Scientists in China and Germany have designed an artificial material mimicking chameleon skin to detect seafood freshness by changing color. 

Smart packaging and technology that can detect food spoilage in real-time could help reduce food waste and ensure food safety for the retailer and consumer.

The material uses luminogens – molecules that make crystals glow – which change color in response to different stimuli. Scientists demonstrated this using amine vapors released by microbes as fish spoils. 

In addition to freshness detection, the researchers also believe this technology can use patterned, colored materials for anti-counterfeiting purposes.

Putting it to the test
Wei Lu, a researcher at the Ningbo Institute of Materials Technology and Engineering at the Chinese Academy of Sciences, along with other researchers, tested the abilities of the chemosensor made from a two-luminogen hydrogel to detect seafood freshness.

They sealed test strips made from the material in boxes with fresh shrimp or fish for 50 hours. 

The test strip stored with seafood at less than -10°C barely changed from its original red fluorescent color, indicating the food was still fresh, while the test strip stored with seafood at 30°C shifted to a vivid green hue, indicating the food had spoiled.

The authors note that the emission color of the blue and green fluorescent layers could be adjusted, enabling the material to display colors from nearly the full visible spectrum.

Creating colors
The research paper, published in the journal Cell Reports Physical Science, describes the two-luminogen hydrogel chemosensor. 

While scientists have long envisioned developing soft materials that can fluctuate between a wide range of fluorescent colors with ease, synthetic materials are rarely able to change hue as artfully as chameleons do.

The color-changing system was designed from the inside out. First, Lu and colleagues synthesized a red fluorescent core hydrogel, which would serve as a template for the other layers. 

This core hydrogel was incubated in various aqueous europium solutions, after which the gel was incubated in a growth solution containing sodium alginate and responsive blue/green fluorescent polymers. 

Spontaneous diffusion of europium ions from the core hydrogel into the surrounding solution triggered the formation of blue and green hydrogel layers.

The core and shell layers of the hydrogels change from red to blue or green when triggered by changes in temperature or pH because of the way they overlap.

“It is highly expected that the proposed synthetic strategy could be expanded to produce other soft color-changing materials, such as smart hydrogels or elastomers with stimuli-responsive structural color or pigment color change,” says Tao Chen, a professor at the Ningbo Institute of Materials Technology and Engineering at the Chinese Academy of Sciences and an author of the study. 

Inspired by nature
The artificial material was designed into different core and shell hydrogel layers instead of one uniform matrix.

“Most artificial color-changing soft materials have been prepared by simultaneously incorporating two or more responsive luminogens into one single elastomer or hydrogel matrix,” says Chen.

“On the other hand, the organization of different iridophores into two superposed core-shell structured layers constitutes an evolutionary novelty for panther chameleons that allows their skins to display complex structural colors.”

“This novel core-shell layout does not require a careful choice of luminogen pairs, nor does it require an elaborate design and regulation of the complex photophysical interactions between different luminogens.” 

These advantages are important to the future construction of robust multicolor material systems with as-yet-unachieved performance, Chen adds. 

In addition to food applications, Chen envisions the chameleon skin-like core-shell hydrogels will be used to help achieve desirable active camouflage, display and alarm functions in robots. 

Last November, researchers in Singapore developed packaging that couples with an app to indicate when fish is no longer safe to eat. Spoilage was similarly detected through the presence of gases released from the decaying food, which then changed colors on a barcode.