Neutral polymeric nanoparticles have found many important applications in fields such as drug delivery, biosensing and environmental research. However, charged polymeric nanoparticles did not yet find such success mainly due to the multi-step and time-consuming conventional production route. Emulsificationevaporation method was described as a fast and reproducible way to obtain neutral polymeric nanosuspensions. We propose here, for the first time, to extend this method to the one-step production of negatively charged polymeric nanoparticles. In this purpose, we compared different processes, such as sonicator, shear mixer and elongational-flow reactor and mixer (µRMX) in order to produce polyelectrolyte nanoparticles (PNPs) of poly(styrene sulfonate). We found that only µRMX allowed the production of highly monodispersed PNPs. We also verified the decrease of nanoparticles' size (from 300 to 150 nm) and polydispersity index by increasing the emulsification's time and decreasing the polymer's molecular weight. Finally, we observed and explained the causes of an unusual behavior: the sudden increase of PNPs' size after a given emulsification time when using the elongational-flow reactor and mixer.