Stretchy, porous crystal can grow on demand to trap more molecules
Engineers at Dartmouth College have developed a new crystal structure that can stretch to twice its size when it encounters a specific chemical. The team says that the material could be used to selectively absorb impurities in water.
The carbon-based crystal belongs to a class of materials known as porous organic frameworks. As the name suggests, these materials are made of organic molecules like carbon, oxygen and nitrogen, mixed with stronger molecules into a scaffold full of pores. By trapping molecules in those pores, these materials could be used to absorb pollutants from air or water, to deliver drugs in the body, or to facilitate chemical reactions.
These kinds of materials (and the similar metal organic frameworks) have been well studied, but for the new work the team added an extra function. They built in “soft joints” made of clusters of bisulfate anions, which normally repel each other but in this structure they’re held in place through interactions with other molecules. However, when a certain chemical comes along, those interactions are disrupted, and the anions push each other away again. That causes the crystal to expand, exposing more pores that can absorb more of the target molecules.
The team tested the material by placing the crystals in a phenol solution, and found that they expanded to more than twice their length within 20 minutes. When the phenol was cleaned off them, they shrank back down to their original size in about 10 minutes.
"Seeing the crystal expand and contract to this extent is remarkable," says Chenfeng Ke, lead author of the study. "Picture a diamond that behaves like a rubber band.”
The researchers say that the new expandable porous organic frameworks could make for more effective nanofilters, by growing to collect more of a specific molecule.
The research was published in the journal Chem.