Large-scale synthesis of thiolated Au25 clusters via ligand exchange reactions of phosphine-stabilized Au11 clusters

Phosphine-stabilized Au11 clusters in chloroform were reacted with glutathione (GSH) in water under a nitrogen atmosphere. The resulting Au:SG clusters exhibit an optical absorption spectrum similar to that of Au25(SG)18, which was isolated as one of the major products from chemically prepared Au:SG clusters (Negishi, Y. et al. J. Am. Chem. Soc. 2005, 127, 5261). Rigorous characterization by optical spectroscopy, electrospray ionization mass spectrometry, and polyacrylamide gel electrophoresis confirms that the Au25(SG)18 clusters were selectively obtained on the sub-100 mg scale by ligand exchange reaction under aerobic conditions. The ligand exchange strategy offers a practical and convenient method of synthesizing thiolated Au25 clusters on a large scale.     

Density-functional, density-functional tight-binding, and wave function calculations on biomolecular systems

Recently, two computational approaches that supply a density-functional-based quantum-chemical method with an empirical term accounting for London dispersion were introduced and found use in the studies of biomolecular systems, namely, DFT-D and SCC-DFTB-D. Here, we examine the performance and usability of these combined techniques for dealing with several tasks typically occurring in the research of biomolecules. The interaction energy of small biomolecular complexes agrees very well with the reference data yielded by correlated ab initio quantum chemical methods. In real-life studies aimed at interaction energy, structure, and infrared spectra, the mentioned methods provide results in good agreement with each other and with experiment (where available). The very favorable time demands of these approaches are discussed, and for each of them, a suitable area of use is proposed on the basis of the results of our analysis.     

Self-consistent field tight-binding model for neutral and (multi-) charged carbon clusters

A semiempirical model for carbon clusters modeling is presented, along with structural and dynamical applications. The model is a tight-binding scheme with additional one- and two-center distance-dependent electrostatic interactions treated self-consistently. This approach, which explicitly accounts for charge relaxation, allows us to treat neutral and (multi-) charged clusters not only at equilibrium but also in dissociative regions. The equilibrium properties, geometries, harmonic spectra, and relative stabilities of the stable isomers of neutral and singly charged clusters in the range n=1-14, for C(20) and C(60), are found to reproduce the results of ab initio calculations. The model is also shown to be successful in describing the stability and fragmentation energies of dictations in the range n=2-10 and allows the determination of their Coulomb barriers, as examplified for the smallest sizes (C(2) (2+),C(3) (2+),C(4) (2+)). We also present time-dependent mean-field and linear response optical spectra for the C(8) and C(60) clusters and discuss their relevance with respect to existing calculations.     

Thiolate ligands for synthesis of water-soluble gold clusters

Water-soluble monolayer-protected gold clusters (MPCs) have been an object of investigation by many research groups since their first syntheses were reported in 1998 and 1999. The basic requirements for a ligand to form a monolayer protecting a gold cluster were established some time ago for alkanethiolate MPCs, but there has been no such information published for water-soluble MPCs. We identify 6 new ligands capable of forming water-soluble MPCs, as well as 22 water-soluble ligands that fail to form MPCs. Our findings contribute not only to the definition of the requirements for MPC formation but also to the variety of MPCs available for applications in chemistry and biology.     

Structural study of gold clusters

Density functional theory (DFT) calculations were carried out to study gold clusters of up to 55 atoms. Between the linear and zigzag monoatomic Au nanowires, the zigzag nanowires were found to be more stable. Furthermore, the linear Au nanowires of up to 2 nm are formed by slightly stretched Au dimers. These suggest that a substantial Peierls distortion exists in those structures. Planar geometries of Au clusters were found to be the global minima till the cluster size of 13. A quantitative correlation is provided between various properties of Au clusters and the structure and size. The relative stability of selected clusters was also estimated by the Sutton-Chen potential, and the result disagrees with that obtained from the DFT calculations. This suggests that a modification of the Sutton-Chen potential has to be made, such as obtaining new parameters, in order to use it to search the global minima for bigger Au clusters.     

Spontaneous alloying of gold clusters into nanometer-sized antimony clusters

Alloying behavior of gold into nm-sized amorphous antimony (a-Sb) clusters has been studied by transmission electron microscopy (TEM), employing gold clusters in contact with a-Sb clusters. In order to produce gold clusters on individual a-Sb clusters, a-Sb clusters on an amorphous carbon film were cooled down to 96 K, and gold was then condenced on the film. When gold clusters in contact with a-Sb clusters are gradually heated from 96 to 290 K, dissolution of gold into a-Sb clusters sets in around 200K and clusters of a-(Sb-Au) alloys are produced. With increasing annealing temperture, more gold is absorbed into individual a-Sb clusters, and when the gold concentration in a-(Sb-Au) clusters reaches to the stoichiometric composition of AuSb₂, these amorphous clusters crystallize into AuSb2 clusters. The crystallization temperature decreases with decreasing size of initial a-Sb clusters.     

The wetting of gold and silicon nanoscale arrays

The relative hydrophobicity of surfaces containing highly regular, nanoscale (<100 nm) topological features (50 nm diameter, 5 nm height, 100 nm spacing) was measured and compared with their flat counterparts. The results are described in the context of both the Wenzel and Cassie models for wetting rough surfaces. Wenzel-type wetting is observed for these high areal density nanostructures (approximately 10(10)/cm2) whose aspect ratios range from 0.2 to 0.7.     

Toward constructing nanoscale hydroxo-lanthanide clusters: syntheses and characterizations of novel tetradecanuclear hydroxo-lanthanide clusters

Two novel tetradecanuclear hydroxo-lanthanide complexes, formulated as Ln14(mu 4-OH)2(mu 3-OH)16(mu-eta 2-acac)8(eta 2-acac)16, Ln = Tb, 1; Eu, 2; acac = acetylacetonato, were synthesized and characterized by single-crystal X-ray diffraction, elemental analysis and phosphorescence spectroscopy.     

Fully ferrocenated hexanethiolate monolayer-protected gold clusters

The synthesis and compositional analysis of four different gold clusters with protecting monolayers comprised solely of ferrocene hexanethiolate ligands is described. The gold nanoparticles have average core diameters of 1.4, 1.6, 2.0, and 2.2 nm with estimated average atom counts of 55, 140, 225, and 314 Au atoms and average monolayer coverages of 37, 39, 43, and 58 ferrocenated ligands, respectively. The data show unequivocally that the number of ferrocene hexanethiolate ligands bound to each core size is constrained by the steric requirements of the ferrocene head group; the ligand numbers are significantly smaller than those for hexanethiolate ligands bonded to analogous-sized Au cores. Voltammetry of dilute solutions of these nanoparticles shows a large ferrocene oxidation wave and, at more negative potentials, smaller one-electron waves for the quantized double-layer charging of the Au cores. Together, the ferrocenes and core of the ferrocenated Au314 nanoparticle deliver 60 electrons at the ferrocene oxidation potential, which amounts to a very large volume charge capacity, 7x10(9) C/m3, for an undiluted nanoparticle sample.     

Structural evolution of larger gold clusters

The preferred structures of larger gold clusters comprised of 100 to 1000 atoms (1.4–C3.0 nm equivalent diameter) have been determined theoretically via exhaustive search and energy-minimization methods and experimentally by synchrotron x-ray diffraction analysis of purified powder samples of small gold nanocrystals passivated by alkylthiol(ate) self-assembled monolayers. Theory predicts a persistent, close competition, across the entire size-range, among three structure-types: Marks-type decahedral (Dh) structures, monocrystals of a particular (TO+) truncated-octahedral (or ‘Wulffi’) morphology, and symmetrically twin-faulted variants (t-TO+) of the second; all other forms are much less stable. Quantitative comparison of the experimental diffraction patterns with patterns calculated from the structures provides clear evidence for a high abundance of the Dh and t-TO+ forms, but also reveals a definite transition from the former to the latter structures in the 1.7 to 2.0 nm range (~ 200 atoms). Further, the observed (mean) lattice contraction is only about half that predicted, suggesting that the surfactant monolayer acts to reduce the surface energy of the clusters. Taken together, these results suggest that the surfactant monolayer may play a small but important role in differentially stabilizing the higher energy {100}-type facets present to a greater extent in the TO-type structures.     

Ligand heterogeneity on monolayer-protected gold clusters

This paper describes the effects of oxidative electronic charging of the Au cores of the monolayer-protected clusters (MPCs), Au140(S(CH2)5CH3)53 and Au38(SCH2CH2Ph)24, on nuclear magnetic resonance (NMR) spectra of their monolayer ligand shells. Previously unresolved fine structure in the 13C NMR hexanethiolate methyl and C5 methylene resonances is seen in spectra of solutions of monodisperse Au140(S(CH2)5CH3)53 MPCs, reflecting magnetically inequivalent ligand sites. Incremented increases in positive cluster core charge, effected by electrochemical charging, cause the spectral fine structure of the methyl resonance to coalesce, becoming a single peak at the Au140(3+) charge state. The spectral changes are reversible; charging back to the original core charge state regenerates the methyl 13C resonance fine structure. Adding an equimolar quantity of a Au(I) thiolate complex, Au(I)[SCH2(C6H4)C(CH3)3], to an uncharged Au140(S(CH2)5CH3)53 MPC solution in d2-methylene chloride causes partial spectral coalescence. 13C NMR spectra of Au38(SCH2CH2Ph)24 MPCs exhibit roughly comparable spectral changes upon positive core charging to the '0', '+1', and '+2' states. The NMR results indicate that exchange between magnetically inequivalent sites occurs at rates of 100 to 400 s(-1), a rate believed to be too fast to be accountable by actual exchanges of ligands between different sites on the Au core. We also describe changes in core electronic spectra of Au140(S(CH2)5CH3)53 induced by positive charging, measured using spectroelectrochemistry.     

'Shape effects' in metal oxide supported nanoscale gold catalysts

We report the activity of shape-controlled metal oxide (CeO(2), ZnO and Fe(3)O(4)) supported gold catalysts for the steam reforming of methanol (SRM) and the water gas shift (WGS) reactions. Metal oxide nanoshapes, prepared by controlled hydrolysis and thermolysis methods, expose different crystal surfaces, and consequently disperse and stabilize gold differently. We observe that similar to gold supported on CeO(2) shapes exposing the {110} and {111} surfaces, gold supported on the oxygen-rich ZnO {0001} and Fe(3)O(4) {111} surfaces shows higher activity for the SRM and WGS reactions. While the reaction rates vary among the Au-CeO(2), Au-ZnO and Au-Fe(3)O(4) shapes, the apparent activation energies are similar, indicating a common active site. TPR data further indicate that the reaction lightoff coincides with the activation of Au-O-M species on the surface of all three oxide supports evaluated here. Different shapes contain a different number of binding sites for the gold, imparting different overall activity.     

CO combustion on supported gold clusters.

Recent progress in the understanding of the fascinating catalysis of CO combustion by supported gold particles is summarized. Focusing on size-selected gold clusters consisting of only a few atoms, that is, the size regime with properties nonscalable from the bulk properties, we discuss the current knowledge of the different factors controlling the reactivity at the molecular level. These factors include the role of the oxide support, its defects, cluster charging as well as the structural fluxionality of clusters, the cluster size dependency, and the promotional effect of water. By combining experimental results with quantum mechanical ab initio calculations, a detailed picture of the reaction mechanism emerges. While similar mechanisms might be active for gold nanoparticles in the scalable size regime, it is shown that for different systems (defined by the cluster size, the support, experimental conditions, etc.) the reaction mechanism differs and, hence, no generalized explanation for the catalytic driving force of small gold particles can be given.     

Quantum-sized gold clusters as efficient two-photon absorbers

The two-photon absorption properties of Au25 cluster has been investigated with the aid of two-photon excited fluorescence in the communication wavelength region with a cross-section of 2700 GM at 1290 nm. Additional visible fluorescence has been discovered for small gold clusters which is two-photon allowed (after excitation at 800 nm), and the absolute cross-section has been determined for gold clusters with number of gold atoms varying from 25 to all the way up to 2406 using one and two-photon excited time-resolved fluorescence upconversion measurements. Record high TPA cross-sections have been measured for quantum sized clusters making them suitable for two-photon imaging as well as other applications such as optical power limiting and lithography.     

Planar gold nanoparticle clusters as microscale mirrors

Dispersible microscale mirror particles were synthesized by linkage of citrate-coated gold nanoparticles and 3-aminopropylsilyl-modified Ca2Nb3O10 perovskite nanoplates. The mirror particles reflect 14-19% of light in the 500-800 nm wavelength interval with retention of polarization. Due to their directional reflection properties, laser-irradiated micromirror dispersions in solvents exhibit Brownian motion-driven multicolor blinking behavior.     

Surfactant layering on mixed monolayer-protected gold clusters

Positively-charged monolayer protected gold clusters (MMPCs) were mixed with sodium dodecyl sulfate (SDS). At lower SDS concentration, the initially water-soluble particles became organic-soluble while remaining discrete. Upon further addition of SDS, the particles aggregate and become water-soluble. NaCN decomposition, TEM, and DLS characterization reveal the morphology and properties of these encapsulated assemblies.     

The minimum-energy structure of nanometer-scale gold clusters

We report results of experiments in which gold clusters with controlled diameters ranging from 1nm to 20nm are grown in a gas aggregation reactor and are subsequently melted and slowly cooled in the gas phase. These clusters are soft landed on thin carbon films and their structure determined by means of HRTEM. All of the clusters down to the smallest whose lattice fringes could be resolved (N≈405) are single fcc crystals. MD calculations using an EAM potential for gold predict that the fcc motif seen in these experiments may indeed be the minimum-energy structure for gold clusters containing more than a few hundred atoms.