Why do some ion sensors perform worse the longer they spend in solution? The answer, say researchers in Australia and Switzerland, is in the water.

Polymeric ion-selective electrodes distinguish between ions by having a polymeric membrane that only allows the intended ion to pass through to the electrode. These electrodes can be made small enough to use in lab-on-a-chip methods. The trouble with these coated-wire electrodes is that their performance deteriorates significantly the longer they spend in solution.

Roland De Marco at the Curtin University of Technology, Perth, Australia and colleagues, believe that the deterioration is caused by a layer of water that forms between the membrane and the electrode. This water layer acts as a reservoir for ions that have already passed through the membrane, and these ions interfere with the normal operation of the sensor.

The polymeric membrane of an ion-selective electrode traps a water layer which can interfere with the operation of the sensor

'Many researchers have speculated about the existence of a detrimental water layer, but no single research group has provided direct structural evidence,' said De Marco. The team used neutron and x-ray reflectometry to demonstrate the existence of this layer.

De Marco also has some ideas about how to prevent the formation of the water layer. 'We have a new approach employing a water-repellent ion sensing polymer film,' he said. 

In terms of characterising ion sensors, De Marco warns that the 'big science' techniques using neutron and x-ray sources are 'inaccessible by industry' and an industrially practical method is needed. 'In this context, we have developed a conventional electrochemical technique, that is, electrochemical impedance spectroscopy, for the characterization of new solid-state polymeric ion sensors.'

Colin Batchelor