Improvement of water-dispersibility of composite nanoparticles composed of magnetic iron oxide coated with noble metal and dot formation by magnetic field.

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We executed studies for improvement of composite nanoparticles consisting of gold and magnetic iron oxide, in which the former is of single-nano-sized grains and the latter is of larger particles. To synthesize the particles, an aqueous solution containing gold ions, preformed iron oxide nanoparticles, adequate amounts of alcohol and polymer is irradiated with ^<60>Co γ-rays, high energy electrons or ultrasonics. Aqueous gold ions are reduced to form metallic grains by radicals in-situ formed by the irradiation, and the grains are stabilized iron oxide particles after meanwhile migration in water. It is a very simple method proceeding in an aqueous solution at room temperature under ambient atmosphere. We tried many experiments and found conditions providing a good dispersiblity, that they kept a dispersion in water for weeks. This dispersibility was given by a wet beads mill. We as well studied their properties and modifying them for application to a quick and selective separation of biomolecules such as DNA, proteins or antigens, by exploiting Au's high affinity to sulfur-containing molecules and magnetism of the iron oxide. It was already demonstrated that the particles are able to magnetically separate nucleotides with sensitivity higher than those of commercially available particles. We also tried experiments that these particles are charged into a channel with sub-millimeter width. The particle dispersing solution was mixed with an another solution and applied with a magnetic field with a small permanent magnet chip. We successfully confirmed the magnetic particles were attracted by the field to form a dot.