HREM analysis of graphite-encapsulated metallic nanoparticles for possible medical applications

Abstract

High resolution electron microscopy has been applied to study the structure of metallic nanoparticles. These have sparked considerable interest as contrast agents in the field of biological imaging, including in magnetic resonance imaging (MRI) and computed tomography (CT). Here, we describe a method of synthesizing sub-10nm superparamagnetic metal and alloy nanoparticles by reduction of metallic salts. Annealing at 900°C in a methane/hydrogen environment forms a thin graphitic-carbon shell which is expected to improve stability, biocompatibility, and functionalization. Subsequent high resolution electron microscopy verifies graphitization and allows for crystallographic analysis. Most particles consist of single crystals in the phase predicted for the bulk material at the annealing temperature. Electron energy loss spectroscopy, energy dispersive X-ray spectroscopy and lattice constant measurements show large variation in composition for alloy nanoparticles from a single synthesis. The magnetization relaxation time (T2) measurements demonstrate that Fe and AuFe nanoparticles compete with commercially available iron oxide MRI contrast agents. X-ray attenuation measurements of an AuFe alloy nanoparticle solution gave a relative radiodensity of 280 Hounsfield Units, demonstrating promise as a dual-purpose contrast agent in CT and MRI. Long term stability in an atmospheric environment was also tested, with no signs of corrosion or oxidation after several years of storage.

Synthesis and characterization of graphite-encapsulated iron nanoparticles from ball milling-assisted low-pressure chemical vapor deposition

“…The emergence of the cementite (Fe 3 C) phase which was generally reported in the XRD patterns of carbon-encapsulated Fe nanoparticles [14,19,28], was not observed in the XRD patterns of the as-blended nor of the ball milled powders after LPCVD (Figure 2e–f), within the detection limit of the diffractometer (≥ 1 wt.%). Moreover, no carbon or graphite peaks were present in the as-synthesized product of the ball milled powders even though encapsulation was found in the TEM results in the following section.…”

“…This is in contrast to the bulk as-blended sample where only BCC iron and unreacted graphite were observed (Figure 2e). Although BCC is the stable phase of bulk iron at room temperature, prior work has found that a fraction of nanoparticles rich in carbon remain in the FCC phase after cooling from elevated temperatures such 900 °C, where the FCC phase is stable in the Fe-C system [14]. Therefore, the partial suppression of the FCC to BCC phase transition after synthesis using the ball milled sample suggests that the milling process promotes the formation of nanoscale iron particles.…”

“…A magnified high resolution transmission electron micrograph (HR-TEM) (Figure 5e) has been taken from the indicated region, illustrating that the particle is FCC Fe in the [110] zone axis orientation. The utility of the high resolution image to identify the iron phase has been described previously [14]. …”

“…In particular, Fe, Ni and Co-based magnetic nanoparticles are preferred candidates for medical applications because of their high magnetization and catalyzing abilities [4,6,9,10]. However, they oxidize readily and the strategy has been to encapsulate them with a protective layer to prevent them from being oxidized or corroded [4,11–14]. Materials such as silica, boron nitride, noble metals and organic polymers have been used for such coating [15–22].…”

“…Chemical vapor deposition (CVD) has also been employed for the synthesis of CEMNPs under different pressure conditions such as high vacuum (10 −4 –10 −6 Torr), atmospheric pressure and low pressure (0.1–1 Torr) originating from metallic salts or metallic salt-loaded silica powders, metallocene precursors and ferrofluid-loaded silicon wafers [6,11–14,16,37–40]. The formation of non-encapsulated particles encountered in the above-mentioned methods can be avoided using CVD processes by increasing the interaction area and reaction kinetics through the introduction of a gaseous carbon source.…”