Tailored optical material from DNA: Nano spiral staircases modify light

There was a lot of excitement a few years ago following the discovery of the DNA origami technique. The approach could be used to build nanoparticles of a given shape and size. However, real applications, such as nano-tweezers, remained out of reach. An international team of researchers led by Professor Tim Liedl of the Ludwig-Maximillians-Universitaet Muenchen and Professor Friedrich Simmel of the Technische Universitaet Muenchen have now succeeded in building nanoparticles using optically active DNA building blocks that can be used to modify light in very specific ways.

Coupling light and nanostructures may help significantly reduce the size of optical sensors for medical and environmental applications, while at the same time making them more sensitive. However, the size of a light wave stretching out over 400 to 800 nanometers is gigantic in comparison to nanostructures of only a few nanometers. Yet in theory, when tiniest structures work together in very specific ways, even small objects can interact very well with light. Unfortunately it is not possible to produce the requisite three-dimensional structures with nano-scale precision in sufficient quantities and purity using conventional methods.

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A team of scientists from Technische Universitaet Muenchen and Ludwig-Maximilians-Universitaet Muenchen succeeded in building up nano spiral staircases from artificial DNA using the DNA-origami method. The DNA strand carries nine gold particles which lead to strong interactions with circular polarized visible light. Credit: Animation: Tim Liedl /LMU

"With DNA origami, we have now found a methodology that fulfills all of these requirements. It makes it possible to define in advance and with nanometer precision the three-dimensional shape of the object being created," says Professor Friedrich Simmel, who holds the Chair for Biomolecular Systems and Bionanotechnology at the TU Muenchen. Programmed solely using the sequence of basic building blocks, the nano-elements fold themselves into the desired structures." Friedrich Simmel's team successfully built nano spiral staircases 57 nanometers high and 34 nanometers in diameter with 10 nanometer gold particles attached at regular intervals.

On the surface of the gold particles the electrons react with the electromagnetic field of the light. The small clearance between the particles ensures that the gold particles of a DNA strand work in unison, thereby amplifying the interactions many fold. Professor Alexander O. Govorov, theoretical physicist at the Ohio State University in Athens, USA, had predicted that the effect should depend on the spacing, size and composition of the metal particles. Using the DNA origami method, the Munich physicists built up nanostructures in which they varied these parameters.

The results of these experiments confirm the predictions of their colleagues in every regard: Aqueous solutions of right-handed and left-handed nano spiral staircases differ visibly in their interactions with circular polarized light. Spiral staircases with large particles show a significantly stronger optical response than those with small particles. The chemical composition of the particles also had a large effect: When the gold particles were coated with a layer of silver, the optical resonance shifted from the red to the shorter wave blue domain.

By combining theoretical calculations and the possibilities of DNA origami, the researchers are now able to produce nano-optical materials with precisely specified characteristics. Professor Tim Liedl describes the path the research might follow: "We will now investigate whether we can use this method to influence the refraction index of the materials we manufacture. Materials with a negative refractive index could be used to develop novel optical lens systems – so-called super lenses."

More information: DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response. Anton Kuzyk, Robert Schreiber, Zhiyuan Fan, Günther Pardatscher, Eva-Maria Roller, Alexander Högele, Friedrich C. Simmel, Alexander O. Govorov und Tim Liedl.
Nature Volume 482, 7389, pp 311-314. DOI: 10.1038/nature10889

Provided by Technische Universitaet Muenchen

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Graphene and DNA: 'Wonder material' may hold key to fast, inexpensive genetic sequencing

But when scientists found a way—using, essentially, a piece of ordinary sticky tape—to peel off a layer of graphite that was just a single atom thick, they called the two-dimensional material graphene and, in 2010, won the Nobel Prize in physics for the discovery.

Now, researchers at the University of Delaware have conducted high-performance computer modeling to investigate a new approach for ultrafast DNA sequencing based on tiny holes, called nanopores, drilled into a sheet of graphene.

"Graphene is a two-dimensional sheet of carbon atoms arranged in a honeycomb pattern" Branislav Nikolic, associate professor of physics and astronomy, said. "The mechanical stability of graphene makes it possible to use an electron beam to sculpt a nanopore in a suspended sheet of graphene, as demonstrated in 2008 by Marija Drndic at the University of Pennsylvania.”

Graphene has been among the fastest-growing areas of study in nanoscience and technology over the past five years, Nikolic said. He calls it a wonder material that has remarkable mechanical, electronic and optical properties and is being investigated for a variety of applications as diverse as plastic packaging and next-generation gigahertz transistors.

In the sequencing that he and other physicists have proposed, a tiny hole a few nanometers in diameter is drilled into a sheet of graphene and DNA is threaded through that nanopore. Then, a current of ions flowing vertically through the pore or an electronic current flowing transversely through the graphene is used to detect the presence of different DNA bases within the nanopore.

“Since graphene is only one atom thick, the nanopore through which DNA is threaded has contact with only a single DNA base,” Nikolic said.

In 2010, three experimental teams—led by Jene Golovchenko of Harvard, Cees Dekker of Delft and Drndic—demonstrated DNA detection using nanopores in large-area graphene. However, Nikolic said, the process moved too quickly for the existing electronics to detect single DNA bases.

The new device concept proposed by the UD researchers uses graphene nanoribbons—thin strips of graphene that are less than 10 nanometers wide—with a nanopore drilled in their interior. Chemists, engineers, materials scientists and physicists have devised various methods over the past three years to fabricate nanoribbons with a specific zigzag pattern of carbon atoms along their edges, Nikolic said. Nanoribbons could enable fast and low-cost (less than $1,000) DNA sequencing, he said, because of the quantum-mechanically generated electronic currents that flow along those edges.

Such quick and inexpensive DNA sequencing could usher in an era of personalized medicine, Nikolic said.

"We used the knowledge acquired from several years of theoretical and computational research on the electronic transport in graphene to increase the magnitude of the detection current in our biosensor by a thousand to million times when compared to other recently considered devices," Nikolic said. "Two years ago, scientists would have told me our device was impossible, but there are so many people working on graphene that nothing is impossible anymore.

"Every time physicists think something is impossible, materials scientists or chemists come to the rescue—and vice versa."

Nikolic said he and postdoctoral researcher Kamal Saha have employed their home-grown massively parallel computational codes to simulate the operation of the proposed nanoelectronic biosensor from first principles, using the supercomputer Chimera that UD acquired with support from a National Science Foundation grant.

"This project has to run on 500-1,000 processors for several months continuously," he said. "We couldn't have done it without UD Chimera becoming fully operational in early 2011."

Nikolic, Saha and Drndic have recently published the results of this research in an article in the prestigious Nano Letters, a journal with an impact factor of 12.219 published by the American Chemical Society.  Colleagues, led by Drndic at the University of Pennsylvania, will now seek to fabricate the biosensors in their lab, guided by the simulations presented in the article. Nikolic said that this research synergy will, in turn, allow for simulations of improved device designs. 

Provided by University of Delaware (news : web)

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