A new UB study indicates that intact cadmium selenide quantum dots, like the ones pictured here, including those with a “protective” zinc sulfide shell, will partially degrade in soil over time. |
Quantum dots made from cadmium and selenium degrade in soil,
unleashing toxic cadmium and selenium ions into their surroundings, a
University at Buffalo
study has found.
The research, accepted for publication in Environmental Science and Technology, demonstrates the importance
of learning more about how quantum dots—and other nanomaterials—interact with
the environment after disposal, says Diana Aga, the chemistry professor who led
the study.
Quantum dots are semiconductor nanocrystals with diameters of
about 2 nm to 100 nm. Though 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.
“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 Aga, who presented the findings at a National Science
Foundation-funded workshop on nanomaterials in the environment. “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.
Aga’s research into the afterlife of quantum dots is funded by a
$400,000 Environmental Protection Agency grant to investigate the environmental
transport, biodegradation, and bioaccumulation of quantum dots and oxide
nanoparticles.
Her collaborators on the new study in Environmental Science and Technology include PhD student Divina
Navarro, Assistant Professor Sarbajit Banerjee, and Associate Professor David
Watson, all of the UB Department of Chemistry.
Working in the laboratory, the team tested two kinds of quantum
dots: 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’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. The
researchers 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% of that in the column holding
unshelled quantum dots, and more than 70% of that in the column holding shelled
quantum dots—remained in the top 1.5 cm of the soil.
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 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.
Aga says that even if the quantum dots remain in top soil,
without contaminating underground aquifers, the particles’ degradation still
poses a risk to the environment.
In a separate study submitted for publication in a different
journal, she and her colleagues tested the reaction of Arabidopsis plants to quantum dots with zinc sulfide shells. The
team found that 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.
