Design and fabrication of a chitosan based integrated optical device for humidity sensing

Abstract

ABSTRACT Here we report on preparation of planar optical waveguid es based on chitosan (DD=80.5%, MW=500kDa) in different salt forms, chitosan/gold nanoparticles, chitosan/gold nanopar ticles/silica hybrids with layered structure and modification of Na/K ion-exchange waveguides with thin chitosan/carrag eenan multilayers. Chitosan-based optical waveguides with thickness of 0.5- 1.5 m were obtained on quartz, glass and MgF 2 substrates by spin-coating and dip-coating. For investigation of optical properties, light (wavelength 632 or 532 nm) was coupled into the planar waveguide via the flint glass prism using goniometer. A number of modes, effective refractive index, waveguide propagation losses were determined for all samples in the range of relative humidity 10-99%. Keywords: polymer, waveguide, thin film, chitosan, sensor INTRODUCTION Silica has been widely used for many years as the main material for fabrication of optical fiber. There has always been an interest in the development of plastic optical fibers for both telecommunications and sensor uses, but this technology has tended to be somewhat ‘in the shadow’ of the silica-based approach. Recently, polymers and polym er/inorganic hybrids have become of great in terest for development of optical waveguides with tailored optical properties for telecommunications and sens or applications. Despite the fact that the polymer optics is based on synthetic polymers, whose functional composition can be controlled to provide materials with refractive indices ranging from ~ 1.3 to ~ 2.0, some of the natural polymers can be considered as an alternative to synthetic polymers for optoelectronic applications due to their high availability, relatively low cost and good film-forming properties. Aminopolysaccharide chitosan is one of the most promising biopolymer-candidates for development of optical waveguides and sensors. Advantages of chitosan are not limited to its easy processability, low cost and excellent film-forming properties. Chitosan is rather transparent material, whereas its refractive index can be tailored via metal ions binding, in-situ reduction and stabilization of metal nanoparticles or incorporation of optically active inorganic nanocrystals into the polymer thin film. This approach allows producing optical materials for various purposes with required mechanical and optical properties. Moreover, inter actions of chitosan, as a hydrophilic polybase, with organic solvents, water vapor, mineral and organic acids change level of polymer hydration and/or protonation degree, and thus, optical properties of the film that is beneficial for sensing application.