EVANSTON, Ill., Jan. 19, 2018 — Researchers have developed a technique for creating optical structures that could lead to new classes of optical materials and devices. Their technique combines the traditional fabrication method of top-down lithography with a new method, programmable self-assembly driven by DNA. They used the technique to build optically active superlattices of discrete nanoparticle architectures on a gold surface over large areas. These superlattices, which would be difficult to construct using typical assembly methods, could provide a platform to systematically study and control light-matter interactions in nanoparticle-based optical materials.

A Northwestern University team took individual colloidal plasmonic nanoparticles of different shapes and sizes and arranged them in two and three dimensions to form the superlattices. Using lithography methods, the  researchers drilled tiny holes — just one nanoparticle wide — in a polymer resist, creating “landing pads” for nanoparticle components modified with strands of DNA. Configuration of the structures was controlled through DNA molecules containing “locked” nucleic acids and confined environments (landing pads) provided by the polymer pores.

Northwestern University researchers have developed a new method to precisely arrange nanoparticles of different sizes and shapes in two and three dimensions, resulting in optically active superlattices. Courtesy of Northwestern University.

The nanoscopic landing pads were modified with one sequence of DNA, and the gold nanoparticles were modified with complementary DNA. The DNA-modified gold nanoparticles were positioned on a prepatterned template made from DNA. Stacks of structures were built by introducing a second and then a third DNA-modified particle with DNA that complemented the subsequent layers.

By alternating nanoparticles with complementary DNA, the researchers were able to build nanoparticle stacks with tremendous positional control and over a large area. The particles could be different sizes and shapes, such as spheres, cubes or disks.

The resulting architectures featured tunable arrangements and independently controllable distances at both nanometer and micrometer length scales. 

“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,” said professor Chad Mirkin. 

Researchers used numerical simulations and optical spectroscopy techniques to identify which wavelengths of visible light were absorbed by the different superlattices. They reported that the structures could exhibit almost any color across the visible spectrum.

In addition, the materials were shown to be stimuli-responsive, in other words, the DNA strands that hold them together change in length when exposed to new environments, such as solutions of ethanol that vary in concentration. The change in DNA length, the researchers found, caused a change of color from black to red to green, providing extreme tunability of optical properties.

“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,” said professor Koray Aydin.

The team believes that the technique could be used to build metamaterials for a range of applications including sensors for medical and environmental uses.

“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,” Aydin said.

“Architecture is everything when designing new materials, and we now have a new way to precisely control particle architectures over large areas,” said Mirkin. “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.”