Robust approaches for model-free small-angle scattering data analysis.

The small-angle neutron scattering data of nanostructured magnetic samples contain information regarding their chemical and magnetic properties. Often, the first step to access characteristic magnetic and structural length scales is a model-free investigation. However, due to measurement uncertainties and a restricted q range, a direct Fourier transform usually fails and results in ambiguous distributions. To circumvent these problems, different methods have been introduced to derive regularized, more stable correlation functions, with the indirect Fourier transform being the most prominent approach. Here, the indirect Fourier transform is compared with the singular value decomposition and an iterative algorithm. These approaches are used to determine the correlation function from magnetic small-angle neutron scattering data of a powder sample of iron oxide nanoparticles; it is shown that with all three methods, in principle, the same correlation function can be derived. Each method has certain advantages and disadvantages, and thus the recommendation is to combine these three approaches to obtain robust results.

Time-dependent AC magnetometry and chain formation in magnetite: the influence of particle size, initial temperature and the shortening of the relaxation time by the applied field.

Magnetite nanoparticles (MNPs) with 12, 34 and 53 nm sizes have been measured by AC-magnetometry at 50 kHz and 57 mT maximum applied field. The MNPs form chains under the AC-field, and the dynamics of the formation can be studied by measuring hysteresis cycles at different times. The measurement time has been varied from 5 ms to 10 s and for different initial temperatures of 5, 25 and 50 °C. The chain formation, identified by the increase of susceptibility and remanence with the measurement time, appears only for 34 nm particles. It has been observed that saturation, remanence and susceptibility at low (high) fields increase (decrease) with time. For the other two samples, these magnitudes are independent of time. At low fields, the heating efficiency is higher at 5 °C than at 50 °C, whereas it shows an opposite behaviour at higher fields; the origin of this behaviour is discussed in the article. Additionally, the relaxation times, τ N and τ B, have been calculated by considering the influence of the applied field. Chain formation requires translation and rotation of MNPs; therefore, the Brownian mechanism plays a fundamental role. It is found that magnetic reversal for 12 nm MNPs is mainly due to Néel relaxation. However, in the case of 34 nm MNPs, both mechanisms, Néel and Brownian relaxation, can be present depending on the amplitude of the field; for µ 0 H < 22 mT, the physical rotation of the particle is the dominant mechanism; on the other hand, for µ 0 H > 22 mT, both mechanisms are present within the size distribution. This highlights the importance of taking the field intensity into account to calculate relaxation times when analysing the relaxation mechanisms of magnetic colloids subjected to AC fields.

Improving the reliability of the iron concentration quantification for iron oxide nanoparticle suspensions: a two-institutions study.

Most iron oxide nanoparticles applications, and in special biomedical applications, require the accurate determination of iron content as the determination of particle properties from measurements in dispersions is strongly dependent on it. Inductively coupled plasma (ICP) and spectrophotometry are two typical worldwide used analytical methods for iron concentration determination. In both techniques, precise determination of iron is not straightforward and nanoparticle digestion and dilution procedures are needed prior to analysis. The sample preparation protocol has been shown to be as important as the analytical method when accuracy is aimed as many puzzling reported results in magnetic, colloidal, and structural properties are simply attributable to inadequate dissolution procedures. Therefore, a standard sample preparation protocol is needed to ensure the adequate and complete iron oxide nanoparticle dissolution and to harmonize this procedure. In this work, an interlaboratory evaluation of an optimized iron oxide nanoparticle digestion/dilution protocol was carried out. The presented protocol is simple, inexpensive, and does not involve any special device (as microwave, ultrasound, or other high-priced digestion devices). Then, iron concentration was measured by ICP-OES (performed in ICMM/CSIC-Spain) and spectrophotometry (NanoPET-Germany) and the obtained concentration values were analyzed to determine the most probable error causes. Uncertainty values as low as 1.5% were achieved after the optimized method was applied. Moreover, this article provides a list of recommendations to significantly reduce uncertainty in both sample preparation and analysis procedures. Graphical abstract ᅟ.

High Frequency Hysteresis Losses on γ-Fe2O3 and Fe3O4: Susceptibility as a Magnetic Stamp for Chain Formation.

In order to understand the properties involved in the heating performance of magnetic nanoparticles during hyperthermia treatments, a systematic study of different γ-Fe2O3 and Fe3O4 nanoparticles has been done. High-frequency hysteresis loops at 50 kHz carried out on particles with sizes ranging from 6 to 350 nm show susceptibility χ increases from 9 to 40 for large particles and it is almost field independent for the smaller ones. This suggests that the applied field induces chain ordering in large particles but not in the smaller ones due to the competition between thermal and dipolar energy. The specific absorption rate (SAR) calculated from hysteresis losses at 60 mT and 50 kHz ranges from 30 to 360 W/gFe, depending on particle size, and the highest values correspond to particles ordered in chains. This enhanced heating efficiency is not a consequence of the intrinsic properties like saturation magnetization or anisotropy field but to the spatial arrangement of the particles.

Field-dependent dynamic responses from dilute magnetic nanoparticle dispersions.

The response of magnetic nanoparticles (MNPs) to an oscillating magnetic field outside the linear response region is important for several applications including magnetic hyperthermia, magnetic resonance imaging and biodetection. The size and magnetic moment are two critical parameters for the performance of a colloidal MNP dispersion. We present and demonstrate the use of optomagnetic (OM) and AC susceptibility (ACS) measurements vs. frequency and magnetic field strength to obtain the size and magnetic moment distributions including the correlation between the distributions. The correlation between the size and the magnetic moment contains information on the morphology and intrinsic structure of the particle. In OM measurements, the variation of the second harmonic light transmission through a dispersion of MNPs is measured in response to an oscillating magnetic field. We solve the Fokker-Planck equations for MNPs with a permanent magnetic moment, and develop analytical approximations to the ACS and the OM signals that also account for the change in the curve shapes with increasing field strength. Further, we describe the influence of induced magnetic moments on the signals, by solving the Fokker-Planck equation for particles, which apart from the permanent magnetic moment may also have an induced magnetic moment and shape anisotropy. Using the results from the Fokker-Planck calculations we fit ACS and OM measurements on two multi-core particle systems. The obtained fit parameters also describe the correlations between the magnetic moment and size of the particles. From such an analysis on a commercially available polydisperse multicore particle system with an average particle size of 80 nm, we find that the MNP magnetic moment is proportional to the square root of the hydrodynamic size.

Time-course assessment of the aggregation and metabolization of magnetic nanoparticles.

To successfully develop biomedical applications for magnetic nanoparticles, it is imperative that these nanoreagents maintain their magnetic properties in vivo and that their by-products are safely metabolized. When placed in biological milieu or internalized into cells, nanoparticle aggregation degree can increase which could affect magnetic properties and metabolization. To evaluate these aggregation effects, we synthesized citric acid-coated iron oxide nanoparticles whose magnetic susceptibility can be modified by aggregation in agar dilutions and dextran-layered counterparts that maintain their magnetic properties unchanged. Macrophage models were used for in vitro uptake and metabolization studies, as these cells control iron homeostasis in the organism. Electron microscopy and magnetic susceptibility studies revealed a cellular mechanism of nanoparticle degradation, in which a small fraction of the particles is rapidly degraded while the remaining ones maintain their size. Both nanoparticle types produced similar iron metabolic profiles but these profiles differed in each macrophage model. Thus, nanoparticles induced iron responses that depended on macrophage programming. In vivo studies showed that nanoparticles susceptible to changes in magnetic properties through aggregation effects had different behavior in lungs, liver and spleen. Liver ferritin levels increased in these animals showing that nanoparticles are degraded and their by-products incorporated into normal metabolic routes. These data show that nanoparticle iron metabolization depends on cell type and highlight the necessity to assess nanoparticle aggregation in complex biological systems to develop effective in vivo biomedical applications. STATEMENT OF SIGNIFICANCE: Magnetic iron oxide nanoparticles have great potential for biomedical applications. It is however imperative that these nanoreagents preserve their magnetic properties once inoculated, and that their degradation products can be eliminated. When placed in a biological milieu nanoparticles can aggregate and this can affect their magnetic properties and their degradation. In this work, we showed that iron oxide nanoparticles trigger the iron metabolism in macrophages, the main cell type involved in iron homeostasis in the organism. We also show that aggregation can affect nanoparticle magnetic properties when inoculated in animal models. This work confirms iron oxide nanoparticle biocompatibility and highlights the necessity to assess in vivo nanoparticle aggregation to successfully develop biomedical applications.

SAXS analysis of single- and multi-core iron oxide magnetic nanoparticles.

This article reports on the characterization of four superparamagnetic iron oxide nanoparticles stabilized with dimercaptosuccinic acid, which are suitable candidates for reference materials for magnetic properties. Particles p1 and p2 are single-core particles, while p3 and p4 are multi-core particles. Small-angle X-ray scattering analysis reveals a lognormal type of size distribution for the iron oxide cores of the particles. Their mean radii are 6.9 nm (p1), 10.6 nm (p2), 5.5 nm (p3) and 4.1 nm (p4), with narrow relative distribution widths of 0.08, 0.13, 0.08 and 0.12. The cores are arranged as a clustered network in the form of dense mass fractals with a fractal dimension of 2.9 in the multi-core particles p3 and p4, but the cores are well separated from each other by a protecting organic shell. The radii of gyration of the mass fractals are 48 and 44 nm, and each network contains 117 and 186 primary particles, respectively. The radius distributions of the primary particle were confirmed with transmission electron microscopy. All particles contain purely maghemite, as shown by X-ray absorption fine structure spectroscopy.

Degradation of magnetic nanoparticles mimicking lysosomal conditions followed by AC susceptibility.

BACKGROUND: A deeper knowledge on the effects of the degradation of magnetic nanoparticles on their magnetic properties is required to develop tools for the identification and quantification of magnetic nanoparticles in biological media by magnetic means. METHODS: Citric acid and phosphonoacetic acid-coated magnetic nanoparticles have been degraded in a medium that mimics lysosomal conditions. Magnetic measurements and transmission electron microscopy have been used to follow up the degradation process. RESULTS: Particle size is reduced significantly in 24 h at pH 4.5 and body temperature. These transformations affect the magnetic properties of the compounds. A reduction of the interparticle interactions is observed just 4 h after the beginning of the degradation process. A strong paramagnetic contribution coming from the degradation products appears with time. CONCLUSIONS: A model for the in vivo degradation of magnetic nanoparticles has been followed to gain insight on the changes of the magnetic properties of iron oxides during their degradation. The degradation kinetics is affected by the particle coating, in our case being the phosphonoacetic acid-coated particles degraded faster than the citric acid-coated ones.

Ultrasmall iron oxide nanoparticles for biomedical applications: improving the colloidal and magnetic properties.

A considerable increase in the saturation magnetization, M(s) (40%), and initial susceptibility of ultrasmall (<5 nm) iron oxide nanoparticles prepared by laser pyrolysis was obtained through an optimized acid treatment. Moreover, a significant enhancement in the colloidal properties, such as smaller aggregate sizes in aqueous media and increased surface charge densities, was found after this chemical protocol. The results are consistent with a reduction in nanoparticle surface disorder induced by a dissolution-recrystallization mechanism.

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.