Bioinspired Crystallization of Guanine.

Biological guanine crystals in organisms exhibit excellent optical properties and functions, including broad-band and narrow-band reflectors, band-tunable reflectors, mirrors, and stimuli-responsive structural colors, attributed to the high refractive index of guanine (1.85) and the exquisite control of the polymorphs, morphologies, sizes, exposed planes, and the hierarchically ordered assembly of biological guanine crystals in the organisms. Herein, the controlled synthesis of guanine crystals with defined polymorphs and morphologies and their formation processes in organic and aqueous solutions are summarized in detail. In particular, the controlled synthesis of microplatelets or nanoplatelets of the thermodynamically metastable β form of anhydrous guanine (β-AG) exposing the (100) plane in the presence of additives, twinned crystals, and the occlusion of hypoxanthine in β-AG were investigated to mimic biological guanine crystals with superior optical properties. One-dimensional assembly of β-AG microrods was studied as a preliminary work to mimic the highly ordered assembly of guanine crystals with superior optical properties.