Scientists in China and Germany have designed an artificial color-changing material that mimics chameleon skin, with luminogens (molecules that make crystals glow) organized into different layers of core and shell hydrogel instead of a uniform matrix. The results, published on May 6 in the journal Cell Reports Physical Science, demonstrate that a dual luminogen hydrogel chemosensor developed with this design can detect the freshness of seafood by changing color in response to amine vapors released by microbes when fish spoil. The material can also be used to advance the development of expandable electronics, dynamic camouflage robots, and anti-counterfeiting technologies.
“This new nucleus-shell arrangement does not require careful choice of luminogen pairs, nor elaborate design and regulation of the complex photophysical interactions between different luminogens,” says Tao Chen, professor at the Ningbo Institute of Materials Technology and Engineer at the Chinese Academy of Sciences and author of the study. “These advantages are important for the future construction of robust multi-color material systems with unmatched performance yet.”
While scientists have long contemplated developing flexible materials that can easily fluctuate between a wide range of fluorescent colors, synthetic materials are rarely able to change hue as skillfully as chameleons.
“Most flexible artificial color change materials have been prepared by simultaneously incorporating two or more reactive luminogens into a single elastomeric or hydrogel matrix,” explains Chen. “On the other hand, the organization of different iridophores into two superimposed structured core-shell layers is an evolutionary novelty for panther chameleons that allows their skins to display complex structural colors.”
To determine whether artificial color-changing materials could be imbued with the natural core-shell structure of chameleon skin, Wei Lu, a researcher at the Ningbo Institute of Materials Technology and Engineering at the Chinese Academy of Sciences , and his colleagues have developed a multi-luminogen. hydrogen system layered from the inside to the outside. First, the researchers synthesized a hydrogel with a red fluorescent core, which would serve as a template for the other layers. This core hydrogel was incubated in various aqueous solutions of Europium, after which the gel was incubated in a growth solution containing sodium alginate and reactive blue / green fluorescent polymers. The spontaneous diffusion of Europium ions from the central hydrogel into the surrounding solution triggered the formation of blue and green hydrogel layers.
Due to the way the central and shell layers of hydrogels overlapped, they could turn from red to blue or green when triggered by changes in temperature or pH. The authors also note that the emission color of the blue and green fluorescent layers could be adjusted, allowing the material to display colors of almost the entire visible spectrum.
“The diffusion-induced interfacial polymerization proposed to prepare core-shell materials is found to be general,” says Chen. “It is therefore expected that the proposed synthetic strategy will be extended to produce other soft color change materials, such as smart hydrogels or elastomers with stimuli sensitive structural color or pigment color change. “
To test the abilities of a chemosensor made from a dual-luminogen hydrogel to detect the freshness of seafood, Lu and his colleagues sealed test strips made from the material in boxes containing fresh shrimp or of fish for 50 hours. The test strip stored with seafood below -10 ° C barely changed from its original fluorescent red color, indicating that the food was still fresh, while the test strip stored with seafood at 30 ° C changed to a bright green tint, indicating that the food was spoiled.
Chen suggests that the new core-shell hydrogels and the diffusion-induced interfacial polymerization strategy used to make them could prove useful in a wide range of scientific fields, including robotics.
“In the near future, we plan to use the chameleon skin-shaped core-shell hydrogels to prepare biomimetic soft camouflage skins, which can be used to mimic the various color-changing functions of the skins of organisms. alive and to help achieve desirable assets. camouflage, display and alarm functions in robots, ”explains Chen.
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