The Ligand-Exchange Reactions of Rod-Like Au25-nMn (M=Au,Ag, Cu,Pd, Pt) Nanoclusters with Cysteine – A Density Functional Theory Study

The atomic precision of ultrasmall noble-metal nanoclusters (NMNs) is fundamental for elucidating structure-property relationships and probing their practical applications. So far, the atomic structure of NMNs protected by organic ligands has been widely elucidated, whereas the precise atomic structure of NMNs protected by water-soluble ligands (such as peptides and nucleic acid), has been rarely reported. With the concept of “precision to precision”, density functional theory (DFT) calculations were performed to probe the thermodynamic plausibility and inherent determinants for synthesizing atomically precise, water-soluble NMNs via the framework-maintained two-phase ligand-exchange method. A series of rod-like Au_25-nM_n (M=Au, Ag, Cu, Pd, Pt) NMNs with the same framework but varied ligands and metal compositions was chosen as the modeling reactants, and cysteine was used as the modeling water-soluble ligand. It was found that the acidity of the reaction remarkably affects the thermodynamic facility of the ligand exchange reactions. Ligand effects (structural distortion and acidity) dominate the overall thermodynamic facility of the ligand-exchange reaction, while the number and type of doped metal atom(s) has little influence.     

原子级精确的币金属纳米团簇在光催化应用方面的研究进展

光催化技术可以直接将太阳能转化为化学能,制造化学燃料或环境友好的产品。然而,常用的光催化剂大多为具有宽能隙的半导体材料,所需光源大多在紫外区,对太阳光的利用率不高;并且电子-空穴复合率高,导致光催化反应效率低。币金属纳米团簇具有超小尺寸（<2 nm）和分立能级,能够实现电子和空穴的分离,电子结构可调,可以通过调节其电子结构进而提高其光催化性能。同时,精确的原子级组成和结构使其成为一种在原子水平上探索光催化机制的理想模型。本文报道了基于币金属纳米团簇的光催化反应的现状,包括水分解产氢、有机污染物降解和光催化氧化胺等。通过探讨调节币金属纳米团簇的光催化性能的策略,对币金属纳米团簇光催化剂的发展前景予以展望。     

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.     

Atomically Precise Metal Nanoclusters: Novel Building Blocks for Hierarchical Structures

Atomically precise ligand-protected nanoclusters (NCs) constitute an important class of compounds that exhibit well-defined structures and, when sufficiently small, evident molecular properties. NCs provide versatile building blocks to fabricate hierarchical superstructures. The assembly of NCs indeed offers opportunities to devise new materials with given structures and able to carry out specific functions. In this Concept article, we highlight the possibilities offered by NCs in which the physicochemical properties are controlled by the introduction of foreign metal atoms and/or modification of the composition of the capping monolayer with functional ligands. Different approaches to assemble NCs into dimers and higher hierarchy structures and the corresponding changes in physicochemical properties are also described.     

Pillar[5]arene‐Stabilized Silver Nanoclusters: Extraordinary Stability and Luminescence Enhancement Induced by Host–Guest Interactions

Herein, we report the synthesis of a new class of functional silver nanoclusters (AgNCs) capped with pillar[5]arene (P5)‐based host ligands. These NCs are readily prepared through direct synthesis or ligand exchange synthesis and are stable at room temperature for over 4 months. The pillar[5]arene‐stabilized NCs (Ag₂₉(LA‐P5)₁₂(TPP)₂) endorse reversible host–guest interactions with neutral alkylamines and cationic quaternary ammonium guests. This results in the formation of spherical assemblies with unparalleled changes in their optical properties including an astonishing circa 2000‐fold luminescence enhancement. This is the highest luminescence enhancement ratio reported so far for such atomically precise NCs. Our synthetic protocol paves the way for the preparation of a new generation of metal nanoclusters protected by macrocyclic ligands with molecular recognition and selectivity toward specific guests.     

Electrocatalytic and photocatalytic applications of atomically precise gold-based nanoclusters

With advances in cluster chemistry, atomically precise gold nanoclusters(NCs) with well-defined composition and tunable structure provide an exciting opportunity to uncover the specific roles of the geometrical and electronic structures as well as the capped ligands in overall catalytic performances. The Au NCs possess quantum energy levels and unique optical properties,which have exhibited unexpected photocatalytic and electrocatalytic activities. In this review, we first highlight the electrocatalytic applications of Au NCs, including hydrogen evolution reaction, oxygen reduction reaction, CO_2 reduction and catalytic oxidation reactions, and then present Au NCs-driven photocatalytic applications such as selective organic reactions, decomposition of pollutants and energy conversion reactions. Finally, we conclude this review with a brief perspective on the catalytic field of Au NCs.     

Coherent Vibrational Dynamics of [Au25(SR)18]- Nanoclusters?

Coherent vibrational dynamics can be observed in atomically precise gold nanoclusters using femtosecond time-resolved pump-probe spectroscopy. It can not only reveal the coupling between electrons and vibrations, but also reflect the mechanical and electronic properties of metal nanoclusters, which holds potential applications in biological sensing and mass detection. Here, we investigated the coherent vibrational dynamics of [Au₂₅(SR)₁₈]^- nanoclusters by ultrafast spectroscopy and revealed the origins of these coherent vibrations by analyzing their frequency, phase and probe wavelength distributions. Strong coherent oscillations with frequency of 40 cm^-1 and 80 cm^-1 can be reproduced in the excited state dynamics of [Au₂₅(SR)₁₈]^-, which should originate from acoustic vibrations of the Au₁₃ metal core. Phase analysis on the oscillations indicates that the 80 cm^-1 mode should arise from the frequency modulation of the electronic states while the 40 cm^-1 mode should originate from the amplitude modulation of the dynamic spectrum. Moreover, it is found that the vibration frequencies of [Au₂₅(SR)₁₈]^- obtained in pump-probe measurements are independent of the surface ligands so that they are intrinsic properties of the metal core. These results are of great value to understand the electron-vibration coupling of metal nanoclusters.     

Template‐Free Supracolloidal Self‐Assembly of Atomically Precise Gold Nanoclusters: From 2D Colloidal Crystals to Spherical Capsids

We report supracolloidal self‐assembly of atomically precise and strictly monodisperse gold nanoclusters involving p‐mercaptobenzoic acid ligands (Au₁₀₂‐pMBA₄₄) under aqueous conditions into hexagonally packed monolayer‐thick two‐dimensional facetted colloidal crystals (thickness 2.7 nm) and their bending to closed shells leading to spherical capsids (d ca. 200 nm), as controlled by solvent conditions. The 2D colloidal assembly is driven in template‐free manner by the spontaneous patchiness of the pMBA ligands around the Au₁₀₂‐pMBA₄₄ nanoclusters preferably towards equatorial plane, thus promoting inter‐nanocluster hydrogen bonds and high packing to planar sheets. More generally, the findings encourage to explore atomically precise nanoclusters towards highly controlled colloidal self‐assemblies.     

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.     

Interstitial Hydrides in Nanoclusters can Reduce M(I) (M=Cu,Ag, Au) to M(0) and Form Stable Superatoms

High-nuclearity clusters resemble the closest model between the determination of atomically precise chemical species and the bulk metallic version thereof, and both impacts on a variety of applications, including catalysis, optics, sensors, and new energy sources. Our interest lies with the nanoclusters of the Group 11 (Cu, Ag, Au) metals stabilized by dichalcogenido and hydrido ligands. Herein, we describe superatoms formed by the clusters and their relationship with precursor hydrido clusters. Specifically, our concept seeks to demonstrate a possible correlation that exist between hydrido clusters (and nanoalloys) and the formation of superatoms, with the loss of hydrides and typically with release of H₂ gas. These reactions appear to be internal self-redox reactions and require no additional reducing agent, but does seem to require a similar core structure. Knowledge of such processes could provide insight into how clusters grow and an understanding in bridging the atomically precise cluster – metal nanoparticle mechanism.     

Controlling ultrasmall gold nanoparticles with atomic precision

Gold nanoparticles are probably the nanoparticles that have been best studied for the longest time due to their stability, physicochemical properties and applications. Controlling gold nanoparticles with atomic precision is of significance for subsequent research on their structures, properties and applications, which is a dream that has been pursued for many years since ruby gold was first obtained by Faraday in 1857. Fortunately, this dream has recently been partially realized for some ultrasmall gold nanoparticles (nanoclusters). However, rationally designing and synthesizing gold nanoparticles with atomic precision are still distant goals, and this challenge might rely primarily on rich atomically precise gold nanoparticle libraries and the in-depth understanding of metal nanoparticle chemistry. Herein, we review general synthesis strategies and some facile synthesis methods, with an emphasis on the controlling parameters determined from well-documented results, which might have important implications for future nanoparticle synthesis with atomic precision and facilitate related research and applications.

The synthesis strategy, methods and parameters for atomically precise gold nanoclusters were reviewed, and future outlook was also proposed.     

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.     

Highly Efficient Circularly Polarized Luminescence from Chiral Au13 Clusters Stabilized by Enantiopure Monodentate NHC Ligands

Chiral Au nanoclusters have promising application prospects in chiral sensing, asymmetric catalysis, and chiroptics. However, enantiopure superatomic homogold clusters with crystallographic structures emitting bright circularly polarized luminescence (CPL) remain challenging. In this study, we designed chiral N-heterocyclic carbenes (NHCs), and for the first time enantioselectively synthesized a pair of monovalent cationic superatomic Au₁₃ clusters. This new enantiomeric pair of clusters has a quasi-C₂ symmetric core and exhibited CPL with an unprecedent solution-state quantum yield (QY) of 61 % among those of the atomically precise Au nanoclusters. DFT calculations provided insights into the circular dichroism behavior, and revealed the origin of CPL from superatomic Au clusters. This work opens a new avenue for developing novel homochiral nanoclusters using chiral NHC ligands and provides fundamental understanding of the origin of the chiroptics of metal clusters.     

原子数精确的金属纳米团簇在电催化领域的应用研究进展

金属纳米团簇(MNCs)是由几个到数百个金属原子组成,其尺寸一般小于2nm.金属纳米团簇在许多催化反应中表现出高的催化活性和选择性,这与金属纳米团簇具有高的比表面积、较多暴露的活性原子,以及与金属纳米粒子(MNPs)不同的电子结构有关.金属纳米团簇确定的组成和结构使其成为一种新型模型催化剂,对纳米团簇的催化性能研究有利...     

Atomically Precise Nanocluster Assemblies Encapsulating Plasmonic Gold Nanorods

The self‐assembled structures of atomically precise, ligand‐protected noble metal nanoclusters leading to encapsulation of plasmonic gold nanorods (GNRs) is presented. Unlike highly sophisticated DNA nanotechnology, this strategically simple hydrogen bonding‐directed self‐assembly of nanoclusters leads to octahedral nanocrystals encapsulating GNRs. Specifically, the p‐mercaptobenzoic acid (pMBA)‐protected atomically precise silver nanocluster, Na₄[Ag₄₄(pMBA)₃₀], and pMBA‐functionalized GNRs were used. High‐resolution transmission and scanning transmission electron tomographic reconstructions suggest that the geometry of the GNR surface is responsible for directing the assembly of silver nanoclusters via H‐bonding, leading to octahedral symmetry. The use of water‐dispersible gold nanoclusters, Au_≈250(pMBA)_n and Au₁₀₂(pMBA)₄₄, also formed layered shells encapsulating GNRs. Such cluster assemblies on colloidal particles are a new category of precision hybrids with diverse possibilities.     

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.     

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.     

Ag2Au50(PET)36 Nanocluster: Dimeric Assembly of Au25(PET)18 Enabled by Silver Atoms

The assembly of atomically precise metal nanoclusters offers exciting opportunities to gain fundamental insights into the hierarchical assembly of nanoparticles. However, it is still challenging to control the assembly of individual nanoclusters at a molecular or atomic level. Herein, we report the dimeric assembly of Au₂₅(PET)₁₈ (PET=2‐phenylethanethiol), where two Au₂₅(PET)₁₈ monomers are bridged together by two Ag atoms to form the Ag₂Au₅₀(PET)₃₆ dimer. The Ag₂Au₅₀(PET)₃₆ dimer is a unique mesomer, which has not been found in any other chiral metal nanoclusters. Furthermore, the Ag₂Au₅₀(PET)₃₆ dimer is distinct from the Au₂₅(PET)₁₈ monomer in its optical, electronic, and catalytic properties. This study is expected to provide a feasible strategy to precisely modulate the assembly of metal nanoclusters with controllable structures and properties.     

Ligand Engineering in Metal Nanoclusters: from Structural Control to Functional Modulation

Ligand-protected metal nanoclusters have drawn increasing research interest because of their unique physicochemical properties and practical applications. Great efforts have been made in pursuing rational synthesis of metal nanoclusters and establishing the structure-property relationships. As an indispensable part of ligand-protected metal nanoclusters, ligands play multiple roles in determining their structures and properties. In this perspective, we demonstrate the importance of ligand engineering in terms of the control of structures, optical and catalytic properties of metal nanoclusters. Furthermore, we will show that ligand engineering is prospective in structural design and preorganization of surface metal sites.     

Metal–Organic Gels from Silver Nanoclusters with Aggregation-Induced Emission and Fluorescence-to-Phosphorescence Switching

Luminescent metal nanoclusters (NCs) are emerging as a new class of functional materials that have rich physicochemical properties and wide potential applications. In recent years, it has been found that some metal NCs undergo aggregation-induced emission (AIE) and an interesting fluorescence-to-phosphorescence (F-P) switching in solutions. However, insights of both the AIE and the F-P switching remain largely unknown. Now, gelation of water soluble, atomically precise Ag₉ NCs is achieved by the addition of antisolvent. Self-assembly of Ag₉ NCs into entangled fibers was confirmed, during which AIE was observed together with an F-P switching occurring within a narrow time scale. Structural evaluation indicates the fibers are highly ordered. The self-assembly of Ag₉ NCs and their photoluminescent property are thermally reversible, making the metal–organic gels good candidates for luminescent ratiometric thermometers.