At first glance it seemed like a joke: when Norman Lüchinger told his supervisor that the thin copper strips he was studying in his laboratory would turn blue if he put his hand on them, Wendelin Stark’s initial response was disbelief. However, on closer inspection it became apparent that the young researcher really was telling the truth: the metal strip changes colour depending on how moist the environment is.

Lüchinger and Stark realised immediately that this was an interesting observation. Together with other researchers from the Institute for Chemical Engineering and Bio-engineering of ETH Zurich (1), the pair have since shown that such copper strips can be used to measure air humidity remarkably precisely. (2) The reason lies in the special structure of the strips. In fact they are not made from ordinary copper but consist of countless minute copper particles deposited on a polymer substrate. Between the copper particles there are tiny cavities that fill with water in moist air. Stark explains that “The process is comparable to a piece of blotting paper which absorbs water.” The uptake of water into the pores has a direct effect on the surface resonance of the material. The visible consequence: there is a striking change in the colour of the copper strip.

Two effects combined

When the pores are completely full of water, a second additional effect occurs: a thin film of water forms on the strips, like an oil film on a puddle. When light falling on this water film is reflected, interference effects occur and make the surface appear in quite different colours again. Stark explains enthusiastically "The whole colour spectrum can be observed from an air humidity of 60 percent onwards.” At the same time the metal strip’s response is remarkably sensitive: in the region between 80 and 85 percent air humidity, differences of one percent can be detected solely on the basis of the colour differences.

The copper platelet changes colour depending on the air humidity. A similar effect is observed when the copper comes into contact with ethanol vapour. (© American Chemical Society) large

However, the new sensor is not only sensitive, it also responds to changes very quickly. Stark has observed that "The colour adjusts within a few milliseconds.” The colour change is also reversible. “The measurements can be repeated any number of times.”

Specific modifications

The scientists can think of various practical applications. Stark thinks "The new sensor could be used as a simple way to determine the air humidity in a room and this, by the way, without any of the kind of electronics that is usual with modern instruments." The new sensor could also be attractive for the food industry. It is important that many foodstuffs are stored in the correct environment. The sensor would be a simple way of checking whether the air in the package is moist enough or whether the goods are in danger of drying out.

However, the copper sensor that has now been developed is able to cover only a particular range. Below 60 percent air humidity its colour remains constant. Stark now wants to modify the strips by adding other substances so they can also display high sensitivity for other ranges. Several strips with different sensitivities could be combined together if there is a need to cover the entire measuring range. Stark is also convinced that the measuring principle could also be used to detect other substances, e.g. petrol vapours. The two researchers have already shown that ethanol can also cause the copper platelets to change colour.