Nanocrystalline infrared-to-visible up-conversion phosphors, ytterbium and erbium co-doped sodium yttrium fluoride, were synthesized. Spherical particles with narrow size distribution were prepared by a co-precipitation method in the presence of ethylenediaminetetraacetic acid (EDTA). Particles of controlled size in the range of 37 to 166 nm diameter were obtained by adjusting the molar ratio of EDTA to total lanthanides. Although the as-prepared nanoparticles emit very weak up-conversion fluorescence when excited by infrared light, the emission was enhanced by up to 40-fold after they were annealed at temperatures between 400 and 600 degreesC. By comparison with the bulk phosphor, the luminescence efficiency of the nanoparticle was estimated to be 1%. Factors affecting the particle size and their up-conversion fluorescence intensity were investigated by various microscopic and spectroscopic techniques. Preliminary results demonstrated the nanoparticles as promising up-converting fluorescence labels in the detection of biological interactions.
