Cross-shaped molecules that change colour when exposed to amines could be used to detect compounds used in the manufacture of drugs, pesticides, dyes, preservatives and disinfectants.

The so-called cruciforms have phenol rings at the end of their arms. An amine can react with one of these phenol rings to remove a proton. This subtle change to the structure of the cruciform alters its energy levels and hence the colour of the light it emits. Different amines are more or less efficient at removing protons, leading to different colour changes. For example, piperazine causes the solution to glow yellow when exposed to UV light and imidazole causes it to glow pink.

Uwe Bunz at the Georgia Institute of Technology, Atlanta, US, and colleagues created the cruciforms to study the effects of anionic compounds on their interesting energy level structure. Light emission usually involves an electron moving between two energy levels of a molecule; in the case of the cruciforms the two energy levels are located on different arms of the cross. This means they can be tuned independently to change the light emission of the cruciform and change the compounds which it is sensitive to.

Bunz plans to make libraries of cruciform molecules that can be used for staining biological specimens. 'The cruciforms are very photostable,' he said. 'The problem with current biological labelling molecules is they undergo photobleaching - they are eventually destroyed by the light used to measure them.' He also added that it's difficult to predict what molecules will work best. 'I take the approach of make it and see what happens,' he said.

'There is no question that the cruciforms are new candidates for fluorescence sensors,' said Junwu Chen, professor in polymer optoelectronic materials and devices at the South China University of Technology, Guangzhou. 'Though sensitivity and selectivity are challenging aspects for practical sensors, construction of cruciforms with additional binding sites for targeted amines would pave the way for this.'

Clare Boothby