A hierarchical self-assembly of eumelanin

Atomic force microscopy is used to examine the structure of a natural eumel anin isolated from the ink sacs of cuttlefish (Sepia officinalis). The expe rimental data presented clearly show that the 100-200 nm spherical eumelani n 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 thes e 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 ass embling into hundred-nanometer structures, which then aggregate to form the morphology of the macroscopic pigment.