Organic nanoparticles of a carbocyanine dye (JC-1) are synthesized via ion-association between the dye cation and tetrakis(4-fluorophenyl)borate anion in the presence/absence of a neutral stabilizing polymer, poly(vinylpyrrolidone), in water. In particular, their fluorescence properties can be tuned by the size of the nanoparticles, which is controlled by both the concentration of the borate anion added and the stabilizing polymer.