High-Performance Ferrite Nanoparticles through Nonaqueous Redox Phase Tuning

Abstract

From magnetic resonance imaging to cancer hyperthermia and wireless control of cell signaling, ferrite nanoparticles produced by thermal decomposition methods are ubiquitous across biomedical applications. While well-established synthetic protocols allow for precise control over the size and shape of the magnetic nanoparticles, structural defects within seemingly single-crystalline materials contribute to variability in the reported magnetic properties. We found that stabilization of metastable wüstite in commonly used hydrocarbon solvents contributed to significant cation disorder, leading to nanoparticles with poor hyperthermic efficiencies and transverse relaxivities. By introducing aromatic ethers that undergo radical decomposition upon thermolysis, the electrochemical potential of the solvent environment was tuned to favor the ferrimagnetic phase. Structural and magnetic characterization identified hallmark features of nearly defect-free ferrite nanoparticles that could not be demonstrated through postsynthesis oxidation with nearly 500% increase in the specific loss powers and transverse relaxivity times compared to similarly sized nanoparticles containing defects. The improved crystallinity of the nanoparticles enabled rapid wireless control of intracellular calcium. Our work demonstrates that redox tuning during solvent thermolysis can generate potent theranostic agents through selective phase control in ferrites and can be extended to other transition metal oxides relevant to memory and electrochemical storage devices.

Thermal Decomposition Synthesis of Iron Oxide Nanoparticles with Diminished Magnetic Dead Layer by Controlled Addition of Oxygen

“…39 Furthermore, a recent study showed that tuning the electrochemical potential of the solvent can influence the nature of the iron oxide phase obtained from synthesis. 49 …”

Alkaline Metal Reagent-Assisted Synthesis of Monodisperse Iron Oxide Nanostructures

“…Among advanced approaches for magnetic nanocrystal synthesis including co-precipitation reactions [8,9], sol-gel [10], hydrothermal/solvothermal [11], and microemulsion or microwave-assisted methods [12], the formation of monodisperse Fe 3 O 4 nanoparticles has been demonstrated with two typical wet chemical synthetic approaches: (I) solvothermal decomposition of iron (III) oleate complexes; and (2) reductively enhanced reaction using iron (III) acetylacetonate reagents with oleylamine under the reflux condition of high-temperature organic solvents [13][14][15]. While abundant chemical synthetic routes have been reported to obtain uniform magnetic nanostructures with size, shape and chemical composition control, however, the tailoring of enhanced pure phase over various iron oxide polymorphology remains a synthetic challenge without the constraint of size due to unstable nanoscale surface and interface which involved phase transformation resulting in unwanted core-shell nanostructures [13,16,17]. Here, we report a well-defined www.mdpi.com/journal/metals dimensional and phase-controlled synthetic method for magnetite nanostructures, assisted by alkaline metal precursors that show higher reduction potentials than that of iron complexes reagent.…”

“…Here, we report a well-defined www.mdpi.com/journal/metals dimensional and phase-controlled synthetic method for magnetite nanostructures, assisted by alkaline metal precursors that show higher reduction potentials than that of iron complexes reagent. Fe 3 O 4 nanoparticle is a common magnetic iron oxide nanostructure with an inverse spinel cubic structure in which oxygen forms a face-centered cubic (FCC) closed packing, and Fe cations occupy interstitial tetrahedral sites and octahedral sites [16]. In the case of organic phase synthesis, the high-temperature thermal decomposition of iron (III) oleate complexes in alkene hydrocarbon solvent (e.g., 1-octadecene) is a versatile method for monodisperse magnetite nanoparticles [13].…”

“…To increase the size of magnetite nanoparticles, the synthesis using iron (III) oleate precursors frequently obtains biphasic nanostructures consisting of a wüstite (antiferromagnetic (AFM) below Neel temperature) core and a spinel (ferrimagnetic (FiM)) shell at which the overall magnetization significantly lowers compared to single-phase Fe 3 O 4 nanoparticles [13,18]. The weakness of the oleate-based approach was improved via applying partial oxygen condition and post-treatment using magnetic field-stimulated phase transformation in ambient conditions [16,18,23]. However, purging of oxygen within the as-prepared crude solution enables the formation of a maghemite (γ-Fe 2 O 3 ) phase even at 200 • C [24].…”

“…However, purging of oxygen within the as-prepared crude solution enables the formation of a maghemite (γ-Fe 2 O 3 ) phase even at 200 • C [24]. In addition, the reductive effect of oleylamine on iron (III) acetylacetonate is critical for obtaining a uniform nucleation and the redox activity of aromatic ether solvents at reflux temperature enables generation of intermediated radical products including aldehyde group for potential oxidative agents [16]. The harsh conditions with refluxed aromatic ether solution resulted in excess oxidation on the synthesized nanostructures, which formed maghemite phase surface depending on the reaction time [24].…”