Fig. 2.

(a) Chemical structure of eumelanin, according to recent overviews [41,46], showing 4 units of the linear 4,7’-linked indole-5,6-quinone (and 5,6-dihydroxy)-2-carboxylic acid polymer, with atom numbering. (b) Comparative absorption spectra of sepia ink (A) and China ink (B), both diluted 1:2000 (v/v) with distilled water, showing the position of the 808-nm laser excitation (dashed line). The broad-band absorption curves decrease monotonically going from UV to NIR, a typical feature of these black polymeric chromophores.