Flexible Molecular Links Offer a New Way to Organize Nanoparticles

Researchers at the Massachusetts Institute of Technology have reported a way to arrange nanoparticles into ordered materials by making the molecular links between them more flexible, not less. The team's corresponding author describes the approach as a reversal of long-standing practice in the field.

The study, published Nov. 2 in Nature Materials, describes particles coated with bendable polymer chains whose tips carry positive or negative charges. As the particles come together, the chains reorganize, and that reorganization limits how many neighbors each particle can bind. By changing particle size and chain length, the researchers said, they could control those connections and produce crystal structures they had not observed before.

Nanoparticles can be made in many sizes, shapes and compositions, and individual particles are already used in drug delivery, energy storage, computing and sensing. Assembling them into larger ordered arrays, known as superlattices, offers a way to build materials whose properties depend on both the particles and their arrangement. But spherical particles tend to pack tightly, and controlling the number and direction of the links between neighbors has proved difficult.

Researchers have tried changing the particles' shapes or making only certain patches of their surfaces sticky. In MIT's account, such approaches rely on rigid connector molecules, called ligands, that hold the particles at fixed angles. Robert Macfarlane, a professor of materials science and engineering at MIT whose group did the work, said in a university news release that such techniques tend to work for one material, at one size and under one set of conditions, which makes them hard to scale.

The new study inverts that logic. "It's the exact opposite of what the dogma has been for 20 years," Macfarlane said in the release, referring to the view that ligands must be rigid for assembly to succeed.

How it works

The key quantity is coordination: the number of particles that bind to a given particle in the lattice. In the MIT system, the flexible chains bend toward oppositely charged partners and reorganize where the particles meet. Macfarlane said that process lets the team lower the number of bonds each particle forms without designing the particles' shapes or bonding sites in advance.

According to the paper's abstract, the resulting structures have lower coordination, packing density and symmetry than the packing of hard spheres would predict. The paper's title describes the effect as spontaneous valency, borrowing the chemistry term for the number of bonds an atom forms.

Nicholas Sbalbi, an MIT doctoral candidate, led the experiments. The other authors are Stephen Kronenberger, a doctoral student at the University of Delaware; Hana Shinzawa, an MIT undergraduate; Matthew Ye, who completed his MIT doctorate this year; and Arthi Jayaraman of IBM France. The work paired laboratory experiments with computer simulations and was supported in part by the National Science Foundation, the Department of Energy, the Army and the Navy. The paper

Related work

The MIT group is not the only one exploring charged polymer coatings. On January 17, a separate team with Binay P. Nayak as first author reported in Nature Communications that grafting nanoparticles with oppositely charged polymers of adjustable length produced a range of cubic lattices. Among them were diamond-like structures, which are of interest for photonic devices, formed without building directional bonding into the particles. That study also combined experiments with theory and simulations. The new paper cites the earlier work. The Nayak team's manuscript

In both studies, a particle's coating helps determine the structure that forms, alongside its core. A 2022 Nature Materials study by Chad Mirkin's and Sharon Glotzer's groups had also generated low-symmetry crystals using DNA-coated particles.

Possible uses, and open questions

MIT says the approach could be useful in photonics, chemical catalysis, medicine and agriculture. Macfarlane said densely packed nanoparticles limit what a material can do, and that less dense arrangements are where many of the promising applications lie.

In photonics, for example, a material's optical behavior depends on the spacing and geometry of its repeating structure as well as on what its particles are made of. In catalysis, more open lattices could, in principle, give other molecules greater access to particle surfaces.

The reported advance concerns control over structure; its value in a device remains an application question. Several questions would need answers before the method could support products: whether it yields the same structures reliably from batch to batch, whether those structures hold up under working conditions, whether it works with the particle materials particular applications require, and how a given structure translates into a measured property.

Macfarlane described the new method as a step toward a longstanding goal in nanoparticle assembly: controlling a particle's makeup and the structure it forms independently.