Photocatalitic disinfectant, the next tool against the COVID-19

In these times of COVID-19, masks and disinfectant gel have increased awareness about safety and hygiene measures to avoid infections. It is in this scenario that incredible improvements appear in the sector and known technologies are reviewed and take on a new role, reaching new heights. One of the most innovative disinfection systems is the photocatalysis process, which in recent years has seemed to be efficient in removing organic components, from bacteria and fungi, to viruses, prions or even harmful particles or contaminating molecules. Photocatalytic disinfectants not only act when applied, but can continue to react for up to several months later.

Photocatalysis is the process of activation of mineral compounds by UV light from the sun (although they can also be activated by the light energy provided by low intensity artificial light). This quality of Titanium nanoparticles was discovered during the second half of the last century. Since then its mechanism and its applications have been studied. When these compounds from the group of semiconductor metals receive photons from a light source, they take energy in their valence shell. The energy excites an electron (e) - and will create a charged electron hole (h +). This change in its surface allows its interaction at a chemical level with water or oxygen molecules. The e- will reduce molecular oxygen creating superoxide anions. While h + will oxidize water molecules, dissociating them into their hydroxyl (OH), superoxide and hydrogen peroxide radicals. These reactive oxygen species (ROS) cause the inactivation and oxidation of infectious agents by breaking down matter. Turning out to be a powerful disinfectant that interacts with bacterial cell walls and virus coatings, altering their composition and undoing them.

To generate this electric potential capable of transforming water or molecular oxygen into reactive species, semiconductor metals are used. Among the most used in this type of technology we find Cadmium, Zinc, but the one that is giving the best results is undoubtedly Titanium, in the form of titanium dioxide (TiO2). In turn, the presence of other metals such as copper, silver or gold, zinc oxides or cadmium sulphides mixed with titanium oxides seem to improve photocatalysis. In fact, since the beginning of the pandemic, copper has been revalued as a material for hospitals because it has biocidal properties.

Studies on the biocidal capacity two months after their application of these compounds show that they are 100% effective against fungi and a wide variety of GRAM negative and GRAM positive bacteria (E. coli, Salmonella spp, Pseudomonas, Enterobacteria). In adenovirus type 5 (the most resistant because it lacks an envelope) it has been proven that it maintains an efficiency close to 85% after two months. A long list of viruses from different taxonomic groups (both DNA and RNA single or double stranded) have been inactivated by titanium oxide photocatalysis processes. For example: herpes group viruses, rotavirus A, poliovirus, or bacterial phage T4 and MS2.

It should be said that the effectiveness of photocatalysis is not homogeneous in all viruses, with non-enveloped viruses being the least affected by the process. In viruses with capsules, it can interact with the different membrane, integument and capsid structures, destabilizing them. In non-enveloped viruses, it can only interact with the capsid. SARS-CoV-2 is an enveloped virus. In principle, it would interact more easily with ROS and therefore the use of photocatalytic disinfectants would have greater lethality in these viruses. The inactivation of viruses occurs in a time-dependent manner. Tests carried out with the human and avian influenza (H5N2) viruses have shown great promise in this regard, completely eliminating the virus from a surface in just 30 minutes. For these experiments, other variables that can influence the speed of the photocatalysis process have been taken into account, such as the intensity of light, temperature or the initial amount of virus on the surface. While differences in pH did not cause significant differences in the process.

The continuous disinfecting capacity of photocatalytic compounds makes them one of the best cleaning systems for surfaces and water today. It is possible that thanks to them the possible spread of COVID-19 by surfaces or contact can be avoided. The use of air filters with nanoparticles with photocatalytic capacity could be an interesting solution to the spread of the virus indoors. In many health centers or care centers for the elderly, these products are beginning to be used to guarantee the hygiene of the facilities. In fact, work is beginning on self-disinfecting surfaces with integrated photocatalytic particles. Several companies are starting to prepare their own photocatalitic disinfectants to release to the global market to clean against the SARS-CoV-2. On the other hand, the bactericidal activity of these compounds could be the solution to the increase in resistance to traditional disinfectants. Another line of research in the applications of photocatalytic materials was working on their ability to eliminate organic molecules from pollution (CO, NOx and SOx).

Biblio:

Foster HA, Ditta IB, Varghese S, Steele A. Photocatalytic disinfection using titanium dioxide: spectrum and mechanism of antimicrobial activity. Appl Microbiol Biotechnol. 2011 90(6):1847-68. 2011.

P.Venkata Laxma Reddy, Beluri Kavitha, Police Anil Kumar Reddy, Ki-Hyun Kim. TiO2-based photocatalytic disinfection of microbes in aqueous media: A review.  Environmental Research. 2017. 154, 296-303.

Nakano R, Ishiguro H, Yao Y, Kajioka J, Fujishima A, Sunada K, Minoshima M, Hashimoto K, Kubota Y. Photocatalytic inactivation of influenza virus by titanium dioxide thin film. Photochem Photobiol Sci. 2012. 11(8):1293-8.

Aziz Habibi-Yangjeh, Soheila Asadzadeh-Khaneghah, Solmaz Feizpoor, Afsar Rouhi. Review on heterogeneous photocatalytic disinfection of waterborne, airborne, and foodborne viruses: Can we win against pathogenic viruses? Journal of Colloid and Interface Science, 2020. 580, 503-14.

Bogdan J, Zarzyńska J, Pławińska-Czarnak J. Comparison of Infectious Agents Susceptibility to Photocatalytic Effects of Nanosized Titanium and Zinc Oxides: A Practical Approach. Nanoscale Res Lett. 2015. 10(1):1023.

K.G. Linden, M. Mohseni. Assuring Purity of Drinking Water. Comprehensive Water Quality and Purification, 2014. 1st ed.

Xiang Zheng, Zhi-peng Shen, Can Cheng, Lei Shi, Rong Cheng, Dong-hai Yuan. Photocatalytic disinfection performance in virus and virus/bacteria system by Cu-TiO2 nanofibers under visible light. Environmental Pollution.  2018. 237, 452-9.

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