Hidden triangular grating structures in biogenic guanine platelet

A platelet made of guanine is used to control light reflection from the bodies of fish, and an assembly of the platelets acts as an efficient photonic crystal. This work reveals that internal structures exist in addition to the smooth platelet surface. A fresh fish guanine platelet resembles a mono-plate. However, in a digested platelet, guanine nano-beams that form triangular array networks are discovered by scanning electron microscopy and transmission electron microscopy in the guanine platelets of two fish species. It is found for the first time that the nano-beams have widths of 30 nm–50 nm and sheets cover the internal gratings. The newly found internal grating structures of the biogenic guanine platelet may provide further information on the optical property of the reflection in living creatures.