Let light do the work!

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Titanium(IV) oxide opens up limitless opportunities. Chemists from the Jagiellonian University are working on its new applications.

Titanium dioxide (TiO2) is a material with well-recognized optical properties. Titanium white, a commonly used white pigment, gives paints unique optical properties (whiteness), good coating, and durability. It also has found many other common applications in the cosmetics and food industry, but a real renaissance of TiO2 is emerging. It's the light-induced activity that makes titanium(IV) oxide really special.

Chemists from the Jagiellonian University developed new nano- and microcrystalline visible light-active materials based on titanium dioxide, which can be used for purifying and sterilizing surfaces, water and air. Such materials share a common name: photocatalysts. In the future, people might be able to use them as photoantibiotics or photodrugs used in anti-cancer and other advanced therapies, but almost the same materials can find applications as optoelectronic devices for data processing.

New colors of white pigment

Titanium dioxide in nano- and microcrystalline form can be used to create a uniform coating on various materials like glass, ceramics and metals, but also on polymers. However, coating production is not sufficient to obtain the desired effect. TiO2, like most stable photocatalysts, is white. This color implies its photoactivity occurs only under UV light illumination. Therefore it is beneficial "to sensitize it" to visible light. In other words, it is desirable to change the color of the white paint. If this is done in a smart manner, the applicability of this material can be significantly increased (see image). Sensitization can be achieved in different ways, depending on the expected properties and applications of the new material. The team from the Department of Inorganic Chemistry, Faculty of Chemistry (www.photocatalysis.eu), has developed several methods of TiO2 sensitization.

"The significance of the project depends on the planned applications of the new photocatalysts. Until now, our most promising materials have proven to be useful in photo-cleaning and photodisinfecting applications, in particular as coatings of self-sterilizing, single-use medical items like syringes, catheters, etc.," said Professor Wojciech Macyk, the leader of the research team. "But we hope to use our sensitized TiO2 colloids also as photoactive drug components," he added. In fact, a demonstrated phototoxicity of new materials may lead to the development of an effective photoactive component of drugs for therapies used to destroy viruses, bacteria, fungi or tumor cells.

Water and oxygen molecules (left) are photocatalytically converted
at developed TiO2 coatings into highly reactive species (right),
enabling the oxidation of pollutants and elimination of microorganisms. Graphics: P. Łabuz

How does it work?

The research also expands our basic knowledge of the methods of photosensitization. The results prove that chemical and physical processes, which are crucial for photocatalysis, may be controlled by an appropriate modification of the surface of photocatalysts. Since the overall photocatalytic reactions usually involve the activation of small molecules as primary steps, extensive basic studies on these processes are also in progress. They are supported by several funding bodies: the Foundation for Polish Science, the Ministry of Science and Higher Education, the National Science Center and FP7. As a result of these studies, not only have new sensitization modes been elaborated, but a better understanding of reactive oxygen species photogeneration on the surface of titanium dioxide has also been obtained. In particular, the conditions for a more efficient formation of socalled "singlet oxygen" have been determined. Singlet oxygen plays a particular role in therapies involving light.

Not only titanium dioxide

Photocatalysis are not limited to titanium dioxide-based materials; other, relatively simple chemicals may show unique, photocatalytic properties. Reactive oxygen species can be formed in the presence of other excited oxides, like zinc oxide. On the other hand, zinc sulfide (ZnS) can be considered as an interesting photocatalyst for solardriven carbon dioxide reduction. Its applicability in artificial photosynthesis of carboxylic acids from CO2 and other substrates was recently demonstrated as the result of a fruitful collaboration with Profs. A. Dibenedetto and M. Aresta from Department of Chemistry, University of Bari, Italy. The synthesis of carboxylic acids involves processes inaccessible on the surface of titanium dioxide.

Several patent applications have been submitted so far and there are plans to commercialize the results of studies that are conducted in collaboration with other research teams within an international and interdisciplinary collaboration (in particular with the group of Prof. P. B. Heczko and Prof. M. Strus, Chair of Microbiology, Jagiellonian University Medical College). Recent results have proven the visible light-induced activity of coatings produced at organic polymers on bacteria inactivation. Will this finding result in safer products in the medicine or food industries? Will artificial photosynthesis compete with natural processes? Only time will tell.

Research team:Wojciech Macyk, PhD; Professor Grażyna Stochel; Przemysław Łabuz, PhD; Marta Buchalska, PhD; Rafał Sadowski, MSc, Eng.; Tomasz Baran, MSc