One-step hydrothermal synthesis of highly water-soluble secondary structural Fe3O4 nanoparticles

► MNPs are prepared in a low-temperature hydrothermal synthesis procedure. ► Synthesis of MNPs is hybridized with their in situ surface functionalization. ► Each of the secondary structural MNPs is composed of monodisperse primary particles. ► The resulting MNPs show good magnetic properties combining that of primary one. ► The size of MNPs is controllable and the mechanism is elaborated.