nanomaterials Article Bactericidal Activity of Multilayered Hybrid Structures Comprising Titania Nanoparticles and CdSe Quantum Dots under Visible Light Ekaterina Kolesova 1, * , Anastasia Bulgakova 1 , Vladimir Maslov 1 , Andrei Veniaminov 1 , Aliaksei Dubavik 1 , Yurii Gun’ko 2 , Olga Efremenkova 1,3 , Vladimir Oleinikov 1,4 and Anna Orlova 1, *   Citation: Kolesova, E.; Bulgakova, A.; Maslov, V.; Veniaminov, A.; Dubavik, A.; Gun’ko, Y.; Efremenkova, O.; Oleinikov, V.; Orlova, A. Bactericidal Activity of Multilayered Hybrid Structures Comprising Titania Nanoparticles and CdSe Quantum Dots under Visible Light. Nanomaterials 2021, 11, 3331. https://doi.org/10.3390/ nano11123331 Academic Editor: Maria Violetta Brundo Received: 24 October 2021 Accepted: 30 November 2021 Published: 8 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 School of Photonics, ITMO University, 197101 St. Petersburg, Russia; anastasiya.makovectkaya@mail.ru (A.B.); maslov04@bk.ru (V.M.); avveniaminov@itmo.ru (A.V.); adubavik@yandex.ru (A.D.); ovefr@yandex.ru (O.E.); voleinik@mail.ru (V.O.) 2 School of Chemistry, Trinity College Dublin, D02 PN40 Dublin, Ireland; igounko@tcd.ie 3 Department of Microbiology, FSBI Gause Institute of New Antibiotics, 119021 Moscow, Russia 4 Department of Biomaterials and Bionanotechnology, Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, 117997 Moscow, Russia * Correspondence: e.p.kolesova@gmail.com (E.K.); a.o.orlova@gmail.com (A.O.) Abstract: Titania nanoparticle/CdSe quantum dot hybrid structures are a promising bactericidal coating that exhibits a pronounced effect against light-sensitive bacteria. Here, we report the results of a comprehensive study of the photophysical properties and bactericidal functionality of these hybrid structures on various bacterial strains. We found that our structures provide the efficient generation of superoxide anions under the action of visible light due to electron transfer from QDs to titania nanoparticles with ~60% efficiency. We also tested the antibacterial activity of hybrid structures on five strains of bacteria. The formed structures combined with visible light irradiation effectively inhibit the growth of Escherichia coli, Bacillus subtilis, and Mycobacterium smegmatis bacteria, the last of which is a photosensitive causative agent model of tuberculosis. Keywords: hybrid nanostructures; quantum dots; photo-induced electron transfer; reactive oxygen species; antibacterial activity 1. Introduction Bacterial infections, despite the high level of development of modern medicine, remain among the most dangerous socially significant diseases in the current era [1], threatening human health, food security, and the development of society in general. The inappropri- ate use of antibiotics has become one of the reasons for the manifestation of antibiotic resistance in bacteria [2,3]. Many new bacterial strains have been found to be resistant to one or several types of antibiotics every year [4]. That is why in 2017, the World Health Organization (WHO) announced the need to develop new bactericidal systems to combat the growing threat of antimicrobial resistance [5]. In particular, in this statement, the importance of funding the treatment of tuberculosis was noted since multiple strains of tuberculosis pathogens that are resistant to antibiotics, according to WHO statistics, are registered in more than 100 countries around the world. The current situation of the COVID-19 pandemic and the risk of respiratory infections has increased manifold, also indicating the need for new approaches for the treatment of not only bacterial but also viral infections, including respiratory infections, which can significantly complicate the course of coronavirus infection. These facts determine the urgent need to search for new approaches and alternative systems for effective therapy for bacterial infections. Many pathogenic bacteria can spread with airborne droplets or can settle on various surfaces. This increases the risk of infection through touching objects, walls, and furniture, followed by touching mucous membranes. The creation of bactericidal self-cleaning Nanomaterials 2021, 11, 3331. https://doi.org/10.3390/nano11123331 https://www.mdpi.com/journal/nanomaterials