Optical material tailored from DNA

Ashley N. Paddock, ashley.paddock@photonics.com

The DNA-origami technique has enabled the construction of nanospiral staircases from artificial blocks of DNA molecules. The approach could be used to modify light in very specific ways and could lead to the development of “superlenses.”

An international team of scientists led by professor Tim Liedl of Ludwig Maximilian University of Munich and professor Friedrich C. Simmel of Technical University of Munich built the 57 x 34-nm staircases with 10-nm gold particles attached at regular intervals.

Although chemically alike, solutions of right- and left-handed nanospiral staircases interact in specific ways with circular polarized light. The staircases were built using the DNA-origami method. Courtesy of Tim Liedl, Ludwig Maximilian University.

Coupling light and nanostructures may help to significantly reduce the size of optical sensors for medical and environmental applications, while also making them more sensitive. However, a light wave that stretches over 400 to 800 nm is quite large in comparison with nanostructures of only a few nanometers. Yet, in theory, when the tiniest of structures work together in very specific ways, they, too, can interact well with light. Until now, it has not been possible to produce the requisite 3-D structures with nanoscale precision in sufficient quantities and purity using conventional methods.

“With DNA origami, we have now found a methodology that fulfills all of these requirements,” Simmel said. “It makes it possible to define in advance and with nanometer precision the three-dimensional shape of the object being created. Programmed solely using the sequence of basic building blocks, the nanoelements fold themselves into the desired structures.”

Electrons on the surface of the gold particles react with the electromagnetic field of light. The small clearance between the particles ensures that the gold particles of a DNA strand work in unison, amplifying the interactions manyfold. Theoretical physicist Alexander O. Govorov of Ohio University in Athens previously said that the effect should depend on the spacing, size and composition of the metal particles. The Munich physicists varied these parameters as they built the nanostructures using the DNA-origami method.

Their findings confirmed that aqueous solutions of right- and left-handed nanospiral 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.

Fluids containing gold particles organized in chiral configurations exhibit designable optical activity. The particles are assembled with nanometer precision in space with the help of a rigid DNA-origami construct. Courtesy of Kuzyk, Schreiber, Hohmann.

They also discovered that the particles' chemical composition plays a significant role: When the gold particles were coated with a layer of silver, the optical resonance shifted from the red to the shorter-wave blue domain.

“We will now investigate whether we can use this method to influence the refraction index of the materials we manufacture,” Liedl said. “Materials with a negative refractive index could be used to develop novel optical lens systems — so-called superlenses.”

The work was funded by the Volkswagen Foundation, the DFG Cluster of Excellence Nanosystems Initiative Munich and the National Science Foundation.