However, the potential of chitosan and chitosan–metal composites in optics, which is not limited to above mentioned fabrication of SERS supports [3,24], is not yet realized to a sufficient degree. Recently, we have shown [25] that planar optical waveguides based on chitosan thin films can be used as efficient humidity sensors with fast response time. In this manuscript we report a promising approach to modify chitosan optical properties for fabrication of planar optical waveguides (POW) and sensors by turning its refractive index via ‘‘in situ’’ reduction of silver ions directly in chitosan films with controlled thickness.