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Opal in stone

Opal-like material could be used in range of sensors

Image credit: Dreamstime

UK researchers have developed a colour-changing material inspired by iridescent butterfly wings and peacock features, which could be the 'cornerstone' of smart sensors.

The material – photonic crystals containing a miniscule amount of graphene – has a range of highly desirable qualities, including its outputs being directly observable by the naked eye. The crystals are intensely green under natural light, but change to blue when stretched and become transparent when heated.

This makes use of a natural phenomenon known as structural colour - the production of colour by tiny structured surfaces which interfere with visible light. For example, peacock feathers have brown pigment, but also reflect blue and green light due to photonic mechanisms within the structures on their surfaces.

Scientists at the Universities of Surrey and Sussex led an international team of scientists to develop the material; their method is described in an Advanced Functional Materials paper. As the material is fabricated, a tiny amount of graphene within the crystal lattice results in the formation of crystals with structural colour. These crystals can reversibly change colour as a function of temperature.

“Our research here has taken inspiration from the amazing biomimicry abilities in butterfly wings, peacock feathers and beetle shells, where the colour comes from structure and not from pigments,” said Professor Alan Dalton, a University of Sussex physicist. “Whereas nature has developed these materials over millions of years, we are slowly catching up in a much shorter period.”

The colour-changing, flexible crystals could be incorporated into sensors capable of responding very sensitively to light, temperature, strain and many other physical and chemical stimuli. Possible applications include time-temperature indicators for 'smart packaging'; fingerprint analysis; testing devices for respiratory viruses; health and fitness monitoring, and healthcare safety - e.g. a wristband that changes colour to indicate whether a clinician has recently washed their hands.

University of Surrey physicist Dr Izabela Jurewicz explained that this was the first demonstration of polymer-based opals with these precise qualities, adding: “While these crystals are beautiful to look at, we’re also very excited about the huge impact they could make to people’s lives.”

The university researchers are working alongside Sussex-based Advanced Materials Development (AMD) Ltd to commercialise the technology.

“Given the versatility of these crystals, this method represents a simple, inexpensive and scalable approach to produce multi-functional, graphene-infused synthetic opals and opens up exciting applications for novel nanomaterial-based photonics,” said John Lee, CEO of AMD. “We are very excited to be able to bring it to market in near future.”

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