We report single-atom doping of gold nanoclusters (NCs), and its drastic effects on the optical, electronic, and catalytic properties, using the 25-atom system as a model. In our synthetic approach, a mixture of Pt(1)Au(24)(SC(2)H(4)Ph)(18) and Au(25)(SC(2)H(4)Ph)(18) was produced via a size-focusing process, and then Pt(1)Au(24)(SC(2)H(4)Ph)(18) NCs were obtained by selective decomposition of Au(25)(SC(2)H(4)Ph)(18) in the mixture with concentrated H(2)O(2) followed by purification via size-exclusion chromatography. Experimental and theoretical analyses confirmed that Pt(1)Au(24)(SC(2)H(4)Ph)(18) possesses a Pt-centered icosahedral core capped by six Au(2)(SC(2)H(4)Ph)(3) staples. The Pt(1)Au(24)(SC(2)H(4)Ph)(18) cluster exhibits greatly enhanced stability and catalytic activity relative to Au(25)(SC(2)H(4)Ph)(18) but a smaller energy gap (E(g) ≈ 0.8 eV vs 1.3 eV for the homogold cluster).

Peptide nucleic acid (PNA) is a synthetic analogue of DNA and RNA, developed more than a decade ago in which the naturally occurring sugar phosphate backbone has been replaced by the N-(2-aminoethyl) glycine units. Unlike DNA or RNA in the unhybridized state (single strand) which can adopt a helical structure through base-stacking, although highly flexible, PNA does not have a well-defined conformational folding in solution. Herein, we show that a simple backbone modification at the gamma-position of the N-(2-aminoethyl) glycine unit can transform a randomly folded PNA into a helical structure. Spectroscopic studies showed that helical induction occurs in the C- to N-terminal direction and is sterically driven. This finding has important implication not only on the future design of nucleic acid mimics but also on the design of novel materials, where molecular organization and efficient electronic coupling are desired.

Despite the recent crystallographic determination of the crystal structure of Au(25)(SCH(2)CH(2)Ph)(18) clusters, the question--whether all thiolate-capped, 25-atom gold clusters adopt the same structure, regardless of the types of thiols (e.g., long-chain alkylthiols, aromatic thiols, or other functionalized ones)--still remains unanswered. To crystallize long-chain or bulky ligand (e.g., glutathione)-capped Au(25)(SR)(18) clusters has proven to be difficult due to the major amorphousness caused by such ligands; therefore, one needs to seek other strategies to probe the structural information of such gold clusters. Herein, we report a strategy to probe the Au(25) core structure and surface thiolate ligand distribution by means of NMR in combination with mass spectrometry. We use glutathione-capped Au(25)(SG)(18) clusters as an example to demonstrate the utility of this strategy. One-dimensional (1D) and two-dimensional (2D) correlation NMR spectroscopic investigation of Au(25)(SG)(18) reveals fine spectral features that explicitly indicate two types of surface binding modes of thiolates, which is consistent with the ligand distribution in the Au(25)(SCH(2)CH(2)Ph)(18) cluster. Laser desorption ionization (LDI) mass spectrometry analysis shows that Au(25)(SG)(18) exhibits an identical ionization and core fragmentation pattern with phenylethylthiolate-capped Au(25) clusters. The charge state of the native Au(25)(SG)(18) clusters was determined to be -1 by comparing their optical spectrum with those of [Au(25)(SCH(2)CH(2)Ph)(18)](q) of different charge states (q = -1, 0). Taken together, our results led to the conclusion that glutathione-capped Au(25)(SG)(18) clusters indeed adopt the same structure as that of Au(25)(SCH(2)CH(2)Ph)(18). This conclusion is also valid for other types of thiolate-capped Au(25) clusters, including hexyl- and dodecylthiolates. Interestingly, the chiral optical responses (e.g., circular dichroism (CD) signals in the visible wavelength region) from the Au(25)(SG)(18) clusters seem to be imparted by the chiral glutathione ligands because no similar CD signals were observed in Au(25)(SCH(2)CH(2)Ph)(18).

We report the structure determination of a large gold nanocluster formulated as Au130(p-MBT)50, where p-MBT is 4-methylbenzenethiolate. The nanocluster is constructed in a four-shell manner, with 55 gold atoms assembled into a two-shell Ino decahedron. The surface is protected exclusively by -S-Au-S- staple motifs, which self-organize into five ripple-like stripes on the surface of the barrel-shaped Au105 kernel. The Au130(p-MBT)50 can be viewed as an elongated version of the Au102(SR)44. Comparison of the Au130(p-MBT)50 structure with the recently discovered icosahedral Au133(p-TBBT)52 nanocluster (where p-TBBT = 4-tert-butylbenzenethiolate) reveals an interesting phenomenon that a subtle ligand effect in the para-position of benzenethiolate can significantly affect the gold atom packing structure, i.e. from the 5-fold twinned Au55 decahedron to 20-fold twinned Au55 icosahedron.

The [Au37(PPh3)10(SR)10X2](+) nanocluster (where SR = thiolate and X = Cl/Br) was theoretically predicted in 2007, but since then, there has been no experimental success in the synthesis and structure determination. Herein, we report a kinetically controlled, selective synthesis of [Au37(PPh3)10(SC2H4Ph)10X2](+) (counterion: Cl(-) or Br(-)) with its crystal structure characterized by X-ray crystallography. This nanocluster shows a rod-like structure assembled from three icosahedral Au13 units in a linear fashion, consistent with the earlier prediction. The optical absorption and the electrochemical and catalytic properties are investigated. The successful synthesis of this new nanocluster allows us to gain insight into the size, structure, and property evolution of gold nanoclusters that are based upon the assembly of icosahedral units (i.e., cluster of clusters). Some interesting trends are identified in the evolution from the monoicosahedral [Au13(PPh3)10X2](3+) to the bi-icosahedral [Au25(PPh3)10(SC2H4Ph)5X2](2+) and to the tri-icosahedral [Au37(PPh3)10(SC2H4Ph)10X2](+) nanocluster, which also points to the possibility of achieving even longer rod nanoclusters based upon assembly of icosahedral building blocks.