Silica-coated super-paramagnetic iron oxide nanoparticles (SPIONPs): a new type contrast agent of T1 magnetic resonance imaging (MRI)

Abstract

Magnetic resonance imaging (MRI), a sophisticated promising three-dimensional tomographic noninvasive diagnostic technique, has an intrinsic advantage in safety compared with radiotracer and optical imaging modalities; however, MRI contrast agents are less sensitive than complexes used in other imaging techniques. Usually the clinically used Gd-based complexes MRI-T₁ contrast agents are toxic; therefore, the demand for nontoxic novel T₁-weighted MRI candidates with ultrasensitive imaging and advanced functionality is very high. In this research, silica-coated ultra-small monodispersed super-paramagnetic iron oxide nanoparticles were synthesized via a thermal decomposition method, which demonstrated themselves as a high performance T₁-weighted MRI contrast agent for heart, liver, kidney and bladder based on in vivo imaging analyses. Transmission electron microscopy (TEM) results illustrated that the diameter of the SPIONPs was in the range of 4 nm and the average size of Fe₃O₄@SiO₂ was about 30–40 nm. X-ray diffraction (XRD) and Raman spectroscopy analyses revealed the phase purity of the prepared SPIONPs. These magnetite nanoparticles exhibited a weak magnetic moment at room temperature because of the spin-canting effect, which promoted a high positive signal enhancement ability. MTT assays and histological analysis demonstrated good biocompatibility of the SPIONPs in vitro and in vivo. In addition, the silica-coated ultra-small (4 nm sized) magnetite nanoparticles exhibited a good r₁ relaxivity of 1.2 mM⁻¹ s⁻¹ and a low r₂/r₁ ratio of 6.5 mM⁻¹ s⁻¹. In vivo T₁-weighted MR imaging of heart, liver, kidney and bladder in mice after intravenous injection of nanoparticles further verified the high sensitivity and biocompatibility of the as-synthesized magnetite nanoparticles. These results reveal silica-coated SPIONPs as a promising candidate for a T₁ contrast agent with extraordinary capability to enhance MR images.