A neutron reflectometry study of human serum albumin adsorption in situ

Reflectometry is used to characterize silicon-wafer-supported NH3+-terminated self-assembled silane monolayers (SAMs) and silicon oxide (SiOx) surfaces against air, water, and human serum albumin (HSA) solutions. X-ray reflectometry (XR) of the NH3+-terminated SAM in air revealed a 14 Å thick, close-packed silane monolayer consistent with an all-trans extended hydrocarbon chain packing tilted about 30° with respect to the surface normal. Neutron reflectometry (NR) of the NH3+ surface against D2O was consistent with (XR) results if it was assumed that labile protons exchange with deuterium resulting in an interface comprised largely of ND3+. NR results obtained, with the SiOx surface against D2O were interpreted in terms of a porous, hydrated oxide layer about 20 A thick. NR of HSA adsorbed from D2O buffer onto NH3+ terminated SAM surfaces revealed a two-layer adsorption regime. The first layer directly adjacent to the solid surface was about 40 Å thick and was essentially independent of bulk liquid concentration whereas the second layer was more strongly dependent on liquid phase concentration studied, extending an additional 40 Å into solution at the maximum solution concentration (0.1% wt/V). NR results are thus consistent with multilayer or mixed-layer protein adsorption mechanism and formation of an interphase region about 80 Å in thickness that separates bulk solid and bulk liquid phases. By contrast, no such interphase could be resolved for an HSA solution in contact with a water-wettable SiOx surface, suggesting that HSA does not adsorb to fully-water-wettable surfaces from water. Implications of these results in biomaterials science, especially blood contact phenomena, are briefly discussed in the summary.