Functionalized tris-dipicolinate europium(III) complexes present a very important two-photon absorption cross-section (up to 775 GM) but exhibit a weak stability in aqueous solutions due to the ligand substitution by water molecules. This dissociation decreases radically the luminescence quantum yield and lifetime of the complex. In this paper, we present a one-step alternative route to stabilize these lanthanide complexes in aqueous solutions or in aqueous buffer solutions by embedding them in silica nanoparticles (NPs). This simple one-step method is based on the spray-drying of sol–gel solutions involving silicon alkoxides as precursors of the silica NPs, hydrolyzed through a small amount of water, an organic solvent and the dissolved Eu(III) complex. These atomized sols are dried under laminar air flow in a tubular furnace while the resulting NPs were collected with an electrostatic filter. Different types of silicon alkoxides were used to adjust the hydrophilic character and porosity of NPs, to optimize their biocompatibility and dispersion in aqueous solutions and to avoid the Eu(III) complexes dissociation. The chemical stability in aqueous solutions of the Eu-complexes inserted in the different NPs was followed through their luminescence spectroscopy and two-photon microscopy to determine the optimal chemical composition of the starting sols for the preparation of highly luminescent water stable bio-labels.

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.