For roughly two decades, chemists building ordered crystals out of nanoparticles have worked from a simple rule: the molecular links between particles must be rigid, or the structure will not come out as designed. A team at MIT led by Robert Macfarlane flipped that rule by tethering particles together with highly flexible polymer chains that carry electrical charges at their tips. The approach produced nanoparticle crystals never observed before, and it was reported in the journal Nature Materials.
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Macfarlane, the Richard P. Simmons Professor in Metallurgy in MIT’s Department of Materials Science and Engineering, is the senior author. His description of the work was blunt: it is the exact opposite of what the dogma has been for 20 years.
Rigid links were the 20-year orthodoxy
Nanoparticles can be arranged into repeating three-dimensional patterns called superlattices, and the arrangement decides how a material handles light, heat and chemical reactions. To steer particles into a chosen pattern, earlier methods either reshaped the particles themselves or coated only parts of their surfaces with sticky chemistry so that bonds form in set directions. Macfarlane called those strategies complicated, hard to scale and “always bespoke,” as ScienceDaily relayed from MIT’s release. Both strategies share a premise. Bond directions have to be designed in advance, and rigid connector molecules, known as ligands, hold the angles in place. That premise is the 20-year habit the new work overturns, and it explains why designers of nanoparticle crystals have spent so much effort on precise surface chemistry: if the links must be stiff, the geometry has to be engineered into every particle before assembly begins.
Charged tips on flexible polymer ligands
The MIT team went the other direction. Their nanoparticles carry long, floppy polymer ligands, and the tip of each chain bears either a positive or a negative charge. Opposite charges attract, so a ligand bends toward a partner on a neighboring particle and links up. Because the chain can deform, the bonds settle into whatever arrangement is most stable. In the words Macfarlane gave MIT News, the bonds will be as flexible as they want to be.
That freedom is what sets the number of neighbors each particle binds, a property chemists call valency. Nobody programs the valency in. The ligands reorganize as particles come together and settle on the valency that maximizes bond stability, which is why the paper calls the effect “spontaneous valency.” The study’s title, “Breaking symmetry in nanoparticle superlattices via spontaneous valency,” appears in Nature Materials.
Why would floppiness help? A rigid ligand fixes a geometry before the particles ever meet, while a flexible one lets the assembly decide. As particles approach each other the ligands deform, and that deformation caps how many neighbors a particle can reach. The polymer’s flexibility therefore determines the bond count and allows fewer bonds per particle than a densely packed crystal would force. A designer no longer carves bond directions into each particle; the balance of charge, chain length and particle size does the sorting.
Dials for size and chain length, and the structures that resulted
Nicholas Sbalbi, an MIT doctoral candidate, led the experiments. He varied two settings: the size of the nanoparticles and the length of the polymer chains. Those two dials controlled how many neighbors each particle bonded with. Fewer bonds per particle yielded more open, less densely packed lattices, and those open lattices are the new structures the group had not seen before. In a dense crystal every particle touches as many neighbors as geometry allows. An open lattice leaves gaps between particles, and the release lists photonics, the manipulation of light, and chemical catalysis among the uses that stand to gain. Co-authors include Stephen Kronenberger of the University of Delaware, MIT undergraduate Hana Shinzawa, Matthew Ye (PhD ’26) and Arthi Jayaraman, a former Delaware professor.
Sbalbi’s group built on years of lab work in the same area. A 2024 paper from Macfarlane’s group, with Sbalbi and Ye among the authors, used molecular additives to loosen the hydrogen-bonded links that hold polymer-grafted gold nanoparticles together, which let ordered superlattices form at room temperature and grow into faceted single crystals, according to the National Science Foundation’s public-access archive. The newer work removes the need to design each bond direction at all.
Macfarlane argued that dense packing limits what nanoparticle materials can do, and said open arrangements are where many interesting applications sit. The authors point to photonics, chemical catalysis, medicine, agriculture, electronics and energy technologies, though the release says further development is needed before anything reaches commercial use. Funding came in part from the National Science Foundation, the Department of Energy, the U.S. Army and the U.S. Navy. The Macfarlane lab publications page shows the same group has worked on nanoparticle superlattices, polymer-grafted nanoparticle composites and DNA-programmed building blocks for years, so the flexible-ligand result grows out of a long line of assembly work rather than appearing from nowhere.
The unanswered question is scale. The release names no specific crystal structures and gives no performance data for the open lattices, so how well they work as optical or catalytic materials will have to come from follow-up papers. Macfarlane’s own argument is that the interesting applications sit in the less dense formations, which makes those measurements the next test of whether a structural novelty becomes a useful material.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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