Aqueous Ferrofluids Research Articles

On the impact of surfactant type on the structure of aqueous ferrofluids

The impact of surfactant type on the structure organization of aqueous ferrofluids was considered based on the data of small-angle neutron scattering (SANS). The aggregates of different sizes and types were observed for magnetite magnetic nanoparticles of in heavy water stabilized by sodium oleate (SO) or dodecylbenzene sulphonic acid (DBSA). According to the surface tension measurements it was shown the significant difference of the critical micelle concentrations for SO and DBSA solutions. The observed structural difference of the aggregates in the ferrofluids was related to the behaviour of surfactant molecules in aqueous solutions including the structure and interaction parameters of micelles (micelle aggregation number, fractional charge, charge per micelle and surface potential, etc.) derived from the SANS analysis.

On the adsorption of magnetite nanoparticles on lysozyme amyloid fibrils

An adsorption of magnetic nanoparticles (MNP) from electrostatically stabilized aqueous ferrofluids on amyloid fibrils of hen egg white lysozyme (HEWL) in 2mg/mL acidic dispersions have been detected for the MNP concentration range of 0.01⿿0.1vol.%. The association of the MNP with amyloid fibrils has been characterized by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS) and magneto-optical measurements. It has been observed that the extent of adsorption is determined by the MNP concentration. When increasing the MNP concentration the formed aggregates of magnetic particles repeat the general rod-like structure of the fibrils. The effect is not observed when MNP are mixed with the solution of lysozyme monomers. The adsorption has been investigated with the aim to clarify previously found disaggregation activity of MNP in amyloid fibrils dispersions and to get deeper insight into interaction processes between amyloids and MNP. The observed effect is also discussed with respect to potential applications for ordering lysozyme amyloid fibrils in a liquid crystal phase under external magnetic fields.

Facile synthesis of magnetic agarose microfibers by directed self-assembly in W/O emulsions

We propose a new versatile approach for the fabrication of flexible magnetic microfibers having tunable size and mechanical properties. The methodology is based on the magnetic field-driven self-assembly of agarose-loaded kinetically unstable aqueous ferrofluid-in-oil emulsions. Agarose is dissolved at a temperature above its gelling point in an aqueous ferrofluid and the resulting suspension is then emulsified in oil and later exposed to a DC uniaxial magnetic field. Once the fibers have formed, the emulsion is quenched to force the gelification of the agarose, and a second addition of surfactant is carried out to prevent lateral aggregation of the fibers. Resulting fibers have diameters of the order of a few to tens of micrometers. A phase diagram is constructed and the influence of surfactant concentration and residence time under field in the synthesis is discussed.

Colloidal properties of water dispersible magnetite nanoparticles by photon correlation spectroscopy

Temperature and time trend analyses by light scattering technique over a year on aqueous ferrofluid, synthesized by coprecipitation technique, showed unambiguously that they are ultrastable and monodisperse.

Biomimetically Synthesized Aqueous Ferrofluids Having Antibacterial and Anticancer Properties

Synthesis of functional iron oxide nanoparticles, well dispersed in aqueous fluids still remains a challenge as its stability requires a delicate balance between electrostatic and magnetic interactions. Templated synthesis using biomolecules is useful because the biomolecules have their unique arrangement in aqueous systems that enhance stability, commonly called “biomimetic synthesis”. We have developed a one-pot in-situ, low energy process for the synthesis of highly monodispersed, Collagen Functionalized Ferrofluids (CFF) as a templating agent in an aqueous medium. The nanoparticles so obtained were characterized by X-ray diffraction (XRD), dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR). The antibacterial activity in terms of minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC) and growth inhibition has been assessed against gram positive, Staphylococcus aureus, ATCC 13709 (native strain) and in Escherichia coli, DH5α gram negative bacteria. The cytotoxicity of the CFFs on cancer cell lines human embryonic kidney (HEK), breast adenocarcinoma (MCF-7) and Ehrlich ascitic carcinoma (EAC) have also been investigated. CFFs indicated variable MIC and MBC values against S. aureus and E. coli being minimum for 1.5% CFF (MIC:23.43 μg/ml and 93.75 μg/ml and MBC: 46.87 μg/ml and 187.5 μg/ml). The observed cytotoxicity in mammalian cells indicated the susceptibility of MCF-7 breast cancer cells when compared to HEK cells.

SiO2 versus chelating agent@ iron oxide nanoparticles: interactions effect in nanoparticles assemblies at low magnetic field

Hydrophilic magnetic nanoparticles present many interest for various medical applications due to their unique properties: immunoassays, imaging and hyperthermia. With regards to their applicability in the biomedical field, colloidal stability is a key parameter related to nanoparticle surface functionalization. In this paper, we report the water transfer of hydrophobic oleic acid coated iron oxide nanoparticles comparing two methodologies to obtain water dispersible iron oxide nanoparticles: exchange ligands with small strong chelating agent (caffeic acid) and SiO2 shell passivation. Both strategies are leading to stable aqueous ferrofluid but differing by their interactions. The non linear magnetic behavior at high and low magnetic field and second derivative signature of water dispersed superparamagnetic Fe304 nanoparticles samples are studied using conventional SQUID equipment and miniaturized detector MIAplex® device. We demonstrated those samples differing only by their interparticle interactions present different magnetic behavior at very low magnetic field whereas at high magnetic field both samples are very similar.

The Oxygen Reduction Reaction in Ferrofluids: Towards Membrane-less and Spill-less Gas Sensors

In this study, a “membrane-less and spill-less” gas-sensing device has been evaluated for the electrochemical detection of oxygen. Iron oxide magnetic nanoparticles were prepared by chemical co-precipitation and used to prepare an aqueous ferrofluid. The iron oxide nanoparticles were subsequently stabilised and passivated with a cationic polymer, namely, poly(diallyldimethyl ammonium chloride). The resulting ferrofluid was evaluated as an electrolyte for the analytical quantification of oxygen on screen-printed carbon electrodes. An applied magnetic field immobilised the ferrofluid electrolyte in place to result in a “membrane-less and spill-less” ferrofluid-based gas sensor. The polymer poly(diallyldimethyl ammonium chloride) was found to result in an apparent enhancement in the electrocatalysis of the system towards the oxygen reduction reaction. Furthermore, as the strength of the applied magnetic field was increased, the oxygen reduction current also increased owing to the response of the polymer-coated nanoparticles. The oxygen reduction current was linear from 0 to 100 % oxygen content.

Investigation of Fe3O4 Aqueous Ferrofluids Before and After Freezing

The magnetization and magnetoviscous properties of Fe3O4 aqueous ferrofluids before and after zero-field freezing are investigated. The results show that the magnetization of ferrofluids is less than that of Fe3O4 nanoparticles. The saturation magnetization of ferrofluids after freezing is greater than that of ferrofluids before freezing due to the breakage of aggregations in ferrofluids. Our experimental data agree well with the model of gas-like compression. The viscosity-magnetic field hysteretic curves of condensed ferrofluids after freezing are initially studied. The results demonstrate a special curve with increasing and decreasing fields. The viscosity changes under magnetic fields are related to the magnetically induced structures.

Flows and torques in Brownian ferrofluids subjected to rotating uniform magnetic fields in a cylindrical and annular geometry

Ferrofluid flow in cylindrical and annular geometries under the influence of a uniform rotating magnetic field was studied experimentally using aqueous ferrofluids consisting of low concentrations (<0.01 v/v) of cobalt ferrite nanoparticles with Brownian relaxation to test the ferrohydrodynamic equations, elucidate the existence of couple stresses, and determine the value of the spin viscosity in these fluids. An ultrasound technique was used to measure bulk velocity profiles in the spin-up (cylindrical) and annular geometries, varying the intensity and frequency of the rotating magnetic field generated by a two pole stator winding. Additionally, torque measurements in the cylindrical geometry were made. Results show rigid-body like velocity profiles in the bulk, and no dependence on the axial direction. Experimental velocity profiles were in quantitative agreement with the predictions of the spin diffusion theory, with a value of the spin viscosity of ∼10−8 kg m/s, two orders of magnitude larger than the value estimated earlier for iron oxide based ferrofluids, and 12 orders of magnitude larger than estimated using dimensional arguments valid in the infinite dilution limit. These results provide further evidence of the existence of couple stresses in ferrofluids and their role in driving the spin-up flow phenomenon.

Magnetically modified coffee silverskin for the removal of xenobiotics from wastewater

Coffee silverskin (CS), a solid waste from the coffee industry, was magnetically modified by contact with an aqueous ferrofluid containing magnetite nanoparticles. The resulting material was characterized by SEM-EDX analysis and X-ray diffraction. To assess the suitability of the magnetically modified CS for the removal of pollutants from wastewaters, both equilibrium and kinetic adsorption experiments were performed. Methylene blue was used as a model contaminant. Equilibrium and kinetic data were fitted by the Langmuir equation and the pseudo-secondorder model. A maximum adsorption capacity of 556 mg g –1 and a half-adsorption time ranging from 12.8 to 35.3 min were determined. The magnetically modified adsorbent was regenerated by washing with 0.01 N HCl and recovered from the aqueous solution by a permanent magnet. Overall, the results obtained strongly support the use of the magnetic material developed as a new lowcost adsorbent for the large-scale removal of pollutants from wastewater.

Preparation and Characterization of Magnetically Responsive Biosorbents from Coffee Industry Residues

Spent coffee grounds (SCG) and coffee silverskin (CS), the two main coffee industry residues, were magnetically modified by contact with an aqueous ferrofluid containing magnetite nanoparticles. The materials were characterized by SEMEDX analysis, X-ray diffraction and FTIR spectroscopy. Batch adsorption experiments were performed to assess their suitability as biosorbents, using methylene blue as a model pollutant. Adsorption equilibrium data were analysed by the Langmuir model, which allowed estimation of the maximum adsorption capacity. The magnetic biosorbents were easily regenerated by treatment with hydrochloric acid.

Experimental investigation on laminar forced convection heat transfer of ferrofluids under an alternating magnetic field

Abstract This research study presents an experimental investigation on forced convection heat transfer of an aqueous ferrofluid flow passing through a circular copper tube in the presence of an alternating magnetic field. The flow passes through the tube under a uniform heat flux and laminar flow conditions. The primary objective was to intensify the particle migration and disturbance of the boundary layer by utilizing the magnetic field effect on the nanoparticles for more heat transfer enhancement. Complicated convection regimes caused by interactions between magnetic nanoparticles under various conditions were studied. The process of heat transfer was examined with different volume concentrations and under different frequencies of the applied magnetic field in detail. The convective heat transfer coefficient for distilled water and ferrofluid was measured and compared under various conditions. The results showed that applying an alternating magnetic field can enhance the convective heat transfer rate. The effects of magnetic field, volume concentration and Reynolds number on the convective heat transfer coefficient were widely investigated, and the Optimum conditions were obtained. Increasing the alternating magnetic field frequency and the volume fraction led to better heat transfer enhancement. The effect of the magnetic field in low Reynolds numbers was higher, and a maximum of 27.6% enhancement in the convection heat transfer was observed.

Synthesis of silica-coated aqueous ferrofluids through ligand exchange with a new organosilica precursor

A new method for the production of aqueous dispersions of superparamagnetic iron oxide nanoparticles with applications in biomedicine is reported. The method is based on the use of a triethoxisilyl dodecanoic acid ligand that has been specially synthesized for this purpose. The nanoparticles were grown in organic medium using oleic acid as surfactant. Subsequently, oleic acid was exchanged for the alkoxysilane ligand, then hydrolysis was performed in a hydrocarbon solvent, and the nanoparticles were transferred into water. The organic and aqueous ferrofluids have been characterized and their magnetic properties have been determined. The resulting maghemite/silica nanoparticles were single core, and stable in aqueous suspension.

Formation of Columnar Structures by the Magnetically Directed Assembly of Cobalt Ferrite Nanoparticles

We have studied the assembly of cobalt ferrite nanoparticles in a magnetic field. Nanoparticles of different sizes, i.e., 5-40 nm, were synthesized with coprecipitation or under hydrothermal conditions. Their saturation magnetization values ranged between 31 and 68 Am2 /kg, respectively, resulting in a strong magnetic attraction and agglomeration between the nanoparticles. In aqueous ferrofluids this was prevented by the adsorption of citric acid on the nanoparticles' surfaces. The estimated interaction energies show that the DLVO (Derjaguin, Landau, Verwey, Overbeek) theory fails to explain the stability of cobalt ferrite ferrofluids and that the solvation interaction cannot be neglected. Cobalt ferrite ferrofluids were deposited on alumina substrates and dried in a magnetic field of 0.03 or 0.5 T. The homogeneity of the deposits decreased with the increasing saturation magnetization of the nanoparticles and/or with the increasing magnetic field, until columnar structures were formed from the nanoparticles with Ms ≥ 55 Am2/kg under a magnetic field of B = 0.5 T.

Ferrohydrodynamic evaluation of rotational viscosity and relaxation in certain ferrofluids

A significant effect of aggregation dynamics for aqueous ferrofluid (AF) and kerosene based ferrofluid (KF) using magnetic field dependent capillary viscosity and magneto-optical relaxation measurements is studied. For better comparison parameters of AF and KF are kept similar. Ferrohydrodynamic equations of chain forming ferrofluids, dilute ferrofluids, and Brownian dynamic simulations are compared. It is observed that the rotational viscosity of AF is larger than that of KF due to field induced aggregates in it and strong dipolar interactions. It is also observed that at Ωτ ~ 0.04 both AF and KF viscosity becomes almost similar, suggesting similar behavior at that shear rate. The magneto-optical relaxation in AF exhibits nonexponential behavior when relaxed from higher magnetic field and follows irreversible thermodynamics, whereas for KF the relaxation is exponential and follows the effective field method. This discrepancy is explained based on aggregation dynamics of magnetic particles. Results are well described by the corresponding theoretical models.

Magnetic Field Controlled Composite Paramagnetic–Diamagnetic Colloidal Phases

We report on differently ordered colloidal phases of a mixture of paramagnetic and diamagnetic colloids subject to a quickly varying time dependent magnetic field. Effectively paramagnetic and effectively diamagnetic colloids are created from paramagnetic and nonmagnetic colloids immersed into a thin film of aqueous ferrofluid. The time-averaged dipole interaction between induced dipoles can be characterized by a uniaxial external precession angle and a biaxial eccentricity characterizing the anisotropy of the external field modulation. The variation of both control parameters causes a sequence of transitions between differently correlated orientation order between the paramagnetic and diamagnetic colloids. We observe the formation of bonds between paramagnets and diamagnets along one or two directions with a staggered order of the magnetic moments. Bonds between similar particles with uniform order of the magnetic moments form along directions orthogonal to bonds between different particles along the staggered directions. When the external precession angle passes the magic angle, the particle order rearranges and staggered directions with bonds between different particles change into uniformly ordered directions with bonds between similar particles and vice versa. The transition in order occurs in two steps with a biaxial phase intervening between the two uniaxial ordering phases.

Stability of biomimetic ferrofluids established by a systematic study using microwave irradiation at defined wattages

An established biomimetic process for the synthesis of aqueous ferrofluids using polymers has been subjected to systematic microwave irradiation at different wattages primarily to see if the magnetization could be increased by microwave irradiation and if so how would it affect the stability of the fluid. Care has been taken to maintain ambient conditions of synthesis even after three cycles of microwave irradiation before oxidation and ten cycles after it, so as not to violate the basic principles of the process. Detailed characterization using, x-ray diffractometry, transmission electron microscopy, fourier transform infra-red spectroscopy, dynamic light scattering, thermo-gravimetric analysis, differential thermal analysis and vibrating sample magnetometry showed that these fluids containing iron oxide nanoparticles-poly(vinyl) alcohol nanocomposites are highly stable in the proportions established by us. Measurements at five different wattages double the saturation magnetization but the stability remains unaffected compared to the one without microwave irradiation, forcing us to believe that the incubation of the iron salt solution and the polymer in the right proportion before oxidation is what contributes to the stability and that increasing the number of cycles of microwave irradiation at this stage, perhaps, would have led to a more pronounced effect.

Particle size and magnetic properties dependence on growth temperature for rapid mixed co-precipitated magnetite nanoparticles

Magnetite nanoparticles have been prepared by co-precipitation using a custom-designed jet mixer to achieve rapid mixing (RM) of reactants in a timescale of milliseconds. The quick and stable nucleation obtained allows control of the particle size and size distribution via a more defined growth process. Nanoparticles of different sizes were prepared by controlling the processing temperature in the first few seconds post-mixing. The average size of the nanoparticles investigated using a Tecnai transmission electron microscope is found to increase with the temperature from 3.8 nm at 1 ± 1 °C to 10.9 nm for particles grown at 95 ± 1 °C. The temperature dependence of the size distribution follows the same trend and is explained in terms of Ostwald ripening of the magnetite nanoparticles during the co-precipitation of Fe2+ and Fe3+. The magnetic properties were studied by monitoring the blocking temperature via both DC and AC techniques. Strikingly, the obtained RM particles maintain the high magnetization (as high as ∼88 A m2 kg−1 at 500 kA m−1) while the coercivity is as low as ∼12 A m−1 with the expected temperature dependence. Besides, by adding a drop of tetramethylammonium hydroxide, aqueous ferrofluids with long term stability are obtained, suggesting their suitability for applications in ferrofluid technology and biomedicine.

Synthesis of aqueous ferrofluids of ZnxFe3−xO4 nanoparticles by citric acid assisted hydrothermal-reduction route for magnetic hyperthermia applications

Superparamagnetic and monodispersed aqueous ferrofluids of Zn substituted magnetite nanoparticles (ZnxFe3−xO4, x=0, 0.25, 0.3, 0.37 and 0.4) were synthesized via hydrothermal-reduction route in the presence of citric acid, which is a facile, low energy and environmental friendly method. The synthesized nanoparticles were characterized by X ray diffraction (XRD) analysis, Fourier transform infrared (FTIR) spectroscopy, scanning and transmission electron microscopy (SEM and TEM) and the dynamic light scattering (DLS) method. The results showed that a certain amount of citric acid was required to obtain single phase Zn substituted magnetite nanoparticles. Citric acid acted as a modulator and reducing agent in the formation of spinel structure and controlled nanoparticle size and crystallinity. Mean particle sizes of the prepared nanoparticles were around 10nm. The results that are obtained from XRD, magnetic and power loss measurements showed that the crystallinity, saturation magnetization (MS) and loss power of the synthesized ferrofluids were all influenced by the substitution of Zn in the structure of magnetite. The Zn substituted magnetite nanoparticles obtained by this route showed a good stability in aqueous medium (pH 7) and hydrodynamic sizes below 100nm and polydispersity indexes below 0.2. The calculated intrinsic loss power (ILP) for the sample x=0.3 (e.g. 2.36nHm2/kg) was comparable to ILP of commercial ferrofluids with similar hydrodynamic sizes.

Modified double emulsion process as a new route to prepare submicron biodegradable magnetic/polycaprolactone particles for in vivo theranostics

A modified double emulsion evaporation method was devised to encapsulate active ingredients to be used for theranostic purposes. In comparison with classical double emulsion methods, the solvents used for this modified method are miscible. Aqueous and organic ferrofluids having different solid contents of iron oxide nanoparticles were prepared by the co-precipitation method. Different materials were screened for the preparation of biodegradable and biocompatible nanoparticles via this new modified double emulsion evaporation method. Factors affecting the final particle size were investigated by encapsulating a hydrophilic model drug that permitted us to prepare a recipe using polycaprolactone (PCL) as polymer, dichloromethane (DCM) as organic solvent and polyvinyl alcohol (PVA) as stabilizing agent. The characterization of aqueous and organic ferrofluids alone, Stilbene and iron oxide loaded nanoparticles was done by studying particle size, size distribution, zeta potential and morphology, the magnetic properties and chemical composition. All the characterization results showed that the process is praiseworthy for encapsulation of iron oxide nanoparticles (in both aqueous and organic medium) along with an active ingredient for theranostic purposes with a final size of about 300 nm.