AuAg alloy nanomolecules with 38 metal atoms

Site-specific doping of silver atoms into a Au25 nanocluster as directed by ligand binding preferences

For the first time site-specific doping of silver into a spherical Au25 nanocluster has been achieved in [Au19Ag6(MeOPhS)17(PPh3)6] (BF4)2 (Au19Ag6) through a dual-ligand coordination strategy. Single crystal X-ray structural analysis shows that the cluster has a distorted centered icosahedral Au@Au6Ag6 core of D 3 symmetry, in contrast to the I h Au@Au12 kernel in the well-known [Au25(SR)18]- (R = CH2CH2Ph). An interesting feature is the coexistence of [Au2(SPhOMe)3] dimeric staples and [P-Au-SPhOMe] semi-staples in the title cluster, due to the incorporation of PPh3. The observation of only one double-charged peak in ESI-TOF-MS confirms the ordered doping of silver atoms. Au19Ag6 is a 6e system showing a distinct absorption spectrum from [Au25(SR)18]-, that is, the HOMO-LUMO transition of Au19Ag6 is optically forbidden due to the P character of the superatomic frontier orbitals.

Synthesis of metal nanoclusters and their application in Hg2+ ions detection: A review

Mercuric (Hg²⁺) ions released from human activities, natural phenomena, and industrial sources are regarded as the global pollutant of world's water. Hg²⁺ ions contaminated water has several adverse effects on human health and the environment even at low concentrations. Therefore, rapid and cost-effective method is urgently required for the detection of Hg²⁺ ions in water. Although, the current analytical methods applied for the detection of Hg²⁺ ions provide low detection limit, they are time consuming, require expensive equipment, and are not suitable for in-situ analysis. Metal nanoclusters (MNCs) consisting of several to ten metal atoms are important transition missing between single atoms and plasmonic metal nanoparticles. In addition, sub-nanometer sized MNCs possess unique electronic structures and the subsequent unusual optical, physical, and chemical properties. Because of these novel properties, MNCs as a promising material have attracted considerable attention for the construction of selective and sensitive sensors to monitor water quality. Hence this review is focused on recent advances on synthesis strategies, and optical and chemical properties of various MNCs including their applications to develop optical assay for Hg²⁺ ions in aqueous solutions.

Master key to coinage metal nanoclusters treasure chest: 38-metal clusters

Atomically precise metal nanoclusters with specific chemical compositions have become a popular research topic due to their precise structures, attractive properties, and wide range of applications in various fields. Currently, among more than 100 reported metal nanoclusters with precise formulas, 38-atom coinage metal nanoclusters stand out due to their unique structural diversities, such as face-centered cubic (FCC) and body-centered cubic (BCC) arrangements. Among them, the formation of the metal cores includes vertex-sharing, face-fusion, and FCC cubes fusion. Due to their geometrical features, 38-atom coinage metal nanoclusters exhibit attractive properties, making them an ideal model for exploring structure-property relationships. Therefore, 38-atom coinage metal nanoclusters are a universal key to the treasure trove of nanoclusters, which can open almost all fields and are of great research significance. This paper focuses on the structure of 38-atom coinage metal nanoclusters and reviews the preparation and crystallization methods, excellent properties, and practical applications. Finally, future research prospects and development opportunities are provided.

Programmable Metal Nanoclusters with Atomic Precision

With the recent establishment of atomically precise nanochemistry, capabilities toward programmable control over the nanoparticle size and structure are being developed. Advances in the synthesis of atomically precise nanoclusters (NCs, 1-3 nm) have been made in recent years, and more importantly, their total structures (core plus ligands) have been mapped out by X-ray crystallography. These ultrasmall Au nanoparticles exhibit strong quantum-confinement effect, manifested in their optical absorption properties. With the advantage of atomic precision, gold-thiolate nanoclusters (Au_n (SR)_m ) are revealed to contain an inner kernel, Au-S interface (motifs), and surface ligand (-R) shell. Programming the atomic packing into various crystallographic structures of the metal kernel can be achieved, which plays a significant role in determining the optical properties and the energy gap (E_g ) of NCs. When the size increases, a general trend is observed for NCs with fcc or decahedral kernels, whereas those NCs with icosahedral kernels deviate from the general trend by showing comparably smaller E_g . Comparisons are also made to further demonstrate the more decisive role of the kernel structure over surface motifs based on isomeric Au NCs and NC series with evolving kernel or motif structures. Finally, future perspectives are discussed.

Physical Origin of Dual-Emission of Au-Ag Bimetallic Nanoclusters

On the origin of photoluminescence of noble metal NCs, there are always hot debates: metal-centered quantum-size confinement effect VS ligand-centered surface state mechanism. Herein, we provided solid evidence that structural water molecules (SWs) confined in the nanocavity formed by surface-protective-ligand packing on the metal NCs are the real luminescent emitters of Au-Ag bimetal NCs. The Ag cation mediated Au-Ag bimetal NCs exhibit the unique pH-dependent dual-emission characteristic with larger Stokes shift up to 200 nm, which could be used as potential ratiometric nanosensors for pH detection. Our results provide a completely new insight on the understanding of the origin of photoluminescence of metal NCs, which elucidates the abnormal PL emission phenomena, including solvent effect, pH-dependent behavior, surface ligand effect, multiple emitter centers, and large-Stoke's shift.

Self-Assembly of Precision Noble Metal Nanoclusters: Hierarchical Structural Complexity, Colloidal Superstructures, and Applications

Ligand protected noble metal nanoparticles are excellent building blocks for colloidal self-assembly. Metal nanoparticle self-assembly offers routes for a wide range of multifunctional nanomaterials with enhanced optoelectronic properties. The emergence of atomically precise monolayer thiol-protected noble metal nanoclusters has overcome numerous challenges such as uncontrolled aggregation, polydispersity, and directionalities faced in plasmonic nanoparticle self-assemblies. Because of their well-defined molecular compositions, enhanced stability, and diverse surface functionalities, nanoclusters offer an excellent platform for developing colloidal superstructures via the self-assembly driven by surface ligands and metal cores. More importantly, recent reports have also revealed the hierarchical structural complexity of several nanoclusters. In this review, the formulation and periodic self-assembly of different noble metal nanoclusters are focused upon. Further, self-assembly induced amplification of physicochemical properties, and their potential applications in molecular recognition, sensing, gas storage, device fabrication, bioimaging, therapeutics, and catalysis are discussed. The topics covered in this review are extensively associated with state-of-the-art achievements in the field of precision noble metal nanoclusters.

The synthesis of novel fluorescent bimetal nanoclusters for aqueous mercury detection based on aggregation-induced quenching

In this research, new bimetal nanoclusters (DAMP-AuAg BNCs) with 4,6-diamino-2-mercaptopyrimidine (DAMP) as a reducing agent and stabilizer ligand were exploited. The nanoclusters displayed excellent fluorescent properties, very small size, good stability, and water solubility. It was found that the as-prepared DAMP-AuAg BNCs exhibited strong fluorescent emission at 640 nm under an excitation wavelength of 473 nm with a large Stokes shift of 167 nm, and the red fluorescence could be readily quenched with aqueous Hg²⁺. The DAMP-AuAg BNCs showed good specificity and sensitivity toward Hg²⁺ in aqueous solution, and the fluorescence analysis of Hg²⁺ showed a wide linear range from 0.85 μM to 246 μM and a detection limit of 20 nM. It is demonstrated that strong Hg²⁺-Au⁺ interactions led to the aggregation of nanoclusters, which caused the quenching of the fluorescence, and the affinity of Hg²⁺ for nitrogen should also be considered. Due to the relevant good performance of DAMP-AuAg BNCs, they were applied to the fluorescence analysis of Hg²⁺ in real water samples and were found to be a potential fluorescent sensor for aqueous mercury ions.

Dopamine-Modified AuCu Bimetallic Nanoclusters as Charge Transfer-Based Biosensors for Highly Sensitive Glycine Detection

Glycine is the simplest amino acid in living organisms and plays important roles in biology and medicine. However, few biosensors for glycine sensing have been reported. Herein, we present a facile strategy to construct dopamine-modified AuCu bimetallic nanoclusters (denoted as AuCu NC-DA) as charge transfer-based biosensors for highly sensitive glycine sensing. The AuCu NCs stabilized by bovine serum albumin (BSA) exhibited a fluorescence maximum at 400 nm. Because of the high affinity of BSA for dopamine (DA), the surface of the AuCu NCs was modified with DA without any complicated chemical reactions, resulting in fluorescence quenching through a charge transfer process. Among 20 amino acids, AuCu NC-DA exhibited an off/on fluorescence switching response specifically toward glycine through the formation of hydrogen bonds with oxidized DA, which inhibited the charge transfer process, leading to the emergence of a new emission peak at 475 nm. Spectroscopic and thermodynamic results combined with molecular docking analyses provided comprehensive understanding of the sensing mechanism. Furthermore, we showed that AuCu NC-DA was able to sense glycine in cells by imaging. Finally, the practicability of AuCu NC-DA for glycine detection was validated in milk drink samples. This study presents a promising type of a charge transfer-based sensor.

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.

Solvent-mediated assembly of atom-precise gold-silver nanoclusters to semiconducting one-dimensional materials

Bottom-up design of functional device components based on nanometer-sized building blocks relies on accurate control of their self-assembly behavior. Atom-precise metal nanoclusters are well-characterizable building blocks for designing tunable nanomaterials, but it has been challenging to achieve directed assembly to macroscopic functional cluster-based materials with highly anisotropic properties. Here, we discover a solvent-mediated assembly of 34-atom intermetallic gold-silver clusters protected by 20 1-ethynyladamantanes into 1D polymers with Ag-Au-Ag bonds between neighboring clusters as shown directly by the atomic structure from single-crystal X-ray diffraction analysis. Density functional theory calculations predict that the single crystals of cluster polymers have a band gap of about 1.3 eV. Field-effect transistors fabricated with single crystals of cluster polymers feature highly anisotropic p-type semiconductor properties with ≈1800-fold conductivity in the direction of the polymer as compared to cross directions, hole mobility of ≈0.02 cm² V^-1 s^-1, and an ON/OFF ratio up to ≈4000. This performance holds promise for further design of functional cluster-based materials with highly anisotropic semiconducting properties.

Atomic-level separation of thiolate-protected metal clusters

Fine metal clusters have attracted much attention from the viewpoints of both basic and applied science for many years because of their unique physical/chemical properties and functions, which differ from those of bulk metals. Among these materials, thiolate (SR)-protected gold clusters (Au_n(SR)_m clusters) have been the most studied metal clusters since 2000 because of their ease of synthesis and handling. However, in the early 2000s, it was not easy to isolate these metal clusters. Therefore, high-resolution separation methods were explored, and several atomic-level separation methods, including polyacrylamide gel electrophoresis (PAGE), high-performance liquid chromatography (HPLC), and thin-layer chromatography (TLC), were successively established. These techniques have made it possible to isolate a series of Au_n(SR)_m clusters, and much knowledge has been obtained on the correlation between the chemical composition and fundamental properties such as the stability, electronic structure, and physical properties of Au_n(SR)_m clusters. In addition, these high-resolution separation techniques are now also frequently used to evaluate the distribution of the product and to track the reaction process. In this way, high-resolution separation techniques have played an essential role in the study of Au_n(SR)_m clusters. However, only a few reviews have focused on this work. This review focuses on PAGE, HPLC, and TLC separation techniques, which offer high resolution and repeatability, and summarizes previous studies on the high-resolution separation of Au_n(SR)_m and related clusters with the purpose of promoting a better understanding of the features and the utility of these techniques.

Development of a near infrared Au-Ag bimetallic nanocluster for ultrasensitive detection of toxic Pb2+ ions in vitro and inside cells

Although the research activities pertaining to the synthesis of fluorescent noble metal nanoclusters (NCs) and their applications in biological optics have been growing, only limited information is available in the near IR (NIR) region. However, fluorescence spectroscopy and microscopy in the NIR region offer significant advantages over UV and visible wavelengths. In this manuscript, we demonstrate bio-mineralized synthesis of stable Au-Ag bimetallic NCs with tunable NIR fluorescence using bovine serum albumin (BSA) as a protein template. We also demonstrate its application in the detection of toxic heavy metal ions Pb²⁺ in vitro and inside cells. The tunability of the fluorescence emission between 680 nm and 815 nm is achieved by systematically varying the ratio of Au and Ag in the composite NCs. The bimetallic NCs when interacting with Pb²⁺ offered a large increase in fluorescence intensity, which enabled sensitive detection of Pb²⁺. We determined a limit of detection (LOD) of 96 nM for the detection of Pb²⁺ under in vitro conditions, which is significantly less than the safe level in drinking water. Its applicability has also been demonstrated successfully in real water samples collected from local water bodies.

Rapid synthesis of Au/Ag bimetallic nanoclusters with highly biochemical stability and its applications for temperature and ratiometric pH sensing

Herein, we developed a simple and rapid strategy to synthesize gold/silver bimetallic nanoclusters (Au/Ag NCs) with highly biochemical stability by a one-pot route. The Au/Ag NCs were obtained via a chemical reduction procedure in alkaline aqueous solution at 75 °C within only 20 min by employing bovine serum albumin (BSA) as both ligand and reductant. The as-obtained Au/Ag NCs displayed bright orange fluorescence with an emission peak located at 570 nm and temperature-dependent fluorescence property, which were utilized as fluorescent thermometer directly. More intriguingly, the Au/Ag NCs were very stable against various pH values, ions, biothiols, H₂O₂, fetal bovine serum (FBS), RPMI 1640 medium and amino acids. Taking advantage of the excellent biochemical stability, a ratiometric fluorescence biosensor, fluorescein-5-isothiocyanate (FITC)-Au/Ag NCs, was constructed for pH sensing based on the incorporation of FITC into the Au/Ag NCs. Furthermore, the ratiometric pH sensor was also successfully applied on the model of HeLa cells.

Bimetallic AuCu nanoclusters-based florescent chemosensor for sensitive detection of Fe3+ in environmental and biological systems

Assays of ferric ion (Fe³⁺) with high sensitivity and selectivity have been required to evaluate its amount in environmental and biological systems. Herein, a novel fluorometric penicillamine-capped bimetallic gold-copper nanoclusters (PA-AuCu bi-MNCs) sensor was constructed for facile, environmentally friendly and quantitative detection of Fe³⁺ through inner filter effect (IFE) mechanism. One-step green synthetic approach was applied for the synthesis of AuCu bi-MNCs by using d-penicillamine (D-PA) as template and stabilizer. In the presence of Fe³⁺, the emission of the PA-AuCu bi-MNCs was hindered that caused selective quenching of the fluorescence intensity. The response to Fe³⁺ allows for two linear dynamic ranges of 5.0 × 10^-7 M-7.0 × 10^-6 M and 7.0 × 10^-6 M-1.0 × 10^-4 M with a detection limit of 0.1 μM, which is approximately 53 times lower than the maximum level (5.37 μM) of Fe³⁺ in drinking water that had been reported by the World Health Organization. The independency of the system from most of the interferences is the important feature of this work. Beside the appropriate selectivity of the proposed method, it shows a considerable operation in various environmental samples including rain water, three types of river water and also in human blood serum as a biological matrix.

Doping of thiolate protected gold clusters through reaction with metal surfaces

A new technique is introduced for doping gold nanoclusters by using a metal surface such as Ag, Cu and Cd as a source of heteroatoms. The importance of the thiol ligand in the doping process is examined by following the reactions with MALDI-TOF mass spectrometry in the presence and the absence of the thiols on the surface. The doping reactions depend greatly on the type of the cluster and the availability of the ligand which is a crucial element for alloying. The thiol acts as a messenger exchanging the metal atoms between the cluster and the metal surface as revealed by the XPS studies performed on the metal surfaces.

Single Au Atom Doping of Silver Nanoclusters

Ag₂₉ nanoclusters capped with lipoic acid (LA) can be doped with Au. The doped clusters show enhanced stability and increased luminescence efficiency. We attribute the higher quantum yield to an increase in the rate of radiative decay. With mass spectrometry, the Au-doped clusters were found to consist predominantly of Au₁Ag₂₈(LA)₁₂^3-. The clusters were characterized using X-ray absorption spectroscopy at the Au L₃-edge. Both the extended absorption fine structure (EXAFS) and the near edge structure (XANES) in combination with electronic structure calculations confirm that the Au dopant is preferentially located in the center of the cluster. A useful XANES spectrum can be recorded for lower concentrations, or in shorter time, than the more commonly used EXAFS. This makes XANES a valuable tool for structural characterization.

Ultrasmall Noble Metal Nanoparticles: Breakthroughs and Biomedical Implications

As a bridge between individual atoms and large plasmonic nanoparticles, ultrasmall (core size <3 nm) noble metal nanoparticles (UNMNPs) have been serving as model for us to fundamentally understand many unique properties of noble metals that can only be observed at an extremely small size scale. With decades'efforts, many significant breakthroughs in the synthesis, characterization and functionalization of UNMNPs have laid down a solid foundation for their future applications in the healthcare. In this review, we aim to tightly correlate these breakthroughs with their biomedical applications and illustrate how to utilize these breakthroughs to address long-standing challenges in the clinical translation of nanomedicines. In the end, we offer our perspective on the remaining challenges and opportunities at the frontier of biomedical-related UNMNPs research.

M4Au12Ag32(p-MBA)30 (M = Na, Cs) bimetallic monolayer-protected clusters: synthesis and structure

Crystals of M₄Au₁₂Ag₃₂(p-MBA)₃₀ bimetallic monolayer-protected clusters (MPCs), where p-MBA is p-mercapto-benzoic acid and M⁺ is a counter-cation (M = Na, Cs) have been grown and their structure determined. The mol-ecular structure of triacontakis[(4-carboxylatophenyl)sulfanido]dodecagolddotriacontasilver, Au₁₂Ag₃₂(C₇H₅O₂S)₃₀ or C₂₁₀H₁₅₀Ag₃₂Au₁₂O₆₀S₃₀, exhib-its point group symmetry at 100 K. The overall diameter of the MPC is approximately 28 Å, while the diameter of the Au₁₂Ag₂₀ metallic core is 9 Å. The structure displays ligand bundling and inter-molecular hydrogen bonding, which gives rise to a framework structure with 52% solvent-filled void space. The positions of the M⁺ cations and the DMF solvent mol-ecules within the void space of the crystal could not be determined. Three out of the five crystallographically independent ligands in the asymmetric unit cell are disordered over two sets of sites. Comparisons are made to the all-silver M₄Ag₄₄(p-MBA)₃₀ MPCs and to expectations based on density functional theory.

Manifestation of Geometric and Electronic Shell Structures of Metal Clusters in Intercluster Reactions

Monolayer protected clusters exhibit rich diversity in geometric and electronic structures. However, structure-reactivity relationships in these clusters are rarely explored. In this context, [Ag₄₄(SR)₃₀]^4-, where -SR is an alkyl/aryl thiolate, is an interesting system due to its geometrically and electronically closed-shell structures and distinct charge states. We demonstrate that these structural features of [Ag₄₄(SR)₃₀]^4- are distinctly manifested in its solution-state reaction with another cluster, [Au₂₅(SR)₁₈]^-. Through this reaction, an alloy cluster anion, [Au₁₂Ag₃₂(SR)₃₀]^4-, evolves spontaneously as revealed by high-resolution electrospray ionization mass spectrometry. Ultraviolet-visible absorption spectroscopy and density functional theory calculations indicate that [Au₁₂Ag₃₂(SR)₃₀]^4- is formed by the substitution of all of the Ag atoms in the innermost icosahedral shell of [Ag₄₄(SR)₃₀]^4- and the abundance is attributed to its higher stability due to closed geometric as well as electronic shell structure, similar to the reactant clusters. We further demonstrate that the substitution of metal atoms in the middle dodecahedral shell and the outermost mount sites are also possible, however such substitutions produce Au_xAg_44-x(SR)₃₀ alloy clusters with geometrically and electronically open shells. Depending on specific sites of substitution, an unexpected superatom-nonsuperatom transition occurs in the distribution of Au_xAg_44-x(SR)₃₀ alloy clusters formed in this reaction. Our results present a unique example of a structure-reactivity relationship in the metal atom substitution chemistry of monolayer protected clusters, wherein a systematic trend, reflecting the geometric and the electronic shell structures of the reactant as well as the product clusters, was observed.

Synthesis of Metal Nanoparticles and Metal Fluoride Nanoparticles from Metal Amidinate Precursors in 1-Butyl-3-Methylimidazolium Ionic Liquids and Propylene Carbonate

Decomposition of transition-metal amidinates [M{MeC(NiPr)2} n ] [M(AMD) n ; M=MnII, FeII, CoII, NiII, n=2; CuI, n=1) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (triflate) ([BMIm][TfO]), and 1-butyl-3-methylimidazolium tosylate ([BMIm][Tos]) or in propylene carbonate (PC) gives transition-metal nanoparticles (M-NPs) in non-fluorous media (e.g. [BMIm][Tos] and PC) or metal fluoride nanoparticles (MF2-NPs) for M=Mn, Fe, and Co in [BMIm][BF4]. FeF2-NPs can be prepared upon Fe(AMD)2 decomposition in [BMIm][BF4], [BMIm][PF6], and [BMIm][TfO]. The nanoparticles are stable in the absence of capping ligands (surfactants) for more than 6 weeks. The crystalline phases of the metal or metal fluoride synthesized in [BMIm][BF4] were identified by powder X-ray diffraction (PXRD) to exclusively Ni- and Cu-NPs or to solely MF2-NPs for M=Mn, Fe, and Co. The size and size dispersion of the nanoparticles were determined by transmission electron microscopy (TEM) to an average diameter of 2(±2) to 14(±4) nm for the M-NPs, except for the Cu-NPs in PC, which were 51(±8) nm. The MF2-NPs from [BMIm][BF4] were 15(±4) to 65(±18) nm. The average diameter from TEM is in fair agreement with the size evaluated from PXRD with the Scherrer equation. The characterization was complemented by energy-dispersive X-ray spectroscopy (EDX). Electrochemical investigations of the CoF2-NPs as cathode materials for lithium-ion batteries were simply evaluated by galvanostatic charge/discharge profiles, and the results indicated that the reversible capacity of the CoF2-NPs was much lower than the theoretical value, which may have originated from the complex conversion reaction mechanism and residue on the surface of the nanoparticles.

Au38Cu1(2-PET)24 nanocluster: synthesis, enantioseparation and luminescence

Two different species of the chiral Au₃₈Cu₁(2-PET)₂₄ adduct, showing enhanced fluorescence, were successfully separated using chiral HPLC.

Synthesis of metal-fluoride nanoparticles supported on thermally reduced graphite oxide

Metal-fluoride nanoparticles, (MF _x -NPs) with M = Fe, Co, Pr, Eu, supported on different types of thermally reduced graphite oxide (TRGO) were obtained by microwave-assisted thermal decomposition of transition-metal amidinates, (M{MeC[N(iPr)]₂} _n ) or [M(AMD) _n ] with M = Fe(II), Co(II), Pr(III), and tris(2,2,6,6-tetramethyl-3,5-heptanedionato)europium, Eu(dpm)₃, in the presence of TRGO in the ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF₄]). The crystalline phases of the metal fluorides synthesized in [BMIm][BF₄] were identified by powder X-ray diffraction (PXRD) to be MF₂ for M = Fe, Co and MF₃ for M = Eu, Pr. The diameters and size distributions of MF _x @TRGO were from (6 ± 2) to (102 ± 41) nm. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) were used for further characterization of the MF _x -NPs. Electrochemical investigations of the FeF₂-NPs@TRGO as cathode material for lithium-ion batteries were evaluated by galvanostatic charge/discharge profiles. The results indicate that the FeF₂-NPs@TRGO as cathode material can present a specific capacity of 500 mAh/g at a current density of 50 mA/g, including a significant interfacial charge storage contribution. The obtained nanomaterials show a good rate capacity as well (220 mAh/g and 130 mAh/g) at a current density of 200 and 500 mA/g, respectively.

Gold-doped silver nanocluster [Au3Ag38(SCH2Ph)24X5]2- (X = Cl or Br)

Here we report the single-crystal structure, experimental and theoretical characterization of a 41-metal atom Au-Ag alloy nanocluster [Au₃Ag₃₈(SCH₂Ph)₂₄X₅]^2- (1, X = Cl or Br). The nanocluster 1 is co-protected by thiolate and halogen atoms and features an all-metallic face-fused biicosahedral Au₂@AuAg₂₀ rod-like kernel enwrapped by the outermost Ag₁₈(SCH₂Ph)₂₄X₃ shell. Two sites on the surface of the biicosahedral kernel are partially occupied by Au and Ag atoms. The outer Ag₁₈(SCH₂Ph)₂₄X₃ shell is composed of two Ag₆S₆ cycles at the two poles and one Ag₆S₂X₃ arc at the equator with both 2- and 3-coordinated Ag atoms, which has not been observed in gold or silver nanoclusters ever before. Theoretical calculations elucidate its electronic structure as well as optical properties, thus producing informative correlations between its structure and properties. This nanocluster exhibits near-infrared (NIR) emission around 825 nm. This work (i) snapshots a rare crystal structure of an Au-doped silver alloyed nanocluster; (ii) gives a deep insight to understand how the capping ligand or anions affect the structure of the alloy nanocluster; and (iii) provides precise information about gold atom doping site that is very significant in the recognition of potential active catalytic sites of the alloy nanoparticles.

Symmetry breaking in ligand-protected gold clusters probed by nonlinear optics

The first hyperpolarizabilities of [Au25(SR)18](-1/0) and Au38(SR)24 clusters were determined by Hyper-Rayleigh Scattering. A strong dependence on the molecular symmetry was observed, and we explore two strategies to destroy the center of inversion in [Au25(SR)18](-1/0), protection by chiral ligands and alloying of the cluster with silver. This may open new avenues to applications of Au : SR clusters in second-order nonlinear optics.

Synergism of gold and silver invites enhanced fluorescence for practical applications

Synergism of gold and silver improves fluorescence behavior of gold–silver bimetallic clusters with practical applications.

High-resolution separation of thiolate-protected gold clusters by reversed-phase high-performance liquid chromatography

This perspective summarizes our work on high-resolution separation of thiolate-protected gold clusters using reversed-phase high-performance liquid chromatography, new findings obtained by those separation, and future prospects for this field.

Thiolated Au18 cluster: preferred Ag sites for doping, structures, and optical and chiroptical properties

Silver doping of thiolated Au₁₈ cluster occurs in the inner core.

Controlling the selectivity of catalytic oxidation of styrene over nanocluster catalysts

The water is discovered to control the selectivity by changing the surface composition of Au nanocluster or the valence state of Ag nanocluster.

Ion-precursor and ion-dose dependent anti-galvanic reduction

Controlling alloy nanoparticles with atomic monodispersity is challenging, and the recently revealed anti-galvanic reduction (AGR) provides a unique solution to this challenge. Herein we demonstrate that AGR is ion-precursor and ion-dose dependent, which offers novel strategies to tune the composition, structure and properties of nanoparticles by varying the ion-precursor and ion-dose in the AGR reaction.

X-ray Crystal Structure of Au38-xAgx(SCH2CH2Ph)24 Alloy Nanomolecules

Herein, we report the X-ray crystallographic structure of a 38-metal atom Au-Ag alloy nanomolecule. The structure of monometallic Au38(SR)24 consists of 2 central Au atoms and 21 Au atoms forming a bi-icosahedral core protected by 6 dimeric and 3 monomeric units. In Au38-xAgx(SR)24,where x ranges from 1 to 5, the silver atoms are selectively incorporated into the Au21 bi-icosahedral core. Within the Au21 core, the silver atoms preferentially occupy nine selected locations: (a) the two vertex edges, three atoms on each edge and six atoms total, and (b) the middle face-shared three-atom ring, adding to a total of nine locations. X-ray crystallography yielded a composition of Au34.04Ag3.96(SCH2CH2Ph)24. The crystal structure of the alloy nanomolecule can be described in terms of shells as Au2@Au17.04Ag3.96@ 6×[-SR-Au-SR-Au-SR] 3×[-SR-Au-SR-].

Atomically precise metal nanoclusters: stable sizes and optical properties

Controlling nanoparticles with atomic precision has long been a major dream of nanochemists. Breakthroughs have been made in the case of gold nanoparticles, at least for nanoparticles smaller than ∼3 nm in diameter. Such ultrasmall gold nanoparticles indeed exhibit fundamentally different properties from those of the plasmonic counterparts owing to the quantum size effects as well as the extremely high surface-to-volume ratio. These unique nanoparticles are often called nanoclusters to distinguish them from conventional plasmonic nanoparticles. Intense work carried out in the last few years has generated a library of stable sizes (or stable stoichiometries) of atomically precise gold nanoclusters, which are opening up new exciting opportunities for both fundamental research and technological applications. In this review, we have summarized the recent progress in the research of thiolate (SR)-protected gold nanoclusters with a focus on the reported stable sizes and their optical absorption spectra. The crystallization of nanoclusters still remains challenging; nevertheless, a few more structures have been achieved since the earlier successes in Au102(SR)44, Au25(SR)18 and Au38(SR)24 nanoclusters, and the newly reported structures include Au20(SR)16, Au24(SR)20, Au28(SR)20, Au30S(SR)18, and Au36(SR)24. Phosphine-protected gold and thiolate-protected silver nanoclusters are also briefly discussed in this review. The reported gold nanocluster sizes serve as the basis for investigating their size dependent properties as well as the development of applications in catalysis, sensing, biological labelling, optics, etc. Future efforts will continue to address what stable sizes are existent, and more importantly, what factors determine their stability. Structural determination and theoretical simulations will help to gain deep insight into the structure-property relationships.

Optical properties of nanoalloys

Optical absorption spectra of bare (left) and monolayer-protected (right) metal nanoalloys.

Recent Progress in the Functionalization Methods of Thiolate-Protected Gold Clusters

Nanomaterials that exhibit both stability and functionality are currently considered to hold great promise as components of nanotechnology devices. Thiolate-protected gold clusters (Aun(SR)m) have long attracted attention as functional nanomaterials. Magic Aun(SR)m clusters are an especially stable group of thiolate-protected clusters that have particularly high potential as functional materials. Although numerous application experiments have been conducted for magic Aun(SR)m clusters, it is important that functionalization methods are also established to allow for effective utilization of these materials. The results of recent research on heteroatom doping and the use of other chalcogenide ligands strongly suggest that these strategies are promising as functionalization methods of magic Aun(SR)m clusters. In this Perspective, we focus on studies relating to three representative types of magic clusters-Au25(SR)18, Au38(SR)24, and Au144(SR)60-and discuss the recent progress and future issues.

Au99(SPh)42 nanomolecules: aromatic thiolate ligand induced conversion of Au144(SCH2CH2Ph)60

A new aromatic thiolate protected gold nanomolecule Au99(SPh)42 has been synthesized by reacting the highly stable Au144(SCH2CH2Ph)60 with thiophenol, HSPh. The ubiquitous Au144(SR)60 is known for its high stability even at elevated temperature and in the presence of excess thiol. This report demonstrates for the first time the reactivity of the Au144(SCH2CH2Ph)60 with thiophenol to form a different 99-Au atom species. The resulting Au99(SPh)42 compound, however, is unreactive and highly stable in the presence of excess aromatic thiol. The molecular formula of the title compound is determined by high resolution electrospray mass spectrometry (ESI-MS) and confirmed by the preparation of the 99-atom nanomolecule using two ligands, namely, Au99(SPh)42 and Au99(SPh-OMe)42. This mass spectrometry study is an unprecedented advance in nanoparticle reaction monitoring, in studying the 144-atom to 99-atom size evolution at such high m/z (∼12k) and resolution. The optical and electrochemical properties of Au99(SPh)42 are reported. Other substituents on the phenyl group, HS-Ph-X, where X = -F, -CH3, -OCH3, also show the Au144 to Au99 core size conversion, suggesting minimal electronic effects for these substituents. Control experiments were conducted by reacting Au144(SCH2CH2Ph)60 with HS-(CH2)n-Ph (where n = 1 and 2), bulky ligands like adamantanethiol and cyclohexanethiol. It was observed that conversion of Au144 to Au99 occurs only when the phenyl group is directly attached to the thiol, suggesting that the formation of a 99-atom species is largely influenced by aromaticity of the ligand and less so on the bulkiness of the ligand.