Antibodies Irreversibly Adsorb to Gold Nanoparticles and Resist Displacement by Common Blood Proteins

Gold nanoparticles (AuNPs) functionalized with proteins to impart desirable surface properties have been developed for many nanobiotechnology applications. A strong interaction between the protein and nanoparticle is critical to the formation of a stable conjugate to realize the potential of these emerging technologies. In this work, we examine the robustness of a protein layer adsorbed onto gold nanoparticles while under the stress of a physiological environment that could potentially lead to protein exchange on the nanoparticle surface. The adsorption interaction of common blood plasma proteins (transferrin, human serum albumin, and fibrinogen) and anti-horseradish peroxidase antibody onto AuNPs is investigated by nanoparticle tracking analysis. Our data show that a monolayer of protein is formed at saturation for each protein, and the maximum size increase for the conjugate, relative to the AuNP core, correlates with the protein size. The binding affinity of each protein to the AuNP is extracted from a best fit of the adsorption isotherm to the Hill equation. The antibody displays the greatest affinity (K_d = 15.2 ± 0.8 nM) that is ∼20-65 times stronger than the affinity of the other plasma proteins. Antibody-AuNP conjugates were prepared, purified, and suspended in solutions of blood plasma proteins to evaluate the stability of the antibody layer. An enzyme-mediated assay confirms that the antibody-AuNP interaction is irreversible, and the adsorbed antibody resists displacement by the plasma proteins. This work provides insight into the capabilities and potential limitations of antibody-AuNP-enabled technologies in biological systems.