Toward the Tailoring Chemistry of Metal Nanoclusters for Enhancing Functionalities

[Cu15 (PPh3 )6 (PET)13 ]2+ : a Copper Nanocluster with Crystallization Enhanced Photoluminescence

Due to their atomically precise structure, photoluminescent copper nanoclusters (Cu NCs) have emerged as promising materials in both fundamental studies and technological applications, such as bio-imaging, cell labeling, phototherapy, and photo-activated catalysis. In this work, a facile strategy is reported for the synthesis of a novel Cu NCs coprotected by thiolate and phosphine ligands, formulated as [Cu₁₅ (PPh₃ )₆ (PET)₁₃ ]²⁺ , which exhibits bright emission in the near-infrared (NIR) region (≈720 nm) and crystallization-induced emission enhancement (CIEE) phenomenon. Single crystal X-ray crystallography shows that the NC possesses an extraordinary distorted trigonal antiprismatic Cu₆ core and a, unique among metal clusters, "tri-blade fan"-like structure. An in-depth structural investigation of the ligand shell combined with density functional theory calculations reveal that the extended CH···π and π-π intermolecular ligand interactions significantly restrict the intramolecular rotations and vibrations and, thus, are a major reason for the CIEE phenomena. This study provides a strategy for the controllable synthesis of structurally defined Cu NCs with NIR luminescence, which enables essential insights into the origins of their optical properties.

Understanding the Chemical Insights of Staple Motifs of Thiolate-Protected Gold Nanoclusters

Improving the fundamental understanding of the basic structures of ligand-protected gold nanoclusters is essential to their bottom-up synthesis as well as their further application explorations. The thiolate ligands that cover the central metal core in staple motifs are vital for the stability of the gold clusters. However, the knowledge about the geometrical and bonding characters of the thiolate ligands has not been fully uncovered yet. In this work, density functional theory calculations and molecular orbital analysis are applied to show that the Au atoms in the thiolate ligands are hypervalent. The chemical insights of the linear SAuS configuration as well as the lengthened AuS bond by combining the 3-center 4-electron (3c-4e) model and the well-recognized valence shell electron pair repulsion theory are revealed. Valence bond formulations of the motifs are given to provide more chemical insights, for example, the resonant structures, to show how the thiolate motif forms one covalent bond and one dative covalent bond with the Au core. This work provides a thorough understanding of the structure and bonding pattern of thiolate ligands of Au nanoclusters, which is important for the rational design of ligands-protected Au nanoclusters.

Designing New Metal Chalcogenide Nanoclusters through Atom-by-Atom Substitution

Atom-by-atom substitution is a promising strategy for designing new cluster-based materials, which has been used to generate new gold- and silver-containing clusters. Here, the first study focused on atom-by-atom substitution of Fe and Ni to the core of a well-defined cobalt sulfide superatom [Co₆ S₈ L₆ ]⁺ ligated with triethylphosphine (L = PEt₃ ) to produce [Co₅ MS₈ L₆ ]⁺ (M = Fe, Ni) is reported. Electrospray ionization mass spectrometry confirms the substitution of 1-6 Fe atoms with the single Fe-substituted cluster being the dominant species. The Fe-substituted clusters oxidize in solution to generate dicationic species. In contrast, only a single Ni-substituted cluster is observed, which remains stable as a singly charged species. Collision-induced dissociation experiments indicate the reduced stability of the [Co₅ FeS₈ L₆ ]⁺ toward ligand loss in comparison with the unsubstituted and Ni-substituted counterparts. Density functional theory calculations provide insights into the effect of metal atom substitution on the stability and electronic structures of the clusters. The results indicate that Fe and Ni have a different impact on the electronic structure, optical, and magnetic properties, as well as ligand-core interaction of [Co₆ S₈ L₆ ]. This study extends the atom-by-atom substitution strategy to the metal chalcogenide superatoms providing a direct path toward designing novel atomically precise core-tailored superatoms.

Self-Assembly of Amphiphilic Copper Nanoclusters Driven by Cationic Surfactants

Amphiphilicity is an excellent physicochemical property, which is yet to be explored from traditional surfactants to nanoparticles. This article shows that the amphiphilicity of copper nanoclusters (CuNCs) can be readily tuned by electrostatic interactions with cationic surfactants and cetyltrimethylammonium cations (CTA⁺) with counterions Br^-, Cl^-, and C₇H₈O₃S^-. Due to the role of surface ligands, the complexes of glutathione-capped CuNCs (GSH-CuNCs) and the surfactants exhibit good amphiphilicity, which enables them to self-assemble like a molecular amphiphile. This could significantly increase the utility of metal nanoclusters in basic and applied research. As the concentration of the surfactant changes, the aggregates change from nanoparticles to network-like structures. After the formation of supramolecular self-assemblies by hydrophobic interactions, the enhancement of fluorescence intensity was observed, which can be ascribed to the suppression of intramolecular vibrations based on aggregation-induced emission (AIE) and combined with the compactness of GSH-CuNCs in self-assemblies. Our study provides a facile way to generate solid fluorescent materials with excellent fluorescence performance, which may find applications in light-emitting diodes (LEDs).

Cubic aromaticity in ligand-stabilized doped Au superatoms

The magnetic response of valence electrons in doped gold-based M@Au₈L₈ ^q superatoms (M = Pd, Pt, Ag, Au, Cd, Hg, Ir, and Rh; L = PPh₃; and q = 0, +1, +2) is studied by calculating the gauge including magnetically induced currents (GIMIC) in the framework of the auxiliary density functional theory. The studied systems include 24 different combinations of the dopant, total cluster charge, and cluster structure (cubic-like or oblate). The magnetically induced currents (both diatropic and paratropic) are shown to be sensitive to the atomic structure of clusters, the number of superatomic electrons, and the chemical nature of the dopant metal. Among the cubic-like structures, the strongest aromaticity is observed in Pd- and Pt-doped M@Au₈L₈ ⁰ clusters. Interestingly, Pd- and Pt-doping increases the aromaticity as compared to a similar all-gold eight-electron system Au₉L₈ ⁺¹. With the recent implementation of the GIMIC in the deMon2k code, we investigated the aromaticity in the cubic and butterfly-like M@Au₈ core structures, doped with a single M atom from periods 5 and 6 of groups IX-XII. Surprisingly, the doping with Pd and Pt in the cubic structure increases the aromaticity compared to the pure Au case not only near the central atom but encompassing the whole metallic core, following the aromatic trend Pd > Pt > Au. These doped (Pd, Pt)@Au₈ nanoclusters show a closed shell 1S²1P⁶ superatom electronic structure corresponding to the cubic aromaticity rule 6n + 2.

Optical Properties and Excited-State Dynamics of Atomically Precise Gold Nanoclusters

Understanding the excited-state dynamics of nanomaterials is essential to their applications in photoenergy storage and conversion. This review summarizes recent progress in the excited-state dynamics of atomically precise gold (Au) nanoclusters (NCs). We first discuss the electronic structure and typical relaxation pathways of Au NCs from subpicoseconds to microseconds. Unlike plasmonic Au nanoparticles, in which collective electron excitation dominates, Au NCs show single-electron transitions and molecule-like exciton dynamics. The size-, shape-, structure-, and composition-dependent dynamics in Au NCs are further discussed in detail. For small-sized Au NCs, strong quantum confinement effects give rise to relaxation dynamics that is significantly dependent on atomic packing, shape, and heteroatom doping. For relatively larger-sized Au NCs, strong size dependence can be observed in exciton and electron dynamics. We also discuss the origin of coherent oscillations and their roles in excited-state relaxation. Finally, we provide our perspective on future directions in this area.

Photoluminescence-enhanced cholesteric films: Coassembling copper nanoclusters with cellulose nanocrystals

Iridescent and luminescent composite films were fabricated through a coassembly strategy, in which glutathione-stabilized copper nanoclusters (GSH-CuNCs) were incorporated into chiral nematic structures of a cellulose nanocrystal (CNC) film. Through variations in the helical pitch, these composite films exhibited broadband reflection. The fluorescence emission spectrum of the composite film exhibited peaks at 439 and 600 nm, corresponding to crystallization-induced emission from CNCs and assembly-induced emission from CuNCs. The enhanced luminescence and prolonged lifetime of the composite film were attributed to the confinement effect of solid layers and attendant intermolecular interactions. By tuning the reaction time, temperature, and pH of the solution, the emission color and intensity of the CuNCs could be changed. At appropriate GSH and Cu²⁺ concentrations, the chiral organization of GSH-CuNCs enabled the composite CNC film to exhibit right-handed chiral fluorescence with an asymmetry factor of -0.16. Luminescent composite films were employed to fabricate LEDs with custom colors and patterns.

Discriminating detection of dissolved ferrous and ferric ions using copper nanocluster-based fluorescent probe

Discrimination and detection of specific metal ions that belong to the same metallic element with different valence states in a complex matrix is challenging. In the present work, a fluorescence method using polyvinylpyrrolidone stabilized copper nanocluster (CuNCs@PVP) as a probe for discriminating detection of ferrous (Fe³⁺) and ferric (Fe²⁺) ions was developed. The CuNCs@PVP exhibited an excellent selective response to Fe³⁺ ions in contrast to Fe²⁺ ions and other metal ions when the pH value of solution was less than 4.0. Furthermore, the fluorescence of the CuNCs@PVP could be more sensitively quenched by Fe²⁺ ions by virtue of Fenton reaction. The different response of CuNCs@PVP towards Fe³⁺ and Fe²⁺ ions under different conditions offered the potential for the discriminating detection of Fe³⁺ and Fe²⁺ ions. Based on detailed optimization of detection conditions, an excellent linear relationship between the fluorescence quenching efficiency (F/F₀) of the CuNCs@PVP and the concentration of Fe³⁺ ions over the range of 0.4-20.0 μM and of Fe²⁺ ions in the range of 0.01-0.4 μM were obtained, respectively. The detection limits for the Fe³⁺ and Fe²⁺ ions were 0.14 μM and 0.008 μM, respectively. The developed probe showed good selectivity and presented an alternative strategy for discriminating detection of Fe³⁺ and Fe²⁺ ions in complex samples.

Atomically Precise Alkynyl- and Halide-Protected AuAg Nanoclusters Au78Ag66(C≡CPh)48Cl8 and Au74Ag60(C≡CPh)40Br12: The Ligation Effects of Halides

Reported herein are the synthesis and structures of two high-nuclearity AuAg nanoclusters, namely, [Au₇₈Ag₆₆(C≡CPh)₄₈Cl₈]^q- and [Au₇₄Ag₆₀(C≡CPh)₄₀Br₁₂]^2-. Both clusters possess a three-concentric-shell Au₁₂@Au₄₂@Ag₆₀ structure. However, the dispositions of the metal atoms, and the ligand coordination modes, of the outermost shells of these clusters are distinctly different. These structural differences reflect the bonding characteristics of the halide ligands. As revealed by density functional theory analysis, these clusters exhibit superatomic electron shell closings at magic numbers of 92 (for q = 4) and 84, respectively, consistent with their spherical shapes. Both clusters exhibit unusual multivalent redox properties.

Controllable synthesis and formation mechanism study of homoleptic alkynyl-protected Au nanoclusters: recent advances, grand challenges, and great opportunities

In the past decade, atomically precise coinage metal nanoclusters have been a subject of major interest in nanoscience and nanotechnology because of their determined compositions and well-defined molecular structures, which are beneficial for establishing structure-property relationships. Recently ligand engineering has been extended to alkynyl molecules. Homoleptic alkynyl-protected Au nanoclusters (Au NCs) have emerged as a hotspot of research interest, mainly due to their unique optical properties, molecular configuration, and catalytic functionalities, and more importantly, they are used as a counterpart object for fundamental study to compare with the well-established thiolate Au NCs. In this review, we first summarize the recently reported various controllable synthetic strategies for atomically precise homoleptic-alkynyl-protected Au NCs, with particular emphasis on the ligand exchange method, direct reduction of the precursor, one-pot synthesis, and the synchronous nucleation and passivation strategy. After that, we switch our focus to the formation mechanism and structure evolution process of homoleptic alkynyl-protected Au NCs, where Au144(PA)60 and Au36(PA)24 (PA = phenylacetylide) are given as examples, along with the prediction of the possible formation mechanism of some other cluster molecules. In the end of this review, the outlook and perspective of this rapidly developing field including grand challenges and great opportunities are discussed. This review can stimulate more research efforts towards developing new synthetic strategies to enrich the limited examples and unravel the formation/growth mechanism of homoleptic alkynyl-protected Au NCs.

New atomically precise (M = Au/Ag) nanoclusters as excellent oxygen reduction reaction catalysts

By introducing 1,1'-bis-(diphenylphosphino)ferrocene (dppf) as an activating ligand, two novel nanoclusters, M1Ag21 (M = Au/Ag), have been controllably synthesized and structurally characterized. The atomically precise structures of the M1Ag21 nanoclusters were determined by SCXC and further confirmed by ESI-TOF-MS, TGA, XPS, DPV, and FT-IR measurements. The M1Ag21 nanoclusters supported on activated carbon (C) are exploited as efficient oxygen reduction reaction (ORR) catalysts in alkaline solutions. Density functional theory (DFT) calculations verify that the catalytic activities of the two cluster-based systems originate from the significant ensemble synergy effect between the M13 kernel and dppf ligand in M1Ag21. This work sheds lights on the preparation of cluster-based electrocatalysts and other catalysts that are activated and modified by peripheral ligands.

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.

Engineering amino-mediated copper nanoclusters with dual emission and assembly-to-monodispersion switching by pH-triggered surface modulation

We synthesized l-cysteine (Cys)-mediated copper nanoclusters (CuNCs) with assembly-to-monodispersion switching by pH-triggered surface modulation, where aggregated red emissive (R)-CuNCs@Cys at pH = 3 turns to monodispersed blue emissive (B)-CuNCs@Cys at pH = 10.

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.

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.

Development of coinage metal nanoclusters as antimicrobials to combat bacterial infections

Infections from antibiotic-resistant bacteria have caused huge economic loss and numerous deaths over the past decades. Researchers are exploring multiple strategies to combat these bacterial infections. Metal nanomaterials have been explored as therapeutics against these infections owing to their relatively low toxicity, broad-spectrum activity, and low bacterial resistance development. Some coinage metal nanoclusters, such as gold, silver, and copper nanoclusters, can be readily synthesized. These nanoclusters can feature multiple useful properties, including ultra-small size, high catalytic activity, unique photoluminescent properties, and photothermal effect. Coinage metal nanoclusters have been investigated as antimicrobials, but more research is required to tap their full potential. In this review, we discuss multiple advantages and the prospect of using gold/silver/copper nanoclusters as antimicrobials.

Synthesis, Structural Characterization, and DFT Investigations of [MxM'5-xFe4(CO)16]3- (M, M' = Cu, Ag, Au; M ≠ M') 2-D Molecular Alloy Clusters

Miscellaneous 2-D molecular alloy clusters of the type [M_xM'_5-xFe₄(CO)₁₆]^3- (M, M' = Cu, Ag, Au; M ≠ M') have been prepared through the reactions of [Cu₃Fe₃(CO)₁₂]^3-, [Ag₄Fe₄(CO)₁₆]^4- or [M₅Fe₄(CO)₁₆]^3- (M = Cu, Ag) with M'(I) salts (M' = Cu, Ag, Au). Their formation involves a combination of oxidation, condensation, and substitution reactions. The total structures of several [M_xM'_5-xFe₄(CO)₁₆]^3- clusters with different compositions have been determined by means of single crystal X-ray diffraction (SC-XRD) and their nature in solution elucidated by electron spray ionization mass spectrometry (ESI-MS) and IR and UV-visible spectroscopy. Substitutional and compositional disorder is present in the solid state structures, and ESI-MS analyses point out that mixtures of isostructural clusters differing by a few M/M' coinage metals are present. SC-XRD studies indicate some site preferences of the coinage metals within the metal cores of these clusters, with Au preferentially in corner sites and Cu in the central site. DFT studies give theoretical support to the experimental structural evidence. The site preference is mainly dictated by the strength of the Fe-M bonds that decreases in the order Fe-Au > Fe-Ag > Fe-Cu, and the preferred structure is the one that maximizes the number of stronger Fe-M interactions. Overall, the molecular nature of these clusters allows their structures to be fully revealed with atomic precision, resulting in the elucidation of the bonding parameters that determine the distribution of the different metals within their metal cores.

Toward the Formation of N-Heterocyclic-Carbene-Protected Gold Clusters of Various Nuclearities. A Comparison with Their Phosphine-Protected Analogues from Density Functional Theory Calculations

The structure and bonding of a series of selected phosphine-protected gold clusters (Au_n-P) of nuclearity varying from n = 6 to 13 were investigated by density functional theory (DFT) calculations and compared to those of the hypothetical homologues in which phosphines were replaced by N-heterocyclic carbene (NHC) analogues (Au_n-C). Both the Au_n-P and Au_n-C series exhibit similar stabilities and structural features, except for n = 6, where some differences are noted. The NHC ligands are found to be even slightly more strongly bonded to the gold core (by a few kilocalories per mole per ligand) than phosphines. Investigation of the optical properties of both series using time-dependent DFT calculations indicates similarities in the nature and energies of the UV-vis optical transitions and, consequently, relatively similar shapes of the simulated spectra, with a general blue-shift tendency when going from Au_n-P to Au_n-C. The fluorescence behavior observed experimentally for some of the Au_n-P species is expected to occur also for their Au_n-C analogues, which can be extended to other carbene-ligand-protected nanoclusters. Our results show that it should be possible to stabilize gold clusters with NHC ligands, in relation to the seminal Au₁₃-ligand-protected core, offering novel building blocks for the design of nanostructured materials with various properties.

Total Structure Determination of the Largest Alkynyl-Protected fcc Gold Nanocluster Au110 and the Study on Its Ultrafast Excited-State Dynamics

Great attention has been paid to nanoclusters having face-centered-cubic (fcc) metal kernels, because of the similarity of metal packing to that of bulk gold. So far, there is no precedent example of an all-alkynyl-protected fcc gold nanocluster with more than 100 gold atoms. We report the synthesis and total structure determination of an alkynyl-protected gold nanocluster [NEt₃H]₂[Au₁₁₀(p-CF₃C₆H₄C≡C)₄₈] (Au₁₁₀). It has an fcc Au₈₆ kernel with 24 peripheral Au(C≡CR)₂ staples. The Au₈₆ kernel consists of six close packing layers in the pattern of Au₆:Au₁₆:Au₂₁:Au₂₁:Au₁₆:Au₆. Electronic absorption spectroscopy shows Au₁₁₀ has a molecular-like discrete electronic structure, and transient absorption experiments reveal its nonmetallic nature.

Ultrasmall Au and Ag Nanoclusters for Biomedical Applications: A Review

Noble metal (e.g., Au, Ag, Pt, Pd, and their alloys) nanoclusters (NCs) have emerged as a new type of functional nanomaterial in nanoscience and nanotechnology. Owing to their unique properties, such as their ultrasmall dimension, enhanced photoluminescence, low toxicity, and excellent biocompatibility, noble metal NCs-especially Au and Ag NCs-have found various applications in biomedical regimes. This review summarizes the recent advances made in employing ultrasmall Au and Ag NCs for biomedical applications, with particular emphasis on bioimaging and biosensing, anti-microbial applications, and tumor targeting and cancer treatment. Challenges, including the shared and specific challenges for Au and Ag NC toward biomedical applications, and future directions are briefly discussed at the end.

Absolute Templating of M(111) Cluster Surrogates by Galvanic Exchange

The precise preparation of monodisperse nanomaterials is among the most fundamental tasks in inorganic synthesis and materials science. Achieving this goal by galvanic exchange is hardly predictable and often results in major structural changes and polydisperse mixtures. Taking advantage of the enhanced stability imparted by ambiphilic carbenes, we report and rationalize the absolute templating, the complete exchange of metals in a template, of group 11 clusters across the entire coinage metal family by means of galvanic exchange. We further delineate that these species provide a molecular model for better understanding the reduction of CO₂ at M(111) coinage metal surfaces.

Sub-monolayer Au9 cluster formation via pulsed nozzle cluster deposition

Submonolayer coverages of chemically synthesised triphenylphosphine-protected Au₉ clusters on mica and TiO₂ substrates were achieved through the development of a Pulsed Nozzle Cluster Deposition (PNCD) technique under high vacuum conditions. This method offers the deposition of pre-prepared, solvated clusters directly onto substrates in a vacuum without the potential for contamination from the atmosphere. AFM and TEM were used to investigate the rate of gold cluster deposition as a function of cluster solution concentration and the number of pulses, with pulse number showing the most effective control of the final deposition conditions. TEM and XPS were used to determine that the clusters retained their unique properties through the deposition process. Methanol solvent deposited in the PNCD process has been shown to be removable through post-deposition treatments. A physical model describing the vapour behaviour and solvent evaporation in a vacuum is also developed and presented.

Co13O8—metalloxocubes: a new class of perovskite-like neutral clusters with cubic aromaticity

Exploring stable clusters to understand structural evolution from atoms to macroscopic matter and to construct new materials is interesting yet challenging in chemistry. Utilizing our newly developed deep-ultraviolet laser ionization mass spectrometry technique, here we observe the reactions of neutral cobalt clusters with oxygen and find a very stable cluster species of Co₁₃O₈ that dominates the mass distribution in the presence of a large flow rate of oxygen gas. The results of global-minimum structural search reveal a unique cubic structure and distinctive stability of the neutral Co₁₃O₈ cluster that forms a new class of metal oxides that we named as ‘metalloxocubes’. Thermodynamics and kinetics calculations illustrate the structural evolution from icosahedral Co₁₃ to the metalloxocube Co₁₃O₈ with decreased energy, enhanced stability and aromaticity. This class of neutral oxygen-passivated metal clusters may be an ideal candidate for genetic materials because of the cubic nature of the building blocks and the stability due to cubic aromaticity.

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.

Ligand accommodation causes the anti-centrosymmetric structure of Au13Cu4 clusters with near-infrared emission

We synthesized an [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ nanocluster co-capped by phosphine and thiolate ligands. Interestingly, this Au₁₃Cu₄ cluster corresponds to an anti-centrosymmetric structure with the four copper atoms coordinated to the mixed ligands on the same side of the Au₁₃ icosahedron, which is in sharp contrast to the [Au₁₃Cu₄(PPh₂Py)₄(SPhtBu)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ clusters which possess highly symmetric structures with well-separated Cu adatoms. Both [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ clusters correspond to 8 valence electron superatoms with large HOMO-LUMO gaps, respectively. The difference in structure is rooted in the nature of the mixed ligands, with the bidentate SPy binding strongly to Cu on both binding sites (-N-Cu and Au-SR-Cu) leading to the co-linking of adjacent Cu atoms, while the bidentate PPh₂Py binds Cu on one site and Au on the other giving rise to a separation of the Cu atoms even in the presence of relatively higher monomer concentration. Both [Au₁₃Cu₄(PPh₃)₄(SPy)₈]⁺ and [Au₁₃Cu₂(PPh₃)₆(SPy)₆]⁺ display emissions in the near-IR regions. TD-DFT calculations reproduce the spectroscopic results with specified excited states, shedding light on the geometric and electronic behaviors of the ligand-protected Au₁₃M_x clusters.

An Au8 Cluster Fortified by Four Ferrocenes

Atomically precise metal clusters are now in the research spotlight, owing to the precise correlation between the physicochemical properties and their atomic-packing structures at an atomic-level. Herein we synthesized an Au₈ cluster capped by four ferrocene ligands (DPPF), in which the ferrocene not only can direct the precise formation of the Au₈ cluster, but also can solidify the structural pattern of the Au₈ cluster. The Au₈(DPPF)₄ clusters as heterogeneous catalysts can achieve efficiently catalytic performances for the CO oxidation reaction, mainly due to the resistance to aggregation into large particles under reaction conditions. Our results suggest that the homolytic phosphine dissociation nature and the postdissociation reconstruction effect induced by Fe may enhance the catalytic performances of Au₈(DPPF)₄.

Gold nanoclusters as electrocatalysts: size, ligands, heteroatom doping, and charge dependences

To establish an ultimate energy conversion system consisting of a water-splitting photocatalyst and a fuel cell, it is necessary to further increase the efficiencies of the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR). Recently, it was demonstrated that thiolate (SR)-protected gold clusters, Aun(SR)m, and their related alloy clusters can serve as model catalysts for these three reactions. However, as the previous data have been obtained under different experimental conditions, it is difficult to use them to gain a deep understanding of the means to attain higher activity in these reactions. Herein, we measured the HER, OER, and ORR activities of Aun(SR)m and alloy clusters containing different numbers of constituent atoms, ligand functional groups, and heteroatom species under identical experimental conditions. We obtained a comprehensive set of results that illustrates the effect of each parameter on the activities of the three reactions. Comparison of the series of results revealed that decreasing the number of constituent atoms in the cluster, decreasing the thickness of the ligand layer, and substituting Au with Pd improve the activities in all reactions. Taking the stability of the cluster into consideration, [Au24Pd(PET)18]0 (PET = 2-phenylethanethiolate) can be considered as a metal cluster with high potential as an HER, OER, and ORR catalyst. These findings are expected to provide clear design guidelines for the development of highly active HER, OER, and ORR catalysts using Aun(SR)m and related alloy clusters, which would allow realization of an ultimate energy conversion system.

Rational Design of "Three-in-One" Ratiometric Nanoprobes: Protein-Caged Dityrosine, CdS Quantum Dots, and Gold Nanoclusters

Recently, multiplexed ratiometric fluorescence sensors for detecting several analytes have received much interest because of their multifunctionality. Here, we fabricate a novel trinity fluorescent nanoprobe in which one small-molecule fluorophore, blue-emissive dityrosine (diTyr) residues, and two nanomaterial fluorophores, green-emissive CdS quantum dots (CdSQDs) and red-emissive gold nanoclusters (AuNCs), are cocaged in a bovine serum albumin (BSA) molecule. The large differences of Stokes shifts among diTyr residues, CdSQDs, and AuNCs ensure their emission at a single excitation wavelength. The nanoprobes can be facilely integrated using two-step synthetic reactions. DiTyr residues and AuNCs are formed and bound to the protein cage through the redox reaction between Au3+ and tyrosine residues of BSA, and the CdSQDs are followed to be conjugated to the modified BSA cage-templated CdS combination reaction. With established benign biocompatibility, the nanoprobes can ratiometrically detect intracellular glutathione by significantly enhancing the green emission of the conjugated CdSQDs. Likewise, the ratiometric sensing of solution alkalinity and tris(2-carboxyethyl)phosphine can be achieved using blue-emitted diTyr residues and red-emitted AuNCs as the responsive units, respectively, and the corresponding other two fluorophores as the reference signals. This study addresses a concept of trinity fluorescence ratiometric sensing system with multiple targets and optional references, which should be a promising pathway to meet the challenges from complexing biochemical environments and multivariate analysis.

A small bimetallic Ag3Cu2 nanocluster with dual emissions within and against Kasha's rule

Single crystals of a small bimetallic Ag₃Cu₂ nanocluster protected by six ligands of 2,4-dimethylbenzene thiol are synthesized by a one-pot procedure of wet chemistry. This Ag₃Cu₂ nanocluster bears a trigonal bipyramid metallic core with two copper atoms located on both sides of a triangular Ag₃. Interestingly, the six Cu-Ag side edges of the trigonal bipyramid are fully protected by the six ligands giving rise to reinforced stability and high chemical purity. More interestingly, this Ag₃Cu₂ cluster shows strong dual fluorescence emissions in both ultraviolet visible (UV-vis) and near infrared (NIR) regions. Theoretical calculations reproduce the absorption and fluorescence spectra where the NIR emission at 824 nm is assigned to the S₁→ S₀ transition, while the simultaneous emission in the visible band is due to the radiation of highly excited states and is against Kasha's rule.

Unravelling the formation mechanism of alkynyl protected gold clusters: a case study of phenylacetylene stabilized Au144 molecules

Despite recent progress in the preparation of alkynyl protected Au clusters with molecular purity (e.g., Na[Au₂₅(C[triple bond, length as m-dash]CAr)₁₈, Ar = 3,5-(CF₃)₂C₆H₃^-, Au₃₆(C[triple bond, length as m-dash]CPh)₂₄, Au₄₄(C[triple bond, length as m-dash]CPh)₂₈, and Au₁₄₄(C[triple bond, length as m-dash]CAr)₆₀, Ar = 2-F-C₆H₄^-), the formation mechanism still remains elusive. Herein, a new molecule-like alkynyl Au cluster was successfully prepared, and its formula was determined as Au₁₄₄(PA)₆₀ (PA = PhC[triple bond, length as m-dash]C-, phenylacetylene). In the formation of Au₁₄₄(PA)₆₀, the introduction of ethanol in post-synthesis treatment to manipulate the aggregation state of the precursor was found to play a critical role in producing the Au144 clusters. During the Au₁₄₄(PA)₆₀ formation process, the contents of PA, (PA)₂ and (PA)₄ were monitored by absorbance and gas chromatography-mass spectrometry (GC-MS), disclosing that Au₁₄₄(PA)₆₀ molecules were generated in sync with (PA)₄. Finally, the formation mechanism of Au₁₄₄(PA)₆₀ molecules has been tentatively proposed, of which three major stages are involved. This study can shed light on the formation mechanism that may be exploited for the precise control of the synthesis of alkynyl protected coinage metal clusters.

Origin of the Photoluminescence of Metal Nanoclusters: From Metal-Centered Emission to Ligand-Centered Emission

Recently, metal nanoclusters (MNCs) emerged as a new class of luminescent materials and have attracted tremendous interest in the area of luminescence-related applications due to their excellent luminous properties (good photostability, large Stokes shift) and inherent good biocompatibility. However, the origin of photoluminescence (PL) of MNCs is still not fully understood, which has limited their practical application. In this mini-review, focusing on the origin of the photoemission emission of MNCs, we simply review the evolution of luminescent mechanism models of MNCs, from the pure metal-centered quantum confinement mechanics to ligand-centered p band intermediate state (PBIS) model via a transitional ligand-to-metal charge transfer (LMCT or LMMCT) mechanism as a compromise model.

Gold Nanoclusters as Electrocatalysts for Energy Conversion

Gold nanoclusters (Aun NCs) exhibit a size-specific electronic structure unlike bulk gold and can therefore be used as catalysts in various reactions. Ligand-protected Aun NCs can be synthesized with atomic precision, and the geometric structures of many Aun NCs have been determined by single-crystal X-ray diffraction analysis. In addition, Aun NCs can be doped with various types of elements. Clarification of the effects of changes to the chemical composition, geometric structure, and associated electronic state on catalytic activity would enable a deep understanding of the active sites and mechanisms in catalytic reactions as well as key factors for high activation. Furthermore, it may be possible to synthesize Aun NCs with properties that surpass those of conventional catalysts using the obtained design guidelines. With these expectations, catalyst research using Aun NCs as a model catalyst has been actively conducted in recent years. This review focuses on the application of Aun NCs as an electrocatalyst and outlines recent research progress.

Photo/electrocatalysis and photosensitization using metal nanoclusters for green energy and medical applications

Owing to the rapidly increasing demand for sustainable technologies in fields such as energy, environmental science, and medicine, nanomaterial-based photo/electrocatalysis has received increasing attention. Recently, synthetic innovations have allowed the fabrication of atomically precise metal nanoclusters (NCs). These NCs show potential for green energy and medical applications. The present article primarily focuses on evaluation of the recent developments in the photo/electrocatalytic and photosensitizing characteristics of metal and alloy NCs. The review comprises two sections: (i) photo/electrocatalysis for green energy and (ii) photosensitization for biomedical therapy applications. Finally, the challenges associated with the use of metal NCs are presented on the basis of current developments.

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.