Scientists in Japan have developed a new antiviral coating that inactivates various types of coronavirus variants under both indoor light and darkness.

The coating developed by researchers at Nara Medical University, Kanagawa Institute of Industrial Science and Technology, and Tokyo Institute of Technology is made using a combination of titanium dioxide (TiO2) and copper oxide (CuxO) compounds.

They noted that the main transmission pathway of the SARS-CoV-2 virus is through droplets released by infected people into the air, adding these droplets exist on various surfaces as well.

Viral infections mainly occur in indoor environments where many people gather, the researchers said. Antiviral chemicals, such as alcohol and hydrogen peroxide, are often used to decontaminate regularly touched surfaces, they said.

These chemicals essentially render the virus inactive by breaking down its proteins. However, they are volatile in nature, and therefore evaporate away. As a result, the disinfection process has to be carried out regularly.

The study, published in the journal Scientific Reports, described a solid-state photocatalyst as an alternative defence against the virus. Unlike chemical disinfectants, solid-state coatings remain for a long time, and have the advantage of being non-toxic, abundant, and chemically and thermally stable, the researchers said.

Many of these solid-state coatings use TiO2 photocatalysts, that when exposed to ultraviolet (UV) light, cause an oxidation reaction that can destroy organic matter like the spike protein that helps the coronavirus to enter and infect human cells.

However, these coatings are activated only when exposed to UV light, which is not present in typical indoor environments. To get the coating to work under visible light as well dark conditions, the team developed a composite consisting of TiO2 and CuxO nanoclusters.

CuxO nanoclusters are composed of a mixed oxide, in which Cu(I) and Cu(II) species are present. The Cu(II) species in CuxO contributes to the visible-light-driven photocatalysis reaction, whereas the Cu(I) species plays a crucial role in denaturing virus proteins, thereby causing their inactivation under dark conditions.

By coating the CuxO/TiO2 powder on a glass, the researchers showed that it could inactivate even the highly virulent Delta variant of SARS-CoV-2. They also confirmed the inactivation of Alpha, Beta, and Gamma variants by CuxO/TiO2 in addition to the wild type strain.

The team's analysis strongly suggests that the Cu(I) species in CuxO denaturalises spike proteins and also causes RNA fragmentation of SARS-CoV-2, even under dark conditions.