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Mechanochromic, thermochromic, piezochromic … or just pretty

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POLINA POTOCHEVSKA, EDITORIAL INTERN [email protected]

It took nature millions of years to develop the tiny scales that give butterfly wings — which are themselves transparent — their intense coloration. Like peacock feathers, they derive their signature hues from structures rather than pigments.

A different sort of butterfly effect is evident in the development of nanocrystals that produce intense, changeable colors from tiny structures rather than pigments. Courtesy of the University of Surrey.


A different sort of butterfly effect is evident in the development of nanocrystals that produce intense, changeable colors from tiny structures rather than pigments. Courtesy of the University of Surrey.

Now, a team led by scientists at the University of Surrey and the University of Sussex in England has borrowed from nature’s bag of tricks to develop a flexible photonic crystal that can not only display brilliant colors, but also reversibly change them.

The photonic crystals contain graphene — a one-atom-thick crystalline form of carbon — and are created using evaporation-driven self-assembly of soft polymer colloids, according to a paper published in Advanced Functional Materials titled “Mechanochromic and Thermochromic Sensors Based on Graphene Infused Polymer Opals.”

The title refers to the feature that makes the opalescent crystals more than a pretty bauble. Their color can respond to light, temperature, strain, or other physical and chemical stimuli, which offers interesting options for cost-effective, robust visual sensors. The sensors appear green under natural light but shift to blue when stretched. They can even turn transparent at higher temperatures. Their crystals act as shape-memory polymers, allowing them to memorize and recover their original shape and color.


The researchers added graphene into a colloidal crystal lattice to form photonic crystals with angle-dependent structural color and a reversible stopband. These unique features make the crystals good candidates for a variety of visual sensing applications.

Their sensitivity to thermal changes offers a visual indicator when perishables, such as food or pharmaceuticals, have experienced undesirable time-temperature histories, for example. Their pressure-responsive shape-memory characteristics also enable them to precisely reveal fingerprints, offering new options for biometric and anti-counterfeiting applications. If functionalized with biomolecules, the crystals could enable the creation of highly sensitive point-of-care testing devices for respiratory viruses, offering inexpensive, reliable, and user-friendly biosensing systems.

The crystals’ color can respond to light, temperature, strain or other physical and chemical stimuli, which offers interesting options for cost-effective, robust visual sensors.
The list of possible applications goes on. As Izabela Jurewicz, a lecturer in soft matter physics at Surrey, said: “While these crystals are beautiful to look at, we’re also very excited about the huge impact they could make to people’s lives.”

It’s clear that nature can serve as a powerful inspiration to scientists, and the functionality of nature’s colorful creatures and materials can be translated into new and exciting technologies today.
 

Published: September 2020
Glossary
graphene
Graphene is a two-dimensional allotrope of carbon consisting of a single layer of carbon atoms arranged in a hexagonal lattice pattern. It is the basic building block of other carbon-based materials such as graphite, carbon nanotubes, and fullerenes (e.g., buckyballs). Graphene has garnered significant attention due to its remarkable properties, making it one of the most studied materials in the field of nanotechnology. Key properties of graphene include: Two-dimensional structure:...
Lighter SideMaterialsSensors & DetectorsUniversity of SurreyUniversity of Sussexphotonic crystalgraphene

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