Investigations of Biogenic Optically Functional Purines and Pteridines by Cryogenic Scanning Electron Microscopy

Organisms produce diverse optical systems via biomineralization of purines and pteridines, forming high-refractive-index crystals that enable structural coloration, visual image formation and photosynthesis enhancement. Distinct from conventional inorganic biomineralization, these biogenic organic crystals and their hierarchical assemblies are highly susceptible to conventional electron microscopy sample treatments, which cause dehydration, structural distortion and other artifacts formation. Cryogenic scanning electron microscopy (cryo-SEM) combined with high-pressure freezing effectively preserves native hydrated states and vitrifies intracellular water, avoiding common artifacts and enabling high-resolution visualization (∼ 10 nm) of delicate photonic structures. This review summarizes recent cryo-SEM investigations on guanine, xanthine, isoxanthopterin and 7,8-dihydroxanthopterin across a wide range of organisms. We elaborate their crystal packing modes, anisotropic optical properties, and structure–function correlations in biogenic optical systems. We also discuss developmental crystallization pathways of these organic crystals, revealing how organisms regulate nucleation, growth and assembly to design and fabricate targeted optical systems for specialized functions. This work demonstrates that cryo-SEM is a powerful tool for exploring organic biomineralization, and provides fundamental insights for developing bioinspired optically functional artificial materials.