Nanometer-scaled up-converting fluoride phosphor particles and processes of making them are disclosed. In the process, an aqueous solution consisting of soluble salts of rare-earth metal ions at a molar ratio of (yttrium, lanthanum or gadolinium): ytterbium:(erbium, holmium, terbium or thulium)=(70-90):(0-29):(0.001-15) is mixed a rare-earth metal chelator and a soluble fluoride salt to form precipitates, which are then annealed at an elevated temperature to produce nanometer-scaled up-converting fluoride phosphor particles. The particle size is between 35 nm and 200 nm, and can be controlled by the amount of the metal chelator added to the solution. The nanometer-sized particle is applicable to many biological assays.