Development of Uniform Chitosan Thin-Film Layers on Silicon Chips

Nature creates selectively refractive materials in butterfly wings and crab shells by alternating layers of chitin with different refractive indices. To mimic this construction, control of both thickness and refractive index of chitosan films is required. Methods were developed for adhering the chitosan layer to glass and silicon, polymerizing the chitosan, stabilizing the chitosan films against environmental changes in temperature and humidity, and depositing a uniform chitosan layer with defined thickness. First, commercial chitosan was hydrolyzed to obtain lower molecular weight polymers and further purified. Then chitosan layers were prepared on clean silicon chips pretreated with polyvinyl butyral resin for adhesion. The chitosan solution with a polymerizing component and a plasticizer consisted of 2.5% purified chitosan/2.5% Resimene/0.25% tetraethylene glycol. Spin-coating at varying speeds after an incubation period of 1 h at room temperature produced chitosan layer thicknesses in the ideal range of 1200−2000 Å. The different thicknesses were reproducible, with a standard deviation across the film between 3 and 5%. The experimental index of refraction calculated for these layers was 1.59.