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In this work, a magnetic fluid (Fe3O4/water) moving through a square tube in a steady-state and fluctuating electromagnetic field was investigated experimentally. The relevant parameter effects are considered, including rotating direction, flux, frequency, and frequency on the heat removal ability and flow resistance. The electromagnetic field induces the particles to move to the square tube wall, greatly influencing the boundary layer disturbance. A greater electromagnetic flux and rotating direction enhance nanoparticle turbulence intensity and local thermal conductivity. Compared to when it is fixed, the Nu of an electromagnetic field oscillating at 1.25 hz, 2.3 µT, is 9.16% more. Additionally, compared to the other modes, the Nu is greater by 3.41% when the mixed mode of the electromagnetic field rotates at 1.25 hz and 0.5 µT. The heat-removal capacity may be significantly increased by increasing the electromagnetic flux and frequency, which raises the Nu. The f is also further increased by the disruption of electromagnetic flow. [ABSTRACT FROM AUTHOR]

Magnetic soap films under the forcing of an inhomogeneous magnetic field are governed by a wide range of interconnected physics. The study of magnetic soap films requires the development of comprehensive models to support experimental observations. In this study, the thin film approximation is applied to the Navier–Stokes equations to derive a model for the film thickness of magnetic soap films that incorporates the effects of interfacial mobility, surfactant transport and magnetite nanoparticle (NP) transport. This derived model consists of a coupled system of equations for the film thickness, interfacial velocity, interfacial surfactant concentration and magnetite NP concentration. Simulations are performed for both soap films and magnetic soap films by solving the system of equations using the Galerkin finite element method, and results are compared with experiments. Simulation results highlight that interfacial flows can dominate the rate of film thinning and that accounting for the dependence of the magnetisation on the local magnetite NP concentration can influence the predicted speed of magnetically driven flows. Furthermore, simulation results demonstrate that the model is able to predict marginal regeneration in qualitative agreement with the experiments for soap films; the model also predicts the same flow pattern as seen in the experiments for magnetic soap films. Overall, this study advances the state of soap film and magnetic soap film modelling and will contribute to acquiring control over the drainage and stability of magnetic soap films in the long term. [ABSTRACT FROM AUTHOR]

Three compositions Ni0.5Fe2.5O4, Ni0.7Fe2.3O4, and Fe2O3 were prepared by the co- precipitation method and checked by XDR, SEM, and VSM. The nanoparticles show a good match with the XRD standards. The particle sizes were 25, 32, and 18nm for the above compositions, respectively. The first composition showed the highest magnetization saturation. Suspensions containing the mentioned nanoparticles were prepared with two carrier fluids, distilled water, and dimethylsulphoxide (DMSO). A transmission to ultraviolet-visible light was tested and showed good transparency of more than 60 % in the red light region. The transparency to 623nm laser light under different magnetic fields of the six suspensions was tested. A simple device was fabricated to perform this test. The water suspension in water Ni0.5Fe2.5O4 has a larger variation in the transmitted power. The suspension of hematite shows no sensible change in the transmitted power. The magnetic properties of the nanoparticles and the dispersion property of the carrier fluid were behind the explanation of the suspension behaviours under the influence of the magnetic field. [ABSTRACT FROM AUTHOR]

We used ac-susceptibility measurements to study the superspin relaxation in Fe3O4/Isopar M nanomagnetic fluids of different concentrations. Temperature-resolved data collected at different frequencies, χ″ vs. T|f, reveal magnetic events both below and above the freezing point of the carrier fluid (TF = 197 K): χ″ shows peaks at temperatures Tp1 and Tp2 around 75 K and 225 K, respectively. Below TF, the Néel mechanism is entirely responsible for the superspin relaxation (as the carrier fluid is frozen), and we found that the temperature dependence of the relaxation time, τN(Tp1), is well described by the Dorman–Bessais–Fiorani (DBF) model: τ N T = τ r exp ⁡ E B + E a d k B   T . Above TF, both the internal (Néel) and the Brownian superspin relaxation mechanisms are active. Yet, we found evidence that the effective relaxation times, τeff, corresponding to the Tp2 peaks observed in the denser samples do not follow the typical Debye behavior described by the Rosensweig formula 1 τ e f f = 1 τ N + 1 τ B . First, τeff is 5 × 10−5 s at 225 K, almost three orders of magnitude more that its Néel counterpart, τN~8 × 10−8 s, estimated by extrapolating the above-mentioned DBF analysis. Thus, 1 τ N ≫ 1 τ e f f , which is clearly not consistent with the Rosensweig formula. Second, the observed temperature dependence of the effective relaxation time, τeff(Tp2), is excellently described by τ B − 1 T = T γ 0 exp ⁡ − E ′ k B T − T 0 ′ , a model solely based on the hydrodynamic Brown relaxation, τ B (T) = 3 η T V H k B T , combined with an activation law for the temperature variation of the viscosity, η T = η 0 exp ⁡ E ′ / k B (T − T 0 ′ . The best fit yields γ 0 = 3 η V H k B = 1.6 × 10−5 s·K, E′/kB = 312 K, and T0′ = 178 K. Finally, the higher temperature Tp2 peaks vanish in the more diluted samples (δ ≤ 0.02). This indicates that the formation of larger hydrodynamic particles via aggregation, which is responsible for the observed Brownian relaxation in dense samples, is inhibited by dilution. Our findings, corroborating previous results from Monte Carlo calculations, are important because they might lead to new strategies to synthesize functional magnetic ferrofluids for biomedical applications. [ABSTRACT FROM AUTHOR]

An efficient synthesis of new (methylsulfonyl)benzopyranophenazin-3-amines via the condensation of 2-hydroxynaphthoquinone, 1,2-phenylenediamine, methylsulfonylacetonitrile, and arylaldehydes derivatives catalyzed by ionic liquid [(EtO)3Si–CH2–CH2–CH2–NH3+][HCOO−] immobilized on magnetic Fe3O4@SiO2, as a novel heterogeneous nanocatalyst, was described. The novel nanocatalyst was characterized by FE-SEM, XRD, FT-IR, TGA‐DTG, and VSM techniques. The environmentally friendly protocol presented some advantages, such as a simple work-up and excellent yields. This nanocatalyst can be easily reused several times and recovered by an external magnet. The activity of the recoverable nanocatalyst remains intact even after five repeated runs. [ABSTRACT FROM AUTHOR]

Magnetic fluid is an optical functional material which comprised of magnetic nanoparticles that are dispersed in water-base fluid or oil-base fluid. For a magnetic fluid film, when magnetic field is applied perpendicularly to the surface of film, the magnetic nanoparticles can agglomerate to columns from initial dispended state, resulting in the microstructure changes which represent the different optical characteristics of the magnetic fluid film. And this characteristic can be applied for optical sensors. So the study on microstructure of magnetic fluid is very important and necessary. In this paper, the molecular dynamic method based on both spherical magnetic nanoparticle and Rod-like magnetic nanoparticle was respectively used to simulate the microstructure of magnetic fluid. The calculating results showed that for spherical computation model, the magnetic nanoparticles could agglomerate to form chains along the direction of magnetic field, as for Rod-like computation model, however, not only the process of chaining but also its periodic structure can be reflected. Finally, the reason for the difference was discussed. [ABSTRACT FROM AUTHOR]

Magnetic nanofluids, also referred to as ferromagnetic particle levitation systems, are materials with highly responsive magnetic properties. Due to their magnetic responsiveness, excellent controllability, favorable thermal characteristics, and versatility, magnetic nanofluids have sparked considerable interest in both industrial manufacturing and scientific research. Magnetic nanofluids have been used and developed in diverse areas such as materials science, physics, chemistry and engineering due to their remarkable characteristics such as rapid magnetic reaction, elastic flow capacities, and tunable thermal and optical properties. This paper provides a full and in-depth introduction to the diverse uses of ferrofluids including material fabrication, fluid droplet manipulation, and biomedicine for the power and machinery sectors. As a result, magnetic nanofluids have shown promising applications and have provided innovative ideas for multidisciplinary research in biology, chemistry, physics and materials science. This paper also presents an overview of the device construction and the latest developments in magnetic-nanofluid-related equipment, as well as possible challenging issues and promising future scenarios. [ABSTRACT FROM AUTHOR]

Magnetic fluids, a new type of energy transfer fluid with tunable properties, have garnered significant interest from researchers worldwide. Hybrid magnetic fluids prepared by adding different types of nanoparticles exhibit superior thermophysical properties and functional characteristics. In this paper, we prepared a water-based magnetic fluid loaded with multi-walled carbon nanotubes (MCNTs), silver (Ag), and copper (Cu) to enhance thermal conductivity. Using a transient double hot-wire method, we designed and built an experimental measurement system for the thermal conductivity of magnetic fluids with an average measurement error of less than 5%. We studied the thermal conductivity of hybrid magnetic fluids under different conditions and evaluated the advantages and disadvantages of various models, including the Maxwell model, H&C model, Tim model, Y&C model, and Evans model. Our results show that MF+MCNTs, MF+Ag, and MF+Cu nanofluids can all improve the thermal conductivity of the carrier fluid, with MF+MCNTs exhibiting the best improvement effect of 10.93%. Among the five models evaluated, the Evans model had the best predictive effect with a deviation range within 5%. This work provides theoretical and practical reference for enhancing the thermal conductivity of magnetic fluids and provides a more accurate theoretical model for calculating the thermal conductivity of hybrid magnetic fluids. [ABSTRACT FROM AUTHOR]

Magnetic pore fabrics (MPFs) are a promising approach to explore the 3D pore space of rocks. There exist empirical relationships between the orientation, degree, and shape of MPF with pore space geometry and permeability anisotropy. Nevertheless, the precise nature of these relationships remains elusive. A common assumption in MPF studies is that ferrofluid uniformly fills the pore space, establishing a constant nanoparticle density and magnetic properties in space and time. However, this assumption is challenged by observations of particle sedimentation and time-dependent magnetic properties, which render the quantitative interpretation of MPFs challenging. This study explores the physical and chemical stability of ferrofluid, mobility of particles after impregnation, and the magnetic properties of impregnated rocks over time. Water-based ferrofluids are physically more stable than oil-based ferrofluids, though with higher magnetic variability. In impregnated rock, magnetic nanoparticles display a certain degree of mobility, resulting in changes in MPF degree and shape. When ferrofluid is mixed with epoxy, particles are less mobile, though some changes were observed both during polymerization and aging. The susceptibility of ferrofluid-epoxy mixtures is lower than for ferrofluid and carrier liquid at the same concentration. Interestingly, the susceptibility of impregnated rock increases over time, regardless of the ferrofluid used for the impregnation. Understanding and controlling these processes will enhance the reliability of MPF interpretations and increase the applicability of the method. [ABSTRACT FROM AUTHOR]

In this study, the improved hyperthermia properties of a magnetic, biocompatible core/shell nanostructure containing 12 -nm hard magnetic CoFe2O4 nanoparticles as the core and soft magnetic Fe3O4 nanoparticles as the shell were reported. By simply using thermal decomposition synthesis method, CoFe2O4 nanoparticles were coated layer-by-layer by Fe3O4 nanoparticles to result the core/shell structure with a variable shell thickness. XRD analysis showed that the as-synthesized nanoparticles have face-centered cubic structure with ferrite spinel single phase. TEM images showed that particle size of the core–shell structure increased by 0.7 − 2.1 nm and monotonously with the Fe3O4 coating. The coercivity (Hc) of the core/shell nanoparticles decreased remarkably compared to that of CoFe2O4 core nanoparticles while saturation magnetization Ms significantly increased, indicating the successful formation of the soft magnetic Fe3O4 shell layer. Dynamic light scattering (DLS) analysis revealed that the uniform size and good stability of the magnetic fluids based on the as-synthesized CoFe2O4/Fe3O4 core/shell nanoparticles using poly(maleic anhydride-alt-1-octadecene) (PMAO) as the phase transfer ligand were achieved. Comparing to the core CoFe2O4 nanoparticle, the specific absorption rate (SAR) values of the core/shell nanoparticles were significantly higher and, especially, were more than doubled with shell thickness of 2 nm. Additionally, the cytotoxicity test indicated low toxicity of the magnetic fluid on HEK-293 normal cells. These results showed that the studied CoFe2O4/Fe3O4 core/shell nanoparticles with their high biocompatibility and improved hyperthermia properties are expected to be usable in biomedicine, especially in hyperthermia cancer treatment. [ABSTRACT FROM AUTHOR]

Rheological measurements under an applied magnetic field were used to investigate the changes to the internal structure and stability of an aqueous ferrofluid. The ferrofluid was prepared by dispersing 1.8 wt.% of maghemite nanoparticles with a size of d = 14 ± 3 nm and a saturation magnetization MS = 68 emu/g in water using citric acid as the surfactant. In this study, oscillatory tests were used to investigate the internal structural changes and the stability of ferrofluid under the influence of the magnetic field B. In a magnetic field of approximately 50 mT, the G′ became higher than the loss modulus G″ as the ferrofluid exhibited a gel-like character. However, at a magnetic field of approximately 200 mT, the character of the ferrofluid reverted to that of a liquid. The change in the character of the ferrofluid in this high magnetic field was associated with a gradual change from chain agglomerates to the energetically more favourable globular agglomerates, using a calculation based on a model described in a separate work. The globular agglomerates impeded the flow to a much lesser degree than the chains, causing a reduction in the viscosity. Further increase of the magnetic field resulted in sedimentation of agglomerates and loss of magneto-rheological effect. [ABSTRACT FROM AUTHOR]

We used dc magnetization and ac susceptibility to investigate the magnetic relaxation of ferrofluids made of 8 nm average-diameter Fe3O4 nanoparticles dispersed in hexane. Samples of different concentrations (δ) spanning two orders of magnitude ranging from 0.66 to 0.005 mg (Fe3O4)/mL (hexane) were used to vary the interparticle interaction strength. Our data reveal a critical concentration, δc = 0.02 mg/mL, below which the ferrofluid behaves like an ideal nanoparticle ensemble where the superspins relax individually according to a Néel–Brown activation law τ (T)   = τ 0 exp E B k B T with a characteristic time τo ~10−9 s. That is further confirmed by the observed invariance of the relative peak temperature variation per frequency decade ∆ = ∆ T T · ∆ l o g (f) , which stays constant at ~0.185 when δ < δc. At higher concentrations, between 0.02 and 0.66 mg/mL, we found that Δ exhibits a monotonic increase with the inverse concentration, 1 δ , and the collective superspin dynamics is described by a Vogel–Fulcher law, τ (T)   = τ 0 exp E B k B T − T 0 . Within this regime, the dipolar interaction strength parameter T0 increases from T0 = 0 K at δc = 0.02 mg/mL to T0 = 14.7 K at δ = 0.66 mg/mL. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Magnetorheological (MR) fluids were prepared in transformer oil as well as transformer oil based magnetic fluid having 2 wt. % of Fe3O4 particles. The thermal conductivity enhancement (w.r.t. carrier) in the case of transformer oil based MR fluid under a field was found to vary from 219%, 304%, and 356%, respectively, from 15 vol. %, 20 vol. %, and 25 vol. % of iron concentration. These values are much higher than that obtained for spherical-shaped iron particles based MR fluids. This is further enhanced by another 30% at the same magnetic field when the same flakes were dispersed in transformer oil based magnetic fluid. In addition, the magnetic fluid based MR fluids exhibit minimum hysteresis in thermal conductivity when the field is removed. The increase in the thermal conductivity of transformer oil based MR fluid is due to the shape effect as well as the reduction in thermal resistance due to a higher particle-particle interaction compared to spherical particles. In addition, on adding nanoparticles, the friction between the particles reduces and nanoparticles facilitate flake-shaped particles to form a stronger chain. This results in an enhancement in thermal conductivity. The observed reduction in the viscosity of MR fluids with nanoparticles supports the concept of the increased orientation of flakes in the direction of the field. [ABSTRACT FROM AUTHOR]

There are numerous applications of nanomaterials in catalysis, biosensing, biotechnology, electronics, magnetic fluids, energy storage and also in the biomedical field, especially in gene or drug delivery and diagnostics. Nanomaterials have amazing capabilities to stimulate neuronal cells toward neuronal cell proliferation, neuronal cell adhesion, axonal growth, and neuroprotection. Researchers have demonstrated that nanomaterials can also differentiate stem cells into neuronal cells. Recently, the impact of nanomaterials on the proliferation and differentiation of normal, cancer, and stem cells have been investigated greatly. In this study, the effects of titanium dioxide nanoparticles (TiO2NPs) on the differentiation of neural stem cells are examined. Our findings indicate that TiO2 nanoparticles lead to differentiation tendencies biased towards neurons from neural stem cells, suggesting TiO2 nanoparticles might be a beneficial inducer for neuronal differentiation. We found that pheochromocytoma cell line (PC12 cells) exposed to TiO2, Au/TiO2, Ag/TiO2 nanoparticles significantly increased the differentiation of neural stem cells and promoted neurite outgrowth. Our data may have resulted from the stimulation of cell adhesion molecules that are associated with cell-matrix interactions through nanoparticle. The findings of this work proposes the use of the Ag/TiO2 nanoparticles which also have antibacterial and antioxidant characteristics, as a suitable method to improve Nerve Growth Factor (NGF) activity and efficacy, thus, opening the novel window for substantial neuronal repair therapeutics. [ABSTRACT FROM AUTHOR]

Room‐temperature liquid metals, such as galinstan, have become an emerging material in electronics requiring stretchability and deformability due to its excellent fluidity and conductivity. Eutectic gallium‐indium‐tin (galinstan)‐based composites with certain metallic fillers can obtain magnetism. However, the alloying reaction between galinstan and metals leads to the solidification and reduction in stability of the composite. In this study, Ni@SiO2 nanoparticles with the core–shell structure are used as fillers and galinstan as matrix. The obtained galinstan‐Ni@SiO2 paste is fairly stable and this is attributed to the silica shell, which acts as a barrier to isolate the direct contact between the Ni and galinstan. In addition, flexible circuits composed of as‐prepared galinstan‐Ni@SiO2 pastes are fabricated, which exhibited outstanding durability, retrievability, and self‐healing ability under the drive of magnetic field. [ABSTRACT FROM AUTHOR]

The CrxFe 3 − x O4 ferrofluids have been successfully synthesized using natural iron sand through co-precipitation method as a voltage sensor. The use of chromium dopants is intended to increase the ferrofluids response when used as a voltage sensor. A series of characterizations were carried out to support this justification, including structure, morphology, functional groups, optical properties, and sensor performance of the CrxFe 3 − x O4 ferrofluids. The increase in the Cr 3 + molar fraction has an impact on decreasing in structural parameters of the CrxFe 3 − x O4 ferrofluids. Visually, the particles show an agglomeration phase by the presence of interparticle forces. The study of functional groups showed that the CrxFe 3 − x O4 ferrofluids had been successfully formed, marked by the appearance of all precursor functional groups. Analysis of optical properties based on absorption shows the phenomenon of intervalence charge transfer and causes a decrease in the value of the gap energy. Interestingly, the CrxFe 3 − x O4 ferrofluids sensor sensitivity shows a good and prospective response making them suitable for new sensor candidates in the future. CrxFe3–xO4 ferrofluids were synthesized through co-precipitation method as voltage sensor. The increase in the Cr3+ molar fraction has an impact to the decrease in the structural parameters of the CrxFe3–xO4 ferrofluids. Analysis of optical properties based on absorption shows the phenomenon of intervalence charge transfer and causes a decrease in the value of the gap energy. Interestingly, the sensor sensitivity of the CrxFe3–xO4 ferrofluids shows good and prospective response, making them suitable for new sensor candidate in the future. [ABSTRACT FROM AUTHOR]

In this work, the plasmonic properties of magnetic and liquid nanoparticles coated with aluminium are studied for egg-like/multilayer structures. The optical spectra of multilayers nanoparticles under core–shell or bi-layer (Fe/FeO/Fe2O3/Fe3O4@Al, Cs/Cs2O@Al) and egg-like or multilayer (Fe/FeO/Fe2O3/Fe3O4@Cs/Cs2O@Al) nanostructures are theoretically investigated. Variations in materials types and size parameters including inner core, shell, and outer shell have shown their significant influence on enhancement in LSPR peak position, intensity, and broadening. It is observed that the LSPR peak position effectively increased with pure materials and the peak's intensity along with line-width or broadening increased with oxide materials in core–shell configuration. In an egg-like/multilayer configuration, varying sizes of oxide materials in the core, as well as shell, have their direct impact on line-width as well as the intensity of peaks. It is found that the LSPR peaks get red-shifted with increasing core size and blue-shifted with increasing shell thickness and enhanced spectra are seen for Fe2O3-core with varying Al-shell thickness. The plasmonic LSPR peaks at λmax are revealed in the range of 221–1004 nm wavelengths and tuned in the deep UV–visible-NIR region of core–shell and visible region of egg-like (multilayer) nanostructures. Such response of core–shell and egg-like/multilayer nanostructures for optical-absorption properties opens new potential in stretchable plasmonic devices, solar cells, and therapeutic as well as appropriate light absorption-based applications. [ABSTRACT FROM AUTHOR]

A semi‐active T‐type micromixer is designed to intensify micromixing by actuating magnetic nanoparticles (MNPs). Five permanent magnets in a zig‐zag arrangement are located next to the mixing channel of the micromixer to apply the magnetic field to the fluid flow. Micromixing performance is considered in terms of the segregation index (XS) by the Villermaux/Dushman reaction test. The effects of magnetic flux intensity (B = 380–500 mT), the concentration of MNPs (φ = 0.002–0.01 [w/v]), and flow rate ratios on XS and pressure drop are investigated. By increasing MNPs concentration from φ = 0.002–0.008 (w/v), XS decreased and the rise in φ up to 0.008 (w/v) has not been significant on XS. Maximum mixing efficiency (i.e., minimum XS = 0.0088) is achieved for B = 500 mT and φ = 0.01 (w/v). By applying the magnetic field, the mixing performance increased due to the motion of MNPs, but its negative effect is an increase in the pressure drop along the micromixer reactor. Generally, with the formation of MNPs barriers inside the mixing channel, the main fluid flows through these layers and creates the sinusoidal flow paths compared to no magnetic field conditions, and thus, a superior mixing efficiency could be attained. [ABSTRACT FROM AUTHOR]

This study reports on the use of ferromagnetic resonance (FMR), at various microwave frequencies (1.1 GHz, 3.5 GHz, 9.4 GHz, and 33.9 GHz), to investigate nanosized maghemite-based (γ-Fe 2 O 3) samples in both powder (P) and aqueous-based magnetic fluid (MF) presentations. The structure of the as-synthesized sample was confirmed through x-ray diffraction (XRD), revealing the maghemite phase with average diameter of about 12 nm. Analyses of the room temperature (RT) FMR spectra suggest that the samples are in a non-saturated state for microwave frequencies below about 10 GHz (X-band). Comparisons between theoretical modelling (acceptable) parameters and those obtained through fittings of the experimental data revealed significant discrepancies, highlighting an increase in divergence as the measurement condition deviates from the magnetic saturation condition, taking place at microwave frequencies below about 10 GHz. [ABSTRACT FROM AUTHOR]

In this study, the parameters of coating process in the PEGylated chitosan-based doped cobalt ferrite nanoparticles for hyperthermia applications was optimized using the Taguchi method. The effect of chitosan and polyethylene glycol as well as the glutaraldehyde (crosslinker) contents on the structural, microstructural, and magnetic properties and the colloidal stability of the nanoparticles were investigated. Ultimately, to optimize the properties of the nanoparticles, the Taguchi method (L16 orthogonal array) was employed. The incorporation of the Ca2+ and Gd3+ into the spinel structure with an average crystallite size of 21 nm was successfully verified by X-ray diffraction (XRD) and the Rietveld refinement analysis. Moreover, thermogravimetric (TGA) analysis and Fourier-transform infrared spectroscopy (FT-IR) demonstrated the effective functionalization of the chitosan and polyethylene glycol coatings. Vibrating sample magnetometer (VSM) measurements revealed that saturation magnetization (M s) was achieved in the range of 54.91–60.42 emu/g. Also, microstructural observations indicated that the coating layer can affect the particle size distribution and morphology. The Taguchi method identified that the glutaraldehyde content is the most expressive parameter influencing the nanoparticles' properties, with the optimum properties achieved at chitosan, polyethylene glycol, and glutaraldehyde of 0.4 g, 0.08 g, and 1.2 ml, respectively. The antibacterial analysis confirmed the promising potential of the coated nanoparticles for biomedical applications. Furthermore, the magnetic heating efficiency of the synthesized nanoparticles was investigated in various concentrations of magnetic fluid, demonstrating their suitability for magnetic hyperthermia application. [Display omitted] • Coating process in the PEGylated chitosan-based Co0.968Ca0.032Fe1.97Gd0.03O4 nanoparticles was optimized. • The magnetic and colloidal stability of the coated nanoparticles was optimized by Taguchi method. • The glutaraldehyde content was identified as the most effective parameter on the properties. • The antibacterial studies demonstrated that coated nanoparticles possess the capacity to effectively inhibit bacteria. [ABSTRACT FROM AUTHOR]

[Display omitted] • A reusable magnetic nanocatalyst was designed for a transesterification reaction. • Biodiesel yield of 96.94 % was achieved through process optimization. • The reaction exhibited pseudo-first-order kinetics, necessitating less energy input. • The nanocatalyst retained its chemical stability and catalytic activity for 8 runs. • The biodiesel minimum selling price was 0.943 $/kg, with 2.25 years payback period. A unique magnetically reusable basic ionic liquid nanocatalyst was designed to convert algal oil into biodiesel. The imidazolium-based basic ionic liquid (BIL) was immobilized onto the magnetic nanocomposite (Fe 3 O 4 -SiO 2), resulting in Fe 3 O 4 -SiO 2 -BIL nanocatalyst with notable properties of large surface area (21.963 m2/g), high saturation magnetization (28.78 emu/g) and high thermal stability. The nanocatalyst effectively facilitated algal oil transesterification with 96.94 % biodiesel yield under methanol: oil ratio of 8.58:1 and a catalyst dosage of 4.59 % at 60.60 °C for 87.42 min. The transesterification reaction proceeded with pseudo-first-order kinetics, featuring a relatively low activation energy. The nanocatalyst showed excellent reusability for eight recycles. A techno-economic analysis demonstrated that industrial-scale biodiesel production from algal feedstock was economically viable with a minimum payback of 2.25 years. The environmental analysis revealed a low carbon footprint with the emission of 0.074 kg CO 2 per kg of biodiesel produced. This research presents a sustainable approach for biodiesel production from algal feedstock facilitated by Fe 3 O 4 –SiO 2 –BIL nanocatalyst, offering both environmental and economic benefits. [ABSTRACT FROM AUTHOR]

A one-pot synthesis method of polymer–metal–carbon nanomaterials in the interfacial process was developed. Ternary nanocomposites are single-walled carbon nanotubes (SWCNT) with immobilized magnetite nanoparticles, coated with polydiphenylamine-2-carboxylic acid (PDPAC). The chemical structure and morphology of the obtained hybrid Fe3O4/SWCNT/PDPAC nanocomposites were investigated by FTIR spectroscopy, X-ray diffraction, transmission electron microscopy and field emission (FE-SEM) scanning electron microscopy. The size of Fe3O4 nanoparticles is within the range of 2 < d < 14 nm. Multifunctional nanocomposite materials exhibit thermal, electrical and magnetic properties. Fe3O4/SWCNT/PDPAC nanocomposites were used for preparation of stable magnetic fluids in water and ethyl alcohol. [ABSTRACT FROM AUTHOR]

The effective electric charge of a nanoparticle in an ionic magnetic colloidal system (an ionic ferrofluid) is determined by using the impedance spectroscopy technique. The electric response of the samples to a harmonic external electric field excitation is described by means of the Poisson-Nernst-Planck model. The model proposed for the theoretical interpretation of the impedance spectroscopy data considers that the magnetic particles are electrically charged with H+ and have in their vicinity Cl- counterions, resulting in an effective charge Qeff. In the presence of an harmonic, in time, external field (frequency bigger than 104Hz) particles are assumed to be at rest, due to inertial reason. In this framework, the response of the cell is due to the H+ and Cl- present in the solution. From the spectra of the real and imaginary components of the electric impedance of the cell, by means of a best fit procedure to our model, we derive the effective electric charge of the magnetic particles and the bulk density of ions. From an independent measurement of the ζ-potential of the suspension, it is possible to calculate the hydrodynamic radius of the particle, in good agreement with that independently measured. [ABSTRACT FROM AUTHOR]

Evolution of microstructures in magnetite-based ferrofluids with weak dipolar moments (particle size⩽10 nm) is studied with an emphasis on examining the effects of particle concentration (ϕ) and magnetic field strength (H) on the structures. Nanoparticles are dispersed in water at three different concentrations, ϕ=0.15%, 0.48%, and 0.59% (w/v) [g/ml%] and exposed to uniform magnetic fields in the range of H=0.05-0.42 T. Cryogenic transmission electron microscopy is employed to provide in-situ observations of the field-induced assemblies in such systems. As the magnetic field increases, the Brownian colloids are observed to form randomly distributed chains aligned in the field direction, followed by head-to-tail chain aggregation and then lateral aggregation of chains termed as zippering. By increasing the field in low concentration samples, the number of chains increases, though their length does not change dramatically. Increasing concentration increases the length of the linear particle assemblies in the presence of a fixed external magnetic field. Thickening of the chains due to zippering is observed at relatively high fields. Through a systematic variation of concentration and magnetic field strength, this study shows that both magnetic field strength and change in concentration can strongly influence formation of microstructures even in weak dipolar systems. Additionally, the results of two commonly used support films on electron microscopy grids, continuous carbon and holey carbon films, are compared. Holey carbon film allows us to create local regions of high concentrations that further assist the development of field-induced assemblies. The experimental observations provide a validation of the zippering effect and can be utilized in the development of models for thermophysical properties such as thermal conductivity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper is focused on the signature of thermal-assisted cluster disruption while analyzing the inverse alternating current (AC) susceptibility (1/χ) versus temperature (T) curves recorded at lower AC frequencies (f), below 300 Hz. A commercial oil-based magnetic fluid (MF) sample was used in the experiments to investigate the critical temperature (T*) that characterizes the thermal disruption of aggregates suspended within the MF sample. T* was found to reduce as f increased within the frequency range of our investigation (63-263 Hz). Furthermore, T* was found to scale with the square of the applied AC frequency. Both theoretical and experimental evidences support that the excitation field frequency (f) dependence of the critical temperature (T*) is well described by T*(f) = T*(0)-Af2/1+Bf2. The model is based on energy absorption of magnetic nanoparticles in an AC magnetic field. [ABSTRACT FROM AUTHOR]

A self-regulating magnetic fluid with a suitable second-order phase transition temperature for achieving hyperthermia treatment temperatures is investigated for use as a possible mild cancer treatment. Current studies have focused on the optimization of magnetic fluids. In this work, magnetic fluids of Al-doped Sr2FeMoO6 nanoparticles are investigated based on their relatively high saturation magnetization. The nanoparticles are synthesized using a simple sol–gel method, successfully nanomized, and then coated by oleic acid through ball milling to prevent oxidation and improve biocompatibility. The crystal structure, morphology, basic magnetic properties and heating properties of the samples in a high-frequency magnetic field are investigated. It is found that oleic acid-coated Sr1.98Al0.02FeMoO6 nanoparticles with a particle diameter of ~ 140 nm and a maximum heating temperature of 47.8 °C at a concentration of 2 mg·ml−1 possess excellent induction heating properties. The saturation magnetization, coercive force, specific absorption rate and effective specific absorption rate are 18 emu/g, 5.14 mT, 41.8 W·g−1 and 0.0193 W·g−1·kHz·(kA·m−2), respectively. [ABSTRACT FROM AUTHOR]

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Studies highlighting nanoparticles suspensions and flow attributes in the context of their application are the subject of current research. In particular, the utilization of these materials in biomedical rheological models has gained great attention. Magneto nanoparticles have a decisive role in the ferrofluid flows to regulate their viscoelastic physiognomies. Having such substantial interest in the flow of ferrofluids our objective is to elaborate the melting heat transfer impact in a stretched Oldroyd-B flow owing to a magnetic dipole in the presence of entropy generation optimization. Buongiorno nanofluid model expounding thermophoretic and Brownian features are considered. Moreover, activation energy with chemical reaction is also considered. The Cattaneo–Christov heat flux model is affianced instead of conventional Fourier law. The renowned bvp4c function of MATLAB is utilized to handle the nonlinearity of the system. Impacts of miscellaneous parameters are portrayed through graphical fallouts and numeric statistics. Results divulge that the velocity and temperature profiles show the opposite trend for growing estimates of the ferromagnetic parameter. It is also noticed that the temperature ratio parameter diminishes the entropy profile. Moreover, it is seen that the concentration profile displays a dwindling trend for the Brownian motion parameter and the opposite trend is witnessed for the thermophoretic parameter. [ABSTRACT FROM AUTHOR]

Diffraction patterns formed by a ferronematic sample, which contains 4-(trans- 4-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT) liquid crystal doped with Fe3O4 rodlike magnetic nanoparticles, was studied. The studied mixture can be applicated as a liquid magnetic switch. The diffraction patterns were observed from the micro-lines scribed on the polymer layer, which was contained by the liquid crystal mixture cell, with dimensions of 4 nm for depth and 0.32 μm for width and 5000 line per 1 mm² in the case of absence and presence of the DC magnetic field. From the experimental results, it was observed that the application of the magnetic field caused more than 17% expansion in the diffraction patterns area at the focused point, while the number of patterns was increased by 25%. This change in the diffraction area due to the application of a magnetic field gives the motive to sense the low DC magnetic field which could be useful in the application of liquid switching. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging (MRI) has emerged as one of the most promising techniques, which employs nanoscience and imaging technology, for early diagnosis of the cancers. In this work, magnetic nanoparticles of Fe3O4@ poly(acrylic) acid (PAA) were synthesized using a low cost thermal decomposition method from iron ions precursor and utilized as a signal agent for MRI imaging. The Fe3O4 magnetic nanoparticles obtained from FeCl2.4H2O precursor at the reaction temperature of 315 °C in 1‐octadcene solvent are of spherical shape with average diameter of 8.4 nm and high saturation magnetization (Ms) of 60.05 emu/g. After phase transferred with PAA, the magnetic Fe3O4@PAA fluid is highly stable in a wide range of pHs solution (5 ‐ 9) and wide range of NaCl concentrations (50 ‐ 300 mM) with zeta potential of −35.7 mV. The prepared Fe3O4@PAA fluid showed noncytotoxicity to the Hep‐G2, MCF‐7, RD cancer cell line and Vero healthy cell line. MRI in‐vitro results showed that transverse relaxation rate r2 was around 158.4 mM−1s−1 and the signal intensity was not affected by the pHs solution and NaCl concentrations. In‐vivo test on rabbit revealed the clear observation of rabbit's body parts after injection of the Fe3O4@PAA magnetic fluid with retention time of 90 minutes in the body. [ABSTRACT FROM AUTHOR]

Isolation of exosomes from biological samples provides a minimally-invasive alternative for basic understanding, diagnosis, and prognosis of metastatic cancers. The biology and clinical values of exosomes are under intensive investigation, yet most studies are limited by technical challenges in recovering these exosomes with heterogeneous sizes and cargos from biological samples. We report a novel method based on "particle ferrohydrodynamics" and its associated microfluidic device, termed as the FerroChip, which can separate exosome-like nanoparticles from microliters of cell culture media and human serum in a label-free, continuous-flow and size-dependent manner, and achieves a high recovery rate (94.3%) and a high purity (87.9%). Separated exosome-like nanoparticles had diameters, morphology, and protein expressions that were consistent with other reports. This method, upon further molecular characterization, could potentially facilitate basic understanding of exosomes and its clinical application in blood liquid biopsy. [ABSTRACT FROM AUTHOR]

The objective of this study is to examine the role of blood perfusion in magnetic fluid hyperthermia by drug delivery. Although increasing blood perfusion due to high local temperature difference refers to the intelligent response of the human body, some researchers are accustomed to employing a constant rate of blood perfusion in the bioheat transfer equation. In this study, we considered a variable volumetric rate for blood perfusion in tissue, experimentally and numerically by Galerkin method. To validate the results obtained from numerical methods, an experimental procedure proposed. Furthermore, in order to transfer magnetic fluid by drug delivery, infusion pomp by which the saline serum with nanoparticles through the vein and artery was employed to make a spherical distribution in tissue. This circulation made it possible to transfer nanoparticles to tissue without any injection. After 60 minutes and changing the dark color of the saline serum to light clear, high amount of nanoparticles (about 90 %) was transferred to the tissue. After slicing the tissue, the rather spherical distribution of nanoparticles may be illustrated. Afterward, that circulation at the rate equal to blood perfusion has been made to simulate blood perfusion in dead tissue. To measure temperature, seven Lm35 sensors have been inserted into the tissue to report temperature differences. Based on results, there was seen a significant change in hyperthermic temperature difference in comparison with results obtained from Pennes' bioheat transfer equation. Additionally, a difference of about 5–10 % was seen between numerical and experimental results. [ABSTRACT FROM AUTHOR]

In the present study, we propose a new thermal conductivity model for ferrofluids using the aggregation structure of magnetic nanoparticles under external magnetic field. According to this model, in the presence of an external magnetic field, the total ferrofluid volume consists of two parts: a volume with aggregated magnetic nanoparticles - chainlike clusters and a volume with non-aggregated uniformly dispersed nanoparticles. It has been shown that with increasing the external magnetic field the degree of aggregation increases proportionally to the magnetic field strength. It has been proven that the thermal conductivity of ferrofluids along the magnetic field direction is higher than in the perpendicular direction, due to the increased contribution of aggregated magnetic nanoparticles with a magnetic field. The predictions of the proposed model of anisotropic thermal conductivity for ferrofluids were compared with the available experimental data and a good correlation was achieved. [ABSTRACT FROM AUTHOR]

In the present study, we utilized a novel electrochemical sensor for the determination of colchicine in 0.1 M phosphate buffer solution (pH 4.0) using differential pulse voltammetry. 1-butyl-3-methylimidazolium tetrachloroferrate ([Bmim]FeCl4) was synthesized, investigated by FT-IR, Raman and UV–Vis spectrometer and applied as magnetic ionic liquid in electrode modification. The other modifier was copper oxide nanoparticles/carbon nanofiber composite (CuO/CNF) provided by electrospinning and evaluated by scanning electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy (FT-IR). Several electro-analytical parameters such as the electrochemical behavior of the modified electrode, scan rate, type and supporting electrolyte pH were studied for the determination of colchicine. Under the optimized experimental conditions, oxidative peak current increased linearly by increasing colchicine concentration in the range of 1.0 to 100.0 nM and detection limit was obtained 0.25 nM. The relative standard deviation for six replicate analyses of 20.0 nM and 58.0 nM colchicine solution was obtained to be ± 3.17% and ± 2.08%, respectively. The proposed electrochemical sensor was successfully applied for colchicine quantification in colchicine tablet and health human plasma with satisfying results. [ABSTRACT FROM AUTHOR]

The aim of the work is specific surface area optimization of nanoarchitecture of magnetosensitive nanocomposites (NC) of superparamagnetic core– multilevel shell type for application in oncology. The calculation technique is proposed and tested, and its examination is carried out by experimental studies. Ensembles of Fe3O4 nanoparticles, Fe3O4@CP and Fe3O4@CP/Ol.Na/PEG nanocomposites are used for investigations, where CP is an antitumor chemotherapeutical drug cisplatin based on cis-diamminedichloroplatinum, Ol.Na—sodium oleate, PEG—polyethylene glycol, as well as magnetic fluids (MF) filled with the said nanostructures based on water or physiological solution (PS). Realization conditions and formulas for determination of maximal specific surface area of ensembles of NC with single- and bilayer shells are analysed. Scientific and methodological approaches of the work are applicable to NC based on superparamagnetic cores containing other chemotherapeutical drugs in the shell structure, for example, doxorubicin and gemcitabine, for calculations of specific surface area of the layers with arbitrary values of their number and density. The results of the work are also actual for application in development of new magnetically operated adsorption materials of technical, technological, ecological, and medical-biological destination, medical test-systems, and theranostic remedies. [ABSTRACT FROM AUTHOR]

This work was conducted to investigate the effects of the polyethylene oxide (PEO) as a template on the structures, functional groups, magnetic properties, and antibacterial activities of Fe3O4/PEO/TMAH ferrofluids. Synthesis of Fe3O4 nanoparticles was carried out by using the co-precipitation method obtained from iron sand. The ferrofluid synthesis was done by coating Fe3O4 nanoparticles with tetramethylammonium hydroxide (TMAH) and dispersed in H2O. Based on the structural analysis, it was known that all samples had a magnetite phase with particle sizes ranging from 7.8 to 10.5 nm. The Fe3O4/PEO/TMAH ferrofluid functional groups presented appropriate bonds of Fe3O4 as a filler, TMAH as a surfactant, and H2O as a liquid carrier. The magnetic characteristics of Fe3O4 nanoparticles and Fe3O4/PEO/TMAH ferrofluids indicated the superparamagnetic state. The saturation magnetization value of Fe3O4 nanoparticles and Fe3O4/PEO/TMAH ferrofluids decreased as increasing the molecular weight of polyethylene oxide ranging from 1000 to 20,000. Furthermore, the antibacterial activities performed by the dilution method identified that increasing the molecular weight of polyethylene oxide increased the antibacterial performance of the Fe3O4/PEO/TMAH ferrofluids against Escherichia coli and Bacillus subtilis. [ABSTRACT FROM AUTHOR]

Magnetic fluid hyperthermia (MFH) is a noninvasive treatment that destroys cancer cells by heating a ferrofluid-impregnated malignant tissue with an ac magnetic field while causing minimal damage to the surrounding healthy tissue. The strength of the magnetic field must be sufficient to induce hyperthermia but it is also limited by the human ability to safely withstand it. The ferrofluid material used for hyperthermia should be one that is readily produced and is nontoxic while providing sufficient heating. We examine six materials that have been considered as candidates for MFH use. Examining the heating produced by nanoparticles of these materials, barium-ferrite and cobalt-ferrite are unable to produce sufficient MFH heating, that from iron-cobalt occurs at a far too rapid rate to be safe, while fcc iron-platinum, magnetite, and maghemite are all capable of producing stable controlled heating. We simulate the heating of ferrofluid-loaded tumors containing nanoparticles of the latter three materials to determine their effects on tumor tissue. These materials are viable MFH candidates since they can produce significant heating at the tumor center yet maintain the surrounding healthy tissue interface at a relatively safe temperature. [ABSTRACT FROM AUTHOR]

We prepared Fe3O4 magnetic nanoparticles having diameters of approximately 12 nm by chemical coprecipitation, which were coated with three different fatty acid surfactants: oleic acid, lauric acid, and myristic acid. From x-ray diffraction, transmission electron microscopy, and Mössbauer spectroscopy measurements we confirmed that Fe3O4 is the only phase present in the samples. The zero field cooled magnetization curves for the nanoparticles exhibit broad peaks, consistent with superparamagnetic blocking for the polydisperse samples, and a saturation magnetization smaller than that for bulk Fe3O4. Although there are minimal differences in the magnetic properties of the nanoparticles having different surfactants, we find significant changes in the hydrodynamic response depending on chain length. Hyperthermia measurements show considerably larger response for oleic acid-coated samples, while magneto-optical studies indicate that these samples have slower dynamics of aggregation under the influence of a dc field. These results suggest that the magnetohydrodynamic response of ferrofluids can be controlled by judiciously selecting appropriate surfactants. [ABSTRACT FROM AUTHOR]

Magnetization and Mossbauer spectroscopy measurements are performed at low temperature under high field, on nanoparticles with a nickel ferrite core and a maghemite shell. These nanoparticles present finite size and surface effects, together with exchange anisotropy. High field magnetization brings the evidences of a monodomain ordered core and surface spins freezing in disorder at low temperature. Mossbauer spectra at 4.2 K present an extra contribution from the disordered surface which is field dependent. Field and size dependences of this latter show a progressive spin alignment along the ferrite core which is size dependent. The weak surface pinning condition of the nanoparticles confirms that the spin disorder is localized in the external shell. The underfield decrease in the mean canting angle in the superficial shell is then directly related to the unidirectional exchange anisotropy through the interface between the ordered core and the disordered shell. The obtained anisotropy field HEa scales as the inverse of the nanoparticle diameter, validating its interfacial origin. The associated anisotropy constant KEa equals 2.5×10-4 J/m2. [ABSTRACT FROM AUTHOR]

The dependence of the transmission behavior of magnetic fluids on the incident power density of a laser beam is investigated and the conventional Z-scan technique is employed to continuously vary the optical forces induced by the laser beam. We calculate the optical forces exerted on magnetic nanoparticles and compare them with those for gold and silica nanoparticles. It is found that the optical forces for magnetic nanoparticles are comparable to those for gold nanoparticles. In addition, the calculation results show that the absorption force is dominant at low incident power densities while the gradient and scattering forces become significant at high incident power densities when the clustering of magnetic nanoparticles occurs. In Z-scan experiments, it is observed that the evolution of the Z-scan trace of a magnetic fluid with increasing incident power density cannot be explained only by the nonlinear absorption of the magnetic fluid induced by the thermal diffusion of magnetic nanoparticles. Instead, it indicates that the optical forces exerted on magnetic particles play an important role in determining the transmission of the magnetic fluid. This point is verified by comparing the Z-scan traces for magnetic fluids with different particle concentrations and carrier liquids. The effects of optical forces on the transmission of the magnetic fluids are also manifested in the morphology change of the magnetic fluids. [ABSTRACT FROM AUTHOR]

In this study static magnetic birefringence (SMB) was used to investigate the effects of aging of two biocompatible magnetic fluid (BMF) samples after dilution with pure water. The two BMF samples are based on dimercaptosuccinic-coated maghemite nanoparticles, with very similar average core diameters (7.2 and 7.9 nm) but presenting different dimercaptosuccinic acid surface grafting coefficients. Stock samples were submitted to different degrees of dilution and left to age on shelve before recording the SMB signal. Changes in the diluted and aged samples were evaluated via the average Q-cluster size value () and Q-cluster size-dispersion value (σQ) related to a log-normal distribution function P(Q). The and σQ values were obtained from the analysis of the SMB signal. We found strong evidences that the higher the dilution and the higher the surface grafting coefficient the lower both the and σQ values. [ABSTRACT FROM AUTHOR]

A ferrofluid composed of γ-Fe2O3 nanoparticles of an average diameter of 11 nm dispersed in n-decane was prepared. A series of magnetic measurements was carried out on samples frozen in a magnetic field and in zero magnetic field. The influence of interparticle interactions on the magnetic behavior of ferrofluids is addressed and the potential use of magnetic measurements in estimating the extent of nanoparticle fusion into clusters behaving as superspins is proposed. [ABSTRACT FROM AUTHOR]

An approach for studying steady-state nanoparticle concentration gradients arising in magnetic nanoparticle suspensions in response to strong magnetic field gradient is presented. The experimental approach makes use of microscopic optical absorption measurements of ferrofluid interacting with arrays of patterned magnets. Experimental results are found to be consistent with a simple theoretical description that predicts the local nanoparticle concentration over a wide range of magnetic field conditions and ferrofluid volume fractions. [ABSTRACT FROM AUTHOR]

We have investigated the ac magnetic susceptibility and magnetic heating of aqueous suspensions of γ-Fe2O3 nanoparticles embedded in alginate hydrogel matrix and isolated γ-Fe2O3 and Fe3O4 nanoparticles coated with tetramethyl ammonium hydroxide. All three ferrofluids were characterized by measuring the dc magnetization, ac susceptibility, and magnetic heating. We found that significant Néel relaxation is present in all samples, but only the isolated nanoparticle ferrofluids show any significant feature associated with Brownian relaxation near the freezing temperature of the carrier liquid. The heating rate of the ferrofluids varies systematically with the magnitude of the Brownian relaxation peak, despite similar values of the absolute magnetization. These results highlight the importance of the Brownian relaxation for heating applications incorporating magnetic nanoparticles. [ABSTRACT FROM AUTHOR]

Aging phenomena were investigated in three different ferrofluids containing submicron sized magnetic beads consisting of a cluster of iron oxide nanoparticles embedded in a dextrane matrix with primary amine groups on the surface (one fresh sample and one sample aged during three months) and with a plain surface (only dextrane on the surface, aged for three months), suitable for magnetic biosensor applications. The main characterization technique used was measurement of the frequency dependent complex magnetization using a superconducting quantum interference device. The experimental factors considered were the effect of ultrasonication, temperature, dc bias magnetic field, and addition of sodium dodecyl sulphate surfactant. The stability against aging was found to depend strongly on the bead surface, and the reproducibility of a refreshing procedure involving addition of surfactant in combination with ultrasonication was shown to be low. Aggregation was shown to be strongly affected by the presence of even a small bias magnetic field. Applying a dc bias field caused the formation of more or less stable chainlike aggregates with various lengths. [ABSTRACT FROM AUTHOR]