A Tunable Biophotonic Rainbow

A team of researchers from Yale-NUS College, Singapore, and the University of Fribourg, Switzerland, has discovered a color-generation mechanism in “rainbow” weevils that could lead to future applications in fiber optics and screen displays (Small, doi: 10.1002/smll.201802328).

Not your average insect

The rainbow weevil, also known as a snout weevil or Pachyrrhynchus congestus pavonius, is a bit of a biological marvel—it’s rare that a single insect can produce such a vast spectrum of colors on a scale-by-scale basis. Previously, this sort of natural color generation has only been found in cephalopod species such as squid and octopuses, which are known for their color-shifting camouflage.

The distinctive multicolored spots on the weevil’s thorax and wing casings (or elytra) consist of nearly circular scales, concentrically arranged by hue. Scale shades range from red in the outer rings to blue in the center—the same color pattern as a rainbow. The Singapore-Swiss team was interested in uncovering the precise mechanism of color-tuning underlying these structures, because it could have significant implications for biophotonic nanostructure research and could point to new bio-inspired applications.

The weevil’s scales are particularly notable because they “provide high color fidelity regardless of the angle you view it from,” the team’s co-lead, Vinodkumar Saranathan of Yale-NUS College, noted in a press release. Thus, the scales could have applications in digital displays such as those on smartphones or tablets, allowing the user to view the display from any angle and still see an undistorted, true image. Or they could be used to make reflective cladding for optical fibers, minimizing signal loss.

"The ultimate aim of research in this field is to figure out how the weevil self-assembles these structures, because with our current technology we are unable to do so," especially at visible length scales, Saranathan said.

Size + volume = tunability

The researchers turned to small-angle X-ray scattering, scanning electron microscopy and photonic-band-gap modeling to study the weevil’s “literal elytral rainbow.” The team found that each individual scale comprised a network of 3-D photonic crystalline structures made from chitin (the same material found in insect exoskeletons) in air. These biophotonic structures have a single-diamond symmetry. That’s fundamentally interesting to the physics community, according to the team, as single-diamond photonic crystals possess the largest full photonic band gaps of any photonic nanostructure.

Further, the researchers discovered that the scales’ vibrant colors depended on two main factors: the size of the crystal structure, and the volume of chitin used to make the crystal structure. Larger scales have a larger crystalline structure, higher chitin volume and reflect red light, while smaller scales have a smaller crystalline structure, lower chitin volume and reflect blue light.

According to the team, it is this unique, scale-by-scale ability to simultaneously control both size and volume factors to fine-tune reflected color that sets the rainbow weevil apart. "It is different from the usual strategy employed by nature to produce various different hues on the same animal,” explained Saranathan, “where the chitin structures are of fixed size and volume, and different colors are generated by orienting the structure at different angles, which reflects different wavelengths of light.”

Toward multifunctional applications

Given these results, the researchers believe that weevil-derived single-diamond photonic crystals can be used directly as templates for creating these intricate nanostructures using high-index dielectric infiltration, or as models for inorganic crystal synthesis. In the future, the team would like “to biomimic [the scales’] templated intracellular self-assembly in vitro in order to create tunable mesophases for multifunctional applications in photonics and sensing.”