Ill. -- Detecting the presence of hazardous lead paint
could become as simple as pressing a piece of paper
against a wall and noting a color change.
Scientists at the University of Illinois at
Urbana-Champaign have developed a highly sensitive and
selective biosensor that functions in much the same
fashion as a strip of litmus paper. The researchers
report their discovery in a paper that has been accepted
for publication in the Journal of the American Chemical
Society, and posted on its Web site. The colorimetric
sensor is based upon DNA-gold nanoparticle chemistry,
and could be used for sensing a variety of environmental
Using gold nanoparticles laced with DNA,
Illinois chemistry professor Yi Lu and graduate student
Juewen Liu are able to hybridize the nanoparticles into
aggregate clusters that have a characteristic blue
color. In the presence of a specific metal ion, the
catalytic DNA will break off individual gold
nanoparticles, resulting in a dramatic color shift to
red. The intensity of the color depends upon the initial
concentration of contaminant metal ions.
By applying the DNA-gold nanoparticle solution to a
substrate, the researchers can create a biosensor that
functions in the same manner as litmus paper. "These
simple colorimetric sensors eliminate the need for
additional instrumentation, and are well suited for
on-site, real-time detection and quantification," Lu
To obtain the necessary catalytic DNA for their
biosensors, Lu and Liu use a combinatorial approach
called in vitro selection. Simple and cost-effective,
the selection process can sample a very large pool of
DNA (up to 1,000 trillion molecules), amplify the
desired sequence by the polymerase chain reaction and
introduce mutations to improve performance.
While most DNA is double stranded, the catalytic DNA
Lu and Liu use has a single strand that can wrap around
like a protein. In that single strand, the researchers
fashion a specific binding site -- a kind of pocket that
can only accommodate the metal ion of choice.
"In addition to lead, the selection process can be
customized to select catalytic DNA that would be active
for other metal ions, such as mercury, cadmium and
zinc," Lu said.
The dynamic response of the sensor solution can be
tuned over a wide range by introducing inactive
catalytic DNA into the mix, Lu said. Incorporating more
of the inactive DNA will shift the sensor's sensitivity
to higher contaminant concentrations without saturation.
By using various combinations of active and inactive
catalytic DNA, the sensor could be packaged as a
colorimetric array to detect different contaminant
"There are many old houses around the world that
still contain leaded paint," Lu said. "According to the
U.S. Environmental Protection Agency, leaded paint test
kits that are currently available have shown high rates
of both false positive and false negative results when
compared to laboratory results. Our catalytic DNA-gold
nanoparticle sensor can overcome these shortcomings."
Lu is also working with colleagues at the National
Science Foundation's Nanoscale Science and Engineering
Center for Directed Assembly of Nanostructures (a
partnership among Illinois, the Rensselaer Polytechnic
Institute and the Los Alamos National Laboratory) to
further develop the biosensor technology. For example,
Lu is working with Illinois collaborators Paul Braun and
Gerard Wong to produce nanoparticles from different
"Our ultimate goal is to develop a microchip array
with different color schemes for simultaneously
detecting many different metal ions," Lu said.
Funding was provided by the U.S. Department of Energy
and the National Science Foundation.
Contact: James E. Kloeppel, Physical Sciences
of Illinois at Urbana-Champaign