). The larger proteins tested (pyruvate kinase, fibrinogen, and γ-globulin) also adsorbed best to the least wettable surfaces, but adsorbed to some extent on almost all the surfaces; the single exception was a SAM presenting hexa(ethylene glycol) groups at the surface, to which no protein adsorbed. Films of adsorbed proteins were desorbed from the SAMs by treatment with detergent.
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