In the past decade, gold nanoclusters of atomic precision have been demonstrated as novel and promising materials for potential applications in catalysis and biotechnology because of their optical properties and photovoltaics. The Au₃₆(SR)₂₄ nanocluster is one of the most well-known clusters, which is directly converted from the Au₃₈(SR)₂₄ cluster through a "ligand-exchange" process. It consists of an Au₂₈ kernel with a truncated face-centered cubic (FCC) framework exposing the (111) and (100) facets. Here we report a simple protocol to prepare Au₃₆(SR)₂₄ nanoclusters, ligated by aliphatic and aromatic thiolate ligands (SR = SPh, SC₆H₄CH₃, SCH(CH₃)Ph, and SC₁₀H₇) via a "size-focusing" process. First, polydisperse Au nanoparticles were synthesized and isolated, and then reacted under harsh conditions in the presence of excess thiol ligands at relatively high etching temperatures (80 ℃). The as-synthesized Au₃₆(SR)₂₄ nanoclusters were characterized and analyzed by UV-Vis absorption spectroscopy, electrospray ionization (ESI), and matrix-assisted laser desorption ionization (MALDI) mass spectrometry, as well as thermogravimetric analysis (TGA). All the Au₃₆(SR)₂₄ nanoclusters showed two step-like optical absorption peaks at ~370 and 580 nm in the UV-vis spectra. Only one strong set of nanocluster-ion peaks centered at an m/z of 10517.0 was observed in the ESI mass spectrum of the Au₃₆(SCH(CH₃)Ph)₂₄ nanocluster. This could be assigned to the [Au₃₆(SCH(CH₃)Ph)₂₄Cs]⁺ species, and was a strong indicator of the high purity of the as-obtained Au₃₆ cluster samples produced on a small scale. The TGA profile showed 31.67% organic weight loss of the nanocluster, matching well with the expected theoretical value of 31.71%. The Au-SR bond in the gold nanoclusters was broken at ~180 ℃ in a normal air atmosphere. Fragments of the Au₃₆(SR)₂₄ clusters capped with different thiolate ligands, which were mainly caused by the strong laser intensity during the analysis, were detected in the MALDI mass spectra. This interesting phenomenon was also observed in the case of Au₂₅(SR)₁₈, and could be due to the inherent properties of the Au-SR bond on the surface of the gold nanoclusters. Finally, the optical properties of the Au₃₆(SR)₂₄ nanoclusters were found to be influenced by the capping thiolate ligands. Compared to the UV-Vis spectrum of the Au₃₆(SCH(CH₃)Ph)₂₄ cluster, the optical spectra of the other three Au₃₆ clusters were red-shifted (~3 nm for Au₃₆(SPh)₂₄, 5 nm for Au₃₆(SC₆H₄CH₃)₂₄, and 13 nm for the (Au₃₆(SNap)₂₄ clusters). This shift could be explained by the electron transfer occurring from the electron-rich aromatic ligands to the Au kernel. The electron transfer capacity followed the order ―SNap > ―SC₆H₄CH₃ > ―SPh > ―SCH(CH₃)Ph. Overall, this study demonstrates the effectiveness and promising application of ligand engineering for tailoring the electronic properties of Au nanoclusters.

Research on gold nanoclusters is at the frontier of nanoscience and nanotechnology. The introduction of the first phosphine-protected gold nanocluster, Au₁₁(PPh₃)₇(SCN)₃ (where PPh₃ stands for triphenylphosphine and Ph stands for benzene), can be dated back to 1969. As research in the field progressed, many structures of phosphine-protected nanoclusters such as Au₅, Au₈, Au₁₃, and Au₃₉ were reported. However, the stability of these phosphine-protected nanoclusters was not satisfactory, which handicapped their research and application. In an attempt to find alternatives for phosphine-protected nanoclusters, thiolated gold nanoclusters have attracted extensive attention in recent years. So far, there has been great progress primarily owing to the development of wet-chemical synthesis techniques, among which the utilization of ligand-exchange has been proved to be very effective to synthesize thiolated gold nanoclusters. It can be easily understood that phosphine in gold nanoclusters can be exchanged with thiolate because the latter has stronger affinity for gold. However, we recently found that the reverse ligand-exchange, i.e., the exchange of thiolate with phosphine, can also take place. Some questions have naturally arisen: Is the reverse ligand-exchange only applicable to superatomic [Au₂₅(SR)₁₈]⁻ (SR: thiolate) nanoclusters? Can it occur in other thiolated gold nanoclusters? If so, is this reverse ligand-exchange also dependent on the starting nanoclusters? These intriguing issues have inspired us to conduct this work.

We investigated the reactions of PPh₃ with some thiolated gold nanoparticles, including [Au₂₃(SC₆H₁₁)₁₆]⁻, Au₂₄(SC₂H₄Ph)₂₀, Au₃₆(TBBT)₂₈ (where TBBT stands for 4-(tert-butyl) benzene-1-thiolate), Au₃₈(SC₂H₄Ph)₂₀, mixed nanoclusters, and 3 nm Au nanoparticles. Surprisingly, the experimental results showed that under the action of PPh₃, thiolated gold nanoclusters (or nanoparticles) with different compositions, structures, sizes, and protecting thiolates can be uniformly transformed to [Au₁₁(PPh₃)₈Cl₂]⁺ and then [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺. In other words, PPh₃ is a universal converter for these thiolated gold nanoparticles. However, gold nanoparticles that are protected by polyvinylpyrrolidone (PVP) or citrate and [Ag₂₅(SPhMe₂)₁₈]⁻ particles cannot be transformed to [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺ and [Ag₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺, respectively, under the same conditions, which indicated that the special reactivity with PPh₃ is unique to thiolated gold nanoparticles. Our preliminary investigation on a possible reaction path between thiolated gold nanoparticles and PPh₃ also revealed that the peeling process found in Au₂₅ nanoclusters may be applicable to the conversion of [Au₂₃(SC₆H₁₁)₁₆]⁻, but not other nanoclusters like Au₂₄(SC₂H₄Ph)₂₀ and Au₃₆(TBBT)₂₈.

Employing this special chemistry, we synthesized seven [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺ species with various ligands and investigated the effect of the ligand on the luminescence properties of [Au₂₅(PPh₃)₁₀(SR)₅Cl₂]²⁺. We found that the luminescence quantum yields decreased in the following order: [Au₂₅(PPh₃)₁₀(SCH₂Ph-t-Bu)₅Cl₂]²⁺ (1.32 × 10⁻⁴) > [Au₂₅(PPh₃)₁₀(SCH₂Ph)₅Cl₂]²⁺ (8.23 × 10⁻⁵) > [Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (5.35 × 10⁻⁶) > [Au₂₅(PPh₃)₁₀(SC₁₂H₂₅)₅Cl₂]²⁺ (5.02 × 10⁻⁶) > [Au₂₅(PPh₃)₁₀(SPh-t-Bu)₅Cl₂]²⁺ (3.97 × 10⁻⁶) > [Au₂₅(PPh₃)₁₀(SC₆H₁₃)₅Cl₂]²⁺ (3.73 × 10⁻⁶) > [Au₂₅(PPh₃)₁₀(S-c-C₆H₁₁)₅Cl₂]²⁺ (1.53 × 10⁻⁶). Therefore, it can be concluded that SCH₂Ph-t-Bu is the best ligand, while S-c-C₆H₁₁ is the worse one for triggering luminescence from gold nanoparticles in the investigated thiolates.

Since such diversity in surface ligands is not found in other nanoclusters (e.g., [Au₂₅(SR)₁₈]⁻), the special chemistry between thiolated gold nanoparticles and PPh₃ provides excellent opportunities to investigate the effect of ligands on the properties of gold nanoparticles and to screen ligands for special applications.

In summary, in this work we reveal the unique chemistry of thiolated gold nanoparticles with PPh₃ and provide excellent opportunities for investigating ligand effects of gold nanoparticles and screening ligands for special applications.

Atomically precise pieces of metallic matter with nanometer dimensions, which are called nanoclusters, have attracted special research interest as a frontier in nanoscience research. These nanoclusters exhibit unique properties that make them suitable for widespread applications in fields like medical treatments and catalysis. Studies in nanoclusters have been greatly benefited from the use of advanced instrumentation, especially adaptation of mass spectrometry (e.g., matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) and electrospray ionization mass spectrometry (ESI MS)). However, mass spectrometry could not elucidate the bonding between metals and ligands; therefore, single-crystal X-ray diffraction (SC-XRD) analysis has been used. SC-XRD is significant for the development of the nanocluster range in terms of revealing the precise structure of nanoclusters and fully understanding the structure-property relationship. Furthermore, understanding the nature of nanocluster surface has provided possibility to embellish nanocluster surface and to improve their performance. Nowadays, alloy nanoclusters play an important role in catalysis, biology, and materials science. Researchers have synthesized and predicted the alloy structure composed of silver and nickel in ultra-small size (Ag₄Ni₂(DMSA)_4, (DMSA = meso-2, 3-dimercaptosuccinic acid)). However, no precise crystal structure has been reported. Herein, we report the crystal structure of the Ag-Ni alloy nanocluster Ag₄Ni₂(SPhMe₂)₈. The structure was further confirmed by SC-XRD, X-ray photoelectron spectroscopy (XPS), MALDI-TOF MS, ESI MS and thermo gravimetric analysis (TGA) measurements. The stability experiment suggested that the Ag₄Ni₂ nanocluster could be stable in ultra-small sizes. This research on Ag-Ni alloy nanoclusters will contribute to the understanding of the alloy in ultra-small sizes. Specifically, based on the structure determination by SC-XRD, the structure of Ag₄Ni₂(SPhMe₂)₈ could be divided into three layers: upper and lower layers with Ni(SPhMe₂)₄ complexes constituting a parallelogram, and the middle layer with four silver atoms constituting a parallelogram like a sandwich. The Ag―Ni, Ag―S and Ni―S bond distances were 0.31–0.32, 0.23–0.24, and 0.22–0.23 nm, respectively. XPS analyses revealed that the Ag/Ni/S atomic ratio was 5.19/2.55/10.28, consistent with the corresponding expected ratio of 4/2/8 in Ag₄Ni₂(SPhMe₂)₈. In addition, the Ag 3d_3/2 and Ag 3d_5/2 binding energy peaks were located at 375.0 and 369.0 eV, respectively, and the Ni 2p_1/2 and Ni 2p_3/2 are located at 871.50 and 853.90 eV, respectively. Moreover, combined with ESI, the Ag 3d_3/2 and Ag 3d_5/2 binding energies of Ag₄Ni₂(SPhMe₂)₈ were close to the +1 valences, according to previous reports. Meanwhile, the spectra of Ag₄Ni₂(SPhMe₂)₈ illustrated that the valence of nickel was +2. Additionally, the MALDI-TOF mass spectrum was in good agreement with the ESI results. Weight loss upon heating was used to confirm the percentage of organic material in nanoclusters (66.31% weight loss was observed in TGA, consistent with the 66.67% loss calculated according to the formula). In the liquid state, the UV-Vis spectra showed no change after exposure to oxygen for a few weeks. Meanwhile, we used UV-Vis spectroscopy at temperatures under 80 ℃ to test the stability of the Ag₄Ni₂(SPhMe₂)₈. The absorption peaks were almost identical with each other, suggesting high stability of the Ag₄Ni₂(SPhMe₂)₈. Our study proves that small-sized alloy also has the possibility of diversification, which will play an important role in the synthesis of alloy nanoclusters. Moreover, this research on Ag-Ni alloy nanoclusters will contribute to the understanding of alloys in ultra-small sizes.

Singlet oxygen (¹O₂) plays an important role in various applications, such as in the photodynamic therapy (PDT) of cancers, photodynamic inactivation of microorganisms, photo-degradation of toxic compounds, and photo-oxidation in synthetic chemistry. Recently, water-soluble metal nanoclusters (NCs) have been utilized as photosensitizers for the generation of highly reactive ¹O₂ because of their high water solubility, low toxicity, and surface functionalizability for targeted substances. In the case of metal NC-based photosensitizers, the photo-physical properties depend on the core size of the NCs and the core/ligand interfacial structures. A wide range of atomically precise gold NCs have been reported; however, reports on the synthesis of atomically precise silver NCs are limited due to the high reactivity and low photostability (i.e., easy oxidation) of Ag NCs. In addition, there have been few reports on what kinds of metal NCs can generate large amounts of ¹O₂. In this study, we developed a new one-pot synthesis method of water-soluble Ag₇(MBISA)₆ (MBISA = 2-mercapto-5-benzimidazolesulfonic acid sodium salt) NCs with highly efficient ¹O₂ generation ability under the irradiation of white light emitting diodes (LEDs). The molecular formula and purity were determined by electrospray ionization mass spectrometry and gel electrophoresis. To the best of our knowledge, this is the first report on atomically precise thiolate silver clusters (Ag_n(SR)_m) for efficient ¹O₂ generation under visible light irradiation. The ¹O₂ generation efficiency of Ag₇(MBISA)₆ NCs was higher than those of the following known water-soluble metal NCs: bovine serum albumin (BSA)-Au₂₅ NCs, BSA-Ag₈ NCs, BSA-Ag₁₄ NCs, Ag₂₅(dihydrolipoic acid)₁₄ NCs, Ag₃₅(glutathione)₁₈ NCs, and Ag₇₅(glutathione)₄₀ NCs. The metal NCs examined in this study showed the following order of ¹O₂ generation efficiency under white light irradiation: Ag₇(MBISA)₆ > BSA-Ag₁₄ > Ag₇₅(SG)₄₀ > Ag₃₅(SG)₁₈ > BSA-Au₂₅ ≫ BSA-Ag₈ (not detected) and Ag₂₅(DHLA)₁₄ (not detected). For further improving the ¹O₂ generation of Ag₇(MBISA)₆ NCs, we developed a novel fluorescence resonance energy transfer (FRET) system by conjugating Ag₇(MBISA)₆ NCs with quinacrine (QC) (molar ratio of Ag NCs to QC is 1 : 0.5). We observed the FRET process, from QC to Ag₇(MBISA)₆ NCs, occurring in the conjugate. That is, the QC works as a donor chromophore, while the Ag NCs work as an acceptor chromophore in the FRET process. The FRET-mediated process caused a 2.3-fold increase in ¹O₂ generation compared to that obtained with Ag₇(MBISA)₆ NCs alone. This study establishes a general and simple strategy for improving the PDT activity of metal NC-based photosensitizers.

We present the atomic structure of thiolate-protected hollow Au nanosphere (HAuNS), Au₆₀(SR)₂₀, with high symmetry and stability based on the grand unified model (GUM; Nat. Commun. 2016, 7, 13574) and density-functional theory (DFT) calculations. Using C₂₀ fullerene (with I_h symmetry) as a template, 20 tetrahedral Au₄ units were used to replace the C atoms of C₂₀, and three Au atoms of each Au₄ were fused with three neighboring Au₄ units by sharing one Au atom to form an icosahedral Au₅₀ fullerene cage as the inner core. Subsequently, the unfused Au atom in each Au₄ was bonded with the [―RS―Au―SR―] staple to form the completely hollow Au₆₀(SR)₂₀ nanosphere. Therefore, the Au₆₀(SR)₂₀ is composed of an icosahedral Au₅₀ fullerene hollow cage (constructed by fusing 20 tetrahedral Au₄ units) with 10 [―RS―Au―SR―] staples, obeying the "divide and protect" rule. Each Au₄ unit has 2e valence electrons, namely, the tetrahedral Au₄(2e) elementary block in the grand unified model. The DFT calculations showed that this hollow Au₆₀(SR)₂₀ nanosphere had a large HOMO–LUMO (HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital) gap (1.3 eV) and a negative nucleus-independent chemical shift (NICS) value (−5) at the center of the hollow cage, indicating its high chemical stability. Furthermore, the NICS values in the center of the tetrahedral Au₄ units were much more negative than that in the center of the hollow cage, revealing that the overall stability of Au₆₀(SR)₂₀ likely stemmed from the local stability of each tetrahedral Au₄ unit. The harmonic vibrational frequencies were all positive, suggesting that the HAuNS corresponded to the local minimum of the potential energy surface. In addition, the bilayer HAuNS was designed by fusing the tetrahedral Au₄ layers, indicating the feasibility of tuning the thickness of the shell of HAuNS. In bilayer HAuNS, each tetrahedral Au₄ unit in the first layer shared four Au atoms, while those in the second layer shared one Au atom. The other three Au atoms of each tetrahedral unit bonded with the SR groups, demonstrating that each tetrahedral Au₄ unit has 2e valence electrons (namely the tetrahedral Au₄(2e) elementary block in GUM). The HOMO-LUMO gap of the bilayer Au₁₄₀(SH)₆₀ nanosphere is 1.5 eV, indicating its chemical stability. The thicknesses of the shells in monolayer and bilayer HAuNS are about 0.2 and 0.4 nm, respectively. This process could be easily understood in terms of the local stabilities of the tetrahedral Au₄(2e) elementary block in GUM. Finally, the design of larger HAuNS, Au₁₈₀(SR)₆₀, has also been presented. The HOMO-LUMO gap of Au₁₈₀(SH)₆₀ was 0.9 eV, which showed that it was also a stable HAuNS. This work provides a new strategy to controllably design the HAuNS.

Crystalline silver cluster compounds are highly interesting owing to their intriguing structure and potential technological application in luminescence, semiconductivity, and as precursors for nonlinear optical materials. Typically, the synthesis of silver clusters involves the use of protecting ligands such as thiolates, phosphines, and alkynes, which have been found to be critical for cluster size and shape tuning. Among these nanoclusters, pentacosanuclear silver clusters (Ag₂₅) have only been reported with either thiolate ligands or phosphine ligands, while those bearing mixed protecting ligands are rather rare. In the course of our exploration of novel silver clusters, a new silver thiolate precursor (ⁱPrC₆H₄SAg)_n was used to construct nanosized silver clusters. In the presence of 1, 3-bis(diphenyphosphino)propane (dppp) and CF₃SO₃Ag, the pentacosanuclear silver cluster [Ag₂₅(SC₆H₄Prⁱ)₁₈(dppp)₆](CF₃SO₃)₇·CH₃CN (designated as Ag₂₅, HSC₆H₄Prⁱ = 4-t-isopropylthiophenol) ligated both by thiolate and phosphine ligands was obtained under ultrasonic reaction conditions. Yellow block crystals were isolated from the solution, whose molecular structure was elucidated by single-crystal X-ray analysis. The skeleton of the Ag₂₅ cluster comprises a sandwich-like motif containing two structurally very similar cylinders sharing a metal-cluster plane. The core of each cylinder presents the overall shape of a twisted hexagonal cylinder made of two connected Ag₃S₃ units, with six sulfur atoms and six silver atoms alternating on a puckered drum-like surface. The metal-cluster plane contains one type of pure-Ag tetragons showing significant Ag…Ag argentophilic interactions. The optical properties of Ag₂₅ were investigated in the solid state. A band gap of ~2.5 eV was estimated for Ag₂₅ from the optical absorption spectrum, suggesting this cluster to be a potential wide-gap semiconductor. This Ag₂₅ cluster was also found to emit green luminescence at λ = 505 nm and room temperature.

Atomically precise nanoclusters form an important class of functional materials that have recently attracted research interest for their unique properties and easily tunable surface functionalities. Core-shell nanomaterials with precise structural information can be produced to better understand the structure–property relationships for different applications. Polyoxo-titanium clusters (PTCs) are such a kind of nanomaterial for different functional applications in catalysis, photovoltaics, ceramics, etc. However, the high bandgap of semiconductive PTCs is the limiting factor in their practical solar application in the visible region of sunlight. The development of PTCs with different surface-bound ligands is an emerging area of research in the design and synthesis of core-shell nanoclusters with reduced bandgaps. It has been extensively reported that the polynuclear growth of PTCs requires molecular-level water supply in reactions. Moreover, it is important to identify more environment-friendly synthetic methods. Deep eutectic-solvothermal (DES) synthesis is an emerging green method for the synthesis of different crystalline materials. The hygroscopic nature of DES should enhance the provision of water during polynuclear growth of nanoclusters. Hence, we chose to synthesize different kinds of PTCs using DES as solvent. Two nanoclusters, Zr-oxo (PTC-65) and Zr/Ti-oxo (PTC-66) clusters having surface-bound 1, 10-phenanthroline (1, 10-phn) and phenol ligands, were successfully synthesized using this approach; 1, 10-phn was employed as the precursor in the synthetic reaction, and phenol was not employed directly in the chemical reaction, but was supplied from the DES solvent used in the reaction. In the presence of chromophoric ligands, 1, 10-phn and phenol are believed to enhance the light absorption properties of the resulting functional nanomaterials. Their crystal structure revealed that they form core-shell mimics with Zr-oxo and Ti/Zr-oxo core units having surface-bound shell ligands. Based on their different structural characteristics, photocatalytic hydrogen evolution studies were performed on these two functional materials using an aqueous solution of H₂O (50 mL)/triethanol amine (10 mL). Interestingly, PTC-65 formed a turbid solution, whereas PTC-66 formed a clear solution. The possible reasons for their different dispersion behaviors are widely discussed, with emphasis on their structure–property relationships. This study provides a potential tool for the homogenization of Ti-O materials to improve their photocatalytic activities. Moreover, the success of our work confirms that deep eutectic-solvothermal synthesis can be an effective method for cluster preparation. Many other interesting polynuclear complexes like polyoxometalates, chalcogenides, and noble-metal clusters could be obtained by this synthetic methodology.

In recent years, Au nanoclusters have attracted much attention as new nanomaterials, which contain several to two hundred Au atoms and are protected by ligands. The structures and properties of Au nanoclusters are usually sensitive to the particle size due to quantum confinement effect. Au nanoclusters have been applied to different fields, such as optical properties, catalysis, and biology. There are two common methods for the synthesis of atomically precise Au nanoclusters: "size focusing" and "ligand exchange". Although a series of Au nanocluster have been obtained via "size focusing" and "ligand exchange", obtaining high yield of such Au nanoclusters is a challenge. Au₂₁(S-Adam)₁₅ was previously synthesized via etching Au₁₈ nanoclusters with excess thiols, and its crystal structure was determined by X-ray diffraction crystallography; however, the yield of Au nanoclusters was low. In this study, we prepared Au₂₁(S-Adam)₁₅ in high yield via conversion of Au₂₃(S-Adam)₁₆ to Au₂₁(S-Adam)₁₅. Firstly, Au₂₃(S-Adam)₁₆ nanoclusters were synthesized using adamantanethiols(HS-Adam) as the protecting ligand and HAuCl₄ as the gold resource in ethyl acetate solvent. Au₂₃(S-Adam)₁₆ were further etched with excess thiols at room temperature. After reacting for 30 min, highly pure Au₂₁(S-Adam)₁₅, with high yield of ~20% based on HAuCl₄ precursor, were successfully prepared. Au₂₃(S-Adam)₁₆ and Au₂₁(S-Adam)₁₅ were characterized by electrospray ionization (ESI), UV-Vis absorption spectroscopy, matrix-assisted laser desorption ionization (MALDI) mass spectrometry, and thermogravimetric analysis (TGA). ESI-MS and UV-Vis spectra confirm the high purity of the Au₂₃(S-Adam)₁₆. After conversion, UV-Vis spectra show the absorption peaks of Au₂₁(S-Adam)₁₅ at 700, 540, 435 and 380 nm. The MALDI-MS of Au₂₁(S-Adam)₁₅ shows several peaks at 6502, 6471, 6106, 5411, and 5048, assigned to Au₂₁(S-Adam)₁₄S, Au₂₁(S-Adam)₁₄, Au₂₀(S-Adam)₁₃, Au₁₉(S-Adam)₁₀, and Au₁₈(S-Adam)₉, respectively. The fragments of Au nanoclusters were produced by the strong laser intensity, which easily removes carbon tails from HS-Adam. Thermogravimetric analysis (TGA) was also performed to check the purity of Au₂₁(S-Adam)₁₅ nanoclusters. The TGA curve shows a weight loss of 42% (expected value, 38%). UV-Vis absorption spectroscopy was performed to track the conversion of Au₂₃(S-Adam)₁₆ to Au₂₁(S-Adam)₁₅. It was found that Au₂₃(S-Adam)₁₆ can convert to Au₂₁(S-Adam)₁₅ with a conversion efficiency of up to 97%, using excess thiols at room temperature within 30 min. In general, we successfully synthesized highly pure Au₂₁(S-Adam)₁₅ nanoclusters, with high yield of ∼20% based on HAuCl₄, by etching Au₂₃(S-Adam)₁₆ with excess thiols at room temperature.

Metal nanoclusters (MNCs), as a new type of nano-material, possess excellent properties such as facile synthesis, strong light stability, low toxicity, excellent biocompatibility, and high luminous efficiency. Aggregation-induced emission (AIE), which can enhance the luminescence properties of MNCs, has resulted in MNCs attracting significant attention. In this thesis, L-glutathione (GSH)-protected copper nanoclusters (GS@CuNCs) were prepared by a "one-pot" method in aqueous solution without additional reducing agents. The GS@CuNCs were characterized by UV-Vis absorption spectroscopy and fluorescence spectroscopy. Upon excitation at 370 nm, the fluorescence spectrum of GS@CuNCs displayed the maximum emission peak at 610 nm. The as-prepared CuNCs generate a striking fluorescence intensity via aggregation-induced emission (AIE). The AIE property of GS@CuNCs was examined for the aggregates in different organic solvents, such as ethanol, methanol, and dimethylformamide. Since the aggregation degree was controlled by the content of organic solvent, we further measured the fluorescence intensity of GS@CuNCs in different volume ratios of a water-ethanol mixture solution. The fluorescence intensity of GS@CuNCs exhibited an approximately 30-fold increase at 85% of ethanol content, as compared to that in aqueous solution. A possible mechanism may be that intramolecular motions are restricted in ethanol, resulting in a significant increase of fluorescence intensity. Moreover, only very weak emissions were recorded for the CuNC dispersion in aqueous solution; however, an apparent luminescence enhancement was observed in both luminescence spectra and by naked eyes under UV light, with a gradual increase in the ethanol content in the water-ethanol mixture from 0% to 85%. Additionally, we developed a new selective and sensitive turn-on fluorescent sensor for the detection of trivalent aluminum ions (Al³⁺) based on cation-induced aggregation methods. Among the 15 types of metal cations studied, only Al³⁺ visibly increased the fluorescence emission of the GS@CuNCs. These results indicated that the GS@CuNCs were highly selective to Al³⁺ than other metal ions, which may result from the electrostatic and coordination interactions between the trivalent aluminum ions and monovalent carboxylic anions from GSH in the CuNCs. The response of the probe to Al³⁺ exhibited a good linear range of 2–20 μmol·L^-1 and the detection limit was 33 nmol·L^-1. Thus, the weak fluorescence intensity of CuNCs was increased markedly by the AIE of Al³⁺, and could construct an interesting fluorescent platform for sensing aluminum ions. The property of AIE of GS@CuNCs may expand the potential applications of nanocluster materials to biosensors and cell imaging.

Alloy metal nanoclusters (NCs), including bimetallic and multimetallic clusters, have recently emerged as a novel class of multifunctional nanomaterials. They are widely used in catalysis, optical sensing, and biological imaging due to their excellent physicochemical properties such as unique electronic structure, ultrasmall size, strong photoluminescence, and rich surface chemistry. Although much progress has been made in the development of NCs, a major challenge in the synthesis of the relevant multifunctional nanomaterial is to achieve the synthetic methodological breakthrough, especially for controlling the synthesis and structure of NCs with atomic precision. It is evident that by realizing controlled synthesis and structural regulation at the atomic scale, we can better understand and tune the fundamental properties of NCs for efficient use in various application areas; this could also shed light on the development of new functionalized nanomaterials. Most of the recent research on the controlled synthesis and structural characterization of metal clusters with atomic precision has focused on monometallic NCs, and significant progress has been realized with respect to alloy metal NCs. A number of synthetic strategies have been developed for synthesizing high-quality alloy NCs with well-defined compositions, sizes, and architectures. In this review, we have highlighted some recent advances in strategies for the precise synthesis of ligands-protected alloy metal NCs, especially thiolate-stabilized gold-based alloy NCs. We classified the synthetic strategies for alloy NCs into several strategies, which include one-pot synthesis, metal exchange, ligand exchange, chemical etching, intercluster reactions, surface motif exchange reaction, and in situ two-phase ligand exchange strategy. One-pot synthesis is facile and widely used as a synthetic strategy for monodisperse alloy NCs with well-defined compositions, sizes, architectures, and surface chemistries. However, the alloy NCs obtained through the one-pot strategy often exhibits a relatively somber fluorescence. Therefore, other synthesis strategies have been exploited that can fabricate alloy NCs exhibiting strong photoluminescence. Among them, the surface motif exchange reaction is expected to be extended to the fabrication of new binary alloy NCs with precise alloy sites to broaden the physicochemical properties of the NCs; intercluster reactions has been explored as an emerging and efficient strategy to fabricate atomically precise alloy NCs. It is noted that the two or multiple metal species incorporated in a single alloy NC usually show unexpected synergistic properties like adjustable electronic structures and strong photoluminescence. Such unique properties have rapidly motivated the research community to use alloy NCs in many applications such as catalysis, biosensors, and biomedicine.

Motivated by the unusual structure of the [Pd₄(μ₃-SbMe₃)₄(SbMe₃)₄] cluster, which is composed of a tetrahedral (T_d) Pd(0) core with four terminal SbMe₃ ligands and four triply bridging SbMe₃ ligands capping the four triangular Pd₃ faces (J. Am. Chem. Soc. 2016, 138, 6964), we performed a computational study of the structure and bonding characteristics of the T_d [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster and a series of its analogues. The T_d structure of the [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster could be explained by the cluster electron-counting rules based on the 18-electron rule for transition-metal centers; each sp³ hybridized Pd atom contributed ten valence electrons, and eight valence electrons were provided by one terminal SbH₃ and three bridging μ₃-SbH₃ ligands. The [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster had a count of 104 valence electrons in total; chemical bonding analysis indicated that the cluster featured twenty electron lone pairs generated by d orbital of the four Pd atoms, twenty-four Sb―H σ bonds, four terminal Pd―Sb σ bonds, and four delocalized bonds. There were two bonding patterns of the eight delocalized electrons between the four capping Sb atoms and the Pd₄ core. The first pattern was based on the superatom-network (SAN) model, whereby the palladium cluster could be described as a network of four 2e^– superatoms. The second pattern was based on the spherical jellium model, whereby the cluster could be rationalized as an 8e^– [Pd₄(μ₃-SbH₃)₄] superatom with 1S²1P⁶ electronic configuration. The density functional theory (DFT) calculations showed that the T_d [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster had a large HOMO-LUMO (HOMO: highest occupied molecular orbital; LUMO: lowest unoccupied molecular orbital) energy gap (2.84 eV) and a negative nucleus-independent chemical shift (NICS) value (-12) at the center of the [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] cluster, indicating its high chemical stability and aromaticity. Furthermore, the NICS values in the range of 0–0.30 nm of the [Pd₄(μ₃-SbH₃)₄] motifs were much more negative than those of [Pd₄(SbH₃)₄] in the same range, revealing that the overall stability of [Pd₄(μ₃-SbH₃)₄(SbH₃)₄] was likely derived from the local stability of Pd₄(μ₃-SbH₃)₄. Meanwhile, the d¹⁰…d¹⁰ interaction played a critical role in stabilizing the Pd₄ tetrahedron structure, which is similar to the aurophilicity in Au-Au clusters. It was also found that there is a large difference in the stability of transition metal and non-transition metal clusters with a tetrahedron structure. The structures and bonding patterns of the designed analogues were similar to those of [Pd₄(μ₃-SbH₃)₄(SbH₃)₄]. To summarize, this study was relevant for deciphering the nature of the bonds in a tetrahedral complex with four cores and eight ligands, and predicting a series of analogues. It is expected that this work will provide more options for the synthesis of tetrahedral 4-core transition metal compounds.

Gold nanoclusters are promising materials for a variety of applications because of their unique "superatom" structure, extraordinary stability, and discrete electronic energy levels. Controlled synthesis of well-defined Au nanoclusters strongly depends on rational design and implementation of their synthetic chemistry. Among the numerous approaches for the synthesis of monodisperse Au nanoclusters, etching of pre-formed polydisperse clusters has been widely employed as a top-down method. Understanding the formation mechanism of metal nanoclusters during the etching process is important. Herein, we synthesized monodisperse Au₁₃(L₃)₂(SR)₄Cl₄ nanoclusters via an etching reaction between polydisperse 1, 3-bis(diphenyl-phosphino)propane (L₃)-protected polydisperse Au_n (15 ≤ n ≤ 60) clusters and a mixed solution of HCl/dodecanethiol (SR). The Au₁₃ product, with a mean size of (1.1 ± 0.2) nm, shows pronounced step-like multiband absorption peaks centered at 327, 410, 433, and 700 nm. The synthetic protocol has a suitable reaction rate that allowss for real-time spectroscopic studies. We used a combination of in situ X-ray absorption fine structure (XAFS) spectroscopy, UV-Vis absorption spectroscopy, and matrix-assisted laser desorption ionization mass-spectrometry (MALDI-MS) to study the kinetic formation process of monodisperse Au₁₃ nanoclusters. Emphasis was given to the detection of reaction intermediates. The study revealed that the size-conversion of the Au₁₃ nanoclusters can be divided into three stages. In the first stage, the polydisperse Au₁₅–Au₆₀ clusters, covering a wide m/z range of 3000-13000, are prominently decomposed into smaller Au₈-Au₁₁ (within a m/z range of 3000–4000) species owing to the etching effect of HCl. They are immediately stabilized by the absorbed SR, L₃, and Cl^- ligands to form metastable intermediates, as indicated by the high intensity of the Au-ligand coordination peak at 0.190 nm as well as the low intensity of the Au–Au peaks (0.236 and 0.288 nm) in the Fourier-transform (FT) EXAFS spectra. In the second stage, these Au₈–Au₁₁ intermediates are grown into Au₁₃ cores. The experimental X-ray absorption near-edge spectra, totally different from that of Au(Ⅰ)-SR polymer, could be well reproduced by the calculated spectrum of the Au₁₃P₈Cl₄ cluster. The Au-ligand coordination number (1.0) obtained from the EXAFS fitting is much closer to the nominal values in Au₁₃(L₃)₂(SR)₄Cl₄ (0.92) than to that in Au(Ⅰ)-SR polymers (2.0), suggesting that majority of the Au atoms are in the form of Au₁₃ clusters. The driving force for this growth process is primarily the geometric factor to form a complete icosahedral Au₁₃ skeleton through the incorporation of Au(Ⅰ) ions or Au(Ⅰ)-Cl oligomers pre-existing in the solution. In the third stage, the composition of the clusters is nearly unchanged as indicated by the MALDI-MS and the UV-vis spectra; however, their atomic structure undergoes rearrangement to the energetically stable structure of Au₁₃(L₃)₂(SR)₄Cl₄. During this structural rearrangement, the central-peripheral and peripheral-peripheral Au–Au bond lengths (R_Au-Au(c-p) and R_Au-Au(p-p)) decrease from 0.272 to 0.267 nm and 0.295 to 0.289 nm, respectively, resulting in considerable structural distortion of the original icosahedral Au₁₃ skeleton. This distortion is also reflected by the slightly increased disorder degree of the Au-Au bonds from 0.00015 to 0.00017 nm². This work expands our understanding of the kinetic growth process of metal nanoclusters and promotes design and synthesis of metal nanomaterials in a controllable manner.

Heterometallic d-4f nanoclusters are currently of interest due to their potential use in material science and as probes in biology. Self-assembly by metal-ligand coordination is one of the most efficient processes that organize individual molecular components into nanosized species. However, for lanthanide-based self-assemblies, their stoichiometries and structures are difficult to control during synthesis, because the Ln(Ⅲ) ions often display high and variable coordination numbers. As a result, the structures of lanthanide complexes are commonly influenced by a variety of factors, such as the type of metal ions, the formation of ligands, and the nature of counter anions. In this article, two Zn-Ln nanoclusters [Ln₂Zn₂L₂(OAc)₆] (Ln = Yb (1) and Er (2)) were prepared using a new long Schiff base ligand with a Ph(CH₂)Ph backbone. These nanoclusters show interesting rectangular-like structures. The long Schiff base ligand displays a "stretched" configuration and is bound to the metal ions through its N and phenoxide and methoxy O atoms. As a result, large clusters (0.7 nm × 1.1 nm × 2.2 nm for 1) were formed. In the crystal structures of 1 and 2, each Ln³⁺ ion and its closer Zn²⁺ ion are linked by one OAc^- anion and phenolic oxygen atoms of two long Schiff base ligands, forming a ZnLn unit. Two such ZnLn units are then bridged by two Schiff base ligands to form the nano-rectangular structures. Energy dispersive X-ray spectroscopy (EDX) analyses of the clusters indicate that the molar ratio of Zn : Ln is about 1 : 1, in agreement with their crystal structures. Thermogravimetric analyses show that the clusters lose about 5% of the weight when heated to below 100 ℃. Melting point measurements show that the clusters are thermodynamically stable. Upon excitation of the ligand-centered absorption bands, 1 and 2 exhibit the NIR luminescence of Yb³⁺ and Er³⁺, respectively. The clusters show two excitation bands from 250 to 500 nm, in agreement with their absorption spectra, confirming that energy transfer occurs from the Zn/L centers to Ln³⁺ ions. These results indicate that the chromogenic Zn/L components in these nanoclusters can act as efficient sensitizers for lanthanide luminescence. The efficiencies of the energy transfer from Zn/L-centers to Yb³⁺ is higher than that to Er³⁺, being 75.71% and 25.00% for 1 and 2, respectively. These results provide new insights into the design of polynuclear nanoclusters with interesting luminescence properties.

Recent progress in the research of atomically-precise metal nanoclusters has identified a series of exceptionally stable nanoclusters with specific chemical compositions. Structural determination on such "magic size" nanoclusters revealed a variety of unique structures such as decahedron, icosahedron, as well as hexagonal close packing (hcp) and body-centered cubic (bcc) packing arrangements in gold nanoclusters, which are largely different from the face-centered cubic (fcc) structure in conventional gold nanoparticles. The characteristic geometrical structures enable the nanoclusters to exhibit interesting properties, and these properties are in close correlation with their atomic structures according to the recent studies. Experimental and theoretical analyses have been applied in the structural identification aiming to clarify the universal principle in the structural evolution of nanoclusters. In this mini-review, we summarize recent studies on periodic structural evolution of fcc-based gold nanoclusters protected by thiolates. A series of nanoclusters exhibit one-dimensional growth along the [001] direction in a layer-by-layer manner from Au₂₈(TBBT)₂₀ to Au₃₆(TBBT)₂₄, Au₄₄(TBBT)₂₈, and to Au₅₂(TBBT)₃₂ (TBBT: 4-tert-butylbenzenethiolate). The optical properties of these nanoclusters also evolve periodically based on steady-state and ultrafast spectroscopy. In addition, two-dimensional growth from Au₄₄(TBBT)₂₈ toward both [100] and [010] directions leads to the Au₉₂(TBBT)₄₄ nanocluster, and the recently reported Au₅₂(PET)₃₂ (PET: 2-phenylethanethiol) also follows this growth pattern with partial removal of the layer. Theoretical predictions of relevant fcc nanoclusters include Au₆₀(SCH₃)₃₆, Au₆₈(SCH₃)₄₀, Au₇₆(SCH₃)₄₄, etc, for the continuation of 1D growth pattern, as well as Au₆₈(SR)₃₆ mediating the 2D growth pattern from Au₄₄(TBBT)₂₈ to Au₉₂(TBBT)₄₄. Overall, this mini-review provides guidelines on the rules of structural evolution of fcc gold nanoclusters based on 1D, 2D and 3D growth patterns.