Abstract: The invention provides a method for continuously producing composite nanoparticles, the method comprising heating a precursor mixture with supercritical water, wherein the mixture contains a first compound capable of transitioning from a monoclinic to a tetragonal rutile crystal state; cooling the heated mixture to obtain core particles of a predetermined shape and size; encapsulating the core particle with a second precursor to create a core-shell construct; and encapsulating the construct with an organic material. Also provided is a device for continuously synthesizing composite nanoparticles, the device comprising a water supply and a precursor supply; a means for heating the water, a continuous flow hydrothermal reaction chamber adapted to receive the heated water and precursor, a means for chilling the heated water and precursor, and a capping agent supply positioned downstream of the reaction chamber.