Size and Surface Effects on the MRI Relaxivity of Manganese Ferrite Nanoparticle Contrast Agents

Abstract

Superparamagnetic MnFe2O4 nanocrystals of different sizes were synthesized in high-boiling ether solvent and transferred into water using three different approaches. First, we applied a ligand exchange in order to form a water soluble polymer shell. Second, the particles were embedded into an amphiphilic polymer shell. Third, the nanoparticles were embedded into large micelles formed by lipids. Although all approaches lead to effective negative contrast enhancement, we observed significant differences concerning the magnitude of this effect. The transverse relaxivity, in particular r2*, is greatly higher for the micellar system compared to the polymer-coated particles using same-sized nanoparticles. We also observed an increase in transverse relaxivities with increasing particle size for the polymer-coated nanocrystals. The results are qualitatively compared with theoretical models describing the dependence of relaxivity on the size of magnetic spheres.

Clusters of Superparamagnetic Iron Oxide Nanoparticles Encapsulated in a Hydrogel: A Particle Architecture Generating a Synergistic Enhancement of the T2 Relaxation

“…6,9À15 One study using PEGylated SPIONS found that the relaxation rate decreased with increasing PEG length, while their simulations predicted that the coating causes competing effects that can either increase or decrease relaxivities. 14 Other recent studies reported that relaxivity values are nearly independent of polymer coatings for large magnetic particles (100 nm), while the relaxation rates of smaller particles (10 nm) were dramatically reduced when coated with silica.…”

MRI‐Visual Order–Disorder Micellar Nanostructures for Smart Cancer Theranostics

“…The relative MR imaging contrast properties of SPION was not changing significantly for visual differentiation with respect to pH, whereas there is a change (~5 times in terms of R 2 ) in case of composite. Further, the folic acid surfacing also helps in the unintentional agglomeration of micellar structures (via electrostatic repulsion force of analogous charges) to protect in the differential MRI behavior as observed by Tromodrof [37] .…”

Engineering imaging probes and molecular machines for nanomedicine

“…The magnetization of SPIOs can be enhanced by using cores formed by elementary iron or by doping iron oxide with other magnetic elements such as nickel, cobalt and manganese [96,108,109]. Further, the core size of SPIOs can be increased by employing controllable crystallization through thermo-decomposition of iron complex in organic solvents [94,95,109,110].…”

“…The coating layer must render nanoparticles water-dispersible, prevent aggregation, reduce non-specific adsorption in biological systems, and provide a platform for conjugation of targeting ligands or other functionalities. The size, charge, hydrophilicity and flexibility of the coating molecules are critical mediators for the in vitro and in vivo performance of nanoparticles [109,114,115]. For example, the coating molecules of a SPIO can exclude water from its surface, hinder water diffusion, or immobilize nearby water molecules by forming hydrogen bonds, all may affect the nuclear relaxation of water protons.…”