The new system is comprised of a commercial strip of glass covered with a film of “hairy” nanoparticles. A kind of “nano-velcro,” it can be dipped into water to trap the pollutant and render the film electrically conductive.

When
mercury is dumped into rivers and lakes, the toxic heavy metal can end
up in the fish we eat and the water we drink. To help protect consumers
from the diseases and conditions associated with mercury, researchers at
Northwestern University in collaboration with colleagues at Ecole
Polytechnique Fédérale de Lausanne (EPFL) in Switzerland, have developed
a nanoparticle system that is sensitive enough to detect even the
smallest levels of heavy metals in our water and fish.

The research was published September 9 in the journal Nature Materials.

“The
system currently being used to test for mercury and its very toxic
derivative, methyl mercury, is a time-intensive process that costs
millions of dollars and can only detect quantities at already toxic
levels,” said Bartosz Grzybowski, lead author of the study. “Ours can
detect very small amounts, over million times smaller than the
state-of-the-art current methods. This is important because if you drink
polluted water with low levels of mercury every day, it could add up
and possibly lead to diseases later on. With this system consumers would
one day have the ability to test their home tap water for toxic
metals.”

Grzybowski
is the Kenneth Burgess Professor of Physical Chemistry and Chemical
Systems Engineering in the Weinberg College of Arts and Sciences and the
McCormick School of Engineering and Applied Science.

The
new system is comprised of a commercial strip of glass covered with a
film of “hairy” nanoparticles, a kind of a “nano-velcro,” that can be
dipped into water. When a metal cation—a positively charged entity, such
as a methyl mercury—gets in between two hairs, the hairs close up,
trapping the pollutant and rendering the film electrically conductive.

A
voltage-measuring device reveals the result; the more ions there are
trapped in the “nano-velcro,” the more electricity it will conduct. To
calculate the number of trapped particles, all one needs to do is
measure the voltage across the nanostructure film. By varying the length
of the nano-hairs covering the individual particles in the film, the
scientists can target a particular kind of pollutant that is captured
selectively. With longer “hairs,” the films trap methyl mercury, shorter
ones are selective to cadmium. Other metals also can be selected with
appropriate molecular modifications.

The
nanoparticle films cost somewhere between $1 to $10 to make, and the
device to measure the currents costs a few hundred dollars, Grzybowski
said. The analysis can be done in the field so the results are
immediately available.

Researchers
were particularly interested in detecting mercury because its most
common form, methyl mercury, accumulates as one goes up the food chain,
reaching its highest levels in large predatory fish such as tuna and
swordfish. In the United States, France and Canada, public health
authorities advise pregnant women to limit fish consumption because
mercury can compromise nervous system development in the fetus.

Researchers
used this system to detect levels of mercury in water from Lake
Michigan, near Chicago, among other samples. Despite the high level of
industry in the region, the mercury levels were extremely low.

“The
goal was to compare our measurements to FDA measurements done using
conventional methods,” said Francesco Stellacci of EPFL,
co-corresponding author of the study. “Our results fell within an
acceptable range.”

The
researchers also tested a mosquito fish from the Florida Everglades,
which is not high on the food chain and thus does not accumulate high
levels of mercury in its tissues. The U.S. Geological Survey reported
near-identical results after analyzing the same sample.

“This
technology provides an inexpensive and practical alternative to the
existing cumbersome techniques that are being utilized today,” said
Jiwon Kim, graduate student in Grzybowski’s lab in the department of
chemistry at Northwestern. “I went to Lake Michigan with our sensor and a
hand-held electrometer and took measurements on-site in less than a
minute. This direct measurement technique is a dream come true for
monitoring toxic substances.”

This
work was supported by the Non-equilibrium Energy Research Center, which
is an Energy Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences under grant
number DE-SC0000989.

Authors
of this study include: Jiwon Kim, Baudilio Tejerina, Thomas M. Hermans,
Hideyuki Nakanishi, Alexander Z. Patashinski and Bartosz A. Grzybowski
from the Department of Chemical and Biological Engineering and
Department of Chemistry, Northwestern University; Eun Seon Cho and
Francesco Stellacci, Institute of Materials, Ecole Polytechnique
Fédérale de Lausanne EPFL Switzerland and Hao Jiang and Sharon C.
Glotzer, Department of Chemical Engineering and Department of Materials
Science and Engineering, University of Michigan.

Source: Northwestern University