Pyramid-shaped gold nanoparticles assemble into complex crystals

The most complex crystal designed and built from nanoparticles has been reported by researchers at Northwestern University and the University of Michigan (U-M). Their work, which is reported in a paper in Science, demonstrates that some of nature's most complicated structures can be deliberately assembled if researchers can control both the shape of the nanoparticles and the way they connect using DNA.

"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 Chad Mirkin, professor of chemistry at Northwestern.

Nanotechnology promises to bring materials together in new ways, forging new capabilities by design. One potential application for crystals built of nanoparticles, such as these newly reported ones, is controlling light – nanoparticles interact well with light waves because they are similar in size. This could lead to materials that can change colors or patterns on command, or block certain wavelengths of light while transmitting or amplifying others. New types of lenses, lasers and even Star Trek-like cloaking materials could be possible.

"This work shows that nanoparticle crystals of extraordinary complexity are possible with DNA technology, once one begins to exploit particle shape," said Sharon Glotzer, professor of engineering and professor of chemical engineering at U-M. "And, it's a great example of what can be achieved by experimentalists and simulators teaming up."

While natural materials exhibit a dizzying array of crystal structures, most nanotechnology labs struggle to get past simple designs. The new structures produced by Haixin Lin, now a postdoctoral fellow in Mirkin's lab, are far more interesting. They are composed of clusters of up to 42 gold nanoparticles, forming larger polyhedral, such as the great dodecahedron. These clusters connect into cage-like crystal structures called clathrates.

Clathrates are known for possessing chambers that can house small molecules, and so have been used for capturing pollutants from the environment, for example. The nanoparticle clusters also possess such chambers, which the authors suggest could be useful for storing, delivering and sensing materials for environmental, medical diagnostic and therapeutic applications.

Still, the story isn't the crystal itself: it's how the crystal came to be. Mirkin's group has pioneered many structures through the use of DNA strands as a sort of smart glue, linking nanoparticles together in a particular way. The nanoparticle acts as both a building block and a template that directs bonding interactions. Meanwhile, Glotzer's group has championed the role of nanoparticle shape in guiding the assembly of crystal structures through computer simulation.

"Chad's group got the idea of exploring new phases by looking at predictions we had made," Glotzer said. "One day, I got a phone call from him. 'We just got these incredible structures!' he said. And he texted me micrograph after micrograph – they just kept popping up. He said, we need to figure out a way to definitively assign their structures."

The electron microscope images, or micrographs, showed complex crystalline structures that formed in large part thanks to the unusual shape of the gold nanoparticles. The triangular bipyramidal shape of the nanoparticles, like two flattened tetrahedrons stuck together at their bases, was similar to a shape Glotzer's group had predicted would form a quasicrystal. Quasicrystals are prized in the field of nanoassembly because they are as complex as crystals get.

The bipyramidal gold nanoparticles had just the right angles to make clathrate structures, which often turn up in molecular systems that form quasicrystals. But to do so, they needed strands of DNA attached to their sides at just the right length.

Lin systematically made the gold bipyramids of consistent size and shape, with edges 250nm long – half the wavelength of blue light. He then modified the bipyramids by adding different length sequences of DNA. If the DNA strands were too short, the nanoparticles assembled into disordered, ill-defined structures. But when longer strands produced exotic patterns in the electron microscope images, Lin brought the results to Mirkin, who was both thrilled and intrigued.

"These are stunning – no one has made such structures before," said Mirkin, director of Northwestern's International Institute for Nanotechnology.

It was clear they had made phases never observed before, but getting the structure accurately identified was essential. After Mirkin alerted Glotzer at U-M, Sangmin Lee and Michael Engel 3D printed Lin's bipyramids and glued them together to explore how they might make the structures in the electron micrographs. Lee is a doctoral student in chemical engineering, and Engel was then an assistant research scientist, both in Glotzer's group.

Once they saw how the shapes fit together, they hypothesized the clathrate structures. To confirm their suspicions, they built a computer model of the hypothesized clathrates from bipyramids and compared it to the Northwestern micrographs. They were a perfect match.

As a definitive test, Lee and Matthew Spellings, also a doctoral student in chemical engineering at U-M, developed a molecular model of the DNA-linked nanoparticles, and Lee then 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," Glotzer said. "Once we saw the computer crystals, I knew we had nailed it."

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.