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