Concentrated colloidal solutions of well-dispersed YVO4:Eu nanoparticles are synthesized by precipitation reactions at room temperature and stabilized by sodium hexametaphosphate. X-ray diffraction and electron microscopy characterizations show that the crystalline nanoparticles exhibit an ellipsoidal form with two characteristic dimensions of around 15 and 30 nm. In comparison with the bulk, a lower luminescence efficiency as well as a higher concentration quenching are observed. These deviations are explained as the variations of some characteristics of the colloidal samples, such as the crystallinity and the surface chemistry. When these parameters are optimized, the quantum yield of the luminescence reaches 38% for the nanoparticles containing a europium concentration of 15%.

The luminescence properties of colloidal YVO4:Eu nanoparticles (8 nm in diameter) are investigated and compared to those of the bulk materials. The emission quantum yield of nanoparticles is improved after the transfer of the colloidal particles into D2O, showing that surface OH groups act as efficient quenchers of the Eu3+ emission. The growth of a silicate shell around the nanoparticles decreases the optimum europium concentration, showing that energy transfers within the nanoparticles are limited by the quenching of the excited states of the vanadate groups. Nanoparticles also exhibit structural distortions directly related to the small size of the particles. No clear evidence is found concerning the influence of these distortions on the energy-transfer processes, since the improvement of the emission properties observed after thermal annealing of both crude and silicated powders seems to result mainly from the elimination of Eu3+ and vanadate quenchers from the surface. This latter effect is greatly enhanced in the presence of the silicate shell compared to bare particles.

We present a new process for the synthesis of colloidal europium-doped yttrium vanadate with a particle diameter of about 10 nm. Nanocrystals are produced by precipitation of citrate complexes of rare-earth salts with sodium orthovanadate. NMR and IR studies show that the interaction between citrate ligands and lanthanide ions limits the growth of particles and ensures the stability of the colloidal solutions through electrostatic and steric repulsions. The optimized process leads to stable and highly concentrated transparent colloidal solutions in water (up to 400 g·L-1).

Lanthanide ion-doped oxide nanoparticles were functionalized for use as fluorescent biological labels. These nanoparticles are synthesized directly in water, which facilitates their functionalization, and are remarkably photostable without emission intermittency. Nanoparticles functionalized with guanidinium groups act as artificial toxins and specifically target sodium channels. They are individually detectable in live cardiac myocytes, revealing a heterogeneous distribution of sodium channels. Functionalized oxide nanoparticles appear to be a novel tool that is particularly attractive for long-term single-molecule tracking.

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The optical properties of Yb 3+ and Ho 3+ co-doped Y 2 BaZnO 5 , synthesized by solid-state reactions, are investigated in detail. Under 977 nm excitation (~2.5 W/cm 2 ), bright green upconversion emission is observed. Concentration dependence studies at room temperature show that relatively high infrared to visible upconversion efficiencies are obtained with values up to ~2.6%. The results of power dependence studies and temperature dependent lifetime measurements allow us to determine the dominant upconversion mechanisms in Yb 3+ : Ho 3+ co-doped Y 2 BaZnO 5 oxides. The materials presented in this paper constitute new and efficient upconversion phosphors which may find utility in a variety of applications.

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