Single-domain Magnetic Nanoparticles Research Articles

Magnetic nanoparticles in square-wave fields for breakthrough performance in hyperthermia and magnetic particle imaging

Driving immobilized, single-domain magnetic nanoparticles at high frequency by square wave fields instead of sinusoidal waveforms leads to qualitative and quantitative improvements in their performance both as point-like heat sources for magnetic hyperthermia and as sensing elements in frequency-resolved techniques such as magnetic particle imaging and magnetic particle spectroscopy. The time evolution and the frequency spectrum of the cyclic magnetization of magnetite nanoparticles with random easy axes are obtained by means of a rate-equation method able to describe time-dependent effects for the particle sizes and frequencies of interest in most applications to biomedicine. In the presence of a high-frequency square-wave field, the rate equations are shown to admit an analytical solution and the periodic magnetization can be therefore described with accuracy, allowing one to single out effects which take place on different timescales. Magnetic hysteresis effects arising from the specific features of the square-wave driving field results in a breakthrough improvement of both the magnetic power released as heat to an environment in magnetic hyperthermia treatments and the magnitude of the third harmonic of the frequency spectrum of the magnetization, which plays a central role in magnetic particle imaging.

Study of biopolymer encapsulated Eu doped Fe3O4 nanoparticles for magnetic hyperthermia application

An exciting prospect in the field of magnetic fluid hyperthermia (MFH) has been the integration of noble rare earth elements (Eu) with biopolymers (chitosan/dextran) that have optimum structures to tune specific effects on magnetic nanoparticles (NPs). However, the heating efficiency of MNPs is primarily influenced by their magnetization, size distribution, magnetic anisotropy, dipolar interaction, amplitude, and frequency of the applied field, the MNPs with high heating efficiency are still challenging. In this study, a comprehensive experimental analysis has been conducted on single-domain magnetic nanoparticles (SDMNPs) for evaluating effective anisotropy, assessing the impact of particle-intrinsic factors and experimental conditions on self-heating efficiency in both noninteracting and interacting systems, with a particular focus on the dipolar interaction effect. The study successfully reconciles conflicting findings on the interaction effects in the agglomeration and less agglomerated arrangements for MFH applications. The results suggest that effective control of dipolar interactions can be achieved by encapsulating Chitosan/Dextran in the synthesized MNPs. The lower dipolar interactions successfully tune the self-heating efficiency and hold promise as potential candidates for MFH applications.

Insights into thermally-induced disruption of magnetic-nanoparticle agglomerates

We investigated the temperature dependence of agglomeration systems consisting of interacting single-domain magnetic nanoparticles (MNPs). In our study, we utilized electron spin resonance (ESR) experiments to measure how temperature-induced change in agglomerate state affects the peak-to-peak width of ESR spectra (Hpp) and validated these observations through simulations. Micromagnetic simulations were conducted by combining the classical magnetic dipolar Metropolis Monte Carlo method with the semi-classical macro-spin Landau-Lifshitz-Gilbert finite element method, and the results were compared with experimental findings. Our research elucidates the behavior of thermally-induced disruption of MNP agglomerates. We discovered that the temperature at which MNPs are completely dispersed might correspond to one at which the temperature dependence of Hpp disappears.

A Study of Hyaluronic Acid's Theoretical Reactivity and of Magnetic Nanoparticles Capped with Hyaluronic Acid.

Hyaluronic acid (HA) has attracted much attention in tumor-targeted drug delivery due to its ability to specifically bind to the CD44 cellular receptor, which is widely expressed on cancer cells. We present HA-capped magnetic nanoparticles (HA-MNPs) obtained via the co-precipitation method, followed by the electrostatic adsorption of HA onto the nanoparticles' surfaces. A theoretical study carried out with the PM3 method evidenced a dipole moment of 3.34 D and negatively charged atom groups able to participate in interactions with nanoparticle surface cations and surrounding water molecules. The ATR-FTIR spectrum evidenced the hyaluronic acid binding to the surface of the ferrophase, ensuring colloidal stability in the water dispersion. To verify the success of the synthesis and stabilization, HA-MNPs were also characterized using other investigation techniques: TEM, EDS, XRD, DSC, TG, NTA, and VSM. The results showed that the HA-MNPs had a mean physical size of 9.05 nm (TEM investigation), a crystallite dimension of about 8.35 nm (XRD investigation), and a magnetic core diameter of about 8.31 nm (VSM investigation). The HA-MNPs exhibited superparamagnetic behavior, with the magnetization curve showing saturation at a high magnetic field and a very small coercive field, corresponding to the net dominance of single-domain magnetic nanoparticles that were not aggregated with reversible magnetizability. These features satisfy the requirement for magnetic nanoparticles with a small size and good dispersibility for long-term stability. We performed some preliminary tests regarding the nanotoxicity in the environment, and some chromosomal aberrations were found to be induced in corn root meristems, especially in the anaphase and metaphase of mitotic cells. Due to their properties, HA-MNPs also seem to be suitable for use in the biomedical field.

Dynamic hysteretic properties and specific loss power of magnetic nanoparticles in a viscous medium at different thermal baths

A system of magnetite single-domain magnetic nanoparticles in an aqueous colloidal suspension at different temperatures is simulated. In this framework, we study the response of the magnetization of the system to the presence of a time-dependent magnetic field at certain frequency. To do that, a Hamiltonian that includes the Zeeman interaction and the uniaxial magneto-crystalline anisotropy energy is considered. The dynamics of the system is driven according to the solution of the stochastic Landau–Lifshitz–Gilbert differential equation, in combination with a torque term acting on the nanoparticle in order to consider the Brown rotation influenced by the viscosity of the solvent. Thus, both Néel and Brown rotation mechanisms are contemplated. Our results allow us to conclude that Brownian relaxation mediates the alignment of the anisotropy axes with the external field. Thus, depending on the solid or liquid state of the solvent, it is possible to determine the conditions under which a reinforcement of the magnetic anisotropy can take place in order to increase the remanence and squareness of the hysteresis loops.

In vitro evaluation of magnetic fluid hyperthermia therapy on breast cancer cells using monodispersed Mn0.5Zn0.5Fe2O4 nanoflowers

Magnetic fluid hyperthermia (MFH) is emerging as an alternate treatment therapy for cancer of different types, due to its potential low side effects and relatively less painless treatment option. However, the implementation of standalone therapy is still challenging and needs research to optimize the effective killing of 100% cancerous tissues, number of therapy sessions, time duration of the therapy, controlled hyperthermia window temperature, etc. We report here the potential of nanoclusters (nanoflowers) based dispersion and on an in vitro study on breast cancer cells (MDA-MB-231). The nanoclusters were characterized using XRD, TEM, DLS, VSM and MFH, before investigating them with breast cancer cells. The effect of MFH, before and after the treatment of induction heating on MDA-MB-231 cells, were also studied. Results show that the MFH is capable of killing almost 87% of cells within 30 min of treatment on cells in the presence of magnetic nanoclusters. This shows the potential of developing nanoflowers for the treatment of cancer over other single domain magnetic nanoparticles.

Dynamic magnetic properties of magnetosomes

Magnetosomes are found in magnetotactic bacteria, and consist of linear chains of tens of single-domain magnetic nanoparticles of (magnetite) or (greigite), enclosed in a lipid bilayer membrane. The particles are typically in the size range 30–, and are practically monodisperse. Harnessing the power of biomineralisation could lead to efficient strategies for synthesising semiflexible dipolar filaments, and the development of optimum materials for applications, including in biomedicine. Brownian dynamics simulations of noninteracting magnetosomes, containing ferromagnetic nanoparticles, have been used to determine static properties, and the dynamical response to a weak AC magnetic field. Results are presented for the radius of gyration , the static magnetic susceptibility , the dynamic magnetic susceptibility , and the effective Brownian rotation time , all as functions of N. The results are compared to theoretical predictions of the flexibility, susceptibility, and rotational dynamics of such magnetic filaments.

Role of site selective substitution, magnetic parameter tuning, and self heating in magnetic hyperthermia application: Eu-doped magnetite nanoparticles.

Various researchers have provided considerable insight into the fundamental mechanisms behind the power absorption of single-domain magnetic nanoparticles (MNPs) in magnetic hyperthermia applications. However, the role of all parameters pertinent to magnetic relaxation continues to be debated. Herein, to explore the role of magnetic anisotropy with the site selective substitution related to magnetic relaxation has generally been missing, which is critically essential in respective of hyperthermia treatment. Our study unravels contradictory results of rare earth (RE) interaction effects in ferrite to that of recently reported literature. Despite this, rare earth atoms have unique f-block properties, which significantly impact the magnetic anisotropy as well as the relaxation mechanism. Here, we use appropriate Eu doping concentration in magnetite and analyze its effect on the matrix. Furthermore, a positive SAR can effectively reduce the relative dose assigned to a patient to a minimal level. This study indicates that the introduction of Eu ion positively influenced the heating efficiency of the examined magnetite systems.

Bacterial Magnetosomes Release Iron Ions and Induce Regulation of Iron Homeostasis in Endothelial Cells.

Magnetosomes (MAGs) extracted from magnetotactic bacteria are well-defined membrane-enveloped single-domain magnetic nanoparticles. Due to their superior magnetic and structural properties, MAGs constitute potential materials that can be manipulated via genetic and chemical engineering for use in biomedical and biotechnological applications. However, the long-term effects exerted by MAGs on cells are of concern in the context of in vivo applications. Meanwhile, it remains relatively unclear which mechanisms are employed by cells to process and degrade MAGs. Hence, a better understanding of MAGs' degradation and fundamental signal modulations occurring throughout this process is essential. In the current study, we investigated the potential actions of MAGs on endothelial cells over a 10-day period. MAGs were retained in cells and found to gradually gather in the lysosome-like vesicles. Meanwhile, iron-ion release was observed. Proteomics further revealed a potential cellular mechanism underlying MAGs degradation, in which a group of proteins associated with vesicle biogenesis, and lysosomal enzymes, which participate in protein hydrolysis and lipid degradation, were rapidly upregulated. Moreover, the released iron triggered the regulation of the iron metabolic profiles. However, given that the levels of cell oxidative damage were relatively stable, the released iron ions were handled by iron metabolic profiles and incorporated into normal metabolic routes. These results provide insights into the cell response to MAGs degradation that may improve their in vivo applications.

Magnetic relaxation of a system of interacting magnetic nanoparticles at finite temperature

We investigate the magnetic relaxation of a system of interacting single-domain magnetic nanoparticles, where the uniaxial energy density depends on temperature. Particles of different sizes are placed in random positions along a linear chain. For each value of temperature, the stochastic Landau–Lifshitz–Gilbert equation is numerically integrated. Applying an appropriate scaling function we determine the magnetic relaxation as a function of time and temperature as well as the energy barrier distribution of the system as a function of the mean distance between neighbor particles. We show that the typical energy barrier is not a monotonic function of the dipolar coupling. It decreases with the separation between particles until it reaches a minimum. For weaker dipolar interactions, the antiferromagnetic part could be responsible for enhancing the local uniaxial energy barriers. The decrease of the energy barrier for small values of the dipolar coupling was seen experimentally for the magnetoferritin nanoparticle.

A theoretical examination of localized nanoscale induction by single domain magnetic particles

Single domain magnetic nanoparticles are increasingly investigated as actuators of biological and chemical processes that respond to externally applied magnetic fields. Although their localized effects have often been attributed to nanoscale heating, recent experimental evidence suggests the need to consider alternative hypotheses. Here, using the stochastic Landau–Lifshitz–Gilbert equation and finite element modeling, we investigate and critically examine an alternative hypothesis that localized effects may instead involve the induced electric fields arising from the dynamical behavior of individual single domain magnetic particles. We model the magnetization dynamics and resulting induced electric fields for two relevant and distinct cases of magnetic nanoparticles in alternating magnetic fields: (1) magnetogenetic stimulation of channel proteins associated with ferritin and (2) catalytic enhancement of electrochemical hydrolysis. For the first case, while the local electric fields that ferritin generates are shown to be insufficient to perturb the transmembrane potential, fields on the surface of its mineral core on the order of 102–103 V/m may play a more plausible role in mass transport of iron ions that indirectly lead to stimulation. For the second case, our model indicates that the highest interfacial electric field strengths, on the order of 102 V/m, are expected during reversal events. Thus, nanoparticles well suited for hysteresis heating can also act as intermittent sources of localized induced electric fields in response to an alternating applied field. Finally, we compare the magnitude and timescale of these electric fields to technologically relevant phenomena, showing that they are generally weaker and faster.

Structural and magnetic equilibrium properties of a semi-dilute suspension of magnetic multicore nanoparticles

The idea to create smart magnetically controllable nanostructured materials is highly appealing from both fundamental science point of view and application perspectives. Ample investigations of single-domain magnetic nanoparticles embedded in liquid and gel-like magneto-passive carriers revealed a plethora of new phenomena from controllable equilibrium self-assembly to a designed heat dissipation on the application of AC magnetic fields. However, all the effects in such systems are limited by the physical energy and entropy scales of nanoparticles and are very sensitive to temperature changes and confinement. As a result, among new types of complex magnetic materials, magnetic multicore nanoparticles (MMNPs) made of rigidly bound single-domain nanograins were proposed. The properties of such systems are resulting from inter-grain and grain-field interactions, and are not yet fully understood. In this study, employing Langevin dynamics computer simulations, we analyse the interplay of the aforementioned interactions in a dilute suspension of MMNPs made of grains with negligible magnetic anisotropy. We find that the stronger are the inter-grain interactions the lower is the initial magnetic response of MMNPs. The response is though hindered, rather than suppressed: at a certain intensity of an applied magnetic field a sufficient degree of the alignment of the grain magnetic moments and the field is achieved and a large-scale self-assembly accompanied by a rapid growth of magnetisation is observed. The strength of interactions essential for this crossover is much higher than that for a suspension of single-domain grains with the same concentration and magnetic saturation.

Deep Eutectic Solvent Functionalized Graphene Oxide Based Ferrofluid for the Liquid Phase Microextraction of Fluoroquinolones from Water Samples

The development of smart materials have a significant impact on sample preparation and preconcentration methods. Ferrofluid or magnetic fluids (FF) are smart colloidal suspensions of single domain magnetic nanoparticles in a polar or non-polar liquid carrier. In this study, graphene oxide magnetite (GO@Fe3O4) and deep eutectic solvent-based choline chloride and ethylene glycol as a carrier liquid were utilized to form GO@Fe3O4-DES FF. The synthesised GO@Fe3O4-DES FF was characterized using FTIR, SEM, TEM and vibrating sample magnetometer (VSM). GO@Fe3O4-DES FF was further developed for the application of GO@Fe3O4-DES FF-liquid phase microextraction (LPME) for enrofloxacin as test compound. Several parameters such as type of FF composition and volume, extraction time, desorption solvent volume, desorption time and solution pH were optimised and analysed using HPLC-UV. Under optimum conditions, the developed GO@Fe3O4-DES FF-LPME method showed good linearity, R2 ≥ 0.9921, repeatability, RSD 0.57 – 9.40 %. The developed GO@Fe3O4-DES FF-LPME method was applied for the determination of enrofloxacin in water samples from Langat River Basin, Selangor and the recovery of 71.6 – 112.3% was obtained. In conclusion, the developed GO@Fe3O4-DES FF-LPME method for the determination of enrofloxacin showed excellent sensitivity and precision and may be an excellent alternative method for the extraction on water samples.

Advanced characterization of magnetization dynamics in iron oxide magnetic nanoparticle tracers.

Characterization of the magnetization dynamics of single-domain magnetic nanoparticles (MNPs) is important for magnetic particle imaging (MPI), magnetic resonance imaging (MRI), and emerging medical diagnostic/therapeutic technologies. Depending on particle size and temperature, nanoparticle magnetization relaxation time constants span from nanoseconds to seconds. In solution, relaxation occurs via coupled Brownian and Néel relaxation mechanisms. Even though their coexistence complicates analysis, the presence of two timescales presents opportunities for more direct control of magnetization behavior if the two processes can be understood, isolated, and tuned. Using high frequency coils and sample temperature tunability, we demonstrate unambiguous determination of the specific relaxation processes for iron oxide nanoparticles using both time and frequency domain techniques. Furthermore, we study the evolution of the fast dynamics at ≈ 10 nanosecond timescales, for magnetic field amplitudes relevant to MPI.

μSR-Study of a 3% CoFe2O4 Nanoparticle Concentration Ferrofluid

Magnetic fluids based on single-domain magnetic spinel ferrite nanoparticles dispersed in various liquid media are of particular practical and scientific interest. This paper presents a muon spectroscopy study of a ferrofluid based on magnetic nanoparticles of CoFe2O4 molecules dispersed in water (H2O) with a nanoparticle concentration of 3%. In this study, it was determined that the structure and magnitude of the magnetization of a ferrofluid depend on the viscosity of the liquid itself. It was shown that, at room temperature (290 K) and under an external magnetic field of 527 G, the observed additional magnetization was ~20 G. In a small fraction of the sample under study (~20%), negative magnetization (diamagnetism) was observed. At low temperatures (~30 K), the sample acted as a paramagnet in a magnetic field. For the first time, the magnetic field inside and in the immediate vicinity of a CoFe2O4 nanoparticle has been measured experimentally using the μSR method: the value was 1.96 ± 0.44 kG; thus, direct measurement of the magnetization of a nanoscale object was performed.

Thermal relaxation in a disordered one-dimensional array of interacting magnetic nanoparticles

We consider a system of single-domain magnetic nanoparticles placed along a linear chain and address the effect of the distance between them on the magnetic and thermal relaxation properties of the system. The nanoparticles interact with each other through the dipolar interaction and we introduce disorder into the system by allowing random sizes of the nanoparticles and random directions of their uniaxial anisotropy axes. In particular, the radius of the spherical nanoparticles are drawn from a log-normal distribution. The dynamical behavior of the magnetic moments of the system is studied through numerical simulations of the stochastic Landau-Lifshitz-Gilbert equation, taking into account explicit temperature dependence of the saturation magnetization and uniaxial anisotropy energy density to describe the magnetic properties of perovskite manganite oxide nanoparticles. Hysteresis, zero-field-cooled and field-cooled curves are measured, from which we can find the coercive field and the blocking temperature as a function of the magnitude of the dipolar coupling, controlled by the distance between the particles. We show that both the coercive field and blocking temperature decrease as a function of the separation between the nanoparticles. The magnetic relaxation is measured as a function of both temperature and time, from which we estimate the effective energy barrier distribution of the system. We show that the peak of the energy barrier distribution moves to lower energies as the separation between neighboring particles increases, or equivalently, as the dipolar interaction is reduced.

Filtration of Nanoparticle Agglomerates in Aqueous Colloidal Suspensions Exposed to an External Radio-Frequency Magnetic Field.

The study investigated the phenomenon of the fast aggregation of single-domain magnetic iron oxide nanoparticles in stable aqueous colloidal suspensions due to the presence of a radio-frequency (RF) magnetic field. Single-domain nanoparticles have specific magnetic properties, especially the unique property of absorbing the energy of such a field and releasing it in the form of heat. The localized heating causes the colloid to become unstable, leading to faster agglomeration of nanoparticles and, consequently, to rapid sedimentation. It has been shown that the destabilization of a stable magnetic nanoparticle colloid by the RF magnetic field can be used for the controlled filtration of larger agglomerates of the colloid solution. Two particular cases of stable colloidal suspensions were considered: a suspension of the bare nanoparticles in an alkaline solution and the silica-stabilized nanoparticles in a neutral solution. The obtained results are important primarily for biomedical applications and wastewater treatment.

Role of Anisotropy, Frequency, and Interactions in Magnetic Hyperthermia Applications: Noninteracting Nanoparticles and Linear Chain Arrangements

Efforts by numerous research groups have provided a deeper insight into the physical mechanisms behind the power absorption of single-domain magnetic nanoparticles in magnetic-fluid-hyperthermia applications and theoretical models now account for the main experimental observations. However, the role of all parameters relevant to the magnetic relaxation remains a matter of debate. Here, we employ a nonlinear model for the magnetic relaxation of single-domain magnetic nanoparticles with uniaxial effective anisotropy and evaluate the influence of particle-intrinsic parameters as well as experimental conditions on the power absorption of both noninteracting and interacting systems (linear arrangements). These effects are assessed through the enclosed hysteresis area of the magnetization loops as a function of relative anisotropy ${h}_{K}$ (the anisotropy field with respect to the amplitude of the ac field), i.e., the ``area curve'' of the system. These curves can be divided into four regions with distinct magnetic responses and boundaries that depend on the particle size, frequency of the applied field and interactions. Interactions change the effective anisotropy of the system and shift the area curve towards lower ${h}_{K}$ values. For the low relative anisotropy range, dipolar interactions increase the area of the hysteresis loops [thus, the specific power absorption (SPA)], while they are detrimental or produce nonsignificant effects for the range of high relative anisotropy. Our study resolves seemingly contradictory results of interaction effects in linear arrangements recently reported in the literature. Simulations of randomly oriented particles and chains were contrasted with the oriented cases. An analytical approach and the thermal interpretation of its validity range are discussed, both aimed at the design of nanoparticles and the choice of the experimental conditions for optimal heating. We find that systems with low-thermal-fluctuation influence are better candidates for the application due to their high SPA values.

Microfluidic enrichment of bacteria coupled to contact-free lysis on a magnetic polymer surface for downstream molecular detection.

We report on a microsystem that couples high-throughput bacterial immunomagnetic capture to contact-free cell lysis using an alternating current magnetic field (AMF) to enable downstream molecular characterization of bacterial nucleic acids. Traditional methods for cell lysis rely on either dilutive chemical methods, expensive biological reagents, or imprecise physical methods. We present a microchip with a magnetic polymer substrate (Mag-Polymer microchip), which enables highly controlled, on-chip heating of biological targets following exposure to an AMF. First, we present a theoretical framework for the quantitation of power generation for single-domain magnetic nanoparticles embedded in a polymer matrix. Next, we demonstrate successful bacterial DNA recovery by coupling (1) high-throughput, sensitive microfluidic immunomagnetic capture of bacteria to (2) on-chip, contact-free bacterial lysis using an AMF. The bacterial capture efficiency exceeded 76% at 50 ml/h at cell loads as low as ∼10 CFU/ml, and intact DNA was successfully recovered at starting bacterial concentrations as low as ∼1000 CFU/ml. Using the presented methodology, cell lysis becomes non-dilutive, temperature is precisely controlled, and potential contamination risks are eliminated. This workflow and substrate modification could be easily integrated in a range of micro-scale diagnostic systems for infectious disease.

The effects of anisotropy on solvent-suspended ‘superparamagnetic’ nanoparticles: Magnetization step on melting

The magnetic anisotropy of single-domain magnetic nanoparticles can influence their behaviour significantly even at temperatures above the blocking temperature as conventionally defined. We compare the magnetic properties of such nanoparticles that are free to rotate, and the same nanoparticles with random but fixed orientations. When free to rotate, the particles show Langevin behaviour as expected, but when the orientations are fixed, their magnetic anisotropy causes deviations from this behaviour. These deviations may be observed directly in the M-H curves. They also cause a step in the M-T curve measured for a zero-field cooled sample of nanoparticles suspended in a solvent at the solvent’s melting point. The step occurs because magnetic anisotropy causes M for particles with random but fixed orientation to be lower than for the same particles that are free to rotate when the solvent melts. The size of the step reaches a maximum at a finite applied field. This phenomenon is important because it can be used to determine the fraction of magnetic nanoparticles that are immobilized, for example by adsorption to ice in a freeze-concentrated solution.