Methylene blue doped SiO2 nanoparticles: binding, speciation and ageing

HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

La Plata, Argentina

Otro; 1948-2008: 60 ANIVERSARIO DE ACTIVIDADES EN EL CAMPO DE LA FISICOQUÍMICA EN EL INIFTA, “Fronteras en Fisicoquímica. Un enfoque interdisciplinario”; 2008

Dye-doped nanoparticles show potential applications in photosensitization, photodynamic therapy, photovoltaics and photocatalysis. Dyes may be adsorbed, attached to the surface or entrapped into the matrix. Interactions may involve covalent bond formation or physical forces. High dye loadings are usually needed to increase light absoption. The adsorption of charged dyes may be enhanced on oppositely charged surfaces. However, interactions with the surface and among dye molecules may affect the photophysics directly or, indirectly, through the formation of inactive aggregates. The understanding of how these interactions control the photophysical behavior is essential to evaluate the energy economy of the system and to design strategies for its improvement. Adsorption studies in nanoparticulate systems are typically carried out by separation of the solid adsorbent and the liquid phase, assuming that equilibrium has been achieved. However, this procedure does not allow establishing the state of the dye on the surface in contact with the liquid phase and may even disturb any existing equilibrium. In order to gain insight into surface-dye, solvent-dye and dye-dye interactions, the adsorption of methylene blue (MB) on negatively charged silica nanoparticles (Ludox SM-30, NP) was studied in situ through absorption and fluorescence spectroscopies and fluorescence anisotropy of its aqueous suspensions. In this way, the fraction of monomers in solution and monomers and dimers bound to the particle could be obtained. Samples were prepared following different protocols: a) by the suspension of increasing amounts of NPs from a concentrated solution into an aqueous solution of MB; b) by the addition of increasing amounts of a concentrated solution of MB to an aqueous suspension of NPs; c) by mixing equal volumes of MB solutions and NP suspensions. The driving force for adsorption is mainly electrostatic because dye loading increases on increasing pH, as the negative charge density of the particles does. Dye charge neutralization on adsorption induces dye aggregation, mainly in the form of non-fluorescing nearly H-type dimers, which slowly deaggregate yielding adsorbed monomers. The absorption and emission properties of these monomers are indistinguishable from those in aqueous solution (FF = 0.05), probably because the dye perceives a similar environment as in water. This behavior is observed in systems prepared following protocol (a) at different MB concentrations. However, on increasing the concentration of NPs above 0.25 g L-1 agglutination takes place, leading to the formation of dye higher aggregates. Adsorption is a very fast process but redistribution of the dye from the silica surface into the micropores is very slow. Starting from strongly aggregated systems obtained by protocol (c), more than 7 days are needed to approach thermodynamic equilibrium.  Results are discussed in view of the nature of amorphous silica, taking into account its porous structure and the nature of the silica-water interface.