Determination of optical parameters of colloidal TiO2 nanocrystals-based thin films by using surface plasmon resonance measurments for sensing applications

Surface plasmon resonance (SPR) study in angular modulation was used to optically characterize porous thin films based on TiO2 colloidal nanocrystals. Organic solutions of surfactant-capped TiO2 nanocrystals prepared by means of a colloidal chemistry route, were deposited as thin film by spin coating. Optical parameters of the obtained thin layers were calculated by using SPR measurements on both as deposited and annealed in dry-air thin layers. By using direct fitting of a set of the SPR resonant curves with Fresnelʹs theory, the thickness and refractive index of the prepared thin films were determined. The application of the prepared layers as optical sensing materials was evaluated by detecting the variation of the optical parameters in the presence of different saturated alcohol vapours. Morphological and structural characterization of the thin layers was carried out and related to the optical behaviour of the sensing material. Adsorption of alcohol vapours onto the surface of the sensing layer was found to induce a variation of both thickness and refractive index of TiO2-based nanocrystals films. The presence of the organic passivating layer at the nanocrystal surface was demonstrated to play an effective role in the interaction between alcohol molecules and the sensing material.