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New ribbon like nanostructures assembled by DNA binding method
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U.S. Department of Energy's Brookhaven National Laboratory scientists have made a novel discovery; DNA strands used as linkers can coax nano-sized rods to line up in way previously unseen in any other spontaneous arrangement of rod-shaped objects. The arrangement, with the rods forming "rungs" on ladder-like ribbons linked by multiple DNA strands, results from the sum of interactions of the flexible DNA tethers and may be unique to the nanoscale.

This new discovery could result in the fabrication of new nanostructured materials with desired properties.

Brookhaven physicist Oleg Gang said: "This is a completely new mechanism of self-assembly that does not have direct analogs in the realm of molecular or microscale systems,", Gang is the lead author on the paper, and conducted the bulk of the research at the Lab's Center for Functional Nanomaterials.

Many different classes of rod-like objects, from molecules to viruses, often exhibit classical liquid-crystal-like behavior in similar systems, where the rods align with a dependence on direction, sometimes with the aligned crystals forming two-dimensional planes over a given area. Rod shaped objects with strong directionality and attractive forces between their ends, resulting, for example, from polarized charge distribution, may also sometimes line up end-to-end forming linear one-dimensional chains.
The interesting thing is that neither of these typical arrangements is found in the DNA-tethered nanorods.
"Our discovery shows that a qualitatively new regime emerges for nanoscale objects decorated with flexible molecular tethers of comparable sizes-a one-dimensional ladder-like linear arrangement that appears in the absence of end-to-end affinity among the rods," Gang said.

Alexei Tkachenko, the CFN scientist, who worked up a theory to explain such exceptional behavior, explains:
"Remarkably, the system has all three dimensions to live in, yet it chooses to form the linear, almost one-dimensional ribbons. It can be compared to how extra dimensions that are hypothesized by high-energy physicists become 'hidden,' so that we find ourselves in a 3-D world. Once a nanorod connects to another one side-by-side, it loses the cylindrical symmetry it had when it had free tethers all around. Then, the next nanorod will preferentially bind to another side of the first, where there are still DNA linkers available."

The central approach of Gang's research at the CFN has been the idea of using synthetic DNA as a form of molecular glue to guide nanoparticle assembly.

In his previous work he has shown that strands of DNA can pull nanoparticles together when strands bearing complementary sequences of nucleotide bases are used as linkers, or inhibit binding when unmatched strands are used. Carefully controlling those adhering or repulsing forces can lead to fine-tuned nanoscale engineering.

In this study, the scientists used nanorods made of gold and single strands of DNA to explore the arrangements made with complementary tethers attached to adjacent rods. They also experimented with the effects of using linker strands of different lengths to serve as the tethering glue.

They studied the resulting arrangements using ultraviolet-visible spectroscopy at the CFN, and also with small-angle x-ray scattering at Brookhaven's National Synchrotron Light Source.
To better understand how the process progresses over time they used a technique to freeze the assembly in several points and then use scanning electron microscopy to get the images.

This specific assembly process, called hierarchical, is reminiscent of self-assembly in many biological systems.

Stopping the assembly process at the ladder-like ribbon stage could potentially be applied for the fabrication of linear structures with engineered properties. For example by controlling plasmonic or fluorescent properties-the materials' responses to light-we might be able to make nanoscale light concentrators or light guides, and be able to switch them on demand." Gang said.

Paper published online in ACS Nano, a journal of the American Chemical Society
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