Vase-like β‑Polymorph Guanine Crystal Aggregates Formed at the Air–Water Interface

Guanine in the anhydrous β-form is a common light manipulator in organisms to produce structural colors for a variety of purposes. The level of control over crystal nucleation and growth of guanine exhibited by these organisms to produce crystals with precise morphology, size, and polymorphism far surpasses what has so far been achieved synthetically from aqueous solution. Here, we show that the morphology of guanine crystals can be altered significantly by inducing crystallization at the air–water interface to produce vase-like morphologies with a few hundred nanometers thick, micron-longcrystals pointing away from the vase opening. The exact morphology is highly dependent on the initial guanine concentration with increasing templating efficiency from the air–water interface at decreasing sample volumes. Scanning nanobeam X-ray diffraction experiments reveal that the vases consist exclusively of the anhydrous β-polymorph and that the crystal long axes coincide with a*. While the morphology does not match the biogenic counterparts that usually are thin along a*, the results demonstrate that interfacial crystallization templates the aggregate morphology and dictates the polymorph of guanine.