Ever wondered why there is a little packet marked 'Do not eat' inside the box for your new DVD player? It's there to absorb moisture that could potentially damage the equipment, but how do you tell if it has been exposed? Chemists from the US think they have the answer with a humidity sensor that irreversibly changes colour on contact with water.

Many coloured dye molecules exhibit different properties when isolated from, or associated with, one another. This effect has been known for many years, but now Christoph Weder and colleagues from Case Western Reserve University have used this property to develop a moisture sensor.

Dye molecules within a polymer matrix can be effectively isolated from one another by rapidly cooling the polymer-dye mixture below its glass transition temperature (think of how inflexible a piece of rubber tubing becomes if you freeze it in liquid nitrogen). If the polymer is heated above its glass transition temperature, then the dye molecules can aggregate and cause a colour change - an effect used in the past for temperature sensors.

The trick to making this sense water was to design a polymer system that changes its glass transition temperature on contact with water. Exposure of the team's polyamide to a humid atmosphere causes a change in the glass transition temperature from ~50°C to well below 0°C, and a corresponding irreversible colour change from green to orange.

'What we'd like to do next is investigate whether we can change the properties to create specificity for other analytes,' said Jill Kunzelman, one of Weder's team. This seems to be a very realistic goal: Qinetiq research fellow Ian Sage said 'The new sensor shows good potential for a low cost device whose properties can be tuned to a particular purpose.'

Stephen Davey