Efficient Singlet Oxygen Generation in Metal Nanoclusters for Two-Photon Photodynamic Therapy Applications.

The generation of singlet oxygen (O) has been established as the principal mechanism of photodynamic therapy (PDT). Various dyes, metal nanoparticles, and clusters have been shown to sensitize O. However, metal nanoclusters are even more promising candidates as photosensitizers for this purpose. By understanding the optical properties that lead to efficient O generation, one can fully realize their potential as PDT photosensitizers. Three different metal nanoclusters, Au, Ag, and Au, are investigated for their O generation efficiency. The Au showed a O generation rate that is 2 orders of magnitude higher than that for Au and Ag, and several orders of magnitude higher than nanoparticles (>5 nm) due to Au's high absorption cross section-to-volume ratio. The effectiveness of PDT in live cells with nanoclusters was demonstrated by two-photon excitation compared to one-photon excitation. The implication of these results points toward new efficient two-photon O sensitizers for photodynamic therapy.