4.6 Article

Probing chemical interactions at the single-molecule level in mesoporous silica thin films

Journal

JOURNAL OF PHYSICAL CHEMISTRY C
Volume 111, Issue 18, Pages 6772-6780

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jp068232t

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Single-molecule fluorescence spectroscopy is used to study dye molecule diffusion and surface interactions in 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 by exposure to high-humidity environments are studied. Fluorescence correlation spectroscopy (FCS) is used to assess the relative importance of diffusion and reversible binding in each material, to measure the dye diffusion coefficients, and to determine the duration of reversible adsorption events. In CTAB-containing as-synthesized materials, SPDI is found to be immobile under all conditions explored, while Nile Red is always mobile. The mobility of DiI is dramatically dependent on the level of film hydration. In SDS-containing materials, DiI is immobile 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 are hydrated. In calcined films, all three dyes remain at fixed locations under dry conditions, but become mobile as the ambient humidity is increased. These results and the associated discussion reveal new information on how dye diffusion and adsorption are influenced by materials content, composition, and surface chemistry in these technologically relevant films.

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