Fig. 3: Structural and optical properties of lignin and LNPs photonic crystal.

a SEM image of the surface of photonic crystals. b Refractive index of lignin solution in acetone. Dashed line represents a liner fit (intercept = 1.3590; slope = 0.00187; Pearson’s r = 0.9949). Error bars represent standard deviation based on the entire population (n = 3). c Reflectance spectra of the photonic crystals from three distinct pieces with different colors of violet, blue, and green.