Doped silica nanoparticles containing two-photon luminescent Eu(III) complexes for the development of water stable bio-labels

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.