The present invention refers to nanoparticles having optically fluorescent activity. In more detail, the invention refers to a nanoparticle matrix comprising a co-aggregate of at least one charged polyelectrolyte and at least one oppositely charged active agent, wherein the active agent is a hydrophilic optically fluorescent agent, and the invention further refers to a nanoparticle comprising said nanoparticle matrix. Optionally, the nanoparticle is surface modified. The invention also refers to a method for preparing said nanoparticle, and to a method of surface modification. Furthermore, the invention refers to uses of said nanoparticle in vitro and in vivo, and to methods for in vitro and in vivo diagnosis.