金原子簇催化碳——碳偶联反应的研究进展

Well-defined gold nanoclusters have been documented as new and promising materials in the field of nanoscience. They have been well explored for the nanocatalysis of reactions like selective oxidation and hydrogenation, carbon-carbon coupling, etc. These Au nanoclusters possess unique electronic properties and crystal structure, which provide an excellent opportunity to correlate atomic structure with intrinsic catalytic property and to investigate the mechanisms of reactions over Au nanoclusters. In this review, we generalize the catalytic application of gold nanoclusters in carbon-carbon coupling reactions, including Ullmann, Sonogashira, Suzuki, and A³-coupling reactions. Herein, we have discussed ligand engineering (e.g., aromatic and aliphatic thiolate) as well as the effect of metal dopants (e.g., Cu, Ag, Pt, and Pd). Finally, the tentative catalytic mechanisms and the structure-performance relationships were discussed at the atomic level, which will give some clue for the design of efficient gold cluster catalysts.     

Biomolecules as promising ligands in the synthesis of metal nanoclusters: Sensing,bioimaging and catalytic applications

The use of metal nanoclusters as sensing probes has recently attracted considerable interest from researchers. In particular, metallic nanoclusters (e.g., Au, Ag, Cu, Pt) have been noticed a wide range of applications in the field of fluorescence sensing and bioimaging. The stabilization of metal nanoclusters with organic molecules, proteins, and amino acids enhances their optical properties and analytical applications. In this review, synthetic routes for the fabrication of metal nanoclusters are summarized. This review also describes the metal nanoclusters properties including aggregation-induced emission, optical absorption, non-linear optical, and chiral properties. We discussed the analytical applications of metal nanoclusters for sensing of wide variety of analytes including drugs, biomolecules, biomarkers. Further, the catalytic applications of metal nanoclusters are also briefly summarized. Finally, we summarize the challenges and future perspectives of metal nanoclusters in analytical chemistry.     

Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries

Sub-nanometre sized metal clusters, with dimensions between metal atoms and nanoparticles, have attracted more and more attention due to their unique electronic structures and the subsequent unusual physical and chemical properties. However, the tiny size of the metal clusters brings the difficulty of their synthesis compared to the easier preparation of large nanoparticles. Up to now various synthetic techniques and routes have been successfully applied to the preparation of sub-nanometre clusters. Among the metals, gold clusters, especially the alkanethiolate monolayer protected clusters (MPCs), have been extensively investigated during the past decades. In recent years, silver and copper nanoclusters have also attracted enormous interest mainly due to their excellent photoluminescent properties. Meanwhile, more structural characteristics, particular optical, catalytic, electronic and magnetic properties and the related technical applications of the metal nanoclusters have been discovered in recent years. In this critical review, recent advances in sub-nanometre sized metal clusters (Au, Ag, Cu, etc.) including the synthetic techniques, structural characterizations, novel physical, chemical and optical properties and their potential applications are discussed in detail. We finally give a brief outlook on the future development of metal nanoclusters from the viewpoint of controlled synthesis and their potential applications.     

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.     

Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis,self-assembly,and applications

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of “metallic molecules”. Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)–SR core–shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M–S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

The interactions/reactions of thiolate-protected noble metal nanoclusters with diverse ions, molecules and other metal nanoclusters have been deciphered.     

Insight of the photoluminescence of atomically precise bimetallic nanoclusters with free electrons

Noble-metal nanoclusters with emission properties are promising candidtes for cell imaging, biosensing, photo-therapy applications due to their ultra-small size, good photostability, and high biocompatibility. It is of great importance to realize the atomic precision of such metal nanoclusters (NCs) because they allow us to investigate the origin of fluorescence at the atomic level. However, compared to quantum dots (QDs) and organic dyes, noble metal nanoclusters usually suffered from low quantum yield, which significantly limited their applications. Doping with other metal atoms is an effective strategy to enhance the fluorescent properties of metal NCs. Therefore, bimetallic nanoclusters possess enhanced emission properties relative to their monometallic counterparts. Although a couple of reviews have existed for bimetallic nanocluster, few of them concern their emissive properties. In this context, the emissive properties of atomically precise bimetallic nanoclusters are summarized in this minireview. Synergistic effect, state of aggregation, assembly, and isomerism on the fluorescence of bimetallic clusters and structure-photoluminescence property correlations are also discussed.     

Enhanced Emission of Silver Nanoclusters Through Quantitative Phase Transfer

Silver nanoclusters composed of only a few metal atoms present appealing properties such as fluorescence. We have previously reported on aqueous solutions of this fluorophore using poly(methacrylic acid) as scaffold and their sensing properties. Here we report on the preparation of organic solutions of fluorescent silver nanoclusters by quantitative transfer from aqueous solution to an immiscible organic solvent. The fluorescent silver nanoclusters in the organic phase present enhanced emission properties and increased purity, which may expand the range of applications of this promising fluorophore.     

Protection of Ag Clusters by Metal-Oxo Modules

Monodisperse and atomically precise Ag nanoclusters have attracted considerable recent research interest. A conventional silver cluster usually consists of a silver metallic kernel and an organic peripheral ligand shell. Nevertheless, the present inevitable problem is the unsatisfied stability of such nanoclusters. In this concept, we will give an introduction to Ag clusters protected by metal-oxo modules, which exhibit enhanced stability and unique properties. Accordingly, three different types of clusters are summarized: (1) Ag clusters protected by mononuclear oxometallates; (2) Ag clusters protected by block-like metal-oxo clusters; (3) Ag clusters protected by hollow-like metal-oxo clusters. The aim of this concept is to offer possible general guidance and insight into future rational design of more metal-oxo clusters protected silver clusters or even other coinage metal nanoclusters.     

DNA-Templated Fluorescent Silver Nanoclusters Inhibit Bacterial Growth While Being Non-Toxic to Mammalian Cells

Silver has a long history of antibacterial effectiveness. The combination of atomically precise metal nanoclusters with the field of nucleic acid nanotechnology has given rise to DNA-templated silver nanoclusters (DNA-AgNCs) which can be engineered with reproducible and unique fluorescent properties and antibacterial activity. Furthermore, cytosine-rich single-stranded DNA oligonucleotides designed to fold into hairpin structures improve the stability of AgNCs and additionally modulate their antibacterial properties and the quality of observed fluorescent signals. In this work, we characterize the sequence-specific fluorescence and composition of four representative DNA-AgNCs, compare their corresponding antibacterial effectiveness at different pH, and assess cytotoxicity to several mammalian cell lines.     

Systematically Tuning the Electronic Structure of Gold Nanoclusters through Ligand Derivatization

While the ability to crystallize metal nanoclusters has revealed their geometric structure, the lack of a similarly precise measure of their electronic structure has hampered the development of synthetic design rules to precisely engineer their electronic properties. We track the evolution of highly‐resolved electronic absorption spectra of gold nanoclusters with precisely mass‐selected chemical composition in a controlled environment. Simple derivatization of the ligands yields larger spectral changes than varying the overall atomic composition of the cluster for two clusters with similar symmetry and size. The nominally metal‐localized HOMO–LUMO transition of these nanoclusters lowers in energy linearly with increasing electron donation from the exterior of the ligand shell for both cluster sizes. Very weak surface interactions, such as binding of He or N₂, yield significant state‐dependent shifts, identifying states with significant interfacial character. These observations demonstrate a pathway for deliberate tuning of interfacial chemistry for chemical and technological applications.     

金属纳米簇在CO2光催化还原中的研究进展

化石燃料的大量燃烧不仅造成能源危机，而且排放的二氧化碳（CO2）会使气候变暖。以清洁、储量丰富的太阳光作为能量来源，将CO2光催化还原为高附加值的化学产品是缓解当前环境问题和能源问题的主要方法之一。然而，CO2在常温常压下非常的稳定，因此需要设计并构筑高效光催化剂来捕捉和转化CO2，以达到高效光催化CO2还原的目的。在众多研究的光催化剂中，金属纳米簇因其具有独特的结构特点、优异的物理和化学性质，所以在光催化CO2还原领域得到了广泛的应用。基于此，我们首先对金属纳米簇进行了分类，将其分为贵金属纳米簇和非贵金属纳米簇；然后分别对贵金属和非贵金属纳米簇在光催化CO2还原中的研究进展进行了归纳与总结。本文通过及时全面概述近几年该领域的研究进展，从而为未来研究方向提供新思路。     

锌-稀土矩形纳米簇的构筑及近红外发光性能

本文设计了一个新型含苯-甲基-苯骨架的席夫碱配体,构筑了两个具有矩形结构的锌-稀土纳米簇[Ln₂Zn₂L₂(OAc)₆] (Ln = Yb (1)和Er (2))。该席夫碱配体以“伸展型”配位模式与稀土离子进行配位,使这些锌-稀土纳米簇表现出较大的分子尺寸结构(0.7 nm × 1.1 nm × 2.2 nm)。荧光性质研究表明,由Zn/L组成的发色基团能有效敏化1和2中Yb³⁺和Er³⁺离子的近红外发光。通过对荧光量子产率及寿命进行分析发现,Zn/L对Yb³⁺离子的传能效率要高于Er³⁺离子。     

Variable control of the electronic states of a silver nanocluster via protonation/deprotonation of polyoxometalate ligands

The properties of metal nanoclusters depend on both their structures and electronic states. However, in contrast to the significant advances achieved in the synthesis of structurally well-defined metal nanoclusters, systematic control of their electronic states is still challenging. In particular, stimuli-responsive and reversible control of the electronic states of metal nanoclusters is attractive from the viewpoint of their practical applications. Recently, we developed a synthesis method for atomically precise Ag nanoclusters using polyoxometalates (POMs) as inorganic ligands. Herein, we exploited the acid/base nature of POMs to reversibly change the electronic states of an atomically precise {Ag₂₇} nanocluster via protonation/deprotonation of the surrounding POM ligands. We succeeded in systematically controlling the electronic states of the {Ag₂₇} nanocluster by adding an acid or a base (0–6 equivalents), which was accompanied by drastic changes in the ultraviolet-visible absorption spectra of the nanocluster solutions. These results demonstrate the great potential of Ag nanoclusters for unprecedented applications in various fields such as sensing, biolabeling, electronics, and catalysis.

The electronic states of Ag nanoclusters were reversibly controlled driven by protonation/deprotonation of polyoxometalate ligands.     

Structural Evolution Patterns of FCC-Type Gold Nanoclusters

Recent progress in the research of atomically-precise metal nanoclusters has identified a series of exceptionally stable nanoclusters with specific chemical compositions. Structural determination on such "magic size" nanoclusters revealed a variety of unique structures such as decahedron, icosahedron, as well as hexagonal close packing(hcp) and body-centered cubic(bcc) packing arrangements in gold nanoclusters, which are largely different from the face-centered cubic(fcc) structure in conventional gold nanoparticles. The characteristic geometrical structures enable the nanoclusters to exhibit interesting properties, and these properties are in close correlation with their atomic structures according to the recent studies. Experimental and theoretical analyses have been applied in the structural identification aiming to clarify the universal principle in the structural evolution of nanoclusters. In this mini-review, we summarize recent studies on periodic structural evolution of fcc-based gold nanoclusters protected by thiolates. A series of nanoclusters exhibit one-dimensional growth along the [001] direction in a layer-by-layer manner from Au_(23)(TBBT)_(20) to Au_(36)(TBBT)_(24),Au_(44)(TBBT)_(28), and to Au_(52)(TBBT)_(32)(TBBT: 4-tert-butylbenzenethiolate). The optical properties of these nanoclusters also evolve periodically based on steady-state and ultrafast spectroscopy. In addition, two-dimensional growth from Au_(44)(TBBT)_(28) toward both [100] and [010] directions leads to the Au_(92)(TBBT)_(44) nanocluster, and the recently reported Au_(52)(PET)_(32)(PET: 2-phenylethanethiol) also follows this growth pattern with partial removal of the layer. Theoretical predictions of relevant fcc nanoclusters include Au_(60)(SCH_3)_(36), Au_(68)(SCH_3)_(40), Au_(76)(SCH_3)_(44), etc, for the continuation of 1 D growth pattern, as well as Au_(68)(SR)_(38)mediating the 2 D growth pattern from Au_(44)(TBBT)_(28) to Au_(92)(TBBT)_(44). Overall, this mini-review provides guidelines on the rules of structural evolution of fcc gold nanoclusters based on 1 D, 2 D and 3 D growth patterns.     

New types of multishell nanoclusters with a Frank-Kasper polyhedral core in intermetallics

A comprehensive study of the occurrence of two-shell clusters with the first shell as a Frank-Kasper polyhedron Z12, Z14, Z15, or Z16 (Frank-Kasper nanoclusters) is performed for 22,951 crystal structures of intermetallics containing only metal atoms. It is shown that besides the familiar Bergman and Mackay clusters, two more types of high-symmetrical icosahedron-based nanoclusters are rather frequent; they both have a 50-atom second shell. Moreover, two types of high-symmetrical Frank-Kasper nanoclusters with a Friauf-polyhedron (Z16) core are revealed; these nanoclusters have 44 and 58 atoms in the second shell. On the contrary, Z14 and Z15 Frank-Kasper polyhedra have been found to be rare and improper to form distinct nanoclusters in crystals. The second shells of Frank-Kasper nanoclusters have been revealed possessing their own stability: they can be realized in nanoclusters with different internal polyhedra and can shift around the core shell. The role of Frank-Kasper nanoclusters in assembling intermetallic crystal structures is illustrated by several examples.     

Controlled synthesis of Bi- and tri-nuclear Cu-oxo nanoclusters on metal–organic frameworks and the structure–reactivity correlations

Precisely tuning the nuclearity of supported metal nanoclusters is pivotal for designing more superior catalytic systems, but it remains practically challenging. By utilising the chemical and molecular specificity of UiO-66-NH₂ (a Zr-based metal–organic framework), we report the controlled synthesis of supported bi- and trinuclear Cu-oxo nanoclusters on the Zr₆O₄ nodal centres of UiO-66-NH₂. We revealed the interplay between the surface structures of the active sites, adsorption configurations, catalytic reactivities and associated reaction energetics of structurally related Cu-based ‘single atoms’ and bi- and trinuclear species over our model photocatalytic formic acid reforming reaction. This work will offer practical insight that fills the critical knowledge gap in the design and engineering of new-generation atomic and nanocluster catalysts. The precise control of the structure and surface sensitivities is important as it can effectively lead to more reactive and selective catalytic systems. The supported bi- and trinuclear Cu-oxo nanoclusters exhibit notably different catalytic properties compared with the mononuclear ‘Cu₁’ analogue, which provides critical insight for the engineering of more superior catalytic systems.

The controlled synthesis of novel bi- and trinuclear Cu-oxo nanoclusters supported on UiO-66-NH₂ that show notably different catalytic properties in the photocatalytic formic acid decomposition reaction is reported.     

The density of surface ligands regulates the luminescence of thiolated gold nanoclusters and their metal ion response

The fascinating luminescence properties of gold nanoclusters(AuNCs) have drawn considerable research interests,and been widely harnessed for a wide range of applications.However,a fundamental understanding towards ligand density's role in the luminescence properties of these ultrasmall AuNCs remains unclear yet.In this communication,through systematic investigation of surface chemistries of glutathione-protected Au NCs(GSH-Au NCs) with diffe rent density of GSH as well as other thiolates,it is discovered that the density of surface ligands can significantly regulate the luminescence properties of AuNCs.Fluorescence lifetime spectroscopy and X-ray photoelectron spectroscopy showed that AuNCs with a higher density of electron-rich ligands facilitate their luminescence generation.Moreover,differences in the surface coverage of AuNCs can also affect their interactions with foreign species,as illustrated by significantly different fluorescence quenching capability of GSH-AuNCs with different ligand density towards Hg~(2+).This study provides new insight into the intriguing luminescence properties of metal NCs,which is hoped to stimulate further research on the design of metal NCs with strong luminescence and sensitive/specific responses for promising optoelectronic,sensing and imaging applications.     

Synthesis, structure and properties of metal nanoclusters

Metal nanoclusters have physical properties differing significantly from their bulk counterparts. Metallic properties such as delocalization of electrons in bulk metals which imbue them with high electrical and thermal conductivity, light reflectivity and mechanical ductility may be wholly or partially absent in metal nanoclusters, while new properties develop. We review modern synthetic methods used to form metal nanoclusters. The focus of this critical review is solution based chemical synthesis methods which produce fully dispersed clusters. Control of cluster size and surface chemistry using inverse micelles is emphasized. Two classes of metals are discussed, transition metals such as Au and Pt, and base metals such as Co, Fe and Ni. The optical and catalytic properties of the former are discussed and the magnetic properties of the latter are given as examples of unexpected new size-dependent properties of nanoclusters. We show how classical surface science methods of characterization augmented by chemical analysis methods such as liquid chromatography can be used to provide feedback for improvements in synthetic protocols. Characterization of metal clusters by their optical, catalytic, or magnetic behavior also provides insights leading to improvements in synthetic methods. The collective physical properties of closely interacting clusters are reviewed followed by speculation on future technical applications of clusters. (125 references).     

Growth morphology and properties of metals on graphene

Graphene, a single atomic layer of graphite, has been the focus of recent intensive studies due to its novel electronic and structural properties. Metals grown on graphene also have been of interest because of their potential use as metal contacts in graphene devices, for spintronics applications, and for catalysis. All of these applications require good understanding and control of the metal growth morphology, which in part reflects the strength of the metal–graphene bond. Also of importance is whether the interaction between graphene and metal is sufficiently strong to modify the electronic structure of graphene. In this review, we will discuss recent experimental and computational studies related to deposition of metals on graphene supported on various substrates (SiC, SiO₂, and hexagonal close-packed metal surfaces). Of specific interest are the metal–graphene interactions (adsorption energies and diffusion barriers of metal adatoms), and the crystal structures and thermal stability of the metal nanoclusters.     

Chiral functionalization of optically inactive monolayer-protected silver nanoclusters by chiral ligand-exchange reactions

We report the ligand-exchange reaction between the optically inactive racemic penicillamine monolayer on a silver nanocluster surface and enantiopure D- or L-penicillamine dissolved in solution. Emergence of the identical band positions in the gel electrophoresis separation assures the presence of size-invariant silver nanoclusters (1.05 and 1.30 nm in core diameter) during the ligand-exchange reaction and allows us to further examine the optical/chiroptical properties of these nanoclusters. Consequently, chiral functionalization of the achiral silver nanoclusters has been demonstrated, yielding large Cotton effects in metal-based electronic transitions with an almost mirror-image relationship between the enantiomeric compounds. The ligand-exchange experiments as well as the normal syntheses of the silver nanoclusters revealed that their absorption profiles and anisotropy factors were strongly dependent on the enantiomeric purity (or enantiomeric excess) of surface chiral penicillamine, so that (several-fold) larger chiroptical responses of the silver nanoclusters as compared to those of the analogous gold clusters with a comparable size could be induced by the metal core deformation or rearrangement along with a universally influential vicinal contribution from the chiral ligand field.