Size-tunable phosphorescent particles may be formed through self-assembly of biocompatible linear polymers, such as chitosan and other linear polymers, that bear positive surface charges, through polyelectrolytic complexation to a polyanionic metal phosphor, such as polyanionic gold(I) phosphor (AuP). The phosphorescent hydrogel nanoparticles and thin films thereof are useful for imaging, sensing of biological molecules, detection of hypoxia, and light-emitting devices. The phosphorescent hydrogel particles can be formed from a variety of linear polymers by physical cross-linking using polyelectrolytic light-emitting species, without the need for the phosphorescent complex to be entrapped in an existing microsphere or nanosphere polymer particle.