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)(17)SiCl3), 18-nonadecenyltric hlorosilane (NTS, CH2=CH(CH2)(17)SiCl3), [2-(perfluorooctyl)ethyl] trichlor osilane (FOETS, CF3(CF2)(7)CH2CH2SiCl3) monolayers, and their mixed monolay ers were used as the model substrates for the study of protein adsorption m echanism. Surface plasmon resonance (SPR) spectroscopy was applied to analy ze the protein adsorption behavior onto the surface of the monolayers. Atom ic force microscope (AFM) was used to observe the monolayer surfaces after exposure of these monolayers to bovine serum albumin (BSA) and gamma -globu lin(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 amoun t of adsorbed protein in the charged state was lower than that in the neutr al state. These results indicate that the preferential adsorption of protei n onto the FOETS phase for the mixed monolayer systems at either below or a bove pi is due to (1) the minimization of interfacial free energy between t he monolayer surface and the buffer solution, and (2) the electrostatic rep ulsion among protein molecules bearing charges.