Superparamagnetic ferrite nanoparticles (MFe 2 O 4 , where M = Fe, Co, Mn) were synthesized through a novel one-step aqueous coprecipitation method based on the use of a new type of alkaline agent: the alkanolamines isopropanolamine and diisopropanolamine. The role played by the bases on the particles' size, chemical composition, and magnetic properties was investigated and compared directly with the effect of the traditional inorganic base NaOH. The novel MFe 2 O 4 nanomaterials exhibited high colloidal stability, particle sizes in the range of 4−12 nm, and superparamagnetic properties. More remarkably, they presented smaller particle sizes (up to 6 times) and enhanced saturation magnetization (up to 1.3 times) relative to those prepared with NaOH. Furthermore, the nanomaterials exhibited improved magnetic properties when compared with nanoferrites of similar size synthesized by coprecipitation with other bases or by other methods reported in the literature. The alkanolamines were responsible for these achievements by acting both as alkaline agents and as complexing agents that controlled the particle size during the synthesis process and improved the spin rearrangement at the surface (thinner magnetic "dead" layers). These results open new horizons for the design of waterdispersible MFe 2 O 4 nanoparticles with tuned properties through a versatile and easily scalable coprecipitation route.

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NiO nanoparticles (NPs) were prepared by a sol-gel process using the citrate route. The sol-gel parameters were tuned to obtain samples with different average particle sizes, ranging from 12 to 70 nm. Magnetic characterization revealed an increase in the blocking temperature with the diameter of the NPs and an increase in the effective magnetic anisotropy (K(eff)) with decreasing particle size. The magnetic moment per particle was calculated for all samples using the susceptibility value at T = 300 K. The number of uncompensated spins per NP was found to be proportional to n (n(S)≡ total number of spins), indicating that they are randomly distributed on the NP surface. For small diameters (<30 nm) the surface anisotropy constant was estimated, using, for NiO NPs, a recent model describing the evolution of K(eff) with particle size. Hysteretic loops performed at low temperatures after field cooling displayed loop shifts (∼6.5 kOe in the field axis and ∼0.18 emu g(-1) vertically), coercive field enhancement (H(C)≈ 4.8 kOe) and training effects for the smaller NPs. The sample with NPs of larger diameters presented magnetic properties close to those of bulk NiO.

This work reports a detailed investigation about the physicochemical properties of superparamagnetic gamma-Fe(2)O(3) nanomaterial synthesized by the co-precipitation method and coated with two silica shells, and its application as support for the immobilization of oxovanadium(IV) acetylacetonate ([VO(acac)(2)]). The influence of the silica coatings on the surface composition and physicochemical interactions of the core-shell nanocomposites is discussed based on the combination of several techniques: electron microscopy techniques (SEM and TEM with EDS), DLS, powder XRD, XPS, FTIR and magnetic characterization. The identity of the iron oxide, gamma-Fe(2)O(3), was confirmed by XPS, FTIR and by the Rietveld refinement of the PXRD pattern. The results obtained by electron microscopy techniques, XRD and magnetization indicated that the gamma-Fe(2)O(3) nanoparticles are superparamagnetic and present an average size of approximately 6.5 nm. The first silica coating leads to a core-shell nanomaterial with an average particle size of 21 nm and upon the second coating, the average size increases to 240 nm. Magnetic measurements revealed that the silica-coated nanomaterials maintain the superparamagnetic state at room temperature, although with an expected reduction of the magnetization saturation due to the increase of the silica shell thickness. Furthermore, a numerical fit of the temperature dependence of magnetization was performed to determine the core size distribution and the effect of the silica coatings on the dipolar magnetic interactions. [VO(acac)(2)] was covalently immobilized on the surface of the silica-coated magnetic nanoparticles functionalized with amine groups, as confirmed by chemical analysis and XPS. In a proof-of-principle experiment, we demonstrated the catalytic performance of the novel magnetic hybrid nanomaterial in the epoxidation of geraniol, which presented high selectivity towards the 2,3-epoxygeraniol product and easy recovery by magnetic separation.

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