Antibacterial photodynamic therapy under visible light using N-doped titanium dioxide and gold nanoparticles: study of photodegradation effects on model molecules

Photodynamic and photothermal therapies are gaining attention as innovative antimicrobial strategies since their mechanisms of action does not allow bacteria to develop further resistance. With the aim of developing new hybrid systems with effective activity against localised infections, we present an experimental study on the photocatalytic properties of nitrogen-doped titanium dioxide (N-TiO2), as photodynamic agent to produce reactive oxygen species (ROS), in combination with gold nanoparticles (AuNPs) as suitable enhancers with well-known biocidal and tunable optical properties. N-TiO2 nanostructures were prepared by a green sol-gel synthesis to obtain a visible light sensitive photocatalyst, suitable for application in the biological context. The nitrogen doping procedure allowed the use of a blue LED (centered at 420 nm) as illumination source to trigger ROS production, overcoming the need for UV light of the pristine TiO2. The photocatalytic activity of N-TiO2 nanostructures was assessed by means of spectroscopy methods on methyl-orange (MO), in aqueous solution. Then, following the interesting results observed on the photodegradation of MO, we have moved towards the investigation of the potential impact of the material on target biomolecules such as DNA, lipids, and proteins. The most significant effect was found on DNA which represents a relevant target as it is essential for bacterial survival and replication. Interestingly, the observed degradation was enhanced in the presence of AuNPs, likely due to the synergistic effect of N-TiO2 photocatalyst with AuNPs. Similar results were also observed by studying the photodegradation of DNA, and as for MO, this biomacromolecule undergoes critical modifications, enhanced dramatically in the presence of AuNPs. AuNPs appear to improve the photocatalytic efficiency of N-TiO2 by stabilizing charge carriers on gold surface then preventing electron–hole pairs recombination. Further studies are ongoing to better understand the observed phenomena and to highlight key effectors (e.g. geometry, concentration) of the improved photocatalytic activity.