U. BUFFALO (US) — In their afterlife, quantum dots made from cadmium and selenium release toxic ions into the earth within 15 days of entering the soil.
The findings, published in the journal Environmental Science and Technology, demonstrate the importance of learning how quantum dots and other nanomaterials interact with the environment after disposal.
Quantum dots are semiconductor nanocrystals with diameters of about 2 to 100 nanometers. Though not yet commonly used in consumer products, scientists are exploring the particles’ applications in a variety of technologies, including solar panels and biomedical imaging.
“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,” says Diana Aga, professor of chemistry at the University at Buffalo.
“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.”
“We can conclude from our research that there is potential for some negative impacts, since the quantum dots biodegrade. But there is also a possibility to modify the chemistry, the surface of the nanomaterials, to prevent degradation in the future,” she says.
Researchers tested two kinds of quantum dots in the lab: Cadmium selenide quantum dots, and cadmium-selenide quantum dots with a protective, zinc-sulfide shell. Though the shelled quantum dots are known in scientific literature to be more stable, Aga says both varieties 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 and then added calcium chloride solution to mimic rain.
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 centimeters of the soil.
But how the nanomaterials moved depended on what else was present. 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 top soil are unlikely to burrow their way down 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.
Even if the quantum dots remain in top soil, without contaminating underground aquifers, the particles’ degradation still poses a risk to the environment, Aga says.
In a separate study submitted for publication in a different journal, Aga and colleagues tested the reaction of Arabidopsis plants to quantum dots with zinc sulfide shells. While the plants did not absorb the nanocrystals into their root systems, the plants 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.
The research was funded by the Environmental Protection Agency.
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