Pavel Anzenbacher and co-workers at Bowling Green State University, US, have designed a sensor array for sensing metal ions in water.

Some heavy metals such as mercury have no known vital or beneficial effect on organisms and their accumulation over time in the body can cause serious illness. Because they cannot be degraded or destroyed, they have significant potential to impact human health and the whole environment. Thus, sensing of heavy metal ions in water is important for maintaining well-being of humans and protecting the environment, said Anzenbacher. 

A sensor array is made up of several sensor elements. These elements are not highly selective toward specific analytes, instead the specificity of the device comes from recognition of response patterns (e.g. fluorescence). This pattern originates from interactions of the analyte with each of the sensors in the array. This response pattern is unique to the analyte, a fingerprint. 

Conventional sensors are usually selective to just one analyte. Take the situation where water is contaminated by a mixture of heavy metals, a battery of selective sensors or complicated instrumentation must be used. Alternatively, sensor arrays could provide an analysis of the complex mixture from one single analysis. Sensor arrays, due to the lower selectivity of their sensor elements can also respond to unknown or unexpected analytes. 

The sensors designed here comprise an 8-hydroxyquinoline ligand and blue emitting-residues, i.e. 2-pyrenyl or fluorene bridges. 8-hydroxyquinoline forms luminescent chelates with a number of metal ions. It also shows a handy turn-on signal, it is non-fluorescent in water but emits a blue-green colour when it binds to a metal. The colour depends on the 8-hydroxyquinoline substituents and the metal ion. Less desirable sensors show an on-off signal, said Anzenbacher. The loss of fluorescence may but does not have to be from association with the analyte, but due to competing processes such as precipitation of the sensors from the solution taking place. 

By extending the length of these conjugated chromophores (the colour-emitting groups) they were able to vary the blue-emission to green emission of the metal chelates. Changes in the blue and green emissions of the metal complexes are then used to distinguish the cations. 

'The novelty of our approach is the use of one receptor (ligand) type and varying the chromophore to generate variable signal output', said Anzenbacher. 

Sarah Corcoran