A polymeric nanoparticle that has a size of 30-600 nm in outer diameter and contains a polymeric shell, less than 25 nm in thickness and impermeable to water, one or more aqueous cores enclosed by the polymeric shell, and a bioactive agent encapsulated in each of the one or more aqueous cores. Also disclosed are a method of preparing the polymeric nanoparticle and a method of using it for treating a disease.