MIT researchers and collaborators have shown that flexible, charged molecular connectors can organize nanoparticles into unusual crystal structures—contrary to the long-standing assumption that rigid connectors are essential. By allowing the connectors to bend and rearrange as particles approach one another, the team created a previously unobserved nanoparticle superlattice and demonstrated a potentially simpler route to materials with tailored properties.
Turning a materials-science assumption upside down
For roughly 20 years, researchers have generally assumed that the molecular connectors joining nanoparticles should be stiff. Rigid connectors can preserve carefully controlled angles between particles, helping them assemble into a planned arrangement. The MIT-led team investigated the opposite strategy: connectors flexible enough to bend and adjust as nanoparticles come together.
The results indicate that flexibility and electrical charge can determine how nanoparticles organize themselves. In an upcoming paper in Nature Materials, the researchers show that this approach can produce crystalline structures that have not previously been observed.
“People are interested in nanoparticle superlattices because they hold promise for building designer materials where they have more say in what kinds of structures they can actually build,” said Rob McFarlane, the Richard P. Simmons (1953) Professor of Metallurgy in MIT’s Department of Materials Science and Engineering. “This is a new approach to nanoparticle assembly that has never been explored before. We are very excited to have a counterintuitive new tool. This goes against the 20-year-old dogma that the connector ligand must be stiff and that any deviation will cause the assembly to fail. We showed that if you just take it to the other extreme, it works just as well.”
The research team also includes MIT doctoral candidate Nicholas Sbarbi, University of Delaware doctoral student Stephen Cronenberger, MIT undergraduate Hana Niizawa, PhD ’26 Matthew Yeh, and former University of Delaware professor Arti Jayaraman.
Why nanoparticle assembly is difficult
A material’s properties depend heavily on how its smallest components are arranged. If scientists know what those components are and how they are organized, they can begin to explain—and potentially control—the resulting electrical, optical, chemical and other properties.
That principle is especially important for nanoparticles. These extremely small building blocks can be tailored by changing their size, shape and composition. Individual nanoparticles are already being explored for uses including drug delivery, energy storage, computing and sensing. However, assembling them into larger, ordered materials remains challenging.
Scientists must control both the number of neighboring particles connected to each nanoparticle and the directions of those connections. Without that control, nanoparticles tend to form tightly packed aggregates. Such arrangements can limit the range of structures and properties that researchers can achieve.
Traditional approaches have included changing the shape of the nanoparticles or modifying selected regions of their surfaces so that only specific areas attract neighboring particles. The goal is similar to designing puzzle pieces that can connect only in particular orientations.
These methods can produce useful structures, but they are often complex, specialized and difficult to adapt to large-scale manufacturing.
“The challenge with both of these approaches is that they are difficult to do synthetically when making the particles,” McFarlane explained. “The methods of defining stickiness that are developed are always bespoke. They work on one material, one size scale, and one condition. They are good for research, but you can’t do much with them.”
Both strategies rely on molecular connectors known as ligands. These molecules attach to nanoparticle surfaces and help bind neighboring particles together. Researchers have traditionally tried to make the ligands rigid so that they maintain controlled angles between particles. While that can help preserve a desired alignment, it also restricts the types of structures that can form.
Flexible, charged connectors provide a new form of control
The MIT team wondered whether molecular connectors with almost complete freedom to bend could offer a different way to control nanoparticle assembly.
“We thought, ‘Let’s do exactly the opposite, and make the bonds so flexible that it doesn’t matter which direction they’re initially oriented,’” McFarlane said. “Then, as the particles come together, the ligands can reorganize, and that reorganization itself limits the number of neighbors each particle can bind to.”
Rather than specifying the direction of every connection in advance, the researchers allowed the connectors to adjust naturally as the particles interacted.
McFarlane’s laboratory has spent years developing methods that use linked polymer chains to construct different types of nanoparticles. Polymers are large molecules made from repeating units, and their flexibility makes it possible to create molecular connections with tunable behavior.
For this study, the researchers equipped nanoparticles with highly flexible ligands carrying electrical charges at their ends. Some ligands had positive charges, while others had negative charges. Because opposite charges attract, the connectors bend toward partners with the opposite charge.
This attraction helps bind the nanoparticles, while the flexibility of the connectors influences how many particles can connect effectively.
“The idea was to say, ‘The bond will be as flexible as you want, and will spontaneously proceed to any valency to maximize bond stability,’ rather than defining a specific direction in which the bond will occur in a complex synthetic method,” McFarlane said.
Here, valency refers to the number of neighboring nanoparticles bonded to a given particle. By allowing the connectors to move freely, the researchers found that the system could settle into a stable arrangement without requiring every connection to be designed in advance.
A previously unseen nanoparticle crystal
Experiments led by Sbarbi showed that the connections between nanoparticles could be tuned by changing two relatively simple parameters: particle size and the length of the flexible molecular chains.
These changes influence how the particles approach one another and determine how many neighbors each particle can connect to. Most significantly, the researchers used the method to produce a crystal structure that had not previously been observed.
A crystal consists of components arranged in an organized, repeating pattern. In this case, the repeating arrays are made from nanoparticles rather than atoms or small molecules. Such ordered nanoparticle networks are commonly called superlattices.
By reducing the number of connections between adjacent particles, the team produced more open arrangements instead of the dense structures that typically emerge during nanoparticle assembly.
“We can control the number of bonds each particle has without having to pre-design where the particles connect or the shape of the particles,” McFarlane explained. “The amount of flexibility in a polymer determines the number of bonds it has. This allows us to reduce the number of bonds each particle has, creating lattices unlike anything we’ve seen before.”
The findings suggest that molecular flexibility, traditionally viewed as an obstacle to precise assembly, can instead become a tool for designing new material structures.
Toward customizable designer materials
The researchers believe the method could help address a long-standing goal in materials science: choosing the properties of individual nanoparticles separately from the larger structures they form collectively.
In principle, scientists could select nanoparticles for their electrical, optical, chemical or other useful characteristics and then arrange them in the most effective configuration for a particular application. Existing assembly methods, however, make that level of independent control difficult.
“We in the nanoparticle assembly community are talking about having complete independent control over what a particle is and what kind of assembly it goes into,” McFarlane said. “In theory it should be, but in practice it definitely isn’t.”
The new strategy could help narrow that gap. By adjusting connector flexibility and the attraction between opposite charges, researchers may be able to create a wider variety of structures without relying on the complex manufacturing techniques used in traditional approaches.
The team also sees the method as a possible route to producing customizable materials at scale, potentially helping nanoparticle technologies move beyond laboratory demonstrations.
Potential applications in electronics, medicine and energy
More open and diverse nanoparticle structures could be useful in the development of advanced materials. In photonics, the study of controlling and manipulating light, precisely organized nanoparticles can help researchers engineer specialized optical properties.
New particle arrangements could also create opportunities to improve catalysts, which accelerate chemical reactions. The approach may further support future developments in medicine, agriculture, electronics and energy technology.
The potential advantage comes from the broad range of properties that nanoparticles already offer. Controlling how they assemble could allow scientists to combine those individual capabilities in new ways.
“Nanoparticle superlattices offer a way to create designer materials whose building blocks have all sorts of unique properties,” McFarlane said. “However, simply packing these nanoparticles to a high density limits their applications. This provides a way to achieve the formation of less dense nanoparticles, and there are many interesting applications there.”
Further development will be needed before the method can be translated into commercial technology. Still, the discovery establishes molecular flexibility as a potential strategy for creating materials with precisely tailored structures. Instead of treating flexible molecular bonds as a problem to eliminate, researchers may be able to use them to unlock configurations that are difficult to achieve with rigid designs.
The research received partial support from the National Science Foundation, the Department of Energy, the U.S. Army and the U.S. Navy.
Source: www.sciencedaily.com
