Combined near infrared photothermolysis and photodynamic therapy by association of gold nanoparticles and an organic dye

We investigated the combination of near infrared (NIR) photothermolysis and photodynamic therapy against different models of bacteria (S. aureus, S. epidermidis both methicillin susceptible and resistant), in order to discover possible synergistic pathways in the fight against cancer. Photothermolysis was mediated by NIR light absorption from gold nanorods, which were coated with polyethylene glycol to gain biocompatibility and provide for a convenient interface with the bacterial cell walls. At the same time photodynamic therapy was delivered by administration of Indocyanine Green (ICG), whose spectrum of molecular excitation overlaps the plasmonic oscillations of gold nanorods (∼ 800 nm). Therefore irradiation with NIR light from a low power diode laser resulted into simultaneous photothermolysis and generation of reactive oxygen species and cytotoxic byproducts of ICG. We assessed the inhibition of the bacterial colony forming ability under different NIR light exposures, and compared the performance of the combined treatment (gold nanorods plus ICG) with the projected addition of the separate treatments (either gold nanorods or ICG). Our preliminary results may originate from the interplay of synergistic and conflicting interactions, which may include e.g. the enhanced intake of cytotoxic species due to permeabilization of the bacterial cell walls, quenching of ICG and modification of the bleaching of ICG due to the noble metal surface.