Titanium dioxide drafted in COVID fight

Researchers in a Swiss laboratory have developed a membrane made of titanium oxide nanowires with antibacterial and antiviral properties.

The membrane, which resembles filter paper, may be used in the fight to curtail the COVID-19 pandemic more effectively than paper masks, which are increasingly becoming made mandatory, according to scientists at Ecole Polytechnique Federale de Lausanne in Lausanne, Switzerland.

While the relative effectiveness of paper masks is no longer in question, the scientists contend that the widespread use of the masks has a number of drawbacks. These include the environmental impact of disposable masks made from layers of non-woven polypropylene plastic microfibers, which only trap pathogens instead of destroying them.

"In a hospital setting, these masks are placed in special bins and handled appropriately," said László Forró, head of EPFL's Laboratory of Physics of Complex Matter." However, their use in the wider world – where they are tossed into open waste bins and even left on the street – can turn them into new sources of contamination."

Though the current COVID-19 pandemic was a motivating factor, Forró's team directed their research at technology that could mitigate the effects of all threats coming from viruses and bacteria, which have become more frequent and more devastating recently. They said this tsunami of dangerous microorganisms is due primarily to the increased mobility of people and goods moving around the globe, which enhances the spread of infections.

In an article published in Advanced Functional Materials Aug. 7, the scientists noted that in the past 20 years, the world has seen several viral outbreaks resulting in epidemics and pandemics causing thousands of human causalities, including the current COVID‐19 outbreak, which represents an unprecedented crisis.

"One may have in mind the outbreak of SARS‐CoV in 2004 of H1N1 in 2009, and the Ebola virus in 2018. In the case of bacteria, the O104:H4 strain of Escherichia coli[ caused causalities, but the danger is also coming from antibiotics resistant species like Legionella bacteria, Klebsiella pneumoniae, and Staphylococcus aureus," they wrote.

The researchers further noted that the most sinister of these threats is the current pandemic outbreak of Covid‐19, caused by the highly contagious coronavirus SARS‐CoV‐2, which has rapidly spread worldwide, leading to infections on a millions‐scale, along with hundreds of thousands of human losses.

"As of today, the world is far behind in finding an efficient fight against this disease. Our society needs to employ all possible means in reducing the number of infections and mortalities," they wrote. "In this struggle, one possibility is to create specialized surfaces, which by photophysical effects could damage and disable airborne pathogens such as bacteria and viruses, as well as SARS‐CoV‐2."

Titanium and UV light

The new paper-like membrane developed by the Swiss team has the photocatalytic properties of titanium dioxide, which can make it a bactericidal‐virucidal surface. When exposed to ultraviolet light, fibers of the membrane convert resident moisture into oxidizing agents such as hydrogen peroxide, which have the ability to destroy pathogens.

Titanium oxide nanowires are normally attained via hydrothermal synthesis, where nanoparticles of titanium dioxide are treated in a closed vessel at high temperatures during several days of reaction time. Unfortunately, the yield of such synthesis is very low, several few grams per run, which is a strong handicap for large-scale applications, the researchers noted.

Through an original synthesis method, the scientists were able to greatly upscale production. In a base solution, titanium-containing starting material is added, and the mixture is heated in a base resistive vessel to 70 degrees Celsius (158 degrees Fahrenheit). A shear mixing results in the formation of a high fraction of mesoporous titanates, and a lower fraction of macroporous material. The textural properties can be tuned by temperature, with and by the time of shear mixing. After the synthesis, the jelly titanium oxide composite is transferred into a press or centrifuge to separate it from the residues of the reaction. In a single shot, the quantity of titanate nanowires is in the 0.3- to 1-kilogram range.

The nanowires can be processed into films with a thickness less than 2 micrometers.

A titanium dioxide nanowire filter paper made from this film can be used for water and air filters which, when exposed to ultraviolet light, can be disinfected from viruses, bacteria, and generally all organic species.

"Since our filter is exceptionally good at absorbing moisture, it can trap droplets that carry viruses and bacteria," said László Forró, head of EPFL's Laboratory of Physics of Complex Matter, in announcing the discovery. "This creates a favorable environment for the oxidation process, which is triggered by light."

The researchers' work included experiments that demonstrate the membrane's ability to destroy E. coli, the reference bacterium in biomedical research, and DNA strands in a matter of seconds. Based on these results, the scientists say the process would be equally successful on a wide range of viruses, including the SARS-CoV-2 responsible for COVID-19. No experiments, however, have yet been conducted that demonstrate the membrane's effectiveness in destroying SARS-CoV-2 cells.

Large-scale output possible

The EPFL team also reported that the kilogram‐scale production capability of titanium dioxide nanowires gives credibility to its massive application potentials. Manufacturing the membranes would be feasible on a large scale, they wrote, noting that the laboratory's equipment alone is capable of producing up to 200 square meters of filter paper per week, or enough material for up to 80,000 facial masks per month. Moreover, the masks could be sterilized and reused up to 1,000 times.

Touting the benefits of their innovation, the researchers said the antiviral material would alleviate shortages and substantially reduce the amount of waste and potential contamination created by disposable surgical masks. Moreover, the manufacturing process, which involves calcining the titanite nanowires, makes them stable and prevents the risk of nanoparticles being inhaled by the user.

The researchers have created a start-up company, Swoxid SA, and are already preparing to move the technology out of the lab.

"The membranes (also could) be used in air treatment applications such as ventilation and air conditioning systems as well as in personal protective equipment," added Endre Horváth, the article's lead author and co-founder of Swoxid.