Posted in | Nanomaterials

Nanoparticles Acting like Electrons Could Lead to Novel Materials Design

It is not an electron. However, it does behave like one. A team of scientists from Northwestern University have made a peculiar and surprising discovery that nanoparticles built with DNA in colloidal crystals — when very small — act in the same manner as electrons. Not only has this discovery flipped over the existing, accepted notion of matter, it also paves the way towards new opportunities in materials design.

Monica Olvera de la Cruz (Image credit: Northwestern University)

“We have never seen anything like this before,” said Northwestern’s Monica Olvera de la Cruz, who made the original observation via computational work. “In our simulations, the particles look just like orbiting electrons."

With this finding, the scientists introduced a new word called “metallicity,” which points to the mobility of electrons in a metal. In colloidal crystals, minute nanoparticles travel in the same way as electrons and serve as a glue that keeps the material together.

“This is going to get people to think about matter in a new way,” said Northwestern’s Chad Mirkin, who led the experimental work. “It’s going to lead to all sorts of materials that have potentially spectacular properties that have never been observed before. Properties that could lead to a variety of new technologies in the fields of optics, electronics, and even catalysis.”

The paper has been published in the June 21st issue Science.

Olvera de la Cruz is the Lawyer Taylor Professor of Materials Science and Engineering in Northwestern’s McCormick School of Engineering. Mirkin is the George B. Rathmann Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences.

Mirkin’s team earlier had invented the chemistry for manufacturing colloidal crystals with DNA, which has forged new opportunities for materials design. In these structures, DNA strands serve as a kind of smart glue to bond together nanoparticles in a lattice pattern.

“Over the past two decades, we have figured out how to make all sorts of crystalline structures where the DNA effectively takes the particles and places them exactly where they are supposed to go in a lattice,” said Mirkin, founding director of the International Institute of Nanotechnology.

In these earlier studies, the diameters of the particles are on the tens of nanometers length scale. Particles in these structures are stationary, fixed in position by DNA. In the present study, however, Mirkin and Olvera de la Cruz miniaturized the particles down to 1.4 nm in diameter in computational simulations. Here is where the magic transpired.

The bigger particles have hundreds of DNA strands linking them together. The small ones only have four to eight linkers. When those links break, the particles roll and migrate through the lattice holding together the crystal of bigger particles.

Olvera de la Cruz, Lawyer Taylor Professor of Materials Science and Engineering, McCormick School of Engineering, Northwestern University

When Mirkin’s team carried out the experiments to image the small particles, they discovered that Olvera de la Cruz’s team’s computational observations proved spot-on. Since this behavior is indicative of how electrons act in metals, the scientists term it “metallicity.”

“A sea of electrons migrates throughout metals, acting as a glue, holding everything together,” Mirkin explained. “That’s what these nanoparticles become. The tiny particles become the mobile glue that holds everything together.”

Olvera de la Cruz and Mirkin’s subsequent plan is to investigate how to manipulate these electron-like particles so as to design new materials with beneficial properties. Although their research used gold nanoparticles, Olvera de la Cruz said “metallicity” applies to other groups of particles in colloidal crystals.

“In science, it’s really rare to discover a new property, but that’s what happened here,” Mirkin said. “It challenges the whole way we think about building matter. It’s a foundational piece of work that will have a lasting impact.”

The research, “Particle analogs of electrons in colloidal crystals,” was aided by the Center for Bio-Inspired Science, an Energy Frontier Research Center funded by the US Department of Energy (award number DE-SC0000989); the Air Force Office of Scientific Research (award number FA9550-17-1-0348); the Office of Naval Research (award number 00014-15-1-0043) and the Sherman Fairchild Foundation. Martin Girard, a PhD graduate from Olvera de la Cruz’s laboratory and current postdoctoral scholar at the Max Planck Institute for Polymer Research in Germany, is the first author of the paper.

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