The generation of highly reactive singlet oxygen (1O2) is of major importance for a variety of applications such as photodynamic therapy (PDT) for cancer treatment, water treatment, catalytic oxidation, and others. Herein, we demonstrate that 1O2 can be efficiently produced through the direct photosensitization by Au25(SR)18 – clusters (H−SR = phenylethanethiol or captopril) without using conventional organic photosensitizers under visible/near-IR (532, 650, and 808 nm) irradiation. 1O2 was successfully detected by direct observation of the characteristic 1O2 emission around 1276 nm as well as three different 1O2-selective probes. Water-soluble Au25(captopril)18 – clusters were explored for cytocompatibility and photodynamic activity toward cancer cells. In addition, selective catalytic oxidation of organic sulfide to sulfoxide by 1O2 was demonstrated on the photoexcited Au25(SC2H4Ph)18 – clusters. It is suggested that the optical gap of Au25(SR)18 clusters (∼1.3 eV) being larger than the energy of 1O2 (0.97 eV) allows for the efficient energy transfer to 3O2. In addition, the long lifetime of the electronic excited states of Au25(SR)18 and the well-defined O2 adsorption sites are the key factors that promote energy transfer from Au25(SR)18 – to molecular oxygen, thus facilitating the formation of 1O2. Finally, neutral Au25(SR)18 0 can also produce 1O2 as efficiently as does the anionic Au25(SR)18 −.

Magic-number theories, developed to explain the anomalous stability of clusters in the gas phase, are being successfully applied to explain the stability of families of condensed phase Au clusters. To test the generalizability of these theories, we have synthesized a family of magic-numbered Ag clusters. Silver clusters ligated with glutathione (GSH) were synthesized by reduction of silver glutathiolate in water and then separated by polyacrylamide gel electrophoresis (PAGE). The raw synthetic product consisted of a family of discrete Ag:SG clusters, each forming a band in the PAGE gel. Varying reaction conditions changed the relative abundance of the family members but not their positions and colors within the gel, indicating the molecular precision of magic-number clusters. Absorption onsets for the most abundant clusters monotonically decreased with increasing cluster size, and spectra contained a small number of peaks that corresponded to single electron transitions. Although these Ag:SG clusters are related to Au:SG clusters, the distribution of cluster sizes and the optical absorption spectra were markedly different for the two families. This suggests that the Ag:SG clusters are not a simple extension of the Au:SG system, possibly due to differences in Au and Ag chemistry. Alternatively, condensed-phase magic-number cluster theories may need to be more complex than currently believed.

We report the visible light photocatalytic properties of a composite material consisting of Au25(SR)18 nanoclusters (R:CH2CH2Ph) and TiO2 nanocrystals. The effects of Au25(SR)18 nanoclusters on the photocatalytic activity of TiO2 nanocrystals were evaluated in the reaction of photocatalytic degradation of methyl orange. The loading of Au25(SR)18 nanoclusters onto TiO2 results in strong visible light absorption by the composite and, more importantly, a 1.6 times increase in visible light photocatalytic activity. Furthermore, the Au25(SR)18/TiO2 composite nanostructure exhibits high stability in recycling tests. The Au25(SR)18 nanolusters dispersed on the TiO2 surface can act as a small-band-gap semiconductor to absorb visible light, giving rise to electron–hole separation and producing singlet oxygen (1O2). Both the generated hydroxyl radicals (HO•) and 1O2 are rationalized to be responsible for the decomposition of the dye.

This work was motivated by the unsatisfactory stability of Au(25)(SG)(18) in solution under thermal conditions (e.g. 70-90 °C for DNA melting). Thus, we searched for a better, water-soluble thiol ligand. Herein, we report a one-pot synthesis and investigation of the stability and optical properties of captopril (abbreviated Capt)-protected Au(25)(Capt)(18) nanoclusters. The Au(25)(Capt)(18) (anionic, counterion: Na(+)) nanoclusters were formed via size focusing under ambient conditions. Significantly, Au(25)(Capt)(18) nanoclusters exhibit largely improved thermal stability compared to the glutathione (HSG) capped Au(25)(SG)(18). Both Au(25)(Capt)(18) and Au(25)(SG)(18) nanoclusters show fluorescence centered at ∼700 nm. The chiral ligands (Capt, SG, as well as chirally modified phenylethanethiol (PET*)) give rise to distinct chiroptical features. The high thermal stability and distinct optical properties of Au(25)(Capt)(18) nanoclusters render this material quite promising for biological applications.

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