Electrostatic and Steric Forces Drive Reversible Aggregation of Gold Nanoparticles

We report insights into the mechanisms for the aggregation of citrate-capped gold nanoparticles (AuNPs) with cationic peptides as well as their subsequent dissociation with thiol-PEGs (HS-PEGs). Prior work has suggested that AuNPs aggregated into a secondary energy minimum by peptides could be redispersed by the addition of HS-PEGs, but the AuNPs could not be dissociated when aggregated into a primary minimum with salt ions. To further discern the role of the energy minimum and the mechanisms of this process, we empirically tuned the strength of the intermolecular and surface forces governing the interactions between the peptides, HS-PEGs, and the AuNPs. We evaluated the behavior of AuNPs ranging in size from 10 to 175 nm. We then analyzed how these changes would alter the depth of the energy minimum and the colloidal stability of the system by modulating the strength of the attractive van der Waals forces and the repulsive electric double layer forces. UV–vis spectroscopy and ζ-potential measurements showed that larger AuNPs require higher concentrations of peptide to induce aggregation. Moreover, the ζ-potential for the onset of aggregation appeared roughly at −30 ± 6 mV, suggesting similarities in the surface charge of the AuNPs when the attractive forces overtake the repulsive forces. We also investigated the mechanism by which the HS-PEGs dissociate the AuNPs. We show that the HS-PEGs displace bound peptides when dissociating the AuNPs and that steric forces exerted by larger peptides hinder dissociation. These findings inform the development of more advanced nanomaterials for sensing applications and guide computational models in analyzing the behavior of colloidal nanoparticle systems.