Photocatalytic activity of N-doped TiO2-based materials embedded with gold NPs for applications in antibacterial photodynamic therapy (aPDT)

Antibacterial photodynamic therapy (aPDT) is a potential treatment for antibiotic-resistant bacterial infections. It is based on the photosensitization of bacterial cells with exogenous agents that, when exposed to light, produce reactive oxygen species (ROS), such as OH-, O2-, H2O2. ROS can induce complex oxidative-reductive chains of reactions, resulting in damage of cellular components in target tissues1. Photocatalysts, like inorganic semiconductor oxides, represent an interesting class of materials to design new strategies for aPTD. As exposed to light of proper wavelengths, photocatalysts induce the formation of electron-hole pairs capable of producing a cascade of reactions suitable for aPDT2. In this context, titanium dioxide (TiO2) nanostructures proved to be among the most promising materials, due to its effective photocatalytic activity, which leads to an efficient and fast degradation of organic compounds upon UV light irradiation3. However, the need of UV excitation sources to trigger the TiO2 activity represents a major limitation for some biomedical applications. Here, we present an experimental study aimed at the production and characterization of combined nanostructured materials that can be used to selectively damage biomolecules, bacteria, and more complex structures, such as biofilms. To this aim, we combined the photocatalytic properties of TiO2-based materials, with the well-known biocide properties of gold nanoparticles, whose photo-thermal effects under infrared illumination can be exploited to broad the spectrum of molecular targets and increase antibacterial efficiency. For our purposes, TiO2 was doped with nitrogen atoms by a sol-gel synthesis. N-doped TiO2 is promising visible light-responsive photocatalyst, which allowed to avoid the use of UV radiation as excitation source, a key point for most biomedical applications. N-doped TiO2 and gold nanostructures were characterized alone and in combination by means of spectroscopic techniques (UV-Vis absorption, fluorescence, FTIR), and microscopy (optical, SEM). The photoinduced activity of the nanostructures was investigated in aqueous suspension by using a blue LED (centred at 420 nm) as illumination source, and methyl orange as model molecule. Interestingly, the results showed an enhancement of the photocatalytic activity of N-doped TiO2, in presence of the gold nanoparticles. A possible mechanism which explains this effect is that the combination of gold and TiO2 is supposed to be facilitate the separation of photogenerated electron–hole pairs and promote interfacial charge transfer4. This hinders electron-hole recombination resulting in the improvement of the photocatalytic activity. Further studies are needed to deeply investigate these phenomena also in composite materials where titanium and gold are embedded in hydrogels, nanofibers produced by electrospinning or films, together with suitable components selected with the aim of enhancing biocide performances in the field of photodynamic therapy. 1. M. Wainwright, T. Maisch, S. Nonell, K. Plaetzer, A. Almeida, G. P. Tegos and M. R. Hamblin, The Lancet Infectious Diseases, 2017, 17, e49–e55. 2. Y.-T. Lin, C.-H. Weng, Y.-H. Lin, C.-C. Shiesh and F.-Y. Chen, Effect of C content and calcination temperature on the photocatalytic activity of C-doped TiO2 catalyst, Separation and Purification Technology, 2013, 116, 114–123. 3. C. Liao, Y. Li and S. C. Tjong, Nanomaterials, 2020, 10. 4. Z. Duan, Y. Huang, D. Zhang and S. Chen, Electrospinning Fabricating Au/TiO2 Network-like Nanofibers as Visible Light Activated Photocatalyst, Scientific Reports.