Optical properties of chitin and chitosan biopolymers with application to structural color analysis

Optical properties of the biopolymers chitin and chitosan have been considered for wavelengths between 250 and 750 nm. First, by inverting published refractive index data for composite chitosan–chitin samples of two independent sources, we have been able to obtain the spectral dependence of both the chitosan and chitin refractive indices. Then light reflection and transmission measurements were carried out for samples obtained from fresh shrimp shells. From these spectrophotometric measurements the chitin refractive index and its extinction coefficient have been obtained for the mentioned spectral range. Absorption of light by chitin is negligible for visible wavelengths. Chitin extinction coefficient displays absorption bands in the near ultraviolet, and it is attributed to the proteinaceous content of chitin. Cuticle proteins have been isolated from these chitin samples, and absorbance measurements support the presence of the aforementioned absorption band. The chitin optical constants are used to model the structural color of a shield bug: the Poecilocoris lewisi.

The optical constants (refractive index and extinction coefficient) of chitin are obtained following two approaches: first, from inversion of published data about refractive index of chitosan–chitin samples, and then from inversion of spectrophotometric reflection and transmission measurements. The chitin refractive index spectral dependence is incorporated in structural color analysis to consider the coloration of a shield bug.Figure optionsDownload high-quality image (180 K)Download as PowerPoint slideHighlights
► We report on the optical constants of chitin and chitosan for near ultraviolet and visible wavelengths.
► The spectral regions of normal and anomalous dispersion are established for the chitin refractive index.
► Light absorption in the ultraviolet is resolved for chitin, and it is attributed to its proteinaceous content.
► Structural color calculations are carried out by incorporating the wavelength dependence of the chitin refractive index.