Adsorption of proteins on gold nanoparticles: One or more layers?

Adsorption of proteins on nanoparticles is a complex and poorly studied process. The mechanisms of protein layer formation can fundamentally differ depending on the composition of the medium, the nanoparticles' structure, the protein's nature, and other factors. In particular, monolayer or multilayer immobilization may occur. In the present work, the composition of conjugates of bovine serum albumin and immunoglobulin G with gold nanoparticles obtained by the Turkevich-Frens method are analyzed. The composition was studied by protein fluorescence measurement for particles ranging in size from 20 to 48 nm, depending on the pH of the immobilization medium (from 4 to 5 to 8-10). It was found that a pH shift of the immobilization medium from acidic to alkaline values is accompanied by a change in the mechanism of protein adsorption on the gold surface. In acidic pH conditions (4-5), effective binding of bovine serum albumin and gold nanoparticles occurs throughout the entire range of studied protein concentrations. In alkaline pH conditions (8-10), however, effective binding occurs only at concentrations of >10 μg/mL. This effect is not observed for immunoglobulin G, which is efficiently adsorbed onto nanoparticles throughout the entire range of studied concentrations and pH values. For acidic pH values, the surface of the particles is saturated with the amount of bound proteins, which approximately corresponds to the amount the monolayer is filled. For neutral and alkaline pH values, saturation is not observed and the amount of adsorbed protein certainly exceeds the monolayer filling, leading to multilayer immobilization.