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Despite much progress made in the self-assembly of
nanomaterials, defects that occur during the assembly process still
present major obstacles for applications such as molecular electronics
and photonics. Efforts to overcome this problem have focused on
optimizing the assembly process to minimize errors, and designing
devices that can tolerate errors.
"Instead of trying to avoid defects or work around
them, it makes more sense to accept defects as part of the process and
then correct them during and after the assembly process," said Yi Lu,
a chemistry professor at Illinois and a researcher at the Beckman
Institute for Advanced Science and Technology. "This procedure is
analogous to how nature deals with defects, and can be applied to the
assembly of nanomaterials using biomolecules or biomimetic compounds."
In protein synthesis, nature ensures accuracy by
utilizing a proofreading unit that detects and corrects errors in
translation, often through hydrolysis of incorrect amino acid building
blocks. In a similar fashion, Lu and graduate students Juewen Liu and
Daryl Wernette utilized catalytic DNA to locate and remove errors in a
DNA-templated gold nanoparticle assembly process. The researchers
describe the procedure in a paper accepted for publication in the
journal Angewandte Chemie International Edition, and posted on its Web
site.
Catalytic DNA contains a substrate strand and an
enzyme strand. In the presence of certain ions, the substrate is
cleaved by the enzyme into two pieces of unequal length. The cleaved
fragment with the shorter binding arm can be easily released. This
catalytic DNA serves as a template for assembly of nanoparticles.
There are three kinds of nanoparticles encoded by
different DNA in the system: two are defined as "correct" particles
and one is defined as a "wrong" particle. Besides the difference in
coding DNA, the nanoparticles can also be different in other aspects,
such as size.
"To allow the catalytic DNA substrate to be a
template for nanoparticle assembly, the substrate strand must be
complementary to the DNA attached to the nanoparticles," Lu said. "A
defect can occur in a DNA-templated gold nanoparticle assembly when
the wrong particle is incorporated into the structure."
When a particle of the correct size is encountered,
binding of the longer arm of the enzyme to the DNA template is
permitted, while binding of the shorter arm to the DNA template is
inhibited. "The active structure of the catalytic DNA cannot form," Lu
said. "As a result, the template is not cleaved and the particle is
incorporated into the assembly."
When a particle of the wrong size is mistakenly
incorporated into the assembly, the enzyme can bind both its arms to
the substrate template and form an active structure to cleave the
substrate and remove the particle.
By showing that defects -- the wrong size particles,
in this case -- can be identified and removed, the researchers
demonstrated that proofreading and error-correction can take place
during and after the assembly of nanoparticles.
"This was a small, but definite, step in the right
direction," Lu said. "The error-correction procedure can be expanded
to include many other biomolecules and biomimetic compounds for
controlling the assembly of nanoparticles of defined particle sizes,
shapes or compositions; as well as other nanomaterials, such as
nanotubes and nanowires." |
