Abstract: The subject invention provides novel fluorescent core-shell nanoparticles comprising an encapsulated fluorescent core comprising an ionically bound fluorescent dye and a metal oxide shell. In one exemplary embodiment of the invention a core containing indocyanine green (ICG) with a silica shell that displays excellent photostability for generation of a near infrared fluorescence signal. The fluorescent core-shell nanoparticle can be further modified to act as an MRI, x-ray, or PAT contrast agent. The ICG nanoparticles can also be used as photodynamic therapeutic agent. Other embodiments of the invention directed to methods of making the novel core-shell nanoparticles and to the use of the core-shell nanoparticles for in vitro or in vivo imaging.