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Exploring the environmental impact of quantum dots

19 January 2017

The speed with which microbes in a simplified ecosystem absorb the nanomaterials is raising concerns about the effect on organisms higher in the food chain.

Quantum dots are rapidly finding their way into consumer products. The physical, chemical, and optical properties of the nano-sized semiconductor crystals make them desirable for medical devices, solar panels, display screens, and more.

Quantum dots can also find their way into the aquatic ecosystem, where the changes they bring may not be so desirable. A few toxicity studies have suggested that quantum dots accumulate in microorganisms and then get carried up the food chain.

Now researchers led by Rishi Shanker of Ahmedabad University and Alok Dhawan of CSIR–Indian Institute of Toxicology Research, both in India, have developed a two-level, predator–prey food chain in the lab to elucidate the toxicological impact of quantum dots. Their miniature ecosystem (see micrographs below) consists of a dish of water populated by the bacterium Escherichia coli and the protozoan Paramecium caudatum (the predator). A unicellular organism, P. caudatum controls the bacterial population in natural water systems and represents an important link between microbes and multicellular organisms. Cadmium telluride quantum dots, which are used in some solar cells and lasers, were introduced at a nonlethal concentration to serve as the contaminant.

Although CdTe is far less toxic than either of its individual components, bioaccumulation clearly hindered P. caudatum’s growth and feeding rate. After 24 hours of exposure, the P. caudatum had absorbed 65% of the quantum dots and thereby doubled the cellular concentration of cadmium. Over time the protozoans became significantly less able to ingest their bacterial food source.

The results suggest that scientists and policymakers need to consider more seriously the effects of CdTe that reaches waterways and trickles up the food chain. Beyond quantum dots, the study establishes a useful approach for assessing nanomaterials’ impact on aquatic ecosystems. (G. S. Gupta et al., Environ. Sci. Technol., in press.)

P. caudatum and E. coli

P. caudatum is less able to consume E. coli (inset) when exposed to quantum dots. Credit: Govind Sharan Gupta
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