Fabrication and optical properties of chitosan/Ag nanoparticles thin film composites

•Chitosan based planar optical waveguides (POW) have been fabricated.•Chitosan/Ag composites have been obtained by “in situ” reduction of Ag+ ions.•Refractive index of Chitosan/Ag composites can be tuned in a range from 1.53 to 1.69.•Chitosan/Ag composites can be used as POW up to a silver volume fraction 1.3%.

Chitosan thin films with controlled thickness have been fabricated by spin-coating chitosan acetate and citrate solutions for application as planar optical waveguides (POW). “In situ” reduction of preadsorbed in chitosan film Ag+ ions with sodium borohydride resulted in formation of chitosan/Ag composites, in which silver nanoparticles of a size below 10 nm were homogeneously distributed. In contrast to chitosan/Ag composites formed in solution, UV–vis spectra of the thin film composites have depicted a single narrow adsorption band at 412–413 nm independently of the silver content. The absence of long-wavelength shoulders in the spectra, which are associated with oscillations of non-spherical particles or plasmon coupling of aggregates, has confirmed SEM data on high uniformity of the Ag/chitosan composite films. We have shown that the refractive index of the composites at a wavelength of a HeNe laser (633 nm) increases from 1.53 to 1.69 with the increase of the silver volume fraction up to 2.55%. Investigations of composites waveguiding properties revealed that the light propagation is feasible up to a silver volume fraction of 1.3%, which corresponds to the maximum refractive index 1.58.

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