Effect of flow on human serum albumin adsorption to self-assembled monolayers of varying packing density

Adsorption of human serum albumin (HSA) to self-assembled monolayers (SAMs) of different packing densities under dynamic conditions was investigated in situ using optical reflectometry and neutron reflectometry. When optical reflectometry was used, it was observed that HSA has a greater apparent steady-state surface concentration and higher initial rate of adsorption on a less densely packed SAM than on a densely packed SAM. These results are consistent with the contention that HSA specifically binds to the loosely packed chains. Changes in the morphology of an adsorbed protein layer with the imposition of continuing flow during adsorption were studied for the first time using neutron reflectometry. HSA interpenetrates the less densely packed SAM but sits on top of the densely packed SAM when adsorbing under persistent flow or without persistent flow. However, the orientation of the adsorbed molecules changes with the details of the flow. In the absence of persistent flow, a side-on orientation prevails. The adsorbed layers are thicker when adsorbed under persistent flow, consistent with a reasonably uniform tilting of the molecules up away from the surface. The variation in the adsorption tenacity on the SAMs with an alkyl chain packing density is greater when the adsorbed layer structure is equilibrated under quiescent conditions than when the adsorbed layer is subjected to continuing flow.