| Abstract
| - Single-molecule fluorescence spectroscopy is used to study dye molecule diffusion and surface interactionsin mesoporous silica thin films. Three different dyes (nile red, DiI, and a sulfonated perylene diimide (SPDI)),selected for their hydrophobicity and charge characteristics, are employed. As-synthesized (surfactant-containing) and calcined (surfactant-free) mesoporous films that are either dry or have been hydrated byexposure to high-humidity environments are studied. Fluorescence correlation spectroscopy (FCS) is used toassess the relative importance of diffusion and reversible binding in each material, to measure the dye diffusioncoefficients, and to determine the duration of reversible adsorption events. In CTAB-containing as-synthesizedmaterials, SPDI is found to be immobile under all conditions explored, while Nile Red is always mobile. Themobility of DiI is dramatically dependent on the level of film hydration. In SDS-containing materials, DiI isimmobile in dry films while SPDI and Nile Red are mobile. Both DiI and SPDI exhibit strong humidity-dependent mobilities, yielding diffusion coefficients that increase by up to a factor of 10 when the films arehydrated. In calcined films, all three dyes remain at fixed locations under dry conditions, but become mobileas the ambient humidity is increased. These results and the associated discussion reveal new information onhow dye diffusion and adsorption are influenced by materials content, composition, and surface chemistry inthese technologically relevant films.
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