Synthesis of phase-pure and monodisperse iron oxide nanoparticles by thermal decomposition

Abstract

Superparamagnetic iron oxide nanoparticles (SPIONs) are used for a wide range of biomedical applications requiring precise control over their physical and magnetic properties, which are dependent on their size and crystallographic phase. Here we present a comprehensive template for the design and synthesis of iron oxide nanoparticles with control over size, size distribution, phase, and resulting magnetic properties. We investigate critical parameters for synthesis of monodisperse SPIONs by organic thermal decomposition. Three different, commonly used, iron containing precursors (iron oleate, iron pentacarbonyl, and iron oxyhydroxide) are evaluated under a variety of synthetic conditions. We compare the suitability of these three kinetically controlled synthesis protocols, which have in common the use of iron oleate as a starting precursor or reaction intermediate, for producing nanoparticles with specific size and magnetic properties. Monodisperse particles were produced over a tunable range of sizes from approximately 2-30 nm. Reaction parameters such as precursor concentration, addition of surfactant, temperature, ramp rate, and time were adjusted to kinetically control size and size-distribution, phase, and magnetic properties. In particular, large quantities of excess surfactant (up to 25 : 1 molar ratio) alter reaction kinetics and result in larger particles with uniform size; however, there is often a trade-off between large particles and a narrow size distribution. Iron oxide phase, in addition to nanoparticle size and shape, is critical for establishing magnetic properties such as differential susceptibility (dm/dH) and anisotropy. As an example, we show the importance of obtaining the required size and iron oxide phase for application to Magnetic Particle Imaging (MPI), and describe how phase purity can be controlled. These results provide much of the information necessary to determine which iron oxide synthesis protocol is best suited to a particular application.

Figure 1

(a) TGA (W, Weight), derivative of TGA (dW/dT), and DSC (Heat Flow) of iron (III) oleate. (b) Size versus temperature for iron oxide nanoparticles synthesized by thermal decomposition of iron (III) oleate in 1-octadecene with a concentration of 0.1698 mmols of iron per gram of solution. Precursor was heated at 10°C/min until the specified temperature was reached, and a 1mL aliquot was removed from heat and quenched. Where indicated, the solution was allowed to age for a number of hours and another aliquot was taken. (c) and (d) are TEM micrographs of the particles synthesized at 290°C and 320°C, respectively. (e) Selected Area Diffraction Pattern from the 320°C sample, indexed as an inverse-spinel structure.

Figure 2

Superparamagnetic iron oxide nanoparticles produced by thermal decomposition of iron (III) oleate in the presence of excess oleic acid. Size is shown as a function of (a) precursor concentration, (b) excess oleic acid, and (c) aging time. All sizes are median diameter (D_M) and error bars represent the first standard deviation of the log-normal size distribution (σ), determined by fitting VSM measurements. TEM images of particles of various sizes (d, scale bar 100 nm). High resolution TEM of nanoparticles synthesized from FeOOH (e), Fe(CO)₅(f), and iron(III) oleate (g) (5 nm scale).

Figure 3

Size and time to nucleation of iron oxide nanoparticles by thermal decomposition of Fe(CO)₅ as a function of molar ratio of oleic acid in the precursor. Increasing the excess oleic acid ratio delays nucleation and results in larger particles. TEM micrographs show highly monodisperse particles until, in this case, approximately 2.5:1.

Figure 4

Particle size versus excess oleic acid ratio for iron oxide nanoparticles produced by thermal decomposition of iron oxyhydroxide. Median diameter and distribution is fit from VSM measurements. Insets are TEM micrographs from each sample.

Figure 5

Differential susceptibility (dm/dH) from MPS (a) and m(H) from VSM (b) of iron oxide nanoparticles with comparable physical size (D_M ~ 25 nm from TEM) but differing iron oxide phase: pure magnetite (Fe₃O₄), mixed wüstite/magnetite (FeO@Fe₃O₄), or commercially available Resovist®.

Figure 6

(a) Raman spectroscopy of iron oxide nanoparticles synthesized from iron(III) oleate and characterized at nucleation and after 24 hours of aging. (b) Synthesized from FeOOH, and oxidized to maghemite (c) XRD, θ - 2θ scans, of iron oxide nanoparticles synthesized form FeOOH. Particles were annealed at 100°C for various times to optimize phase and crystallinity. The peaks observed on annealing can be readily indexed as magnetite. (d) XRD of iron oxide nanoparticles produced by thermal decomposition of Fe(CO)₅. Aliquots were taken throughout the synthesis (§2.1.1), and during subsequent oxidation (§2.1.4). Particles initially form as wüstite (FeO_1-x), as indicated by the (111), (200) and (220) wüstite peaks in aliquots 1–5. During oxidation (aliquots 6–9) magnetite peaks clearly develop. A second reflux step (aliquots 10 and 11) optimizes the crystallinity, as indicated by sharpening of peaks.

Figure 7

Electron energy loss spectra (EELS) from iron oxide nanoparticles synthesized from FeOOH before and after annealing at 100°C for 12 hours, showing the oxygen K-edge (onset ~532 eV) and iron-L edge (onset ~708 eV). The ratio of iron L₃ to L₂ core-shell transitions is proportional to iron valence, increasing as nanoparticles are annealed from magnetite to maghemite.