Tailoring Fluorescence and Singlet Oxygen Generation of a Chlorophyll Derivative and Gold Nanorods via a Silica Shell

Gold nanorods deserve special attention as they exhibit tunable longitudinal localized surface plasmon resonances (LSPRs). In our study, gold nanorods of the aspect ratio of 2.25 (maximum of LSPR band at 660 nm) and of controllable SiO₂ thickness in the range of 6–14 nm were mixed with pheophorbide (chlorophyll derivative) in order to create a hybrid system. Energy transfer and singlet oxygen generation were studied for different SiO₂ thicknesses of the nanorod shell. The spectral properties of the hybrid mixture were characterized, and the overlapping of the pheophorbide fluorescence and the longitudinal LSPR band of nanorods on the fluorescence emission, energy transfer, and generation of singlet oxygen were studied. Two independent approaches were used to determine the quantum yield and enhanced factor of singlet oxygen generation. For a certain thickness of the SiO₂ shell and for certain concentrations of gold nanorods, the effect of the plasmon-enhanced singlet oxygen production was observed. Moreover, the enhanced of singlet oxygen yield enhancement was correlated with the far-field optical properties of the gold nanorods. The results obtained indicate the significance of further studies of dye-photosensitizers in hybrid mixtures, taking into account the spectral overlap between dye emission and longitudinal LSPR bands as well as the character of coatings (type and thickness) and scattering yields of gold nanorods.