A new hydrogel can measure the freshness of seafood by changing color as temperature and ammonia levels fluctuate, much like a chameleon's skin shifts its hue to blend into different environments.

The hydrogel's three distinct layers let scientists visualize changes in response to different stimuli in the surrounding environment. Almost the full visible spectrum of light is covered in the current material, which changes its fluorescence from red to blue to green depending on the presence of heat and NH3, commonly known as ammonia. The novel system also shows exciting potential to improve anti-counterfeit technology, the authors explained in the study, published Thursday in Cell Reports Physical Science.

"Our institute is located in a coastal city in China. Many people in this city love seafood very much, but seafood easily spoils due to microbial growth that produces volatile amine vapors (for example, ammonia)," corresponding author Tao Chen told The Academic Times. Chen is a researcher at the Key Laboratory of Marine Materials and Related Technologies, in Ningbo, a name that translates to "peaceful waves."

Ammonia detection is a standard way of assessing whether shrimp, fish and other types of seafood are unsafe to eat. Those marine organisms naturally have a low level of ammonia that is not harmful to humans. But if fish or shrimp are left out at room temperature and start to spoil, the acidity changes as ammonia is released within. At some stage, people can smell the ammonia from volatile gases in the seafood, at which point it has definitely spoiled.

However, the current standard for detecting volatile amine and other volatile gases in shrimp and fish takes about four hours for one sample. This lengthy process is often impractical for seafood production on a larger scale, leading to seafood products being visually inspected by workers — a method that can be highly inaccurate and potentially dangerous to consumers.

Because of its chameleon-like structure, the hydrogel developed by Chen and his colleagues clearly displays if a sample has been exposed to various amine vapors. "Up to now, the responsive color-changing capacity of synthetic materials was still far inferior to that of the natural chameleon skin," co-author Patrick Théato told The Academic Times. Théato studies synthetic macromolecular chemistry at the Karlsruhe Institute of Technology, in Germany; he collaborated with the team in China for this bio-inspired project.

Théato and Chen noted that their respective research groups have long been intrigued by the prospect of designing soft materials with the ability to change colors, but the process was challenging for both chemists and material engineers — until the team separated the colors into layers.

A previous study showed that organizing crystals into "core@shell structured layers is an evolutionary novelty for natural panther chameleons that allows their skins to display complex structural colors," Chen said. (Yes, core@shell structures are supposed to have the 'at' sign in the name: It represents the way the shell wraps around the core.) "Is it possible to mimic this unique core@shell structure into artificial color-changing materials? As described in our paper, the answer is yes."

"Spatial organization [is] the key novelty for the present chameleon skin-inspired multicolor material," Théato explained. The researchers broke the existing pattern in fluorescent materials by separating the reactive molecules, known as luminogens, from each other to see a clearer color response. A luminogen is any type of molecule or atom that lights up when it is added to a crystal. "Since each luminogen is separately incorporated into a different layer, the fluorescence intensity of every luminogen can be controlled independently," explained Théato.

The new hydrogel is composed of a core with two layers that are visually similar to the Earth's core, mantle and crust. At the center is a red sphere, and immediately surrounding the core are two hydrogen shells. The hydrogel layers — one blue and one green — change colors in response to changes in the environment, while the red core stays true to its hue as a static base. 

The blue and green shells morph, in part, because of the materials themselves: The blue shell is made of a porous polymer that is sensitive to temperature, while the green shell is a gelatin-based polymer that is sensitive to acidity. The two layers allowed the researchers to test ammonia levels at different temperatures instead of combining the two environmental variables, which is the standard way to detect the freshness of seafood. Thus, changes in acidity alerted the observer that the shrimp had spoiled and was unsafe to eat, regardless of the seafood's temperature.

In response to temperature changes, the blue hydrogel layer showed a distinct "'purplish red to blue' emission color change," according to the authors. At 20 degrees Celsius, the hydrogel emitted purple or red fluorescent light, but at 50 degrees Celsius, it reflected blue light. The material was also a success in its reaction to amine vapors produced by shrimp. The core@shell turned green when the hydrogel was stored at 30 degrees Celsius over a period of 50 hours, signifying the presence of ammonia. When the same shrimp was stored below minus 10 degrees Celsius for the same period, the core@shell did not change color, as the seafood had been properly refrigerated and was not yet spoiling.

Chen and Théato noted that their core@shell layout does not require an elaborate design, which is a welcome improvement from the multicolor hydrogels currently on the market. The separation of different luminogens into their own layers also means that the interactions between these molecules and crystals are far easier to regulate. "These advantages are important to the future construction of robust multicolor material systems ... which have many frontier applications, such as stretchable electronics and anti-counterfeiting," Théato and Chen said.

Chen and Théato said their hydrogels could one day act as a dynamic camouflage for robots by creating a realistic display on a nonhuman object. "In the near future, we plan to use the core@shell hydrogels to prepare biomimetic soft camouflaging skins, which can be used to mimic the diverse color-changing functions of living organisms' skins," said Théato and Chen.

The study, "A panther chameleon skin-inspired core@shell supramolecular hydrogel with spatially organized multi-luminogens enables programmable color change," published Thursday in Cell Reports Physical Science, was authored by Wei Lu, Ningbo Institute of Materials Technology and Engineering, South China University of Technology and University of the Chinese Academy of Sciences; Muqing Si, Hao Liu, Huiyu Qiu, Shuxin Wei, Baoyi Wu, Jiawei Zhang and Tao Chen, Ningbo Institute of Materials Technology and Engineering and University of the Chinese Academy of Sciences; Ruijia Wang and Guangqiang Yin, Ningbo Institute of Materials Technology and Engineering; Patrick Théato, Karlsruhe Institute of Technology; and Yen Wei, Tsinghua University.