Ion-Specific Stability of Gold Nanoparticle Suspensions

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 and photography) and by electron microscopy. The large range of salt concentrations results in a large variety of colloidal stability, i.e., from stable suspensions to fast destabilization followed by sedimentation, due to the impact of the anions on the interaction between the negatively charged AuNPs. At intermediate and high salt concentrations, strong ion-specific effects emerge that are nonmonotonous with respect to the Hofmeister series. In particular, the chaotropic salts, NaI and NaSCN, strongly alter the absorption spectra very differently. NaI fuses AuNPs together, influencing the primary absorption, while NaSCN retains the 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 the total available surface. Even the covalently bound MPA capping cannot 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. These findings are crucial for the fabrication of inorganic/organic composites by the targeted assembly of AuNPs in a preexisting matrix controlled by the presence of salt.