Fluorescent silica nanoparticles

Abstract

We report our latest progresses in the design and synthesis of fluorescent silica nanoparticles. Two different approaches are proposed: the first one is adopted for the inclusion of the dye to prepare doped silica nanoparticles (DSN). The second strategy allows the grafting of the surface of nanoparticles with the dye molecules and is suitable for the synthesis of covered silica nanoparticles (CSN). The two families of nanoparticles are radically different. While DSN are water soluble, surface coverage can dramatically lower the solubility in aqueous solvents. From the point of view of inter-chromophoric interactions, inclusion in DSN allows a good control of the average distance between the dye molcecules but, on the other hand, by surface modification a higher density of fluorophores can be reached making effective short range interactions (in particular electron transfer processes). Finally because of segregation, the interection of dye molecules with the external environment and macromolecules is less effective in the case of DSN. Three different examples are reported. In the first one energy transfer between fluorescein molecules in DSN is demonstrated though fluorescence anisotropy studies. The average distance between the fluorophores was tuned by controlling the degree of loading in order to have energy transfer inside the nanoparticles and in the mean time avoid a too large quenching because of self quenching processes. In the second example extended quenching via electron transfer processes on the surface of CSN is reported showing a simple case of 'amplified' quenching of the fluorescence upon protonation. Finally this last concept was applied to increase the sensibility in the field of metal ion sensing: a case of a water soluble nanosensor for metal ion with amplified response is, in fact, reported.