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 Angstrom thick, close-packed silane monolayer consistent with an all-trans extended hydrocarbon chain packing tilted about 30 degrees 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 Angstrom 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 Angstrom 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 Angstrom 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 Angstrom 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.