Engineering of bioinspired hierarchical guanine crystals

Guanine crystals are renowned as one of the most appealing biogenic organic optical crystals in living organisms, widely known for their high refractive index, rivaling inorganic crystals. Since their optical excellence is governed by specific morphologies, it is imperative to comprehend the methods for controlling the growth and assembly of guanine crystals. However, the artificial control over their morphology, especially in repeating the biogenic ones, remains a formidable challenge. Herein, we present a biomimetic synthesis strategy to engineer hierarchical guanine crystal polymorphs by controlling the primary nuclei growth processes, followed by the structural modulation under the competition of hydrogen bonding and π-π interactions. Five typical crystal morphologies, ranging from dense spherical polycrystalline aggregates to thin flake crystals, were successfully synthesized. The crystallization mechanism was analyzed using principal component analysis (PCA) method in machine learning technology, where the crystal nucleation behaviors were found to be critical in determining the crystal hierarchies and a three-dimensional synthesis-field diagram mapping 30,000 possible parameter combinations has been predicted. Additionally, these morphologies significantly affect the birefringence and the reflection of the crystals. This work provides a way for tailoring biogenic crystals through precise crystal nucleation regulation and growth-directed assemblies.