Colourful crystals monitor humidity

Chinese chemists have developed a material that changes colour according to the humidity of the air around it. Whereas conventional humidity sensors require electrical power, this technique uses a simple polymer-based photonic crystal hydrogel that could find many practical applications. 

Photonic materials are made up of alternating regions of high and low dielectric constant - a nanostructure that affects the flow of light through the material. This process can be observed in the vibrant colours of the precious stone opal, a natural photonic crystal. 

The key to developing a humidity-sensitive photonic material was in the nanostructure design. The team, led by Yanlin Song at the Chinese Academy of Sciences, Beijing, used a styrene-based polymer to make tiny spheres approximately 150nm in size. These spheres were then soaked in a solution of acrylamide and bathed in UV light to trigger polymerization. 

The result of this process is a ’composite opal’ - a photonic crystal hydrogel that responds to water levels in the air. ’When the humidity increases or decreases, the polyacrylamide swells or shrinks, which extends or contracts the structure,’ explained Song. ’As a result, the colour of the hydrogel responds to the humidity change.

Yadong Yin, an expert in colour-changing photonic crystals at the University of California, Riverside, US, told Chemistry World: ’Although the idea of using hydrogels to control the optical properties of a colloidal crystal is not new, the system developed in this paper has the advantage of a large tuning range that covers the entire visible spectrum, as well as being completely reversible.’

Humidity control is important in many areas of industry, such as food and electronics, in order to prevent harmful condensation from occurring. Developing cheap and accurate humidity sensors could prove very useful in these areas. 

The research team is continuing to develop the technology, and hope to create different detectors to respond to gas leaks or low levels of poisons in the air. 

Lewis Brindley