Extraordinarily small (2.4 nm) cobalt ferrite nanoparticles (ESCIoNs) were synthesized by a one-pot thermal decomposition approach to study their potential as magnetic resonance imaging (MRI) contrast agents. Fine size control was achieved using oleylamine alone, and annular dark-field scanning transmission electron microscopy revealed highly crystalline cubic spinel particles with atomic resolution. Ligand exchange with dimercaptosuccinic acid rendered the particles stable in physiological conditions with a hydrodynamic diameter of 12 nm. The particles displayed superparamagnetic properties and a low r2/r1 ratio suitable for a T1 contrast agent. The particles were functionalized with bile acid, which improved biocompatibility by significant reduction of reactive oxygen species generation and is a first step toward liver-targeted T1 MRI. Our study demonstrates the potential of ESCIoNs as T1 MRI contrast agents.
Ferrite nanoparticles are promising candidates for a range of bio‐medical applications due to their interesting physical and magnetic properties. In order for their nano‐scale properties to be utilised however, synthesis methods which enable fine control over size, shape and composition are required. In this work we explore the size‐controlled synthesis of iron‐oxide and cobalt ferrite nanoparticles by thermal decomposition and their characterisation by advanced microscopy techniques. The tailored synthesis of the nanoparticles was addressed by exploring the parameters in the reduction of both an iron precursor (Fe(III)(acac) 3 ) and a cobalt precursor (Co(II)(acac) 3 ) in oleylamine. In this thermal decomposition approach, oleylamine acts as the sole surfactant, reducing agent, and solvent. By controlling the surfactant‐to‐precursor ratio, we have produced ultra‐small nanoparticles with a narrow size distribution. The size, shape and atomic structure of the resulting particles were examined by aberration‐corrected annular dark‐field scanning‐transmission electron microscopy; and the results show that their average size is <3 nm and they are highly crystalline. Electron energy loss spectroscopy was used to analyse the composition of the resulting nanoparticles and the oxidation state of the metals, confirming that they consist of Fe 3 O 4 and CoFe 2 O 4 . Our results serve to form a solid support for future studies into the size‐dependent interaction of iron‐oxide and cobalt ferrite nanoparticles with cells.
Cobalt ferrite (CoFe 2 O 4 ) nanoparticles have recently emerged as a potential candidate for a range of bio‐medical applications due to their interesting physical and magnetic properties. The high magneto‐crystalline anisotropy of CoFe 2 O 4 nanoparticles in particular offers improved efficiency over iron‐oxide nanoparticles, allowing for smaller particles to be used. Synthesis of cobalt ferrite nanoparticles is conventionally achieved using thermal decomposition in oleic acid and oleylamine. Recent methods using oleylamine alone demonstrate greater suitability for biomedical applications, as oleylamine facilitates the phase‐transfer process required to make the nanoparticles water‐soluble. However changing the surfactant is known to have a significant effect on the crystal structure and morphology of metal‐oxide nanoparticles, and the crystal structure of oleylamine‐capped cobalt ferrite nanoparticles has not been studied in detail before. Here we demonstrate the inverse spinel structure of cobalt ferrite nanoparticles synthesised with oleylamine as the sole surfactant, by aberration‐corrected annular dark‐field scanning‐transmission electron microscopy. The crystal structure is resolved with atomic‐level detail which has not been demonstrated with cobalt ferrite nanoparticles previously. Furthermore the distribution of cobalt and iron atoms is shown by atomic‐resolution EELS spectroscopy mapping. This data serves to form a solid support for future studies into the size‐dependent interaction of cobalt ferrite nanoparticles with cells.
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