Investigations into the electrostatically induced aggregation of Au nanoparticles

The aggregation of Au nanoparticles in solution is induced and influenced b y cationic and oligocationic species. This solution-state aggregation bears similarities to multilayer formation on surfaces but is more facile becaus e of the nanoparticles' intrinsic instability in solution. Aggregation is f ollowed by transmission electron microscopy (TEM) and the appearance of fea tures at lambda = 600-900 nm in the absorbance spectrum. It is found that t hese features are a function of factors such as the aggregant size, charge, and concentration, and the method of mixing the components, and they can b e related to aggregate morphology. It seems that there are two mechanisms t hat can act to cause aggregation. Multiply charged aggregants can bind nano particles together into dense aggregates, displaying a defined absorbance a t ca. lambda = 700 nm, whereas singly charged aggregants cause a slower agg regation into string-like aggregates with a less defined absorbance. Wherea s multiply charged aggregants can "cross-link" the layers in a multilayer s tructure on a surface, singly charged aggregants cannot.