Protoporphyrin IX-Functionalized AgSiO₂ Core-Shell Nanoparticles: Plasmonic Enhancement of Fluorescence and Singlet...

Metal-enhanced processes arising from the coupling of a dye with metallic nanoparticles (NPs) have been widely reported. However, few studies have simultaneously investigated these mechanisms from the viewpoint of dye fluorescence and photoactivity. Herein, protoporphyrin IX (PpIX) is grafted onto the surface of silver core silica shell NPs in order to investigate the effect of silver (Ag) localized surface plasmon resonance (LSPR) on PpIX fluorescence and PpIX singlet oxygen (¹ O₂ ) production. Using two Ag core sizes, we report a systematic study of these photophysical processes as a function of silica (SiO₂ ) spacer thickness, LSPR band position and excitation wavelength. The excitation of Ag NP LSPR, which overlaps the PpIX absorption band, leads to the concomitant enhancement of PpIX fluorescence and ¹ O₂ production independently of the Ag core size, but in a more pronounced way for larger Ag cores. These enhancements result from the increase in the PpIX excitation rate through the LSPR excitation and decrease when the distance between PpIX and Ag NPs increases. A maximum fluorescence enhancement of up to 14-fold, together with an increase in photogenerated ¹ O₂ production of up to five times are obtained using 100 nm Ag cores coated with a 5 nm thick silica coating.