Porosity control and surface sensitivity of titania/silica mesoporous multilayer coatings: applications to optical Bragg resonance tuning and molecular sensing

Mesoporous dielectric multilayer films exhibit an optical response (Bragg resonance) that is highly sensitive to the chemical environment. Using sol-gel preparation and spin-coating deposition method, titania/silica mesoporous layers were alternatively deposited and consolidated to form a Bragg stack on a glass substrate. Tuning of the effective refractive index of titania layers through the control of their porosity allowed us to tailor the optical response of the multilayer film, and in particular its color, to infiltrate liquids and/or molecules dissolved in a solvent. Selective grafting of hydrophobic molecules on the pore surfaces of titania mesoporous layers was likely to be at the origin of unusual behavior of the Bragg resonance spectral shift and intensity variation. The pore accessibility of the different layers forming the stack was investigated through the infiltration of Rhodamine 6G fluorescent dye. Confocal microscopy revealed a homogeneous distribution of the dye across all the layers of the stack. Capillary attraction induced by adequate pore size distribution between adjacent layers was found to play a key role in liquid infiltration throughout the entire mesoporous dielectric structure.