Bismuth Oxide Nanorings: A Potential Material for Hyperthermia-Based Tumor Therapy

Relative to bulk material, bismuth oxide nanoparticles (Bi2O3–NPs), as spheres or rods, have shown quantum confinement effects and unique optical, electrical, and magnetic properties, as well as high surface reactivity, high refractive index, good photoconductivity, and wide energy band gaps. In this work, we present the first observation and report on the synthesis of Bi2O3 nanorings by using a simple and inexpensive sonochemical route. In the presence of trisodium citrate (TSC), the hydrothermal treatment of Bi(NO3)3 gives rise to a controlled spherical morphology of Bi2O3–NPs, while the absence of TSC leads to the formation of nanorods, highlighting the critical role of the coating layer in influencing the crystal growth of NPs. Subsequently, exposing the spherical Bi2O3–NPs to ultrasonication in water led to a rearrangement and self-assembly of toroidally shaped particles, giving rise to the formation of nanorings, as observed through scanning electron microscopy (SEM). The UV–vis analysis demonstrated enhanced absorption of nanorings in the near-infrared (NIR) range compared to other solid structures. Moreover, we demonstrated the direct role of the pore size on the light-absorbing properties of Bi2O3 nanorings. Additionally, we showed that the band gap energies of Bi2O3 nanoparticles can be effectively tuned, depending on their structure. With the distinctive features and hollow morphology of Bi2O3 nanorings, they hold great potential for innovative anticancer treatments. The pronounced absorption of nanorings in the visible and NIR regions can translate into high photothermal conversion efficiency, which can lead to effective hyperthermia-based destruction of the cancer cells. Moreover, the unique hollow architecture of the nanorings makes them highly suitable as excellent nanoscale carriers for chemotherapeutic agents, offering high encapsulation efficiency, as well as stimuli-responsive drug release and targeted delivery against cancer cells.