Air-stable superparamagnetic metal nanoparticles entrapped in graphene oxide matrix

Abstract

Superparamagnetism is a phenomenon caused by quantum effects in magnetic nanomaterials. Zero-valent metals with diameters below 5 nm have been suggested as superior alternatives to superparamagnetic metal oxides, having greater superspin magnitudes and lower levels of magnetic disorder. However, synthesis of such nanometals has been hindered by their chemical instability. Here we present a method for preparing air-stable superparamagnetic iron nanoparticles trapped between thermally reduced graphene oxide nanosheets and exhibiting ring-like or core-shell morphologies depending on iron concentration. Importantly, these hybrids show superparamagnetism at room temperature and retain it even at 5 K. The corrected saturation magnetization of 185 Am(2) kg(-1) is among the highest values reported for iron-based superparamagnets. The synthetic concept is generalized exploiting functional groups of graphene oxide to stabilize and entrap cobalt, nickel and gold nanoparticles, potentially opening doors for targeted delivery, magnetic separation and imaging applications.

Figure 1. Electron microscopy and chemical mapping of the nanocomposites.

(a) Representative TEM image (scale bar, 50 nm) of the Fe(5)/TRGO hybrid system with the inset showing a magnified HRTEM image (scale bar, 5 nm) of a single iron nanoparticle with a core-shell structure. (b) STEM/EDS chemical mapping (Fe and O; scale bar, 4 nm) of a single iron nanoparticle in the Fe(5)/TRGO nanocomposite. (c) Representative TEM image (scale bar, 50 nm) of the Fe(10)/TRGO hybrid system with the inset showing the SAED pattern. (d) Representative HRTEM image (scale bar, 20 nm) of the Fe(10)/TRGO hybrid system demonstrating the ring-link assembly of iron nanoparticles inside the TRGO matrix with insets showing a single iron nanoparticle with marked atomic layers separated by 2.86Å (scale bar, 5 nm) and the particle size distribution. (e) High-angle annular dark-field (HAADF) images (scale bar, 20 nm) of the Fe(10)/TRGO nanocomposite. (f) STEM/EDS chemical mapping (Fe and O; scale bar, 4 nm) of a single iron nanoparticle in the Fe(10)/TRGO hybrid system. (g) Representative TEM image (scale bar, 200 nm) of the Fe(20)/TRGO nanocomposite with the inset (scale bar, 20 nm) showing the ring-like arrangement of iron nanoparticles inside the TRGO matrix. (h) Representative HRTEM image (scale bar, 20 nm) of the Fe(20)/TRGO hybrid system with the inset showing STEM/EDS chemical mapping (Fe; scale bar, 10 nm) of iron nanoparticles in their ring-like assembly.

Figure 2. ⁵⁷Fe Mössbauer spectra of the Fe(10)/TRGO nanocomposite.

⁵⁷Fe Mössbauer spectra acquired at temperatures of (a) 300 K, (b) 50 K and (c) 5 K.

Figure 3. Magnetization measurements of the Fe(10)/TRGO nanocomposite.

(a) Hysteresis loops of the Fe(10)/TRGO nanocomposite, recorded at a temperature of 5 and 300 K, with the insets showing the behaviour of hysteresis loops in weak applied fields and hysteresis loops recalculated with respect to the proportion (by mass) of iron nanoparticles in the TRGO network. (b) ZFC/FC magnetization curves of the Fe(10)/TRGO nanocomposite with insets showing the profile of the ZFC/FC magnetization curves at low temperatures and a schematic illustration of the blocked and SP states of self-assembled iron nanoparticles trapped inside the TRGO network.

Figure 4. Biocompatibility tests and in vivo MRI measurements.

(a) Viability of cells exposed to the Fe(10)/TRGO and the Fe(10)/TRGO/PEG nanocomposite determined by the MTT assay (data presented are the mean±s.d. of triplicate assays). (b) Schematic image of the mouse's abdominal region with marked organs. (c) In vivo MRI images of a healthy mouse in the coronal plane, collected before and after intravenous injection of the Fe(10)/TRGO/PEG nanocomposite, showing clearly the reduction in the signal intensity for the kidneys and liver in the T₂-weighted MRI images.

Figure 5. Characterization of the nanocomposites containing other metals.

(a) Representative TEM image (scale bar, 50 nm) of the Co(10)/TRGO hybrid system with the inset (scale bar, 5 nm) showing a single cobalt nanoparticle with marked atomic layers separated by 2.50Å. (b) Representative HRTEM image (scale bar, 10 nm) of the Co(10)/TRGO hybrid system demonstrating the ring-like arrangement of cobalt nanoparticles inside the TRGO matrix. (c) Representative HRTEM image (scale bar, 20 nm) of the Ni(5)/TRGO hybrid system. (d) HAADF image (scale bar, 6 nm) of a single nickel nanoparticle in the Ni(5)/TRGO nanocomposite. (e) STEM/EDS chemical mapping (Ni and O; scale bar, 6 nm) of the Ni(5)/TRGO hybrid system demonstrating the core-shell architecture of the trapped nickel nanoparticle inside the TRGO network. (f) Representative HRTEM image (scale bar, 20 nm) of the Au(10)/TRGO nanocomposite. (g) HAADF image (scale bar, 4 nm) of a single gold nanoparticle in the Au(10)/TRGO nanocomposite. (h) STEM/EDS chemical mapping (Au and O; scale bar, 4 nm) of the Au(10)/TRGO hybrid system showing a single gold nanoparticle inside the TRGO matrix.

Figure 6. Schematic representation of the approach.

A schematic representation of the universal approach for an entrapment of SP nanometals in TRGO matrix starting from a metal-containing molecular precursor and GO. As an example, the scheme depicts the formation of an Fe/TRGO hybrid with ring-like assembly of iron nanoparticles (red) trapped among GO sheets (black). Note: In this schematic representation, the size ratios of individual atoms do not correspond to real conditions. In the structure of iron(III) nitrate, Fe is shown in red, N in yellow and O in blue.