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We have successfully introduced biological gelatin particles as center cores acting as shape and structure directors to form superparamagnetic hollow and paramagnetic porous Gd 2O 3particles, which have average sizes below 200 nm. Two approaches involving sol−gel processes and precursor deposition induced by solvent evaporation were employed to fabricate hollow and porous particles, respectively. The magnetization measurements including ZFC-FC curves and magnetization vs H/ Tas well as their usefulness for in vitro MR imaging were investigated for Gd 2O 3particles. With the presence of carbon residue on the hollow Gd 2O 3surface, the hollow Gd 2O 3particles exhibited superparamagnetic behavior. However, these hollow species turned paramagnetic after removal of the carbon-based residue. The hollow and porous Gd 2O 3nanospheres all indicated satisfactory biocompatibility from the measurements of the leaching behavior of Gd 3+ions and cell culture viability. We found, from positive contrast images in in vitro MR assays, that they are capable of providing high-contrast MR images. Furthermore, this synthetic strategy using biological gelatin particles as templates could be readily extended to form TiO 2(porous and hollow) particles. [ABSTRACT FROM AUTHOR]