Stability and removal of water soluble CdTe quantum dots in water

Commercial use of quantum dots (QDs) will lead to their entry into aquatic environments. This study examines the characteristics and stability of CdTe QDs with thioglycolate capping ligands in water as well as their removal by alum The capping ligands of QDs are a key factor in determining their fate in water. Protonated thioglycolate capping ligands cause QDs to aggregate. The stability of QDs depends more their ionic composition in water than on the ionic strength. In KCl solution, QDs remain stable even under 0.15 M ionic strength. Relatively low concentrations (<= 2meq/L) of divalent (Mg2+ and Ca2+) or trivalent (Al3+) cations, however, can induce aggregation. The proposed mechanism for this phenomenon is that multivalent metal cations (or their species) react with capping ligands to form complexes that bridge QDs or neutralize their surface charges. Because the complexation of hydrated Al3+ with capping ligands inhibits formation of Al(OH)(3) precipitates, alum dosages higher than the Al3+ solubility are required to form settleable flocs and QDs from nanopure water by sedimentation. Divalent cations (Mg2+ and Ca2+) in tap water induce the formation of QD flocs such that 70-80% of the QDs by mass settle out.