Structural Color Production Process Supports Soft Photonics Applications

JAKE SALTZMAN, NEWS EDITOR
jake.saltzman@photonics.com

By coating droplets of an aqueous solution of hydroxpropyl cellulose with silica nanoparticles, and placing these “liquid marbles” inside an organic solvent, scientists have successfully made and displayed structural colors from cellulose-based polymers. Liquid marbles, which are millimeter-size droplets of liquid crystalline solutions, serve as a surrounding fluid in the system, and are coated to prevent them from mixing with the solvent.

The marbles still interact with the fluid, however, producing colors in a process that yielded results that could be used as an alternative to dyeing.

The two-liquid system responded to environmental changes, meaning the discovery could hold applications in/with soft photonic elements and biobased sensors. Instead of using a dye, structural colors rely on transparent materials to generate colors, through the arrangement of its molecules or elements. A surface displays structural colors when the transparent material’s structural elements reflect light.

This occurs in the scale-ripples of certain colorful fish, in butterflies, and in nanocrystals arranged at specific distances — such as the color-changing skin of chameleons.

The new method improves upon an existing technique for achieving pitch-generated structural colors that controls the pitch, the distance of a polymer’s full helical turn, as a structural element on which reflection may occur and structural colors may then appear. It depends on many parameters, however, some of which are difficult to control, and takes a long time to achieve equilibrium. The technique specifically relies on preparing liquid crystalline biopolymer phases, called cholesteric phases.

In the new system, the scientists used a commercial polymer, derived from cellulose, that orients itself in cholesteric phases (hydroxopropyl cellulose) to form the liquid marbles. The coated marbles were initially colorless in the solvent. Over time, the solvent extracted water from the liquid marbles, causing the biopolymer to adopt a crystal form suitable for the emergence of structural colors green, blue, and red.

The scientists were led by Manos Anyfantakis and colleagues at the University of Luxembourg. They say the colors depend only on the volume of the organic solvent, and emphasize the significance of the system’s slowness. Their ability to continuously control the system is also important, to ensure the polymer molecules can sufficiently adjust to a change in concentration and organize with a new equilibrium pitch.

When the scientists introduced external stimuli such as heat, pressure, and/or exposure to the chemicals, they observed characteristic color shifts corresponding to a varying pitch size.

The researchers believe that the biopolymer-based liquid marbles could offer a route to synthesize sustainable sensors.

The research was published in Angewandte Chemie (www.doi.org/10.1002/anie.202008210).