Synthesis of Bio-Inspired Guanine Microplatelets: Morphological and Crystallographic Control

β-Phase anhydrous guanine (β-AG) crystals are one of the most widespread organic crystals to construct optical structures in organisms. Currently, no synthetic method is available that allows for producing guanine crystals with similar control in size, morphology, and crystallography as in biological ones. Herein, a facile one-step synthesis route to fabricate bio-inspired guanine microplatelets with (100) exposing planes in almost pure β-phase is reported. The synthesis is based on a precipitation process of a guanine sodium hydroxide solution in formamide with poly(1-vinylpyrrolidone-co-vinyl acetate) as a morphological additive. Due to their uniform size (ca. 20 μm) and thickness (ca. 110 nm), the crystals represent the first synthetic guanine microplatelets that exhibit strong structural coloration and pearlescent lusters. Moreover, this synthesis route was utilized as a model system to investigate the effects of guanine analogues, including uric acid, hypoxanthine, xanthine, adenine, and guanosine, during the crystallization process. Our results indicate that the introduction of guanine analogues not only can reduce the required synthesis temperature but also provide a versatile control in crystal morphology and polymorph selection between the α-phase AG (α-AG) and β-AG. Turbidity experiments show that the β-AG microplatelets are formed with a fast precipitation rate in comparison to α-AG, suggesting that the formation of β-AG crystals follows a kinetically driven process.