It is well-known that titanium dioxide (TiO₂) can destroy bacteria and viruses, as well as decompose harmful chemical and biological contaminants in air and water. This property of TiO₂ is based on the phenomenon of photocatalysis, which involves light (photo)-stimulated chemical reactions. The photocatalytic effect encompasses both the dissociation of water molecules on the TiO₂ surface and the generation of reactive oxygen species, such as hydroxyl radicals (OH•) and superoxide radicals (O₂•⁻). In detail, when ultraviolet light (with a wavelength of 388 nanometers or less) irradiates the TiO₂ surface, the energy of each photon is sufficient to eject an electron from the valence band to the conduction band. This electron transition creates a hole (h⁺) in the valence band, which acts as a strong oxidizer and rapidly captures an electron from the surrounding environment. Specifically, it splits water molecules in the air into hydroxyl groups (OH⁻) and hydrogen ions (H⁺). The photocatalytic reaction can be expressed as H₂O + h⁺ → •OH + H⁺. Additionally, a hydroxyl group combined with a hole produces a hydroxyl radical: OH⁻ + h⁺ → •OH. Similarly, an oxygen molecule capturing an electron forms a superoxide radical: O₂ + e⁻ → O₂•⁻. Both hydroxyl radicals (OH•) and superoxide anions (O₂•⁻) are highly reactive free radicals that cause the breakdown of bacteria, viruses, and harmful chemical and biological substances, resulting in the release of carbon dioxide and water. The efficiency of TiO₂ is significantly enhanced when used in the form of nanoparticles, as this increases the surface area in contact with the environment. While TiO₂ is typically activated by ultraviolet light, doping TiO₂ nanoparticles with specific proportions of certain materials allows activation in the visible spectrum, further increasing its effectiveness against various pathogens and pollutants.

We propose the idea of doping TiO2 nanoparticles with specific types and percentages of nanomaterials, both organic (e.g., luminescent and non-luminescent dyes, dendrimers, etc.) and inorganic (e.g., silicon, iron, zinc, gold, silver, copper nanoparticles, etc.), to achieve a photocatalytic effect in desired regions of the optical spectrum, including visible and near-infrared, while enhancing the efficiency coefficient. Based on this concept, it is possible to develop hydroxyl and superoxide radical-generating devices that are cost-effective, technologically simple to manufacture, and operable in the near-ultraviolet, visible, and near-infrared regions of the optical spectrum. These generators show promise for air purification and pathogen neutralization in enclosed spaces. Additionally, their efficiency is notable for use in inhalation or ventilation treatments for individuals affected by viruses (e.g., COVID-19) or bacteria, as well as for treating tumors and other diseases.