Plasma protein adsorption behavior onto the surface of phase-separated organosilane monolayers on the basis of scanning force microscopy

The n-octadecyltrichlorosilane (OTS, CH₃(CH₂)₁₇SiCl₃), 18-nonadecenyltrichlorosilane (NTS, CH₂.CH(CH₂)₁₇SiCl₃), [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS, CF₃(CF₂)₇CH₂CH₂SiCl ₃) monolayers, and their mixed monolayers were used as model surfaces for the study of protein adsorption mechanism. Surface plasmon resonance (SPR) spectroscopy was applied to analyze the protein adsorption behavior onto the monolayer surfaces. The surfaces after exposure of these monolayers to bovine serum albumin (BSA) and γ-globulin(IgG) solutions were observed with atomic force microscope(AFM). AFM observation revealed that the charged protein either below or above pI was preferentially adsorbed onto the FOETS phase of the phase-separated (OTS/FOETS) mixed monolayer. in situ AFM observation of monolayer surfaces in BSA solution also revealed the preferential adsorption of BSA onto the hydrophobic FOETS surface. SPR clarified that the amount of adsorbed protein in the charged state was lower than that in the neutral state. Adhesion force was not detected in the force-distance curve measurement between negatively-charged HOOC(CH₂)₉SH chemisorbed cantilever tip and the OTS phase in the presence of adsorbed BSA on FOETS phase of mixed monolayer. These results indicate that the preferential adsorption of protein onto the FOETS phase for the mixed monolayer systems at either below or above pI is due to, (1) the minimization of interfacial free energy between the monolayer surface and the buffer solution; and (2) the electrostatic repulsion among protein molecules bearing charges.