A simple, inexpensive molecule can be used as a luminescent sensor, which goes dark in the presence of specific metal ions. This could lead to significant improvements in the way we detect pollutants.

Metal cations — positively charged atoms — are present everywhere in our bodies and in the environment. In some cases they are beneficial, but in others — for example in the case of heavy metals such as lead —they can be harmful or even poisonous. Metals get into the water supply as a result of the natural weathering of soil and rocks, and as a result of human activities such as mining, or as a by-product of some industrial processes.

Scientists are looking for straightforward, effective, and inexpensive ways of detecting metal ions in the environment. One way is to design a molecule known as a chemical sensor, which transmits a signal — for example by emitting light — when it detects the metal ion in question. Many of these already exist, but they often involve expensive metals or rare earth metals, or large, complex molecules such as polymers. The search is therefore still on for a cheap, easy-to-make chemical sensor.

Jerome Lacour is based at the University of Geneva and has worked with colleagues there and at the University of Pisa in Italy to design a new type of chemical sensor that could pave the way for a new generation of simple chemical probes.

“Our cyclic molecules are like locks that emit a particular kind of (circular polarized) light to signal that the lock is ready to function. When a key is inserted inside (a metal ion), the geometry of the lock changes and the light disappears.

“In fact, the locks are composed of two parts. A ring (a crown ether) that can encircle metal ions such as sodium and two twisted arms that extend from the rim and act as light bulbs (chromophores) able to emit a specific type of (circularly polarized) light. This type of special emission allows us to ‘see’ and analyses whether metal ions are present or not.

“When no metal ion is present (no key inside), these two light bulbs are close to each other and display intense polarized luminescence. However, when a cation is added (key inside the lock), the molecule changes geometry. The light bulbs move apart and this dramatically shuts down the emitted polarized light — like Dumbledore’s Deluminator.

“It is also possible to remove the key by adding a scavenger. The original signal (luminescence) of the two bulbs is then totally recovered. This on/off ability can be repeated over several cycles — making our molecules effective light switches with easy read-out.”

Source: Royal Society of Chemistry