In Soil, Quantum Dots Spill Their Toxic Guts
BUFFALO, N.Y., July 20, 2011 — Within 15 days of entering soil, discarded cadmium and selenium quantum dots — even those with a protective shell — begin to leak toxic elements.
The research, conducted by University at Buffalo chemistry professor Diana Aga, highlights the importance of learning more about how quantum dots and other nanomaterials interact with the environment after their useful life is over.
Quantum dots are semiconductor nanocrystals with diameters of about 2 to 100 nm. Although quantum dots are not yet commonly used in consumer products, scientists are exploring the particles’ applications in technologies ranging from solar panels to biomedical imaging.
A new UB study indicates that intact cadmium selenide quantum dots, such as the ones pictured here, including those with a “protective” zinc sulfide shell, will partially degrade in soil over time. (Images: University at Buffalo)
“Quantum dots are not yet used widely, but they have a lot of potential, and we can anticipate that the use of this nanomaterial will increase,” Aga said. “We can also anticipate that their occurrence in the environment will also increase, and we need to be proactive and learn more about whether these materials will be a problem when they enter the environment.”
Aga’s research suggests that, although there is potential for negative impacts, there is also a possibility to modify the surface of the nanomaterials to prevent degradation.
Her team tested two kinds of quantum dots: cadmium selenide and cadmium-selenide with a protective, zinc-sulfide shell. Although the shelled quantum dots are known in scientific literature to be more stable, Aga’s team found that both varieties of quantum dot leaked toxic elements within 15 days of entering soil.
In a related experiment designed to predict the likelihood that discarded quantum dots would leach into groundwater, the scientists placed a sample of each type of quantum dot at the top of a narrow soil column, then added calcium chloride solution to mimic rain.
What they observed: Almost all the cadmium and selenium detected in each of the two columns — more than 90 percent of that in the column holding unshelled quantum dots and more than 70 percent of that in the column holding shelled quantum dots — remained in the top 1.5 cm of the soil.
Diana Aga, a chemist at the University at Buffalo.
But how the nanomaterials moved depended on what else was in the soil. When the team added ethylenediaminetetraacetic acid (EDTA) to test columns instead of calcium chloride, the quantum dots traveled through the soil more quickly. EDTA is a chelating agent, similar to the citric acid often found in soaps and laundry detergents.
The data suggest that, under normal circumstances, quantum dots resting in topsoil are unlikely to burrow their way into underground water tables, unless chelating agents such as EDTA are introduced on purpose or naturally occurring organic acids (such as plant exudates) are present.
Aga said that even if the quantum dots remain in topsoil, without contaminating underground aquifers, the particles’ degradation still poses a risk to the environment.
The research has been accepted for publication in the journal Environmental Science and Technology. In a separate study submitted to a different journal, she and her colleagues tested the reaction of Arabidopsis plants (a member of the mustard family widely used as a model organism in plant biology) to quantum dots with zinc sulfide shells. The team found that, although the plants did not absorb the nanocrystals into their root systems, they still displayed a typical phytotoxic reaction upon coming into contact with the foreign matter; in other words, the plants treated the quantum dots as a poison.
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