Heteroaggregation between citrate-stabilized gold nanoparticles and hematite colloids : mechanisms and quantification of nanoparticle-colloid interactions

Graduation date: 2013Understanding the aggregation behavior of engineered nanomaterials (ENMs) in aqueous environments is essential to understanding their fate, transport, and toxicity. Because naturally occurring colloids will likely be present at much higher concentrations than ENMs in environmental systems, heteroaggregation and attachment with these natural colloids could significantly influence ENM fate and impact on water bodies. This study investigated the heteroaggregation and attachment between hematite (α-Fe₂O₃) and citrate-capped gold nanoparticles (Cit-AuNPs) in 1 mM KCl. The influence of Cit-AuNP concentration was investigated at pH 6 and pH 8. Attachment was quantified by separation of hematite and hematite-Cit-AuNP aggregates from free Cit-AuNPs via a novel technique utilizing track-etched membrane filtration; quantification of the free Cit-AuNPs allowed the calculation of a maximum adsorption capacity. Aggregation of hematite induced by the addition of Cit-AuNPs was investigated using time-resolved dynamic light scattering. Heteroaggregate electrophoretic mobility was measured, and the corresponding zeta potential was calculated. Evidence of enhanced aggregation and non-zero zeta potential corresponding to the maximum aggregation conditions suggested that an "electrostatic patch" model may explain the heteroaggregation mechanism. In the absence of natural organic matter at the given conditions, Cit-AuNPs were completely associated with hematite at the low Cit-AuNP concentrations expected in environmental systems, indicating an attachment efficiency of unity