Size and morphological effect of Au–Fe3O4 heterostructures on magnetic resonance imaging

Various sizes and morphologies of Au–Fe3O4 heterostructures including dumbbell-like nanoparticles (DBNPs) and flower-like nanoparticles (FLNPs) have been synthesized via thermal decomposition of iron–oleate complex in the presence of different sizes of Au seeds for MR imaging application. The size of Au seeds and the concentration of iron–oleate complex have been proved to be crucial parameters for controlling the morphology of Au–Fe3O4 heterostructures. The use of 5-nm Au seeds can only fabricate Au–Fe3O4 DBNPs, while both DBNPs and FLNPs are produced by using 10-nm Au NPs as the seeds. The obtained Au–Fe3O4 heterostructures are then modified with 8-arm PEG-amine and show a good phase transfer with satisfactory hydrodynamic size for further MR imaging application. The Au–Fe3O4 DBNPs with small Au seeds result in an increase in spin–spin relaxivity, and the r 2 values are 125.5 ± 13.5, 118.3 ± 2.3, and 143.8 ± 3.2 mM−1 s−1 for 5-nm Au–Fe3O4 DBNPs, 10-nm Au–Fe3O4 DBNPs, and 10-nm Au–Fe3O4 FLNPs, respectively. The Au–Fe3O4 FLNPs shows an increased relaxivity when compared with that of Au–Fe3O4 DBNPs, presumably attributed to the magnetic coupling effect among Fe3O4 domains on the surface of Au seeds.