A hierarchical self-assembly of eumelanin

Atomic force microscopy is used to examine the structure of a natural eumelanin isolated from the ink sacs of cuttlefish (Sepia officinalis). The experimental data presented clearly show that the 100-200 nm spherical eumelanin particles imaged previously by SEM are not a fundamental structural unit. While these spherical particles are stable structures, as is evidenced by their cohesiveness under mechanical stress, the AFM images reveal that these particles are composed of smaller constituents. Taking recent scattering and mass spectrometry results into consideration, we conclude that the self-assembly of Sepia eumelanin is a hierarchical process with small units assembling into hundred-nanometer structures, which then aggregate to form the morphology of the macroscopic pigment.