A preparation method for Gd-ethylenediaminetetraacetic acid disodium salt dihydrate (ETDA) complex-immobilized silica particles (Gd-EDTA/SiO2) is proposed. Preparation of spherical silica particles was performed by a sol-gel method at 35°C using 0.2？M tetraethylorthosilicate, 25？M H2O, and 0.01？M NaOH in ethanol, which produced silica particles with an average size of ？nm. Immobilization of Gd-EDTA on the silica particles was conducted at 35°C by introducing amino groups on the silica particles with (3-aminopropyl)trimethoxysilane at pH 3 (NH2/SiO2) and then making Gd-EDTA act on the NH2/SiO2 particles at pH 5. The as-prepared Gd-EDTA/SiO2 particle colloid solution was concentrated up to a Gd concentration of 0.347？mM by centrifugation. The sphere structure of Gd-EDTA/SiO2 particles was undamaged, and the colloid solution was still colloidally stable, even after the concentrating process. The concentrated Gd-EDTA/SiO2 colloid solution revealed good MRI properties. A relaxivity value for T1-weighted imaging was as high as 5.15？mM？1？s？1, that was comparable to that for a commercial Gd complex contrast agent. 1. Introduction Gadolinium complexes (GdC) function as contrast agents for magnetic resonance imaging (MRI) [1–8], and solutions dissolving the GdC homogeneously at molecular level are available commercially. The GdC molecules flow fast in living bodies. This means that they are quickly discharged from the living bodies, which results in a difficulty in taking high-contrast images for a long period with the use of the solution-type commercial agents. Not the GdC solution but colloid solution of GdC immobilized with particles is promising as a new contrast agent that can be expected to stay in living bodies for a long period, because the particles are dragged from fluid to a direction opposite to the fluid due to their projected area larger than molecules, which controls flow of the particles in liquid. Consequently, the immobilization will increase residence time of GdC, which makes it possible to take clear MRI images for a long term. Particles used for the immobilization are required to unaggregate in living bodies, because aggregation of the particles prevents smooth flow of body fluid. Since silica particles prepared by a sol-gel method are colloidally stable [9–11], GdC-immobilized silica particles may function as the MRI contrast agent that shows long residence time and nonaggregation in living bodies. Lux et al. produced particles composed of Gd oxide core and silica-related compound shell for bioimaging [12]. Our group also proposed a method
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