Quantum dot weathering results in microbial toxicity.

Quantum dots (QDs) are increasingly being used for electronics, solar energy generation, and medical imaging applications. Most QDs consist of a heavy metal core/shell coated with amphiphilic organics that stabilize the nanoparticles and allow conjugation with biological molecules. In this study, QDs were evaluated for their effects on bacterial pure cultures, which serve as models of cell toxicity and indicators of potential impact to ecosystem health. QDs with intact surface coatings decreased growth rates of Gram positive Bacillus subtilis and Gram negative Escherichia coli but were not bactericidal. In contrast, weathering of various types of QDs under acidic (pH < or = 4) or alkaline (pH > or = 10) conditions significantly increased bactericidal activity due to the rapid (< 1 min) release of cadmium and selenite ions following QD destabilization upon loss of the organic coating. Toxicity was mitigated by humic acids, proteins, and other organic ligands that reduced metal bioavailability. The best available science, which is limited, suggests that QDs are potentially safe materials when used in their intended applications at near-neutral pH. These results forewarn us that even moderately acidic or alkaline conditions could lead to significant and localized organism effects due to toxic exposure to dissolved heavy metals. Thus, biocompatibility and ecotoxicity tests for QDs should consider in vivo and/or in situ transformations to fully characterize the potential risks to environmental health.