The role of dipole interactions in hyperthermia heating colloidal clusters of densely-packed superparamagnetic nanoparticles

Effect of magnetic anisotropy and interaction on spatial focused hyperthermia for rotating and oscillating fields

The behavior of magnetic nanoparticles in a time-varying magnetic field has several practical applications. One of these is hyperthermia used in the treatment of cancer. The nanoparticles injected in the tumor cells release the energy absorbed from the time dependent external magnetic field in the form of heat to its environment in a well-localized way. The aim of the research in this area is to maximize the amount of the dissipated energy. Using a combination of an oscillating and static magnetic field, this dissipated energy can be more focused in space. In this article, we investigated whether this spatial focusing is also present using a rotating and static field together. Furthermore, we investigated the effects of anisotropy and interaction between nanoparticles on this spatial focusing effect using the jump-diffusion model for Néel relaxation in both cases. This kinetic Monte Carlo (MC) method was validated and compared with the stochastic Landau-Lifshitz-Gilbert (SLLG) equation based model. We have shown that the spatial focusing effect is also present for these non-idealized experimentally realizable cases. Also, the effect of rotating magnetic field on magnetic nanoparticles was not investigated in kinetic Monte Carlo simulations before.

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.

Influence of Controlled Dipolar Interaction for Polymer-Coated Gd-Doped Magnetite Nanoparticles toward Magnetic Hyperthermia Application

To maximize heat release from immobilized nanoparticles (NPs), a detailed understanding of the controlled dipolar interaction is essential for challenging magnetic hyperthermia (MH) therapies. To design optimal MH experiments, it is necessary to precisely determine magnetic states impacted by the inevitable concurrence of magnetic interactions under a common experimental form. In this work, we describe how the presence of dipolar interaction significantly alters the heating mechanism of host materials when NPs are embedded in them for MH applications. The concentration of the NPs and the intensity of their interaction can profoundly impact the amplitude and shape of the heating curves of the host material. The heating capability of interacting NPs might be enhanced or diminished, depending on their concentration within the host material. We propose chitosan- and dextran-coated Gd-doped Fe3O4 NPs directing dipole interactions effective for the linear regime to enlighten the pragmatic trends. The outcomes of our study may have substantial implications for cancer therapy and could inspire novel approaches for maximizing the effectiveness of MH.

A comprehensive scrutiny to controlled dipolar interactions to intensify the self-heating efficiency of biopolymer encapsulated Tb doped magnetite nanoparticles

An exciting prospect in the field of magnetic fluid hyperthermia (MFH) has been the integration of noble rare earth elements with biopolymers (chitosan/dextran) that have optimum structures to tune specific effects on magnetic nanoparticles (MNPs). Remarkably, it has been demonstrated that dipole-dipole interactions have a significant influence on nanoparticle dynamics. In this article, we present an exhaustive scrutiny of dipolar interactions and how this affects the efficiency of MFH applications. In particular, we prepare chitosan and dextran-coated Tb-doped MNPs and study whether it is possible to increase the heat released by controlling the dipole-dipole interactions. It has been indicated that even moderate control of agglomeration may substantially impact the structure and magnetization dynamics of the system. Besides estimating the specific loss power value, our findings provide a deep insight into the relaxation mechanisms and bring to light how to tune the self-heating efficacy towards magnetic hyperthermia.

The Curie temperature: a key playmaker in self-regulated temperature hyperthermia

The Curie temperature is an important thermo-characteristic of magnetic materials, which causes a phase transition from ferromagnetic to paramagnetic by changing the spontaneous re-arrangement of their spins (intrinsic magnetic mechanism) due to an increase in temperature. The self-control-temperature (SCT) leads to the conversion of ferro/ferrimagnetic materials to paramagnetic materials, which can extend the temperature-based applications of these materials from industrial nanotechnology to the biomedical field. In this case, magnetic induction hyperthermia (MIH) with self-control-temperature has been proposed as a physical thermo-therapeutic method for killing cancer tumors in a biologically safe environment. Specifically, the thermal source of MIH is magnetic nanoparticles (MNPs), and thus their biocompatibility and Curie temperature are two important properties, where the former is required for their clinical application, while the latter acts as a switch to automatically control the temperature of MIH. In this review, we focus on the Curie temperature of magnetic materials and provide a complete overview beginning with basic magnetism and its inevitable relation with Curie's law, theoretical prediction and experimental measurement of the Curie temperature. Furthermore, we discuss the significance, evolution from different types of alloys to ferrites and impact of the shape, size, and concentration of particles on the Curie temperature considering the proposed SCT-based MIH together with their biocompatibility. Also, we highlight the thermal efficiency of MNPs in destroying tumor cells and the significance of a low Curie temperature. Finally, the challenges, concluding remarks, and future perspectives in promoting self-control-temperature based MIH to clinical application are discussed.

Iron oxide magnetic aggregates: Aspects of synthesis, computational approaches and applications

Superparamagnetic magnetite nanoparticles have been central to numerous investigations in the past few decades for their use in many applications, such as drug delivery, medical diagnostics, magnetic separation, and material science. However, the properties of single magnetic nanoparticles are sometimes not sufficient to accomplish tasks where a strong magnetic response is required. In light of this, aggregated magnetite nanoparticles have been proposed as an alternative advanced material, which may expand and combine some of the advantages of single magnetic nanoparticles, including superparamagnetism, with an enhanced magnetic moment and increased colloidal stability. This review comprehensively discusses the current literature on aggregates made of magnetic iron oxide nanoparticles. This review is divided into three sections. First, the current synthetic strategies for magnetite nanoparticle aggregates are discussed, together with the influence of different stabilizers on the primary crystals and the final aggregate size and morphology. The second section is dedicated to computational approaches, such as density functional methods (which permit accurate predictions of electronic and magnetic properties and shed light on the behavior of surfactant molecules on iron oxide surfaces) and molecular dynamics simulations (which provide additional insight into the influence of ligands on the surface chemistry of iron oxide nanocrystals). The last section discusses current and possible future applications of iron oxide magnetic aggregates, including wastewater treatment, water purification, medical applications, and magnetic aggregates for materials displaying structural colors.

Field- and concentration-dependent relaxation of magnetic nanoparticles and optimality conditions for magnetic fluid hyperthermia

The field-dependent relaxation dynamics of suspended magnetic nanoparticles continues to present a fascinating topic of basic science that at the same time is highly relevant for several technological and biomedical applications. Renewed interest in the intriguing behavior of magnetic nanoparticles in response to external fields has at least in parts be driven by rapid advances in magnetic fluid hyperthermia research. Although a wealth of experimental, theoretical, and simulation studies have been performed in this field in recent years, several contradictory findings have so far prevented the emergence of a consistent picture. Here, we present a dynamic mean-field theory together with comprehensive computer simulations of a microscopic model system to systematically discuss the influence of several key parameters on the relaxation dynamics, such as steric and dipolar interactions, the external magnetic field strength and frequency, as well as the ratio of Brownian and Néel relaxation time. We also discuss the specific and intrinsic loss power as measures of the efficiency of magnetic fluid heating and discuss optimality conditions in terms of fluid and field parameters. Our results are helpful to reconcile contradictory findings in the literature and provide an important step towards a more consistent understanding. In addition, our findings also help to select experimental conditions that optimize magnetic fluid heating applications.

Effect of the Size and Shape of Dendronized Iron Oxide Nanoparticles Bearing a Targeting Ligand on MRI, Magnetic Hyperthermia, and Photothermia Properties—From Suspension to In Vitro Studies

Functionalized iron oxide nanoparticles (IONPs) are increasingly being designed as a theranostic nanoplatform combining specific targeting, diagnosis by magnetic resonance imaging (MRI), and multimodal therapy by hyperthermia. The effect of the size and the shape of IONPs is of tremendous importance to develop theranostic nanoobjects displaying efficient MRI contrast agents and hyperthermia agent via the combination of magnetic hyperthermia (MH) and/or photothermia (PTT). Another key parameter is that the amount of accumulation of IONPs in cancerous cells is sufficiently high, which often requires the grafting of specific targeting ligands (TLs). Herein, IONPs with nanoplate and nanocube shapes, which are promising to combine magnetic hyperthermia (MH) and photothermia (PTT), were synthesized by the thermal decomposition method and coated with a designed dendron molecule to ensure their biocompatibility and colloidal stability in suspension. Then, the efficiency of these dendronized IONPs as contrast agents (CAs) for MRI and their ability to heat via MH or PTT were investigated. The 22 nm nanospheres and the 19 nm nanocubes presented the most promising theranostic properties (respectively, r2 = 416 s−1·mM−1, SARMH = 580 W·g−1, SARPTT = 800 W·g−1; and r2 = 407 s−1·mM−1, SARMH = 899 W·g−1, SARPTT = 300 W·g−1). MH experiments have proven that the heating power mainly originates from Brownian relaxation and that SAR values can remain high if IONPs are prealigned with a magnet. This raises hope that heating will maintain efficient even in a confined environment, such as in cells or in tumors. Preliminary in vitro MH and PTT experiments have shown the promising effect of the cubic shaped IONPs, even though the experiments should be repeated with an improved set-up. Finally, the grafting of a specific peptide (P22) as a TL for head and neck cancers (HNCs) has shown the positive impact of the TL to enhance IONP accumulation in cells.

Insights into the Effect of Magnetic Confinement on the Performance of Magnetic Nanocomposites in Magnetic Hyperthermia and Magnetic Resonance Imaging

The combination of superparamagnetic iron oxide nanoparticles (SPIONs) and lipid matrices enables the integration of imaging, drug delivery, and therapy functionalities into smart theranostic nanocomposites. SPION confinement creates new interactions primarily among the embedded SPIONs and then between the nanocomposites and the surroundings. Understanding the parameters that rule these interactions in real interacting (nano)systems still represents a challenge, making it difficult to predict or even explain the final (magnetic) behavior of such systems. Herein, a systematic study focused on the performance of a magnetic nanocomposite as a magnetic resonance imaging (MRI) contrast agent and magnetic hyperthermia (MH) effector is presented. The effect of stabilizing agents and magnetic loading on the final physicochemical and, more importantly, functional properties (i.e., blocking temperature, specific absorption rate, relaxivity) was studied in detail.

Photothermal conversion and transfer in photothermal therapy: From macroscale to nanoscale

Photothermal therapy (PTT) is a promising alternative therapy for benign or even malignant tumors. To improve the selective heating of tumor cells, target-specific photothermal conversion agents are often included, especially nanoparticles. Meanwhile, some indirect methods by manipulating the radiation and heat delivery are also adopted. Therefore, to gain a clear understanding of the mechanism, and to improve the controllability of PTT, a few issues need to be clarified, including bioheat and radiation transfer, localized and collective heating of nanoparticles, etc. In this review, we provide an introduction to the typical bioheat transfer and radiation transfer models along with the dynamic thermophysical properties of biological tissue. On this basis, we reviewed the most recent advances in the temperature control methods in PTT from macroscale to nanoscale. Most importantly, a comprehensive introduction of the localized and collective heating effects of nanoparticle clusters is provided to give a clear insight into the mechanism for PPT from the microscale and nanoscale point of view.

Fluorescent Single-Core and Multi-Core Nanoprobes as Cell Trackers and Magnetic Nanoheaters

Iron oxide magnetic nanoparticles (MNPs) have been widely studied due to their versatility for diagnosis, tracking (magnetic resonance imaging (MRI)) and therapeutic (magnetic hyperthermia and drug delivery) applications. In this work, iron oxide MNPs with different single-core (8–40 nm) and multi-core (140–200 nm) structures were synthesized and functionalized by organic and inorganic coating materials, highlighting their ability as magnetic nanotools to boost cell biotechnological procedures. Single core Fe3O4@PDA, Fe3O4@SiO2-FITC-SiO2 and Fe3O4@SiO2-RITC-SiO2 MNPs were functionalized with fluorescent components with emission at different wavelengths, 424 nm (polydopamine), 515 (fluorescein) and 583 nm (rhodamine), and their ability as transfection and imaging agents was explored with HeLa cells. Moreover, different multi-core iron oxide MNPs (Fe3O4@CS, Fe3O4@SiO2 and Fe3O4@Citrate) coated with organic (citrate and chitosan, CS) and inorganic (silica, SiO2) shells were tested as efficient nanoheaters for magnetic hyperthermia applications for mild thermal heating procedures as an alternative to simple structures based on single-core MNPs. This work highlights the multiple abilities offered by the synergy of the use of external magnetic fields applied on MNPs and their application in different biomedical approaches.

Hyperthermia Effect of Nanoclusters Governed by Interparticle Crystalline Structures

Magnetic nanoparticles have an important role as heat generators in magnetic fluid hyperthermia, a type of next-generation cancer treatment. Despite various trials to improve the heat generation capability of magnetic nanoparticles, iron oxide nanoparticles are the only approved heat generators for clinical applications, which require a large injection dose due to their low hyperthermia efficiency. In this study, iron oxide nanoclusters (NCs) with a highly enhanced hyperthermia effect and adjustable size were synthesized through a facile and simple solvothermal method. Among the samples, the NCs with a size of 25 nm showed the highest hyperthermia efficiency. Differently sized NCs exhibit inconsistent interparticle crystalline alignments, which affect their magnetic properties (e.g., coercivity and saturation magnetization). As a result, the optimal NCs exhibited a significantly enhanced heat generation efficiency compared with that of isolated iron oxide nanoparticles (ca. 7 nm), and their hyperthermia effect on skin cancer cells was confirmed.

Reliable evaluation method of heating power of magnetic nanofluids to directly predict the tumor temperature during hyperthermia

Reliable measurement of heating power of magnetic nanofluids (MNs) to accurately predict the AC heat-induction performance in tumors is highly desirable for clinical magnetic nanofluids hyperthermia (MNFH) application because it can save time for screening the performance of newly developed MNFH agent and minimize the over-use of animals dramatically. Here, a bio-mimicking phantom model, called Pseudo-Tumor Environment System (P-TES), biochemically designed by considering the external and internal critical factors related to the complex biological environments is proposed to provide a highly reliable evaluation method of heating performance of MNs for in-vivo MNFH applications. According to the experimentally analyzed results, the heating power of MNs measured using the P-TES is well accorded with the heating temperature measured in the tumors during in-vivo MNFH. This result strongly demonstrates that the proposed P-TES can be recommended as a standardized measurement method of heating performance of MNs for clinical MNFH application.

Superferromagnetic Nanoparticles Enable Order-of-Magnitude Resolution & Sensitivity Gain in Magnetic Particle Imaging

Magnetic nanoparticles have many advantages in medicine such as their use in non-invasive imaging as a Magnetic Particle Imaging (MPI) tracer or Magnetic Resonance Imaging contrast agent, the ability to be externally shifted or actuated and externally excited to generate heat or release drugs for therapy. Existing nanoparticles have a gentle sigmoidal magnetization response that limits resolution and sensitivity. Here it is shown that superferromagnetic iron oxide nanoparticle chains (SFMIOs) achieve an ideal step-like magnetization response to improve both image resolution & SNR by more than tenfold over conventional MPI. The underlying mechanism relies on dynamic magnetization with square-like hysteresis loops in response to 20 kHz, 15 kAm^-1 MPI excitation, with nanoparticles assembling into a chain under an applied magnetic field. Experimental data shows a "1D avalanche" dipole reversal of every nanoparticle in the chain when the applied field overcomes the dynamic coercive threshold of dipole-dipole fields from adjacent nanoparticles in the chain. Intense inductive signal is produced from this event resulting in a sharp signal peak. Novel MPI imaging strategies are demonstrated to harness this behavior towards order-of-magnitude medical image improvements. SFMIOs can provide a breakthrough in noninvasive imaging of cancer, pulmonary embolism, gastrointestinal bleeds, stroke, and inflammation imaging.

Nanoparticles for Magnetic Heating: When Two (or More) Is Better Than One

The increasing use of magnetic nanoparticles as heating agents in biomedicine is driven by their proven utility in hyperthermia therapeutic treatments and heat-triggered drug delivery methods. The growing demand of efficient and versatile nanoheaters has prompted the creation of novel types of magnetic nanoparticle systems exploiting the magnetic interaction (exchange or dipolar in nature) between two or more constituent magnetic elements (magnetic phases, primary nanoparticles) to enhance and tune the heating power. This process occurred in parallel with the progress in the methods for the chemical synthesis of nanostructures and in the comprehension of magnetic phenomena at the nanoscale. Therefore, complex magnetic architectures have been realized that we classify as: (a) core/shell nanoparticles; (b) multicore nanoparticles; (c) linear aggregates; (d) hybrid systems; (e) mixed nanoparticle systems. After a general introduction to the magnetic heating phenomenology, we illustrate the different classes of nanoparticle systems and the strategic novelty they represent. We review some of the research works that have significantly contributed to clarify the relationship between the compositional and structural properties, as determined by the synthetic process, the magnetic properties and the heating mechanism.

Magnetoliposomes Based on Shape Anisotropic Calcium/Magnesium Ferrite Nanoparticles as Nanocarriers for Doxorubicin

Multifunctional lipid nanocarriers are a promising therapeutic approach for controlled drug release in cancer therapy. Combining the widely used liposome structure with magnetic nanoparticles in magnetoliposomes allies, the advantages of using liposomes include the possibility to magnetically guide, selectively accumulate, and magnetically control the release of drugs on target. The effectiveness of these nanosystems is intrinsically related to the individual characteristics of the two main components—lipid formulation and magnetic nanoparticles—and their physicochemical combination. Herein, shape-anisotropic calcium-substituted magnesium ferrite nanoparticles (Ca_0.25Mg_0.75Fe₂O₄) were prepared for the first time, improving the magnetic properties of spherical counterparts. The nanoparticles revealed a superparamagnetic behavior, high saturation magnetization (50.07 emu/g at 300 K), and a large heating capacity. Furthermore, a new method for the synthesis of solid magnetoliposomes (SMLs) was developed to enhance their magnetic response. The manufacturing technicalities were optimized with different lipid compositions (DPPC, DPPC/Ch, and DPPC/DSPE-PEG) originating nanosystems with optimal sizes for biomedical applications (around or below 150 nm) and low polydispersity index. The high encapsulation efficiency of doxorubicin in these magnetoliposomes was proven, as well as the ability of the drug-loaded nanosystems to interact with cell membrane models and release DOX by fusion. SMLs revealed to reduce doxorubicin interaction with human serum albumin, contributing to a prolonged bioavailability of the drug upon systemic administration. Finally, the drug release kinetic assays revealed a preferable DOX release at hyperthermia temperatures (42 °C) and acidic conditions (pH = 5.5), indicating them as promising controlled release nanocarriers by either internal (pH) and external (alternate magnetic field) stimuli in cancer therapy.

Iron Oxide Nanorings and Nanotubes for Magnetic Hyperthermia: The Problem of Intraparticle Interactions

Magnetic interactions can play an important role in the heating efficiency of magnetic nanoparticles. Although most of the time interparticle magnetic interactions are a dominant source, in specific cases such as multigranular nanostructures intraparticle interactions are also relevant and their effect is significant. In this work, we have prepared two different multigranular magnetic nanostructures of iron oxide, nanorings (NRs) and nanotubes (NTs), with a similar thickness but different lengths (55 nm for NRs and 470 nm for NTs). In this way, we find that the NTs present stronger intraparticle interactions than the NRs. Magnetometry and transverse susceptibility measurements show that the NTs possess a higher effective anisotropy and saturation magnetization. Despite this, the AC hysteresis loops obtained for the NRs (0-400 Oe, 300 kHz) are more squared, therefore giving rise to a higher heating efficiency (maximum specific absorption rate, SARmax = 110 W/g for the NRs and 80 W/g for the NTs at 400 Oe and 300 kHz). These results indicate that the weaker intraparticle interactions in the case of the NRs are in favor of magnetic hyperthermia in comparison with the NTs.

Reversible magnetism switching of iron oxide nanoparticle dispersions by controlled agglomeration

The controlled agglomeration of superparamagnetic iron oxide nanoparticles (SPIONs) was used to rapidly switch their magnetic properties. Small-angle X-ray scattering (SAXS) and dynamic light scattering showed that tailored iron oxide nanoparticles with phase-changing organic ligand shells agglomerate at temperatures between 5 °C and 20 °C. We observed the concurrent change in magnetic properties using magnetic particle spectroscopy (MPS) with a temporal resolution on the order of seconds and found reversible switching of magnetic properties of SPIONs by changing their agglomeration state. The non-linear correlation between magnetization amplitude from MPS and agglomeration degree from SAXS data indicated that the agglomerates' size distribution affected magnetic properties.

Plasmonic Optical Tweezers for Particle Manipulation: Principles, Methods, and Applications

Inspired by the idea of combining conventional optical tweezers with plasmonic nanostructures, a technique named plasmonic optical tweezers (POT) has been widely explored from fundamental principles to applications. With the ability to break the diffraction barrier and enhance the localized electromagnetic field, POT techniques are especially effective for high spatial-resolution manipulation of nanoscale or even subnanoscale objects, from small bioparticles to atoms. In addition, POT can be easily integrated with other techniques such as lab-on-chip devices, which results in a very promising alternative technique for high-throughput single-bioparticle sensing or imaging. Despite its label-free, high-precision, and high-spatial-resolution nature, it also suffers from some limitations. One of the main obstacles is that the plasmonic nanostructures are located over the surfaces of a substrate, which makes the manipulation of bioparticles turn from a three-dimensional problem to a nearly two-dimensional problem. Meanwhile, the operation zone is limited to a predefined area. Therefore, the target objects must be delivered to the operation zone near the plasmonic structures. This review summarizes the state-of-the-art target delivery methods for the POT-based particle manipulating technique, along with its applications in single-bioparticle analysis/imaging, high-throughput bioparticle purifying, and single-atom manipulation. Future developmental perspectives of POT techniques are also discussed.

Review on the advancements of magnetic gels: towards multifunctional magnetic liposome-hydrogel composites for biomedical applications

Magnetic gels have been gaining great attention in nanomedicine, as they combine features of hydrogels and magnetic nanoparticles into a single system. The incorporation of liposomes in magnetic gels further leads to a more robust multifunctional system enabling more functions and spatiotemporal control required for biomedical applications, which includes on-demand drug release. In this review, magnetic gels components are initially introduced, as well as an overview of advancements on the development, tuneability, manipulation and application of these materials. After a discussion of the advantages of combining hydrogels with liposomes, the properties, fabrication strategies and applications of magnetic liposome-hydrogel composites (magnetic lipogels or magnetolipogels) are reviewed. Overall, the progress of magnetic gels towards smart multifunctional materials are emphasized, considering the contributions for future developments.

Magnetite (Fe3O4) Nanoparticles in Biomedical Application: From Synthesis to Surface Functionalisation

Nanotechnology has gained much attention for its potential application in medical science. Iron oxide nanoparticles have demonstrated a promising effect in various biomedical applications. In particular, magnetite (Fe3O4) nanoparticles are widely applied due to their biocompatibility, high magnetic susceptibility, chemical stability, innocuousness, high saturation magnetisation, and inexpensiveness. Magnetite (Fe3O4) exhibits superparamagnetism as its size shrinks in the single-domain region to around 20 nm, which is an essential property for use in biomedical applications. In this review, the application of magnetite nanoparticles (MNPs) in the biomedical field based on different synthesis approaches and various surface functionalisation materials was discussed. Firstly, a brief introduction on the MNP properties, such as physical, thermal, magnetic, and optical properties, is provided. Considering that the surface chemistry of MNPs plays an important role in the practical implementation of in vitro and in vivo applications, this review then focuses on several predominant synthesis methods and variations in the synthesis parameters of MNPs. The encapsulation of MNPs with organic and inorganic materials is also discussed. Finally, the most common in vivo and in vitro applications in the biomedical world are elucidated. This review aims to deliver concise information to new researchers in this field, guide them in selecting appropriate synthesis techniques for MNPs, and to enhance the surface chemistry of MNPs for their interests.

Adjusting the Néel relaxation time of Fe3O4/Zn x Co1-x Fe2O4 core/shell nanoparticles for optimal heat generation in magnetic hyperthermia

In this work it is shown a precise way to optimize the heat generation in high viscosity magnetic colloids, by adjusting the Néel relaxation time in core/shell bimagnetic nanoparticles, for magnetic fluid hyperthermia (MFH) applications. To pursue this goal, Fe3O4/Zn x Co1-x Fe2O4 core/shell nanoparticles were synthesized with 8.5 nm mean core diameter, encapsulated in a shell of ∼1.1 nm of thickness, where the Zn atomic ratio (Zn/(Zn + Co) at%) changes from 33 to 68 at%. The magnetic measurements are consistent with a rigid interface coupling between the core and shell phases, where the effective magnetic anisotropy systematically decreases when the Zn concentration increases, without a significant change of the saturation magnetization. Experiments of MFH of 0.1 wt% of these particles dispersed in water, in Dulbecco modified Eagles minimal essential medium, and a high viscosity butter oil, result in a large specific loss power (SLP), up to 150 W g-1, when the experiments are performed at 571 kHz and 200 Oe. The SLP was optimized adjusting the shell composition, showing a maximum for intermediate Zn concentration. This study shows a way to maximize the heat generation in viscous media like cytosol, for those biomedical applications that require smaller particle sizes.

Fine tuning and optimization of magnetic hyperthermia treatments using versatile trapezoidal driving-field waveforms

Applying trapezoidal driving-field waveforms to activate magnetic nanoparticles optimizes their performance as heat generators in magnetic hyperthermia, with notable advantages with respect to the effects of harmonic magnetic fields of the same frequency and amplitude. A rate equation approach is used to determine the hysteretic properties and the power released by monodisperse and polydisperse magnetite nanoparticles with randomly oriented easy axes subjected to a radio-frequency trapezoidal driving field. The heating ability of the activated nanoparticles is investigated by means of a simple model in which the heat equation is solved in radial geometry with boundary conditions simulating in vivo applications. Changes of the inclination of the trapezoidal waveform's lateral sides are shown to induce controlled changes in the specific loss power generated by the activated nanoparticles. Specific issues typical of the therapeutic practice of hyperthermia, such as the need for fine tuning of the optimal treatment temperature in real time, the possibility of combining sequential treatments at different temperatures, and the ability to substantially reduce the heating transient in a hyperthermia treatment are suitably addressed and overcome by making use of versatile driving fields of a trapezoidal shape.

Enhanced magnetic heating efficiency at acidic pH for magnetic nanoemulsions stabilized with a weak polyelectrolyte

HYPOTHESIS

Magnetic fluid hyperthermia has attracted considerable attention for cancer therapeutics. Magnetic nanoemulsions are potential candidates for multi-modal hyperthermia due to the possibility of volumetric loading with suitable chemo/photo-therapy agents. Often, the nanocarriers are stabilized using organic molecules that behave differently under varying pH and hence, an understanding of their interfacial behaviour is important for practical applications.

EXPERIMENTS

We probe the magnetic heating efficiency of poly acrylic acid (PAA) stabilized oil-in-water magnetic nanoemulsions, as a function of pH, where the conformational changes of the PAA molecules are studied using dynamic light scattering and inter-droplet force measurements.

FINDINGS

A ~50% enhanced heating efficiency is observed when solution pH is reduced from ~9 to 3, which is attributed to the coil-to-globule conformational changes of the PAA molecules. The increased ionization of the carboxylic acid groups, at higher pH, leads to reduced hydrophobicity that results in an increase in the interfacial thermal resistance causing a lower magneto-thermal heating efficiency at higher pH. The proposed interfacial heat transfer hypothesis is experimentally verified using thermal imaging, where a lower rate of heat transfer is obtained at higher pH. The observed enhanced hyperthermia efficiency at low pH is beneficial for designing efficient pH-responsive nano-carriers for multi-modal hyperthermia.

The potential of magnetic heating for fabricating Pickering-emulsion-based capsules

Pickering emulsions (particle-stabilized emulsions) have been widely explored due to their potential applications, one of which is using them as precursors for the formation of colloidal capsules that could be utilized in, among others, the pharmacy and food industries. Here, we present a novel approach to fabricating such colloidal capsules by using heating in the alternating magnetic field. When exposed to the alternating magnetic field, magnetic particles, owing to the hysteresis and/or relaxation losses, become sources of nano- and micro-heating that can significantly increase the temperature of the colloidal system. This temperature rise was evaluated in oil-in-oil Pickering emulsions stabilized by both magnetite and polystyrene particles. When a sample reached high enough temperature, particle fusion caused by glass transition of polystyrene was observed on surfaces of colloidal droplets. Oil droplets covered with shells of fused polystyrene particles were proved to be less susceptible to external stress, which can be evidence of the successful formation of capsules from Pickering emulsion droplets as templates.

Enhancing the magnetic and inductive heating properties of Fe3O4 nanoparticles via morphology control

Fe₃O₄ nanoparticles (NPs) with different shapes have been prepared by a 'solventless' synthesis approach to probe shape anisotropy effects on the magnetic and inductive heating properties. Various shapes including spheres, octahedrons, cubes, rods, wires, and multipods are obtained through alterations in reaction conditions such as the ratio of precursor to surfactant content and heating rate. Magnetic and Mössbauer measurements reveal better stoichiometry in anisotropic-shaped Fe₃O₄ NPs than that in the spherical and multipod NPs. As a result, the magnetization value of the anisotropic-shaped NPs approaches the value for bulk material (∼86 emu g^-1). More surprisingly, the Verwey transition, which is a characteristic phase transition of bulk magnetite structure, is observed near 120 K in the anisotropic-shaped NPs, which further corroborates the fact that these NPs possess better stoichiometry compared to the spherical and multipod-shaped NPs. Other than the improved magnetic properties, these anisotropic-shaped NPs are more effective for hyperthermia applications. For example, compared to the conventional spherical NPs, the nanowires show much higher SAR value up to 846 W g^-1, making them a potential candidate for practical hyperthermia treatment. In particular, the octahedral NPs shows an SAR value higher than the same size spherical NPs, which demonstrates the importance of occurrence of the Verwey transition in Fe₃O₄ NPs for better stoichiometric and higher heating.

Role of zinc substitution in magnetic hyperthermia properties of magnetite nanoparticles: interplay between intrinsic properties and dipolar interactions

Optimizing the intrinsic properties of magnetic nanoparticles for magnetic hyperthermia is of considerable concern. In addition, the heating efficiency of the nanoparticles can be substantially influenced by dipolar interactions. Since adequate control of the intrinsic properties of magnetic nanoparticles is not straightforward, experimentally studying the complex interplay between these properties and dipolar interactions affecting the specific loss power can be challenging. Substituting zinc in magnetite structure is considered as an elegant approach to tune its properties. Here, we present experimental and numerical simulation results of magnetic hyperthermia studies using a series of zinc-substituted magnetite nanoparticles (Zn_xFe_1-xFe₂O₄, x = 0.0, 0.1, 0.2, 0.3 and 0.4). All experiments were conducted in linear regime and the results were inferred based on the numerical simulations conducted in the framework of the linear response theory. The results showed that depending on the nanoparticles intrinsic properties, interparticle interactions can have different effects on the specific loss power. When dipolar interactions were strong enough to affect the heating efficiency, the parameter σ = K_effV/k_BT (K_eff is the effective anisotropy and V the volume of the particles) determined the type of the effect. Finally, the sample x = 0.1 showed a superior performance with a relatively high intrinsic loss power 5.4 nHm²kg^-1.

Esterase-Cleavable 2D Assemblies of Magnetic Iron Oxide Nanocubes: Exploiting Enzymatic Polymer Disassembling To Improve Magnetic Hyperthermia Heat Losses

Here, we report a nanoplatform based on iron oxide nanocubes (IONCs) coated with a bioresorbable polymer that, upon exposure to lytic enzymes, can be disassembled increasing the heat performances in comparison with the initial clusters. We have developed two-dimensional (2D) clusters by exploiting benchmark IONCs as heat mediators for magnetic hyperthermia and a polyhydroxyalkanoate (PHA) copolymer, a biodegradable polymer produced by bacteria that can be digested by intracellular esterase enzymes. The comparison of magnetic heat performance of the 2D assemblies with 3D centrosymmetrical assemblies or single IONCs emphasizes the benefit of the 2D assembly. Moreover, the heat losses of 2D assemblies dispersed in water are better than the 3D assemblies but worse than for single nanocubes. On the other hand, when the 2D magnetic beads (2D-MNBs) are incubated with the esterase enzyme at a physiological temperature, their magnetic heat performances began to progressively increase. After 2 h of incubation, specific absorption rate values of the 2D assembly double the ones of individually coated nanocubes. Such an increase can be mainly correlated to the splitting of the 2D-MNBs into smaller size clusters with a chain-like configuration containing few nanocubes. Moreover, 2D-MNBs exhibited nonvariable heat performances even after intentionally inducing their aggregation. Magnetophoresis measurements indicate a comparable response of 3D and 2D clusters to external magnets (0.3 T) that is by far faster than that of single nanocubes. This feature is crucial for a physical accumulation of magnetic materials in the presence of magnetic field gradients. This system is the first example of a nanoplatform that, upon exposure to lytic enzymes, such as those present in a tumor environment, can be disassembled from the initial 2D-MNB organization to chain-like assemblies with clear improvement of the heat magnetic losses resulting in better heat dissipation performances. The potential application of 2D nanoassemblies based on the cleavable PHAs for preserving their magnetic losses inside cells will benefit hyperthermia therapies mediated by magnetic nanoparticles under alternating magnetic fields.

The Effect of Tissue-Mimicking Phantom Compressibility on Magnetic Hyperthermia

During hyperthermia, magnetite nanoparticles placed in an AC magnetic field become a source of heat. It has been shown that in fluid suspensions, magnetic particles move freely and generate heat easily. However, in tissues of different mechanical properties, nanoparticle movement is limited and leads to a small temperature rise in tissue. Therefore, it is crucial to conduct magnetic hyperthermia experiments in similar conditions to the human body. The effect of tissue-mimicking phantom compressibility on the effectiveness of magnetic hyperthermia was investigated on agar phantoms. Single and cluster nanoparticles were synthesized and used as magnetic materials. The prepared magnetic materials were characterized by transmission electron microscopy (TEM), and zeta potential measurements. Results show that tissue-mimicking phantom compressibility decreases with the concentration of agar. Moreover, the lower the compressibility, the lower the thermal effect of magnetic hyperthermia. Specific absorption rate (SAR) values also proved our assumption that tissue-mimicking phantom compressibility affects magnetic losses in the alternating magnetic field (AMF).

Clustering of MnFe2O4 nanoparticles and the effect of field intensity in the generation of heat for hyperthermia application

MnFe₂O₄ nanoparticles thinly coated with oleic acid were successfully synthesized via a co-precipitation technique. Morphological analysis shows the co-existence of different nanoparticle assemblies such as dense agglomeration, chain clustering, and random clustering, the effect of which was then reflected in the FTIR spectrum and magnetization behavior of the MnFe₂O₄ nanoparticles. Induction heating study collectively discusses the association of dipolar energy and magnetic anisotropy energy with the applied field intensity in various nanoparticle clustering systems. The highest applied field intensity (H₂₅₀: 14.98 kAm^-1) at a nanoparticle concentration of 2 mg ml^-1 shows a maximum specific absorption rate of 98.37 W g^-1, which is attributed to the effect of threshold field amplitude surpassing the dipolar field energy. The nanoparticle clustering due to an interdigitated effect and chain clustering can effectively contribute to heat generation via Neelian and hysteresis loss mechanism at a suitable high field intensity.