Monitoring the Formation of Polymer Nanoparticles with Fluorescent Molecular Rotors

We present an experimental approach to studying the kinetics of polymer nanoparticle formation using molecular rotor molecules as fluorescence probes. By combining fluorescence lifetime analysis and fluorescence correlation spectroscopy, it is possible to simultaneously monitor the evolution of the local environment in the nanoparticles and their size. To assess the generality of the method, we select two common but very different methods for nanoparticle preparation: emulsion-solvent evaporation and miniemulsion polymerization. In both cases, three stages of the nanoparticle formation process were identified on the basis of their polymer content. In the initial stage of the process, the polymer content is low. Then it increases rapidly because of solvent evaporation or polymerization during the second stage. Finally, it reaches a plateau value. Furthermore, significant heterogeneities in the final nanoparticles were found that can be attributed to remaining solvent or monomer and the spatial variation of the polymer-free volume.