Virucidal Properties of Photocatalytic Coating on Glass against a Model Human Coronavirus

Abstract

The antimicrobial properties of photocatalysts have long been studied. However, most of the available literature describes their antibacterial properties, while knowledge of their antiviral activity is rather scarce. Since the outset of the coronavirus disease 2019 (COVID-19) pandemic, an increasing body of research has suggested their antiviral potential and highlighted the need for further research in this area. In this study, we investigated the virucidal properties of a commercial TiO₂-coated photocatalytic glass against a model human coronavirus. Our findings demonstrate that the TiO₂-coated glass consistently inactivates coronaviruses upon contact under daylight illumination, in a time-dependent manner. A 99% drop in virus titer was achieved after 3.9 h. The electron micrographs of virus-covered TiO₂-glass showed a reduced number of virions compared to control glass. Morphological alterations of TiO₂-exposed viruses included deformation, disruption of the viral envelope, and virion ghosts, endorsing the application of this material in the construction of protective elements to mitigate the transmission of viruses. To the best of our knowledge, this is the first report showing direct visual evidence of human coronaviruses being damaged and morphologically altered following exposure to this photocatalyst. IMPORTANCE Surface contamination is an important contributor to SARS-CoV-2 spread. The use of personal protective elements and physical barriers (i.e., masks, gloves, and indoor glass separators) increases safety and has proven invaluable in preventing contagion. Redesigning these barriers so that the virus cannot remain infectious on them could make a difference in COVID-19 epidemiology. The introduction of additives with virucidal activity could potentiate the protective effects of these barriers to serve not only as physical containment but also as virus killers, reducing surface contamination after hand touch or aerosol deposition. We performed in-depth analysis of the kinetics of photocatalysis-triggered coronavirus inactivation on building glass coated with TiO₂. This is the first report showing direct visual evidence (electron microscopy) of coronaviruses being morphologically damaged following exposure to this photocatalyst, demonstrating the high potential of this material to be incorporated into daily-life high-touch surfaces, giving them an added value in decelerating the virus spread.

Keywords: human coronavirus; photocatalytic coating; titanium dioxide; virucidal activity.

FIG 1

Virucidal profile of TiO₂-coated glass against GFP-expressing human coronavirus 229E. Time course survival of HCoV-229E/GFP on TiO₂-coated glass and uncoated float glass when the experiment was performed in the absence of light (A) or when the glass samples were preactivated with UV-A light and further incubated with viruses in the presence of D65 light throughout the experiment (B). Virus titers (lines [left axis]) recovered from both uncoated control float glass (●) and TiO₂-coated glass (▪) surfaces were compared at each time point using Sidak’s multiple mean comparison test (P values shown above the lines). The virucidal indexes between time points (bars [right axis]) were compared using the Tukey’s multiple mean comparison test. The P values shown refer to statistical comparison between adjacent virucidal values (see details in the text). (C) Graph of virucidal activity versus elapsed time, showing the kinetics of HCoV-229E/GFP virus inactivation on TiO₂-coated surfaces. (D) Exponential phase of virus inactivation on TiO₂-coated surface (see details in the text). Data are presented as means plus standard deviations.

FIG 2

Virucidal performance of TiO₂-coated glass against GFP-expressing human coronavirus 229E when illuminated with D65 light throughout the incubation with virus, in the absence of previous activation with UV-A light. This experiment was performed in duplicate. Virus titers (lines [left axis]) recovered from both uncoated control float glass (●) and TiO₂-coated (▪) surfaces were compared at each time point using Sidak’s multiple mean comparison test (P values shown above the lines). The virucidal indexes between time points (bars [right axis]) were compared using the Tukey’s multiple mean comparison test. The P values shown refer to statistical comparison between adjacent virucidal values (see details in the text).

FIG 3

Quantification of viral genomic RNA detected in virus preparations recovered from the virucidal assays described in Materials and Methods (“Test glass and virucidal assays”), performed under different illumination conditions. (A) Amounts of viral RNA recovered after incubation of virus on TiO₂-coated and float glasses in the absence of light; (B) amounts of virus RNA detected in virus recovered from the test glasses subjected to preactivation with UV-A light and further incubation with D65 light; (C) amounts of virus RNA from samples recovered from the test glasses in the presence of D65 light without UV-A preactivation. The viral genomic RNA in the samples was quantified using RT-qPCR by extrapolation from the standard curve (Fig. S2), run in triplicate together with the rest of samples (see details in the text).

FIG 4

Scanning electron microscopy (SEM) images of gold sputter-coated control float glass (A) and TiO₂-coated glass (B), after 20 h incubation each with 5 × 10⁴ TCID₅₀ of HCoV-229E/GFP in the presence of D65 light. Magnifications, ×50,000 (left panels) and ×100,000 (right panels). See further details in the text.

FIG 5

Transmission electron microscopy (TEM) images of negatively stained HCoV-229E/GFP recovered from float glass and TiO₂-coated glass, after 2 and 26 h exposure. Two different fields are shown for each sample at the 26-h time point. Bars, 0.1 μm.

FIG 6

Schematic model for photocatalysis-triggered virucidal activity of TiO₂ against coronaviruses. Upon excitation by light, the photon energy (hv) generates an electron-hole pair on the TiO₂ surface, with holes located in the valence band (VB) of the semiconductor and electrons accumulating in the conduction band (CB). An electron-hole pair is a highly unstable state with strong oxidation/reduction power that converts water and oxygen into hydroxyl radicals (^·OH), superoxide ions (O₂^·−) and hydrogen peroxide (H₂O₂) in the vicinity of holes within the valence band. These reactive oxygen species (ROS) exert their virucidal action through oxidative stress, causing virus genome damage, lipid peroxidation with the consequent disruption of the viral envelope, virion osmotic shock, and lysis, which is ultimately associated with the release of the nucleocapsid and other virion components and the loss of viral infectivity (virus inactivation).