Fundamentals of Medicinal Application of Titanium Dioxide Nanoparticles

Titanium dioxide (TiO2), a semiconducting material, is a well-known photocatalyst. A nanoparticle (NP) of TiO2 also demonstrates photocatalytic activity. Photo-irradiated TiO2 NPs induce the formation of various reactive species, leading to the damage of biomacromolecules. These reactive species include h+, either free or trapped hydroxyl radicals (OH⋅), superoxide (O2), hydrogen peroxide (H2O2), and singlet oxygen (O2), among others. TiO2 NPs photocatalyze DNA oxidation. A relatively small concentra‐ tion of TiO2 NPs frequently induces tandem base oxidation at guanine and thymine residues through H2O2 generation in the presence of a copper(II) ion. A copper–per‐ oxo complex is considered to be an important reactive species responsible for this DNA damage. In the case of a high concentration of TiO2 NPs, OH⋅ contributes to DNA damage without sequence specificity. In the presence of sugars, TiO2 NPs indi‐ rectly induce DNA damage by the secondary H2O2, which is produced through an au‐ toxidation process of the product of sugar photooxidized by TiO2 NPs. Furthermore, O2 is also produced by photo-irradiated TiO2 NPs. The photocatalyzed formation of O2 might contribute to the oxidation of the membrane protein. These mechanisms of photocatalytic formation of the reactive species may be involved in the photocytotox‐ icity of TiO2 NPs.