Effects of carboxylic acids on nC60 aggregate formation

The discovery that negatively charged aggregates of C₆₀ fullerene (nC₆₀) are stable in water has raised concerns regarding the potential environmental and health effects of these aggregates. In this work, we show that nC₆₀ aggregates produced by extended mixing in the presence of environmentally relevant carboxylic acids (acetic acid, tartaric acid, citric acid) have surface charge and morphologic properties that differ from those produced by extended mixing in water alone. In general, aggregates formed in the presence of these acids have a more negative surface charge and are more homogeneous than those produced in water alone. Carboxylic acid identity, solution pH, and sodium ion concentration, which are all intricately coupled, play an important role in setting the measured surface charge. Comparisons between particle sizes determined by analysis of TEM images and those obtained by dynamic light scattering (DLS) indicate that DLS results require careful evaluation when used to describe nC₆₀ aggregates.