New core-shell hybrid nanoparticles for biophotonics: fluorescent organic nanocrystals confined in organosilicate spheres

We have developed a versatile, one-step process to synthesize in large amounts hybrid core-shell nanoparticles (NPs) made up of an organic crystallized core embedded in amorphous organosilicate shells. These NPs exhibit a hydrophilic surface and a high fluorescence intensity arising from the organic crystal core. Their preparation is based on a controlled drying process of micrometre-sized droplets (spray) from sol-gel solutions leading simultaneously to the confined nucleation and growth of dye nanocrystals and to the formation of silica-based shells. In this paper, we present several experimental optimizations of the spray-drying process to reduce the average NP size and size distribution. This was achieved by combining several physical parameters of spray-drying and the chemistry of the initial sols. Original precursors were developed for the formation of optimized silicate shells. TEM analysis in diffraction mode showed that organic cores are single crystals. On the other hand, the negatively charged silicate shells led to colloidally stable NPs which is promising for the development of bright fluorescent tracers excited by two-photon absorption for in vivo imaging.