Development of Gold Nanoparticles Coated with Bismuth Oxide for X-ray Computed Tomography Imaging

Au nanoparticles are promising X-ray contrast agents as their properties could render them useful for long-term diagnostic imaging. However, the surface modification of Au nanoparticles is necessary to improve their biocompatibility and hence, their retention in the body and contrast-enhancing ability. Therefore, in this study, we prepared a colloidal solution of Au nanoparticles coated with bismuth oxide (Bi₂O₃) under stabilization of the coated nanoparticles using polyvinylpyrrolidone (PVP) (Au/Bi₂O₃/PVP) and performed X-ray imaging of a mouse using the Au/Bi₂O₃/PVP nanoparticle colloidal solution. Au nanoparticles with a particle size of 21.9 nm were prepared by reducing Au ions (III) using sodium citrate in water at 80 °C. The Bi₂O₃ coating of the Au nanoparticles was performed in the presence of Au nanoparticles and PVP by precipitation via the reaction of Bi(NO₃)₃ and NaOH in water (Au/Bi₂O₃/PVP). The loading efficiencies of Au and Bi in the Bi₂O₃-coated Au nanoparticles were dependent on the Bi concentration in the Bi₂O₃-coating process, and were 67.2 and 61.8%, respectively, at a Bi concentration of 0.66 × 10⁻³ M. The X-ray imaging ability of the Au/Bi₂O₃/PVP nanoparticle colloidal solution was superior to that of a commercial X-ray contrast agent. One Au/Bi₂O₃/PVP nanoparticle exhibited a computed tomography value larger than that of a Au nanoparticle. The popliteal lymph node of a mouse was identified using the Au/Bi₂O₃/PVP nanoparticle colloidal solution, highlighting the possibility of accurate, long-term observation of the sentinel lymph node using this solution.