Synthesis, characterization, and X-ray attenuation properties of polyacrylic acid-coated ultrasmall heavy metal oxide (Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3) nanoparticles as potential CT contrast agents

Ultrasmall heavy metal oxide nanoparticles are potential candidate materials for X-ray computed tomography (CT) contrast agents because they possess high X-ray attenuation powers owing to high X-ray attenuation coefficients of heavy metal atoms and high density of heavy metal atoms per nanoparticle. In this study, five kinds of polyacrylic acid (PAA)-coated ultrasmall heavy metal oxide (Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3 nanoparticles were synthesized and their X-ray attenuation properties were investigated. The estimated average particle diameters were 2.3 +/- 0.1, 1.7 +/- 0.1, 1.5 +/- 0.1, 1.8 +/- 0.1, and 1.9 +/- 0.1 nm for PAA-coated ultrasmall Bi2O3, Yb2O3, NaTaO3, Dy2O3, and Gd2O3 nanoparticles, respectively. All of the nanoparticle suspension samples exhibited a high colloidal stability, a high biocompatibility, and X-ray attenuation powers which were stronger than that of a commercial iodine contrast agent Ultravist (R) at the same atomic concentration and much stronger, at the same number density. The effectiveness of the nanoparticle suspension samples as CT contrast agents was demonstrated by acquiring in vivo CT images by using one of the samples (i.e., PAA-coated ultrasmall Bi2O3 nanoparticles). After intravenous injection into the mouse tail vein, positive contrast enhancements in various organs were observed.