Polyelectrolyte multilayers as nanocontainers for functional hydrophilic molecules

We report on the introduction of small organic hydrophilic molecules (fluorescein, rhodamine B, and two coumarin-based dyes) in multilayers of strong polyelectrolytes, studied by X-ray reflectometry, UV/visible spectroscopy, and fluorescence measurements. Very low diffusion coefficients (about 10(-17) cm(2.)s(-1)) were found for the inward diffusion of fluorescein in preformed multilayers. In addition, diffusion was accompanied by substantial variations of the thickness of the multilayer (up to 300%), ruling out the practical significance of inward diffusion as a tool to dope multilayers. We then attempted to coadsorb the fluorophores simultaneously with the polyions during the construction of the multilayer. However, displacement of small molecules by polyions of identical charge and outward diffusion of the fluorophores during the rinsing step resulted in very limited inclusion of the dye by this procedure. This issue was solved by introducing the fluorophore in all baths, including the rinsing ones. Then, the concentration of the multilayers in dye is directly related to the concentration of the dipping solutions and is dependent on the nature of the dye and of the multilayer. The outward diffusion of the fluorophores from these multilayers was studied, and very low diffusion coefficients were again determined, depending on the net charge of the dye. The ability to load rapidly polyelectrolyte multilayers with a variety of hydrophilic organic molecules of small molar mass, in tunable concentration, is a major outcome of the present study. It offers new opportunities to use these multilayers as templates for the confinement of active molecules in functional devices.