Fig. 1
From: Nanoparticles of Titanium and Zinc Oxides as Novel Agents in Tumor Treatment: a Review
Mechanisms of generating reactive oxygen species (ROS). ROS are generated on the surface of titanium dioxide or zinc oxide nanoparticles (TiO2 NPs, ZnO NPs), exposed to ultraviolet (UV) radiation or ultrasound (US) wave in aqueous solutions. Semiconductor NPs can be excited by UV (photoexcitation) or US (sonoexcitation). The acoustic wave, while propagating in the liquid, induces the formation and the subsequent collapse of cavitation bubbles. During their implosion, the emission of the sonoluminescent light takes place. As a result of the TiO2 NPs or ZnO NPs photo- or sonoexcitation, electron (e−) transfers from valence band to conduction band, leaving behind hole (h+). The follow-up process can be twofold. The electrons (e−) together with holes (h+) react with adjacent molecules (O2, H2O, OH−), generating the formation of ROS (•O2 −, •OH, H2O2). Alternatively, radiative recombination of electron-hole pair (e− + h+) results in the emission of photon that transforms the oxygen molecule in the ground state (O2) to the oxygen molecule’s excited state, singlet oxygen (1O2)