Effects of surface coating character and interactions with natural organic matter on the colloidal stability of gold nanoparticles

Aggregation is one of the dominant processes controlling the environmental fate of engineered nanomaterials (ENMs) in aquatic systems. Engineered coatings and coronas obtained through interactions with natural organic matter (NOM) and other macromolecules are known to play a significant role in controlling these processes. However, our ability to predict environmental fate on the basis of nanoparticle properties and the properties of the surrounding medium is still developing. To better understand the interplay between ENM surface coatings and their interaction with NOM, the aggregation of gold nanoparticles (AuNPs) with four different surface coatings-polyethylene glycol of varying molecular weight (PEG-AuNPs), carboxylated PEG-AuNPs (PEG-COOH-AuNPs), aminated PEG-AuNPs (PEG-amine-AuNPs), and branched polyethylenimine (bPEI-AuNPs)-was investigated as a function of pH, ionic strength and the presence of a model organic matter (Suwannee River NOM, SRNOM). Time-resolved dynamic light scattering and electrophoretic mobility titrations were used to investigate how changes in the solution chemistry affect the ability of the different AuNPs to resist aggregation. Under the conditions investigated, the PEG-AuNPs and PEG-COOH-AuNPs remained stable across a range of conditions and in the presence of SRNOM due to steric and/or electrosteric stabilization. In contrast, the PEG-amine-AuNPs and bPEI-AuNPs were destabilized in the presence of SRNOM at circumneutral pH. It is hypothesized that aggregation of these AuNP types occurs via adsorption and interparticle bridging that is strongly dependent on pH and the concentration of SRNOM. At mass ratios of NOM to ENMs expected in natural systems, all the AuNP types investigated here are expected to be stable with respect to homoaggregation in the presence of SRNOM. These findings provide a foundation to investigate more complex systems where competing interactions between NOM, ENMs and natural colloids are expected to control ENM environmental fate. Character of engineered surface coatings plays a significant role in controlling ENM interactions with model NOM macromolecules.