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Reaction Mechanisms

Mechanochemistry strips cargo molecules from a loaded rotaxane

Using polymer strands to pull a ring along an axle could release repair molecules in self-healing materials

by Mark Peplow, special to C&EN
April 28, 2024 | A version of this story appeared in Volume 102, Issue 13


A schematic shows how a pillararene ring is pulled along a molecular axle by polymer strands to strip cargo molecules from a series of docking stations.
Credit: Adapted from Nature
When polymer strands pull a pillararene ring (beige) along a molecular axle, the ring strips cargo molecules (blue) from a series of docking stations.

Researchers have created a nanoscale system that might be used to deliver drugs or release molecules that repair damaged materials (Nature 2024, DOI:10.1038/s41586-024-07154-0).

The system relies on mechanochemistry, in which a mechanical force causes a chemical reaction. In this effort, the researchers rely on long polymer strands tethered to a force-sensitive molecule known as a mechanophore. “The idea is to use polymers as a rope in a tug-of-war,” says Guillaume De Bo of the University of Manchester, who led the new work.

In solids, an impact can tug the polymer strands apart and trigger the mechanophore’s reaction. Researchers have previously used this strategy to release fluorescent molecules to signal damage in a material. In solution, bursts of ultrasound have the same mechanochemical effect. But each tug-of-war bout can free only a single molecule, which limits the concentration of released molecules.

De Bo’s team has now overcome this drawback with the help of a rotaxane, a structure containing a molecular ring threaded onto an axle-like structure. The long axle contains up to five furan groups that act as docking stations so that maleimide-based cargo molecules can be added by a Diels-Alder reaction. The molecular ring, a pillararene, is fitted with a long strand of polymethyl methacrylate that runs roughly parallel to the axle. Another polymer strand is attached to one end of the axle so that it can pull in the opposite direction.

When ultrasound pulses tug these two polymers apart, theirmovement hauls the pillararene along the axle so hard that it triggers a reverse Diels-Alder reaction at each docking station, stripping off all the cargo molecules. “Being able to release multiple molecules in a single elongation event—this is completely new in mechanochemistry,” De Bo says.

For rotaxanes with a single docking station, ultrasound freed up to 71% of the maleimide cargo units in solution, while compression of a bulk sample released 30%. “This is already a very good efficiency,” says Anne-Sophie Duwez, an expert in single-molecule force spectroscopy at the University of Liège, who peer-reviewed the paper. “It’s a very elegant system.”

The researchers then showed that the system also worked with three useful cargo molecules—a fluorescent tag, a catalyst, and the anticancer agent doxorubicin. This proof of principle suggests that the method might find use in an ultrasound-activated drug delivery system.

For now, De Bo’s team is creating rotaxanes with more docking stations, and also hopes to load the rotaxane’s axle with a mixture of monomers and a catalyst. In principle, a sudden force could release these ingredients to generate new polymers that repair a damaged region of material.



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