Masayori Suwa

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.

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.

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.

Magnetophoretic study of photo-induced spin transition of single crystalline particles of cobalt?iron Prussian blue analogues

Abstract The photo-induced spin transition of crystalline microparticle of Co–Fe Prussian blue analog was investigated bymeans of magnetophoresis. An apparatus to observe the photo-induced spin transition of a single microcrystal in suspension was constructed, and the magnetophoretic velocities before and after the irradiation with 532 nm pulse laser were observed. The photo-induced spin transition from a low-spin state to a high-spin state of the microparticle was observed from theabrupt increase of the magnetophoretic velocity after the irradiation. The threshold value for spin transition was estimated to be lower than 10 mJ cm−2 by this method, more precisely than that reported from SQUID magnetometer measurements. Moreover, the magnetic susceptibility of an individual particle after irradiation was calculated from the magnetophoretic velocity change. The magnetic susceptibility values determined by the present method were highly scattered. This was caused not only by the experimental error resulting from Brownian motion but also by the difference in the spin transition probability for individual particles. The effect of size on the spin transition phenomena was discussed.

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.

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.

Optical extinction change of magnetic nanoparticle suspension under pulsed magnetic field

We investigated the optical extinction change of γ-Fe2O3 magnetic nanoparticle suspension induced by a pulsed magnetic field. The Extinction decrease accompanied by a pulsed magnetic field showed clear relationship with the magnetization of magnetic nanoparticle. However, there observed a different response to the magnetic field between increasing and decreasing of the magnetic field. This indicated that the extinction change followed the time variation of magnetic field with some time constant. The time constant was estimated to be 4.5 μs, but it was different from either of rotational Brownian relaxation (0.23 μs) and Neel relaxation (10 ns) of individual particle. It was thought that the optical density decrease might be attributed to the decrease of Rayleigh scattering cross section by the rotation of the magnetic nanoparticle aggregates.

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

Abstract 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.

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.