18 Biotechnology
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particles – both with high yields and low levels of other, “The size and distance between the linked particles
unwanted assemblies. affect the plasmonic behaviour,” said Gang. By adjusting
“When we arrange a few nanoparticles in a these parameters, scientists might engineer clusters
particular structure, new properties can emerge,” Gang for absorbing a range of wavelengths in solar-energy
emphasised. “Nanoparticles in this case are analogous conversion devices. Modulations in the plasmonic
to atoms, which, when connected in a molecule, often response could also be useful as a new means for
exhibit properties not found in the individual atoms. Our transferring data, or as a signal for a new class of highly
approach allows for rational and efficient assembly of specific biosensors.
nano-'molecules'. The properties of these new materials Asymmetric clusters, which were also assembled
may be advantageous for many potential applications.” by the Brookhaven team, allow an even higher level of
For example, in the paper, the scientists describe control, and therefore open new ways to design and
an optical effect that occurs when nanoparticles are engineer functional nanomaterials.
linked as dimer clusters. When an electromagnetic field Because of its reliability and precision control,
interacts with the metallic particles, it induces a collective Brookhaven’s nano-assembly method would be scalable
oscillation of the material’s conductive electrons. This for the kind of high-throughput production that would be
phenomenon, known as a plasmon resonance, leads to essential for commercial applications. (Fig. 2) Brookhaven
strong absorption of light at a specific wavelength. Lab has applied for a patent on the assembly method as
well as several specific applications of the technology.
This latest advance builds on work carried out at
Brookhaven last year. At that stage the team had for the
first time used DNA to guide the creation of 3D, ordered,
crystalline structures of nanoparticles. This is essential to
produce functional materials that take advantage of the
properties that exist at the nanoscale.
First, the scientists attached to nanoparticles hair-like
extensions of DNA with specific 'recognition sequences'
of complementary bases. Then they mixed the
DNA-covered particles in solution. When the recognition
sequences find one another in solution, they bind together
to link the nanoparticles.
This first binding is necessary, but not sufficient, to
produce the organised structures the scientists were
seeking. To achieve ordered crystals, the scientists altered
the properties of DNA and borrowed some techniques
known for traditional crystals.
Importantly, they heated the samples of DNA-linked
particles and then cooled them back to room temperature.
Such thermal processing allows the nanoparticles
to unbind, reshuffle, and find more stable binding
arrangements.
The team also experimented with different degrees of
DNA flexibility, recognition sequences, and DNA designs
in order to find a 'sweet spot' of interactions where a
stable, crystalline form would appear.
Fig. 2. Using DNA to assemble nanoclusters: (a) (1) Results from a variety of analysis techniques,
DNA linker strands (squiggly lines) are used to attach including small angle x-ray scattering at the National
DNA-coated nanoparticles to a surface. (2) Linker Synchrotron Light Source and dynamic light scattering
strands are attached to the top side of the nanoparticle. and different types of optical spectroscopies and electron
(b) (3a) A nanoparticle of a second type with microscopy, were combined to reveal the detail of the
complementary DNA encoding recognises the exposed ordered structures and the underlying processes for their
linker strands and attaches to the surface-anchored formation.
nanoparticle. (4a and 5a) The assembled structure These results indicated that the scientists had
is released from the surface support, resulting in a indeed found that sweet spot to create 3D nanoparticle
two-particle, dimer cluster. (c) (3b) Alternatively, the assemblies with long-range crystalline order using
immobilised particles produced in step (a) are released DNA. The crystals were remarkably open, with the
from the surface, leaving the opposite-side linker nanoparticles themselves occupying only five per cent of
strands free to bind with multiple particles (4b) to form the crystal lattice volume, and DNA occupying another
asymmetric ‘Janus’ clusters. five per cent. u
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