Orientational Dynamics of Magnetic Iron Oxide Nanoparticles in a Hydrogel: Observation by Magnetic Linear Dichroism under Oscillating Field.

For the success of biomedical applications of magnetic iron oxide nanoparticles (MION), such as magnetic hyperthermia and magnetic particle imaging, it is essential to understand the orientational dynamics of MION in a complex fluid under an alternating field. Here, using the magnetic linear dichroism (MLD) measurement, we directly observed the orientational behavior of MION in a hydrogel under a damped oscillating magnetic field (DOMF) of 33 kHz in frequency. Hydrophobically modified ethoxylated urethane (HEUR) is examined as the network polymer because the mesh size of the network is controllable with its concentration. We used two MIONs: a bare MION (MION1) and a MION coated with an amphiphilic polymer (MION2). Where the mesh size of the gel network is larger than the particle's hydrodynamic diameter, MION1 in the hydrogel rotates in the same manner in a simple solution, although the macroscopic rheological property of the medium is quite different. Meanwhile, the orientational behavior of MION2 is dramatically changed by the addition of HEUR molecules even below the minimum gelation concentration, indicating that MION2 is associated with the flower micelles of HEUR. By analyzing the MLD waveform, the orientational behavior of MION1 in the HEUR gel under a DOMF can be explained with single-mode relaxation, whereas that of MION2 is complicated; a rapid partial rotation near the particle and a whole slow rotation of the particle-flower micelle associate are superimposed. It is hard to distinguish this difference in orientational behaviors from the dynamic magnetization curve because the dominant magnetization reversal process is Néel rotation, the rotation of the magnetic moment in the particle. The MLD measurement is a potential tool for optimizing biomedical techniques utilizing MIONs and for nanorheology or colloid science in a complex matrix such as a hydrogel or cytoplasmic matrix.

Anchoring Energy Measurements at the Aqueous Phase/Liquid Crystal Interface with Cationic Surfactants Using Magnetic Fréedericksz Transition.

We constructed the apparatus to observe the Fréedericksz transition of liquid crystal in contact with water. The Fréedericksz transition is a distortion of nematic liquid crystals (LCs) induced by external fields. In the present system, sweeping homogeneous magnetic field was applied to the sample, and the distortion of the LC was visualized with a polarized light microscope with the crossed Nichols configuration. The anchoring energy (WAQ/LC) at the aqueous phase/LC interface was measured in the presence of surfactant from the threshold magnetic field of the Fréedericksz transition. We studied two cationic surfactants: dodecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide. A nematic LC, 4-cyano-4'-pentylbiphenyl (5CB), was examined, which was confined in a copper grid on an octadecyltrichlorosilane-treated microscope glass plate. Measured WAQ/LC were reproducible and showed consistence with the reported region for the water/LC interface. Interfacial excess of surfactants was also measured by the pendant drop method, and the relationship between the obtained WAQ/LC and the interfacial excess was investigated. Experiments showed that an increase in the anchoring energy depends on the surfactant and its interfacial excess. The region of the interfacial coverage, at which WAQ/LC increases, varied with the chain length of the surfactant. The measurement of the anchoring energy will provide new fundamental information on aqueous phase/LC interface.

Electromagnetophoretic Micro-convection around a Droplet in a Capillary.

The electromagnetophoretic behavior of organic droplets in an electrolyte solution was investigated in a silica capillary cell using a superconducting bulk magnet (3.5 T) and a magnetic circuit (2.7 T). The initially dispersed emulsion droplets of dodecane migrated to the wall of the capillary, responding to the direction of an electric current, and coalesced to form smaller and larger droplets after some repeated migrations. When the electric current was applied continuously, the larger droplets became arranged with regular intervals on the wall, and smaller droplets rotated around the larger droplets. These interesting behaviors were analyzed while taking into account the local electric current density determined by the flow velocity of the ionic current around a droplet, which was lowest on the electrode sides of the droplet. The difference in the local electric current density generated the Lorentz-force difference in the medium, which lead to local micro-convection around the droplet, and also the alignment of larger droplets by a repelling effect between the adjacent micro-convections.

Faraday rotation dispersion microscopy imaging of diamagnetic and chiral liquids with pulsed magnetic field.

We have constructed an experimental setup for Faraday rotation dispersion imaging and demonstrated the performance of a novel imaging principle. By using a pulsed magnetic field and a polarized light synchronized to the magnetic field, quantitative Faraday rotation images of diamagnetic organic liquids in glass capillaries were observed. Nonaromatic hydrocarbons, benzene derivatives, and naphthalene derivatives were clearly distinguished by the Faraday rotation images due to the difference in Verdet constants. From the wavelength dispersion of the Faraday rotation images in the visible region, it was found that the resonance wavelength in the UV region, which was estimated based on the Faraday B-term, could be used as characteristic parameters for the imaging of the liquids. Furthermore, simultaneous acquisition of Faraday rotation image and natural optical rotation image was demonstrated for chiral organic liquids.

Faraday rotation dispersion measurements of diamagnetic organic liquids and simultaneous determination of natural optical rotatory dispersion using a pulsed magnetic field.

We constructed an apparatus to measure the wavelength dispersion of the Faraday rotation in the visible region, and determined the Verdet constants of diamagnetic organic liquids, including aliphatic compounds, benzene derivatives, and naphthalene derivatives. These three groups were easily distinguished by the magnitudes of their Verdet constants. Based on the theory developed by Serber, we determined the enhancing effect of π*←π transitions on the visible-light Faraday rotation angles observed for aromatic compounds. Furthermore, we propose a novel approach for simultaneously observing Faraday rotation dispersion and natural optical rotatory dispersion.

Magnetoanalysis of micro/nanoparticles: a review.

Application of magnetic field on the separation and analysis of nano/microparticles is a growing subject in analytical separation chemistry. The migration phenomenon of a particle under inhomogeneous magnetic field is called magnetophoresis. The migration velocity depends on the magnetic susceptibility and the size of a particle. Therefore, magnetophoresis allows us to determine the magnetic susceptibility of particles, and to separate particles based on the magnetic properties. Magnetic separation of ferromagnetic particles in liquid has been utilized for a long time. For example, a high gradient magnetic separation under the non-uniform magnetic field generated by ferromagnetic mesh has been utilized in a wide region from chemical industry to bioscience. Recent progress on magnetic nanoparticles and microfluidic devices has made it possible to extend the range of application. Furthermore, it has been demonstrated that the very sensitive measurement of the magnetic susceptibility of microparticles can be performed by observing magnetophoretic velocity. In this review, we mainly introduce novel separation and detection methods based on magnetophoresis, which have been invented in this decade, and then new principles of particle migration under magnetic field are presented.

Effective transition probability for the Faraday effect of lanthanide(III) ion solutions.

The Faraday effects of 14 lanthanide(III) ion solutions were systematically analyzed on the basis of the Faraday C term. The effective transition probability, K, which measures the magneto-optical contribution of the 4f(n) --> 4f(n-1)5d transition to the molar Verdet constant, was determined. Linear correlations between K and the square root of the molar magnetic susceptibility of the lanthanide(III) ions, chi(m)(1/2), were obtained. From the observed new regularity, K for promethium(III) was estimated.

Pulsed magnetic field faraday imaging of diamagnetic liquids.

The Faraday rotation of diamagnetic liquids has been measured by applying a pulsed magnetic field. The observed results for water, alcohol, and aliphatic and aromatic organic liquids suggested a possible discrimination of such liquids by Faraday imaging. Actually, we have succeeded to observe Faraday images using a magneto-optical microscope system. The discrimination of several diamagnetic liquids including chiral liquids has been demonstrated.

Sensitive light-scattering detection-magnetophoretic acceleration mass analysis of single microparticles in an atmosphere.

Optically detected magnetophoretic acceleration mass analysis of an individual micro-particle in an atmosphere has been remarkably improved in sensitivity by using a reflective microscope objective, by which forward scattered light from a particle could be effectively collected. From the light-scattering simulation, the detection limit for the radius of a micro-particle was estimated to be smaller than 0.4 µm, and about 60 times intensity enhancement was observed for a polystyrene particle with a radius of 2.8 µm. For both paramagnetic and diamagnetic micro-particles, the mass magnetic susceptibility and the relaxation time could be determined without knowing any parameters of the particles. From the relaxation time, the mass of a particle was obtained if the radius or the density of the particle was known. For a test sample silica particles were used to adsorb paramagnetic dysprosium(III), the surface concentration of dysprosium(III) on a single particle could be successfully determined by use of this method.

Magnetophoretic evaluation of interfacial adsorption of dysprosium(III) on a single microdroplet.

Magnetophoretic velocimetry is a novel technique to measure the magnetic susceptibility of a single microparticle. This technique could be applied to study the interfacial adsorption equilibria of a paramagnetic dysprosium(III) ion with capric, lauric or stearic acid for a single 2-fluorotoluene microdroplet. The observed magnetic susceptibility of the micro-organic droplets was reciprocally proportional to its radius in each case. From the proportional constant, the interfacial concentration of Dy(III) was determined. Furthermore, the dependences of the interfacial concentration on the initial Dy(III) concentration and pH were examined in order to analyze the adsorption equilibrium. Finally, the saturated interfacial concentration and the interfacial adsorption constant at the infinite dilution of Dy(III)-laurate complex were evaluated as 4.8 x 10(-10) mol cm(-2) and 3.4 x 10(-2) dm, respectively.

Magnetic susceptibility measurement of single iron/cobalt carbonyl microcrystal by atmospheric magnetophoresis.

In this study, the use of an innovative atmospheric magnetophoresis, which enables us to measure the mass magnetic susceptibility and mass of a microparticle simultaneously, was demonstrated. Using this technique, we determined the magnetic susceptibility of a crystalline deposit of iron/cobalt carbonyl, mainly composed of Fe2(CO)9, which was prepared photochemically from a gaseous mixture of iron pentacarbonyl (Fe(CO)5) and cobalt tricarbonyl nitrosyl (Co(CO)3NO). The mass magnetic susceptibility and the characteristic relaxation time of the microcrystal were (7.0±1.9)×10-9 m3 kg-1 and (5.6±2.2)×10-4 s, respectively. The observed magnetic susceptibility shows that the microparticle was paramagnetic. Assuming that the density was equal to that of Fe2(CO)9 (2.1×103 kg m-3) and that the shape of the particle was spherical, a hydrodynamic radius of 4.7 µm and a mass of 0.91 ng were observed. It was suggested that Co was incorporated in Fe2(CO)9.

Magnetophoretic velocity modulation mass analysis of a single microparticle in an atmosphere.

A new principle of the magnetophoretic velocity modulation mass analysis of microparticles, which can determine simultaneously the mass and magnetic susceptibility of a single microparticle, has been proposed, and the measurement system was constructed by applying a magnetophoretic force on a falling microparticle through a magnetic field gradient in an atmosphere. A polystyrene microparticle as a test particle adsorbed on a glass plate was selectively knocked off by a pulsed Nd:YAG laser impact into a narrow gap of pole pieces of permanent magnets having a magnetic field gradient with a maximum intensity of 850 T2 m(-1). The falling particle was irradiated by a He-Ne laser, and the scattered light was detected through a slit array mask as a function of time. A bundle of spiked signals of scattered light intensity was analyzed to obtain velocities, which gave acceleration and deceleration of the falling particle. On the basis of the equation of motion under the magnetic field gradient, the mass and magnetic susceptibility of the test particle were reasonably determined.

Raman microprobe spectrometer installed in a super-conducting magnet.

A Raman microprobe spectrometer that could be installed in the bore of a cryogen free super-conducting magnet (10 T) was designed and constructed for the investigation of the external magnetic field effect on the Raman spectra of molecular aggregates in solutions and at interfaces. The performance of the present instrument was demonstrated by measuring the magnetic field effect (0 - 10 T) on the resonance Raman spectra of diprotonated meso-tetra-(sulfonatophenyl)porphine aggregates in an acidic solution. The Raman shifts of the aggregates were not significantly influenced even in 10 T. However, the relative intensity of 1123 cm(-1) peak (nu(C(a)-N)) was interestingly enhanced about 20% under the magnetic fields higher than 2.5 T.

New principle of magnetophoretic velocity mass analysis.

We propose a novel principle of velocity mass analysis of a micro-particle using magnetophoretic force. The new method can determine the mass of a particle from its magnetophoretic velocity change in a high magnetic field gradient in a low viscous medium such as air. In the present study, the new principle was demonstrated by the magnetophoretic acceleration of an aqueous manganese(II) chloride micro-droplet and the deceleration of a water micro-droplet in the atmosphere. The observed velocity change was analyzed taking into account the mass of the droplet through the acceleration term of the equation of motion. The experimental results proved that the inertia force in the magnetophoretic velocity of a micro-particle could be detected in air. The present method provided an innovative mass analysis method without any ionization of the sample.

Magnetophoretic detection of photo-induced spin transition.

Magnetophoretic velocimetry detected the spin transition of a single Co-Fe Prussian Blue analogous micro-crystal in water induced by a single-shot pulse laser.

Migration analysis of micro-particles in liquids using microscopically designed external fields.

The recent development of new migration methods of micro-particles in liquids using various external fields is reviewed. The combination of a laser scattering force and a photothermal effect produced photothermal-conversion laser-photophoresis. A dielectric field generated in a planer or a capillary quadrupole electrode realized dielectrophoresis. Using a micrometer-scaled magnetic field gradient, the "Magnetophoretic velocimetry" of micro-particles was invented. Furthermore, the Lorentz force generated by combining an electric field and a magnetic field was utilized for electromagnetophoresis. These new methods were overlooked and the advantages in analytical use were discussed.

High-magnetic-field electromagnetophoresis of micro-particles in a capillary flow system.

The electromagnetophoretic migration of micro-particles in a capillary flow system was demonstrated using a homogeneous magnetic field applied at right angles to an electric current. We utilized a high-magnetic-field of 10 T for observing this phenomenon. When the direction of the electric current was alternatively changed, polystyrene latex particles in a flowing aqueous medium migrated zigzag affected by a Lorentz force exerted on the medium. Carbon particles also migrated in the same manner with polystyrene particles. Further, we tried the electromagnetophoretic migration of biological particles, such as yeasts and human red blood cells. The migration velocity component perpendicular to the flow was proportional to both the electric current and the magnetic flux density. These results proved that the dominant force of the zigzag migration was an electromagnetophoretic buoyancy generated in the flowing medium. Moreover, it was found that the force exerted on the particles in the magnetic field of 10 T was sufficient for the desorption of particles adsorbed on the capillary wall.

Magnetophoresis and electromagnetophoresis of microparticles in liquids.

The magnetic field-induced migration of particles in liquids is a highly-promising technique for the micro-separation analysis of bioparticles, such as cells and large DNA. Here, new methods that make use of magnetophoresis and electromagnetophoresis to induce the migration of microparticles in liquids are briefly reviewed. Magnetic force and Lorentz force are utilized in the new methods. Some typical examples of the use of these methods are described, and the advantages of using a superconducting magnet for them are demonstrated.

Magnetophoretic velocity of microorganic droplets adsorbed by dysprosium(III) laurate in water.

By using an improved apparatus for the observation of magnetophoresis, the magnetophoretic velocity of 2-fluorotoluene droplets including lauric acid was measured in aqueous dysprosium(III) solution. The magnetophoretic velocity of pure 2-fluorotoluene droplets was proportional to the square of the radius. On the other hand, the velocity of the organic droplets including lauric acid in the dysprosium(III) solution showed a deviation from the square radius relationship, more remarkably in smaller droplets than 2 microm in radius. These results indicated that the dysprosium(III)-laurate complex was formed at the liquid-liquid interface. This study is the first report on the detection of the interfacial complex by the magnetophoresis of the microdroplet.

Magnetophoretic velocimetry of manganese(II) in a single microdroplet in a flow system under a high gradient magnetic field generated with a superconducting magnet.

An experimental system for magnetophoretic velocimetry, which could determine the volume magnetic susceptibility of a single particle dispersed in a liquid phase from a magnetophoretic velocity, has been developed. A micrometer-sized high-gradient magnetic field could be generated in a capillary by a pair of iron pole pieces in a superconducting magnet (10 T). The magnetophoretic behavior of a single particle in a capillary flow system was investigated under the inhomogeneous magnetic field. From the magnetophoretic velocity of a polystyrene latex particle dispersed in a MnCl2 aqueous solution, the product of the magnetic flux density and the gradient, B(dB/dx), was determined as a function of the position along the capillary. The maximum value of B(dB/dx) was 4.7 x 10(4) T2 m(-1), which was approximately 100 times higher than that obtained by two Nd-Fe-B permanent magnets (0.4 T). Organic droplets extracting manganese(II) with 2-thenoyltrifluoroacetone and tri-n-octylphosphine oxide from MnCl2 solution were used as test samples. The difference of the volume magnetic susceptibility between the droplet and the medium could be determined from the magnetophoretic velocity. This method allowed us to continuously measure a volume magnetic susceptibility of 10-6 level for a picoliter droplet and to determine manganese(II) in the single droplet at the attomole level.