Self-assembled block copolymer nanoparticles (NPs) have emerged as a major potential nanoscale vehicle for fluorescence bioimaging purpose. Crucial to their practical implementation is to prepare with high yields NPs possessing a high kinetic stability to prevent the leakage of fluorophore molecules. Here, we report a photomediated RAFT polymerization-induced self-assembly (PISA) yielding uniform and nanosized poly((oligo(ethylene glycol) acrylate)-block-poly(benzyl acrylate) particles (POEGA-b-PBzA) with a concentration of 22 w%, over 20 times more than micellization and nanoprecipitation. The spherical diblock copolymer nanoparticles have an average size of 10-50 nm controllable through the degree of polymerization of the stabilizing POEGA block. Subsequent dialysis against water and swelling with Nile red solution lead to highly stable fluorescent NPs able to withstand changes in concentration, ionic strength, pH or temperature. The PBzA/water interfacial tension of 48.6 mN/m hinders the exchange between copolymer chains, resulting in the trapping of NPs in a “kinetically frozen” state responsible for the high stability. A spectroscopic study combining fluorescence and UV-vis absorption agrees with a preferential distribution of fluorophore in the outer POEGEA shell despite its hydrophobic nature. Nile red-doped POEGA-b-PBzA micelles stripped of initiator residue and unimers with high structural stability turn out to be non cytotoxic, and can be used for the optical imaging of cells. Real-time confocal fluorescent microscopy shows a fast cellular uptake using C2C12 cell lines in the matter of minutes, and a preferential localization in the perinuclear region, in particular in the vesicles