Adsorption of lysozyme onto the silicon oxide surface chemically grafted with a monolayer of pentadecyl-1-ol

The adsorption of chicken egg white lysozyme at the functionalized silicon oxide-solution interface has been studied using the combined measurement of spectroscopic ellipsometry and neutron reflection. The solid oxide surface was modified by coating a self-assembled monolayer of pentadecyltrichlorosilane with terminal hydroxyl groups (abbreviated to C₁₅OH). Neutron reflection measurement at the solid-D₂O interface showed that the C₁₅OH layer was 16 ± 2 angstrom thick and the volume fraction was 0.94 ± 0.05, suggesting the formation of a close-packed monolayer. The adsorption of lysozyme was made at pH 4 and 7 with lysozyme concentration ranging from 0.03 to 4 g dm^-3. The results were then compared with those from previous studies at the hydrophilic SiO₂-water and the hydrophobed SiO₂-water interfaces, with the latter formed by coating a monolayer of octadecyl trichlorosilane (abbreviated to OTS). At 0.03 g dm^-3 and pH 7 the surface excess was found to be 0.6 ± 0.3 mg m^-2 at the C₁₅OH-water interface, as compared with 1.7 mg m^-2 at the SiO₂-water interface and 1.9 mg m^-2 at the OTS-water interface. As lysozyme concentration is increased to 4 g dm^-3, the surface excess at the C₁₅OH-water interface reaches 2.1 mg m^-2, as compared with 4.7 mg m^-2 at the hydrophilic SiO₂-water interface and 5.1 mg m^-2 at the OTS-water interface. These values demonstrate the attainment of the minimum surface excess on the hydroxyl surface. Shifting solution pH from 7 to 4 reduces adsorption on all the surfaces studied, but the lowest level of adsorption is again obtained on the hydroxyl surface. The reversibility of the adsorption at the C₁₅OH-water interface was examined by cycling the solution pH at different lysozyme concentrations. Adsorption was found to be completely reversible at the low lysozyme concentration of 1 g dm^-3, while at the high concentration of 4 g dm^-3 the adsorption was irreversible.