Frequency-domain birefringence measurement of biological binding to magnetic nanoparticles

Optical detection of the frequency-dependent magnetic relaxation signal is used to monitor the binding of biological molecules to magnetic nanoparticles in a ferrofluid. Biological binding reactions cause changes in the magnetic relaxation signal due to an increase in the average hydrodynamic diameter of the nanoparticles. To allow the relaxation signal to be detected in dilute ferrofluids, measurements are made using a balanced photodetector, resulting in a 25 μV/√Hz noise floor, within 50% of the theoretical limit imposed by photon shot noise. Measurements of a ferrofluid composed of magnetite nanoparticles coated with anti-IgG antibodies show that the average hydrodynamic diameter increases from 115.2 to 125.4 nm after reaction with IgG.     

Ferrofluids Containing Fe3O4 Nanoparticles in Solidified Photoresist Carrier

Ferrofluids, formed by magnetic nanoparticles uniformly dispersed in a liquid carrier, respond to an external magnetic field, which enable the fluid's position by applying a magnetic field. Here, ferrofluids composed of Fe₃O₄ nanoparticles with oleic acid and oleylamine as the surfactant and photoresist, respectively, were prepared. Under an external magnetic field, the movement and the position of ferrofluids and the injection of the fluids into complex shapes were easily achieved. The ferrofluid surfaces were distorted under the magnetic field, and the surface structue was controlled by the applied field strength. Using a photoresist as the liquid carrier, it was possible to solidify the ferrofluids by UV irradiation. The shape and the position of the solid superparamagnetic nanoparticles/polymer composites were also determined by the external magnetic field.     

Study of magnetic and structural properties of ferrofluids based on cobalt-zinc ferrite nanoparticles

Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 30 nm, dispersed in a liquid carrier. Magnetic Co_(1−x)Zn_xFe₂O₄ (x=0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive X-ray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy (TEM) studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer (VSM); and finally, a magnetic force microscope was used to visualize the magnetic domains of Co_(1−x)Zn_xFe₂O₄ nanoparticles. X-ray diffraction patterns of Co_(1−x)Zn_xFe₂O₄ show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe₂O₄. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5±0.3) nm to (5.4±0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co-Zn ferrite nanoparticles obtained by TEM is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer's formula. The magnetic properties investigated with the aid of a VSM at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co_(1−x)Zn_xFe₂O₄ magnetic nanoparticles, the crystal and nanoparticle sizes determined by X-ray Diffraction and TEM, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is small and for this reason Co_(1−x)Zn_xFe₂O₄ nanoparticles are considered as soft magnetic material.     

Versatile ferrofluids based on polyethylene glycol coated iron oxide nanoparticles

Versatile ferrofluids based on polyethylene glycol coated iron oxide nanoparticles were obtained by a facile protocol and thoroughly characterized. Superparamagnetic iron oxide nanoparticles synthesized using a modified forced hydrolysis method were functionalized with polyethylene glycol silane (PEG silane), precipitated and dried. These functionalized particles are dispersable in a range of solvents and concentrations depending on the desired properties. Examples of tunable properties are magnetic behavior, optical and magneto-optical response, thermal features and rheological behavior. As such, PEG silane functionalized particles represent a platform for the development of new materials that have broad applicability in e.g. biomedical, industrial or photonic environments. Magnetic, optical, magneto-optical, thermal and rheological properties of several ferrofluids based on PEG coated particles with different concentrations of particles dispersed in low molecular mass polyethylene glycol were investigated, establishing the applicability of such materials.     

CoFe2O4自形成磁性液体场致结构化对磁化的影响

因为磁性液体的磁性微粒有着很强的相互作用,Langevin顺磁理论不能很好描述磁性液体的磁化强度随外磁场的变化.研究认为影响磁化的主要因素是磁性液体内微粒整体的结构化,其结构的形成储存了部分磁化功,直接或间接地影响了磁化.在此基础上提出“压缩”模型,修正了描述磁性液体常用的Langevin函数,得出了与实验较好符合的曲线.所提出的一个压缩后等效体积分数与外磁场强度的关系式,近似地描述了磁性液体在磁场中磁化的过程.由修正式得出了近似初始磁化率随体积分数变化关系.     

Ferrofluid magnetoviscous control of wall flow channeling in oorous media

We analyzed the phenomenon of ferrofiuid magnetoviscosity in high-permeability wall-region non-magnetic porous media of the Müller kind. After upscaling the pore-level ferrohydrodynamic model, we obtained a simplified volume-average zero-order axisymmetric model for non-Darcy non-turbulent flow of steady-state isothermal incompressible Newtonian ferrofluids through a porous medium experiencing external constant bulk-flow oriented gradient magnetic field, ferrofluid self-consistent demagnetizing field and induced magnetic field in the solid. The model was explored in contexts plagued by wall flow maldistribution due to low column-to-particle diameter ratios. It was shown that for proper magnetic field arrangement, wall channeling can be reduced by inflating wall flow resistance through magnetovisco-thickening and Kelvin body force density which reroute a fraction of wall flow towards bed core.     

Molecular dynamics study of the primary ferrofluid aggregate formation

Investigations of the phase transitions and self-organization in the magnetic aggregates are of the fundamental and applied interest. The long-range ordering structures described in the Tománek's systematization (M. Yoon, and D. Tománek, 2010 [1]) are not yet obtained in the direct molecular dynamics simulations. The resulted structures usually are the linear chains or circles, or, else, amorphous (liquid) formations. In the present work, it was shown, that the thermodynamically equilibrium primary ferrofluid aggregate has either the long-range ordered or liquid phase. Due to the unknown steric layer force and other model idealizations, the clear experimental verification of the real equilibrium phase is still required. The predicted long-range ordered (crystallized) phase produces the faceting shape of the primary ferrofluid aggregate, which can be recognized experimentally. The medical (antiviral) application of the crystallized aggregates has been suggested. Dynamic formation of all observed ferrofluid nanostructures conforms to the Tománek's systematization.     

Magnetic and optical response of tuning the magnetocrystalline anisotropy in Fe3O4 nanoparticle ferrofluids by Co doping

Co_xFe_3−xO₄ (0?x?0.10) nanoparticles coated with tetramethyl ammonium hydroxide as a surfactant were synthesized by a co-precipitation technique. The Fe:Co ratio was tuned up to x=0.10 by controlling the Co²⁺ concentration during synthesis. The mean particle size, determined by transmission electron microscopy, ranged between 15±4 and 18±4 nm. The superparamagnetic blocking temperature and the magnetocrystalline anisotropy constant of the ferrofluids, determined using ac and dc magnetic measurements, scale approximately linearly with cobalt concentration. We also find distinct differences in the optical response of different samples under an applied magnetic field. We attribute changes in field-induced optical relaxation for the x=0 and 0.05 samples to differences in the anisotropic microstructure under an applied magnetic field.     

Copper adsorption on magnetite-loaded chitosan microspheres: A kinetic and equilibrium study

A composite of Fe₃O₄ nanoparticles and the biopolymer chitosan, chemically crosslinked, was prepared as microspheres and used to adsorb copper ions, which were chosen as a model of contaminant metal in water.     

Mixed Finite Element Methods for the Ferrofluid Model with Magnetization Paralleled to the Magnetic Field

Mixed finite element methods are considered for a ferrofluid flow model with magnetization paralleled to the magnetic field. The ferrofluid model is a coupled system of the Maxwell equations and the incompressible Navier-Stokes equations. By skillfully introducing some new variables, the model is rewritten as several decoupled subsystems that can be solved independently. Mixed finite element formulations are given to discretize the decoupled systems with proper finite element spaces. Existence and uniqueness of the mixed finite element solutions are shown, and optimal order error estimates are obtained under some reasonable assumptions. Numerical experiments confirm the theoretical results.