Doublet Nanoplate Structure of Phoroncidia rubroargentea Spider-Derived Guanine Crystals for Enhanced Broadband Reflectivity

Biogenic guanine nanocrystal-based multilayers offer intriguing biological structural models for designing dielectric optical systems with extraordinary optical effects, such as polarizing-insensitive broadband reflections and tunable and dynamic structural colors. Single crystals and crystals of nonmonocrystalline structures have been identified for biogenic guanine crystals. Although biogenic guanine crystals have been intensively investigated in relevant structure–function relationships, the role of various nonmonocrystalline structures of biogenic guanine crystals remains to be further explored. Here, we examine a doublet structure of the guanine crystal nanoplates responsible for the epidermal silvery coloration in a model spider, Phoroncidia rubroargentea, in both two and three dimensions. This doublet structure is composed of two tightly adhered crystallized layers. Half- and full-thickness defects are observed in these crystals. Combining structural characterizations and finite-difference time-domain (FDTD) optical modeling, here, we report a prospective role of this doublet structure. Simulation results indicate that defects in the doublet crystal nanoplates allow for enhancing light reflectance in the short-wavelength region while maintaining high reflectance at longer wavelengths for this natural optical system, as well as reducing the polarization degree of the reflected light in the visible range. Compared with previously reported undefined thickness variations of the nanocrystals for silvery coloration, this doublet structure provides an alternative strategy to optimize the broadband reflection in a more controllable way. This study also paves an avenue for structural optical design with guanine crystals at the individual crystal level.