Water-Soluble Phosphine-Protected Au₁₁ Clusters: Synthesis, Electronic Structure, and Chiral Phase Transfer in a Synergistic Fashion

Electrochemical Analysis of the Thermal Stability of 0.9-4.1 nm Diameter Gold Nanoclusters

Here we report the thermal properties of weakly stabilized 0.9, 1.6, and 4.1 nm Au nanoparticles (NPs)/nanoclusters (NCs) attached to indium-tin-oxide- or fluorine-doped-tin-oxide-coated glass electrodes (glass/ITO or glass/FTO). The peak oxidation potential (E_p) for Au measured by anodic stripping voltammetry (ASV) is indicative of the NP/NC size. Heating leads to a positive shift in E_p due to an increase in NP/NC size from thermal ripening. The size transition temperature (T_t) decreases with decreasing NP/NC size following the order of 4.1 nm (509 °C) > 1.6 nm (132 °C) > 0.9 nm (90 °C/109 °C, two transitions) as compared to the bulk melting point (T_m,b) for Au of 1064 °C. The T_t generally agrees with models describing the size-dependent melting point of Au NPs (T_m,NP) for 4.1 and 1.6 nm diameter Au NPs but is higher than the models for 0.9 nm Au NCs. Scanning electron microscopy (SEM) and UV-vis size analysis confirm the electrochemical results. The thermal stability of electrode-supported metal NPs/NCs is important for their effective use in catalysis, sensing, nanoelectronics, photovoltaics, and other applications.

Asymmetric transformation of achiral gold nanoclusters with negative nonlinear dependence between chiroptical activity and enantiomeric excess

The investigation of chirality at the nanoscale is important to bridge the gap between molecular and macroscopic chirality. Atomically precise metal nanoclusters provide an ideal platform for this research, while their enantiopure preparation poses a challenge. Here, we describe an efficient approach to enantiopure metal nanoclusters via asymmetric transformation, that is, achiral Au₂₃(SC₆H₁₁)₁₆ nanoclusters are converted into chiral and enantiopure Au₂₄(L)₂(SC₆H₁₁)₁₆ nanoclusters by a chiral inducer phosphoramidite (L). Two enantiomers of Au₂₄(L)₂(SC₆H₁₁)₁₆ are obtained and the crystal structures reveal their hierarchical chirality, which originates from the two introduced chiral L molecules, the transformation-triggered asymmetric rearrangement of the staple motifs on the surface of the gold core, and the helical arrangement of nanocluster molecules. The construction of this type of enantiomerically pure nanoclusters is achieved based on the easy-to-synthesize and modular L. Lastly, the chirality-related chiroptical performance was investigated, revealing a negative nonlinear CD-ee dependence.

Mixed-ligand strategy for synthesizing water-soluble chiral gold clusters with phosphine ligands

Water-soluble chiral metal clusters have drawn much attention by virtue of their fascinating physicochemical properties and potential biomedical applications, but currently, phosphine-protected Au clusters with both chirality and water-solubility are still very limited. In this article, we demonstrate a mixed-ligand strategy for the facile synthesis of atomically precise, water-soluble chiral Au clusters protected by phosphine alone. The clusters are obtained by the reduction of aurate ions in the presence of a phosphine mixture consisting of highly hydrophilic monophosphine (i.e., triphenylphosphine trisulfonate; TPPTS) and hydrophobic chiral diphosphine (i.e., S-Segphos or S-BINAP), both of which are commercially available. The clusters are size/composition-separated via gel electrophoresis, and notably, heptanuclear cluster Au₇(S-Segphos)₃(TPPTS)₂ exhibits a large chiroptical activity with the maximum anisotropy factor (g-factor) of 4.7 × 10^-3, one of the largest values in such Au clusters. Quantum chemical calculations for model Au₇ cluster species suggest two important factors to obtain large chiroptical activity: (i) more than two axially-chiral diphosphine ligands, and (ii) the absence of configurational isomer averaging. Consequently, despite the experimental use of a mixture containing both chiral and achiral phosphines, a large chiroptical activity can be created in Au clusters with high water-solubility.

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.

Insight into the Effects of Chiral Diphosphine Ligands on the Structure and Optical Properties of the Au24Cd2 Nanocluster

Introduction of chiral ligands has been regarded as an effective strategy to obtain nanoclusters with optical purity. However, how the chiral ligands work is still unclear due to the lack of structural comparison between racemic nanoclusters and the corresponding optically active ones. In this work, three structurally related Au₂₄Cd₂ nanoclusters, including one racemic and two homochiral nanoclusters, were synthesized, and their crystal structures were characterized using single-crystal X-ray crystallography (SC-XRD). Based on their crystal structures, the origin of the chirality in Au₂₄Cd₂ was found to be the twist of the kernel and the chiral arrangement of the metal-ligand surface. Au₂₄Cd₂ protected with chiral ligands exhibits a more twisted kernel than the racemic one. Therefore, the chirality of chiral diphosphine was found to transfer from the ligands to the metal-ligand interface and then to the metal core, inducing its distortion to produce enhanced chirality. In addition, the optical properties including optical absorption and circular dichroism of these structurally related Au₂₄Cd₂ nanoclusters were compared.

Ligand exchange reactions on thiolate-protected gold nanoclusters

As a versatile post-synthesis modification method, ligand exchange reaction exhibits great potential to extend the space of accessible nanoclusters. In this review, we summarized this process for thiolate-protected gold nanoclusters. In order to better understand this reaction we will first provide the necessary background on the synthesis and structure of various gold clusters, such as Au25(SR)18, Au38(SR)24, and Au102(SR)44. The previous investigations illustrated that ligand exchange is enabled by the chemical properties and flexible gold-sulfur interface of nanoclusters. It is generally believed that ligand exchange follows a SN2-like mechanism, which is supported both by experiments and calculations. More interesting, several studies show that ligand exchange takes place at preferred sites, i.e. thiolate groups -SR, on the ligand shell of nanoclusters. With the help of ligand exchange reactions many functionalities could be imparted to gold nanoclusters including the introduced of chirality to achiral nanoclusters, size transformation and phase transfer of nanoclusters, and the addition of fluorescence or biological labels. Ligand exchange was also used to amplify the enantiomeric excess of an intrinsically chiral cluster. Ligand exchange reaction accelerates the prosperity of the nanocluster field, and also extends the diversity of precise nanoclusters.

Reversing the Thermodynamics of Galvanic Replacement Reactions by Decreasing the Size of Gold Nanoparticles

Here, we describe the surprising reactivity between surface-attached (a) 0.9, 1.6, and 4.1 nm diameter weakly stabilized Au nanoparticles (NPs) and aqueous 1.0 × 10^-4 M Ag⁺ solution, and (b) 1.6 and 4.1 nm diameter weakly stabilized Au NPs and aqueous 1.0 × 10^-5 M PtCl₄^2-, which are considered to be antigalvanic replacement (AGR) reactions because they are not thermodynamically favorable for bulk-sized Au under these conditions. Anodic Stripping Voltammetry (ASV) and Scanning Transmission Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (STEM-EDS) mapping provide quantitation of the extent of Ag and Pt replacement as a function of Au NP diameter. The extent of the reaction increases as the Au NP size decreases. The percentage of Ag in the AuAg alloy following AGR based on ASV is 17.8 ± 0.6% for 4.1 nm diameter Au NPs, 87.2 ± 2.9% for 1.6 nm Au NPs, and an unprecedented full 100% Ag for 0.9 nm diameter Au NPs. STEM-EDS mapping shows very close agreement with the ASV-determined compositions. In the case of PtCl₄^2-, STEM-EDS mapping shows AuPt alloy NPs with 3.9 ± 1.3% and 41.1 ± 8.7% Pt following replacement with 4.1 and 1.6 nm diameter Au NPs, respectively, consistent with qualitative changes to the ASV. The size-dependent AGR correlates well with the negative shift in the standard potential (E⁰) for Au oxidation with decreasing NP size.

Magnetic Circular Dichroism in Nanomaterials: New Opportunity in Understanding and Modulation of Excitonic and Plasmonic Resonances

The unique capability of magnetic circular dichroism (MCD) in revealing geometry and electronic information has provided new opportunities in exploring the relationship between structure and magneto-optical properties in nanomaterials with extraordinary optical absorption. Here, the representative studies referring to application of the MCD technique in semiconductor and noble metal nanomaterials are overviewed. MCD is powerful in elucidating the structural information of the excitonic transition in semiconductor nanocrystals, electronic transitions in noble metal nanoclusters, and plasmon resonance in noble metal nanostructures. By virtue of these advantages, the MCD technique shows its unrivalled ability in evaluating the magnetic modulation of excitonic and plasmonic optical activity of nanomaterials with varied chemical composition, geometry, assembly conformation, and coupling effect. Knowledge of the key factors in manipulating magneto-optical properties at the nanoscale acquired with the MCD technique will largely boost the application of semiconductor and noble nanomaterials in the fields of sensing, spintronic, nanophotonics, etc.

Metal-Ligand Interface in the Chemical Reactions of Ligand-Protected Noble Metal Clusters

We discuss the role of the metal-ligand (M-L) interfaces in the chemistry of ligand-protected, atomically precise noble metal clusters, a new and expanding family of nanosystems, in solution as well as in the gas phase. A few possible mechanisms by which the structure and dynamics of M-L interfaces could trigger intercluster exchange reactions are presented first. How interparticle chemistry can be a potential mechanism of Ostwald ripening, a well-known particle coarsening process, is also discussed. The reaction of Ag₅₉(2,5-DCBT)₃₂ (DCBT = dichlorobenzenethiol) with 2,4-DCBT leading to the formation of Ag₄₄(2,4-DCBT)₃₀ is presented, demonstrating the influence of the ligand structure in ligand-induced chemical transformations of clusters. We also discuss the structural isomerism of clusters such as Ag₄₄(SR)₃₀ (-SR = alkyl/aryl thiolate) in the gas phase wherein the occurrence of isomerism is attributed to the structural rearrangements in the M-L bonding network. Interfacial bonding between Au₂₅(SR)₁₈ clusters leading to the formation of cluster dimers and trimers is also discussed. Finally, we show that the desorption of phosphine and hydride ligands on a silver cluster, [Ag₁₈(TPP)₁₀H₁₆]²⁺ (TPP = triphenylphosphine) in the gas phase, leads to the formation of a naked silver cluster of precise nuclearity, such as Ag₁₇⁺. We demonstrate that the nature of the M-L interfaces, i.e., the oxidation state of metal atoms, structure of the ligand, M-L bonding network, and so forth, plays a key role in the chemical reactivity of clusters. The structure, dynamics, and chemical reactivity of nanosystems in general are to be explored together to obtain new insights into their emerging science.

A comprehensive study of catalytic, morphological and electronic properties of ligand-protected gold nanoclusters using XPS, STM, XAFS, and TPD techniques

A combination of microscopy and spectroscopy techniques comprehensively elucidates the unique properties of distinct ligand-protected gold nano clusters.

Chirality in adsorption on solid surfaces

In the present review we survey the main advances made in recent years on the understanding of chemical chirality at solid surfaces. Chirality is an important topic, made particularly relevant by the homochiral nature of the biochemistry of life on Earth, and many chiral chemical reactions involve solid surfaces. Here we start our discussion with a description of surface chirality and of the different ways that chirality can be bestowed on solid surfaces. We then expand on the studies carried out to date to understand the adsorption of chiral compounds at a molecular level. We summarize the work published on the adsorption of pure enantiomers, of enantiomeric mixtures, and of prochiral molecules on chiral and achiral model surfaces, especially on well-defined metal single crystals but also on other flat substrates such as highly ordered pyrolytic graphite. Several phenomena are identified, including surface reconstruction and chiral imprinting upon adsorption of chiral agents, and the enhancement or suppression of enantioselectivity seen in some cases upon adsorption of enantiomixtures of chiral compounds. The possibility of enhancing the enantiopurity of adsorbed layers upon the addition of chiral seeds and the so-called "sergeants and soldiers" phenomenon are presented. Examples are provided where the chiral behavior has been associated with either thermodynamic or kinetic driving forces. Two main approaches to the creation of enantioselective surface sites are discussed, namely, via the formation of supramolecular chiral ensembles made out of small chiral adsorbates, and by adsorption of more complex chiral molecules capable of providing suitable chiral environments for reactants by themselves, via the formation of individual adsorbate:modifier adducts on the surface. Finally, a discussion is offered on the additional effects generated by the presence of the liquid phase often required in practical applications such as enantioselective crystallization, chiral chromatography, and enantioselective catalysis.

Fluorescent metal quantum clusters: an updated overview of the synthesis, properties, and biological applications

A brief history of metal quantum clusters, their synthesis methods, physical properties, and an updated overview of their applications is provided.

Amplification of the Optical Activity of Gold Clusters by the Proximity of BINAP

Despite recent progress in the synthesis and characterization of optically active gold clusters, the factor determining optical rotatory strength has not been clarified due to the lack of structurally resolved, enantiomerically pure Au clusters. We addressed this issue by studying the correlation between the optical activity and geometrical structures of two types of Au clusters that were protected by chiral diphosphines: [Au₁₁(R/S-DIOP)₄Cl₂]⁺ (DIOP = 1,4-bis(diphenylphosphino)-2,3-o-isopropylidene-2,3-butanediol) and [Au₈(R/S-BINAP)₃(PPh₃)₂]²⁺ (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl). [Au₈(BINAP)₃(PPh₃)₂]²⁺ showed stronger rotatory strengths than [Au₁₁(DIOP)₄Cl₂]⁺ in the visible region, while the Hausdorff chirality measure calculated from the crystal data indicated that the Au core of the former is less chiral than that of the latter. We propose that the optical activity in the Au core-based transition due to the deformed core is further amplified by chiral arrangement of the binaphthyl moiety near the Au core.