Concentrated colloidal solutions of well dispersed YVO 4 :Ln (Ln=Nd, Yb, and Yb-Er) nanocrystals are synthesized through the precipitation of citrate complexes of rare earth salts with sodium orthovanadate. Luminescence spectra of aqueous suspensions of YVO 4 :Nd or Yb are very similar to the bulk material, with lower lifetimes of the emitting level. After heating at 600 o C, the luminescence properties of silica capped YVO 4 :Ln nanoparticles are significantly improved. In these conditions, YVO 4 :Yb, Er nanoparticles exhibit green up-conversion luminescence and 1.55 μm emission of the erbium ion under infrared excitation of the ytterbium ion.