Impact of Incoherent Coupling within Localized Surface Plasmon Resonance on Singlet Oxygen Production in Rose Bengal-Modified Silica-Coated Silver Nanoshells (SiO₂@Ag@SiO₂‑RB)

The use of metal-containing nanoparticles capable of localized surface plasmon resonance (LSPR)-enhanced singlet oxygen (¹O₂) production, via photosensitization, are currently promising candidates for cytotoxic medical purposes. Despite the ongoing advances in fabricating plasmonic nanomaterials, much of the insight into the fundamental mechanisms governing the photosensitizer (PS)–plasmon interactions remain unexplored. Silver nanoshells (SiO₂@Ag) possess seemingly promising optical properties to investigate the underlying LSPR effects on photosensitization, while also raising questions about the nature of LSPR generation in such dielectric-core metallic-shell nanostructures. In the present study, we synthesize and use silica-coated hybrid SiO₂@Ag nanoparticles with an outer surface-conjugated PS, and Ag shell densities ranging from ∼50 to >90%, to experimentally investigate these nanostructures’ LSPR-enhanced ¹O₂ generation properties. Using both direct and indirect ¹O₂ detection approaches, and with rigorous consideration of ¹O₂ kinetics, we show that the SiO₂@Ag-based structures produce severe inner filter effects in bulk-solution measurements, making them impractical for photodynamic applications. Furthermore, we make a case for the SiO₂@Ag nanoparticles’ poor LSPR generation efficiency to be the consequence of the species’ incoherently coupled plasmonic field and resultant lack of enhancement of the nearby chromophore’s photophysical properties.