Effects of ligand species and cluster size of biomolecule-protected Au nanoclusters on eff iciency of singlet-oxygen generation

Thiolate-protected Au25 nanoclusters (NCs) have been shown to be efficient 1O2 photosensitizers. The 1O2-generation efficiency of thiolate-protected Au NCs is expected to depend on the thiolate ligand species and size of the gold clusters. In the present study, Au25(glutathione)18 nanoclusters (Au25(SG)18 NCs) and bovine-serum-albumin (BSA)-protected Au25 nanoclusters (BSA-Au25 NCs) were synthesized to investigate the effect of different thiolate ligands on the 1O2 generation. The efficiency of 1O2 generation by the Au25 NCs that were photo-excited at 532 nm was evaluated with methotrexate as a chemical probe. The effect of Au NC size on the 1O2-generation efficiency of thiolate-protected Au18 and Au25 NCs was also investigated. The 1O2-generation efficiency of BSA-Au25 NCs was higher than that of Au25(SG)18 NCs. The long lifetime of the electronic excitations and the high-photo-stability were key factors facilitating the formation of 1O2 in BSA-Au25 NCs. The effect of Au NC size on the 1O2-generation efficiency of thiolate-protected Au NCs, Aun(SR)m, decreased in the following order: Au25(SR)18>Au18(SR)14.