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"This is the first
nanotube-based sensor that can detect analytes at the subcellular
level," said Michael Strano, a professor of
chemical and
biomolecular engineering at Illinois and corresponding author of
a paper that appeared in the Jan. 27 issue of the journal Science.
"We also show for the first time that a subtle rearrangement of an
adsorbed biomolecule can be directly detected by a carbon nanotube."
At the heart of the new detection system is the transition of DNA
secondary structure from the native, right-handed B form to the
alternate, left-handed Z form.
"We found that the thermodynamics that drive the switching back
and forth between these two forms of DNA structure would modulate
the electronic structure and optical emission of the carbon nanotube,"
said Strano, who is also a researcher at the
Beckman Institute for
Advanced Science and Technology and at the university's
Micro and Nanotechnology
Laboratory.
To make their sensors, the researchers begin by wrapping a piece
of double-stranded DNA around the surface of a single-walled carbon
nanotube, in much the same fashion as a telephone cord wraps around
a pencil. The DNA starts out wrapping around the nanotube with a
certain shape that is defined by the negative charges along its
backbone.
When the DNA is exposed to ions of certain atoms -- such as
calcium, mercury and sodium -- the negative charges become
neutralized and the DNA changes shape in a similar manner to its
natural shape-shift from the B form to Z form. This reduces the
surface area covered by the DNA, perturbing the electronic structure
and shifting the nanotube's natural, near-infrared fluorescence to a
lower energy.
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 "The change in emission energy indicates how many ions bind to
the DNA," said graduate student Daniel Heller, lead author of the
Science paper. "Removing the ions will return the emission energy to
its initial value and flip the DNA back to the starting form, making
the process reversible and reusable."
The researchers demonstrated the viability of their measurement
technique by detecting low concentrations of mercury ions in whole
blood, opaque solutions, and living mammalian cells and tissues --
examples where optical sensing is usually poor or ineffective.
Because the signal is in the near-infrared, a property unique to
only a handful of materials, it is not obscured by the natural
fluorescence of polymers and living tissues.
"The nanotube surface acts as the sensor by detecting the shape
change of the DNA as it responds to the presence of target ions,"
Heller said.
Co-authors of the paper with Strano and Heller were graduate
student Esther Jeng and undergraduate students Tsun-Kwan Yeung,
Brittany Martinez, Anthonie Moll and Joseph Gastala.
The work was funded by the National Science Foundation..
[News release from
University of Illinois at Urbana-Champaign]
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