Abstract:
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 µm 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.
PREVIEW
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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.