Dissolution and aggregation of zinc oxide nanoparticles at circumneutral pH; a study of size effects in the presence and absence of citric acid

Understanding the size dependent dissolution of engineered nanoparticles is one important aspect in addressing the potential environmental and health impacts of these materials as well as their long-term stability. In this study, experimental measurements of size dependent dissolution of well-characterized zinc oxide (ZnO) nanoparticles with particle diameters in the range of 4 to 130 nm have been measured and compared at circumneutral pH (pH 7.5). Enhanced dissolution was found for the smaller particles with the largest enhancement observed in Zn2+(aq) concentrations for 4 nm diameter ZnO nanoparticles compared to larger-sized particles. Interestingly, size dependent dissolution was observed even though the nanoparticles aggregated with hydrodynamic diameters on the order of 1-3 m in diameter. Although these results are found to be in qualitative agreement with theoretical predictions used to predict the dissolution of solids, a linearized form of the Kelvin equation to calculate a bulk dissolution value for ZnO and a surface free energy yielded quantities inconsistent with known literature values. It is therefore concluded that deviations from solubility behavior from classical thermodynamics are due to a lack of the detailed knowledge of the surface free energy as well as its dependence on the details of the surface structure, surface properties, including the presence of different surface crystal facets and adsorbed ligands, as well of aggregation state. The presence of citric acid significantly enhances the extent of ZnO dissolution for all sizes such that no significant differences were observed for total Zn2+(aq) concentrations for nanoparticles between 4 to 130 nm. This can be attributed to ligand enhanced dissolution of ZnO nanoparticles where there is no dependence on size. Adsorption of citrates onto ZnO nanoparticles was observed using ATR-FTIR spectroscopy. A reversal of surface charge of ZnO nanoparticles was observed upon adsorption of citrates. Adsorption of negatively charged Cit3- onto ZnO nanoparticles make the surfaces negatively charged and this result in a repulsion between nanoparticles eventually leading to a lesser extent of aggregation. Formation of a stable suspension was also observed in the presence of citric acid. These trends observed in aggregation pattern are of great environmental and biological importance as citric acid is abandon in the environment as well as in human body.