Nanoparticle Concentration Effects on Kinetic Stability of CdSe Quantum Dots in Waters

Many recent laboratory studies on nanoparticle (NP) behavior used highly concentrated particle suspensions (up to several mM concentrations), largely to overcome instrument detection limits. In potentially engineered-NP impacted aquatic systems NP-concentrations can vary many orders of magnitude lower depending on the pathways of their release. We investigated NP concentration effects on kinetic stability of CdSe quantum dots (QDs), that have CdSe core diameter of 4.4 ±0.2 nm (by TEM), and the mercaptoundecanoic acid (MUA) coatings. The term stability is used to describe resistance to NP aggregation/coagulation and precipitation. We measured stability ratios of varied NP suspension concentrations, 10 to 0.1 mM (atom-based concentrations), under stability-favorable pH condition, and ionic strength (IS) from 1.0 mM to 2.0 M NaCl. In addition to determining short-term stability (10 seconds to 60 minute), we also measured long-term stability, up to 2 months (in order to observe precipitation). The results were compared with DLVO predictions using calculated Hamaker constants for MUA-coated CdSe QDs). Our data indicate that at the high concentrations (≥ 1.4 mM), CdSe QDs are extremely stable under low IS and favorable pH conditions, staying monodispersed for beyond the long experiment duration. Surprisingly, decreasing NP concentrations to < 1.4 mM resulted in rapid coagulation (< 10 seconds) of the primary NPs under the same low IS and stability-favorable pH. Interestingly, after this initial rapid coagulation the aggregates were relatively stable in suspensions, with larger aggregate sizes in more diluted solutions, suspended in water for beyond the long experiment duration. Measured electrokinetic properties of NP suspensions at different concentrations are presented, and mechanisms are discussed. The phenomena observed in this study may be extended to some of other types of charge-stabilized engineered NPs, and their impacts on transport and bioavailability can be inferred. Funding for this study was provided by the U.S. Department of Energy, the joint BER-EPA-NSF nanoparticulate research program.