MPI offers more advanced imaging than other commonly used techniques such as magnetic resonance imaging (MRI), with high sensitivity and resolution along with near-perfect contrast due to the diamagnetic nature of human tissue. MPI signal strength and efficiency are dependent on the synthesis and size control of large magnetic nanoparticles. Theoretical simulations predicted that the currently used commercial MPI contrast agent, Resovist®, has a limited signal strength similar to that of 3% iron oxide nanoparticles with a 30 nm inorganic diameter or above. However, the synthesis of superparamagnetic iron oxide nanoparticles in the ≥ 30 nm domain is challenging and rarely reported. To this end, the inventors have developed monodisperse and highly magnetic 35 nm iron oxide nanoparticles that are expected to improve the existing MPI signal strength of Resovist® by over 90%.