Large Superparamagnetic FeCo Nanocubes for Magnetic Theranostics

In the past few decades, superparamagnetic nanoparticles (SPMNPs) have attracted increasing attention in a wide range of magnetic theranostics including magnetic biosensors, drug delivery, magnetic separation, magnetic imaging, hyperthermia therapy, and so on. Superparamagnetic iron oxide nanoparticles are currently widely used for these purposes despite their low saturation magnetizations (below 80 emu/g). In pursuit of higher magnetic signals (spatial resolutions) for magnetic imaging, higher sensitivity (limit of detection) for biosensing, higher efficiency, and lower dosage in drug delivery and hyperthermia therapy, magnetic compounds and alloys that generally have higher saturation magnetizations are of interest. FeCo SPMNPs are considered promising candidates for biomedical applications due to their good corrosion resistance, stability, and high saturation magnetizations (over 220 emu/g). However, the critical size for FeCo nanoparticles to be superparamagnetic is limited by a theoretical value of ∼15 nm, making it difficult to further increase the magnetic moment per SPMNP. Herein, we report a method to synthesize large single-crystalline FeCo nanoparticle complexes (NPCs) with an overall size of ∼100 nm while retaining the superparamagnetic properties. These large FeCo NPCs are synthesized by self-assembling 3 nm FeCo nanoparticle units through a DC sputtering-based gas-phase condensation (GPC) method. By controlling the sputtering parameters like sputtering current density, sputtering pressure, and carrying gas velocity in the GPC system, the nucleation and growth of FeCo nanoparticles can be tuned, and different sizes of nanoparticles can be obtained. The large FeCo NPCs are formed from the second crystallization of small FeCo nanoparticle units with well-aligned crystalline axes, which show both high saturation magnetization and superparamagnetic properties suitable for biomedical applications. It is expected that with the superparamagnetic behavior and higher magnetic moment per FeCo NPC, they can potentially bring higher sensitivities to magnetic biosensors that rely on the magnetic labels, higher efficiency in hyperthermia therapy, and lower dose requirements for magnetic imaging and separation.