| The interactions between proteins and various surfaces coated with self-assembled monolayers (SAMs) were studied to assess factors relating to biocompatibility. Surfaces studied include gold surfaces coated with SAMs containing a methyl end group (hexadecane thiol), carboxylic end group (mercaptohexadecanoic acid), and a bovine serum albumin (BSA) functionalized surface. Cantilever tips functionalized with BSA were brought into contact with these surfaces using atomic force microscopy (AFM) to determine the interaction forces for the BSA/SAM pairs. Using AFM, it was possible to examine the forces that attract/repel proteins from the SAMs as the surfaces approach each other (approaching curve) and the forces that are involved in the actual adhesion of the protein to a surface, observed as the surfaces are brought out of contact (pull-off forces). AFM force measurements were taken in various ionic strength solutions in order to separate and quantify the influence of the van der Waals (vdW) and the electrostatic double layer (EDL) forces. Effective Hamaker constants were determined for each system by fitting the approaching curves to a vdW-style model. EDL forces were described by measured zeta potentials and calculated Debye lengths in the EDL force equation developed by Hogg, Healy, and Fuerstenau (the HHF equation). Effective Hamaker constants were also determined from the pull-off force measurements. By comparing the differences between the Hamaker constants obtained via the approaching curve and the pull-off force, the effects of other forces such as hydration, hydrophobic, and steric can be seen. By isolating and quantifying these forces, a more exact characterization of how proteins interact with different types of surfaces can be completed, leading to a fuller understanding of biocompatibility. |
