Optimizing Color Saturation in Colloidal Photonic Crystals by Control of Absorber Amount and Distribution

Nanostructured materials that mimic structural coloration in nature can be synthetically created by colloidal self-assembly. To maximize optical effects, the natural world integrates melanin as a broadband absorber to remove incoherently scattered light. Polydopamine (PDA) is used as a synthetic analog of natural melanin to systematically investigate the influence of absorber quantity and distribution on color saturation in colloidal photonic crystals. The absorbing PDA is integrated into two distinct ways: homogenous colloidal crystals are produced from core-shell particles with incrementally increasing polydopamine shells, and heterogeneous colloidal crystals are formed by co-assembly of varying ratios of polystyrene (PS) and absorbing PS@PDA particles. The chromaticity is quantified by converting the measured spectra to reconstructed colors in the L*a*b* color sphere to identify structures with optimal color saturation. Simulations based on the discrete dipole approximation (DDA) indicate that a homogeneous absorber distribution is most efficient in creating saturated structural coloration. Experiments, however, demonstrate that the heterogeneous absorber incorporation outperforms the homogeneous strategies, as it allows for a more precise adjustment of the absorber content in the required concentration range. These results underline the importance of incorporating absorbers and indicate efficient ways in which colloidal photonic crystals with saturated structural colors can be prepared. The striking coloration in the natural world is typically caused by a combination of light interference at nanostructures and the broadband absorption of melanin. Here, colloidal photonic crystals containing polydopamine are used as synthetic mimics of this structural coloration. The amount of absorber and the way of its incorporation to create the most saturated structural color is investigated. image