Researchers at the University of Durham and Heriot-Watt University, UK, have designed a system that encapsulates chloride anions in a confined 3D space without the need to synthesise a full cage. The method is analogous to an old-fashioned coin purse, where the presence of the chloride guest acts as a template for the host species to change conformation and seal itself up via a hydrogen bonded urea belt. 

The team led by Jonathan Steed has previously studied the binding of anions in conical cavities, in which the anion is able to be readily exchanged from the 'open' face. Steed wanted to find a way in which the exchange of the anion could be slowed down and the guest entirely isolated from the surrounding media, thereby enhancing the binding strength, and to simultaneously provide a tight size fit for a chloride guest. Now, the team have managed to achieve just this. The guest species is trapped into a deep cavity that is sealed at the top, in this case using a hydrogen-bonded 'tape' around the top of the capsule. 

'The major challenge in the field remains selective binding that overcomes the intrinsic bias that arises from anion basicity,' says Steed. 'The ability to fully encapsulate environmentally and biomedically important anions in a reversible manner in synergy with induced changes in the host, as demonstrated in this communication, is a significant step towards this goal'. 

Steed hopes that future developments will include the incorporation of signalling units into this new class of receptors to detect and report the binding event as it occurs. 'The gross conformational change of the molecule should allow us to amplify a fluorescent or electrical signal used to detect the presence of the anion,' explains Steed. 

Kathleen Too