Partitioning of hydrophobic CdSe quantum dots into aqueous dispersions of humic substances: influence of capping-group functionality on the phase-transfer mechanism.
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Studies of the fate and transport of engineered nanomaterials are invaluable in predicting environmental impact, bioavailability, and toxicity. We report on the influence of humic and fulvic acids (models of natural organic matter) on the phase transfer of organic-capped CdSe quantum dots (QDs) from hexane to water. QDs capped with tri-n-octylphosphine oxide, tetradecylphosphonic acid, and oleic acid, which were otherwise insoluble in water, were transferred into aqueous solutions of humic substances (HS) (Suwannee River humic acid and fulvic acid standards) within 1-10 days after mixing. Phase transfer was characterized by infrared and UV/Vis absorption spectroscopy, emission spectroscopy, dynamic light scattering, electron microscopy, and inductively coupled plasma mass spectrometry. Phase-transferred QDs were intact and temporarily stabilized by HS. On longer timescales, Cd(2+) leached into aqueous solution. Our data suggest that two mechanisms promote the phase transfer of QD-HS agglomerates: (1) an overcoating mechanism involving dispersion interactions between non-polar moieties of HS and hydrocarbon chains of organic capping groups and (2) a coordinative mechanism involving displacement of capping groups by Lewis basic functionalities of HS. The structure of the capping group of QDs influenced the relative contributions of the two mechanisms and the extent to which Cd(2+) leached into water.