Smart Nanoparticles for Targeted Photodynamic Therapy of Oral Bacteria - Characterisation

Objectives: Photodynamic therapy is a promising treatment for oral plaque biofilms. However, the treatment causes widespread relatively non-specific bacterial killing via the production of ROS/radical species. To increase specificity for disease-causing organisms we have synthesised gold nanoparticles to which have been conjugated erythrosine (photosensitiser) and antibodies specific for either S. mutans or L. casei. Our previous studies have shown that upon irradiation these conjugates cause enhanced killing of their target organism by up to 2 logs relative to non-target species. The aim of the current study was to use electron paramagnetic resonance (EPR) to characterise the nanoparticles in terms of their ROS/radical production. Methods: EPR was used in conjunction with spin trapping to study short-lived radical species produced as a result of PDT using the conjugated nanoparticles and free erythrosine. The broad-spectrum spin trap, DMPO, was used to detect and trap radicals such as oxygen-, carbon-, sulphur- and nitrogen-centred radicals produced following irradiation with white light. DMSO was used as a radical scavenger. DBNBS was used for the detection of carbon-centred radicals only. Results: Clear radical generation was observed in all the samples containing erythrosine, with radical generation being higher in the presence of the nanoparticles. Addition of the radical scavenger, DMSO, led to significant decreases in the amount of radical trapped by the DMPO. Free erythrosine gave no significant response upon irradiation in the presence of DBNBS. However, both the S. mutans- and L. casei-specific nanoparticles gave results indicative of the generation of anisotropic DBNBS/•tyrosyl-protein adducts. There was no effect of adding the radical scavenger, DMSO, to any of the samples. Conclusions: The EPR data showed that upon irradiation with light, the erythrosine bound to the nanoparticles acts via a mixed-type reaction leading to the production of a variety of ROS/radicals that will contribute to bacterial cell killing.