A calixarene can adsorb nitrogen oxides, making it potentially useful for both storage and sensing applications.  

Pete McGrail and Praveen Thallapally at Pacific Northwest National Laboratory, Washington, US, in collaboration with Jerry Atwood at University of Missouri-Columbia, US, are developing gas storage, separation and sensing materials based on calixarenes. Calixarenes are bowl-shaped macrocyclic organic molecules that can trap guest molecules within their cavity.

The researchers made a low-density form of p-tert-butylcalix[4]arene that can adsorb gases such as carbon dioxide and methane, and now they have shown that nitrogen oxides can be trapped inside the calixarene. Nitrogen oxides, known as NOx, are released from burning fossil fuels and their emission is regulated by the US Environmental Protection Agency.

Voids in the calixarene packing (yellow) make excellent capsules to absorb gases

When crystals of the calixarene are exposed to NOx they change colour because a charge transfer complex is formed. This means that the calixarene might be useful for sensing NOx in the presence of other gases. 

'Nitrogen oxides are important gases that contribute to global warming,' said Thallapally. 'Development of improved NOx sensors would enable better control of the combustion process in power plants and even automobiles to help minimize NOx emissions.'

Further work needs to be carried out before practical devices based on these materials can be developed, said Thallapally.

Caroline Moore