Effects of Environmental Transformations on the Phototoxicity of Cadmium–Zinc-Based Quantum Dots to Freshwater Algae Chlorella Vulgaris

CdSe/ZnS-based quantum dots (QDs) are being increasingly applied in many electronic devices, raising concerns over the ecotoxicity of QDs released into the environment during the end-of-life of such devices. Most (eco)toxicity studies have used pristine QDs with noncommercially relevant coatings or omitted their environmental transformation. We used CdSe/ZnS QDs coated with polyethylenimine, similar to those found in TV screens to examine their transformation and toxicity when aged in the presence of humic acid and/or light. The transformed samples were exposed to Chlorella vulgaris to assess impacts of different aging conditions on oxidative stress and growth inhibition. Light exposure enhanced the dissolution of pristine QDs and the release of Zn and Cd which was 5-10 folds higher than those released under dark conditions. Presence of humic acid, however, decreased the dissolution of QDs when exposed to light because of the light shielding and ion-scavenging effects of humic acid. Pristine QDs aggregated with the algae, which, when exposed to light, caused dissolution and release of metal ions locally around algae cells, causing significant loss of cell viability. However, humic acid in QD exposure media reduced oxidative stress and cell viability loss (by 2.5- fold) as it reduced bioavailability of both QDs and heavy metal ions. These findings highlight the significance of understanding the relationship between environmental transformation and ecotoxicity of QDs.