Manganese Ferrite Nanoparticles (MnFe2O4): Size Dependence for Hyperthermia and Negative/Positive Contrast Enhancement in MRI

Abstract

We synthesized manganese ferrite (MnFe₂O₄) nanoparticles of different sizes by varying pH during chemical co-precipitation procedure and modified their surfaces with polysaccharide chitosan (CS) to investigate characteristics of hyperthermia and magnetic resonance imaging (MRI). Structural features were analyzed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (TEM), selected area diffraction (SAED) patterns, and Mössbauer spectroscopy to confirm the formation of superparamagnetic MnFe₂O₄ nanoparticles with a size range of 5-15 nm for pH of 9-12. The hydrodynamic sizes of nanoparticles were less than 250 nm with a polydispersity index of 0.3, whereas the zeta potentials were higher than 30 mV to ensure electrostatic repulsion for stable colloidal suspension. MRI properties at 7T demonstrated that transverse relaxation (T₂) doubled as the size of CS-coated MnFe₂O₄ nanoparticles tripled in vitro. However, longitudinal relaxation (T₁) was strongest for the smallest CS-coated MnFe₂O₄ nanoparticles, as revealed by in vivo positive contrast MRI angiography. Cytotoxicity assay on HeLa cells showed CS-coated MnFe₂O₄ nanoparticles is viable regardless of ambient pH, whereas hyperthermia studies revealed that both the maximum temperature and specific loss power obtained by alternating magnetic field exposure depended on nanoparticle size and concentration. Overall, these results reveal the exciting potential of CS-coated MnFe₂O₄ nanoparticles in MRI and hyperthermia studies for biomedical research.

Keywords: X-ray diffraction; magnetic resonance angiography; manganese ferrite; nanomaterials; specific loss power.

Figure 1

XRD studies of as-dried MnFe₂O₄ nanoparticles at different pH. (a) XRD patterns of the samples synthesized at pH of 9, 10, 11, and 12, (b) pH dependence particle size (d) and lattice parameter (a), (c) nanoparticle size dependence of X-ray density (d_x) and specific surface area (S), (d) nanoparticle size dependence of hopping length (L) for tetrahedral (A) and octahedral (B) site.

Figure 2

XRD studies of as-dried MnFe₂O₄ nanoparticles synthesized at different pH. (a) nanoparticle size dependence of ionic radii on tetrahedral and octahedral sites (r_A and r_B) (left axis) and bond length (d_AL and d_BL) (right axis), oxygen parameters (u) and shared tetrahedral edge (d_AE), shared octahedral edge (d_BE), unshared octahedral edge (d_BEU), (b) nanoparticle size dependence of interionic distances between cations (Me-Me) (b, c, d, e, f), between cations and anions (p, q, r, s) and bond angles (θ₁, θ₂, θ₃, θ₄, θ₅).

Figure 3

TEM images of the CS-coated MnFe₂O₄ nanoparticles at pH 11. (a) bright field (BF), (b) dark-field (DF), (c) selected area diffraction (SAED) pattern, and (d) high-resolution TEM image.

Figure 4

M-H hysteresis loops of MnFe₂O₄ nanoparticles in the as-dried condition measured at 5 and 300 K with a maximum applied field of 5 Tesla. The M-H loops of the MnFe₂O₄ nanoparticles at different pH are presented, (a) 5 nm, (b) 6 nm, (c) 10 nm, and (d) 15 nm.

Figure 5

(a) Maximum magnetization (b) Bohr magneton, (c) Canting angles, (d) squareness ratio, (e) coercivity, and (f) anisotropy with size variations of MnFe₂O₄ nanoparticles.

Figure 6

Maximum magnetization with (a) inversion parameter, (b) with cation distribution in the octahedral site, and (c) with oxygen parameter.

Figure 7

Mössbauer spectra of MnFe₂O₄ nanoparticles in the as-dried condition for different nanoparticle sizes measured at room temperature and without any applied magnetic field (a) 5 nm, (b) 6 nm, (c) 10 nm, and (d) 15 nm. In the spectrum, hollow red circles represent experimental data, the solid red line represents theoretical fitting, and the other three lines represent sub spectra of three subspecies named a, b, and c to fit the data.

Figure 8

FTIR spectra of uncoated MnFe₂O₄, CS-coated MnFe₂O₄, and CS nanoparticles for sizes of (a) 5 nm, (b) 6 nm, (c) 10 nm, and (d) 15 nm.

Figure 9

Hydrodynamic diameter (H_d) of CS-coated MnFe₂O₄ nanoparticles measured at 25 °C. In the figure, we present (a) distribution of H_d with concentration and (b) with nanoparticle size at 25 °C. Analyzing the data in (a) and (b), we presented (c) concentration and (d) nanoparticle size dependence of hydrodynamic diameter (H_d) and the polydispersity index (PDI).

Figure 10

Zeta potential of CS-coated MnFe₂O₄ nanoparticles for 5 nm, 6 nm, 10 nm, and 15 nm at room temperature during the DLS measurement.

Figure 11

Cytotoxicity results assessed by survival of Hela cells (a) with and without solvent and (b) CS-coated MnFe₂O₄ nanoparticles at different particle sizes, where the corresponding cell culures are pictured on the top panel.

Figure 12

Time dependence temperature curves of CS-coated MnFe₂O₄ nanoparticles with a RF magnetic field of an amplitude of 26 mT and a frequency of 342 kHz. The curve presents time dependence of temperature curves of (a) 1 mg/mL, (b) 2 mg/mL, (c) 3 mg/mL, (d) 4 mg/mL. Subsequently, nanoparticle size dependence of (e) maximum temperature (T_max), and (f) specific loss power (SLP) are presented.

Figure 13

MRI data of CS-coated MnFe₂O₄ spinel ferrites nanoparticles. (a) Images acquired at TE of 14 ms (particle sizes of 5, 6, 10, and 15 nm) for CS-coated MnFe₂O₄ spinel ferrites nanoparticles with different values of concentrations (0.17, 0.34, 0.51, 0.68 and 1.03 mM) inside the five tubes in each image 1 to 5 represents lower to higher concentration of the nanoparticles in solution demonstrating contrast agents at different particle sizes. (b) Absolute R₂ (or 1/T₂) mapping images for particle sizes of 5, 6, 10, and 15 nm at different concentrations. (c) Nanoparticle size dependence of relaxivity (r₂) of CS-coated MnFe₂O₄ nanoparticles.

Figure 14

MRA with TOF 3D in vivo demsontrated in rat brain. Images from maximum intensity projection (MIP) are shown, where A and A’ represents a slice of MIP with and without contrast agents at 3° from the horizontal position, B and B’ represents with and without contrast agents at the sagittal position for the nanoparticle size of (a) 6 nm, (b) 10 nm, and (c) 15 nm.