Direct measurements of interaction forces of bovine serum albumin and lysozyme with stainless steel by atomic force microscopy

The adsorption of proteins on metal surfaces is important in various fields of engineering and science. Thus, it is necessary to understand the very complex phenomena of interactions between proteins and metal surfaces. In this study, we directly measured the interactions between proteins and stainless steel (SS) surfaces, the most common metal in engineering, by atomic force microscopy (AFM). Bovine serum albumin (BSA) and lysozyme (LSZ) were chemically attached to the tip of an AFM probe, and the interaction forces between the probe tip and the SS substrate were measured. The approaching forces between the proteins and metal surfaces were dominated by electrostatic double-layer and steric repulsion. However, the dependence of adhesion force on pH was not totally consistent with the approaching forces for both proteins. The adhesion of BSA to the SS surface was strongest in mildly acidic solutions, whereas that of LSZ was almost independent of pH. This result indicates that local interactions between the functional groups of proteins and SS, particularly the interactions between carboxyl and hydroxyl groups, are significant. The dependence of the adhesion forces on the contact time reveals that the mechanical properties (hardness) of proteins also have a significant effect on their adhesion on surfaces.