This reproduction of The Starry Night contains 65,536 glowing pixels and is just the width of a dime across. Credit: Paul Rothemund and Ashwin Gopinath/Caltech
A hurdle for large-scale integration of molecular devices on chips has been removed by a technique that allows humanmade DNA shapes to be placed wherever desired, to within a margin of error of just 20nm. Using folded DNA to precisely place glowing molecules within microscopic light resonators, researchers at Caltech have created one of the world’s smallest reproductions of Vincent van Gogh’s The Starry Night. The monochrome image – just the width of a dime across – was a proof-of-concept project that demonstrated, for the first time, how the precision placement of DNA origami can be used to build chip-based devices like computer circuits at smaller scales than ever before.
DNA origami, developed 10 years ago by Caltech’s Paul Rothemund (BS ’94), is a technique that allows researchers to fold a long strand of DNA into any desired shape. The folded DNA then acts as a scaffold onto which researchers can attach and organize all kinds of nanometer-scale components, from fluorescent molecules to electrically conductive carbon nanotubes to drugs.
Applications include drug delivery, nanoscale computers etc. But organizing nanoscale components is not enough; the devices have to be wired together into larger circuits and need to have a way of communicating with larger-scale devices. One early approach was to make electrodes first, and then scatter devices randomly on a surface, with the expectation that at least a few would land where desired, a method Rothemund describes as “spray and pray.”
DNA origami, binding sites for placement, and photonic crystal cavities.
In 2009, Rothemund and colleagues at IBM Research first described a technique through which DNA origami can be positioned at precise locations on surfaces using electron-beam lithography to etch sticky binding sites that have the same shape as the origami. For example, triangular sticky patches bind triangularly folded DNA.
Over the last seven years, DNA shapes can be precisely positioned on almost any surface used in the manufacture of computer chips. In the Nature paper, they report the first application of the technique — using DNA origami to install fluorescent molecules into microscopic light sources. In this case, the lamps are microfabricated structures: photonic crystal cavities (PCCs), which are tuned to resonate at a particular wavelength of light. Created within a thin glass-like membrane, a PCC takes the form of a bacterium-shaped defect in a perfect honeycomb of holes.
AFM and schematics show that up to seven origami can be placed in distinct antinodes of a single PCC
“Depending on the exact size and spacing of the holes, a particular wavelength of light reflects off the edge of the cavity and gets trapped inside,” says Gopinath. He built PCCs that are tuned to resonate at around 660 nm, the wavelength corresponding to a deep shade of the color red. Fluorescent molecules tuned to glow at a similar wavelength light up the lamps – provided they stick to exactly the right place within the PCC. “A fluorescent molecule tuned to the same color as a PCC actually glows more brightly inside the cavity, but the strength of this coupling effect depends strongly on the molecule’s position within the cavity. A few tens of nanometers is the difference between the molecule glowing brightly, or not at all,” Gopinath says.
By moving DNA origami through the PCCs in 20-nm steps, the researchers found that they could map out a checkerboard pattern of hot and cold spots, where the molecular light bulbs either glowed weakly or strongly. As a result, they were able to use DNA origami to position fluorescent molecules to make lamps of varying intensity. Similar structures have been proposed to power quantum computers and for use in other optical applications that require many tiny light sources integrated together on a single chip.
Schema for recreation of Vincent van Gogh’s painting The Starry Night (1889).
By creating PCCs with different numbers of binding sites, Gopinath reliably installed any number from 0 to 7 DNA origami, allowing him to digitally control the brightness of each lamp. He treated each lamp as a pixel with one of 8 different intensities, and produced an array of 65,536 of the PCC pixels (a 256 x 256 pixel grid) to create a reproduction of Van Gogh’s “The Starry Night.” Now that the team can reliably combine molecules with PCCs, they are working to improve the light emitters. Currently, the fluorescent molecules last about 45s before reacting with oxygen and “burning out,” and they emit a few shades of red rather than a single pure color. Solving both these problems will help with applications such as quantum computers. “Aside from applications, there’s a lot of fundamental science to be done,” Gopinath says.
http://www.caltech.edu/news/dna-origami-lights-microscopic-glowing-van-gogh-51280
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature18287.html
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