Comprehensive magnetic resonance characteristics of carbon-encapsulated iron nanoparticles: a new frontier for the core-shell–type contrast agents

The development of carbon-encapsulated iron nanoparticles (CEINS) is of considerable interest in many areas of cancer nanotechnology, ranging from basic tumor biology to early detection and treatment of cancer. To meet these challenges, the present study was undertaken to determine the magnetic and relaxometric performance of CEINS used as a new contrast agent for magnetic resonance imaging (MRI) in preclinical phantom models. CEIN samples were synthesized using a carbon arc discharge route, and the as-synthesized nanoparticles were purified and functionalized with surface acidic groups. The presence of various Fe-bearing metallic phases reflecting the net magnetic properties of CEINS was characterized by powder X-ray diffraction (XRD), thermogravimetry (TGA), and vibrating sample magnetometry. The morphological and surface chemistry features were characterized by electron microscopy (transmission electron microscopy [TEM] and scanning electron microscopy [SEM]), Raman spectroscopy, and Fourier transform-infrared (FT-IR) spectroscopy. The textural properties of CEINS, including porosity, surface total charge density, and zeta potentials, were also measured. The as-synthesized different CEIN samples were finally examined as a potent MRI contrast drug candidate. Magnetic resonance relaxation measurements were performed in bovine gelatin-based phantom models by using a 1.5-T MRI scanner equipped with a standard radiofrequency “birdcage” type head coil. To obtain data, T1- and T2-weighted MR images were acquired using the inversion recovery spin echo (SE) and the SE protocol with multiple time of echo (TE), respectively. Chemical characterization showed similarity in morphology and textural properties between as-synthesized CEINS, purified CEINS, and CEINS functionalized with acidic groups. The as-synthesized CEINS had significantly higher Fe content and higher saturation magnetization. The analysis of the relaxometric properties of CEINS revealed that all the CEIN samples decreased T2 relaxation times on the T2-weighted images. The relaxation rate (1/T2) showed some differences between the as-synthesized, purified, and surface-functionalized CEINS containing surface carboxylic groups. Both the as-synthesized and purified CEINS slightly decreased the T1 relaxation times, which was evident through increase in the relaxation rates (1/T1). This study concludes that CEINS may represent a novel “core-shell”–type negative contrast drug candidate for MRI. It should be emphasized that all the studied CEIN samples have acceptable r2 relaxivities at the field strength of 1.5 T where most of the MRI systems operate in clinical radiology.