Designing the Iridescences of Biopolymers by Assembly of Photonic Crystal Superlattices

Structural color in naturally occurring systems is generally constituted by nanoscale lattices of biopolymers that generate beautiful iridescences through their regular structures, often interspersed with defects or period mismatch. Taking inspiration from both formats and materials found in Nature, a series of large-scale, highly reflective biopolymer-based photonic crystal superlattices constituted by stacking layers of 3D nanoscale lattices with different periodicity is presented. These silk photonic crystal superlattices (SPCSs) are fully composed of naturally derived structural proteins (silk fibroin) and exhibit brilliant structural color while being mechanically flexible. Multi-stopbands over broad wavelength ranges or single-stopbands with narrowband spectral responses can be readily realized and precisely controlled by manipulating the hierarchy of the lattice stacks or the repetition periods of the assembled colloidal monolayers. The unique ability to vary the silk protein conformation allows to vary the lattice and controllably “design” the iridescences of the SPCSs with water vapor adding versatility to this biopolymer-based photonic structure.