The present invention relates to a method for synthesizing water-soluble particles, the method includes providing a solution including a lanthanide compound, a halide compound, and a first solvent; introducing a capping agent into the solution to form a mixture; heating the mixture under pressure to produce the particles; and recovering the particles from the mixture. The present invention also relates to a water-soluble particle having a surface functional group. The particles exhibit up-conversion luminescence utilizing NIR excitation, wherein the particles are synthesized in a one-pot process.