Synthesizing upconverting nanoparticles includes heating a precursor solution comprising one or more rare earth salts, an alkali metal salt or alkaline earth salt, and a solvent comprising a plasticizer in a microwave reactor to yield a product mixture, and cooling the product mixture to yield the upconverting nanoparticles. Core-shell upconverting nanoparticles are synthesized by combining the upconverting nanoparticles with a precursor solution comprising one or more rare earth salts, an alkali metal salt or alkaline earth salt, and a solvent comprising a plasticizer to yield a nanoparticle mixture, heating the nanoparticle mixture in a microwave reactor to yield a product mixture, and cooling the product mixture to yield the core-shell upconverting nanoparticles.