Tetrahedral Au20 Cluster Research Articles

Binding Mechanism and Surface-Enhanced Raman Scattering of the Antimicrobial Sulfathiazole on Gold Nanoparticles.

A combined study using the surface-enhanced Raman scattering (SERS) technique and quantum chemical calculations was carried out to elucidate the adsorption behavior of sulfathiazole, an antibiotic drug, on gold nanoparticles. The tetrahedral Au20 cluster was used as a simple model to mimic a nanostructured gold surface. Computations using density functional theory with the PBE functional were performed in both the gas phase and aqueous medium using a continuum model. The drug is found to bind to the Au metals via the nitrogen of the thiazole ring. The interaction is also partially stabilized by the ring-surface π coupling rather than a sideway adsorption as previously proposed. In an aqueous solution, the drug molecule mainly exists as a deprotonated form, which gives rise to a much greater affinity toward Au nanoparticles as compared to the neutral forms. The drug adsorption further induces a significant alteration on the energy gap of the gold cluster Aun, which could result in an electrical noise. Notable SERS signals below 1600 cm-1, which result from a coupling of several vibrations including the ring breathing, C-C stretching, and N-H bending, could be employed for both qualitative and quantitative detection and assessment of sulfathiazole at trace concentrations.

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model.

Computational approaches are employed to elucidate the binding mechanism and the SERS phenomenon of 6-mercaptopurine (6MP) adsorbed on the tetrahedral Au20 cluster as a simple model for a nanostructured gold surface. Computations are carried out in both vacuum and aqueous environments using a continuum model. In the gaseous phase and neutral conditions, interaction of 6MP with the gold cluster is mostly dominated by a covalent Au−S bond and partially stabilized by the Au⋅⋅⋅H−N coupling. However, in acidic solution, the nonconventional Au⋅⋅⋅H−S hydrogen-bond becomes the most favorable binding mode. The 6MP affinity for gold clusters decreases in the order of vacuum > neutral solution > acidic medium. During the adsorption, the energy gap of Au20 substantially declines, leading to an increase in its electrical conductivity, which can be converted to an electrical noise. Moreover, such interaction is likely a reversible process and triggered by either the low pH in sick tissues or the presence of cysteine residues in protein matrices. While N−H bending and stretching vibrations play major roles in the SERS phenomenon of 6MP on gold surfaces in neutral solution, the strongest enhancement in acidic environment is mostly due to an Au⋅⋅⋅H−S coupling, rather than an aromatic ring-gold surface π overlap as previously proposed.

Au20. Effect of a Strong Tetrahedral Field in a Spherical Concentric Bonding Shell Model

The tetrahedral Au20 cluster represents an outstanding landmark in cluster science. Its electronic structure can be described in terms of superatomic orbitals based on a 1s21p62s21d10 electronic configuration. Here we use the concentric bonding shell approach in order to rationalize Au20 in terms of a multilayered architecture accounting for its magic number of 20 valence electrons, which originates from the 2s antibonding combination between two structural layers. As the number of concentric structures increases from [Au4] → [Au4@Au12] → Au20, the superatomic shells are consequently expanded as, 1s1p → 1s1p1d2s2p1f → 1s1p2s1d2p3s1f3p. The role of spin–orbit coupling in affecting the electronic structure is also described. Our results suggest that Au20 can be conveniently viewed as the combination of concentric structures denoted by [{Au4@Au12}Au4] with considerable sharing of the electron density between the different concentric layers. Thus, the presence of the 2s antibonding combination originates from...

Contriving a Catalytically Active Structure from an Inert Conformation: A Density Functional Investigation of Al, Hf, and Ge Doping of Au20 Tetrahedral Clusters

The transformation of an inert structure into a catalytically active conformation has immense potential from a structural engineering point of view. In the present work, we explored methods of achieving such a transformation through a density-functional-based study. Au20 is known be one of the most stable and catalytically inert gold clusters, with a well-known tetrahedral structure. We demonstrate the transformation of the inert Au20 conformation into a highly active catalytic cluster through selective doping with Hf and Ge atoms. Depending on the dopant, the inert tetrahedral Au20 cluster evolves into either an endohedral or a hollow-cage-like conformation. The structural changes are manifested in the catalytic properties as well, with the result that the transformed doped cages exhibit extremely low activation barriers for the environmentally important CO oxidation reaction as compared to reported inert Au20 cluster. The activation barriers for CO oxidation are particularly low (<0.12 eV) when germaniu...

Effect of Hydrogen Atom Doping on the Structure and Electronic Properties of 20-Atom Gold Cluster

We test the validity of gold–hydrogen analogy in a hydrogen-atom-doped larger gold cluster, namely, Au20, which has attracted considerable interest in recent years because of its unique nature. For this purpose, we carry out density functional theory based calculations to determine the structures of various possible isomers of Au19H cluster by employing GGA and meta-GGA functionals. To obtain the optimized structures of Au19H cluster, several possible initial geometries have been explored. We find that the structure of Au19H cluster is very close to that of tetrahedral Au20 cluster, and the dopant H atom prefers to sit on one of the vertices of the tetrahedron. On the other hand, for the cases of Li, coinage metal (Cu and Ag), and Pt atom doping, the dopant atom has been shown to preferably sit on the surface site of the tetrahedral Au20 cluster. The structure and HOMO–LUMO gap of the Au19H cluster are found to be very close to that of the pure Au20 cluster. Moreover, we observe that the adsorption energi...

Binding of Trivalent Arsenic onto the Tetrahedral Au20 and Au19Pt Clusters: Implications in Adsorption and Sensing.

The interaction of arsenic(III) onto the tetrahedral Au20 cluster was studied computationally to get insights into the interaction of arsenic traces (presented in polluted waters) onto embedded electrodes with gold nanostructures. Pollutant interactions onto the vertex, edge, or inner gold atoms of Au20 were observed to have a covalent character by forming metal-arsenic or metal-oxygen bonding, with adsorption energies ranging from 0.5 to 0.8 eV, even with a stable physisorption; however, in aqueous media, the Au-vertex-pollutant interaction was found to be disadvantageous. The substituent effect of a platinum atom onto the Au20 cluster was evaluated to get insights into the changes in the adsorption and electronic properties of the adsorbent-adsorbate systems due to chemical doping. It was found that the dopant atom increases both the metal-pollutant adsorption energy and stability onto the support in a water media for all interaction modes; adsorption energies were found to be in a range of 0.6 to 1.8 eV. All interactions were determined to be accompanied by electron transfer as well as changes in the local reactivity that determine the amount of transferred charge and a decrease in the HOMO-LUMO energy gap with respect to the isolated substrate.

Formation of H2O2 on Au20 and Au19Pd Clusters: Understanding the Structure Effect on the Atomic Level

Supported gold nanoparticles are promising catalysts for production of H2O2 from O2 and H2. Size, structure, and palladium doping effects play the key role in activity and selectivity of a gold catalyst. We performed a study of the influence of Au20 and Au19Pd structure features on the main steps of H2O2 formation on the atomic level, using the DFT/PBE approach with relativistic all electron basis set. The top, edge, and facet atoms of the tetrahedral Au20 cluster as well as a palladium atom of Au19Pd located on the top, edge, and facet of a tetrahedron have been considered as active sites of steps involved in H2O2 synthesis. The thermodynamic and kinetic data including Gibbs free energies and the activation Gibbs free energies were calculated for the steps determining the formation of H2O2 (H(s) + OOH(s) = H2O(2(s)), H2O(2(s)) = H2O(2(g))) and for one step decreasing the selectivity (H2O(2(s)) = OH(s) + OH(s)). Gold tends to have low activity and high selectivity in H2O2 synthesis regardless of the structure of active site. Low coordinated palladium atoms promote H2O2 formation as well as its dissociation. Pd on a facet of a cluster facilitates H2O2 production with high activity and selectivity.

Direct atomic imaging and dynamical fluctuations of the tetrahedral Au20 cluster

We report real-space, atomic-resolution images of Au(20) clusters obtained with the aberration-corrected Scanning Transmission Electron Microscopy. The proposed tetrahedral (FCC segment) pyramid structure is confirmed. The clusters cycle between isomers under the electron beam in the time-lapse images acquired. Disordered variants on the high symmetry forms are commonly observed. We believe that the direct experimental identification of these kinds of atomic structure, and the fluctuations between them, is fundamental to our understanding of nanoparticle structures, as well as applications such as heterogeneous catalysis.

Structures of Neutral Au 7 , Au 19 , and Au 20 Clusters in the Gas Phase

The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Facile Syntheses of Monodisperse Ultrasmall Au Clusters

During our effort to synthesize the tetrahedral Au20 cluster, we found a facile synthetic route to prepare monodisperse suspensions of ultrasmall Au clusters AuN (N < 12) using diphosphine ligands. In our monophasic and single-pot synthesis, a Au precursor ClAu(I)PPh3 (Ph = phenyl) and a bidentate phosphine ligand P(Ph)2(CH2)(M)P(Ph)2 are dissolved in an organic solvent. Au(I) is reduced slowly by a borane-tert-butylamine complex to form Au clusters coordinated by the diphosphine ligand. The Au clusters are characterized by both high-resolution mass spectrometry and UV-vis absorption spectroscopy. We found that the mean cluster size obtained depends on the chain length M of the ligand. In particular, a single monodispersed Au11 cluster is obtained with the P(Ph)2(CH2)3P(Ph)2 ligand, whereas P(Ph)2(CH2)(M)P(Ph)2 ligands with M = 5 and 6 yield Au10 and Au8 clusters. The simplicity of our synthetic method makes it suitable for large-scale production of nearly monodisperse ultrasmall Au clusters. It is suggested that diphosphines provide a set of flexible ligands to allow size-controlled synthesis of Au nanoparticles.

Toward the Solution Synthesis of the Tetrahedral Au20 Cluster

We report the observation in solution of the recently discovered tetrahedral Au20 cluster coordinated with eight PPh3 (Ph = phenyl) ligands. The composition and molecular weight of the Au20(PPh3)8 complex were confirmed by the isotopic pattern and accurate mass measurement of its doubly charged cation using high-resolution mass spectrometry. A collision-induced dissociation experiment showed that four PPh3 ligands can be easily removed from Au20(PPh3)82+, resulting in a highly stable Au20(PPh3)42+ ion. This observation is consistent with the tetrahedral Au20, in which the four apex sites are expected to bond to the PPh3 ligands strongly, and is confirmed by theoretical calculations that predict a highly stable Au20(PH3)4 complex with Au−PH3 bond energies of ∼1 eV. The current experimental and theoretical observations suggest that large quantities of ligand-stabilized tetrahedral Au20 can be obtained, opening the door for exploring its anticipated novel chemical, optical, and catalytic properties.