Predictive gold nanocluster formation controlled by metal-ligand complexes

Exploration of the Atomic Pathway of Seed-Mediated Growth from Icosahedral [Au25(SR)18]- to Bi-Icosahedral Au38(SR)24 and Au44(SR)26 Clusters Based on the 2e- Hopping Mechanism

Size growth is ubiquitous in the gold nanocluster synthesis. However, the atomic-level mechanism of seed-mediated growth of gold clusters remains mysterious. In this study, the seed-mediated growth pathway from the icosahedral [Au₂₅(SR)₁₈]^- cluster to the bi-icosahedral Au₃₈(SR)₂₄ and Au₄₄(SR)₂₆ clusters is studied based on the two-electron (2e^-) hopping mechanism. First, atomic structures of three key intermediate clusters, [Au₂₉(SR)₂₀]^-, [Au₃₃(SR)₂₂]^-, and Au₄₁(SR)₂₅, are predicted based on the 2e^--unit decomposition strategy. The theoretically simulated UV-Vis spectra based on the predicted structure model of [Au₂₉(SR)₂₀]^- and [Au₃₃(SR)₂₂]^- matched well with the experimental curves reported previously. Based on the predicted intermediate cluster structures, the size growth pathway from the eight-electron (8e^-) [Au₂₅(SR)₁₈]^- cluster to 14-electron (14e^-) Au₃₈(SR)₂₄ and 18-electron (18e^-) Au₄₄(SR)₂₆ clusters is determined. In the step of formation of bi-icosahedral Au₃₈(SR)₂₄ from icosahedral [Au₂₅(SR)₁₈]^-, two Au₄ units are first formed. The third 2e^- hopping step results in formation of an icosahedron unit. The present studies offered new insights into the formation and size conversion mechanism of ligand-protected gold nanoclusters containing icosahedral cores.

Synthesis of Highly Accessible and Reactive Sites in Gold Nanoparticles Using Bound Bis(Diphenylphosphine) Ligands

While bound organic ligands provide steric protection against aggregation for metallic nanoparticles in solution, they can block a large fraction of the surface atoms which are needed for binding in catalysis and sensing applications. In this work, highly accessible Au nanoparticles ligated with bis(diphenylphosphine) molecules are synthesized and characterized in solution. Characterization is performed using high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), ultraviolet-visible (UV-Vis) spectroscopy, and fluorescence chemisorption experiments. These synthesized nanoparticles are accessible to a 2-napthalenethiol (2-NT) probe molecule in solution. The highest 2-NT accessibility is observed when using 1,1-bis(diphenylphosphino)methane (dppm) ligand where 61 % of the total gold atoms are accessible. It is hypothesized that increasing the rigidity of the bis(diphenylphosphine) ligand increases the number of binding sites on the Au nanoparticles. These nanoparticles are catalytically active for resazurin reduction, and the resazurin reduction rate scales with the number of binding sites.

Synthesis and Stability of Mixed-Diphosphine Ligated Gold Clusters

Sub-nanometer gold clusters are promising size- and composition-tunable materials that may be used for advanced technological applications such as catalysis, energy generation, and microelectronics. Synthesis and characterization of phosphine ligated gold clusters containing different ligands provide insight into how steric and electronic effects resulting from changes in chemical functionality influence cluster size, stability, and formation in solution. Herein, we demonstrate that synthesizing gold clusters using two different diphosphines in solution at the same time results in a broad distribution of novel mixed-ligand clusters. In comparison, adding a second diphosphine to a solution of gold clusters presynthesized with another diphosphine does not result in extensive formation of mixed-ligand species. Utilizing high-mass resolution electrospray ionization mass spectrometry, we determined novel cluster compositions and observed size-dependent trends in gold clusters that undergo ligand exchange forming mixed diphosphine species. Adjacent peaks in the mass spectra, separated by characteristic mass-to-charge ratios, provide evidence for multiple 1,3-bis(diphenylphosphino)propane (L³) and 1,5-bis(diphenylphosphino)pentane (L⁵) ligands on cationic clusters containing 8, 10, 11, and 22 gold atoms. Energy-resolved collision-induced dissociation experiments provide qualitative insight into how different diphosphine ligands affect the relative stability of specific size gold clusters. Our results indicate that mixed-ligand clusters containing both L³ and L⁵ are generally more stable than their single ligand counterparts containing either L³ or L⁵. These molecular-level insights will facilitate the rational and scalable synthesis of gold clusters for targeted applications.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Understanding the Chemical Insights of Staple Motifs of Thiolate-Protected Gold Nanoclusters

Improving the fundamental understanding of the basic structures of ligand-protected gold nanoclusters is essential to their bottom-up synthesis as well as their further application explorations. The thiolate ligands that cover the central metal core in staple motifs are vital for the stability of the gold clusters. However, the knowledge about the geometrical and bonding characters of the thiolate ligands has not been fully uncovered yet. In this work, density functional theory calculations and molecular orbital analysis are applied to show that the Au atoms in the thiolate ligands are hypervalent. The chemical insights of the linear SAuS configuration as well as the lengthened AuS bond by combining the 3-center 4-electron (3c-4e) model and the well-recognized valence shell electron pair repulsion theory are revealed. Valence bond formulations of the motifs are given to provide more chemical insights, for example, the resonant structures, to show how the thiolate motif forms one covalent bond and one dative covalent bond with the Au core. This work provides a thorough understanding of the structure and bonding pattern of thiolate ligands of Au nanoclusters, which is important for the rational design of ligands-protected Au nanoclusters.

ESI-MS Identification of the Cationic Phosphine-Ligated Gold Clusters Au1-22: Insight into the Gold-Ligand Ratio and Abundance of Larger Clusters

Triphenylphosphine (PPh₃)-ligated gold nanoclusters are valuable for a number of potential applications due to their relative ease of synthesis and usefulness in forming advanced cluster architectures. While previous studies have reported cationic PPh₃-ligated gold clusters with core sizes of Au_1-4, Au_6-11, and Au_13-14, there has not been definitive identification by mass spectrometry of many larger clusters in the Au_12-25 range. Herein, we survey a polydisperse solution of cationic PPh₃-ligated gold clusters using high-mass-resolution (M/ΔM = 60,000) electrospray ionization mass spectrometry (ESI-MS). To improve the sensitivity and mass resolution of larger clusters for unambiguous identification, we increased the number of scan averages and reduced the range of mass collection windows to 200 m/z, thereby mitigating potential mass and ion abundance bias resulting from smaller "building block" gold clusters that are present in much higher abundance in solution. In addition to the previously reported clusters, we identify several new species including Au₅(PPh₃)₅⁺, Au₁₂(PPh₃)₉HCl²⁺, Au₁₅(PPh₃)₉Cl²⁺, Au₁₆(PPh₃)₁₀Cl₂²⁺, Au₁₇(PPh₃)₁₁³⁺, Au₁₈(PPh₃)₁₀²⁺, Au₁₉(PPh₃)₁₀Cl²⁺, Au₂₀(PPh₃)₁₂H₃³⁺, Au₂₁(PPh₃)₁₀Cl²⁺, and Au₂₂(PPh₃)₁₀Cl₂²⁺, indicating that a full range of clusters between Au_1-22 may be observed in a single polydisperse solution. Considering all of the clusters observed, our findings provide evidence that the Au_12-14 size range is a critical transition point in cluster nucleation. While smaller clusters exhibit a 1:1 gold-to-ligand ratio, larger clusters (beginning Au_12-14) feature additional gold atoms without an equal number of accompanying ligands. Our results support previous evidence in the literature indicating that the "magic number" icosahedral Au₁₃ geometry is the smallest cluster size where a ligand-less central gold atom is coordinated by a complete shell of 12 surrounding ligated gold atoms, thereby creating a stable "one-shell" cluster. Furthermore, our findings reinforce growing evidence that ligands may be used to actively direct gold cluster size and abundance during synthesis. While for PPh₃-ligated systems the most abundant species are Au_6-9 clusters, we find that for related methyldiphenylphosphine (PPh₂Me) and dimethylphenylphosphine (PPhMe₂)-ligated systems the most abundant cluster sizes are Au_10-11 and Au_12-14, respectively. Together, we demonstrate that reducing the range of m/z collection windows and increasing the number of scan averages dramatically improves instrument sensitivity for cationic gold clusters, enabling thorough characterization of polydisperse solutions that is not possible using conventional techniques.

Using electrospray ionization-tandem mass spectrometry to explore formation and gas-phase chemistry of silver nanoclusters generated from the reaction of silver salts with NaBH4 in the presence of bis(diphenylarsino)methane

Electrospray ionization-mass spectrometry (ESI-MS) of mixtures of AgBF4 or AgNO3 with the capping ligand bis(diphenylarsino)methane ((Ph2 As)2 CH2 = dpam) in a solution of acetonitrile revealed the formation of the following cations: [Ag(CH3 CN)(dpam)]+ , [Ag(dpam)2 ]+ , [Ag2 (Cl)(dpam)2 ]+ , and [Ag3 (Cl)2 (dpam)3 ]+ . Addition of NaBH4 to these solutions results in the formation of the cluster cations [Ag2 (BH4 )(dpam)2 ]+ , [Ag2 (BH4 )(dpam)3 ]+ , [Ag3 (H)(BH4 )(dpam)3 ]+ , [Ag3 (BH4 )2 (dpam)3 ]+ , [Ag3 (H)(Cl)(dpam)3 ]+ , and [Ag3 (I)(BH4 )(dpam)3 ]+ , as established by ESI-MS. Use of NaBD4 confirmed that borohydride is the source of the hydride in these clusters. An Orbitrap Fusion LUMOS mass spectrometer was used to explore the gas-phase unimolecular chemistry of selected clusters via multistage mass spectrometry (MSn ) experiments employing low-energy collision-induced dissociation (CID) and high-energy collision-induced dissociation (HCD) experiments. The borohydride containing clusters fragment via two competing pathways: (i) ligand loss and (ii) B-H bond activation involving BH3 loss. Density functional theory (DFT) calculations were used to calculate the energetics of the optimized structures for all precursor ions, fragment ions, and neutrals and to estimate the reaction endothermicities. Generally, there is reasonable agreement between the most abundant product ion formed and the predicted endothermicity of the associated reaction channel. The DFT calculations predicted that the novel dimer [Ag2 (BH4 )(dpam)2 ]+ has a paddlewheel structure in which the dpam and BH4 - ligands bridge both silver centers.

Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh₃) ligands on Au₈L₇²⁺ (L = PPh₃) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters.

Synthesis, structure, and condensed-phase reactivity of [Ag3(μ3-H)(μ3-BH4)LPh3](BF4) (LPh = bis(diphenylphosphino)amine) with CS2

Electrospray ionisation mass spectrometry (ESI-MS) was used to monitor the reaction of AgBF₄, bis(diphenylphosphino)amine (dppa = (Ph₂P)₂NH = L^Ph) and NaBH₄ in acetonitrile.

Observing the real time formation of phosphine-ligated gold clusters by electrospray ionization mass spectrometry

Real-time monitoring of the gold cluster synthesis by electrospray ionization mass spectrometry reveals distinct formation pathways for Au₈, Au₉ and Au₁₀ clusters.

Understanding ligand effects in gold clusters using mass spectrometry

This review summarizes recent research on the influence of phosphine ligands on the size, stability, and reactivity of gold clusters synthesized in solution. Sub-nanometer clusters exhibit size- and composition-dependent properties that are unique from those of larger nanoparticles. The highly tunable properties of clusters and their high surface-to-volume ratio make them promising candidates for a variety of technological applications. However, because "each-atom-counts" toward defining cluster properties it is critically important to develop robust synthesis methods to efficiently prepare clusters of predetermined size. For decades phosphines have been known to direct the size-selected synthesis of gold clusters. Despite the preparation of numerous species it is still not understood how different functional groups at phosphine centers affect the size and properties of gold clusters. Using electrospray ionization mass spectrometry (ESI-MS) it is possible to characterize the effect of ligand substitution on the distribution of clusters formed in solution at defined reaction conditions. In addition, ligand exchange reactions on preformed clusters may be monitored using ESI-MS. Collision induced dissociation (CID) may also be employed to obtain qualitative insight into the fragmentation of mixed ligand clusters and the relative binding energies of differently substituted phosphines. Quantitative ligand binding energies and cluster stability may be determined employing surface induced dissociation (SID) in a custom-built Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). Rice-Ramsperger-Kassel-Marcus (RRKM) based modeling of the SID data allows dissociation energies and entropy values to be extracted. The charge reduction and reactivity of atomically precise gold clusters, including partially ligated species generated in the gas-phase by in source CID, on well-defined surfaces may be explored using ion soft landing (SL) in a custom-built instrument combined with in situ time of flight secondary ion mass spectrometry (TOF-SIMS). Jointly, this multipronged experimental approach allows characterization of the full spectrum of relevant phenomena including cluster synthesis, ligand exchange, thermochemistry, surface immobilization, and reactivity. The fundamental insights obtained from this work will facilitate the directed synthesis of gold clusters with predetermined size and properties for specific applications.

In situ studies on controlling an atomically-accurate formation process of gold nanoclusters

Knowledge of the molecular formation mechanism of metal nanoclusters is essential for developing chemistry for accurate control over their synthesis. Herein, the "top-down" synthetic process of monodisperse Au13 nanoclusters via HCl etching of polydisperse Aun clusters (15 ≤ n ≤ 65) is traced by a combination of in situ X-ray/UV-vis absorption spectroscopy and time-dependent mass spectrometry. It is revealed experimentally that the HCl-induced synthesis of Au13 is achieved by accurately controlling the etching process with two distinctive steps, in sharp contrast to the traditional thiol-etching mechanism through release of the Au(i) complex. The first step involves the direct fragmentation of the initial larger Aun clusters into metastable intermediate Au8-Au13 smaller clusters. This is a critical step, which allows for the secondary size-growth step of the intermediates toward the atomically monodisperse Au13 clusters via incorporating the reactive Au(i)-Cl species in the solution. Such a secondary-growth pathway is further confirmed by the successful growth of Au13 through reaction of isolated Au11 clusters with AuClPPh3 in the HCl environment. This work addresses the importance of reaction intermediates in guiding the way towards controllable synthesis of metal nanoclusters.

Spot the difference: engineered and natural nanoparticles in the environment--release, behavior, and fate

The production and use of nanoparticles leads to the emission of manufactured or engineered nanoparticles into the environment. Those particles undergo many possible reactions and interactions in the environment they are exposed to. These reactions and the resulting behavior and fate of nanoparticles in the environment have been studied for decades through naturally occurring nanoparticulate (1-100 nm) and colloidal (1-1000 nm) substances. The knowledge gained from these investigations is nowhere near sufficiently complete to create a detailed model of the behavior and fate of engineered nanoparticles in the environment, but is a valuable starting point for the risk assessment of these novel materials. It is the aim of this Review to critically compare naturally observed processes with those found for engineered systems to identify the "nanospecific" properties of manufactured particles and describe critical knowledge gaps relevant for the risk assessment of manufactured nanomaterials in the environment.

Near-infrared silver cluster optically signaling oligonucleotide hybridization and assembling two DNA hosts

Silver clusters with ~10 atoms form within DNA strands, and the conjugates are chemical sensors. The DNA host hybridizes with short oligonucleotides, and the cluster moieties optically respond to these analytes. Our studies focus on how the cluster adducts perturb the structure of their DNA hosts. Our sensor is comprised of an oligonucleotide with two components: a 5'-cluster domain that complexes silver clusters and a 3'-recognition site that hybridizes with a target oligonucleotide. The single-stranded sensor encapsulates an ~11 silver atom cluster with violet absorption at 400 nm and with minimal emission. The recognition site hybridizes with complementary oligonucleotides, and the violet cluster converts to an emissive near-infrared cluster with absorption at 730 nm. Our key finding is that the near-infrared cluster coordinates two of its hybridized hosts. The resulting tertiary structure was investigated using intermolecular and intramolecular variants of the same dimer. The intermolecular dimer assembles in concentrated (~5 μM) DNA solutions. Strand stoichiometries and orientations were chromatographically determined using thymine-modified complements that increase the overall conjugate size. The intramolecular dimer develops within a DNA scaffold that is founded on three linked duplexes. The high local cluster concentrations and relative strand arrangements again favor the antiparallel dimer for the near-infrared cluster. When the two monomeric DNA/violet cluster conjugates transform to one dimeric DNA/near-infrared conjugate, the DNA strands accumulate silver. We propose that these correlated changes in DNA structure and silver stoichiometry underlie the violet to near-infrared cluster transformation.

Nucleation products of ligated nanoclusters unaffected by temperature and reducing agent

Atomically uniform nucleation products of ligated metal nanoclusters are observed irrespective of reduction conditions for metal-bidentate ligand systems. Monodentate ligands are not reported to wield similar control, indicating steric contributions of complexing ligands may be as important as their electronic structure for synthesizing small nanoclusters.