Scientists have spent decades trying to control how tiny particles connect to form advanced materials. Now, researchers at MIT and their collaborators have discovered that doing the exact opposite of what was previously believed necessary can produce remarkable results.
Instead of relying on rigid molecular connections to hold nanoparticles in carefully planned arrangements, the team developed extremely flexible connectors that bend and adjust as particles come together. This unexpected strategy gives scientists a new way to control how nanoparticles organize themselves, allowing them to create crystal structures that have never been observed before.
The discovery could help overcome a major obstacle in materials science and open new possibilities for creating substances with precisely engineered properties. Potential applications include electronics, medicine, energy technologies, agriculture, and chemical manufacturing.
The findings, described in a paper that will appear in Nature Materials, demonstrate how the flexibility and electrical charges of molecular connectors can determine the structures that nanoparticles form.
MIT Researchers Challenge 20 Years of Materials Science
“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.”
The research team also 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.
At the heart of the discovery is a basic principle of materials science: The way a material’s smallest components are arranged determines how that material behaves.
“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.”
This principle is especially important when working with nanoparticles, which are extremely small pieces of matter whose shapes, sizes, and properties can be adjusted for different purposes. Scientists already use individual nanoparticles in technologies involving drug delivery, energy storage, computing, and sensors.
But turning these tiny building blocks into larger, organized materials presents a difficult challenge.
Why Nanoparticles Are So Difficult to Control
One of the biggest obstacles is controlling how nanoparticles attach to one another. Scientists need to determine not only how many neighboring particles each nanoparticle connects with, but also the directions in which those connections form.
Without that control, nanoparticles tend to assemble into tightly packed arrangements that restrict the variety of structures researchers can produce. This limits their ability to create materials with unusual or specialized properties.
Researchers have traditionally approached the problem in two ways. One strategy involves changing the shapes of nanoparticles to influence how they fit together. Another involves modifying specific areas of their surfaces so that only certain regions attract neighboring particles.
In effect, scientists attempt to control where the particles stick to one another, much like designing puzzle pieces that can connect only in particular positions.
Although these methods can produce useful structures, they are often complicated, highly specialized, and difficult to adapt for large-scale production.
“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 strategies also depend on molecular connectors called ligands. These molecules attach to the surfaces of nanoparticles and help link neighboring particles together.
Traditionally, researchers have tried to make these ligands rigid so the connections remain at carefully controlled angles. This helps maintain a desired arrangement, but it also restricts the types of structures that can form and the functions they can perform.
The MIT team decided to explore a radically different possibility.
A Surprising Discovery About Flexible Molecular Bonds
Rather than forcing molecular connectors to remain in fixed positions, the researchers wondered whether giving them almost complete freedom to bend could offer an entirely new form of control.
“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.
The idea reverses the usual approach to nanoparticle assembly. Instead of controlling the orientation of every connection in advance, the scientists allow the connectors to adjust naturally as the particles interact.
Macfarlane’s laboratory has spent years developing ways to construct different types of nanoparticles using attached polymer chains. Polymers are large molecules made of repeating smaller units, and their flexibility makes them useful for creating adjustable molecular connections.
For the new system, the team equipped nanoparticles with highly flexible ligands that carry electrical charges at their ends. Some have positive charges, while others have negative charges.
Because opposite electrical charges attract, the connector molecules naturally bend toward partners with the opposite charge. This attraction helps the nanoparticles join together, while the flexibility of the connectors influences how many connections each particle can make.
“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 this context, valency refers to the number of connections a nanoparticle forms with its neighbors. By allowing the connectors to move freely, the researchers found that the system could naturally settle into stable arrangements without requiring every connection to be designed beforehand.
Scientists Create Crystal Structures Never Seen Before
Experiments led by Sbalbi demonstrated that the researchers could adjust the connections between nanoparticles by changing two relatively straightforward characteristics: the sizes of the particles and the lengths of their flexible molecular chains.
These adjustments altered how the particles assembled, giving the team control over the number of neighboring particles each one could connect with.
Most importantly, the researchers used this method to produce previously unseen crystal structures.
Crystals are materials whose building blocks form organized, repeating patterns. In this case, the repeating arrangements are made from nanoparticles rather than individual atoms or small molecules. Such ordered nanoparticle networks are often called superlattices.
By reducing the number of connections between neighboring particles, the team was able to create more open arrangements instead of 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 results suggest that molecular flexibility, once viewed as an obstacle to precise nanoparticle assembly, can instead become a powerful tool for engineering new material structures.
A New Path Toward Customizable Designer Materials
The researchers believe their method could bring scientists closer to a long-sought goal in materials science: choosing the properties of individual nanoparticles independently of the larger structures into which they assemble.
Ideally, scientists could select nanoparticles for their electrical, optical, chemical, or other useful characteristics and then organize those particles into whatever arrangement best serves a particular application.
In practice, however, existing assembly methods have made that level of flexibility difficult to achieve.
“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.”
The new approach could help close that gap. Because the researchers can influence nanoparticle arrangements through the flexibility of their connectors and the attraction between electrical charges, they may be able to create a wider variety of structures without the complicated manufacturing techniques required by traditional methods.
The team also sees the discovery as a promising route toward producing customizable materials at larger scales, potentially helping move nanoparticle technologies beyond laboratory experiments and into practical use.
Potential Applications in Electronics, Medicine, and Energy
The ability to build more open and varied nanoparticle structures could prove particularly valuable for developing advanced materials.
In photonics, which involves controlling and manipulating light, precisely organized nanoparticles could help researchers engineer materials with specialized optical properties. In chemical catalysis, where substances accelerate chemical reactions, new particle arrangements could create opportunities to improve the way catalysts operate.
The approach may also support future developments in medicine, agriculture, electronics, and energy technologies.
A major advantage is that nanoparticles can already possess a wide range of desirable properties. By controlling how those particles come together, scientists may be able to combine their individual capabilities in new ways.
“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.”
Although further development will be needed to translate the method into commercial technologies, the discovery establishes molecular flexibility as a new strategy for creating materials with tailored structures.
Instead of treating bending molecular connections as a problem to eliminate, scientists may now be able to use that flexibility to unlock arrangements that rigid designs could not easily achieve.
The research received partial support from the National Science Foundation, the Department of Energy, the U.S. Army, and the U.S. Navy.
