Flexible Bonds, New Materials

Creating better-performing materials comes down to having more control over their structure at the smallest possible scale. One particularly effective way to accomplish that is using nanoparticles, which have highly customizable shapes and sizes and have already shown promise in applications across energy, medicine, and electronics.

But the process of turning nanoparticles into useful new materials has been hampered by the difficulty of controlling the number and direction of the links between each particle and its neighbors. Researchers have tried changing the shape of particles or precisely aligning the rigid connector chains between them, but both of those methods limit the material structures nanoparticles are able to form and are difficult to scale.

A team including researchers from MIT is taking a different approach. The researchers developed a tunable way to use super-flexible connector molecules that regulate the packing of nanoparticle networks by intentionally deforming based on their surrounding environment. In a paper published today in Nature Materials, the researchers showed how connector flexibility and electric charges can be used to tune the network’s structure, and they used the technique to create new kinds of nanoparticle-based materials.

The approach could offer a more versatile way to create high-performing materials for photonics, chemical catalysis, medicine, agriculture, and more.

“People are interested in nanoparticle superlattices because of the promise of building designer materials where you have more say in what types of structures they can actually build into,” says Rob Macfarlane, the Richard P. Simmons (1953) Professor in Metallurgy in MIT’s Department of Materials Science and Engineering. “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.”

Joining Macfarlane on the paper are MIT PhD candidate Nicholas Sbalbi; University of Delaware doctoral student Stephen Kronenberger; MIT undergraduate Hana Shinzawa; Matthew Ye PhD ’26; and former University of Delaware professor Arthi Jayaraman.

More flexible structures

A foundational truth in materials science is that a material’s structure governs its function.

“That means if you know what your building blocks are and how they’re arranged, you can explain what properties materials have,” Macfarlane says. “Controlling that is how you make materials that have new properties.”

Nanoparticles give scientists a flexible building block to make materials with desired properties, and individual nanoparticles are already used in applications as diverse as drug delivery, energy storage, computing, and sensing. But assembling them into larger structures has proven to be a challenge because of how tightly they tend to pack together.

To overcome that challenge, researchers make nanoparticles different shapes, which changes how they connect. They can also make only specific parts of the particles’ surfaces sticky so they form different patterns.

“The challenge with both of those approaches is they’re difficult to do synthetically as we make the particles,” Macfarlane explains. “The methods for defining stickiness that have been developed are always bespoke: They work for one material, at one size scale, in one condition. It’s good for studying, but you can’t do much with it.”

Both methods also require connector molecules, called ligands, at the surface of the nanoparticles to be rigid so they stay at the specific angles needed to maintain the formation researchers want, which further limits their functionality.

For their new study, the researchers wanted to turn that approach on its head.

“We thought, ‘Let’s do the exact opposite: make the bonds so flexible it doesn’t matter what direction they’re initially pointing.’ The ligands can then reorganize as the particles come together, and that reorganization itself limits how many neighbors each particle can bind,” Macfarlane says.

Macfarlane’s group has spent years creating ways to make different kinds of particles from grafts of polymers. The nanoparticles in their new system have flexible ligands with electrically charged tips, some positively charged and others negatively charged. The opposites attract, so the ligands bend to connect with opposite charges.

“The idea is, instead of defining a specific direction the bonds are going to occur in with complicated synthesis methods, we just said, ‘The bonds are going to be as flexible as they want to be, and they’ll spontaneously go to whatever valency is going to maximize bond stability,’” Macfarlane says.

In experiments led by Sbalbi, the researchers showed they could control the connections by changing the nanoparticle sizes and ligand chain lengths. They also used the technique to assemble the nanoparticles into previously unseen crystal structures.

“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.”

A new path toward customizable materials

Macfarlane believes the system is a step toward getting custom nanoparticle-based materials out of the research lab and into the world.

“We in the nanoparticle assembly community talk about having completely independent control over what your particle is and what assembly it goes into,” Macfarlane says. “In theory, that should be the case. In reality, it absolutely has not been.”

More broadly, the researchers say their approach establishes ligand flexibility as a new path to making material structures that can be customized at scale.

“Nanoparticle superlattices offer a way to create designer materials where your building blocks have all sort sorts of unique properties,” Macfarlane says. “But just densely packing these nanoparticles together limits your applications. This offers a way to get less dense formations of nanoparticles, and that’s where a lot of the interesting applications are.”