Synthesis and characterization of metal-carbon core-shell nanoparticles.

摘要

Superparamagnetic nanoparticles have been actively investigated as the contrast agents for Magnetic Resonance Imaging (MRI) since the last decade owing to their relatively high magnetic susceptibility, longer retention in the blood stream and better toxicity profile compared to gadolinium-based contrast agents. Most research in this regard has been focused on iron oxides primarily because nanoparticles made of materials that have higher saturation magnetization are usually prone to degradation due to oxidation and are difficult to retain under ambient conditions. Attempts to preserve the high saturation magnetization phases have been made through passivating the surface of the nanoparticles with Au or oxides. However, these approaches are not very effective. In this work, the synthesis, structures, compositions, magnetic properties and potential applications of some metallic nanoparticles, stabilized with protective graphitic carbons, have been studied.; Fe, Co, FexCo(1-x) and AuxFe (1-x) alloy nanoparticles encapsulated by graphitic carbon are synthesized by chemical vapor deposition. Transmission electron microscopy (TEM) reveals that the nanoparticles are mostly about 10 nm in diameter and each nanoparticle is enclosed by at least one layer of graphitic carbon. Phase identification by high resolution TEM indicates the metallic phases were indeed obtained and preserved, even after three years of exposure to ambient conditions. The Fe-containing nanoparticles were found to be either BCC or FCC or Fe 3C, the Co nanoparticles being FCC, the FexCo(1-x) (0.1 x 0.6) nanoparticles being BCC alloys and AuxFe (1-x) (0.2 x 0.8) nanoparticles being FCC alloys. Energy dispersive spectrometry analysis of the Au0.67Fe0.33 nanoparticles indicates the composition of nanoparticles is consistent with the molar ratio of metal precursors, while quantitative EELS analysis suggest that the composition distribution of FexCo(1-x) rather wide. Preliminary Energy-Filtered TEM analysis suggests that AuFe nanoparticles might be segregated into a Au-rich core and Fe-rich surface. Magnetic properties of these nanoparticles were measured by a superconducting quantum interference device (SQUID). Superparamagnetic behavior at room temperature was observed in all the three types of nanoparticles. Water solutions of phospholipid were used to suspend the nanoparticles. The T2 relaxations of Fe and Au0.67 Fe0.33 nanoparticle suspensions were measured by a 1.5 Tesla MRI scanner. The T2 of Fe nanoparticle suspensions were found to be shorter than that of Feridex, the commercial MRI contrast agent based on Fe3 O4, while the T2 of Au0.67Fe0.33 nanoparticles was found to be comparable to Feridex. Au0.67Fe0.33 nanoparticles provide similar contrast enhancement in computed tomography (CT) to commercial products. It is believed that functionalized AuxFe (1-x) nanoparticles are very promising dual-modality (MRI and CT) contrast agents.