Large Superparamagnetic FeCo Nanocubes for Magnetic Theranostics

Abstract

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.

Synthesis and Characterization of Magnetoplasmonic Air-Stable Au@FeCo

“…It is of note to mention that the pure FeCo nanoparticles synthesized demonstrated an M S value of ∼130 emu/g, as seen in Figure S7, as would be expected from the literature. Superparamagnetic iron oxide nanoparticles demonstrates sub-80 emu/g M S values, while superparamagnetic FeCo has shown M S values of >220 emu/g . The FeCo nanoparticles we produced are larger and are ∼13 nm and have, according to estimates based on the values of M S (Figure S7b), magnetically inactive oxide layers 1–2 nm thick.…”

“…Superparamagnetic iron oxide nanoparticles demonstrates sub-80 emu/g M S values, while superparamagnetic FeCo has shown M S values of >220 emu/g. 66 The FeCo nanoparticles we produced are larger and are ∼13 nm and have, according to estimates based on the values of M S (Figure S7b), magnetically inactive oxide layers 1−2 nm thick. Previous groups who have attempted the synthesis of metaldoped plasmonic FeCo�namely, Ag/FeCo/Ag NPs�have seen M S values of ∼30−40 emu/g, even lower than that of iron oxide nanoparticles.…”

“…Furthermore, as the growth of the metal nanoparticles are well understood through LaMer, Finke–Watzky, and various other models, the fine tuning of the morphological characteristics is further well understood empirically . For example, surfactants with higher zeta potentials would lower the average size of the nanoparticles formed, and vice versa, as well as different metals would cause different speeds of particle formation through modulating the nucleation rate. , The addition of the fatty acid surfactants mechanistically causes stabilization of the Fe/Co/FeCo monomers, thus increasing the concentration necessary for supersaturation of the solvent and allowing more monomers to grow uniformly before nucleation and growth begin through a LaMer burst nucleation model. ,, Additional groups have also demonstrated an ability to control the morphology of the resultant nanoparticle through a variety of surfactants, most notably nanocubes, , demonstrating the crucial ability of the surfactant to control both shape direction and the supersaturation concentration. Thus, if a metal with a higher standard reduction potential was added, one would expect that the metal reaches a point of supersaturation first, and subsequently catalyzes the formation of more reduced metal of the various types to its surface. ,− This is the case for Au + /Au (+1.69 V vs SHE), which has a greatly positive standard reduction potential and implies a faster and more favorable reduction to Au 0 monomers, as the Au + is preferably reduced and the product is then pulled from equilibrium toward the formation of stable clusters and particles.…”

“…58,62 The addition of the fatty acid surfactants mechanistically causes stabilization of the Fe/Co/ FeCo monomers, thus increasing the concentration necessary for supersaturation of the solvent and allowing more monomers to grow uniformly before nucleation and growth begin through a LaMer burst nucleation model. 59,63,64 Additional groups have also demonstrated an ability to control the morphology of the resultant nanoparticle through a variety of surfactants, most notably nanocubes, 65,66 demonstrating the crucial ability of the surfactant to control both shape direction and the supersaturation concentration. Thus, if a metal with a higher standard reduction potential was added, one would expect that the metal reaches a point of supersaturation first, and subsequently catalyzes the formation of more reduced metal of the various types to its surface.…”

Enhanced Magnetic Anisotropy for Reprogrammable High‐Force‐Density Microactuators

“…The remanent magnetization for the MNCs is 15 emu g −1 , which is in agreement with the data in the literature. [ 50,51 ] Thus, the ratio of remanent magnetization to saturation magnetization, $$M_{\text{r}}/M_{\text{$\text{s}$}}$$, is only 0.068 in the MNCs. In contrast, the randomly‐oriented FeCo chain sample exhibits a remanent magnetization of M r = 28 emu g −1 and M r / M s ratio of 0.127, which is almost twice that of FeCo MNCs.…”