Bismuth Oxide Nanorings:\nA Potential Material for\nHyperthermia-Based Tumor Therapy

Relative to bulk material, bismuth oxide nanoparticles\n(Bi<sub>2</sub>O<sub>3</sub>–NPs), as spheres or rods, have\nshown\nquantum confinement effects and unique optical, electrical, and magnetic\nproperties, as well as high surface reactivity, high refractive index,\ngood photoconductivity, and wide energy band gaps. In this work, we\npresent the first observation and report on the synthesis of Bi<sub>2</sub>O<sub>3</sub> nanorings by using a simple and inexpensive\nsonochemical route. In the presence of trisodium citrate (TSC), the\nhydrothermal treatment of Bi(NO<sub>3</sub>)<sub>3</sub> gives rise\nto a controlled spherical morphology of Bi<sub>2</sub>O<sub>3</sub>–NPs, while the absence of TSC leads to the formation of nanorods,\nhighlighting the critical role of the coating layer in influencing\nthe crystal growth of NPs. Subsequently, exposing the spherical Bi<sub>2</sub>O<sub>3</sub>–NPs to ultrasonication in water led to\na rearrangement and self-assembly of toroidally shaped particles,\ngiving rise to the formation of nanorings, as observed through scanning\nelectron microscopy (SEM). The UV–vis analysis demonstrated\nenhanced absorption of nanorings in the near-infrared (NIR) range\ncompared to other solid structures. Moreover, we demonstrated the\ndirect role of the pore size on the light-absorbing properties of\nBi<sub>2</sub>O<sub>3</sub> nanorings. Additionally, we showed that\nthe band gap energies of Bi<sub>2</sub>O<sub>3</sub> nanoparticles\ncan be effectively tuned, depending on their structure. With the distinctive\nfeatures and hollow morphology of Bi<sub>2</sub>O<sub>3</sub> nanorings,\nthey hold great potential for innovative anticancer treatments. The\npronounced absorption of nanorings in the visible and NIR regions\ncan translate into high photothermal conversion efficiency, which\ncan lead to effective hyperthermia-based destruction of the cancer\ncells. Moreover, the unique hollow architecture of the nanorings makes\nthem highly suitable as excellent nanoscale carriers for chemotherapeutic\nagents, offering high encapsulation efficiency, as well as stimuli-responsive\ndrug release and targeted delivery against cancer cells.