ADVANCES
20 Scientific American, March 2020
PETER CHADWICKGetty ImagesMATERIALS SCIENCE
Sculpting
with Light
A new process hollows
tiny crystals to lead reactions
such as carbon capture
For the first time, researchers have used
light to control the shape of nanoparticles
and create micron-size hollow shells from
crystals of cuprous oxide (copper and oxy-
gen). Such particles could have future appli-
cations as a low-cost catalyst to help pull
excess carbon dioxide from the air, a way to
improve microscopic imaging and more,
says Bryce Sadtler, a chemist at Washing-
ton University in St. Louis and senior author
of a study on the new method, published
last October in Chemistry of Materials.
The hollowing process involves visible
light, an alkaline solution and a source
of voltage, Sadtler explains. Illuminating
a cuprous oxide microcrystal excites its
electrons, which join with copper ions to
form regular copper atoms. No longer
bound to oxygen, these atoms are free to
jump to the particle’s surface and form a
copper metal coating that shields parts of
the underlying crystal from the solution.
The crystal’s structure determines which
of its faces are protected and which dis-
solve: Some faces’ atomic makeup lets elec-
trons get excited more easily, bringing met-
al atoms to the surface. But the unprotected
faces dissolve quickly, shaping the crystal
along stark, geometric lines. “A diamond
can only be [easily] cut a certain number of
ways” for similar reasons, Sadtler says. Dia-
monds break most easily in line with rows
of atoms in their crystal structure.
Stephen Maldonado, a chemist at the
University of Michigan, who was not in -
volved in the study, says the group’s find-
ings “could be potentially useful in terms
of designing catalysts for high-efficiency...CO 2 reduction, or something else.”
The large surface area and specific
shape of the hollowed-out crystals could
also be useful beyond facilitating a carbon-
capture reaction, Sadtler says. In micro-
scopic imaging, for example, existing
methods are great for identifying solid,
crystalline materials—but they struggle to
identify biological molecules. According
to Sadtler, similar hollowed structures
could surround organic molecules, possi-
bly in blood or urine samples, and boost
the signal of the hard-to-detect matter.
The researchers are also investigating
different materials that strongly interact
with light, such as iron and manganese
oxides, which hold promise for hydrogen
fuel-cell technology. —Leto Sapunar1 2 3 4 5
Time under Halogen Lamp (minutes)Cuprous oxide microcrystal 1 micronBOTANY
Potato Signals
Sweet potato variety alerts
neighbors to keep pests at bay
When nibbled, the leaves of one type
of sweet potato release a strong-smelling
chemical warning that prompts other
leaves—on the same plant and those
nearby—to produce defensive proteins
that make them hard to digest. New
research tracks this odorous alert system.
“It’s sort of a shortcut,” says Axel
Mithöfer, a plant ecologist at the Max
Planck Institute for Chemical Ecology in
Jena, Germany, and co-author of the study,
which appeared last November in Scientific
Reports. Other plants have chemical warn-
ing systems that prompt neighbors to pre -
pare for attack, but individual leaves often
wait to manufacture defensive compounds
until bitten themselves. But this plant’s
leaves produce the compound im medi -
ately when neighbors are bitten, he says.
To investigate this response, Mithöfer
and his colleagues released caterpillars on
the pest-resistant sweet potato strain Tai-
nong (TN) 57 and its more susceptible cous-
in TN66, both native to Taiwan. Each “ex -
haled” at least 40 chemicals when attacked,
but the TN57 leaves released twice the
amount of a compound called DMNT, also
found in other plant-defense responses.
Next, the scientists placed a healthy
TN57 plant in a closed glass tank with one
whose leaves had been pierced with twee-
zers. Within 24 hours high levels of a proteincalled sporamin formed in both plants’ unin-
jured leaves. Sporamin, also found in sweet
potato tubers themselves, is what makes
it difficult for humans to digest them un -
cooked—and it causes trouble in insect guts,
too. When re searchers released synthesized
DMNT into a tank with healthy plants, the
leaves again readily formed sporamin.
Mithöfer’s team is now probing the
mechanism TN57 leaves use to “smell” and
“recognize” DMNT. The researchers also
hope to test whether other chemicals the
leaves release also elicit defenses.
Cesar Rodriguez-Saona, an entomolo-
gist at Rutgers University, who was not
involved in the study, says this research
showcases an intriguing defense mecha-
nism—although he cautions that DMNT
exposure in closed tanks could be higher
than what plants experience in open,
windy fields. It is also possible, he notes,
that unattacked TN57s may not always
expend the energy to use this direct
defense “shortcut.” — Priyanka RunwalSweet potatoCHU QIN© 2020 Scientific American