One-, Two-, and Three-Dimensional Self-Assembly of Atomically Precise Metal Nanoclusters

Multicolor photoluminescence of Cu14 clusters modulated using surface ligands

Copper nanoclusters exhibit unique structural features and their molecular assembly results in diverse photoluminescence properties. In this study, we present ligand-dependent multicolor luminescence observed in a Cu14 cluster, primarily protected by ortho-carborane-9,12-dithiol (o-CBDT), featuring an octahedral Cu6 inner kernel enveloped by eight isolated copper atoms. The outer layer of the metal kernel consists of six bidentate o-CBDT ligands, in which carborane backbones are connected through μ3-sulphide linkages. The initially prepared Cu14 cluster, solely protected by six o-CBDT ligands, did not crystallize in its native form. However, in the presence of N,N-dimethylformamide (DMF), the cluster crystallized along with six DMF molecules. Single-crystal X-ray diffraction (SCXRD) revealed that the DMF molecules were directly coordinated to six of the eight capping Cu atoms, while oxygen atoms were bound to the two remaining Cu apices in antipodal positions. Efficient tailoring of the cluster surface with DMF shifted its luminescence from yellow to bright red. Luminescence decay profiles showed fluorescence emission for these clusters, originating from the singlet states. Additionally, we synthesized microcrystalline fibers with a one-dimensional assembly of DMF-appended Cu14 clusters and bidentate DPPE linkers. These fibers exhibited bright greenish-yellow phosphorescence emission, originating from the triplet state, indicating the drastic surface tailoring effect of secondary ligands. Theoretical calculations provided insights into the electronic energy levels and associated electronic transitions for these clusters. This work demonstrated dynamic tuning of the emissive excited states of copper nanoclusters through the efficient engineering of ligands.

Recent progress in atomically precise silver nanocluster-assembled materials

In the dynamic landscape of nanotechnology, atomically precise silver nanoclusters (Ag NCs) have emerged as a novel and promising category of materials with their fascinating properties and enormous potential. However, recent research endeavors have surged towards stabilizing Ag-based NCs, leading to innovative strategies like connecting cluster nodes with organic linkers to construct hierarchical structures, thus forming Ag-based cluster-assembled materials (CAMs). This approach not only enhances structural stability, but also unveils unprecedented opportunities for CAMs, overcoming the limitations of individual Ag NCs. In this context, this review delves into the captivating realm of atomically precise nitrogen-based ligand bonded Ag(I)-based CAMs, providing insights into synthetic strategies, structure-property relationships, and diverse applications. We navigate the challenges and advancements in integrating Ag(I) cluster nodes, bound by argentophilic interactions, into highly connected periodic frameworks with different dimensionalities using nitrogen-based linkers. Despite the inherent diversity among cluster nodes, Ag(I) CAMs demonstrate promising potential in sensing, catalysis, bio-imaging, and device fabrication, which all are discussed in this review. Therefore, gaining insight into the silver nanocluster assembly process will offer valuable information, which can enlighten the readers on the design and advancement of Ag(I) CAMs for state-of-the-art applications.

Silver Cluster Assembled Materials: A Model-Driven Perspective on Recent Progress, with a Spotlight on Ag12 Cluster Assembly

The exploration of individual nanoclusters is rapidly advancing, despite stability concerns. To address this challenge, the assembly of cluster nodes through linker molecules has been successfully implemented. However, the linking of the cluster nodes itself introduces a multitude of possibilities, especially when additional factors come into play. While this method proves effective in enhancing material stability, the specific reasons behind its success remain elusive. In our laboratory, we have undertaken extensive studies on Ag cluster-assembled materials. So, here our goal is to establish a model system that allows for the discernment of various factors, eliminating unnecessary complexities during the linking approach. So, we hope that the systematic discourse presented in here will contribute significantly to future endeavors, helping to set clear priorities, and provide solutions to concerns that arise when working with a model system.

Diphosphine-Protected IrAu12 Superatom with Open Site(s): Synthesis and Programmed Stepwise Assembly

One or two phenylacetylide (PA) ligand(s) were successfully removed from the IrAu12 superatomic core of [IrAu12(dppe)5(PA)2]+ (dppe=1,2-bis(diphenylphosphino)ethane) by reaction with controlled amounts of tetrafluoroboric acid. Optical and nuclear magnetic resonance spectroscopies and density functional theory calculations revealed the formation of open Au site(s) on the IrAu12 core of [IrAu12(dppe)5(PA)1]2+ and [IrAu12(dppe)5]3+ with the remaining structure intact. Isocyanide was efficiently trapped at the open electrophilic site on [IrAu12(dppe)5(PA)1]2+, whereas a dimer or trimer of the IrAu12 superatoms was formed using diisocyanide as a linker. These results open the door to designed assembly of chemically modified metal superatoms.

A Silver Nanocluster Assembled by a Superatomic Building Unit

A unique assembly of a two-electron superatom, [Ag10{S2P(OiPr)2}8], as a primary building unit in the construction of a supramolecule [Ag10{S2P(OiPr)2}8]2(μ-4,4'-bpy) through a 4,4'-bipyridine (4,4'-bpy) linker is reported. This approach is facilitated by an open site in the structure that allows for effective pairing. The assembled structure demonstrates a minimal solvatochromic shift across organic solvents with variable polarities, highlighting the influence of self-assembly on the photophysical properties of silver nanoclusters.

Application of gold nanoclusters in fluorescence sensing and biological detection

Gold nanoclusters (Au NCs) exhibit broad fluorescent spectra from visible to near-infrared regions and good enzyme-mimicking catalytic activities. Combined with excellent stability and exceptional biocompatibility, the Au NCs have been widely exploited in biomedicine such as biocatalysis and bioimaging. Especially, the long fluorescence lifetime and large Stokes shift attribute Au NCs to good probes for fluorescence sensing and biological detection. In this review, we systematically summarized the molecular structure and fluorescence properties of Au NCs and highlighted the advances in fluorescence sensing and biological detection. The Au NCs display high sensitivity and specificity in detecting iodine ions, metal ions, and reactive oxygen species, as well as certain diseases based on the fluorescence activities of Au NCs. We also proposed several points to improve the practicability and accelerate the clinical translation of the Au NCs.

Controlled Sequential Doping of Metal Nanocluster

Atomically precise doping of metal nanoclusters provides excellent opportunities not only for subtly tailoring their properties but also for in-depth understanding of composition (structure)-property correlation of metal nanoclusters and has attracted increasing interest partly due to its significance for fundamental research and practical applications. Although single and multiple metal atom doping of metal nanoclusters (NCs) has been achieved, sequential single-to-multiple metal atom doping is still a big challenge and has not yet been reported. Herein, by introducing a second ligand, a novel multistep synthesis method was developed, controlled sequential single-to-multiple metal atom doping was successfully achieved for the first time, and three doped NCs Au25Cd1(p-MBT)17(PPh3)2, Au18Cd2(p-MBT)14(PPh3)2, and [Au19Cd3(p-MBT)18]- (p-MBTH: para-methylbenzenethiol) were obtained, including two novel NCs that were precisely characterized via mass spectrometry, single-crystal X-ray crystallography, and so forth. Furthermore, sequential doping-induced evolutions in the atomic and crystallographic structures and optical and catalytic properties of NCs were revealed.

Modulation of Singlet-Triplet Gap in Atomically Precise Silver Cluster-Assembled Material

Silver cluster-based solids have garnered considerable attention owing to their tunable luminescence behavior. While surface modification has enabled the construction of stable silver clusters, controlling interactions among clusters at the molecular level has been challenging due to their tendency to aggregate. Judicious choice of stabilizing ligands becomes pivotal in crafting a desired assembly. However, detailed photophysical behavior as a function of their cluster packing remained unexplored. Here, we modulate the packing pattern of Ag12 clusters by varying the nitrogen-based ligand. CAM-1 formed through coordination of the tritopic linker molecule and NC-1 with monodentate pyridine ligand; established via non-covalent interactions. Both the assemblies show ligand-to-metal-metal charge transfer (LMMCT) based cluster-centered emission band(s). Temperature-dependent photoluminescence spectra exhibit blue shifts at higher temperatures, which is attributed to the extent of the thermal reverse population of the S1 state from the closely spaced T1 state. The difference in the energy gap (ΔEST ) dictated by their assemblies played a pivotal role in the way that Ag12 cluster assembly in CAM-1 manifests a wider ΔEST and thus requires higher temperatures for reverse intersystem crossing (RISC) than assembly of NC-1. Such assembly-defined photoluminescence properties underscore the potential toolkit to design new cluster- assemblies with tailored optoelectronic properties.

Photoluminescence Properties of a Chiral One-Dimensional Silver Chalcogenolate Chain

Atom-precise metal nanoclusters, which contain a few tens to hundreds of atoms, have drawn significant interest due to their interesting physicochemical properties. Structural analysis reveals a fundamental architecture characterized by a central core or kernel linked to a staple motif with metal-ligand bonding playing a pivotal role. Ligands not only protect the surface but also exert a significant influence in determining the overall assembly of the larger superstructures. The assemblies of nanoclusters are driven by weak interaction between the ligand molecules; it also depends on the ligand type and functional group present. Here, we report an achiral ligand and Ag(I)···Ag(I) interaction-driven spontaneous resolution of silver-thiolate structure, [Ag18(C6H11S)12(CF3COO)6(DMA)2], where silver atoms and cyclohexanethiolate are connected to form a one-dimensional chain with helicity. Notably, silver atoms adopt different types of coordination modes and geometries. The photoluminescence properties of the one-dimensional (1D) chain structure were investigated, and it was found to exhibit excitation-dependent emission properties attributed to hydrogen-bonding interactions. Experimental and theoretical investigations corroborate the presence of triplet-emitting ligand-to-metal charge-transfer transitions.

Size and charge effects of metal nanoclusters on antibacterial mechanisms

Nanomaterials, specifically metal nanoclusters (NCs), are gaining attention as a promising class of antibacterial agents. Metal NCs exhibit antibacterial properties due to their ultrasmall size, extensive surface area, and well-controlled surface ligands. The antibacterial mechanisms of metal NCs are influenced by two primary factors: size and surface charge. In this review, we summarize the impacts of size and surface charge of metal NCs on the antibacterial mechanisms, their interactions with bacteria, and the factors that influence their antibacterial effects against both gram-negative and gram-positive bacteria. Additionally, we highlight the mechanisms that occur when NCs are negatively or positively charged, and provide examples of their applications as antibacterial agents. A better understanding of relationships between antibacterial activity and the properties of metal NCs will aid in the design and synthesis of nanomaterials for the development of effective antibacterial agents against bacterial infections. Based on the remarkable achievements in the design of metal NCs, this review also presents conclusions on current challenges and future perspectives of metal NCs for both fundamental investigations and practical antibacterial applications.

A new two-dimensional luminescent Ag12 cluster-assembled material and its catalytic activity for reduction of hexacyanoferrate(III)

Silver cluster-assembled materials (SCAMs) have garnered significant interest as promising platforms for different functional explorations. Their atomically precise structures, intriguing chemical/physical properties, and remarkable luminescent capabilities make them highly appealing. However, the properties of these materials are primarily determined by their structural architecture, which is heavily influenced by the linker molecules used in their assembly. The choice of linker molecules plays a pivotal role in shaping the structural characteristics and ultimately determining the unique properties of SCAMs. To this end, the first SCAM with an intriguing (3,6)-connected kgd topology, [Ag12(StBu)6(CF3COO)6(TPBTC)6]n (termed TUS 3), TPBTC = benzene-1,3,5-tricarboxylic acid tris-pyridin-4-ylamide, has been synthesized by reticulating C6-symmetric Ag12 cluster cores with C3-symmetric tripodal pyridine linkers. Due to the structutural architecture of the linker molecule, TUS 3 posseses a luminescent porous framework structure where each two-dimensional (2D) layers are non-covalently linked with each other to form a three dimensional (3D) framework and ultimately offers uniaxial open channels. The compact mesoporous structural architecture not only gives the excellent surface area but also offers impressive stability of this material even in water medium. Taking advantage of these properties, TUS 3 shows brilliant catalytic activity in the reduction of hexacyanoferrate(III) using sodium borohydride in aqueous solutions.

Two-Dimensional Silver-Chalcogenolate-Based Cluster-Assembled Material: A p-type Semiconductor

We have synthesized and characterized a new two-dimensional honeycomb architecture resembling a single-layer of atomically precise silver cluster-assembled material (CAM), [Ag12(StBu)6(CF3COO)6(4,4'-azopyridine)3] (Ag12-azo-bpy). The interlayer noncovalent van der Waals interactions within the single-crystals were successfully disrupted, leading to the creation of this unique structure. The optimized Ag12-azo-bpy CAM demonstrates a valence band that is localized on the Ag12 cluster node situated near the Fermi energy level. This localization induces electron injection from the linker to the cluster node, facilitating efficient charge transportation along the plane. Exploiting this single-layer structure as a distinctive platform for p-type channel material, it was employed in a field-effect transistor configuration. Remarkably, the transistor exhibits a high hole mobility of 1.215 cm2 V-1 s-1 and an impressive ON/OFF current ratio of ∼4500 at room-temperature.

Five Novel Silver-Based Coordination Polymers as Photoluminescent Sensing Platforms for the Detection of Nitrobenzene

Nitrobenzene (NB) is a highly toxic chemical and a cause for concern to human health and the environment. Hence, it is worth designing new efficient and robust sensing platforms for NB. In this study, we present three newly synthesized luminescent silver cluster-based coordination polymers, {[Ag10 (StBu)6 (CF3 COO)4 (hpbt)] (DMAc)2 (CH3 CN)2 }n (hpbt=N,N,N',N'N",N"-hexa(pyridine-4-yl)benzene-1,3,5-triamine), [Ag12 (StBu)6 (CF3 COO)6 (bpva)3 ]n (bpva=9,10-Bis(2-(pyridin-4-yl)vinyl)anthracene), and {[Ag12 (StBu)6 (CF3 COO)6 (bpb)(DMAc)2 (H2 O)2 ] (DMAc)2 }n (bpb=1,4-Bis(4-pyridyl)benzene) composed of Ag10 , Ag12 and Ag12 cluster cores, respectively, connected by multidentate pyridine linkers. In addition, two new luminescent polymorphic silver(I)-based coordination polymers, [Ag(CF3 COO)(dpa)]n (dpa=9,10-di(4-pyridyl)anthracene) referred to as Agdpa (H) and Agdpa (R), where H and R denote hexagon- and rod-like crystal shapes, respectively, have been prepared. The coordination polymers exhibit highly sensitive luminescence quenching effects to NB, attributed to the π-π stacking interactions between the polymers and NB as well as the electron-withdrawing character of NB.

Synthesis and luminescence properties of two silver cluster-assembled materials for selective Fe3+ sensing

Silver cluster-assembled materials (SCAMs) are emerging light-emitting materials with molecular-level structural designability and unique photophysical properties. Nevertheless, the widespread application scope of these materials is severely curtailed by their dissimilar structural architecture upon immersing in different solvent media. In this work, we report the designed synthesis of two unprecedented (4.6)-connected three-dimensional (3D) luminescent SCAMs, [Ag₁₂(S^tBu)₆(CF₃COO)₆(TPEPE)₆]_n (denoted as TUS 1), TPEPE = 1,1,2,2-tetrakis(4-(pyridin-4-ylethynyl)phenyl)ethene and [Ag₁₂(S^tBu)₆(CF₃COO)₆(TPVPE)₆]_n (denoted as TUS 2), TPVPE = 1,1,2,2-tetrakis(4-((E)-2-(pyridin-4-yl)vinyl)phenyl)ethene, composed of an Ag₁₂ cluster core connected by quadridentate pyridine linkers. Attributed to their exceptional fluorescence properties with absolute quantum yield (QY) up to 9.7% and excellent chemical stability in a wide range of solvent polarity, a highly sensitive assay for detecting Fe³⁺ in aqueous medium is developed with promising detection limits of 0.05 and 0.86 nM L^-1 for TUS 1 and TUS 2 respectively, comparable to the standard. Furthermore, the competency of these materials to detect Fe³⁺ in real water samples reveals their potential application in environmental monitoring and assessment.

Luminescent [CO2@Ag20(SAdm)10(CF3COO)10(DMA)2] nanocluster: synthetic strategy and its implication towards white light emission

Owing to the quantized size and associated discrete energy levels, atomically precise silver nanoclusters (Ag NCs) hold great potential for designing functional luminescent materials. However, the thermally activated non-radiative transition of Ag(I)-based NCs has faded the opportunities. To acquire the structurally rigid architecture of cluster nodes for constraining such transitions, a new synthetic approach is unveiled here that utilizes a neutral template as a cluster-directing agent to assemble twenty Ag(I) atoms that ensure the maximum number of surface-protecting ligand attachment possibilities in a particular solvent medium. The solvent polarity triggers the precise structural design to circumvent the over-reliance of the templates, which results in the formation of [CO₂@Ag₂₀(SAdm)₁₀(CF₃COO)₁₀(DMA)₂] NC (where SAdm = 1-adamantanethiolate and DMA = N,N-dimethylacetamide) exhibiting an unprecedented room-temperature photoluminescence emission. The high quantum yield of the generated blue emission ensures its candidature as an ideal donor for artificial light-harvesting system design, and it is utilized with the two-step sequential energy transfer process, which finally results in the generation of ideal white light. For implementing perfect white light emission, the required chromophores in the green and red emission regions were chosen based on their effective spectral overlap with the donor components. Due to their favorable energy-level distribution, excited state energy transfers occurred from the NC to β-carotene at the initial step, then from the conjugate of the NC and β-carotene to another chromophore, Nile Blue, at the second step via a sequential Förster resonance energy transfer pathway.

Bifunctional Supertetrahedral Chalcogenolate Cluster-Based Assembly Materials Constructed by a Photoactive Ligand

The assembly of supertetrahedral chalcogenolate clusters (SCCs) and multifunctional organic linkers could lead to the formation of tunable structures and synergistic properties. Two SCC-based assembled materials (SCCAM-1 and -2) constructed by a triangular chromophore ligand, tris(4-pyridylphenyl)amine, were successfully synthesized and characterized. The SCCAMs demonstrate unusually long-lived afterglow at low temperatures (83 K) and efficient activities for the photocatalytic degradation of organic dye in water.

Silver cluster-assembled materials for label-free DNA detection

Herein, we report two newly synthesized silver cluster-assembled materials (SCAMs), [Ag₁₄(S^tBu)₁₀(CF₃COO)₄(bpa)₂]_n (bpa = 1,2-bis(4-pyridyl)acetylene) and [Ag₁₂(S^tBu)₆(CF₃COO)₆(bpeb)₃]_n (bpeb = 1,4-bis(pyridin-4-ylethynyl)benzene) composed of Ag₁₄ and Ag₁₂ chalcogenolate cluster cores, respectively, bridged by acetylenic bispyridine linkers. The linker structures and electrostatic interaction between positively charged SCAMs and negatively charged DNA confer the SCAMs with the ability to suppress the high background fluorescence of single-stranded (ss) DNA probes with SYBR Green I nucleic acid stain, leading to high signal-to-noise ratio for label-free target DNA detection.

A novel fluorescent sensor array to identify Baijiu based on the single gold nanocluster probe

Baijiu occupies a vital position in Chinese food and China's market. Strict evaluation of Baijiu is highly demanded. In this study, we constructed a novel fluorescent sensor array based on the single glutathione-protected gold nanoclusters (AuNCs) probe for the detection of organic acids and Baijiu. The fluorescence of AuNCs was simply modulated by three metal ions (Cu²⁺, Mn²⁺, and Ag⁺), and formed new complexes as sensing elements. These four sensing elements responded variously to nine organic acids, and further chemometric analysis results allowed for the classification and quantification of acids. Moreover, the sensor array successfully identified 21 Baijiu samples of different brands among 11 aroma types. It could also distinguish Baijiu of different qualities as well as pure Baijiu from its adulterations and showed high selectivity among multiple interfering drinks. The results demonstrated that the AuNCs-based sensor array has considerable potential for quality monitoring of Baijiu.

Self-Assembly of Silver Clusters into One- and Two-Dimensional Structures and Highly Selective Methanol Sensing

The development of new materials for the design of sensitive and responsive sensors has become a crucial research direction. Here, two silver cluster-based polymers (Ag-CBPs), including one-dimensional {[Ag22(L1)8(CF3CO2)14](CH3OH)2} n chain and two-dimensional {[Ag12(L2)2(CO2CF3)14(H2O)4(AgCO2CF3)4](HNEt3)2} n film, are designed and used to simulate the human nose, an elegant sensor to smells, to distinguish organic solvents. We study the relationship between the atomic structures of Ag-CBPs determined by x-ray diffraction and the electrical properties in the presence of organic solvents (e.g., methanol and ethanol). The ligands, cations, and the ligated solvent molecules not only play an important role in the self-assembly process of Ag-CBP materials but also determine their physiochemical properties such as the sensing functionality.

Fabricating sub-nanometer materials through cluster assembly

The self-assembly of clusters provides a feasible approach for the bottom-up fabrication of functional materials with tailored properties. Sub-nanometer cluster assembly with a well-defined construction presents a precisely controllable structure and extraordinary properties, which provides an ideal model for the investigation of structures and properties at the molecular level. Non-covalent interactions between clusters may dominate the assembly behavior, appearing as tunable structures different from their nano-counterparts. Interactions between clusters and their superatom orbitals can significantly influence the electronic structures, because of which exceptional properties may emerge. In this paper, recent progress on cluster-based assemblies is introduced, including sub-nanometer building blocks of noble metal and polyoxometalate (POM) clusters. The structures, formation mechanism and properties of these cluster assemblies are discussed from experimental and theoretical aspects. This perspective aims to provide a new insight into the design and manufacture of sub-nanometer materials based on clusters.

Depletion Driven Assembly of Ultrasmall Metal Nanoclusters: From Kinetically Arrested Assemblies to Thermodynamically Stable, Spherical Superclusters

Nanoscale assembly of ultrasmall metal nanoclusters (MNCs) by means of molecular forces has proven to be a powerful strategy to engineer their molecule-like properties in multiscale dimensions. By leveraging depletion attraction as the guiding force, herein, we demonstrate the formation of kinetically trapped NCs assemblies with enhanced photoluminescence (PL) and excited state lifetimes and extend the principle to cluster impregnated cationic nanogels, nonluminescent Au(I)-thiolate complexes, and weakly luminescent CuNCs. We further demonstrate a thermal energy driven kinetic barrier breaking process to isolate these assemblies. These isolated assemblies are thermodynamically stable, built from a strong network among several discrete, ultrasmall AuNCs and exhibit several unusual properties such as high stability in various pH, strong PL, microsecond lifetimes, large Stocks shifts, and higher accumulation in the lysosome of cancer cells. We anticipate our strategy may find wider use in creating a large variety of MNC-based assemblies with many unforeseen arrangements, properties, and applications.

Versatile Superatom Complex Nanocluster for the Construction of Framework Materials

Accurately controlling the assembly of nanometer-sized building blocks presents an important but significant challenge for the construction of functional framework materials, which requires the development of highly stable versatile nanosized assembly modules with multiple coordination sites. In this study, [Ag₂₃(SAdm)₁₂]³⁺ (Ag₂₃, in which SAdm = 1-adamantanethiol, i.e., C₁₀H₁₅S), a chiral superatom complex nanocluster, was synthesized and assembled into various topologies. We constructed two kinds of framework materials, i.e., superatom complex inorganic framework (SCIF) and superatom complex organic framework (SCOF) materials, including [Ag₂₃(SAdm)₁₂](SbF₆)₂X (Ag₂₃-1; X = Cl^-/SbF₆^-, a SCIF), [Ag₂₃(SAdm)₁₂](SbF₆)₃ (Ag₂₃-2, a SCIF), [Ag₂₃(SAdm)₁₂](SbF₆)₃(bpy)₃ (Ag₂₃-bpy, a SCOF, in which bpy = 4,4'-bipyridine, i.e., C₁₀H₈N₂), and [Ag₂₃(SAdm)₁₂](SbF₆)₃(dpbz)₃ (Ag₂₃-dpbz, a SCOF, in which dpbz = 1,4-bis(4-pyridyl)benzene, i.e., C₁₆H₁₂N₂), owing to strong interactions between the versatile Ag₂₃ and the inorganic and organic linkers. Ag₂₃-1, Ag₂₃-2, and Ag₂₃-bpy exhibit two superstructures with interpenetrating frameworks and adamantane-like, hexagonal, and cubic topologies, while Ag₂₃-dpbz displays three superstructures with interpenetrating frameworks and cubic topologies. Ag₂₃-dpbz exhibits the largest specific surface area as well as the strongest photoluminescence and electrochemiluminescence signals owing to its dense network arrangement. This work contributes to the construction of nanocluster-based framework materials and helps to elucidate the effect of the assembly mode on the material properties and functionalities.

Insight into the Mechanism of Single-Metal-Atom Tailoring on the Surface of Au-Cu Alloy Nanoclusters

Tailoring the surface structure of nanomaterials is desirable for investigating their mechanisms and properties from a nanochemistry perspective. The modification of the surface of metal nanoparticles with a single metal atom has proven difficult, which has hindered the understanding of the contribution of different motifs in nanoclusters to their properties. Herein, we report single-metal-atom surface tailoring by thermally etching the nanocluster Au_xCu_15-x(DPPMH)₃(SPhCl₂)₉ (x = 8 or 9) to obtain Au_xCu_16-x(DPPMH)₂(DPPM)(SPhCl₂)₉ (x = 9 or 10) nanoclusters. An Au₇Cu₄ core was observed in both nanoclusters, which can be regarded as part of an icosahedron. Experiments and theoretical simulations revealed the tailoring processes of the icosahedron. Both nanoclusters displayed an NIR-II emission, and the introduction of the surface metal atom led to a red-shift in the emission band from 983 to 1025 nm. This work contributes to the development of precisely tailored nanocluster structures and provides an understanding of structure-property correlations.

Self-Assembled Metal Nanoclusters: Driving Forces and Structural Correlation with Optical Properties

Studies on self-assembly of metal nanoclusters (MNCs) are an emerging field of research owing to their significant optical properties and potential applications in many areas. Fabricating the desired self-assembly structure for specific implementation has always been challenging in nanotechnology. The building blocks organize themselves into a hierarchical structure with a high order of directional control in the self-assembly process. An overview of the recent achievements in the self-assembly chemistry of MNCs is summarized in this review article. Here, we investigate the underlying mechanism for the self-assembly structures, and analysis reveals that van der Waals forces, electrostatic interaction, metallophilic interaction, and amphiphilicity are the crucial parameters. In addition, we discuss the principles of template-mediated interaction and the effect of external stimuli on assembly formation in detail. We also focus on the structural correlation of the assemblies with their photophysical properties. A deep perception of the self-assembly mechanism and the degree of interactions on the excited state dynamics is provided for the future synthesis of customizable MNCs with promising applications.

Development and Functionalization of Visible-Light-Driven Water-Splitting Photocatalysts

With global warming and the depletion of fossil resources, our fossil fuel-dependent society is expected to shift to one that instead uses hydrogen (H2) as a clean and renewable energy. To realize this, the photocatalytic water-splitting reaction, which produces H2 from water and solar energy through photocatalysis, has attracted much attention. However, for practical use, the functionality of water-splitting photocatalysts must be further improved to efficiently absorb visible (Vis) light, which accounts for the majority of sunlight. Considering the mechanism of water-splitting photocatalysis, researchers in the various fields must be employed in this type of study to achieve this. However, for researchers in fields other than catalytic chemistry, ceramic (semiconductor) materials chemistry, and electrochemistry to participate in this field, new reviews that summarize previous reports on water-splitting photocatalysis seem to be needed. Therefore, in this review, we summarize recent studies on the development and functionalization of Vis-light-driven water-splitting photocatalysts. Through this summary, we aim to share current technology and future challenges with readers in the various fields and help expedite the practical application of Vis-light-driven water-splitting photocatalysts.

Metal-nanocluster Science and Technology: My Personal History and Outlook

Metal nanoclusters (NCs) are among the leading targets in research of nanoscale materials, and elucidation of their properties (science) and development of control techniques (technology) have been continuously studied for...

Qualitative and Quantitative Analysis of Tumor Cell Invasion Using Au Clusters

Tumor invasion/metastasis is still the major cause of death in cancer patients. Membrane type-1 matrix metalloproteinase (MT1-MMP) is directly related to tumor invasion/metastasis. To accurately and quickly distinguish the risk of invasion/metastasis of primary tumor cells, it is urgent to develop a simple and precise quantitative method to distinguish the expression level of MT1-MMP. In this work, we have constructed red fluorescent Au clusters with peroxidase-like properties that could specifically bind to MT1-MMP on human cervical cancer cells. After MT1-MMP was labelled with Au clusters, we could visually see red fluorescence of MT1-MMP on cervical cancer cells via fluorescence microscopy and catalytic color imaging using an ordinary optical microscope. The constructed Au clusters contained 26 Au atoms; thus, the amount of MT1-MMP on cervical cancer cells could be accurately quantified using inductively coupled plasma mass spectrometry (ICP-MS). More importantly, the invasion/metastasis capabilities of the cervical cancer Siha, Caski and Hela cells with different MT1-MMP amounts could be accurately distinguished by fluorescence/catalysis qualitative imaging and ICP-MS quantitative analysis. This method of qualitative/quantitative analysis of tumor-associated proteins on cancer cells has great potential for accurately diagnosing aggressive tumor cells and assessment of their invasion/metastasis risk.

Controlling the Chemistry of Nanoclusters: From Atomic Precision to Controlled Assembly

Metal nanoclusters have gained prominence in nanomaterials sciences, owing to their atomic precision, structural regularity, and unique chemical composition. Additionally, the ligands stabilizing the clusters provide great opportunities for linking the clusters in higher order dimensions, eventually leading to the formation of a repertoire of nanoarchitectures. This makes the chemistry of atomic clusters worth exploring. In this mini review, we aim to focus on the chemistry of nanoclusters. Firstly, we summarize the important strategies developed so far for the synthesis of atomic clusters. For each synthetic strategy, we highlight the chemistry governing the formation of nanoclusters. Next, we discuss the key techniques in the purification and separation of nanoclusters, as the chemical purity of clusters is deemed important for their further chemical processing. Thereafter which we provide an account of the chemical reactions of nanoclusters. Then, we summarize the chemical routes to the spatial organization of atomic clusters, highlighting the importance of assembly formation from an application point of view. Finally, we raise some fundamentally important questions with regard to the chemistry of atomic clusters, which, if addressed, may broaden the scope of research pertaining to atomic clusters.

Solvent-Dependent Photophysical Properties of a Semiconducting One-Dimensional Silver Cluster-Assembled Material

Unraveling the total structure of the atom-precise silver cluster-assembled materials (CAMs) is extremely significant to elucidating the structure-property correlation, but it is a very challenging task. Herein, a new silver CAM is synthesized by a facile synthetic pathway with a unique distorted elongated square-bipyramid-based Ag₁₁ core geometry. The core is protected by two different kinds of the surface protecting ligands (adamantanethiolate and trifluoroacetate) and connected through a bidentate organic linker. The crystallographic data show that this material embraces a one-dimensional periodic structure that orchestrates by various noncovalent interactions to build a thermally stable supramolecular assembly. Further characterization confirms its n-type semiconducting property with an optical band gap of 1.98 eV. The impact of an adamantanethiol-protected silver core on the optical properties of this type of periodic framework is analyzed by the UV-vis absorbance and emission phenomena. Theoretical calculations predicted that the occupied states are majorly contributed by Ag-S. Solvent-dependent photoluminescence studies proved that a polar solvent can significantly perturb the metal thiolate and thiolate-centered frontier molecular orbitals that are involved in the electronic transitions.

Ensembles from silver clusters and cucurbit[6]uril-containing linkers

In this work, organic supramolecular linkers involving cucubit[6]urils CB[6] and N,N'-hexamethylene-bis(pyrazinyl hexafluorophosphate) (BPHF@CB[6]) were utilized to assemble dodenuclear silver chalcogenolate clusters into three one-dimensional (1D) materials under different synthesis conditions. These three crystal structures of CB[6]-based sliver cluster-organic rotaxane frameworks were well resolved, and their emission properties were investigated systematically. This construction strategy involving organic supramolecular linkers gives a new methodology for cluster-assembled materials with intriguing structural and functional properties.

Novel class of water-soluble phosphonate silver cluster assembled material for efficient photoelectric sensing and photoacoustic imaging

Owing to the atomic precession and exotic photophysical properties, silver cluster assembled materials (CAMs) have been explored for use as functional nanomaterials in recent years. Although a small number of thiolate protected silver CAMs have previously been investigated, the synthesis of thiol-free analogues and their solubility remain challenging. Here, the structure-property correlation of a newly synthesized one-dimensional phenyl phosphonate protected [Ag₂(PhPO₃H)₂(apy)₂], (in which, 4,4'-azopyridine = apy) CAM is demonstrated. The multifunctional surface protecting ligand is strategically attached to the core for the first time to tailor the solubility, structural stability and charge transfer mechanism. The small size of the cluster building blocks, along with the choice of organic linker molecules, efficiently stabilize the structure via intra-chain π-π stacking while inter-chains π-π interactions create a two-dimensional supramolecular architecture. The advantageous band structure associated with the charge transfer phenomenon and the high structural stability of the material are guided to explore the sustainable photoresponsive character of this CAM, resulting in the generation of an 82 nA photocurrent. Additionally, the unprecedented water solubility, which is very rare for this class of material, provides opportunities for use in biomedical imaging applications. The measured photoacoustic signal strength confirms the blood vessel mimicking capabilities of the portrayed material at a depth of approximately 3 mm inside chicken breast tissue.

[Ag23Pd2(PPh3)10Cl7]0: A new family of synthesizable bi-icosahedral superatomic molecules

Icosahedral noble-metal 13-atom nanoclusters (NCs) can form connected structures, which can be regarded as superatomic molecules, by vertex sharing. However, there have been very few reports on the superatomic molecules formed using silver (Ag) as the base element. In this study, we synthesized [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ (Pd = palladium, PPh₃ = triphenylphosphine, Cl = chloride), in which two icosahedral 13-atom NCs are connected, and elucidated its geometric and electronic structures to clarify what type of superatomic molecules can be synthesized. The results revealed that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ is a synthesizable superatomic molecule. Single crystal x-ray diffraction analysis showed that the metal-metal distances in and between the icosahedral structures of [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ are slightly shorter than those of previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰, whereas the metal-PPh₃ distances are slightly longer. On the basis of several experiments and density functional theory calculations, we concluded that [Ag₂₃Pd₂(PPh₃)₁₀Cl₇]⁰ and previously reported [Ag₂₃Pt₂(PPh₃)₁₀Cl₇]⁰ are more stable than [Ag₂₅(PPh₃)₁₀Cl₇]²⁺ because of their stronger superatomic frameworks (metal cores). These findings are expected to lead to clear design guidelines for creation of new superatomic molecules.

Toward Controlling the Electronic Structures of Chemically Modified Superatoms of Gold and Silver

Atomically precise gold/silver clusters protected by organic ligands L, [(Au/Ag)_x L_y ]^z , have gained increasing interest as building units of functional materials because of their novel photophysical and physicochemical properties. The properties of [(Au/Ag)_x L_y ]^z are intimately associated with the quantized electronic structures of the metallic cores, which can be viewed as superatoms from the analogy of naked Au/Ag clusters. Thus, establishment of the correlation between the geometric and electronic structures of the superatomic cores is crucial for rational design and improvement of the properties of [(Au/Ag)_x L_y ]^z . This review article aims to provide a qualitative understanding on how the electronic structures of [(Au/Ag)_x L_y ]^z are affected by geometric structures of the superatomic cores with a focus on three factors: size, shape, and composition, on the basis of single-crystal X-ray diffraction data. The knowledge accumulated here will constitute a basis for the development of ligand-protected Au/Ag clusters as new artificial elements on a nanometer scale.

Thiolate-Protected Metal Nanoclusters: Recent Development in Synthesis, Understanding of Reaction, and Application in Energy and Environmental Field

Metal nanoclusters (NCs), which are composed of about 250 or fewer metal atoms, possess great potential as novel functional materials. Fundamental research on metal NCs gradually started in the 1960s, and since 2000, thiolate (SR)-protected metal NCs have been the main metal NCs actively studied. The precise and systematic isolation of SR-protected metal NCs has been achieved in 2005. Since then, research on SR-protected metal NCs for both basic science and practical application has rapidly expanded. This review describes this recent progress in the field of SR-protected metal NCs in three areas: synthesis, understanding, and application. Specifically, the recent study of alloy NCs and connected structures composed of NCs is highlighted in the "synthesis" section, recent knowledge on the reactivity of NCs in solution is highlighted in the "understanding" section, and the applications of NCs in the energy and environmental field are highlighted in the "application" section. This review provides insight on the current state of research on SR-protected metal NCs and discusses the challenges to be overcome for further development in this field as well as the possibilities that these materials can contribute to solving the problems facing modern society.

Toward the creation of high-performance heterogeneous catalysts by controlled ligand desorption from atomically precise metal nanoclusters

Ligand-protected metal nanoclusters controlled by atomic accuracy (i. e. atomically precise metal NCs) have recently attracted considerable attention as active sites in heterogeneous catalysts. Using these atomically precise metal NCs, it becomes possible to create novel heterogeneous catalysts based on a size-specific electronic/geometrical structure of metal NCs and understand the mechanism of the catalytic reaction easily. However, to create high-performance heterogeneous catalysts using atomically precise metal NCs, it is often necessary to remove the ligands from the metal NCs. This review summarizes previous studies on the creation of heterogeneous catalysts using atomically precise metal NCs while focusing on the calcination as a ligand-elimination method. Through this summary, we intend to share state-of-art techniques and knowledge on (1) experimental conditions suitable for creating high-performance heterogeneous catalysts (e.g., support type, metal NC type, ligand type, and calcination temperature), (2) the mechanism of calcination, and (3) the mechanism of catalytic reaction over the created heterogeneous catalyst. We also discuss (4) issues that should be addressed in the future toward the creation of high-performance heterogeneous catalysts using atomically precise metal NCs. The knowledge and issues described in this review are expected to lead to clear design guidelines for the creation of novel heterogeneous catalysts.

Unconventional-Phase Crystalline Materials Constructed from Multiscale Building Blocks

Crystal phase, an intrinsic characteristic of crystalline materials, is one of the key parameters to determine their physicochemical properties. Recently, great progress has been made in the synthesis of nanomaterials with unconventional phases that are different from their thermodynamically stable bulk counterparts via various synthetic methods. A nanocrystalline material can also be viewed as an assembly of atoms with long-range order. When larger entities, such as nanoclusters, nanoparticles, and microparticles, are used as building blocks, supercrystalline materials with rich phases are obtained, some of which even have no analogues in the atomic and molecular crystals. The unconventional phases of nanocrystalline and supercrystalline materials endow them with distinctive properties as compared to their conventional counterparts. This Review highlights the state-of-the-art progress of nanocrystalline and supercrystalline materials with unconventional phases constructed from multiscale building blocks, including atoms, nanoclusters, spherical and anisotropic nanoparticles, and microparticles. Emerging strategies for engineering their crystal phases are introduced, with highlights on the governing parameters that are essential for the formation of unconventional phases. Phase-dependent properties and applications of nanocrystalline and supercrystalline materials are summarized. Finally, major challenges and opportunities in future research directions are proposed.

Controlling the Crystallographic Packing Modes of Pt1Ag28 Nanoclusters: Effects on the Optical Properties and Nitrogen Adsorption-Desorption Performances

We herein report the manipulation of the crystallographic packing modes of Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ nanoclusters by altering counterions as different polyoxometalates (POMs). Specifically, the Cl^- anion of the presynthesized Pt₁Ag₂₈ nanocluster was substituted by POM anions including [Mo₆O₁₉]^2-, [W₆O₁₉]^2-, or [PW₁₂O₄₀]^3-. The crystal lattices of these Pt₁Ag₂₈ nanoclusters with diverse anions showed distinct packing modes and thus manifested remarkably distinguishable crystalline-state optical properties and nitrogen adsorption-desorption performances. Overall, the combination of intercluster control in this work and intracluster control reported previously (the control over metal-ligand within the nanocluster framework) accomplished a more comprehensive manipulation over the M₂₉(SR)₁₈(PR'₃)₄ nanocluster system, which enables us to further grasp the structure-property correlations at the atomic level.

Four orders-of-magnitude enhancement in the two-photon excited photoluminescence of homoleptic gold thiolate nanoclusters following zinc ion-induced aggregation

Herein we report unprecedented determination of the molar mass of zinc mediated assemblies of homoleptic gold nanoclusters, based on charge detection mass spectrometry measurements. The accurate determination of the molar mass of zinc coordinated assemblies of gold clusters has further allowed unambiguous determination of the two-photon excited photoluminescence cross section of the same. Furthermore, in line with one-photon excited photoluminescence measurements, four orders-of-magnitude enhancement in two-photon excited photoluminescence of gold nanoclusters has been observed following complexation with zinc ions. The study reported herein is envisioned to not only deepen the fundamental understanding of the multiphoton excitation properties of atomic clusters but also widen their application potential as luminescence markers.

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.

Creation of active water-splitting photocatalysts by controlling cocatalysts using atomically precise metal nanoclusters

With global warming and the depletion of fossil resources, our fossil-fuel-dependent society is expected to shift to one that instead uses hydrogen (H₂) as clean and renewable energy. Water-splitting photocatalysts can produce H₂ from water using sunlight, which are almost infinite on the earth. However, further improvements are indispensable to enable their practical application. To improve the efficiency of the photocatalytic water-splitting reaction, in addition to improving the semiconductor photocatalyst, it is extremely effective to improve the cocatalysts (loaded metal nanoclusters, NCs) that enable the reaction to proceed on the photocatalysts. We have thus attempted to strictly control metal NCs on photocatalysts by introducing the precise-control techniques of metal NCs established in the metal NC field into research on water-splitting photocatalysts. Specifically, the cocatalysts on the photocatalysts were controlled by adsorbing atomically precise metal NCs on the photocatalysts and then removing the protective ligands by calcination. This work has led to several findings on the electronic/geometrical structures of the loaded metal NCs, the correlation between the types of loaded metal NCs and the water-splitting activity, and the methods for producing high water-splitting activity. We expect that the obtained knowledge will lead to clear design guidelines for the creation of practical water-splitting photocatalysts and thereby contribute to the construction of a hydrogen-energy society.

Ligand-protected gold/silver superatoms: current status and emerging trends

Monolayer-protected gold/silver clusters have attracted much interest as nano-scale building units for novel functional materials owing to their nonbulk-like structures and size-specific properties. They can be viewed as ligand-protected superatoms because their magic stabilities and fundamental properties are well explained in the framework of the jellium model. In the last decade, the number of ligand-protected superatoms with atomically-defined structures has been increasing rapidly thanks to the well-established synthesis and structural determination by X-ray crystallography. This perspective summarizes the current status and emerging trends in synthesis and characterization of superatoms. The topics related to synthesis include (1) development of targeted synthesis based on transformation, (2) enhancement of robustness and synthetic yield for practical applications, and (3) development of controlled fusion and assembly of well-defined superatoms to create new properties. New characterization approaches are also introduced such as (1) mass spectrometry and laser spectroscopies in the gas phase, (2) determination of static and dynamic structures, and (3) computational analysis by machine learning. Finally, future challenges and prospects are discussed for further promotion and development of materials science of superatoms.

This perspective summarizes the current status and emerging trends in synthesis and characterization of ligand-protected gold/silver superatoms.

Formation of Free-Standing Inverse Opals with Gradient Pores

We demonstrate the fabrication of free-standing inverse opals with gradient pores via a combination of electrophoresis and electroplating techniques. Our processing scheme starts with the preparation of multilayer colloidal crystals by conducting sequential electrophoresis with polystyrene (PS) microspheres in different sizes (300, 600, and 1000 nm). The critical factors affecting the stacking of individual colloidal crystals are discussed and relevant electrophoresis parameters are identified so the larger PS microspheres are assembled successively atop of smaller ones in an orderly manner. In total, we construct multilayer colloidal crystals with vertical stacking of microspheres in 300/600, 300/1000, and 300/600/1000 nm sequences. The inverse opals with gradient pores are produced by galvanostatic plating of Ni, followed by the selective removal of colloidal template. Images from scanning electron microscopy exhibit ideal multilayer close-packed structures with well-defined boundaries among different layers. Results from porometer analysis reveal the size of bottlenecks consistent with those of interconnected pore channels from inverse opals of smallest PS microspheres. Mechanical properties determined by nanoindentation tests indicate significant improvements for multilayer inverse opals as compared to those of conventional single-layer inverse opals.