Magnetic resonance imaging (MRI), a sophisticated promising three-dimensional tomographic noninvasive diagnostic technique, has an intrinsic advantage in safety when compared with radiotracer and optical imaging modalities. However, MRI contrast agents are less sensitive than complexes used in other imaging techniques and toxicity issues still endure in nanoparticle-based MRI-T1 contrast agents. Therefore, demand for a nontoxic novel (T1 & T2) T1-weighted MRI potential candidate 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 thermal decomposition which demonstrated high-performance T1-weighted MRI contrast agent for heart, liver, kidney, and bladder. Advanced characterization techniques were used to investigate the crystal structure, morphological evaluation, concentration of iron, size distribution, active modes, and magnetization of as-synthesized nanoparticles. Transmission electron microscopy (TEM) results have illustrated that the diameter of SPIONPs was in the range of 4 nm and the average size of Fe3O4@SiO2 was about 30nm~40nm. X-ray diffraction (XRD) and Raman spectroscopy analyses revealed the purity in the phase of the prepared SPIONPs. These magnetite nanoparticles exhibited weak magnetic moment at room temperature because of the spin-canting effect which escorted high positive signal enhancement ability. MCF-7 and HeLa cell viability experiments demonstrated good biocompatibility of the SPIONPs. In addition, the first-ever study has been made on T1 MRI contrast agent of silica-coated ultra-small (4nm-sized) magnetite nanoparticles exhibited a good r1 relaxivity of 1.2 and low r2/r1 ratio of 6.5, attributed to low magnetization, large surface area and 5 unpaired valence electrons on the surface of Fe3+. In vivo T1-weighted MR imaging of the heart, liver, kidney, and bladder after intravenous injection of nanoparticles further verified the high sensitivity and biocompatibility of as-synthesized magnetite nanoparticles. These results reveal silica-coated SPIONPs as a promising candidate for T1 contrast agents with extraordinary capability to enhance MR images.
Keywords: Ultrasmall nanoparticles, Magnetic properties, Co-toxicity, T1-weighted MRI contrast