Ion-specific Stability of Gold Nanoparticle Suspensions

Gold nanoparticles (AuNPs) play an important role in fundamental research and development due to their versatile applications and biocompatibility. This study addresses the aging of three AuNP suspensions after the addition of various sodium salts along the well-known Hofmeister series (NaF, NaCl, NaBr, NaI, NaSCN) at different salt concentrations between 10 mM and 100 mM. The AuNP types differ in size (5 nm vs. 11 nm in diameter) and the capping type (physisorbed citrate vs. covalently bound mercaptopropionic acid (MPA)). We monitor the aggregation of the AuNPs and the suspension stability optically (absorption spectroscopy, photography) and by electron microscopy. The large range of salt concentrations results in a large variety of colloidal stability, e.g., from stable suspensions to fast destabilization followed by sedimentation. At intermediate and high salt concentration strong ion-specific effects emerge that are non-monotonous with respect to the Hofmeister series. In particular, the chaotropic salts, NaI and NaSCN, strongly alter the absorption spectra in very different ways. NaI fuses AuNPs together influencing the primary absorption, while NaSCN retains AuNP structure during aggregation much stronger than the remaining sodium halides, resulting in a secondary absorption peak. Although decreasing the size of AuNPs leads to more stable suspensions, the ion specific effects are even more pronounced due to the increase in total available surface. Even the covalently bound MPA capping is not able to stabilize AuNPs against particle fusion by NaI, although it delays the process. Despite the complex interplay between different effects of ions on the stability of colloidal dispersions, this study disentangles the different effects from electrostatic screening, via adsorption at the interface and bridging of AuNPs, to the competition between ions and the capping agent of the AuNPs.