Fusion growth patterns in atomically precise metal nanoclusters

Cu superparticle-based aggregation induced enhancement strategy with PVDF-HFP/CeVO4 NP sensing interface for miR-103a detection

Aggregation-induced emission (AIE) has been widely used in research on electrochemiluminescence (ECL) due to its excellent luminescence intensity. In this work, copper superparticles (Cu SPs) were used to construct ECL biosensor to detect the microRNA-103a (miRNA-103a) in triple-negative breast cancer (TNBC) tumor tissues. Firstly, GSH-capped copper clusters were used as precursors to prepare Cu SPs by the AIE effect. Compared with clusters, Cu SPs possessed higher luminescence performance and energy stability, making them an ideal choice for ECL nanoprobe. The film of PVDF-HFP/CeVO4 NPs was constructed and modified with CPBA and GSH as the sensing interface (PCCG). The PCCG film displayed good conductivity and hydrophilicity, and desirable mechanical stability. Moreover, the PCCG film can induce high carrier mobility rates and dissociate large amounts of the co-reactant K2S2O8 to enhance the ECL intensity of Cu SPs. As a result, the prepared ECL sensor with the catalyzed hairpin assembly (CHA) strategy was employed to quantify miRNA-103a in the range of 100 fM to 100 nM. The biosensor provided a novel analytical approach for the clinical diagnosis of TNBC.

Chemical Flexibility of Atomically Precise Metal Clusters

Ligand-protected metal clusters possess hybrid properties that seamlessly combine an inorganic core with an organic ligand shell, imparting them exceptional chemical flexibility and unlocking remarkable application potential in diverse fields. Leveraging chemical flexibility to expand the library of available materials and stimulate the development of new functionalities is becoming an increasingly pressing requirement. This Review focuses on the origin of chemical flexibility from the structural analysis, including intra-cluster bonding, inter-cluster interactions, cluster-environments interactions, metal-to-ligand ratios, and thermodynamic effects. In the introduction, we briefly outline the development of metal clusters and explain the differences and commonalities of M(I)/M(I/0) coinage metal clusters. Additionally, we distinguish the bonding characteristics of metal atoms in the inorganic core, which give rise to their distinct chemical flexibility. Section 2 delves into the structural analysis, bonding categories, and thermodynamic theories related to metal clusters. In the following sections 3 to 7, we primarily elucidate the mechanisms that trigger chemical flexibility, the dynamic processes in transformation, the resultant alterations in structure, and the ensuing modifications in physical-chemical properties. Section 8 presents the notable applications that have emerged from utilizing metal clusters and their assemblies. Finally, in section 9, we discuss future challenges and opportunities within this area.

Intermediate Intercluster Bond Orders. Electronic Communication in Au38(SR)24 Superatomic Molecules

Ligand-protected gold clusters remain potential building blocks for envisaged molecular materials. The archetypal Au38(SR)24 cluster can be viewed as a robust template for the fusion of two Au25(SR)18 - cluster units, retaining a bi-icosahedral Au23 core. Via electrochemical properties, the overall charge state can be selectively tuned, enabling the access of 14 valence electron (ve) species featuring a single intercluster bond and nearby charge from -1 to +3, achieving related species bearing 15- to 11-ve with variable intercluster bond orders. Here, we explore the characteristics of intermediate intercluster bond orders in order to provide insights into the plausible electron communication between the constituent building blocks, with Au38(SR)24, as a representative template. Our results denote a small structural variation along -1 to +3 charge states, provided by the core-protecting ligand interaction, which is enhanced towards more oxidized species. The remaining unpaired electron from intermediate intercluster bond orders of 1.5 for Au38(SR)24 1-, 1.5 for Au38(SR)24 1+, and 2.5 for Au38(SR)24 3+, holds delocalized characteristics between the building block units, favoring electron communication for conductive and cooperative cluster aggregates. Such features are relevant for the formation of molecular electronic device applications, favoring the rationalization prior to engaging in explorative synthesis of larger ligand-protected cluster aggregates.

Building Block Metal Nanocluster-Based Growth in 1D Direction

Metal nanoclusters with precisely modulated structures at the nanoscale give us the opportunity to synthesize and investigate 1D nanomaterials at the atomic level. Herein, it realizes selective 1D growth of building block nanocluster "Au13 Cd2 " into three structurally different nanoclusters: "hand-in-hand" (Au13 Cd2 )2 O, "head-to-head" Au25 , and "shoulder-to-shoulder" Au33 . Detailed studies further reveals the growth mechanism and the growth-related tunable properties. This work provides new hints for the predictable structural transformation of nanoclusters and atomically precise construction of 1D nanomaterials.

Thiacalix[4]arene Etching of an Anisotropic Cu70H22 Intermediate for Accessing Robust Modularly Assembled Copper Nanoclusters

Atom-precise metal nanoclusters (NCs) with large bulk (nuclearity >60) are important species for insight into the embryonic phase of metal nanoparticles and their top-down etching synthesis. Herein, we report a metastable rod-shaped 70-nuclei copper-hydride NC, [Cl@Cu70H22(PhC≡C)29(CF3COO)16]2+ (Cu70), with Cl- as the template, in which the Cl@Cu59 kernel adopts a distinctive metal packing mode along the bipolar direction, and the protective ligand shell exhibits corresponding site differentiation. In terms of metal nuclearity, Cu70 is the largest alkynyl-stabilized Cu-hydride cluster to date. As a typical highly active intermediate, Cu70 could undergo a transformation into a series of robust modularly assembled Cu clusters (B-type Cu8, A-A-type Cu22, A-B-type Cu23, and A-B-A-type Cu38) upon etching by p-tert-butylthiacalix[4]arene (H4TC4A), which could not be achieved by "one-pot" synthetic methods. Notably, the patterns of A and B blocks in the Cu NCs could be effectively modulated by employing appropriate counterions and blockers, and the modular assembly mechanism was illustrated through comprehensive solution chemistry analysis using HR-ESI-MS. Furthermore, catalytic investigations reveal that Cu38 could serve as a highly efficient catalyst for the cycloaddition of propargylic amines with CO2 under mild conditions. This work not only enriched the family of high-nuclear copper-hydride NCs but also provided new insights into the growth mechanism of metal NCs.

Total Structure, Structural Transformation and Catalytic Hydrogenation of [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- Constructed from Twisted Cu13 Units

Herein, a remarkable achievement in the synthesis and characterization of an atomically precise copper-hydride nanocluster, [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- via a mild one-pot reaction is presented. Through X-ray crystallography analysis, it is revealed that [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- exhibits a unique shell-core-shell structure. The inner Cu29 kernel is composed of three twisted Cu13 units, connected through Cu4 face sharing. Surrounding the metal core, two Cu6 metal shells, resembling a protective sandwich structure are observed. This arrangement, along with intracluster π···π interactions and intercluster C─H···F─C interactions, contributes to the enhanced stability of [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- . The presence, number, and location of hydrides within the nanocluster are established through a combination of experimental and density functional theory investigations. Notably, the addition of a phosphine ligand triggers a fascinating nanocluster-to-nanocluster transformation in [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- , resulting in the generation of two nanoclusters, [Cu14 (SC6 H3 F2 )3 (PPh3 )8 H10 ]+ and [Cu13 (SC6 H3 F2 )3 (P(PhF)3 )7 H10 ]0 . Furthermore, it is demonstrated that [Cu41 (SC6 H3 F2 )15 Cl3 (P(PhF)3 )6 (H)25 ]2- exhibits catalytic activity in the hydrogenation of nitroarenes. This intriguing nanocluster provides a unique opportunity to explore the assembly of M13 units, similar to other coinage metal nanoclusters, and investigate the nanocluster-to-nanocluster transformation in phosphine and thiol ligand co-protected copper nanoclusters.

Effect of Specific Heavy Doping of Silver Atoms into the Icosahedral Au13 on Electronic Structure and Catalytic Performance

The exploration of specific heavy doping of silver atoms into icosahedral Au13 clusters and their electronic structures and properties has been somewhat limited. Herein, we report two heavily Ag doped nanoclusters, [Au7Ag6(C7H4NOS)4(Dppf)3Cl]0 and [Au7Ag6(C7H4NOS)3(Dppf)3Cl](SbF6) (Au7Ag6-0 and Au7Ag6-1, respectively) [C7H4NOSH = 2-mercaptobenzoxazole, and Dppf = 1,1'-bis(diphenylphosphino)ferrocene]. The electronic structures and superatomic orbitals of nanoclusters were determined by density functional theory (DFT) calculations, and the energy degeneracy of the superatomic orbitals of Au7Ag6-1 is higher than that of Au7Ag6-0. Transient absorption spectroscopy was performed, revealing that Au7Ag6-0 significantly extends the excited-state lifetime. Both nanoclusters were supported on activated carbon for the oxygen reduction reaction. DFT calculations confirm that the catalytic activities mainly stem from the carbon atom of ferrocene rather than the iron atom. This study not only sheds light on the preparation of icosahedral alloy clusters but also provides insights into the regulation of icosahedral superatomic structure and electrocatalytic properties.

Supramolecular Anchored Copper Nanoclusters for a Multipath Electrochemiluminescence Probe

Copper nanoclusters (Cu NCs) are highly promising nanomaterials in the field of electrochemiluminescence (ECL). Nevertheless, their limited stability and efficiency have impeded their practical applications. Here, we introduced a novel supramolecular anchoring strategy resulting in the creation of exceptionally stable Cu NCs (CET-Cu NCs) with remarkable ECL properties. Specifically, CET-Cu NCs exhibited a relative ECL efficiency (ΦECL) of 62% based on the annihilation ECL efficiency of [Ru(bpy)3]2+ (100%), with tripropylamine employed as a coreactant. Moreover, CET-Cu NCs can generate ECL emission through multiple different paths, which enables them to serve as signal probes in a wider range of testing scenarios, thereby enhancing the reliability and robustness of sensing and analytical systems. To demonstrate the practical utility, CET-Cu NCs were selected as an ECL signal probe for a sensing platform that facilitated ultrasensitive detection of progesterone via oriented immobilization technology and antibody/aptamer sandwich assays. This study surmounted the barriers to the practical application of Cu NCs through the implementation of a supramolecular anchoring strategy, thereby providing enhanced utility of Cu NCs in ECL sensing and analysis.

Motif-to-Core Nucleation in a Decahedral Evolution Pattern

The atomic precision of ultrasmall metal nanoclusters has opened the door to elucidating the structural evolution principles of metal nanomaterials at the molecular level. Here, we report a novel set of super-atomic Ag clusters, including [Ag19(TBBT)16(DPPP)4]+ (Ag19), [Ag22(DMAT)8(DPPM)4Cl8]2+ (Ag22), Ag26(SPh3,5-CF3)15(DPPF)4Cl5 (Ag26), and [Ag30(DMAT)12(DPPP)4Cl8]2+ (Ag30). The core structures of these clusters correspond to one decahedral Ag7, perpendicular bi-decahedrons, three-dimensional penta-decahedrons, and hexa-decahedrons, respectively. The Ag atoms in AgS2 blocks show a strong correlation with the decahedral cores: the five equatorial Ag atoms in the decahedral Ag7 core of Ag19 all adopt the AgS2 coordination, while the Ag atoms in AgS2 blocks of Ag22, Ag26, and Ag30 unexceptionally constitute additional decahedral structures with the core Ag atoms. Specifically, two and four core Ag atoms of Ag26 and Ag30 clusters occupy positions that highly resemble that of Ag (in AgS2 motifs) of Ag22. The strong structural correlation demonstrates the motif-to-core evolution of the surface Ag (on AgS2) to build extra-decahedral blocks. Density functional theory calculations indicate that the 2e, 4e, 6e, and 8e clusters (from Ag19 to Ag30) adopt 1S2, 1S21P2, 1S21P4, and 1S21P6 electron configurations, all of which feature excellent super-atomic characters.

Structural predictions of three medium-sized thiolate-protected gold nanoclusters Au44(SR)30, Au56(SR)32, and Au60(SR)34

The knowledge of structural evolution among thiolate-protected gold nanoclusters is not only helpful for understanding their structure-property relationship but also provides scientific evidence to rule-guided structure predictions of gold nanoclusters. In this paper, three new atomic structures of medium-sized thiolate-protected gold nanoclusters, i.e. Au44(SR)30, Au56(SR)32, and Au60(SR)34, are predicted based on the grand unified model and ring model. Two structural evolution rules, i.e., Au44(SR)28 + [Au12(SR)4] → Au56(SR)32 + [Au12(SR)4] → Au68(SR)36 and Au44(SR)30 + [Au8(SR)2] → Au52(SR)32 + [Au8(SR)2] → Au60(SR)34 + [Au8(SR)2] → Au68(SR)36, are explored. The generic growth patterns underlying both sequences of nanoclusters can be viewed as sequential addition of four and three highly stable tetrahedral Au4 units on the cores, respectively. In addition, density functional theory calculations show that these three newly predicted gold nanoclusters have very close formation energies with their adjacent structures, large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, and all-positive harmonic vibration frequencies, indicating their high stabilities.

A heteroleptic fused bi-cuboctahedral Cu21S2 cluster

A new dicationic cluster, [Cu₂₁S₂{S₂CNⁿBu₂}₉(C₂Ph)₆]²⁺, where the Cu21S2 kernel consists of two S@Cu₁₂ cuboctahedra sharing a triangular Cu₃ face is reported. Its waist part is bridged by three dithiocarbamate ligands, each in a hexaconnective, hexametallic (μ₃, μ₃) coordination pattern, an unprecedented feature in Cu nanocluster chemistry.

Mechanistic Study of the Hydride Migration-Induced Reversible Isomerization in Au22(SR)15H Isomers

Unraveling the evolution mechanism of metal nanoclusters is of great importance in understanding the formation and evolution of metallic condensed matters. In this work, the specific evolution process between a pair of gold nanocluster (Au NC) isomers is completely revealed by introducing hydride ligands to simplify the research system. A hydride-containing Au NC, Au₂₂(SR)₁₅H, was synthesized by kinetic control, and the positions of the hydrides were then confirmed by combining X-ray diffraction, neutron diffraction, and DFT calculations. Importantly, a reversible structural isomerization was found to occur on this Au₂₂(SR)₁₅H. By combining the crystal structures and theoretical calculations, the focus was placed on the hydride-binding site, and a [Au-H] migration mechanism of this isomerization process is clearly shown. Furthermore, this [Au-H] migration mechanism is confirmed by the subsequent capture and structural determination of theoretically predicted intermediates. This work provides insight into the dynamic behavior of hydride ligands in nanoclusters and a strategy to study the evolution mechanism of nanoclusters by taking the hydride ligand as the breakthrough point.

Unusual core engineering on a copper hydride nanoball

A neutral polyhydrido copper cluster, [Cu₂₇H₁₅{S₂CNⁿBu₂}₁₂] (abbreviated as [Cu₂₇H₁₅]), was prepared by the reaction of dithiocarbamates (dtc), Cu(I) salts and NaBH₄. The isolated cluster provides insights into core engineering, demonstrating its novel ability to reversibly add or remove one copper atom from the cluster core. Single-crystal X-ray analysis reveals that the new core-shell structure exhibits a Cu₂₄ rhombicuboctahedral outer cage and an inner Cu₃ triangular kernel. The two core-shell clusters, [Cu₂₇H₁₅{S₂CNⁿBu₂}₁₂] and previously published [Cu₂₈H₁₅(S₂CNⁿBu₂)₁₂]⁺ (abbreviated as [Cu₂₈H₁₅]⁺), are only differentiated by one copper atom in their inner core. Importantly, we demonstrate core engineering with the controllable reversible transition between an irregular Cu₄ tetrahedron and a Cu₃ triangle, whilst maintaining their outer Cu₂₄ shell intact. The 15 hydride atoms in [Cu₂₇H₁₅], coordinated in three different modes, are co-incident with the hydride positions in [Cu₂₈H₁₅]⁺. The degradation of [Cu₂₇H₁₅] in solution or the addition of one eq. of Cu(I) ions leads to the conversion of [Cu₂₇H₁₅] into [Cu₂₈H₁₅]⁺, while the reverse transformation can be achieved by the addition of either formic acid or a reducing agent to [Cu₂₈H₁₅]⁺. A dicationic species was observed in the ESI mass spectrum, and the composition is formulated as [Cu₅₆H₃₀(S₂CNⁿBu₂)₂₄]²⁺, a dimer of [Cu₂₇H₁₅(S₂CNⁿBu₂)₁₂ + Cu⁺]₂²⁺. The dimeric species was further explored by DFT calculations, suggesting that the lowest energy structure consists of a [Cu₂₈H₁₅]⁺ and a [Cu₂₇H₁₅] cluster connected through one Cu⁺ atom bridge. As a result, [Cu₂₇H₁₅] is considered an intermediate species in the formation of the more stable [Cu₂₈H₁₅]⁺ nanoball.

Superatomic Three-Center Bond in a Tri-Icosahedral Au36Ag2(SR)18 Cluster: Analogue of 3c-2e Bond in Molecules

Probing the nature of electronic stability for ligand-protected gold clusters is important in gold chemistry. A thermally stable Au₃₆Ag₂(SR)₁₈ nanocluster was synthesized recently. It has a D_3h tri-icosahedral [Au₃₀Ag₂]¹²⁺ core with 20 valence electrons, which does not follow the magic number of gold superatoms. Herein, we propose a superatomic three-center bond to unveil its electronic stability. The [Au₃₀Ag₂]¹²⁺ core is viewed as a union of three face-fused superatoms, and chemical bonding analysis suggests a three-superatom-center two-electron (3sc-2e) bond for the octet rule of each superatom, which mimics the bonding framework of the D_3h O₃^2- molecule. Moreover, a liganded tri-icosahedral [Au₂₇Pt₃Ag₂]¹¹⁺ core with 18 valence electrons is predicted, and three 2sc-2e bonds are formed between each of two superatoms to satisfy the octet rule (analogue of D_3h O₃), indicating the flexibility of superatomic bonding. Such a superatomic three-center bond extends the community of superatomic bonding and gives a new perspective for superatom assembling.

Sulfide Boosting Near-Unity Photoluminescence Quantum Yield of Silver Nanocluster

Silver nanoclusters have emerged as promising candidates for optoelectronic applications, but their room-temperature photoluminescence quantum yield (PLQY) is far from ideal to access cutting-edge device performance. Herein, two supertetrahedral silver nanoclusters with high PLQY in non-degassed solution at room temperature were constructed by interiorly supporting the core with multiple VO₄^3- and E^2- anions as structure-directing agents and exteriorly protecting the core with a rigid ligand shell of PhC≡C^- and Ph₂PE₂^- (E = S, Ag64-S; E = Se, Ag64-Se). Both clusters have similar outer Ag₅₈ tetrahedral cages and [Ag₆E₄@(VO₄)₄] cores, forming a pair of comparable clusters to decrypt the origin of such a high PLQY, particularly in Ag64-S, where the PLQY reached up to 97%. The stronger suppression effect of inner sulfides for nonradiative decay is critical to boost the PLQY to near unity. Transient absorption spectroscopy is employed to confirm the phosphorescence nature. The quadruple-capping assembly mechanism involving Ag₇ secondary building units on a Ag₃₆ truncated tetrahedron was also established by collision-induced dissociation studies. This work not only provides a strategy of core engineering for the controlled syntheses of silver nanoclusters with high PLQY but also deciphers the origin of a near-unity PLQY, which lays a foundation for fabricating highly phosphorescent silver nanoclusters in the future.

[Cu18H3(S-Adm)12(PPh3)4Cl2]: fusion of Platonic and Johnson solids through a Cu(0) center and its photophysical properties

Structural elucidation of atom-precise thiolate-protected copper nanoclusters (Cu NCs) containing Cu(0) is quite challenging. Here, we report a new adamantane-thiol-protected NC, [Cu₁₈H₃(S-Adm)₁₂(PPh₃)₄Cl₂] (Cu₁₈), which represents the first observation of a rare mononuclear Cu(0)-containing Cu₁₀H₃Cl₂ core that is constructed via kernel fusion through vertex sharing of the Platonic-solid- and Johnson-solid-geometry-like kernels and hydride-bridging. The unique core is surrounded by a Cu₈S₁₂P₄ metal–ligand motif shell and adopts a butterfly-like structure. In comparison to its closest structural analogue, the predominant effect of the principal Cu atom vacancy-induced structural rearrangement is evidenced. The occupied orbitals of this NC have a major d-orbital contribution to the distorted Cu₆ octahedral kernel, whereas unoccupied orbitals owe a contribution to the distorted Cu₅ square-pyramidal kernel. Thus, the charge transfer phenomenon is uniquely instigated between the two fused kernels through Cu(d) → Cu(d) transition via the Cu(0) center. This NC exhibits violet emission due to kernel-dominated relaxation at room temperature, which is further enhanced by confining the surface protecting ligands through recognition-site-specific host–guest supramolecular adduct formation by β-cyclodextrin. The unique electronic structure of this NC further facilitates its application toward photocurrent generation. Thus, this study offers a unique strategy for the controllable synthesis of a Cu(0)-containing Cu NC, which enables atomic-level insights into their optoelectronic properties.

An example of a butterfly-like [Cu₁₈H₃(S-Adm)₁₂(PPh₃)₄Cl₂] cluster with vertex-sharing kernels through a Cu(0) center. Combined experimental and theoretical results correlate its photophysical properties with its unique structural architecture.

Tri- and Tetra-superatomic Molecules in Ligand-Protected Face-Fused Icosahedral (M@Au12)n (M = Au, Pt, Ir, and Os, and n = 3 and 4) Clusters

Cluster assembling has been one of the hottest topics in nanochemistry. In certain ligand-protected gold clusters, bi-icosahedral cores assembled from Au₁₃ superatoms were found to be analogues of diatomic molecules F₂, N₂, and singlet O₂, respectively, in electronic shells, depending upon the super valence bond (SVB) model. However, challenges still remain for extending the scale in cluster assembling via the SVB model. In this work, ligand-protected tri- and tetra-superatomic clusters composed of icosahedral M@Au₁₂ (M = Au, Pt, Ir, and Os) units are theoretically predicted. These clusters are stable with reasonable highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps and proven to be analogues of simple triatomic (Cl₃^-, OCl₂, O₃, and CO₂) and tetra-atomic (N≡C-C≡N, and Cl-C≡C-Cl) molecules in both geometric and electronic structures. Moreover, a stable cluster-assembling gold nanowire is predicted following the same rules. This work provides effective electronic rules for cluster assembling on a larger scale and gives references for their experimental synthesis.

Selective ligand exchange synthesis of Au16(2-PET)14 from Au15(SG)13

Replacement of protecting ligands of gold nanoclusters by ligand exchange has become an established post-synthetic tool for selectively modifying the nanoclusters' properties. Several Au nanoclusters are known to additionally undergo size transformations upon ligand exchange, enabling access to cluster structures that are difficult to obtain by direct synthesis. This work reports on the selective size transformation of Au₁₅(SG)₁₃ (SG: glutathione) nanoclusters to Au₁₆(2-PET)₁₄ (2-PET: 2-phenylethanethiol) nanoclusters through a two-phase ligand exchange process at room temperature. Among several parameters evaluated, the addition of a large excess of exchange thiol (2-PET) to the organic phase was identified as the key factor for the structure conversion. After exchange, the nature of the clusters was determined by UV-vis, electrospray ionization-time of flight mass spectrometry, attenuated total reflection-Fourier transform infrared, and extended x-ray absorption fine-structure spectroscopy. The obtained Au₁₆(2-PET)₁₄ clusters proved to be exceptionally stable in solution, showing only slightly diminished UV-vis absorption features after 3 days, even when exposed to an excess of thiol ligands.

Fluorescent noble metal nanoclusters for contaminants analysis in food matrix

Recently, food safety issues caused by contaminants have aroused great public concern. The development of innovative and efficient sensing techniques for contaminants detection in food matrix is in urgent demand. As fluorescent nanomaterials, noble metal nanoclusters have attracted much attention because of their ease of synthesis, enhanced catalytic activity and biocompatibility, and most importantly, excellent photoluminescence property that provides promising analytical applications. This review comprehensively introduced the synthesis method of noble metal nanoclusters, and summarized the application of metal nanoclusters as fluorescent sensing materials in the detection of pollutants, including pesticides, heavy metal, mycotoxin, food additives, and other contaminants in food. The detection mechanism of pesticide residues mostly relies on the inhibition of natural enzymes. For heavy metals, the detection mechanism is mainly related to the interaction between metal ions and nanoclusters or ligands. It is evidenced that metal nanoclusters have great potential application in the field of food safety monitoring. Moreover, challenges and future trends of nanoclusters were discussed. We hope that this review can provide insights and directions for the application of nanoclusters in contaminants detection.

Unraveling the Nucleation Process from a Au(I)-SR Complex to Transition-Size Nanoclusters

Atomically precise noble metal nanoclusters provide a critical benchmark for the fundamental research of the origin of condensed matter because they retain the original state of the metal bonds. Also, knowledge about the transition from organometallic complexes to a nanoclusters is important for understanding the structural evolution of the nanoclusters, particularly their nucleation mechanism. Herein, three transition-size gold nanoclusters are prepared via a controlled diphosphine-mediated top-down routine. Starting from small-size nanoclusters, three new nanoclusters including Au₁₃(SAdm)₈(L⁴)₂(BPh₄) (Au₁₃), Au₁₄(S-c-C₆H₁₁)₁₀L⁴ (Au₁₄), and Au₁₆(S-c-C₆H₁₁)₁₁L^Ph* (Au₁₆) are obtained by controlled clipping on the surface and kernel of initial nanoclusters. Combining their atomically precise structures with DFT theoretical calculations, the overall atom-by-atom structural evolution process from Au₁₂(SR)₁₂ (0 e^-) to Au₁₈(SR)₁₄ (4 e^-) is mapped out. In addition, studies on their electronic structures show that the evolution from an organometallic complex to nanoclusters is accompanied by a dramatic decrease in the HOMO-LUMO gaps. Most importantly, the formation of the first Au-Au bond is captured in the "Au₄S₄ to Au₅" nucleation process from Au₁₂(SR)₁₂ complex to the Au₁₃ nanocluster. This work provides a deep insight into the origin of inner core in Au NCs and their structural transition relationship with metal complexes.

Surface engineering of linearly fused Au13 units using diphosphine and Cd doping

Herein, surface engineering was delicately performed to assemble two new Au-Cd alloy nanoclusters, including [Cd2Au17(S-c-C6H11)12(DPPP)2](BPh4) and Cd2Au29(TBBT)17(DPPF)2. Both the Au13 (in Cd2Au17) and Au25 (in Cd2Au29) cores were covered by two identical Au2Cd(SR)6 motifs and two diphosphine ligands. In addition, their optical properties were explored to give clues on the kernel- and surface-dependent electronic structures.

A construction guide for high-nuclearity (≥50 metal atoms) coinage metal clusters at the nanoscale: bridging molecular precise constructs with the bulk material phase

Synthesis remains a major strength in chemistry and materials science and relies on the formation of new molecules and diverse forms of matter. The construction and identification of large molecules poses specific challenges and has historically lain in the realm of biological (organic)-type molecules with evolved synthesis methods to support such endeavours. But with the development of analytical tools such as X-ray crystallography, new synthesis methods toward large metal-based (inorganic) molecules and clusters have come to the fore, making it possible to accurately determine the precise distribution of hundreds of atoms in large clusters. In this review, we focus on different synthesis protocols used to form new metal clusters such as templating, alloying and size-focusing strategies. A specific focus is on group 11 metals (Cu, Ag, Au) as they currently predominate large metal cluster investigations and related Au and Ag bulk surface phenomena. This review focuses on metal clusters that have very high-nuclearity, i.e. with 50 or more metal centers within the isolated cluster. This size domain, it is believed, will become increasingly important for a variety of applications as these metal clusters are positioned at the interface between the molecular and bulk phases, whilst remaining a classic nanomaterial and retaining unique nano-sized properties.

Atom-by-Atom Evolution of the Same Ligand-Protected Au21, Au22, Au22Cd1, and Au24 Nanocluster Series

Atom-by-atom manipulation on metal nanoclusters (NCs) has long been desired, as the resulting series of NCs can provide insightful understanding of how a single atom affects the structure and properties as well as the evolution with size. Here, we report crystallizations of Au22(SAdm)16 and Au22Cd1(SAdm)16 (SAdm = adamantanethiolate) which link up with Au21(SAdm)15 and Au24(SAdm)16 NCs and form an atom-by-atom evolving series protected by the same ligand. Structurally, Au22(SAdm)16 has an Au3(SAdm)4 surface motif which is longer than the Au2(SAdm)3 on Au21(SAdm)15, whereas Au22Cd1(SAdm)16 lacks one staple Au atom compared to Au24(SAdm)16 and thus the surface structure is reconstructed. A single Cd atom triggers the structural transition from Au22 with a 10-atom bioctahedral kernel to Au22Cd1 with a 13-atom cuboctahedral kernel, and correspondingly, the optical properties are dramatically changed. The photoexcited carrier lifetime demonstrates that the optical properties and excited state relaxation are highly sensitive at the single atom level. By contrast, little change in both ionization potential and electron affinity is found in this series of NCs by theoretical calculations, indicating the electronic properties are independent of adding a single atom in this series. The work provides a paradigm that the NCs with continuous metal atom numbers are accessible and crystallizable when meticulously designed, and the optical properties are more affected at the single atom level than the electronic properties.

Atomically precise alloy nanoclusters: syntheses, structures, and properties

Metal nanoclusters fill the gap between discrete atoms and plasmonic nanoparticles, providing unique opportunities for investigating the quantum effects and precise structure-property correlations at the atomic level. As a versatile strategy, alloying can largely improve the physicochemical performances compared to the corresponding homo-metal nanoclusters, and thus benefit the applications of such nanomaterials. In this review, we highlight the achievements of atomically precise alloy nanoclusters, and summarize the alloying principles and fundamentals, including the synthetic methods, site-preferences for different heteroatoms in the templates, and alloying-induced structure and property changes. First, based on various Au or Ag nanocluster templates, heteroatom doping modes are presented. The templates with electronic shell-closing configurations tend to maintain their structures during doping, while the others may undergo transformation and give rise to alloy nanoclusters with new structures. Second, alloy nanoclusters of specific magic sizes are reviewed. The arrangement of different atoms is related to the symmetry of the structures; that is, different atoms are symmetrically located in the nanoclusters of smaller sizes, and evolve into shell-by-shell structures at larger sizes. Then, we elaborate on the alloying effects in terms of optical, electrochemical, electroluminescent, magnetic and chiral properties, as well as the stability and reactivity via comparisons between the doped nanoclusters and their homo-metal counterparts. For example, central heteroatom-induced photoluminescence enhancement is emphasized. The applications of alloy nanoclusters in catalysis, chemical sensing, bio-labeling, and other fields are further discussed. Finally, we provide perspectives on existing issues and future efforts. Overall, this review provides a comprehensive synthetic toolbox and controllable doping modes so as to achieve more alloy nanoclusters with customized compositions, structures, and properties for applications. This review is based on publications available up to February 2020.

A novel geometric structure of a nanocluster with an irregular kernel: Ag30Cu14(TPP)4(SR)28

Here, we report a bi-ligand protected bimetallic nanocluster Ag₃₀Cu₁₄(TPP)₄(SR)₂₈. It is composed of an Ag₂₇ kernel and Ag₃Cu₁₄(TPP)₄(SR)₂₈ shell. Typically, the metal atoms are arranged irregularly. Both the core and shell exhibit the characteristics of C₂-symmetry.

Effect of thiolate-ligand passivation on the electronic structure and optical absorption properties of ultrathin one and two-dimensional gold nanocrystals

Gold nanomaterials, including one-dimensional (1D) gold nanorods (AuNRs) and nanowires (AuNWs) and two-dimensional (2D) gold nanoprisms with a large surface area and stability, have attracted widespread research interest. A large number of experimental and theoretical studies have shown that the properties of low dimensional gold nanomaterials depend on their anisotropic shape. In this work, we theoretically conceived a new type of gold nanomaterial, namely, thiolate (SR) monolayer passivated quasi-1D and quasi-2D gold nanocrystals and infinite superstructures, which were formed by the fusion of seed clusters Au₂₈(SR)₂₀, Au₃₆(SR)₂₄, Au₄₄(SR)₂₈ and Au₅₂(SR)₃₂ or the layer by layer growth of gold atoms along the [100] and/or [010] directions. By means of DFT and TD-DFT calculations, the structure and properties of these model gold nanocrystals and superstructures are studied in depth. It is found that the passivation of the monolayer of thiolate leads to significantly improved near-infrared absorption properties in comparison with the ligand free gold nanocrystals. Upon passivating the thiolate-monolayer, the ultrathin 1D gold nanowire and 2D gold nanosheet demonstrate a metal to semiconductor transition. The novel electronic structures, optical absorption and semiconductor-to-metal transition found in these thiolate-protected low-dimensional gold nanocrystals suggest that the passivation of the SR ligand is a promising way to tailor the properties of gold nanomaterials.

Atomic-precision engineering of metal nanoclusters

Ultrasmall metal nanoparticles (below 2.2 nm core diameter) start to show discrete electronic energy levels due to strong quantum confinement effects and thus behave much like molecules. The size and structure dependent quantization induces a plethora of new phenomena, including multi-band optical absorption, enhanced luminescence, single-electron magnetism, and catalytic reactivity. The exploration of such new properties is largely built on the success in unveiling the crystallographic structures of atomically precise nanoclusters (typically protected by ligands, formulated as MnLmq, where M = metal, L = Ligand, and q = charge). Correlation between the atomic structures of nanoclusters and their properties has further enabled atomic-precision engineering toward materials design. In this frontier article, we illustrate several aspects of the precise engineering of gold nanoclusters, such as the single-atom size augmenting, single-atom dislodging and doping, precise surface modification, and single-electron control for magnetism. Such precise engineering involves the nanocluster's geometric structure, surface chemistry, and electronic properties, and future endeavors will lead to new materials design rules for structure-function correlations and largely boost the applications of metal nanoclusters in optics, catalysis, magnetism, and other fields. Following the illustrations of atomic-precision engineering, we have also put forth some perspectives. We hope this frontier article will stimulate research interest in atomic-level engineering of nanoclusters.

Towards elucidating structure of ligand-protected nanoclusters

Developing a centralized database for ligand-protected nanoclusters can fuel machine learning and data-science-based approaches towards theoretical structure prediction.