Formation of plasmon quenching dips greatly enhances ¹O₂ generation in a chlorin e6–gold nanorod coupled system

Photodynamic therapy (PDT), as a noninvasive therapeutic method, has been actively explored recently for cancer treatment. However, owing to the weak absorption in the optically transparent windows of biological tissues, most commercial photosensitizers (PSs) exhibit low singlet oxygen (¹O₂) quantum yields when excited by light within this window. Finding the best way to boost ¹O₂ production for clinical applications using light sources within this window is, thus, a great challenge. Herein, we tackle this problem using plasmon resonance energy transfer (PRET) from plasmonic nanoparticles (NPs) to PSs and demonstrate that the formation of plasmon quenching dips is an effective way to enhance ¹O₂ generation. The combination of the photosensitizer chlorin e6 (Ce6) and gold nanorods (AuNR) was employed as a model system. We observed a clear quenching dip in the longitudinal surface plasmon resonance (LSPR) band of the AuNRs when the LSPR band overlaps with the Q band of Ce6 and the spacing between Ce6 and the rods is within the acting distance of PRET. Upon irradiation with 660 nm continuous-wave laser light, we obtained a seven-fold enhancement in the ¹O₂ signal intensity compared with that of a non-PRET sample, as determined using the ¹O₂ electron spin resonance probe 2, 2, 6, 6-tetramethyl-4-piperidine (TEMP). Furthermore, we demonstrated that the PRET effect is more efficient in enhancing ¹O₂ yield than the often-employed local field enhancement effect. The effectiveness of PRET is further extended to the in vitro level. Considering the flexibility in manipulating the localized SPR properties of plasmonic nanoparticles/nanostructures, our findings suggest that PRET-based strategies may be a general way to overcome the deficiency of most commercial organic PSs in biological optically transparent windows and promote their applications in clinical tumor treatments.