An embodiment of the invention is directed to a composition comprising a luminescent noble metal nanoparticle, wherein the surface of the noble metal nanoparticle is coated with a ligand, and wherein the noble metal nanoparticle is about 2 nm to 5 nm in diameter and further wherein a portion of the noble metal is present as its radioactive isotope. In an embodiment of the invention, the radioactive isotope is present at a concentration of up to 2% w/w of the noble metal.
