Adsorption and photophysical characterization of methylene blue in colloidal suspensions of negatively charged SiO2 nanoparticles

HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

Cubatao, SP, Brasil

Otro; IX Encuentro Latino-Americano de Fotoquimica y Fotobiologia (IX-ELAFOT); 2008

Dye-doped nanoparticles received great attention during the last years because of their potential application, among others, 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 photoactivity. 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 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 even may disturb 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. MB concentrations from 1 × 10-7 to 3 × 10-6 M and NP concentrations from 0.001 to 10 g L-1 were examined and the effect of time and pH was evaluated. 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. The absorption and emission properties of MB monomers adsorbed on the NPs are indistinguishable from those in aqueous solution, probably because the dye perceives a similar environment on the silica surface. Adsorption is a very fast process but redistribution of the dye from the silica surface into the micropores, is very slow; accordingly, more than 7 days are needed to reach thermodynamic equilibrium. On increasing the concentration of NPs above 0.25 g L-1, agglutination takes place, leading to the formation of dye higher aggregates. Results are discussed in view of the nature of amorphous silica.