While the color of conventional pigments and dyes is related to the absorption of light, structural color results from selective light scattering by nanostructures with an appropriate periodic variation of refractive index. The latter can be obtained from virtually any material and produce a coloration that appears considerably brighter and is more resistant to fading. We developed colloidal photonic pigments with spherical symmetry from polymer nanoparticles (PNP) of styrene and acrylate monomers, using their surface charge to control the self-assembly by droplet microfluidic emulsification. By changing only the size of the PNPs, we are able to produce colors across the entire visible spectra. Unlike most other spherical photonic pigments, our materials present bright saturated colors and virtually no iridescence. We achieved this by doping the pigments with polydopamine (PD) nanoparticles. The latter can both reduce the long-range order in the photonic pigments and absorb the diffuse light scattering from structural defects, as shown by electronic microscopy and reflective confocal microscopy (RCM). Nanostructured PD-doped spherical photonic pigments can be obtained from sustainable materials and hold enormous potential for technological applications from dye-free color coatings to stimuli-responsive coloring, dye-free colorimetric sensors, and reflective color screens.

The development of appropriate methods to correlate the structure and optical properties of colloidal photonic structures is still a challenge. Structural information is mostly obtained by electron, X-ray, or optical microscopy methods and X-ray diffraction, while bulk spectroscopic methods and low resolution bright-field microscopy are used for optical characterization. Here, we describe the use of reflectance confocal microscopy as a simple and intuitive technique to provide a direct correlation between the ordered/ disordered structural morphology of colloidal crystals and glasses, and their corresponding optical properties.

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