T. González-Carreño

The preparation of magnetic nanoparticles for applications in biomedicine

This review is focused on describing state-of-the-art synthetic routes for the preparation of magnetic nanoparticles useful for biomedical applications. In addition to this topic, we have also described in some detail some of the possible applications of magnetic nanoparticles in the field of biomedicine with special emphasis on showing the benefits of using nanoparticles. Finally, we have addressed some relevant findings on the importance of having well-defined synthetic routes to produce materials not only with similar physical features but also with similar crystallochemical characteristics.

Progress in the preparation of magnetic nanoparticles for applications in biomedicine

This review summarizes recent advances in synthesis routes for quickly and reliably making and functionalizing magnetic nanoparticles for applications in biomedicine. We put special emphasis on describing synthetic strategies that result in the production of nanosized materials with well-defined physical and crystallochemical characteristics as well as colloidal and magnetic properties. Rather than grouping the information according to the synthetic route, we have described methods to prepare water-dispersible equiaxial magnetic nanoparticles with sizes below about 10 nm, sizes between 10 and 30 nm and sizes around the monodomain–multidomain magnetic transition. We have also described some recent examples reporting the preparation of anisometric nanoparticles as well as methods to prepare magnetic nanosized materials other than iron oxide ferrites, for example Co and Mn ferrite, FePt and manganites. Finally, we have described examples of the preparation of multicomponent systems with purely inorganic or organic–inorganic characteristics.

Barium ferrite nanoparticles prepared directly by aerosol pyrolysis

BaFe12O19 nanoparticles, 10 nm in diameter, have been obtained by combination of two methods, the citrate precursor and the aerosol pyrolysis technique. For the first time, well-crystallised barium ferrite particles were obtained by pyrolysis of an aerosol, produced by ultrasonic frequency spraying of a barium iron citrate aqueous solution, in a tubular furnace at 1000°C, without further heat treatment. The reason why the hexaferrite phase forms at lower temperatures than by other methods is the nature of the precursor aerosol solution. The particle size was increased up to 100 nm in diameter by heat treatment at 1000°C in an oven. The obtained particles are spherical aggregates of 400 nm, which can be easily disaggregated by grinding in a mortar. Changes in particle size and aggregation state are reflected as changes in the magnetic properties. Saturation magnetisation and coercivity values obtained for the largest particles were similar to those found for commercial pigments, 50 emu/g and 5600 Oe, respectively.

Preparation of uniform γ-Fe2O3 particles with nanometer size by spray pyrolysis

Aerosol droplets of dilute alcoholic solutions of Fe(III) and Fe(II) salts were used to synthesize γ-Fe2O3 powders by spray pyrolysis at 500°C. A wide variety of particle morphologies with sizes ranging from 5 to 60 nm were obtained depending on the nature of the precursor solution. Aggregated particles of γ-Fe2O3 with a medium diameter of 6 nm forming dense spheres were obtained from nitrate solutions. However, γ-Fe2O3 obtained from acetylacetonate solutions gives monodispersed particles of about 5 nm in diameter. The morphology of the maghemite particles derived from Fe(II) ammonium citrate appears as hollow spheres with a medium diameter of 170 nm, whereas γ-Fe2O3 particles with high degree of crystallinity were obtained from Fe(III) chloride solutions. Mossbauer spectra of the samples show the absence of Fe2+ species corroborating the formation of γ-Fe2O3 particles under the conditions studied.

Comparative study of ferrofluids based on dextran-coated iron oxide and metal nanoparticles for contrast agents in magnetic resonance imaging

Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10?nm) has been obtained with aggregate sizes of ?nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45?emu?g?1) and susceptibilities (4000?emu/g?T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T2 and the corresponding relaxivity R2 value that is of the order of 400?(mmol/l)?1?s?1. It has been shown that R2 value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.

Cytokine adsorption/release on uniform magnetic nanoparticles for localized drug delivery.

Attachment of cytokines to magnetic nanoparticles has been developed as a system for controlled local drug release in cancer therapy. We studied the adsorption/release of murine interferon gamma (IFN-gamma) on negatively charged magnetic nanoparticles prepared by three different methods, including coprecipitation, decomposition in organic media, and laser pyrolysis. To facilitate IFN-gamma adsorption, magnetic nanoparticles were surface modified by distinct molecules to achieve high negative charge at pH 7, maintaining small aggregate size and stability in biological media. We analyzed carboxylate-based coatings and studied the colloidal properties of the resulting dispersions. Finally, we incubated the magnetic dispersions with IFN-gamma and determined optimal conditions for protein adsorption onto the particles, as well as the release capacity at different pH and as a function of time. Particles prepared by decomposition in organic media and further modified with dimercaptosuccinic acid showed the most efficient adsorption/release capacity. IFN-gamma adsorbed on these nanoparticles would allow concentration of this protein or other biomolecules at specific sites for treatment of cancer or other diseases.

Signatures of clustering in superparamagnetic colloidal nanocomposites of an inorganic and hybrid nature.

The individual and co-operative properties of inorganic and hybrid superparamagnetic colloidal nanocomposites that satisfy all the requirements of magnetic carriers in the biosciences and/or catalysis fields are been studied. Essential to the success of this study is the selection of suitable synthetic routes (aerosol and nanocasting) that allow the preparation of materials with different matrix characteristics (carbon, silica, and polymers with controlled porosity). These materials present magnetic properties that depend on the average particle size and the degree of polydispersity. Finally, the analysis of the co-operative behavior of samples allows for the detection of signatures of clustering, which are closely related to the textural characteristics of samples and the methodology used to produce the magnetic carriers.

Preparation by pyrolysis of aerosols and structural characterization of Fe-doped mullite powders

Amorphous Fe-doped 2SiO2·3Al2O3 powders were prepared by pyrolysis of aerosols generated from solutions of TEOS, Al(III) nitrate, and Fe(III) nitrate in methanol. The study of the thermal evolution of these powders revealed that the presence of Fe(III) promotes mullite formation and that these cations enter into the mullite lattice at the first stages of crystallization at ∼900°C. It was also found that the amount of Fe(III) in solid solution with mullite increased as increasing temperature from 900 to 1400°C. The limit of Fe(III) incorporation into mullite in the as-prepared powders was in the range previously reported for samples prepared by the ceramic procedure (∼11 Fe2O3 wt%). The additional iron phase appearing in samples with an iron content above this limit consisted of an iron oxide spinel, which probably contained some Al(III) cations in solid solution. Finally, the analysis of the EXAFS spectra of the heated samples showed that the iron cations incorporated into the mullite lattice are mainly located at octahedral positions.

Magnetic properties of γ-Fe2O3 small particles prepared by spray pyrolysis

Abstract Small particles of γ-Fe2O3 synthesized by spray pyrolysis have been studied by ac susceptibility. They are made out of nanometric crystallites resulting in dense and hollow spherical aggregates. The results show the transition from the superparamagnetic to blocked state. The anisotropy and magnetic moment per aggregate are less than for single particles of the same dimensions.

Structural and Magnetic Properties of Fine γ-Fe2O3 Particles

The magnetic properties of γ-Fe2O3 uniform particles prepared by two different methods; from α-Fe2O3 precursors obtained by homogenous hydrolysis and directly, by spray pyrolysis of iron salt solutions, are analysed and related to their structural characteristics. In the first case, the γ-Fe2O3 particles, obtained by reduction-oxidation from the corresponding α-Fe2O3 precursors, showed different degrees of order in the vacancies distribution depending on the presence of defects inside the α-Fe2O3 particles. It is shown that both the saturation magnetisation and the coercive force decrease with increasing the disorder distribution of the vacancies in γ-Fe2O3.