Core-shell nanoparticles with a shell refractive index (RI) similar to water have been used to tune the spacing between cores to achieve optimal scattering for non-iridescent colors, but only in solution. Even though both bottom-up and top-down methods have been widely used, there is a demand for a scalable process for mass production of structural colors.

Nature provides many spectacular examples of structural colors, such as duck wing feathers and wild turkey feathers. In continuation of their earlier research that unraveled the fundamental properties of melanin — a family of natural pigments found in skin, hair, eyes and the plumage of brightly colored birds — these examples served as bio-inspiration for the design of core-shell synthetic melanin nanoparticles for the production of bright structural colors.

Melanin has been around for millions of years. “Melanin is an important biomaterial that has so far been underutilized in materials science and technological applications,” said The University of Akron’s Dr. Ali Dhinojwala, H.A. Morton Professor of Polymer Science and one of the principal investigators on the project.

In this most recent research, the team has found that tiny packets of synthetic melanin produce structural color when they are packed into semi-ordered spherical particles. Structural color occurs through the interaction of light with materials that have patterns on a sub-micron scale, which reflect light to make some wavelengths brighter and others darker.

“The chemistry we use to make these particles is based on the main ingredient that goes into making melanin,” notes Dhinojwala. “Then we take these melanin particles and self-assemble into a structure using a very straight-forward process. It is similar to things we see in our homes, like mixing oil and water together creating emulsions. Those emulsions essentially allow us to assemble these particles into photonic inks, which we call photonic supraballs.”

These nanoparticles were self-assembled using a one-pot reverse emulsion process, adds Ming Xiao, currently a postdoctoral research fellow. “It resulted in bright and non-iridescent supraballs. With the combination of only two ingredients, synthetic melanin and silica, we can generate a full spectrum of colors. The use of melanin as the core material can increase the brightness and saturation of supraballs due to its unique combination of high RI and broadband absorption of lights. In addition, melanin is biocompatible and can dissipate almost 90% of the UV radiation into heat within a nanosecond, making those melanin-based supraballs suitable for cosmetics or UV-resistant inks.”

Other leading members of the research team are Dr. Nathan C. Gianneschi of Northwestern University and Dr. Matthew Shawkey, formerly with The University of Akron and now an Associate Professor of Biology at the University of Ghent in Belgium. Rounding out the team are scientists from UC San Diego, Sichuan University and Shanghai Jiao Tong University.