Spherical colloidal photonic pigments with saturated structural colors through temperature-controlled polymer nanoparticle assembly

Structural color is based on the reflection of light by materials with a periodic variation of refractive index in the length scale of the wavelength of visible light. Unlike conventional dyes and pigments, there is no absorption of radiation, and therefore structural pigments are impervious to fading by photobleaching. Also, because structural color is mainly dependent on structure, not composition, it can be obtained from materials with low environmental impact and toxicity. Structural pigments with colors covering the full visible spectrum can therefore be obtained from the same colloidal materials, by tuning only their size and the spatial arrangement of individual nanoparticles. Here, we show that using polymer nanoparticles with low size dispersity, and controlling the kinetics and the inter-particle interactions during the assembly process, one can obtain spherical non-iridescent photonic pigment. Using polymer nanoparticles with diameters in the range of 170 nm–330 nm, we obtain structural pigments with bright colors spanning the full visible spectrum, with different hue and saturation. The optical properties of these pigments are directly related to the arrangement of the polymer nanoparticles within the spherical structural pigment. This arrangement, controlled by the surface charge of the nanoparticles and the rate of water evaporation from the template droplets containing the nanoparticles, changes from an ordered face-centered cubic packing at 50 °C, to an almost amorphous packing at 65 °C. Our spherical photonic pigments reflect a plethora of vibrant colors with low angle dependence, with excellent prospects for used as dispersible pigments in the development of environmentally friendly coatings with non-fading color.