Characterization of W20/XD4-SPIONs. a Schematic illustration of W20/XD4-SPIONs. b The morphology of SPIONs and W20/XD4-SPIONs were imaged by TEM. Scale bar 50 nm. c The size distribution of SPIONs and W20/XD4-SPIONs detected by TEM. d The hydrodynamic diameters of SPIONs and W20/XD4-SPIONs were detected by DLS. e Cy7-labeled various SPIONs were injected to nude mice via tail vein with or without mannitol combination. The brains of mice were collected at 6 h post-injection, the Cy7 fluorescence signal was detected using IVIS spectrum imaging system.