‘Chameleon’ material could help develop better sensors

Chameleons, octopuses, and cuttlefish have developed some of the animal kingdom’s most sophisticated skin, which allows them to rapidly change color to blend into the background. Inspired by the color-changing abilities of these critters, NCCR Bio-Inspired Materials researchers at the University of Fribourg have created a gel-like material that varies its hue in response to temperature and pH.

The material could be used as a drug-delivery vector or a colorimetric biosensor — among other applications. “In biology, it’s often difficult to measure alterations in pH or temperature in a very small volume, so having a material that changes in color is a big advantage,” says NCCR Principal Investigator Prof. Marco Lattuada of the University of Fribourg’s Department of Chemistry.

The ‘chameleon’ material, developed by Dr. Golnaz Isapour, a former PhD student in Lattuada’s group, is a gel-like photonic crystal. The key idea behind photonic crystals is that, depending on the exact position, size, and spacing of their components, light interacts with these materials in ways that produce different colors.

Isapour created a photonic crystal made of millions of minuscule water-swollen plastic bubbles, or hydrogel spheres, that measure several tens of micrometers in diameter. These microspheres serve as a scaffold for even smaller hydrogel nanospheres, measuring just a few hundreds of nanometers in diameter.

When temperature increases, the microspheres shrink, pushing the nanospheres closer together. This change in the spacing between nanospheres creates a shift of color that spans the entire range of the visible light spectrum — from red to green to blue. Similarly, when pH increases, the nanospheres shrink, causing the scaffolding microspheres to collapse, which ultimately results in a color change. Variations in hue happen rapidly, over several seconds or a few minutes, the researchers found.

Creating such a smart gel is no trivial task: to be able to see the different colors, the hydrogel spheres have to be packed in a precise three-dimensional arrangement, similar to that observed in snowflakes and other crystals. “You have to make sure that these particles are able to repel each other in such a way that they don’t clump together, so that they can arrange in an ordered manner,” Lattuada says. The researchers detailed the protocol to create the smart material as well as its color-changing properties in the journal ACS Applied Nano Materials. 

Lattuada notes that, with some tweaks, the ‘chameleon’ gel could be employed for drug delivery. He explains that when the hydrogel spheres change their size or spacing, the material expels water, just like a sponge that is squeezed. If researchers can encapsulate a drug in the gel, they could use it to release the drug into a specific tissue after a change in temperature or pH. “You can definitely encapsulate drugs as long as they don’t interfere with the structure of the gel,” Lattuada says. Other applications of the smart gel include colorimetric biosensors — devices that change their color when blood pH or other health indicators are altered.

Reference: Isapour, G.; Lattuada, M. Multiresponsive photonic microspheres formed by hierarchical assembly of colloidal nanogels for colorimetric sensors, ACS Appl. Nano Mater., 2021, 4, 3389–3396.

Author: Giorgia Guglielmi