Water-soluble and fluorescent core-shell nanoparticles (FNP) are synthesized in a miniemulsion reversible addition-fragmentation transfer (RAFT) polymerization and are shown to respond to pH. The particles are obtained from a hydrophilic PEO-b-PAA macromolecular RAFT agent which is block-extended with styrene and a fluorescent BODIPY monomer. A miniemulsion is then formed with the residual hydrophobic monomers. After completion of the polymerization, FNP of ≈ 60 nm in diameter are obtained. The fluorescence of the BODIPY dye in the particles is found to remain (0.2 quantum yield). The particles can be precipitated in acidic pH and redispersed upon addition of base without loss of their integrity or noticeable rearrangement.