Nanoparticle aggregation in a freshwater river: the role of engineered surface coatings

Within aquatic environments, the aggregation of ENPs has been identified as an important process affecting their environmental fate. Previous research using simple model mediums has demonstrated that engineered surface coatings applied to ENPs can alter their aggregation behavior. However, the relevance and effect of these surface coatings on ENPs dispersed in complex aquatic mediums is largely unknown. The objective of the current work was to explore this topic further using gold nanoparticles (AuNPs) with different engineered surface coatings as model ENPs. AuNPs with neutral or negatively-charged, covalently-bound surface coatings (polyethylene glycol [PEG] or carboxylated PEG, respectively) were found to be stable in both raw and filtered river water, while AuNPs with positively-charged (branched polyethylenimine, aminated PEG) or electrostatically-bound (citrate) surface coatings readily aggregated. For the model ENPs that aggregated, their average percent removal after mixing in the filtered river water was similar to that measured after the same period in raw river water, revealing that homoaggregation was dominant relative to heteroaggregation. To quantify the effect of the surface coatings on the colloidal stability of the model ENPs, we attempted to estimate homo- and heteroaggregation attachment efficiency factors (alpha(homo) and alpha(hetero), respectively) using a recently reported functional assay. A number of challenges preventing these direct calculations in this system are discussed. However, from modelling it was inferred that alpha(homo) >= alpha(hetero). We find that ENP colloidal stability was related to eco-corona formation (i.e., adsorption of natural organic matter), which was regulated by the properties of the engineered surface coatings. Overall, the results of the batch experiments demonstrate that engineered surface coatings can affect ENP colloidal stability in a complex medium, further highlighting the need to consider this factor when investigating the environmental fate of ENPs.