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"It is critical to detect mercury quickly,
accurately and at its source," said Chad A. Mirkin, George B. Rathmann
Professor of Chemistry, professor of medicine and professor of
materials science and engineering, who led the study. "Most existing
detection methods require expensive complicated equipment forcing
tests to take place in a lab. Our method is simpler, faster and more
convenient than conventional methods, and results can be read with the
naked eye at the point of use."
The researchers report that they were able to
determine by simple visual inspection if solvated mercuric ion was
present in each sample tested and, if so, in what amount. As an
illustration of the method's selectivity, they also could
differentiate mercury from other metals with similar binding
mechanisms, such as cadmium and copper.
The method is also highly sensitive, capable of
detecting mercuric ions at the 100 nanomolar level. "To the best of my
knowledge, we have set a record for the most sensitive colorimetric
sensor," said Mirkin. "A glucose meter, for example, operates at a
high micromolar scale, with glucose being 100,000 times more
concentrated than the mercury we are detecting."
The Northwestern method takes advantage of gold's
intense color when the metal is measured on the scale of atoms. Mirkin
and his team started with gold nanoparticles, each just 15 nanometers
in diameter, held together by complementary strands of DNA. Because
they are held together within a certain critical distance, the gold
nanoparticles - and the solution they are in - are blue. When the
solution is heated, the DNA breaks apart, and the gold nanoparticles,
no longer in close proximity to each other, are now bright red.
Knowing that mercuric ion binds selectively to the
bases of a thymidine-thymidine (T-T) mismatch, the researchers
designed each strand of DNA, which is attached to a gold nanoparticle,
to have a single thymidine-thymidine (T-T) mismatch. If mercury is
present in the solution it binds tightly to the T-T mismatch site.
The key to the technology is that the blue to red
color change occurs at 46 degrees Celsius if the solution has no
mercury, and it occurs at a higher temperature if mercury is present.
"When mercury binds to the T-T mismatch site it is
like adding some superglue -- the gold nanoparticles are now held
together even more tightly," said Mirkin. "The mercury creates a
stronger bond that requires a higher temperature to break apart the
DNA strands."
The temperature it takes to break apart the strands,
when the color changes from blue to red, also indicates how much
mercury is present - the higher the temperature, the more mercury or
"super glue" that is present.
Their next step, said Mirkin, is to increase the
sensitivity of the method as well as expand the scope of environmental
targets. Using similar principles, the researchers have started
developing a colorimetric screening method for cadmium and lead.
"This is a simple method that we can tailor easily
for other metals," said Mirkin. |
