Structural colouration works thanks to nanoscale pillar-like metallic or dielectric hybrid structures that reflect light at different wavelengths. Such structures are also found in nature – for example, green-winged teal feathers have hexagonal non-close-packed melanosomes and wild turkey have feathers with hollow, high refractive-index contrast melanosomes that brighten their colours.

Researchers led by Ali Dhinojwala of the University of Akron, Nathan Gianneschi from Northwestern University and Matthew Shawkey at the University of Ghent, have now designed nanoparticles with melanin cores and silica shells inspired by such natural structures. They then used a facile one-pot reverse emulsion process to assemble their core-shell synthetic melanin nanoparticles (CS-SMNPs) into semi-ordered micron-sized bright, colourful and non-iridescent photonic supraballs. This process is also easily scalable for producing large quantities of the materials.

Melanin is a natural pigment found in skin, hair, eyes and the feathers of brightly coloured birds. Using this molecule as the core material in the supraballs serves to increase the brightness and saturation of the structures thanks to its high refractive index and the fact that it absorbs a wide range of light wavelengths. Melanin is also of course biocompatible and has the added advantage of dissipating almost 90% of UV light into heat within just a nanosecond, thus making the supraballs suitable for use in cosmetics or UV-resistant inks.

Dhinojwala and colleagues began by examining how duck feathers and iridescent turkey wing feathers reflect light. They then used the FDTD (finite-difference time-domain) technique to created an optical model without iridescence. Many structural colours produced before now were iridescent, which meant that they could not be used for applications such as wide-angle displays. And structural colours that did happen to be non-iridescent required additional absorbing materials to reduce incoherent light scattering.

High-refractive index cores and low refractive index shells are best

The silica shell is analogous to the keratin in duck feathers and helps control the spacing between the melanin nanoparticles, explains Dhinojwala. Although the overall size of the supraballs does not influence its colour, the researchers did find that the thickness of the cores and shells affect how the structures scatter light. Indeed, by measuring the refractive indices of different cores and shells, they found that high refractive index cores and low refractive index shells were the best for increasing light reflectance and so producing brighter colours.

“The colours produced by these supraballs are both bright and saturated compared to other structural colours previously reported in the literature,” Dhinojwala tells nanotechweb.org. “They might be used as photonics inks in paints and textiles, and more interestingly, could be ideal for cosmetics applications thanks to their biocompatibility and the UV protection they afford.”

The new melanin-based supraballs are described in Science Advances DOI: 10.1126/sciadv.1701151.