Ligand Effects on the Structure and the Electronic Optical Properties of Anionic Au₂₅(SR)₁₈ Clusters

This study addresses how ligands module the structure and the electronic optical properties of a large set of the experimentally known anionic thiolate-protected gold clusters, Au₂₅(SR)₁₈^[1−]. Starting from the experimental crystal structure, computational density functional theory calculations reveal that low-polarity R groups do not disturb the Au₂₅S₁₈ framework significantly, such that the inversion symmetry­(<i>C</i>_<i>i</i>) of the crystalline state is retained. In the case of <i>p</i>-thiolphenolate ligands, <i>p</i>-SPhX, a major distortion of the Au₂₅S₁₈ framework, destroys the inversion symmetry, the distortion increasing in the order given X = H, Cl, NO₂ and CO₂H. For branched R groups, linking −CH₃ or −NH₂ groups at the two-position of the phenylethylthiolate ligand, the inversion symmetry is retained and lost, respectively; similarly, the <i>N</i>-acetyl-cysteine ligand also distorts the framework. These results demonstrate a systematic preference of inversion-symmetric versus nonsymmetric framework depending on the ligand-type. The more distorted structures also exhibit significantly reduced HOMO–LUMO gap values and affect the optical absorption spectra accordingly. This study correlates the distortion of the Au₂₅S₁₈ framework with the structure, electronic, and optical properties among the studied clusters

Fully Cationized Gold Clusters: Synthesis of Au₂₅(SR⁺)₁₈

Although many thiolate-protected Au clusters with different numbers of Au atoms and a variety of thiolate ligands have been synthesized, to date there has been no report of a fully cationized Au cluster protected with cationic thiolates. Herein, we report the synthesis of the first member of a new series of thiolate-protected Au cluster molecules: a fully cationized Au₂₅(SR⁺)₁₈ cluster

Ultraviolet Photodissociation of Selected Gold Clusters: Ultraefficient Unstapling and Ligand Stripping of Au₂₅(pMBA)₁₈ and Au₃₆(pMBA)₂₄

We report the first results of ultraviolet photodissociation (UVPD) mass spectrometry of trapped monolayer-protected cluster (MPC) ions generated by electrospray ionization. Gold clusters Au₂₅(pMBA)₁₈ and Au₃₆(pMBA)₂₄ (pMBA = para-mercaptobenzoic acid) were analyzed in both the positive and negative modes. Whereas activation methods including collisional- and electron-based methods produced relatively few fragment ions, even a single ultraviolet pulse (at λ = 193 nm) caused extensive fragmentation of the positively charged clusters. Upon photoactivation using a low number of laser pulses, the staple motifs of both clusters were cleaved and stripped of the protecting ligand portions without removal of any contained gold atoms. This striking process involved Au–S and C–S bond cleavages via a pathway made possible by 6.4 eV photon absorption. Monomer evaporation (neutral gold atom loss) occurred upon exposure to multiple pulses, resulting in a size series of bare gold-cluster ions. All tandem mass spectrometric methods produced the singly charged ring tetramer ion, [Au₄(pMBA)₄ + Na]⁺, for each cluster

Gold Nanocluster Prospecting via Capillary Liquid Chromatography-Mass Spectrometry: Discovery of Three Quantized Gold Clusters in a Product Mixture of “2 nm Gold Nanoparticles”

A nonaqueous reversed phase liquid chromatography-mass spectrometry (LC-MS) method has been developed for extremely hydrophobic MPCs (monolayer-protected clusters), and has been applied to the efficient separation of gold–dodecanethiolate (ddt) assemblies, leading to the identification of three dodecanethiolate-protected gold clusters, Au₁₃₀(ddt)₅₀, Au₁₃₇(ddt)₅₆, and Au₁₄₄(ddt)₆₀, as prominent components of a commercial product of nominally 2 nm (core-diameter) protected gold nanoparticles obtained from nanoComposix, Inc. Various components were separated, according to hydrophobic character, using a linear gradient of methanol–dichloromethane mobile phases, on a C₁₈ HPLC column. Varying concentrations of mobile-phase modifier (triethylammonium acetate) were compared for effect on chromatographic peak shape and cluster retention. Positive electrospray ionization (ESI) was used to ionize all components in the sample. LC separation prior to inline + ESI-MS detection facilitated sample analysis via production of simplified mass spectra for each eluting cluster species and provided insight into the relative polarity of the clusters shown here. UV–vis detection facilitated method development and allowed determination of nonionizing, and/or polydisperse components

Collision-Induced Dissociation of Monolayer Protected Clusters Au₁₄₄ and Au₁₃₀ in an Electrospray Time-of-Flight Mass Spectrometer

Gas-phase reactions of larger gold clusters are poorly known because generation of the intact parent species for mass spectrometric analysis remains quite challenging. Herein we report in-source collision-induced dissociation (CID) results for the monolayer protected clusters (MPCs) Au₁₄₄(SR)₆₀ and Au₁₃₀(SR)₅₀, where R- = PhCH₂CH₂-, in a Bruker micrOTOF time-of-flight mass spectrometer. A sample mixture of the two clusters was introduced into the mass spectrometer by positive mode electrospray ionization. Standard source conditions were used to acquire a reference mass spectrum, exhibiting negligible fragmentation, and then the capillary-skimmer potential difference was increased to induce in-source CID within this low-pressure region (∼4 mbar). Remarkably, distinctive fragmentation patterns are observed for each MPC­[3+] parent ion. An assignment of all the major dissociation products (ions and neutrals) is deduced and interpreted by using the distinguishing characteristics in the standard structure-models for the respective MPCs. Also, we propose a ring-forming elimination mechanism to explain R-H neutral loss, as separate from the channels leading to RS-SR or (AuSR)₄ neutrals

Au₆₇(SR)₃₅ Nanomolecules: Characteristic Size-Specific Optical, Electrochemical, Structural Properties and First-Principles Theoretical Analysis

The preparation of gold nanomolecules with sizes other than Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au₁₀₂(SR)₄₄, and Au₁₄₄(SR)₆₀ has been hampered by stability issues and low yields. Here we report a procedure to prepare Au₆₇(SR)₃₅, for either R = −SCH₂CH₂Ph or -SC₆H₁₃, allowing high-yield isolation (34%, ∼10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI–TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au₆₇(SR)₃₅ composition. Electronic properties were explored using optical absorption spectroscopy (UV–visible–NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy (¹³C and ¹H), and structural characteristics of the metal atom core were determined by powder X-ray measurements. Models featuring a Au₁₇ truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staple-motif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (∼0.75 eV) in the (2−) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au₆₇(SR)₃₅ nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO–LUMO gap (∼0.75 eV) is nearly double that of the larger Au₁₀₂ compound and it is much smaller than that of the Au₃₈ one. The intermediary status of the Au₆₇(SR)₃₅ nanomolecule is also reflected in both its optical and electrochemical characteristics

Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation

Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations

Liquid Chromatography Separation and Mass Spectrometry Detection of Silver-Lipoate Ag₂₉(LA)₁₂ Nanoclusters: Evidence of Isomerism in the Solution Phase

Evidence for the existence of condensed-phase isomers of silver-lipoate clusters, Ag₂₉(LA)₁₂, where LA = (<i>R</i>)-α lipoic acid, was obtained by reversed-phase ion-pair liquid chromatography with in-line UV–vis and electrospray ionization (ESI)-MS detection. All components of a raw mixture were separated according to surface chemistry and increasing size via reversed-phase gradient HPLC methods and identified by their corresponding <i>m</i>/<i>z</i> ratio by ESI in the negative ionization mode. Aqueous and methanol mobile-phase mixtures, each containing 400 mM hexafluoroisopropanol (HFIP)–15 mM triethylamine (TEA), were employed to facilitate the interaction between the clusters and stationary phase via formation of ion-pairs. TEA-HFIP (triethylammonium-<i>hexafluoroisopropoxide</i>) had been shown to provide superior chromatographic peak shape and mass spectral signal compared with alternative modifiers such as TEAA (triethylammonium-<i>acetate</i>) for analysis of oligonucleotide samples. Liquid chromatographic separation prior to mass spectrometry detection facilitated sample analysis by production of simplified mass spectra for each eluting cluster species and provided insight into the existence of at least two major solution-phase isomers of Ag₂₉(LA)₁₂. UV–vis detection in-line with ESI analysis provided independent confirmation of the existence of the isomers and their similar electronic structure as judged from their identical optical spectra in the 300–500 nm range. [Diastereomerism provides a possible interpretation for the near-equal abundance of the two forms, based on a structurally defined nonaqueous homologue.

Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation

Gold–copper (Au–Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core–shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations

Structure determination of superatom metallic clusters using rapid scanning electron diffraction

Experimental analysis of electron diffraction patterns for ligand-stabilized gold clusters in transmission electron microscopy is a cumbersome procedure, due to electron beam–induced irradiation damage. We propose herein a method for instantaneous data collection using scanning nanobeam electron diffraction and the subsequent determination of the crystal metallic clusters. The procedure has been tested on a known structure, namely Au102(p-MBA)44 nanoclusters and has been compared with their structure theoretically determined by ones previously obtained from X-ray diffraction analysis. The method can be unambiguously applied for the case of any nanoscale system susceptible to electron beam damage and it is capable to register the rotation effect on the metallic clusters caused due to the electron beam interaction during the raster scanning on the sample.Fil: Bruma, Alina. The University of Texas at San Antonio; Estados UnidosFil: Santiago, Ulises. The University of Texas at San Antonio; Estados UnidosFil: Alducin, Diego. The University of Texas at San Antonio; Estados UnidosFil: Plascencia Villa, German. The University of Texas at San Antonio; Estados UnidosFil: Whetten, Robert L.. The University of Texas at San Antonio; Estados UnidosFil: Mariscal, Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; ArgentinaFil: José Yacamán, Miguel. The University of Texas at San Antonio; Estados Unido