Chiral Magneto-Optical Properties of Supra-Assembled Fe₃O₄ Nanoparticles

Abstract

Research on the chiral magneto-optical properties of inorganic nanomaterials has enabled novel applications in advanced optical and electronic devices. However, the corresponding chiral magneto-optical responses have only been studied under strong magnetic fields of ≥1 T, which limits the wider application of these novel materials. In this paper, we report on the enhanced chiral magneto-optical activity of supra-assembled Fe3O4 magnetite nanoparticles in the visible range at weak magnetic fields of 1.5 mT. The spherical supra-assembled particles with a diameter of ∼90 nm prepared by solvothermal synthesis had single-crystal-like structures, which resulted from the oriented attachment of nanograins. They exhibited superparamagnetic behavior even with a relatively large supraparticle diameter that exceeded the size limit for superparamagnetism. This can be attributed to the small size of nanograins with a diameter of ∼12 nm that constitute the suprastructured particles. Magnetic circular dichroism (MCD) measurements at magnetic fields of 1.5 mT showed distinct chiral magneto-optical activity from charge transfer transitions of magnetite in the visible range. For the supraparticles with lower crystallinity, the MCD peaks in the 250–550 nm range assigned as the ligand-to-metal charge transfer (LMCT) and the inter-sublattice charge transfer (ISCT) show increased intensities in comparison to those with higher crystallinity samples. On the contrary, the higher crystallinity sample shows higher MCD intensities near 600–700 nm for the intervalence charge transfer (IVCT) transition. The differences in MCD responses can be attributed to the crystallinity determined by the reaction time, lattice distortion near grain boundaries of the constituent nanocrystals, and dipolar interactions in the supra-assembled structures.

Imaging the Magnetization of Single Magnetite Nanoparticle Clusters via Photothermal Circular Dichroism

“…At room temperature, magnetite particles of such size exhibit superparamagnetic behavior. When they form clusters in a wet chemical process, they can maintain their superparamagnetic behavior because of weak magnetic coupling between the individual subunits. ,, …”

“…When they form clusters in a wet chemical process, they can maintain their superparamagnetic behavior because of weak magnetic coupling between the individual subunits. 19,27,28 To show that PT MCD can indeed provide images of single nanoparticles with magnetic contrast and to discern magnetic effects from shape effects (geometric chirality), we spin-coated the 400-nm-diameter magnetite particles on a glass surface at very low surface coverage (∼1 NP/10 μm 2 ). We then obtained a series of four images as displayed on a selected example in Figure 2 The PT scan (Figure 2a) indicates the broad size distribution of the particles.…”

Chiral inorganic nanomaterials for biological applications

“…For example, H. R. Li et al used alanine derivatives as chiral ligands to synthesize twisted mesoporous silica rods that can be used as nanocarriers and loaded with ibuprofen. 57 According to Maqbool et al , supra-assembled Fe 3 O 4 magnetite NPs with enhanced chiral magneto-optical activity exhibited superparamagnetic properties under a weak magnetic field, 58 and were the same as chiral Co 3 O 4 NPs. 13 The variety of chiral ligands also significantly affects the distribution range and intensity of chiral optical activity.…”

Chiral Iron Oxide Supraparticles Enable Enantiomer‐Dependent Tumor‐Targeted Magnetic Resonance Imaging

“…[ 26–28 ] As a bionic strategy, chiral nanomaterials have attracted significant attention in immune regulation, chemical sensing, and chiral catalysis, but have been rarely studied as high‐performance MRI contrast agents. [ 29–36 ]…”