Superparamagnetic iron oxide nanoparticles have received great attention due to their applications as contrast agents for magnetic resonance imaging (MRI). This feature article briefly introduces the concepts of MRI and MRI contrast agents, and then mainly discusses the synthesis, surface modification, surface functionalization, colloidal stability and biocompatibility of iron oxide particles, followed by their MRI applications.

Superparamagnetic iron oxide nanoparticles: from preparations to in vivo MRI applications

Synthesis and physical characterization of magnetite nanoparticles for biomedical applications

Co-precipitation in aqueous solution synthesis of magnetite nanoparticles using iron(III) salts as precursors

Static magnetization and Mossbauer measurements on ultra-fine crystallites (≈ 65 A) of γ-Fe2O3 show them to be superparamagnetic at 296 °K with an average relaxation time of 2 × 10−9 s. The particle size distribution is derived from the magnetization curve which exhibits no hysteresis, and independently from the temperature variation of the ratio of remanence to saturation magnetization. Mossbauer spectra at 5 °K in an applied field of up to 50 kOe show a cation distribution of 1:(1.71 ± 0.05) for A and B sites, and a non-collinear spin arrangement. The low value of the magnetization, 59 e.m.u./g at 4.2 °K for 1/H = 0, is explained by assuming that the spins of cations, which lie in the surface layer, make random angles between 0 and 90° with the direction of the net moment. Allowance is also made for water adsorbed onto the particle surface. When a field is applied to the absorber at 296 °K the spectrum changes from a broad single peak into two broadened hyperfine patterns from A and B sites which are explained by the magnetic polarization and range of ordering temperature of the particle distribution.

Superparamagnetic γ-Fe2O3

The temperature stability of acicular maghemite (γ-Fe2O3) was studied between 20°C and 600°C by x-ray diffraction and magnetic measurements. Synthetic monodomain maghemite was found to be stable with respect to hematite (α-Fe2O3) at temperatures as high as 500°C, inverting to α-Fe2O3 between 510-660°C. However, a complete inversion of the γ to α form for the sample was not achieved even at 660°C, the final product was a two-phase mixture, with 65% of the original γ-Fe2O3 unchanged. Two conclusions are made: (a) the absence of maghemite in a natural sample cannot be proven if heating has been restricted to moderate temperatures (300°-400°C) and (b) the true Curie point of pure maghemite is close to 645°C.

High temperature stability of maghemite (γ-Fe2O3)

The oxidation of magnetite and related spinels. Constitution of gamma ferric oxide

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The oxidation of magnetite and related spinels. Constitution of gamma ferric oxide