Scanning force microscopic study of protein adsorption on the surface of organosilane monolayers prepared by the Langmuir-Blodgett method

The n-octadecyltrichlorosilane (OTS, CH₃(CH₂)₁₇SiCl₃), 18-nonadecenyltrichlorosilane (NTS, CH₂=CH(CH₂)₁₇SiCl₃), [2-(perfluorooctyl)ethyl] trichlorosilane (FOETS, CF₃(CF₂)₇CH₂ CH₂SiCl₃) monolayers, and their mixed monolayers were used as the model substrates for the study of protein adsorption mechanism. Surface plasmon resonance (SPR) spectroscopy was applied to analyze the protein adsorption behavior onto the surface of the monolayers. Atomic force microscope (AFM) was used to observe the monolayer surfaces after exposure of these monolayers to bovine serum albumin (BSA) and Δ-globulin(IgG) solution. AFM observation revealed that the charged protein either below or above the isoelectric point was preferentially adsorbed onto the FOETS phase of the (OTS/FOETS) mixed monolayer. SPR revealed that the amount of adsorbed protein in the charged state was lower than that in the neutral state. 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.