Researchers have developed nano-strips for inexpensive testing, with unprecedented sensitivity, of mercury levels in lakes and oceans. Image: Northwestern University |
Mercury,
when dumped in lakes and rivers, accumulates in fish, and often ends up
on our plates. A Swiss-American team of researchers led by Francesco
Stellacci at the Ecole Polytechnique Fédérale de Lausanne (EPFL) and
Bartosz Grzybowski at Northwestern University has devised a simple,
inexpensive system based on nanoparticles, a kind of nano-velcro, to
detect and trap this toxic pollutant as well as others. The particles
are covered with tiny hairs that can grab onto toxic heavy metals such
as mercury and cadmium. This technology makes it possible to easily and
inexpensively test for these substances in water and, more importantly,
in the fish that we eat. Their new method can measure methyl mercury,
the most common form of mercury pollution, at unprecedentedly small
attomolar concentrations. The system is outlined in an article appearing
September 9, 2012 in the journal Nature Materials.
Methyl mercury, toxic and difficult to monitor
Researchers
are particularly interested in detecting mercury. 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 US, France and Canada, public health authorities advise pregnant
women to limit fish consumption because mercury can compromise nervous
system development in the developing fetus.
“The
problem is that current monitoring techniques are too expensive and
complex,” explains Constellium Chair holder at EPFL and co-author
Francesco Stellacci. “We periodically test levels of mercury in drinking
water, and if those results are good, we make the assumption that
levels are acceptable in between those testing periods.” But industrial
discharge fluctuates.
A simple, inexpensive new technology
The
technology developed by the Swiss-American team is simple to use. A
strip of glass covered with a film of “hairy”nanoparticles is dipped
into the water. When an ion—a positively charged particle, such as a
methyl mercury or cadmium ion—gets in between two hairs, the hairs close
up, trapping the pollutant.
A
voltage-measuring device reveals the result; the more ions there are
trapped in the nano-velcro, the more electricity it will conduct. So to
calculate the number of trapped particles, all one needs to do is
measure the voltage across the nanostructure.
By
varying the length of the nano-hairs, the scientists can target a
particular kind of pollutant. “The procedure is empirical,” explains
Stellacci. Methyl mercury, fortunately, has properties that make it
extremely easy to trap without accidentally trapping other substances at
the same time; thus the results are very reliable.
The
interesting aspect of this approach is that the ‘reading’ glass strip
could costs less than 10 dollars, while the measurement device will cost
only a few hundreds of dollars. The analysis can be done in the field,
so the results are immediately available. “With a conventional method,
you have to send samples to the laboratory, and the analysis equipment
costs several million dollars,” notes Stellacci.
Convincing tests in Lake Michigan and Florida
The
researchers tested the system in Lake Michigan, near Chicago. Despite
the high level of industry in the region, mercury levels were extremely
low. “The goal was to compare our measurements to FDA measurements done
using conventional methods,” explains Stellacci. “Our results fell
within an acceptable range.”
A
mosquito fish from the Everglades in Florida was also tested. This
species is not very high on the food chain and thus does not accumulate
high levels of mercury in its tissues. “We measured tissue that had been
dissolved in acid. The goal was to see if we could detect even
minuscule quantities.” says Bartosz Grzybowski, Burgess Professor of
Chemistry and Director of Non-Equilibrium Energy Research Center at
Northwestern University. The United States Geological Survey reported
near-identical results after analyzing the same sample.
From quantum to real applications
“I
think it is quite incredible,” Grzybowski adds, “how the complex
principles of quantum tunneling underlying our device translate into
such an accurate and practically useful device. It is also notable that
our system—through some relatively simple chemical modifications—can be
readily adapted to detect other toxic species” Researchers have already
demonstrated the detection of cadmium with a very high femtomolar
sensitivity.
“With
this technology, it will be possible to conduct tests on a much larger
scale in the field, or even in fish before they are put on the market,”
says lead author Eun Seon Cho. This is a necessary public health
measure, given the toxic nature of methyl mercury and the extremely
complex manner in which it spreads in the environment and accumulates in
living tissues.
Funding
for this research came from ENI, via the ENI-MIT Alliance; the US
Defense Threat Reduction Agency via a grant to MIT and U Michigan; the
US Department of Energy via a Nonequilibrium Energy Research Center
grant to Northwestern and U Michigan.
Source: EPFL