Floppy Molecules Let MIT Team Build Nanoparticle Crystals Never Seen Before

Researchers led by a team at MIT have built never-before-seen crystal structures from gold nanoparticles by doing the opposite of what the field has long practiced: linking the particles with floppy molecules instead of rigid ones. The work was published in the journal Nature Materials, according to ScienceAlert.

The approach gives scientists finer control over how many links each particle forms and in which directions, which could widen the range of materials that can be designed. The functions of the new structures have not yet been tested.

What the team changed about nanoparticle assembly

Nanoparticles are tiny building blocks that scientists join together into ordered arrangements called superlattices. The connectors between particles, known as ligands, have mostly been designed to be rigid and fixed for about 20 years, ScienceAlert reported.

In the new study, the researchers used gold nanoparticles with ligands made from PEG, a common flexible polymer. The ligand ends were tipped with positive or negative charges to help the particles connect. Computer simulations were run alongside the experiments to follow what the experiments showed.

Why flexibility produced order

With no rigidity built in, the ligands bunched together at the points where particles touched. That limited how many connections could form, because there were only so many particle-to-particle chains to go around, according to the report.

The nature of the links is therefore set by the nanoparticles themselves as they come together, rather than by a layout planned in advance. Materials scientist Rob Macfarlane of MIT said the flexibility of the polymer dictates how many bonds each particle has, which lets the team lower that number and make lattices never seen before.

Macfarlane described the finding as the reverse of what he called a 20-year dogma that connectors must be rigid or assembly fails. He said that pushing to the opposite extreme "works just as well."

The links can still be tuned, the team found. Changing the sizes of the nanoparticles and the lengths of the ligand chains influences the outcome. Smaller particles, for example, led to fewer links.

Why it matters

For general readers, this is early-stage materials science rather than a product. The team created ordered crystal structures that had not been seen before, but ScienceAlert noted that the properties and usefulness of these materials still need to be examined and tested.

The potential lies in design. Materials made of nanoparticles are already widely used, but they can be hard to scale up, because the particles by default pack together tightly and must be carefully engineered for each purpose. Particles that are less tightly packed could make it easier to customize a material at scale, Macfarlane said, adding that this is where many interesting applications lie.

According to the researchers, the approach could eventually contribute to high-performance materials in photonics (technologies that control light), chemical catalysis, medicine and agriculture. That is a stated possibility, not a demonstrated result.

Macfarlane said that knowing what the building blocks are and how they are arranged explains a material's properties, and that controlling the arrangement is how to make materials with new ones.

What happens next

The team plans to experiment with other particle materials and other ligand combinations. The researchers also intend to put the resulting materials to the test in real-world scenarios, according to ScienceAlert.

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Around Prague, based on reporting by ScienceAlert