BUFFALO, N.Y. -- 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 the journal
Environmental Science and Technology, demonstrates the importance
of learning more about how quantum dots -- and other nanomaterials
-- interact with the environment after disposal, said Diana Aga,
the chemistry professor who led the study.
Quantum dots are semiconductor nanocrystals with diameters of
about 2 to 100 nanometers. Though quantum dots are not yet commonly
used in consumer products, scientists are exploring the particles'
applications in technologies ranging from solar panels to
"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," said Aga, who presented the findings in late June
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 said.
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 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 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 said 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.
The University at Buffalo is a premier research-intensive public
university, a flagship institution in the State University of New
York system and its largest and most comprehensive campus. UB's
more than 28,000 students pursue their academic interests through
more than 300 undergraduate, graduate and professional degree
programs. Founded in 1846, the University at Buffalo is a member of
the Association of American Universities.