Formation Mechanism of Maghemite Nanoflowers Synthesized by a Polyol-Mediated Process

Abstract

Magnetic nanoparticles are being developed as structural and functional materials for use in diverse areas, including biomedical applications. Here, we report the synthesis of maghemite (γ-Fe 2 O 3 ) nanoparticles with distinct morphologies: single-core and multicore, including hollow spheres and nanoflowers, prepared by the polyol process. We have used sodium acetate to control the nucleation and assembly process to obtain the different particle morphologies. Moreover, from samples obtained at different time steps during the synthesis, we have elucidated the formation mechanism of the nanoflowers: the initial phases of the reaction present a lepidocrocite (γ-FeOOH) structure, which suffers a fast dehydroxylation, transforming to an intermediate “undescribed” phase, possibly a partly dehydroxylated lepidocrocite, which after some incubation time evolves to maghemite nanoflowers. Once the nanoflowers have been formed, a crystallization process takes place, where the γ-Fe 2 O 3 crystallites within the nanoflowers grow in size (from ∼11 to 23 nm), but the particle size of the flower remains essentially unchanged (∼60 nm). Samples with different morphologies were coated with citric acid and their heating capacity in an alternating magnetic field was evaluated. We observe that nanoflowers with large cores (23 nm, controlled by annealing) densely packed (tuned by low NaAc concentration) offer 5 times enhanced heating capacity compared to that of the nanoflowers with smaller core sizes (15 nm), 4 times enhanced heating effect compared to that of the hollow spheres, and 1.5 times enhanced heating effect compared to that of single-core nanoparticles (36 nm) used in this work.

A Facile One-Pot Synthesis of Versatile PEGylated Platinum Nanoflowers and Their Application in Radiation Therapy

“…The electron diffraction pattern of Pt NFs, shown in Figure 2d, indicated the good crystalline nature of the NFs. In the solution, as indicated in Figure 1e, the hydrodynamic diameter of the as-synthesized Pt NFs was about 34.8 ± 5.3 nm in water, well above the HR-TEM size, indicating the presence of PEG and hydration shell [45]. The zeta potential value at pH = 7 was measured to be 20 ± 2 mV.…”

Aggregation effects on the magnetic properties of iron oxide colloids

“…This type of interactions is especially relevant in the case of multi-core particles prepared directly with a controlled aggregation of the cores forming nanoflowers. 41 In these materials, there is a crystal continuity at the core interfaces that allows a cooperative behaviour due to exchange interactions between the cores, resulting in an enhanced susceptibility while maintaining superparamagnetic behaviour. 42 Exchange interactions are also responsible for the displacement of the blocking temperature in comparison with non-interacting nanoparticles.…”

Influence of clustering on the magnetic properties and hyperthermia performance of iron oxide nanoparticles

“…A special class of multi-core particles is so-called nanoflowers, in which the cores are so densely packed that they are essentially in direct contact (i.e. nanocrystalline nanoparticles) [14,18,19]. The resulting strong interactions between the cores can lead to collective magnetization states within the clusters.…”