Anisotropic Magnetic Nanoparticles Research Articles

Silica-coated magnetic nanorods with zwitterionic surface functionalization to overcome non-specific protein adsorption

Anisotropic magnetic nanoparticles are receiving growing interest for biomedical applications due to their interesting properties, particularly for MRI and magnetic hyperthermia. In this work, a two-step, relatively simple synthesis pathway of magnetite nanorods was optimised. A ligand from the catechol family, known for its high iron oxide surface affinity, was used to transfer them into aqueous phase. The magnetite nanorods were then coated with a layer of fluorescent silica, whose surface was functionalised in two ways: either by short poly(ethylene glycol) (PEG) chains or by sulfobetaine zwitterionic molecules. A wide range of techniques were used to thoroughly characterise their physical structure, magnetic properties and dispersion characteristics. Finally, a study of the adsorption of a fluorescent model protein revealed that zwitterionic sulfobetaine molecules were more effective than PEG chains in limiting the non-specific adsorption of this protein.

Development of pH-Sensitive Magnetoliposomes Containing Shape Anisotropic Nanoparticles for Potential Application in Combined Cancer Therapy.

Late diagnosis and systemic toxicity associated with conventional treatments make oncological therapy significantly difficult. In this context, nanomedicine emerges as a new approach in the prevention, diagnosis and treatment of cancer. In this work, pH-sensitive solid magnetoliposomes (SMLs) were developed for controlled release of the chemotherapeutic drug doxorubicin (DOX). Shape anisotropic magnetic nanoparticles of magnesium ferrite with partial substitution by calcium (Mg0.75Ca0.25Fe2O4) were synthesized, with and without calcination, and their structural, morphological and magnetic properties were investigated. Their superparamagnetic properties were evaluated and heating capabilities proven, either by exposure to an alternating magnetic field (AMF) (magnetic hyperthermia) or by irradiation with near-infrared (NIR) light (photothermia). The Mg0.75Ca0.25Fe2O4 calcined nanoparticles were selected to integrate the SMLs, surrounded by a lipid bilayer of DOPE:Ch:CHEMS (45:45:10). DOX was encapsulated in the nanosystems with an efficiency above 98%. DOX release assays showed a much more efficient release of the drug at pH = 5 compared to the release kinetics at physiological pH. By subjecting tumor cells to DOX-loaded SMLs, cell viability was significantly reduced, confirming that they can release the encapsulated drug. These results point to the development of efficient pH-sensitive nanocarriers, suitable for a synergistic action in cancer therapy with magnetic targeting, stimulus-controlled drug delivery and dual hyperthermia (magnetic and plasmonic) therapy.

Covalent Hybrid Elastomers Based on Anisotropic Magnetic Nanoparticles and Elastic Polymers

Particle-filled elastomers are ubiquitous composite materials with applications ranging from durable materials, such as tires, to smart actuators and sensor materials. By employing magnetic nanoparticles as multifunctional, inorganic cross-linkers, PDMS-based hybrid elastomers with a novel unique architecture and a defined type of particle-matrix interaction are obtained—a direct covalent coupling between magnetic and elastic properties. The resulting particle-cross-linked elastomers possess application potential due to their defined magnetomechanical coupling and their large extensibility. As the filler phase, spindle-shaped hematite nanoparticles with a silica shell are used, and the swelling, thermal, magnetic, and mechanical properties of the resulting particle-cross-linked elastomers are systematically evaluated and compared to the properties of analogous yet conventionally cross-linked particle-filled elastomers of a similar composition. Some unique features are found for the hybrid elastomers, such as a large strain at break of up to εB ≈ 1700%, that are attributed to the exceptional architecture, combining a well-integrated network of long polymer chains that are interconnected by network nodes with a high cross-linker functionality formed by the anisotropic magnetic nanoparticles.

A Versatile Interfacial Coassembly Method for Fabrication of Tunable Silica Shells with Radially Aligned Dual Mesopores on Diverse Magnetic Core Nanoparticles.

Anisotropic magnetic nanoparticles with a mesoporous silica shell have the combined merits of a magnetic core and a robust shell. Preparation of magnetically guidable core–shell nanostructures with a robust silica shell that contains well-defined, large, radially aligned silica pores is challenging, and hence this has rarely been described in detail. Herein, a dynamic soft-templating strategy is developed to controllably synthesize hierarchical, dual-mesoporous silica shells on diverse core nanoparticles, in terms of nanoparticle shape (i.e., spherical, chainlike, and disclike), magnetic properties (i.e., hard magnetic and superparamagnetic), and dimensions (i.e., from 3 nm to submicrometers). The developed interfacial coassembly method allows easy design of applicable silica shells containing tunable pore geometries with pore sizes ranging from below 5 nm to above 40 nm, with a specific surface area of 577 m2 g–1 and pore volume of 1.817 cm3 g–1. These are the highest values reported for magnetically guidable anisotropic nanoparticles. The versatility of the method is shown by transfer of the coating procedure to core particles as diverse as spherical superparamagnetic nanoparticles and their clusters as well as by ferromagnetic 3 nm thick hexaferrite nanoplatelets. This method can serve as a general approach for the fabrication of well-designed mesoporous silica coatings on a wide variety of core nanoparticles.

Dynamic spin freezing and magnetic memory effect in ensembles of interacting anisotropic magnetic nanoparticles

We explore the influence of demagnetization interaction on magnetic memory effect by varying organization geometry of anisotropic ZnFe$_2$O$_4$ nanoparticles in an ensemble. The static and dynamic behaviour of two differently organized ensembles, compact ensemble (CE) and hollow core ensemble (HCE), are extensively studied by both dc and ac susceptibility, magnetic memory effect and spin relaxation. The frequency-dependence peak shifting of freezing temperature in both the systems is analyzed properly with the help of two dynamic scaling models: Vogel-Fulcher law and power law. Presence of cluster spin-glass phase is reflected from Vogel-Fulcher temperature $T_0$ $\simeq$ 142.58 K for CE, $\simeq$ 97 K for HCE and characteristic time constant $\tau_0$ $\simeq$ $8.85\times10^{-9}$ s for CE, $\simeq$ $3.8\times10^{-10}$ s for HCE; along with $\delta$T$_{Th}$ $\sim$ 0.1 for CE and 0.2 for HCE. The power law fitting with dynamic exponent $zv'$ = 6.2 $\pm$ 1.1 for CE, 6.3 $\pm$ 0.5 for HCE and single spin flip $\tau^*$ $\simeq$ $7.7\times10^{-11}$ s for CE, $\simeq$ $1.3\times10^{-10}$ s for HCE provide firm confirmation of cluster spin-glass phase. The progressive spin freezing across multiple metastable states with prominent memory effects is reflected in both the systems via nonequilibrium dynamics study. The hollow core geometry with anisotropic nanoparticles on surface with closer proximity leads to complex anisotropy energy landscape with enhanced demagnetizing field resulting highly frustrated surface spins. As a consequence, more prominent magnetic memory effect is observed in HCE with higher activation energy, reduced blocking temperature and enhanced coercivity than that of CE.

Magnetic-field-assisted synthesis of anisotropic iron oxide particles: Effect of pH.

The synthesis of magnetite (Fe3O4) nanorods using reverse co-precipitation of Fe3+ and Fe2+ ions in the presence of a static magnetic field is reported in this work. The phase composition and crystal structure of the synthesized material were investigated using electron diffraction, Raman spectroscopy, and transmission electron microscopy. It was shown that the morphology of the reaction product strongly depends on the amount of OH− ions in the reaction mixture, varying from Fe3O4 nanorods to spherical Fe3O4 nanoparticles. Fe3O4 nanorods were examined using high-resolution transmission electron microscopy proving that they are single-crystalline and do not have any preferred crystallographic orientation along the axis of the rods. According to the data obtained a growth mechanism was proposed for the rods that consists of the dipole–dipole interaction between their building blocks (small hexagonal faceted magnetite nanocrystals), which are formed during the first step of the reaction. The study suggests a facile, green and controllable method for synthesizing anisotropic magnetic nanoparticles in the absence of stabilizers, which is important for further modification of their surfaces and/or incorporation of the nanoparticles into different media.

The relevance of Brownian relaxation as power absorption mechanism in Magnetic Hyperthermia

The Linear Response Theory (LRT) is a widely accepted framework to analyze the power absorption of magnetic nanoparticles for magnetic fluid hyperthermia. Its validity is restricted to low applied fields and/or to highly anisotropic magnetic nanoparticles. Here, we present a systematic experimental analysis and numerical calculations of the specific power absorption for highly anisotropic cobalt ferrite (CoFe2O4) magnetic nanoparticles with different average sizes and in different viscous media. The predominance of Brownian relaxation as the origin of the magnetic losses in these particles is established, and the changes of the Specific Power Absorption (SPA) with the viscosity of the carrier liquid are consistent with the LRT approximation. The impact of viscosity on SPA is relevant for the design of MNPs to heat the intracellular medium during in vitro and in vivo experiments. The combined numerical and experimental analyses presented here shed light on the underlying mechanisms that make highly anisotropic MNPs unsuitable for magnetic hyperthermia.

Directing the orientational alignment of anisotropic magnetic nanoparticles using dynamic magnetic fields.

The structure-directing influence of static and dynamic, i.e. rotating, magnetic fields on the orientational alignment of spindle-type hematite particles with a high aspect ratio is investigated. Structural characterization using electron microscopy and small-angle X-ray scattering confirms a nearly collinear particle arrangement with orientation of the main particle axis either parallel or perpendicular to the substrate as directed by the magnetic field geometry. The combination of large structural and magnetocrystalline anisotropies results in significantly different, strongly anisotropic magnetic properties of the assemblies revealed by directional magnetization measurements.

Hydrodynamic Properties of Magnetic Nanoparticles with Tunable Shape Anisotropy: Prediction and Experimental Verification

We describe the characterization of the hydrodynamic properties of anisotropic magnetic nanoparticles using a combination of transmission electron microscopy (TEM) and dynamic as well as depolarized dynamic light scattering (DLS/DDLS). The particles used are nearly monodisperse hematite spindles with an average length of 280 nm and a minor axis of 57 nm, coated with a layer of silica of variable thickness that allows us to tune the particle aspect ratio between 5 and 2. Their geometrical dimensions can thus be determined easily and quantitatively from TEM. Moreover, their size is ideal to employ DLS and DDLS to measure the translational and rotational diffusion coefficients D(T) and D(R), while the presence of a magnetic core opens a plethora of opportunities for future studies and applications. We demonstrate that we can successfully predict the hydrodynamic properties of the different particles based on a TEM characterization of their size distribution and using established theoretical models for the hydrodynamic properties of anisotropic particles. When compared with the theoretical predictions, our light scattering measurements are in quantitative agreement. This agreement between theory and experiment is achieved without having to invoke any adjustable free parameter, as the TEM results are used to calculate the corresponding diffusion coefficients on an absolute scale. We demonstrate that this is achieved due to a new and simple method for the statistical weighting of the TEM information, and the use of the correct hydrodynamic models for the observed particle shape. In addition, we also demonstrate an enhanced sensitivity of the rotational diffusion for the surface properties of ellipsoidal nanoparticles, and point out that this may serve as an ideal tool toward characterizing functionalized surfaces.

Correlation between tunneling magnetoresistance and magnetization in dipolar-coupled nanoparticle arrays

The tunneling magnetoresistance (TMR) of a hexagonal array of dipolar coupled anisotropic magnetic nanoparticles is studied using a resistor network model and a realistic micromagnetic configuration obtained by Monte Carlo simulations. Analysis of the field-dependent TMR and the corresponding magnetization curve shows that dipolar interactions suppress the maximum TMR effect, increase or decrease the field-sensitivity depending on the direction of applied field and introduce strong dependence of the TMR on the direction of the applied magnetic field. For off-plane magnetic fields, maximum values in the TMR signal are associated with the critical field for irreversible rotation of the magnetization. This behavior is more pronounced in strongly interacting systems (magnetically soft), while for weakly interacting systems (magnetically hard) the maximum of TMR (Hmax) occurs below the coercive field (Hc), in contrast to the situation for non-interacting nanoparticles or in-plane fields (Hmax=Hc). The relation of our simulations to recent TMR measurements in self-assembled Co nanoparticle arrays is discussed.

Synthesis of nanoparticles in Langmuir monolayer

A novel approach to create ultrathin planar nanoparticulate structures based on the controllable synthesis of nanoparticles directly in Langmuir monolayer at the gas/liquid interface and subsequent deposition of monolayer is introduced. In this approach, all initial reagents are present only in monolayer and reactions of nanoparticles synthesis are initiated by decomposition of organometallic compounds under electromagnetic radiation. The changes in the shape of compression isotherm of stearic acid (SA)/iron pentacarbonyl mixed monolayer under the varying of the time of exposure to the ultraviolet light have been studied. STM images of mixed monolayers deposited onto the graphite substrate revealed synthesized iron isotropic and anisotropic magnetic nanoparticles with the characteristic size varied from 20 to 150 Å in dependence on the ultraviolet exposure time and monolayer compression.