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Inspired by an artist's stencils, researchers have developed atomic-level precision patterning on nanoparticle surfaces, allowing them to "paint" gold nanoparticles with polymers to give them an array of new shapes and functions.

The "patchy nanoparticles" developed by University of Illinois Urbana-Champaign researchers and collaborators at the University of Michigan and Penn State University can be made in large batches, used for a variety of electronic, optical or biomedical applications, or used as building blocks for new complex materials and metamaterials.

Led by Qian Chen, an Illinois professor of materials science and engineering, the researchers report their in the journal Nature.

"One of the holy grails in the field of nanomaterials is making complex, functional structures from nanoscale building blocks. But it's extremely difficult to control the direction and organization of each nanoparticle, especially in achieving materials beyond simple close packing," Chen said.

"Then we got this idea from nature: Proteins have different surface domains, and by their interaction, they can make all the intricate machines we see in biology. So we are adopting that strategy, having patches or distinct domains on the surface of the nanoparticles."

However, the problem of how to attach the patches in a controlled design or at large scales proved a challenge. While wrestling with the problem as a graduate student in Chen's lab, Ahyoung Kim, the co-first author of the paper, took an art class. In the class, she learned a stenciling technique that used a mask to paint a complex design on a curved piece of pottery. She realized such a technique could work on nanoparticle surfaces, too.

"We know that halide atoms, like iodide, chloride or bromide, adsorb to metals. We also know that different facets of a metal nanoparticle have different adsorption affinities. So we can coat some surfaces of a gold nanoparticle in just one layer of iodide, and others in an organic primer. Then we can bring in the polymer, and it just sticks to the facets with the organic primer. The iodide masks the other facets," said Kim, who is now a postdoctoral researcher at the California Institute of Technology.

Chen's group partnered with Penn State professor Kristen Fichthorn's group to explore the competitive binding dynamics of iodide and organic primer to faceted gold nanoparticles and develop masking designs.

"Ionic adsorption is a classical question in surface science," Fichthorn said. "We computed at the atomic level the energetically preferred configurations of iodide and organic primer on various gold facets and predicted a phase diagram for atomic stenciling to occur."

Then the researchers partnered with Michigan professor Sharon Glotzer's group to create a library of what kinds of patchy particles and assemblies the stenciling technique could yield. They used computer simulations to predict how the polymers would arrange within the stencil patterns, and then how the resulting patchy particles would arrange into larger crystal structures. Chen's group validated the simulations experimentally, making more than 20 distinct patchy nanoparticles.

"A computer simulation lets us explore the huge design space of possible patchy particle patterns more quickly than experiments can. By partnering with experimentalists and using their data to help design and validate our computer model, together we can discover much more than with experiment or simulation alone," Glotzer said.

"Atomic stenciling allows for the synthesis of batches of patchy particles with far more intricate patterns than have been possible in the last 25 years of nanoscience research and will make it easier to self-assemble increasingly more sophisticated structures from nanoparticles."

Because the particles have multiple functional areas on their surfaces, they interact in ways other nanoparticles cannot, and they assemble into novel structures with potential for metamaterials—engineered materials with unique light and sound properties—said Illinois graduate student Chansong Kim, a co-first author of the paper. Additionally, the masking technique could apply to many other types of nanoparticles and functional groups, not only gold and polymer, he said.

"You can use different materials for the nanoparticles, and different types of ions as a mask, so that you can generate a huge diversity of materials," Chansong Kim said. "And we can make them in large batches. We believe, based on different materials combinations, this technique can also create unique materials with new properties and applications. It has unlimited potential."