Beetle color secrets help researchers create eco-friendly, vibrant hues

As the world increasingly focuses on sustainability, developing efficient and eco-friendly methods for producing color is essential. Traditional techniques often rely on chemical pigments and materials that can harm the environment. Now, NCCR Bio-Inspired Materials researchers have combined structural color techniques with natural pigments to produce vibrant colors. This approach may enable more sustainable color production for paints and other applications.

“The promise of structural color is that one day, we would be able to make all the colors out of just a few materials, and that would produce a lot less chemical waste and greatly facilitate recycling,” says NCCR Associate PI Eric Dufresne, professor of Materials Science and Engineering at Cornell University in Ithaca, New York. What’s more, he says, structural colors tend to be more stable compared to pigment-based colors, which can fade over time.

The inspiration for the project came from the observations of another NCCR researcher, Bodo Wilts, professor of Chemistry and Physics of Materials at the University of Salzburg. Wilts previously noticed that the vibrant colors in a beetle didn’t align with traditional color theories, which classify colors as either structural — emerging from the arrangement of nanostructures within a material — or based on chemical substances called pigments. This led the researchers to propose that combining structural and pigment-based methods could create even more intense colors. “Sometimes, one plus one equals three,” Dufresne says.

For this study, Wilts and Dufresne teamed up with Frank Scheffold, professor of Physics at the University of Fribourg. The team set out to build an artificial system to test Wilts’ hypothesis and confirmed that merging structural color with pigments produced brighter colors using fewer materials.

In their experiments, the researchers used beta-carotene, the pigment responsible for carrots’ color, alongside structural color techniques. They found that this combination allowed them to achieve high brightness while significantly decreasing the number of material layers needed.

The researchers created a material made up of layers of polyvinyl alcohol and polystyrene, mixed with varying amounts of beta-carotene. By adding beta-carotene to the material, they increased the index of refraction, which is important for creating structural color. For example, to obtain 90% reflection efficiency, they required 31 layers without pigment; this number dropped to just 15 layers when beta-carotene was included.

This approach not only produced vivid colors but also did so with fewer resources, making it a more environmentally friendly option. However, Dufresne notes that while the pigment is plant-based, other materials in the system are derived from oil.

Adding beta-carotene to polystyrene also resulted in a broader range of colors, which included green and orange shades. The team published their findings in the journal Soft Matter.

Before this approach finds applications in the real world, more work is needed to integrate pigments into self-assembled materials for easier processing, Dufresne says. And although the method allows for bright colors such as orange and green, the capacity to create more colors remains limited, he adds.

For his part, Dufresne is focused on understanding the processes that enable organisms such as beetles and butterflies to create the nanostructures responsible for producing structural color. “All these organisms have to produce these crazy structures without a laboratory — they need to do this in the wild and they need to do it reliably,” he says. “This is the thing that I find most fascinating.”

Reference: Sai, T.; Froufe-Pérez, L. S.; Scheffold, F.; Wilts, B. D.; Dufresne, E. R. Structural Color from Pigment-Loaded Nanostructures. Soft Matter 2023, 19 (40), 7717–7723. https://doi.org/10.1039/D3SM00961K.