Polymeric nanoparticles with a hydrophobic core and a hydrophilic shell are formed from: 1) N-isopropyl acrylamide (NIPAAM), at a molar ratio of about 50% to about 90%, and preferably 60% for specific delivery routes such as oral or parenteral; either water-soluble vinyl derivatives like vinylpyrolidone (VP) or vinyl acetate (VA), or water insoluble vinyl derivatives like methyl methacrylate (MMA) or styrene (ST), at a molar ratio of about 10% to about 30%; and acrylic acid (AA), at a molar ratio of about 10% to about 30%. The formed nanoparticles may be optionally surface functionalized using reactive groups present in AA, including PEGylation, or conjugation of moieties such as chemotherapeutics, contrasting agents, antibodies, radionucleides, ligands, and sugars, for diagnostic, therapeutic, and imaging purposes. The polymeric nanoparticles are preferably dispersed in aqueous solutions. The polymeric nanoparticles incorporate one or more types of medicines or bioactive agents in the hydrophobic core; on occasion, the medicine or bioactive agent may be conjugated to the nanoparticle surface via reactive functional groups. The polymeric nanoparticles are capable of delivering the said medicines or bioactive agents through oral, parenteral, or topical routes. The polymeric nanoparticles allow poorly water soluble medicines or bioactive agents, or those with poor oral bioavailability, to be formulated in an aqueous solution, and enable their convenient delivery into the systemic circulation.