Magnetic/plasmonic hybrid nanoparticles are highly desirable for multimodal bioimaging and biosensing. Although the synthesis of heterodimeric nanoparticles has been reported, the products are usually hydrophobic so that post-treatment procedures are required to transfer them into water which are often difficult to perform and cause damages to the structures. Direct synthesis of hydrophilic hybrid nanostructures has remained a grand challenge albeit its immediate advantage of biocompatibility. Herein we report a general seed-mediated approach to the synthesis of hydrophilic and biocompatible M−Fe 3 O 4 (M = Au, Ag, and Pd) heterodimers, in which the size of metals and Fe 3 O 4 can be independently regulated in a wide range. Benefiting from the aqueous synthesis, this approach can be further extended to design more complex heterodimeric structures such as AgPt alloy −Fe 3 O 4 , Au core @Pd shell −Fe 3 O 4 , and Au shell −Fe 3 O 4 . The hydrophilic nature of our heterodimers makes them readily useful for biomedical applications without the need of additional ligand exchange processes in contrast to those prepared in nonpolar solvents. These nanoscale magnetic/ plasmonic heterostructures were shown to be ideally suited for integrated biomedical diagnoses, such as magnetic resonance imaging, photoacoustic imaging, optical coherence tomography, and computed tomography, in virtue of their biocompatibility and combined tunable magnetic and plasmonic properties.

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