Technique to Develop Class of Optical Materials and Devices

A one-of-a-kind method for developing completely new categories of optical materials and devices, which would enable the creation of cloaking and light bending devices, has been devised by scientists from Northwestern University. This news is sure to perk up the ears of Spock from Star Trek.

Chad Mirkin. (Image credit: Northwestern University)

The interdisciplinary team used DNA as an important tool and selected gold nanoparticles of varying shapes and sizes, arranging them in two as well as three dimensions to create optically active superlattices. According to the researchers, structures that have particular configurations can be organized by selection of particle type and both DNA pattern and sequence to display nearly any color over the visible spectrum.

Architecture is everything when designing new materials, and we now have a new way to precisely control particle architectures over large areas,” stated Chad A. Mirkin, the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences at Northwestern. “Chemists and physicists will be able to build an almost infinite number of new structures with all sorts of interesting properties. These structures cannot be made by any known technique.”

The method integrates an old fabrication method, that is, top-down lithography adopted to fabricate computer chips, with a new method, programmable self-assembly induced by DNA. The Northwestern researchers are the first to integrate both the methods to accomplish individual particle control in three dimensions.

The research was published online on January 18, 2018, in the Science journal. Co-corresponding authors of the study are Mirkin and Vinayak P. Dravid and Koray Aydin, both professors in Northwestern’s McCormick School of Engineering.

Researchers can use this dynamic and flexible method to develop metamaterials (or materials not available in nature) for a wide variety of applications, such as sensors for environmental and medical uses.

The team used a blend of optical spectroscopy methods and numerical simulations to recognize specific nanoparticle superlattices with the ability to absorb particular wavelengths of visible light. The DNA-modified nanoparticles, which is gold here, are located on a pre-patterned template formed of complementary DNA. Stacks of these structures can be built if second and third DNA-modified particles including DNA complementary to the subsequent layers are introduced.

Apart from being atypical structures, these materials are responsive to stimuli—the length of the DNA strands holding them intact get altered upon exposure to new environments, for instance, solutions of ethanol of differing concentration. The team discovered that the alteration in the length of the DNA led to change in color from black to red to green, enabling exceptional tunability of optical characteristics.

Tuning the optical properties of metamaterials is a significant challenge, and our study achieves one of the highest tunability ranges achieved to date in optical metamaterials.

Our novel metamaterial platform—enabled by precise and extreme control of gold nanoparticle shape, size and spacing—holds significant promise for next-generation optical metamaterials and metasurfaces,

Koray Aydin, Assistant Professor of Electrical Engineering and Computer Science

The research reports an innovative technique for arranging nanoparticles in two and three dimensions. The team adopted lithography techniques to drill minute holes, with a width of just one nanoparticle, into a polymer resist, thereby forming “landing pads” for nanoparticle components altered with DNA strands. Mirkin stated that the landing pads are crucial because they maintain the vertically grown structures intact.

One sequence of DNA is used to modify the nanoscopic landing pads, and complementary DNA are used to modify the gold nanoparticles. The team alternated nanoparticles with complementary DNA to build nanoparticle stacks with excellent positional regulation and across a large area. The particles can be in distinct shapes—for instance, cubes, spheres, and disks—and sizes.

This approach can be used to build periodic lattices from optically active particles, such as gold, silver and any other material that can be modified with DNA, with extraordinary nanoscale precision,” stated Mirkin, director of Northwestern’s International Institute for Nanotechnology.

Mirkin is also a professor of medicine at Northwestern University Feinberg School of Medicine and professor of chemical and biological engineering, biomedical engineering and materials science and engineering in the McCormick School.

The favorable working of this DNA programmable assembly mandated proficiency with hybrid, or soft-hard, materials and perfect lithographic and nanopatterning potential to accomplish the necessary definition, spatial resolution, and fidelity over large substrate areas. The researchers sought the help of Dravid, a longtime colleague of Mirkin, specializing in advanced microscopy, nanopatterning, and characterization of hard, soft, and hybrid nanostructures.

Dravid offered his proficiency and helped in developing the lithography and nanopatterning strategy and the related portrayal of the innovative, exotic structures. He is the Abraham Harris Professor of Materials Science and Engineering in McCormick and the founding director of the NUANCE center, where the advanced lithography, patterning, and characterization adopted in the DNA-programmed structures are housed.

The first authors of the study titled “Building Superlattices from Individual Nanoparticles via Template-Confined DNA-Mediated Assembly” are Qing-Yuan Lin, Jarad A. Mason, and Zhongyang Li.

The study was supported by the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (award #DE-SC0000989), and the Air Force Office of Scientific Research (award numbers FA9550-12-1-0280, FA9550-14-1-0274, and FA9550-17-1-0348).

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