Polymer cubes recognise each other's surface molecules to partner up and stick together on contact

Molecular recognition, a microscopic process, has been used by Japanese researchers to assemble gels into macroscopic structures. The result is a bit like molecular velcro, the molecules catch each other and hold the gel cubes together. 

Akira Harada, and colleagues at Osaka University, took doughnut shaped cyclodextrin (CD) ’hosts’ and ’guests’ that like to sit inside the CDs and bound each to separate polymer chains. When cubes of the different polymers mix in water, they find each other and stick together. They do this because the guest molecules at the surface of one gel cube find and sit inside the host molecules of the other gel cube.

This kind of host-guest interaction is well known in solution chemistry, but hasn’t been applied on a macroscopic scale before. Phil Gale of Southampton University, UK, points out that the idea is one of those that is so simple and elegant that it seems obvious as soon as you’ve seen it. ’Why didn’t anyone think of this earlier?’ he asks. 

mol recognition nchem-350

Source: © Nature Chemistry

Cubes of polymers incorporating either a cyclodextrin host or a molecular guest stick together on contact

Biological systems use molecular recognition to assemble themselves, and this inspired Harada to use the same process to ’make use of our knowledge of host-guest chemistry in the real world’. 

The host-guest interactions the team used are selective - different guests prefer to sit in the differently sized cyclodextrins. The team bound both alpha-CD and the larger beta-CD to polyacrylamide chains, and did the same with adamantyl, n-butyl or tert-butyl groups. Even when all the different gels are mixed in the same container, the adamantyl and tert-butyl gels only stick to beta-CD gels and n-butyl gels only stick to alpha-CD gels. 

Harada’s gels also show how strong host-guest interactions can be, even though the host and guests are not actually covalently bonded together. When beta-CD and adamantyl gels stick together, the host-guest interactions are so strong that the gel cubes will break before the join can be pulled apart. However, the other gels stick less strongly and can be joined and pulled apart intact. 

’I think this is a beautiful demonstration of how interactions between molecules can lead to macroscopic attraction between objects on the real life scale’ adds Gale, who works on supramolecular chemistry and molecular recognition himself. 

Harada plans to investigate using different intermolecular interactions including using biological molecules and would like to use his system to bind to the surfaces of cells, perhaps to immobilise the cells on surfaces. He also suggests other medical uses for his technology, like sticking together wounds with a polymer that selectively interacts with skin cells to hold them together. After all, as he says, ’all you have to do is just mix the parts in water.’ 

Laura Howes