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.