Modulating Colloidal Adsorption on a Two-Dimensional Protein Crystal

Abstract

The geometric and physicochemical properties of the protein streptavidin make it a useful building block in the construction and manipulation of nanoscale structures and devices. However, one requirement in exploiting streptavidin for "bottom-up" assembly is the capability to modulate protein-nanoparticle interactions. This work examines the effects of pH and the biotin-streptavidin interaction on the adsorption of colloidal gold onto a two-dimensional streptavidin crystal. Particle deposition was carried out below (pH 6), at (pH 7), and above (pH 8) the protein's isoelectric point with both biotinylated and nonbiotinylated nanoparticles. Particle surface coverage depends on deposition time and pH, and increases by 1.4-10 times when biotin is incorporated onto the particle surface. This coverage is highest for both particle types at pH 6 and decreases monotonically with increasing pH. Calculations of interparticle potentials based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory demonstrate that this trend in surface coverage is most likely due to alterations in particle-surface electrostatic interactions and not a result of changes in interparticle electrostatic repulsion. Furthermore, post-adsorption alterations in pH demonstrate that electrostatically adsorbed particles can be selectively desorbed from the surface. Evaluation of the nonspecifically adsorbed fraction of biotinylated particles indicates that the receptor-ligand adsorption mechanism gives a higher rate of attachment to the substrate than nonspecific, electrostatic adsorption. This results in faster adsorption kinetics and higher coverages for biotinylated particles relative to the nonbiotinylated case.