Approaches on Ferrofluid Synthesis and Applications: Current Status and Future Perspectives

Pair correlations of the easy magnetisation axes of superparamagnetic nanoparticles in a ferrofluid/ferrocomposite

The widespread use of magnetic nanoparticles in modern technologies and medical applications highlights the need for reliable theoretical models that can predict their physical properties. The pair correlation function of two randomly selected superparamagnetic nanoparticles in a ferrofluid/ferrocomposite is studied to depict the joint probability density of the easy magnetisation axes across the planes of parameters of major importance; these are the interaction of ferroparticles with an external magnetic field, the energy of magnetic anisotropy inside the superparamagnetic nanoparticle, and the interparticle magnetic dipole-dipole interaction. Assuming the rotational symmetry of the system, we come to the conclusion that the pair correlations of interest are dependent only on the polar angles, determining the inclinations of the ferroparticle easy axes from the direction of an external magnetic field. The dimer configuration, where two ferroparticles are in close contact along a magnetic field with their easy magnetisation axes aligned, is the most probable. This configuration becomes more pronounced with increasing anisotropy energy, dipolar coupling constant, and external magnetic field.

Multi-component liquid-infused systems: a new approach to functional coatings

Antifouling liquid-infused surfaces have generated interest in multiple fields due to their diverse applications in industry and medicine. In nearly all reports to date, the liquid component consists of only one chemical species. However, unlike traditional solid surfaces, the unique nature of liquid surfaces holds the potential for synergistic and even adaptive functionality simply by including additional elements in the liquid coating. In this work, we explore the concept of multi-component liquid-infused systems, in which the coating liquid consists of a primary liquid and a secondary component or components that provide additional functionality. For ease of understanding, we categorize recently reported multi-component liquid-infused surfaces according to the size of the secondary components: molecular scale, in which the secondary components are molecules; nanoscale, in which they are nanoparticles or their equivalent; and microscale, in which the additional components are micrometer size or above. We present examples at each scale, showing how introducing a secondary element into the liquid can result in synergistic effects, such as maintaining a pristine surface while actively modifying the surrounding environment, which are difficult to achieve in other surface treatments. The review highlights the diversity of fabrication methods and provides perspectives on future research directions. Introducing secondary components into the liquid matrix of liquid-infused surfaces is a promising strategy with significant potential to create a new class of multifunctional materials. Keywords: Active surfaces; Antimicrobial; Antifouling; Interfaces; Sensing surfaces.

Microfluidic Detection of SPIONs and Co-Ferrite Ferrofluid Using Amorphous Wire Magneto-Impedance Sensor

The detection of magnetic nanoparticles in a liquid medium and the quantification of their concentration have the potential to improve the efficiency of several relevant applications in different fields, including medicine, environmental remediation, and mechanical engineering. To this end, sensors based on the magneto-impedance effect have attracted much attention due to their high sensitivity to the stray magnetic field generated by magnetic nanoparticles, their simple fabrication process, and their relatively low cost. To improve the sensitivity of these sensors, a multidisciplinary approach is required to study a wide range of soft magnetic materials as sensing elements and to customize the magnetic properties of nanoparticles. The combination of magneto-impedance sensors with ad hoc microfluidic systems favors the design of integrated portable devices with high specificity towards magnetic ferrofluids, allowing the use of very small sample volumes and making measurements faster and more reliable. In this work, a magneto-impedance sensor based on an amorphous Fe73.5Nb3Cu1Si13.5B9 wire as the sensing element is integrated into a customized millifluidic chip. The sensor detects the presence of magnetic nanoparticles in the ferrofluid and distinguishes the different stray fields generated by single-domain superparamagnetic iron oxide nanoparticles or magnetically blocked Co-ferrite nanoparticles.

Individual and collective manipulation of multifunctional bimodal droplets in three dimensions

Spatiotemporally controllable droplet manipulation is vital across numerous applications, particularly in miniature droplet robots known for their exceptional deformability. Despite notable advancements, current droplet control methods are predominantly limited to two-dimensional (2D) deformation and motion of an individual droplet, with minimal exploration of 3D manipulation and collective droplet behaviors. Here, we introduce a bimodal actuation strategy, merging magnetic and optical fields, for remote and programmable 3D guidance of individual ferrofluidic droplets and droplet collectives. The magnetic field induces a magnetic dipole force, prompting the formation of droplet collectives. Simultaneously, the optical field triggers isothermal changes in interfacial tension through Marangoni flows, enhancing buoyancy and facilitating 3D movements of individual and collective droplets. Moreover, these droplets can function autonomously as soft robots, capable of transporting objects. Alternatively, when combined with a hydrogel shell, they assemble into jellyfish-like robots, driven by sunlight. These findings present an efficient strategy for droplet manipulation, broadening the capabilities of droplet-based robotics.

Learning to Control a Three-Dimensional Ferrofluidic Robot

In recent years, ferrofluids have found increased popularity as a material for medical applications, such as ocular surgery, gastrointestinal surgery, and cancer treatment, among others. Ferrofluidic robots are multifunctional and scalable, exhibit fluid properties, and can be controlled remotely; thus, they are particularly advantageous for such medical tasks. Previously, ferrofluidic robot control has been achieved via the manipulation of handheld permanent magnets or in current-controlled electromagnetic fields resulting in two-dimensional position and shape control and three-dimensional (3D) coupled position-shape or position-only control. Control of ferrofluidic liquid droplet robots poses a unique challenge where model-based control has been shown to be computationally limiting. Thus, in this study, a model-free control method is chosen, and it is shown that the task of learning optimal control parameters for ferrofluidic robot control can be performed using machine learning. Particularly, we explore the use of Bayesian optimization to find optimal controller parameters for 3D pose control of a ferrofluid droplet: its centroid position, stretch direction, and stretch radius. We demonstrate that the position, stretch direction, and stretch radius of a ferrofluid droplet can be independently controlled in 3D with high accuracy and precision, using a simple control approach. Finally, we use ferrofluidic robots to perform pick-and-place, a lab-on-a-chip pH test, and electrical switching, in 3D settings. The purpose of this research is to expand the potential of ferrofluidic robots by introducing full pose control in 3D and to showcase the potential of this technology in the areas of microassembly, lab-on-a-chip, and electronics. The approach presented in this research can be used as a stepping-off point to incorporate ferrofluidic robots toward future research in these areas.

Biocompatible Ferrofluid-Based Millirobot for Tumor Photothermal Therapy in Near-Infrared-II Window

Ferrofluidic robots with excellent deformability and controllability have been intensively studied recently. However, most of these studies are in vitro and the use of ferrofluids for in vivo medicinal applications remains a big challenge. The application of ferrofluidic robots to the body requires the solution of many key problems. In this study, biocompatibility, controllability, and tumor-killing efficacy are considered when creating a ferrofluid-based millirobot for in vivo tumor-targeted therapy. For biocompatibility problems, corn oil is used specifically for the ferrofluid robot. In addition, a control system is built that enables a 3D magnetic drive to be implemented in complex biological media. Using the photothermal conversion property of 1064 nm, the ferrofluid robot can kill tumor cells in vitro; inhibit tumor volume, destroy the tumor interstitium, increase tumor cell apoptosis, and inhibit tumor cell proliferation in vivo. This study provides a reference for ferrofluid-based millirobots to achieve targeted therapies in vivo.

Inversion of magnetic diameter distribution of magnetic fluids under high and low temperatures

The magnetic diameter is a crucial factor affecting the magnetic properties of magnetic fluids. The magnetic diameter distribution can be estimated based on the magnetic properties. However, the magnetic dipole interaction of magnetic nanoparticles (MNPs) and the variation of the magnetic diameter with temperature have received relatively little attention in previous research. Hence, this research proposes the AP-MMF1-L method to inverse the magnetic diameter which considers the magnetic dipole interaction and derives the magnetic diameter at different temperatures. Firstly, the AP-MMF1-L uses the least square method between the first-order modified mean-field Langevin function (MMF1-L) and the measured magnetization curve as the objective function. Meanwhile, the hybrid Artificial bee colony-particle swarm (AP) optimization algorithm is introduced to inverse the optimal magnetic diameter distribution. Secondly, the hydrodynamic diameter distribution experimental values are compared with the theoretical values, demonstrating the AP-MMF1-L method obtains accurate inversion results of the magnetic diameter distribution when compared to other models. Finally, the arithmetic mean of the magnetic diameter at different temperatures is investigated, revealing a decreasing trend as the temperature rises, approximately following a linear distribution. The AP-MMF1-L provides a novel and effective tool for accurately determining the magnetic diameter of the MNPs across various temperatures.

Synthesis of Fluorous Ferrofluids and Effects of the Nanoparticle Coatings on Field- and Temperature-Dependent Magnetizations

Ferrofluids have been extensively employed in industrial, environmental, and biomedical areas. Among them, fluorous ferrofluids are of particular interest because of the biorthogonal nature of perfluorocarbons (PFCs). However, the noninteracting nature of PFCs as well as challenges in functionalization of nanoparticle surfaces with fluorous ligands has limited their applications, especially in biomedicine. In particular, commercially available fluorous ferrofluids are stabilized using ionic surfactants with charged groups that physically interact with a wide range of charged biological molecules. In this paper, we developed a unique two-phase ligand attachment strategy to render stable fluorous ferrofluids using nonionic surfactants. The superparamagnetic Fe3O4 or MnFe2O4 core of the magnetic nanoparticles, the magnetic component of the ferrofluid, was coated with a silica shell containing abundant surface hydroxyl groups, thereby enabling the installation of fluorous ligands through stable covalent, neutral, siloxane bonds. We explored chemistry-material relationships between different ligands and PFC solvents and found that low-molecular-weight ligands can assist with the installation of high-molecular-weight ligands (4000-8000 g/mol), allowing us to systematically control the size and thickness of ligand functionalization on the nanoparticle surface. By zero-field-cooled magnetization measurements, we studied how the ligands affect magnetic dipole orientation forces and observed a curve flattening that is only associated with the ferrofluids. This work provided insight into ferrofluids' dependence on interparticle interactions and contributed a methodology to synthesize fluorous ferrofluids with nonionic surfactants that exhibit both magnetic and chemical stability. We believe that the doped MnFe2O4 fluorous ferrofluid has the highest combination of stability and magnetization reported to date.

Using SMART Magnetic Fluids and Gels for Prevention and Destruction of Bacterial Biofilms

Biofouling is a major problem in all natural and artificial settings where solid surfaces meet liquids in the presence of living microorganisms. Microbes attach to the surface and form a multidimensional slime that protects them from unfavorable environments. These structures, known as biofilms, are detrimental and very hard to remove. Here, we used SMART magnetic fluids [ferrofluids (FFs), magnetorheological fluids (MRFs), and ferrogels (FGs) containing iron oxide nano/microparticles] and magnetic fields to remove bacterial biofilms from culture tubes, glass slides, multiwell plates, flow cells, and catheters. We compared the ability of different SMART fluids to remove biofilms and found that commercially available, as well as homemade, FFs, MRFs, and FGs can successfully remove biofilm more efficiently than traditional mechanical methods, especially from textured surfaces. In tested conditions, SMARTFs reduced bacterial biofilms by five orders of magnitude. The ability to remove biofilm increased with the amount of magnetic particles; therefore, MRFs, FG, and homemade FFs with high amounts of iron oxide were the most efficient. We showed also that SMART fluid deposition can protect a surface from bacterial attachment and biofilm formation. Possible applications of these technologies are discussed.

Cancer immunotherapeutic effect of carboxymethylated β-d-glucan coupled with iron oxide nanoparticles via reprogramming tumor-associated macrophages

The cancer immunotherapeutic effect of a carboxymethylated β-d-glucan (CMPTR)/iron oxide nanoparticles (IONPs) system (CMPTR/IONPs) were investigated by using cell culture of bone marrow-derived macrophages (BMDMs) and B16F10 melanoma skin cancer-bearing mouse model. When compared with that of control group, CMPTR/IONPs-treated M2-like BMDMs exhibited upregulated M1 biomarkers expression, significantly inhibited the migration of B16F10 cancer cells (p < 0.05), and had the highest apoptotic percentage of B16F10 cancer cells (80.39 ± 8.73 %) in co-culture system. Intratumoral administration of CMPTR/IONPs significantly (p < 0.05) suppressed tumor growth (46.58 % based on tumor weight) in mice and enhanced the M1/M2 ratio from 0.40 ± 0.09 (control group) to 6.64 ± 1.61 in tumor associated macrophages (TAMs) which was higher than that of in CMPTR (1.27 ± 0.38), IONPs (1.38 ± 0.17). CMPTR/IONPs treatment also promoted apoptosis in cancer cells and increased the infiltration of CD4 and CD8 T-lymphocytes in tumor tissues. These results could be due to the combined effects of CMPTR and IONPs in the CMPTR/IONPs system, possibly mediated by the activation of NF-κB and IRF5 pathways for inducing M1 macrophages polarization and had potential cancer immunotherapeutic applications.

Synthesis and Characterization of Magnetoplasmonic Air-Stable Au@FeCo

The synthesis of FeCo alloys as highly magnetic nanoparticles has been valuable, as far as applications for magnetic nanoparticles are concerned. However, recently, a field of magnetoplasmonics in which magnetic nanoparticles such as the FeCo alloys doped with plasmonic materials such as Au and Ag to create a hybrid nanostructure with both properties has emerged. These magnetoplasmonic metamaterials have greatly enhanced the limit of detection of analytes in spectroscopic methods, as well as providing a more widely applicable nanoparticle to broaden the use of FeCo alloys even further. Herein, we discuss the synthesis of high-yield and fairly monodisperse spherical FeCo and Au-doped FeCo (Au@FeCo) with varying compositions of Au synthesized via the thermal decomposition of iron pentacarbonyl (Fe(CO)₅) and dicobalt octacarbonyl (Co₂(CO)₈), followed by the addition of Au atoms using triphenylphosphine gold(I) chloride ((Ph₃P)AuCl) via both coprecipitation and by delayed addition methods. The products were separated using a hand-held magnet, and then characterized via ultraviolet-visible light (UV-vis), scanning electron microscopy coupled with energy-dispersive X-ray analysis (SEM-EDX), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), flame atomic absorption spectrometry (F-AAS), and magnetization measurements. Optical studies revealed a plasmonic peak at 550 nm in the Au@FeCo nanoparticles that had a gold content (%Au) of >2% (by weight), determined using F-AAS. Colocation of the Fe, Co, and Au were demonstrated through EDX analysis. Location of the Au atoms in the core were seen through high-resolution bright-field imaging. To understand the use of these nanoparticles for potential application in therapeutics and/or electronics, resistance measurements were performed to assess power loss as a function of frequency. We also achieved magnetization values as high as 150 emu/g and as low as 50 emu/g for gold-loaded samples based on %Au by weight. This paves the way to continue to develop magneto-plasmonic structures chemically using these synthesis strategies.

Superparamagnetic Iron Oxide Nanoparticles (SPION): From Fundamentals to State-of-the-Art Innovative Applications for Cancer Therapy

Despite significant advances in cancer therapy over the years, its complex pathological process still represents a major health challenge when seeking effective treatment and improved healthcare. With the advent of nanotechnologies, nanomedicine-based cancer therapy has been widely explored as a promising technology able to handle the requirements of the clinical sector. Superparamagnetic iron oxide nanoparticles (SPION) have been at the forefront of nanotechnology development since the mid-1990s, thanks to their former role as contrast agents for magnetic resonance imaging. Though their use as MRI probes has been discontinued due to an unfavorable cost/benefit ratio, several innovative applications as therapeutic tools have prompted a renewal of interest. The unique characteristics of SPION, i.e., their magnetic properties enabling specific response when submitted to high frequency (magnetic hyperthermia) or low frequency (magneto-mechanical therapy) alternating magnetic field, and their ability to generate reactive oxygen species (either intrinsically or when activated using various stimuli), make them particularly adapted for cancer therapy. This review provides a comprehensive description of the fundamental aspects of SPION formulation and highlights various recent approaches regarding in vivo applications in the field of cancer therapy.

Magnetic forces in paramagnetic fluids

An overview of the effect of a magnetic field gradient on fluids with linear magnetic susceptibilities is given. It is shown that two commonly encountered expressions, the magnetic field gradient force and the concentration gradient force for paramagnetic species in solution are equivalent for incompressible fluids. The magnetic field gradient and concentration gradient forces are approximations of the Kelvin force and Korteweg-Helmholtz force densities, respectively. The criterion for the appearance of magnetically induced convection is derived. Experimental work in which magnetically induced convection plays a role is reviewed.

Magnetic Trails: A Novel Artificial Pheromone for Swarm Robotics in Outdoor Environments

Swarm robotics finds inspiration in nature to model behaviors, such as the use of pheromone principles. Pheromones provide an indirect and decentralized communication scheme that have shown positive experimental results. Real implementations of pheromones have suffered from slow sensors and have been limited to controlled environments. This paper presents a novel technology to implement real pheromones for swarm robotics in outdoor environments by using magnetized ferrofluids. A ferrofluid solution, with its deposition and magnetization system, is detailed. The proposed substance does not possess harmful materials for the environment and can be safely handled by humans. Validation demonstrates that the substance represents successfully pheromone characteristics of locality, diffusion and evaporation on several surfaces in outdoor conditions. Additionally, the experiments show an improvement over the chemical representation of pheromones by using magnetic substances and existing magnetometer sensor technologies, which provide better response rates and recovery periods than MOX chemical sensors. The present work represents a step toward swarm robotics experimentation in uncontrolled outdoor environments. In addition, the presented pheromone technology may be use by the broad area of swarm robotics for robot exploration and navigation.

Fe-Cr-Nb-B Ferrofluid for Biomedical Applications

A ferrofluid based on Fe67.2Cr12.5Nb0.3B20 magnetic particles with a low Curie temperature was prepared. The particles, most of which had dimensions under 60 nm, were dispersed in a calcium gluconate solution, leading to a stable ferrofluid. The obtained ferrofluid had a magnetization of 0.04 to 0.17 emu/cm3, depending on the particles' concentration, and a viscosity that increased nonlinearly with the applied magnetic field. The ferrofluid appeared to be biocompatible, as it showed low cytotoxicity, even at high concentrations and for long intervals of co-incubation with human cells, demonstrating a good potential to be used for cancer therapies through magnetic hyperthermia as well as magneto-mechanical actuation.