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Selective coatings create biological sensors from carbon nanotubes
James
E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
12/13/2004
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CHAMPAIGN, Ill.
— Protein-encapsulated single-walled carbon nanotubes that alter
their fluorescence in the presence of specific biomolecules could generate
many new types of implantable biological sensors, say researchers from
the University of Illinois at Urbana-Champaign who developed the encapsulation
technique.
In a paper accepted for publication in the journal Nature Materials,
and posted on its Web site, the researchers showed the viability of
their technique by creating a near-infrared nanoscale sensor that detects
glucose. The sensor could be inserted into tissue, excited with a laser
pointer, and provide real-time, continuous monitoring of blood glucose
level.
“Carbon nanotubes naturally fluoresce in the near-infrared region
of the spectrum where human tissue and biological fluids are particularly
transparent,” said Michael Strano, a professor of chemical
and biomolecular engineering at Illinois. “We have developed
molecular sheaths around the nanotube that respond to a particular chemical
and modulate the nanotube’s optical properties.”
To make their biological sensors, Strano, postdoctoral research associate
Seunghyun Baik, and graduate students Paul Barone and Daniel Heller
begin by assembling a monolayer of the enzyme glucose oxidase on the
surface of nanotubes suspended in water. The enzyme not only prevents
the nanotubes from sticking together into useless clumps, it also acts
as a selective site where glucose will bind and generate hydrogen peroxide.
Next, the researchers functionalize the surface with ferricyanide, an
ion that is sensitive to hydrogen peroxide. The ion attaches to the
surface through the porous monolayer. When present, hydrogen peroxide
will form a complex with the ion, which changes the electron density
of the nanotube and consequently its optical properties.
“When glucose encounters the enzyme, hydrogen peroxide is produced,
which quickly reacts with the ferricyanide to modulate the electronic
structure and optical characteristics of the nanotube,” Strano
said. “The more glucose that is present, the brighter the nanotube
will fluoresce.”
To prove the practicality of their technique, Strano’s team loaded
some of the sensors into a porous capillary that confined the nanotubes
but allowed glucose to enter. When inserted into human tissue, the fluorescent
emission of the sensor corresponded to the local glucose concentration.
“The advantage of the near-infrared signaling to and from such
a capillary device is its potential for implantation into thick tissue
or whole blood media, where the signal may penetrate up to several centimeters,”
Strano said. “And, because nanotubes won’t degrade like
organic molecules that fluoresce, these nanoparticle optical sensors
would be suitable for long-term monitoring applications.”
One important aspect of the new surface chemistry, Strano said, is that
no bonds are broken on the nanotube. “This allows us to shuttle
electrons in and out without damaging the nanotube itself.”
Another important aspect is that the technique can be extended to many
other chemical systems. “We’ve shown that it is possible
to tailor the surface to make it selective to a particular analyte,”
Strano said. “There are whole classes of analytes that can be
detected in this manner.”
The National Science Foundation funded the work. The researchers have
applied for a patent.
