Jayanta Samanta (left) and Chenfeng Ke (right) with models of their novel porous crystals. Photo: Eli Burakian.
Jayanta Samanta (left) and Chenfeng Ke (right) with models of their novel porous crystals. Photo: Eli Burakian.

Researchers have discovered a new way to make crystals stretchy, a modification that could allow them to act as very effective nanofilters.

"Picture a diamond that behaves like a rubber band," says Chenfeng Ke, assistant professor of chemistry at Dartmouth College. His research team has designed a new type of porous, carbon-based crystals that can stretch to more than twice their length.

Known to chemists as porous organic frameworks, these materials are typically hard. They are built from a scaffold of lightweight organic molecules like carbon, oxygen and nitrogen. Additional molecular crosslinks are then chemically stitched in to strengthen their structure, which resembles open nets full of voids, or pores, that can house a variety of molecules as guests. This allows these crystals to act as filters that can remove certain pollutants from air and water, or separate and store commercially important chemicals. The size of the pores usually determines which molecules can be absorbed and stored.

By tweaking the design of the molecular building blocks, the researchers have now made it possible for specific chemicals to make the crystal expand. It's as if some molecules have a key that can unlock a whole load of extra space inside the crystal that they can now occupy, says Jayanta Samanta, a research associate in the Ke Functional Materials Group.

In a paper in Chem, the researchers describe how they build in this feature by adding what they call 'soft joints' into the crystal's scaffold. These joints are made of ions that repel each other but can be coaxed into place by interactions with other molecules in the scaffold. When the ions encounter the right chemical, however, they are readily disrupted, and push away from each other. This makes the crystal expand, but only as far as its crosslinkers will allow.

Samanta, lead author of the paper, describes the crystals as tiny, hard needles, about half a millimeter in length. He recalls the breakthrough moment when he first placed one in a solution of phenol, an organic compound widely used in household cleaners. The crystal stretched to twice its length in under 20 minutes, he says. When the phenol was washed away, the crystal regained its original shape in half that time.

"Seeing the crystal expand and contract to this extent is remarkable," says Ke, who is particularly struck by the rapidity of the expansion. This physical response to specific chemicals in the environment could be used for interesting applications. Ke is eager to put the new design to work by creating similar crystals that can absorb impurities from water.

This story is adapted from material from Dartmouth College, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.