Distance-Dependent Plasmon-Enhanced Singlet Oxygen Production and Emission for Bacterial Inactivation

Herein, we synthesized a series of 10 core-shell silver-silica nanoparticles with a photosensitizer, Rose Bengal, tethered to their surface. Each nanoparticle possesses an identical silver core of about 67 nm, but presents a different silica shell thickness ranging from 5 to 100 nm. These hybrid plasmonic nanoparticles thus afford a plasmonic nanostructure platform with a source of singlet oxygen (¹O₂) at a well-defined distance from the metallic core. Via time-resolved and steady state spectroscopic techniques, we demonstrate the silver core exerts a dual role of enhancing both the production of ¹O₂, through enhanced absorption of light, and its radiative decay, which in turn boosts ¹O₂ phosphorescence emission to a greater extent. Furthermore, we show both the production and emission of ¹O₂ in vitro to be dependent on proximity to the plasmonic nanostructure. Our results clearly exhibit three distinct regimes as the plasmonic nanostructure moves apart from the ¹O₂ source, with a greater enhancement for silica shell thicknesses ranging between 10 and 20 nm. Moreover, these hybrid plasmonic nanoparticles can be delivered to both Gram-positive and Gram-negative bacteria boosting both photoantibacterial activity and detection limit of ¹O₂ in cells.