Ion mobility mass spectrometry of Au25(SCH2CH2Ph)18 nanoclusters

Bare clusters derived from protein templates: Au25(+), Au38(+) and Au102(+)

A discrete sequence of bare gold clusters of well-defined nuclearity, namely Au25(+), Au38(+) and Au102(+), formed in a process that starts from gold-bound adducts of the protein lysozyme, were detected in the gas phase. It is proposed that subsequent to laser desorption ionization, gold clusters form in the gas phase, with the protein serving as a confining growth environment that provides an effective reservoir for dissipation of the cluster aggregation and stabilization energy. First-principles calculations reveal that the growing gold clusters can be electronically stabilized in the protein environment, achieving electronic closed-shell structures as a result of bonding interactions with the protein. Calculations for a cluster with 38 gold atoms reveal that gold interaction with the protein results in breaking of the disulfide bonds of the cystine units, and that the binding of the cysteine residues to the cluster depletes the number of delocalized electrons in the cluster, resulting in opening of a super-atom electronic gap. This shell-closure stabilization mechanism confers enhanced stability to the gold clusters. Once formed as stable magic number aggregates in the protein growth medium, the gold clusters become detached from the protein template and are observed as bare Au(n)(+) (n=25, 38, and 102) clusters.

Temporal stability of magic-number metal clusters: beyond the shell closing model

The anomalous stability of magic-number metal clusters has been associated with closed geometric and electronic shells and the opening of HOMO-LUMO gaps. Despite this enhanced stability, magic-number clusters are known to decay and react in the condensed phase to form other products. Improving our understanding of their decay mechanisms and developing strategies to control or eliminate cluster instability is a priority, to develop a more complete theory of their stability, to avoid studying mixtures of clusters produced by the decay of purified materials, and to enable technology development. Silver clusters are sufficiently reactive to facilitate the study of the ambient temporal stability of magic-number metal clusters and to begin to understand their decay mechanisms. Here, the solution phase stability of a series of silver:glutathione (Ag:SG) clusters was studied as a function of size, pH and chemical environment. Cluster stability was found to be a non-monotonic function of size. Electrophoretic separations showed that the dominant mechanism involved the redistribution of mass toward smaller sizes, where the products were almost exclusively previously known cluster sizes. Optical absorption spectra showed that the smaller clusters evolved toward the two most stable cluster sizes. The net surface charge was found to play an important role in cluster stabilization although charge screening had no effect on stability, contrary to DLVO theory. The decay mechanism was found to involve the loss of Ag(+) ions and silver glutathionates. Clusters could be stabilized by the addition of Ag(+) ions and destabilized by either the addition of glutathione or the removal of Ag(+) ions. Clusters were also found to be most stable in near neutral pH, where they had a net negative surface charge. These results provide new mechanistic insights into the control of post-synthesis stability and chemical decay of magic-number metal clusters, which could be used to develop design principles for synthesizing specific cluster species.

Traveling wave ion mobility mass spectrometry study of low generation polyamidoamine dendrimers

We reported the use of ion mobility (IM) combined with mass spectrometry (MS) as an analytical tool to investigate low generation polyamidoanine (PAMAM) dendrimers. This analytical approach has been employed to separate ions of defective structures with different charge state but exactly the same m/z value. Tandem mass spectrometry (MS/MS) after IM separation allowed a comprehensive structural characterization of defective dendrimers. In addition, IM was used to evaluate the collision cross-sections of ions of perfect dendrimers. They showed a good correlation with calculated collision cross-sections obtained by the trajectory method (TM) and were also consistent with dimensions reported by other established analytical methods.

Nanocluster isotope distributions measured by electrospray time-of-flight mass spectrometry

Electrospray ionization (ESI) mass spectrometry (MS) is a widely used tool for the characterization of organometallic nanoclusters. By matching experimental mass spectra with calculated isotope distributions it is possible to determine the elemental composition of these analytes. In this work we conduct ESI-MS investigations on M(14)E(13)Cl(2)(tmeda)(6) nanoclusters, where M is a transition metal, E represents a chalcogen, and tmeda is N,N,N',N'-tetramethyl-ethylenediamine. ESI mass spectra of these systems agree poorly with theoretical isotope distributions when data are acquired under standard conditions. This behavior is attributed to dead-time artifacts of the time-of-flight (TOF) analyzer used. It is well-known that excessively high TOF ion count rates lead to dead-time issues. Surprisingly, our data reveal that nanocluster spectra are affected by this problem even at moderate signal intensities that do not cause any problems for other types of analytes. This unexpected vulnerability is attributed to the extremely wide isotope distributions of the nanoclusters studied here. A good match between experimental and calculated nanocluster spectra is obtained only at ion count rates that are more than 1 order of magnitude below commonly used levels. Discrepancies between measured and theoretical isotope distributions have been observed in a number of previous ESI-MS nanocluster investigations. The dead-time issue identified here likely represents a contributing factor to the spectral distortions that were observed in those earlier studies. Using low-intensity ESI-MS conditions we demonstrate the feasibility of analyzing highly heterogeneous nanocluster samples that comprise subpopulations with a wide range of metal compositions.

Observation of cluster size growth in CO-directed synthesis of Au25(SR)18 nanoclusters

The design of an efficient synthesis for large-scale production of atomically precise nanoclusters (NCs) is pivotal in realizing the size-dependent properties of the NCs. A simple and versatile method for producing atomically precise thiolated gold NCs (Au(25)(SR)(18) NCs) in large quantities (~200 mg) is demonstrated in this study. It uses a gaseous reducing agent, carbon monoxide (CO), to support a slow and size-controlled growth of Au(25)(SR)(18) NCs. Absorption measurements of the reaction solution, which underwent distinct color changes (colorless → yellow → orange → brown → red-brown), allowed the formation of thiolated Au(25) NCs to be reconstructed from several key intermediates. The unique reaction environment provided by gaseous CO presents a new synthetic route to fabricate atomically precise metal NCs in quantities large enough for application explorations.

High-performance liquid chromatographic analysis of as-synthesised N,N'-dimethylformamide-stabilised gold nanoclusters product

Reverse-phase high-performance liquid chromatographic (RP-HPLC) separation and analysis of polydisperse water-soluble gold nanoclusters (AuNCs) stabilised with N,N'-dimethylformamide (DMF) were investigated. Under optimal elution gradient conditions, the separation of DMF-AuNCs was monitored by absorption and fluorescence spectroscopy. The UV-vis spectral characteristics of the separated DMF-AuNCs have been captured and they do not possess distinct surface plasmon resonance bands, indicating that all DMF-AuNCs are small AuNCs. The photoluminescence emission spectra of the separated DMF-AuNCs are in the blue-light region. Moreover, cationic DMF-AuNCs are for the first time identified by ion chromatography. Our proposed RP-HPLC methodology has been successfully applied to separate AuNCs of various Au atoms as well as DMF-stabilised ligands. Finally, the composition of the separated DMF-AuNCs was confirmed by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry and electrospray ionisation mass spectrometry, proving that the as-synthesised DMF-AuNCs product consists of Au₁₀⁺, Au₁₀, Au₁₁, Au₁₂, Au₁₃, and Au₁₄ NCs stabilised with various numbers of DMF ligands.

The golden pathway to thiolate-stabilized nanoparticles: following the formation of gold(I) thiolate from gold(III) chloride

Pathways for the formation of gold thiolate complexes from gold(III) chloride precursors AuCl(4)(-) and AuCl(3) are examined. This work demonstrates that two distinct reaction pathways are possible; which pathway is accessible in a given reaction may depend on factors such as the residue group R on the incoming thiol. Density functional theory calculations using the BP86 functional and a polarized triple-ζ basis set show that the pathway resulting in gold(III) reduction is favored for R = methyl. A two-to-one ratio of thiol or thiolate to gold can reduce Au(III) to Au(I), and a three-to-one ratio can lead to polymeric Au(SR) species, which was first suggested by Schaaff et al. J. Phys. Chem. B, 1997, 101, 7885 and later confirmed by Goulet and Lennox J. Am. Chem. Soc., 2010, 132, 9582. Most transition states in the pathways examined here have reasonable barrier heights around 0.3 eV; we find two barrier heights that differ substantially from this which suggest the potential for kinetic control in the first step of thiolate-protected gold nanoparticle growth.

Ag44(SR)30(4-): a silver-thiolate superatom complex

Intensely and broadly absorbing nanoparticles (IBANs) of silver protected by arylthiolates were recently synthesized and showed unique optical properties, yet question of their dispersity and their molecular formulas remained. Here IBANs are identified as a superatom complex with a molecular formula of Ag(44)(SR)(30)(4-) and an electron count of 18. This molecular character is shared by IBANs protected by 4-fluorothiophenol or 2-naphthalenethiol. The molecular formula and purity is determined by mass spectrometry and confirmed by sedimentation velocity-analytical ultracentrifugation. The data also give preliminary indications of a unique structure and environment for Ag(44)(SR)(30)(4-).

Investigating the structural evolution of thiolate protected gold clusters from first-principles

Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) and Au(102)(p-MBA)(44), from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general "inherent structure rule" underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate "staple motifs", as illustrated by a general structural formula [Au](a+a')[Au(SR)(2)](b)[Au(2)(SR)(3)](c)[Au(3)(SR)(4)](d)[Au(4)(SR)(5)](e) where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the "divide-and-protect" concept in general and the "inherent structure rule" in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of Au(25)(SR)(8)(-) and Au(38)(SR)(24) (-R is the alkylthiolate group) have been fully confirmed by later experiments, lending credence to the "inherent structure rule".

Separation and characterization of metallosupramolecular libraries by ion mobility mass spectrometry

The self-assembly of Zn(II) ions and bis(terpyridine) (tpy) ligands carrying 120° or 180° angles between their metal binding sites was utilized to prepare metallosupramolecular libraries with the <tpy-Zn(II)-tpy> connectivity. These combinatorial libraries were separated and characterized by ion mobility mass spectrometry (IM MS) and tandem mass spectrometry (MS(2)). The 180°-angle building blocks generate exclusively linear complexes, which were used as standards to determine the architectures of the assemblies resulting from the 120°-angle ligands. The latter ligand geometry promotes the formation of macrocyclic hexamers, but other n-mers with smaller (n = 5) or larger ring sizes (n = 7-9) were identified as minor products, indicating that the angles in the bis(terpyridine) ligand and within the coordinative tpy-Zn(II)-tpy bonds are not as rigid, as previously believed. Macrocyclic and linear isomers were detected in penta- and heptameric assemblies; in the larger octa- and nonameric assemblies, ring-opened conformers with compact and folded geometries were observed in addition to linear extended and cyclic architectures. IM MS(2) experiments provided strong evidence that the macrocycles present in the libraries were already formed in solution, during the self-assembly process, not by dissociation of larger complexes in the gas phase. The IM MS/MS(2) methods provide a means to analyze, based on size and shape (architecture), supramolecular libraries that are not amenable to liquid chromatography, LC-MS, NMR, and/or X-ray techniques.

Hollow polyhedral structures in small gold-sulfide clusters

Using ab initio methods, we investigate the structural evolution of a family of gold-sulfide cluster anions (Au(m)S(n)(-)). We show that this family of clusters exhibits simple size-evolution rules and novel hollow polyhedron structures. The highly stable Au(m)S(n)(-) species such as Au(6)S(4)(-), Au(9)S(5)(-), Au(9)S(6)(-), Au(10)S(6)(-), Au(11)S(6)(-), Au(12)S(8)(-), and Au(13)S(8)(-) detected in the recent ion mobility mass spectrometry experiment of Au(25)(SCH(2)CH(2)Ph)(18) (Angel et al. ACS Nano2010, 4, 4691) are found to possess either quasi-tetrahedron, pyramidal, quasi-triangular prism, or quasi-cuboctahedron structures. The formation of these polyhedron structures are attributed to the high stability of the S-Au-S structural unit. A unique "edge-to-face" growth mechanism is proposed to understand the structural evolution of the small Au(m)S(n)(-) cluster. A 3:2 ratio rule of Au/S is suggested for the formation of a hollow polyhedron structure among small-sized Au(m)S(n) clusters.