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

Thiolate-protected Au-25 nanoclusters (NCs) have been shown to be efficient O-1(2) photosensitizers. The O-1(2)-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 (Au-25(SG)(18) NCs) and bovine-serum-albumin (BSA)-protected Au-25 nanoclusters (BSA-Au-25 NCs) were synthesized to investigate the effect of different thiolate ligands on the O-1(2) generation. The efficiency of O-1(2) generation by the Au-25 NCs that were photo-excited at 532 nm was evaluated with methotrexate as a chemical probe. The effect of Au NC size on the O-1(2)-generation efficiency of thiolate-protected Au-18 and Au-25 NCs was also investigated. The O-1(2)-generation efficiency of BSA-Au25 NCs was higher than that of Au-25(SG)(18) NCs. The long lifetime of the electronic excitations and the high-photo-stability were key factors facilitating the formation of O-1(2) in BSA-Au-25 NCs. The effect of Au NC size on the O-1(2)-generation efficiency of thiolate-protected Au NCs, Au-n(SR)(m), decreased in the following order: Au-25(SR)(18) > Au-18(SR)(14). (C) 2016 Elsevier B.V. All rights reserved.