MIT researchers use flexible molecular links to create new nanoparticle crystal structures

A reversal of 20 years of materials science thinking

MIT researchers and their collaborators have discovered that flexible molecular connections can create nanoparticle crystal structures that have never been observed before. The finding challenges a long-standing assumption in materials science that connector ligands must be rigid to hold nanoparticles in planned arrangements.

"This is a new approach in nanoparticle assembly that hasn't been investigated before. It's exciting to have a new tool that is so counterintuitive. It's the exact opposite of what the dogma has been for 20 years: that you need to make connector ligands rigid, and any deviation causes assembly to fail. We showed if you just crank it all the way to the opposite extreme, it works just as well," says Rob Macfarlane, the Richard P. Simmons (1953) Professor in Metallurgy in MIT's Department of Materials Science and Engineering.

How the new method works

The team equipped nanoparticles with highly flexible ligands that carry electrical charges at their ends. Some connectors have positive charges and others have negative charges. Because opposite charges attract, the connector molecules bend toward partners with the opposite charge, helping nanoparticles join together.

The researchers used gold nanoparticles with ligands made from a common, floppy polymer called PEG, tipped with positive or negative charges. Computer simulations were also run to follow what the experiment was showing. Without rigidity, the ligands bunched together where the particles touched, limiting the number of connections that could be made.

By changing the sizes of the nanoparticles and the lengths of their flexible molecular chains, the team could control how many neighboring particles each one connected with. Smaller particles led to fewer links.

New crystal structures and potential applications

The method produced ordered crystal structures that have not been seen before. These nanoparticle arrangements, often called superlattices, are more open than the densely packed structures that commonly form during nanoparticle assembly.

"Without having to predesign where the particles connect or what their shapes are, we can control the number of bonds each particle has," Macfarlane explains. "The amount of flexibility of the polymer dictates how many bonds it has. It allows us to lower the number of bonds each particle has and make lattices we have never seen before."

The researchers say the approach could support developments in photonics, chemical catalysis, medicine, agriculture, electronics, and energy technologies. The functionality of the new materials still needs to be examined and tested.

The research team included MIT PhD candidate Nicholas Sbalbi, University of Delaware doctoral student Stephen Kronenberger, MIT undergraduate Hana Shinzawa, Matthew Ye PhD '26, and Arthi Jayaraman, a former professor at the University of Delaware. The findings were published in Nature Materials. The research received partial support from the National Science Foundation, the Department of Energy, the U.S. Army, and the U.S. Navy.

Why it matters

Controlling how nanoparticles arrange themselves determines how a material behaves. The new approach gives scientists a way to create designer materials with precisely engineered properties without the complicated manufacturing techniques required by traditional methods. The next steps will be to experiment with other particle materials and ligand combinations, and to test the materials in real scenarios.

Sources

• sciencedaily.com: MIT scientists break a 20-year rule and unlock never-before-seen crystal structures | ScienceDaily
• sciencealert.com: MIT Scientists Break a 20-Year Rule And Unlock Never-Before-Seen Crystal Structures : ScienceAlert

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