By combining nonhydrolytic reaction with seed-mediated growth, high-quality and monodisperse spinel cobalt ferrite, CoFe(2)O(4), nanocrystals can be synthesized with a highly controllable shape of nearly spherical or almost perfectly cubic. The shape of the nanocrystals can also be reversibly interchanged between spherical and cubic morphology through controlling nanocrystal growth rate. Furthermore, the magnetic studies show that the blocking temperature, saturation, and remanent magnetization of nanocrystals are solely determined by the size regardless the spherical or cubic shape. However, the shape of the nanocrystals is a dominating factor for the coercivity of nanocrystals due to the effect of surface anisotropy. Such magnetic nanocrystals with distinct shapes possess tremendous potentials in fundamental understanding of magnetism and in technological applications of magnetic nanocrystals for high-density information storage.

A correlation between the electron spin-orbital angular momentum coupling and the superparamagnetic properties has been established in MgFe 2 O 4 and CoFe 2 O 4 spinel ferrite nanoparticles. The contribution to the magnetic anisotropy from the Fe 3+ lattice sites is almost the same in both nanocrystallites as neutron diffraction studies have shown a similar cation distribution in these two types of spinel ferrite nanoparticles. Due to the strong magnetic couplings from Co 2+ lattice sites, the blocking temperature of CoFe 2 O 4 nanoparticles is at least 150 deg higher than the same sized MgFe 2 O 4 nanoparticles. Mo ¨ssbauer spectroscopy studies demonstrate that the magnetic anisotropy of CoFe 2 O 4 nanoparticles is higher than that of the same size MgFe 2 O 4 nanoparticles. These studies indicate that the superparamagnetic properties of nanoparticles can be controlled through chemically adjusting the magnetic anisotropy energy.

An atom transfer radical polymerization route is developed for the coating of MnFe2O4 nanoparticles with polystyrene yielding the core-shell nanoparticles with size <15 nm. Magnetic studies show a decrease in coercivity after the formation of polystyrene shell, which is considered due to the reduction of magnetic surface anisotropy upon polymer coating. The MnFe2O4 nanoparticles as the magnetic core were separately prepared by a reverse micelle microemulsion method. Polymerization initiators are chemically attached onto the surface of nanoparticles. The modified nanoparticles are then used as macro-initiators in the subsequent polymerization reaction. This approach provides great flexibility in the selection of magnetic core. Consequently, magnetic tunability is able to be introduced into these core/shell nanoparticulate systems to achieve the desired superparamagnetic response.

A method for coating silica on CoFe2O4 and MnFe2O4 spinel ferrite nanoparticles has been developed by using a reverse micelle microemulsion approach. The ability to controllably synthesize magnetic nanoparticulate cores independent of encapsulation provides great flexibility in tuning the magnetic properties of this magnetic nanocomposite system by controlling the magnetic properties of nanoparticulate cores. For these spinel ferrite nanoparticles, the saturation and remnant magnetizations decrease upon silica coating. The coercivity of silica-coated CoFe2O4 nanoparticles does not show any change after coating, while the coercivity of MnFe2O4 nanoparticles decreases by 10% after they have been coated with silica.

The magnetic properties of spinel nanoparticles are determined by crystal chemistry issues such as cation distribution and oxidation states. The cation distribution and oxidation state of Mn−Fe spinel nanoparticles have been systematically studied at various temperatures by using neutron diffraction and electron energy loss spectroscopy, respectively. The Mn−Fe spinel nanoparticles prepared by coprecipitation have a high degree of inversion with 61% of the tetrahedral sites occupied by Fe3+ cations. The degree of inversion correlates with the distribution expected from random occupancy of cations consisting of Fe (60%) and Mn (40%). After heat treatment in a vacuum, the cation distribution reaches an equilibrium state with a 29% inversion. Initially, one-half of the Mn cations are in the +3 oxidation state and the other half are in the +2 oxidation state. Mn3+ cations are slowly and irreversibly reduced to Mn2+ with increasing temperature. When the temperature approaches 600 °C, all Mn cations are in the +2 state. These results provide direct evidence for the temperature-dependent change of crystal chemistry in Mn−Fe spinel nanoparticles, which has been closely related with the controversy on attributing the changes in the magnetic properties of the nanoparticles to crystallite size effect. These results will also provide an understanding of how to control crystal chemistry in order to control the properties of these magnetic nanoparticles.

MnFe2O4 nanoparticles are synthesized by using sodium dodecylbenzenesulfonate (NaDBS) to form water-in-toluene reverse micelles. The nanoparticles are single crystalline, and the average particle size can be controlled from 4 to 15 nm. High- and low-resolution transmission electron microscopy characterization has shown that the nanoparticles can have a size distribution as narrow as ∼9%. Neutron diffraction and magnetic measurements have been conducted on the nanoparticles with a diameter of 7.7 ± 0.7 nm. The results unambiguously prove that these MnFe2O4 nanoparticles are truly superparamagnetic. The synthesis and characterization of these nanoparticles will facilitate the development of MnFe2O4 nanoparticles for the potential applications such as contrast enhancement agents of magnetic resonance imaging and magnetic carriers for site-specific drug delivery.

Magnetic cobalt spinel ferrite nanoparticles coated with biocompatible polygalacturonic acid were functionalized with ligands specific for targeting expressed EphA2 receptors on ovarian cancer cells. By using such magnetic nanoparticle−peptide conjugates, targeting and extraction of malignant cells were achieved with a magnetic field. Targeting ovarian cancer cells with receptor specific peptide-modified magnetic nanoparticles resulted in cell capture from a flow stream in vitro and from the peritoneal cavity of mice in vivo. Successful removal of metastatic cancer cells from the abdominal cavity and circulation using magnetic nanoparticle conjugates indicate the feasibility of a dialysis-like treatment and may improve long-term survival rates of ovarian cancer patients. This approach can be applied for fighting other cancers, such as leukemia, once the receptors on malignant cells are identified and the efficacy of targeting ligands is established.

MnFe2O4 nanoparticles in a size range of 4−14 nm have been synthesized from water-in-toluene reverse micelles by using sodium dodecylbenzenesulfonate (NaDBS) as surfactant. The blocking temperature, saturation magnetization, and coercivity of the nanoparticles are clearly size-dependent. The blocking temperature increases from 20 to 250 K when the mean size of the nanoparticles increases from 4.4 to 13.5 nm. The coercivity at 20 K increases from 30 to 300 Oe with increasing nanoparticle size. The field-dependent magnetization hysteresis disappears above the blocking temperature. Due to high saturation field, the saturation magnetization of the nanoparticles has been obtained by the extrapolation of the magnetization vs 1/H plot to 1/H = 0. The saturation magnetization decreases with decreasing nanoparticle size. The high saturation field and the size-dependent saturation magnetization suggest the existence of a magnetically inactive layer on MnFe2O4 nanoparticles. The linear fitting of the saturation magnetization vs 1/d plot gives the thickness of this inactive layer as 0.45 nm.