Carlos J. Serna

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.

Structural and magnetic properties of uniform magnetite nanoparticles prepared by high temperature decomposition of organic precursors

Magnetite nanoparticles (Fe3O4) of three different sizes below the limit for single domain magnetic behaviour have been obtained by thermal decomposition of an iron precursor in an organic medium in the presence of a surfactant. Good agreement between mean particle size obtained by TEM, crystal size calculated from x-ray diffraction and magnetic diameter calculated from magnetization curves measured at room temperature shows that the samples consist of uniform, crystalline and isolated magnetite nanoparticles with sizes between 5 and 11 nm. High saturation magnetization and high initial susceptibility values have been found, the latter decreasing as the particle size decreases. The main contribution to the anisotropy is magnetocrystalline and shape anisotropy, since surface anisotropy is suppressed by the oleic acid molecules which are covalently bonded to the nanoparticle surface.

The influence of surface functionalization on the enhanced internalization of magnetic nanoparticles in cancer cells

The internalization and biocompatibility of iron oxide nanoparticles surface functionalized with four differently charged carbohydrates have been tested in the human cervical carcinoma cell line (HeLa). Neutral, positive, and negative iron oxide nanoparticles were obtained by coating with dextran, aminodextran, heparin, and dimercaptosuccinic acid, resulting in colloidal suspensions stable at pH 7 with similar aggregate size. No intracellular uptake was detected in cells incubated with neutral charged nanoparticles, while negative particles showed different behaviour depending on the nature of the coating. Thus, dimercaptosuccinic-coated nanoparticles showed low cellular uptake with non-toxic effects, while heparin-coated particles showed cellular uptake only at high nanoparticle concentrations and induced abnormal mitotic spindle configurations. Finally, cationic magnetic nanoparticles show excellent properties for possible in vivo biomedical applications such as cell tracking by magnetic resonance imaging (MRI) and cancer treatment by hyperthermia: (i) they enter into cells with high effectiveness, and are localized in endosomes; (ii) they can be easily detected inside cells by optical microscopy, (iii) they are retained for relatively long periods of time, and (iv) they do not induce any cytotoxicity.

Ordered ferrimagnetic form of ferrihydrite reveals links among structure, composition, and magnetism

The natural nanomineral ferrihydrite is an important component of many environmental and soil systems and has been implicated as the inorganic core of ferritin in biological systems. Knowledge of its basic structure, composition, and extent of structural disorder is essential for understanding its reactivity, stability, and magnetic behavior, as well as changes in these properties during aging. Here we investigate compositional, structural, and magnetic changes that occur upon aging of “2-line” ferrihydrite in the presence of adsorbed citrate at elevated temperature. Whereas aging under these conditions ultimately results in the formation of hematite, analysis of the atomic pair distribution function and complementary physicochemical and magnetic data indicate formation of an intermediate ferrihydrite phase of larger particle size with few defects, more structural relaxation and electron spin ordering, and pronounced ferrimagnetism relative to its disordered ferrihydrite precursor. Our results represent an important conceptual advance in understanding the nature of structural disorder in ferrihydrite and its relation to the magnetic structure and also serve to validate a controversial, recently proposed structural model for this phase. In addition, the pathway we identify for forming ferrimagnetic ferrihydrite potentially explains the magnetic enhancement that typically precedes formation of hematite in aerobic soil and weathering environments. Such magnetic enhancement has been attributed to the formation of poorly understood, nano-sized ferrimagnets from a ferrihydrite precursor. Whereas elevated temperatures drive the transformation on timescales feasible for laboratory studies, our results also suggest that ferrimagnetic ferrihydrite could form naturally at ambient temperature given sufficient time.

Formation of rod-like zinc oxide microcrystals in homogeneous solutions

The formation of zinc oxide microcrystals obtained from hydrolysis of zinc nitrate and zinc chloride in the presence of hexamethylenetetramine has been studied by X-ray diffraction, electron microscopy and IR spectroscopy. Prisms, needles and spherulitic aggregates are formed depending on several factors such as reactant concentrations, pH and temperature. It is shown that the formation of prisms and needles is mainly determined by kinetic conditions with prisms favoured at lower temperatures (< 80 °C), where faces can be developed from the solution complexes. The formation mechanism of the monodispersed prisms and needles occurs by coupling of microcrystalline spheres through their c axes forming embryonic rod particles which later grow. On the other hand, the formation of spherulitic aggregates is more the consequence of solution heterogeneity, i.e. high reactant concentrations.

Structural considerations about SiO2 glasses prepared by sol-gel

Abstract Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios. In the present work the transformation with temperature of these gels has been studied using IR spectroscopy together with measurements of density and specific surface area, thermal analysis and SEM. The IR spectra of these gels, measured as a function of temperature and H2O/TEOS ratio, confirm that the relative concentration of OH groups is temperature dependent. The total OH content, also depends on the H2O/TEOS ratio in the gel. The combined results of IR, density and specific surface area measurements allow some structural considerations to be advanced about the final SiO2 glasses obtained.

Uniform and water stable magnetite nanoparticles with diameters around the monodomain–multidomain limit

A direct method for the preparation of uniform magnetite nanoparticles with sizes around 30?nm and stable in aqueous media at pH 7 has been developed. This method is based on the precipitation of an iron (II) salt (FeSO4) in the presence of a base (NaOH) and a mild oxidant (KNO3). Reaction rate seems to be controlled by the iron salt concentration and the presence of ethanol in the media. Thus lower iron concentration and a water/ethanol ratio equal to one lead to the formation of the smallest particles, 30?nm in diameter. Colloidal suspensions of these particles were directly obtained by simple ultrasonic treatment of the powders leading to very stable ferrofluids at pH 7. Sulphate anions present at the particle surface seem to be responsible for the colloidal stability, providing a biocompatible character to the suspensions. The structural, morphological and magnetic characterization of the nanoparticles is also described and suggests that the smallest particles have a diameter close to the limit between monodomain?multidomain magnetic structure, which could account for the high powder absorption of magnetic fields. According to this calorimetric experiments resulted in specific power absorption rates of ca 80?95?W?g?1, which are among the highest values reported in the literature and make these nanoparticles very interesting for hyperthermia.

The formation of a –Fe 2 O 3 monodispersed particles in solution

Uniform α–Fe 2 O 3 particles of varying axial ratios have been prepared from hydrolyzed ferric chloride solutions at 100 °C. In the absence of phosphate anions, spherical particles were obtained by a mechanism that follows the classical LaMer and Dinegar scheme. However, in the presence of phosphates ellipsoidal particles were observed, with their formation taking place through an aggregation process from smaller primary particles of α–Fe 2 O 3 . It is also shown that all particles are monocrystalline irrespective of their formation mechanism.

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.