Inactivation of SARS-CoV-2 by Photocatalytic Nanoparticles
Photocatalytic NPs offer another possible approach to the inactivation of SARS-CoV-2. The most described NP in this category is titanium dioxide (TiO2), which shows photocatalytic properties when illuminated under UV light, is considered inert, has low toxicity, and is not susceptible to photocorrosion.191 TiO2 is currently used in paints and lacquers,155 in self-cleaning windows, and for water purification.192 TiO2-containing paints have also been envisaged for purifying ambient air because photocatalytic TiO2 successfully removes volatile organic compounds (VOCs) when exposed to UV light. However, recent findings show that VOC disruption is associated with the release of toxins in the air, which questions the prudence of TiO2-doped paints for air purification purposes.155 If effective, the use of TiO2 photocatalysis for SARS-CoV-2 inactivation would be particularly useful for surface decontamination using TiO2-doped paints, aerosol decontamination using air filtration filters and ventilation systems impregnated with TiO2 that can be exposed to UV light, and for wastewater treatment.
The underlying mechanism of this photocatalytic process relies on the excitation of an electron from the valence band (VB) of the photocatalytic material to the conduction band (CB) when exposed to UV light, which leaves a positive hole (h+) in the VB. The e–/h+ charge carriers migrate to the surface of the photocatalyst and initiate reactions leading to the production of ROS, including the superoxide anion, hydrogen peroxide, the hydroxyl anion, and hydroxyl radical.193 The production of hydroxyl radicals by the oxidation of water molecules on the photocatalysts’ surface accounts for their disinfection activity, owing to their capacity to oxidize many organic constituents of microorganisms, such as lipid peroxidation, leading to damage to cell wall and cell membrane, protein alteration, and/or DNA damage.193 Bare TiO2 exposed to UV light is effective against a broad spectrum of Gram-positive and Gram-negative bacteria, including multi-drug-resistant strains but also against some fungi, viruses, and yeasts. As discussed by Bogdan et al., according to some authors, viruses would be more susceptible to inactivation than bacteria.194 Among viruses, some researchers have found that enveloped viruses would be more protected from photocatalytic inactivation than non-enveloped viruses, whereas other authors reported the opposite.194 Only one article reported the usefulness of this inactivation strategy for treatment of SARS-CoV, using a photocatalytic titanium apatite filter (PTAF). This filter showed effective inactivation of SARS-CoV when exposed for 6 h to UV light.195 One could also imagine that photocatalysts coupled to UV light could damage spike proteins and lead to decreased infectious capacity of the virus.
Because TiO2 shows low solar light activity and a high recombination rate of electron–hole pairs, researchers have developed second-generation photocatalysts in which TiO2 is used in combination with other components, such as metals. This new generation of photocatalysts shows high efficiency of inactivation of a wide range of bacteria and some viruses. Among them, S-doped and N-doped TiO2 show photocatalytic properties when exposed to visible light and, therefore, would possibly be effective under interior lightning. The antimicrobial properties of these photocatalysts have been tested with a variety of bacteria, sometimes indicating good disinfection efficiency (for reviews, see refs (192) and (193)), but to our knowledge, they have not been tested on viruses. Moreover, depositing some Ag NPs on the surface of TiO2 NPs increases their antiviral efficiency against MS2 by means of increased production of hydroxyl radicals.196 A Ag- and Cu-doped TiO2 nanowire membrane is more active in eliminating bacteriophage MS2 from drinking water than are TiO2, Ag-TiO2, or Cu-TiO2 membranes, both in the dark and when exposed to UV light. The underlying mechanism is thought to combine both enhanced photoactivity due to the lower band gap of (Ag, Cu)-TiO2 than that of TiO2197 and antimicrobial activity of free Ag and Cu ions released into the treated water.198 Another strategy to improve the antiviral capability of TiO2 is by increasing its potential to absorb viruses, which has successfully been achieved by mixing TiO2 NPs with SiO2 NPs. Due to the large specific surface area of SiO2, the mixture of NPs inactivated bacteriophage MS2 more effectively than did TiO2 alone, in spite of reduced hydroxyl production.199 Glass slides coated with TiO2 doped with Pt show slightly better efficiency in inactivating aerosols containing influenza A (H3N2) virus than do surfaces coated with only TiO2 when irradiated with UV-A,200 owing to their increased oxidizing photocatalytic properties. Finally, as described by Byrnes et al.,193 new photocatalytic materials that show efficient antibacterial activity have been developed and could be tested for the inactivation of SARS-CoV-2. These new materials include (among others) BiVO4, CuFeO2, CuYxFe2–xO4, LaFeO8, CuMn2O4, ZnMn2O4, BaCr2O4, SrCr2O4, NiCo2O4, CuCo2O4, LaCoO3, and La0.9Sr0.1CoO3. Importantly, before being used for SARS-CoV-2 inactivation, their nontoxicity should be ensured.