Surface-modified heavy metal nanoparticles, including a heavy metal core and a coating layer, the coating layer having at least one ligand, conjugated to polyethylene glycol, the at least one ligand is selected from N-acetyl cysteine, albumin, cysteine, methionine, glutathione, amino thiols, thio-carboxylic acids, ammonia, amines, diamines or any combination thereof. Compositions including surface-modified heavy metal nanoparticles and uses thereof in treatment and diagnosis of various conditions.