Nanoparticle Crystal Used to Control Light

A clathrate colloidal crystal has been created by connecting nanoparticles using DNA. One potential application for the novel crystal structures is the control of light, which could lead to development of materials that could change colors or patterns on demand, or block certain wavelengths of light while amplifying others.

An electron microscope image of a slice of the crystal structure (left). Courtesy of Mirkin Lab, Northwestern University. A matching slice from a simulation of the crystal structure (right). Courtesy of Glotzer Group, University of Michigan / Science.

Designed by researchers at Northwestern University and the University of Michigan, the novel crystal structure is composed of nanoparticle clusters that are connected through DNA strands that act as a sort of “smart glue,” linking the nanoparticles together in a specific way.

The Northwestern team has pioneered the DNA-programmable assembly of nanostructures, while the University of Michigan team has used computer simulation to explore the role of nanoparticle shape in guiding the assembly of crystal structures.

Micrographs taken by the Northwestern team showed complex crystalline structures whose forms were largely based on the shape of the nanoparticles from which they were made. The triangular bipyramidal shape of the Northwestern structures was similar to a shape that the University of Michigan team had predicted would form a quasicrystal. Northwestern’s crystalline structures had the angles necessary to make clathrates, which often turn up in molecular systems that form quasicrystals.

Northwestern researcher Haixin Lin made bipyramids of consistent size and shape, with edge lengths of 250 nm, which he then modified with different length sequences of DNA. When the DNA strands were too short, the nanoparticles made disordered, ill-defined structures. Longer strands were shown in micrographs to produce exotic patterns.

“These are stunning — no one has made such structures before,” said professor Chad A. Mirkin from Northwestern.

The Northwestern team’s next step was to accurately identify the structure. To do so, they turned to the University of Michigan team for assistance.

These are the gold nanoparticle bipyramids assembled into a complex crystal structure, known in chemistry as a clathrate. Courtesy of Glotzer Group, University of Michigan / Science.

University of Michigan researchers 3D-printed the bipyramids and glued them together to explore how they might make the structures that were revealed in the electron micrographs. Once they saw how the shapes fit together, they hypothesized the clathrate structures. For confirmation, they built a computer model of the hypothesized clathrates from bipyramids and compared it to the Northwestern micrographs. It was a perfect match.

As a definitive test, the University of Michigan team developed a molecular model of the DNA-linked nanoparticles and carried out simulations to confirm that the particles would indeed form clathrate structures.

“To really know for sure, we had to run simulations that mimicked the conditions Haixin used in the lab to see if a disordered fluid of DNA-linked bipyramids would assemble into the Northwestern crystals,” professor Sharon C. Glotzer, from the University of Michigan, said. “Once we saw the computer crystals, I knew we had nailed it.”

Novel types of lenses, lasers and even Star Trek-like cloaking materials could be possible using materials made from the clathrate colloidal crystal structures.

“This is a tour de force demonstration of what is possible when one harnesses the chemistry of DNA and combines it with nanoparticles whose shapes encourage a particular crystal structure," said Mirkin.

The research was published in Science (doi: 10.1126/science.aal3919).