Chemistry professor discovers color sensor compound for anions

The University of Huddersfield's Professor Craig Rice has recently discovered a compound that undergoes a colorimetric response to a whole host of different ions. However, the most remarkable facet of the chemistry is that the detecting species is not made directly by the scientist, but because the response spontaneously 'self-assembles' to give a sensor for each specific anion.

"A colorimetric response, which determines the concentration of a chemical element or compound through the aid of a colour reagent, is far more desirable," said Professor Rice, "because as it's easy to interpret and in this newly-discovered system, a large range of colours are observed ions. For example, anions such as nitrate are brown, sulphate yellow and perchlorate blue."

Now, Professor Rice's discovery could lead to a practical colorimetric system for detecting anions.

The phosphate yellow structure pictured is an X-ray of a self-assembled capsule, containing 18 separate species, comprising six organic molecules, six copper ions and six phosphate anions. The phosphate anions are held within the self-assembled system and it is this that gives rise to all the different colours observed with different anions.

In the chemistry, the resultant self-assembled system -- the growing of a large complex molecule from small components -- was dependent on which anion was being investigated.

"We found that when we were investigating phosphate," said Professor Rice, "a capsule was formed from 12 different molecules and it contains six phosphate anions, whereas a nitrate comprised a circular assembly with eight nitrate anions. The phosphate species was yellow and the nitrate green. As a result, a sensor specific to each ion was grown by the molecule itself without any input from the scientist.