Color changes in chitosan and poly(allyl amine) films upon metal binding

The intense coloration of butterflies, snakes, hummingbirds and arthropods is due to reflective interference by stacked thin film layers of alternating high and low index of refraction materials. By controlling film thickness, it is also possible to create a single layer film of similar materials with well-defined color. Cross-linked chitosan and poly(allyl amine) hydrochloride thin films were assembled by dropping a dilute solution onto a spinning silicon substrate. Film quality and reproducibility were investigated, as well as the effects of cross-linking. Control of thickness could be used to determine film color. When dipped into a variety of metal ion solutions, the cross-linked films changed in thickness and color.