Multiple synthesis routes for atomically precise noble metal nanoclusters

Abstract

Well-defined metal nanoclusters protected by thiolates, with sizes between nanocrystals and metal atoms, have attracted enormous attention due to their various structures, controllable compositions, intriguing physical and chemical properties, and potential applications. Atomically precise metal nanoclusters are a type of important nanomaterial that can provide an ideal platform to address some key challenges related to their applications. However, compared to the straightforward synthesis of larger nanoparticles, the preparation of ultra-small metal nanoclusters frequently encounters difficulties owing to the pursuit of monodispersity and atomic accuracy. Although a series of effective synthesis methods have been developed for metal nanoclusters with well-defined sizes, structures and compositions, rational design and successful preparation of atomically precise metal nanoclusters still face challenges that further hinder the enrichment of cluster libraries and the in-depth understanding of structure–property relationships. In this review, we summarize some recent advances in strategies for the synthesis of atomically precise metal nanoclusters, in particular, silver and gold nanoclusters as well as alloy nanoclusters, and emphasize the following synthesis methods including the Brust–Schiffrin method, ligand-exchange, galvanic/anti-galvanic reaction, etching, solid phase synthesis and intercluster reaction.

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1. Introduction

Generally, ultrasmall metal nanoparticles (less than 3 nm in size, so-called nanoclusters), including gold and silver nanoclusters, consist of several to a few hundred metal atoms with a diameter of about the Fermi wavelength of an electron and represent a new type of aggregation material comprising a metal core and an organic ligand shell.^1–11 Owing to their unique structure and size, nanoparticles show characteristic quantum confinement effects, which lead to their discrete energy levels and molecular-like physicochemical properties, such as strong fluorescence with high QYs, HOMO–LUMO transitions, molecular magnetism, chirality, and so on.^12–27 These attractive properties of sub-nanometer sized metal nanoclusters are distinctively different from those of their larger counterparts, and make these clusters ideal nanomaterials which are widely used in potential applications ranging from biological imaging to optical devices and energy conversion.^28–41

For metal nanoclusters, the fine control of inherent features, including the size, shape, composition, state of aggregation, atomic packing model and crystallographic arrangement, can effectively tailor their properties.^42–53 Normally, replacing the central metal atom in the inner structure (e.g., core) of a cluster with a heteroatom (e.g., Au, Pt, Pd, Cu, Cr and Pd) can affect the optical (absorption and fluorescence) properties and HOMO–LUMO gaps owing to the synergistic effect on electronic perturbation induced by the incoming atom.^54–66 Meanwhile, tuning the outer structure of clusters, which is enclosed by an outer protecting ligand, is also meaningful for metal nanoclusters for specific applications, albeit all the difficulties.^67–74 Besides, modifying the type of organic ligand and bonding models on the periphery of the clusters is also a crucial step to render their unique properties.^75–84 Only by successfully realizing the controllable synthesis and structural manipulation of atomically precise metal nanoclusters at the atomic level can researchers deeply understand the structure–property correlation of the clusters and ultimately achieve the purpose of cluster practical application.^85–93 At present, the most feasible strategy to control all of these intriguing properties related to the unique structural features of these clusters is to design a suitable synthesis method. Nevertheless, dexterously designing and synthesizing metal nanoclusters remain a major challenge owing to the lack of synthesis approaches.

The synthetic chemistry of atomically precise metal nanoclusters has been explored for a long time, and a large number of excellent studies in the field of cluster synthesis have been reported.^1,2,94–100 Therefore, it is important to highlight the research progress on the controllable synthesis of metal nanoclusters. This review aims to conduct a comprehensive summary of the recent synthesis advances in atomically precise metal nanoclusters, in particular silver, gold and alloy clusters, based on the Brust–Schiffrin method, ligand-exchange, galvanic/anti-galvanic reaction, etching, solid phase synthesis and intercluster reaction (Scheme 1).

Scheme 1 Schematic representation of multiple routes for controlled synthesis of monodisperse metal nanoclusters.

2. Brust–Schiffrin method

The first described method, the Brust–Schiffrin method, is a simple and conventional method used for generation of highly purified nanoclusters with a high production rate. The pioneering work could be traced back to 1994 when a great breakthrough in the synthesis of gold nanoclusters was achieved by Brust and Schirrin.¹⁰¹ Since then, the popular synthesis method has been widely used to prepare air stable, readily soluble, and isolable metal nanoclusters. After decades of development, the original Brust–Schiffrin method gradually evolved into two variants, including a two-phase system in pure water and a toluene solvent and a modified one-phase system (Fig. 1).^102–112 Generally, gold nanoclusters, such as the “star cluster”-Au₂₅,^113–125 can be prepared via the two-phase method, but silver nanoclusters are formed using the modified one-phase method.^126–135 In detail, the two-phase Brust–Schiffrin method consists of two steps. In the first step, the gold ion provided by the gold salt is sufficiently dissolved in an aqueous solution and the resulting solution further acts as the metal precursor, and then the precursor is transferred to an organic phase under the action of the presence of phase-transfer agents, such as typically tetraoctylammonium bromide. In the second step, both the capping agent (thiolate) and reducing agent (e.g. sodium borohydride) are added in succession to the organic solution to prepare the charge-neutral target nanoclusters capped with thiols. In other words, two steps are employed in the two-phase Brust–Schiffrin method, which are transfer of the metal precursor (eqn (1)) and reduction of the target metal ions (eqn (2)).^{101,102,109–111}

AuC1₄⁻ (aq) + N(C₈H₁₇)₄⁺ (toluene) → N(C₈H₁₇)₄⁺AuC1₄⁻ (toluene) (1)

mAuCl₄⁻ (toluene) + nC₁₂H₂₅SH (toluene) + 3me⁻ → 4m Cl⁻ (aq) + (Au_m)(C₁₂H₂₅SH)_n (toluene) (2)

Fig. 1 (a) Schematic illustration of the synthesis of chalcogenate-protected gold nanoclusters by the Brust–Schiffrin method. Reproduced with permission from ref. 103. Copyright 2011, American Chemical Society. (b) Illustrative diagram of the synthesis of hydrophilic CTAB capped Au₂₅ nanoclusters via two-phase protection deprotection. Reproduced with permission from ref. 104. Copyright 2012, American Chemical Society. (c) The route for preparation of selenolated gold nanoclusters by the solvent-directed phase transfer method. Reproduced with permission from ref. 110. Copyright 2020, Wiley-VCH.

The synthesis mechanism of the two-phase method was studied for a long time because having an insight into the mechanism helps us understand the nucleation mechanisms, growth patterns and passivation procedures in this process of synthesis and even design novel routes for synthesizing other nanoclusters with a high yield. As for the detailed mechanism of the two-phase method, the majority view was that the addition of the capping agent reduces the Au³⁺ to Au⁺ and forms [Au(I)-SR]_n-like intermediate polymers, but some excellent studies held different opinions. For example, the types of precursors in the process of the two-phase system were specifically identified and quantified for metal systems (such as Au, Ag, and Cu) by Goulet and co-workers.¹³⁶ They found that tetraalkylammonium metal complexes were proved to be the precursors of the two-phase reactions, which differed from the widely accepted assumptions, but for the one-phase reactions, M(I) thiolates were shown to be the precursors based on the NMR results. Tong et al. successfully studied the reaction solution compositions in the well-known two-phase synthesis process after addition of the capping ligand and strong reducing agent via multiple techniques, including Raman, NMR, and surface plasmon resonance (SPR) analysis techniques.¹⁰³ Finally, they made a conclusion that there was no metal–sulfur bond formation in the organic reaction solution before addition of the reducing agent, and the two-phase system was actually a reverse micellar synthesis method, which was also beneficial to the synthesis of other metals, including Ag and Cu.

Another highlight of the Brust–Schiffrin method is to prepare alloy nanoclusters. And this method has gradually developed into a mainstream method for preparing alloy nanoclusters. As early as 2009, Murray's group prepared small monolayer-capped AuPd nanoclusters using this method, which were determined to be Au₂₄Pd(SC₂Ph)₁₈ based on positive mode electrospray ionization mass spectrometry (ESI-MS).¹³⁷ Also, Negishi et al. synthesized Au_25−xAg_x(SC₁₂H₂₅)₁₈ bimetal nanoclusters with different Ag–Ag mixing ratios in a binary reaction system.¹³⁸ Initially, after mixing the aqueous solution of gold and silver salts with various molar ratios (22 : 3, 19 : 6, 15 : 10, 10 : 15, 8 : 17 and 5 : 20, respectively), a toluene solution of the phase transfer agent was introduced into the reaction system. When the two phases were separated, a capping agent was added into the separated toluene solution. By reducing the metal–thiolate complexes, nanoclusters were obtained. By the way, the physical properties and electronic structure of Au_25−xAg_x(SC₁₂H₂₅)₁₈ could be continuously tailored by the doped Ag atoms. Similarly, based on this proposed protocol, Xie's group also synthesized a series of fluorescent alloy nanoclusters capped with hydrophilic ligands.¹³⁹

Besides, Brust et al. also extended this well-known method to the synthesis of gold nanoclusters using p-mercaptophenol as a capping agent in a one phase system, which paved the way for the synthesis of various metal nanoclusters (Au, Ag, Cu, Pt) protected by a variety of ligands.¹⁴⁰ Compared to the two-phase method, the one phase method is simple and versatile. For example, Wu and co-workers designed a one-phase method for synthesizing monodisperse thiolate-protected Ag₇ clusters using meso-2,3-dimercaptosuccinic acid (DMSA) as the surface stabilizer.¹⁰⁶ In the typical synthesis, silver nitrate used as the silver source was dissolved in an ethanol solution and was cooled to 0° in an ice bath. Ag_x(DMSA)_y intermediate complexes were formed after addition of DMSA under medium–low speed stirring, and then the intermediates were reduced by solid powder NaBH₄ under intense stirring. Finally, monodisperse Ag₇ nanoclusters were obtained after 12 h of reaction. In their another study, they prepared a highly pure atomically monodisperse Au₂₅ nanoclusters capped with different functionalized surface ligands using a modified one-pot method together with a “size-focusing” process.¹⁴¹ The THF-mediated one-pot synthetic method in this report at least has two obvious merits compared to the conventional two-phase synthetic protocols and ligand-exchange method. On the one hand, the modified one-pot method significantly improves the purity of the Au₂₅ nanocluster product compared to the approach using ethanol as a solvent. On the other hand, this synthetic strategy provides a more simple and convenient access to generation of functionalized Au₂₅ and even other metal nanoclusters using hydrophilic or hydrophobic surface functionalities. Using a similar approach, Negishi et al. prepared small sized monolayer-protected gold nanoclusters via the reducibility and stability of the capping agent (DMSA).¹⁴² The results from the compositional analysis showed that DMSA-stabilized monodisperse gold nanoclusters consist of 10–13 atoms. After the success in synthesizing the star nanoclusters, such as Au₂₅ (ref. 34, 107 and 113–125) and Au₃₈ (ref. 102, 109 and 143–149) nanoclusters, a series of atomically precise Au_n(SR)_m (ref. 88, 89 and 150–157) and Ag_n(SR)_m (ref. 61, 65 and 158–165) and other metal nanoclusters^47,166–170 were obtained in high yield and on a large scale, using the above-mentioned synthetic routes.

3. Ligand-exchange

Besides the initial and modified Brust–Schiffrin methods, ligand exchange is another good strategy to tailor the electronic and geometric structure (including core and shell structures) of ligand protected metal nanoclusters.^171–176 Up to now, the ligand exchange method mainly involves two aspects: the ligand exchange reaction from phosphine-protected (thiol-protected) metal nanoclusters to new (thiol-protected) phosphine-protected metal nanoclusters^177,178 and the ligand exchange reaction from thiol-protected metal nanoclusters to new thiol-protected metal nanoclusters.^{172,179–186} The driving forces of ligand exchange, however, are completely different for these two types of ligand capped metal nanoclusters.¹⁷⁵ For the former, the difference in bonding abilities and coordination modes between the metal–P and metal–S are the decisive driving force of the ligand-exchange reactions. For the latter, the internal dynamics of reactions involves several factors, including the inherent properties (steric hindrance, rigidity, etc.) between the different thiol ligands and modulating factors (e.g., reaction temperature), as well as the fundamental characteristics (size, composition, structure, etc.) of the nanoclusters. According to the literature, PPh₃-stabilized and narrow-sized gold nanoclusters were one of the earliest studied nanomaterials using this approach.¹⁷⁷ As early as 1997, Hutchison and co-workers demonstrated that Au₅₅(PPh₃)₁₂Cl₆ nanoclusters could be transformed into thiol-stabilized 1.7 nm diameter gold clusters with controlled ligand exchange.¹⁸⁷ The ligand exchange reaction was achieved by stirring the parent Au₅₅ nanoclusters in the presence of excess thiol that included 1-propanethiol, 1-octadecanethiol, and 4-mercaptobiphenyl in dry dichloromethane, and the produced gold nanoclusters had excellent thermal and air stability. For a ligand-exchange approach, the critical factors in the process of reactions are the accurate control of thiol dosage and the reaction time, because high thiol/atom molar ratios and a prolonged reaction may cause the decomposition of the target nanoclusters, and low thiol to atom ratios and a short time reaction may lead to incomplete reactions. Although the major reaction factors (e.g., temperature, reaction time and added thiol dose) of the ligand exchange procedure need to be strictly regulated, this method is widely used in the following aspects:

(i) Induced structural transformation of nanoclusters. For example, using the atomically precise pure Au₃₈(PET)₂₄ as the starting nanoclusters, Zeng et al. synthesized new stable Au₃₆(TBBT)₂₄ nanoclusters through the thiol-to-thiol ligand exchange. Interestingly, the crystal structure of the produced Au₃₆(TBBT)₂₄ was completely different from that of the parent nanoclusters (Fig. 2(a)).^180,188 The parent Au₃₈ nanoclusters contained a rod-like face-sharing biicosahedral Au₂₃ core capped by three (–SR–Au–SR–) monomer staple motifs and six (–SR–Au–SR–Au–SR–) dimer staple motifs whereas the produced Au₃₆ nanocluster adopted a more peculiar structure with a truncated tetrahedral face-centered-cubic (fcc) Au₂₈ core and four traditional dimer staple motifs and 12 bridging thiolates. Because this is a groundbreaking study that has a positive effect on the more synthetic fields in nanochemistry, we outline below the basic steps or features and mechanism of this method. Based on the analysis of the MS and UV-vis curves as well as the crystal structures, the transformation mechanism of Au₃₈ to Au₃₆ was proposed. The total ligand exchange reaction could be divided into four stages: (i) the ligand exchange reaction was triggered by the new added TBBT thiol, and up to 12 TBBT ligands were successfully exchanged onto the surface of Au₃₈ during the first 5 minutes. (ii) The ligand exchange reaction continued and the population of exchange product Au₃₈(TBBT)_m(PET)_24−m increased. (iii) This stage was very important because of the transformation of to Au₃₆(SR)₂₄ and Au₄₀(SR)₂₆ and a disproportionation reaction process was identified. (iv) A size conversion occurred together with further ligand exchange toward completion and all of the intermediate clusters which included Au₄₀ and Au₃₈ were converted to the stable Au₃₆(SR)₂₄ nanoclusters. Additionally, the transformation between the structural isomers ((Au₂₈(S-c-C₆H₁₁)₂₀vs. Au₂₈(SPh^tBu)₂₀) has been achieved through the ligand exchange approach.¹⁸⁹ In a typical synthetic process, the Au₂₈(SPh^tBu)₂₀ nanoclusters were reacted with excess cyclohexanethiol for 2 h at 80 °C, and then novel Au₂₈ nanoclusters with quasi-D₂ symmetry were obtained. Interestingly, the reversible transformation can be also achieved between the two gold nanoclusters with isomeric structures via ligand exchange at a high reaction temperature. The theoretical analysis showed that the ligand effects were responsible for the ligand-induced isomerization process and thermal stability of the two gold nanoclusters. In addition to the structural transformation between the two gold nanoclusters, structural inter-conversion among three fcc- and non-fcc-structured gold nanoclusters, including Au₄₄(SPh^tBu)₂₈, Au₄₄(SPhMe₂)₂₆, and Au₄₃(S-c-C₆H₁₁)₂₅ nanoclusters, has also been achieved by Wu's group using the ligand exchange.⁴⁸ From a structural point of view, such structural transformation from an fcc structure to a non-fcc structure was accomplished for the first time. Apart from the gold nanoclusters, the structural transformation in the silver nanocluster field has also made remarkable achievements. For example, Bakr and co-workers investigated the reversible inter-conversion between hollow Ag₄₄(SPhF)₃₀ and nonhollow Ag₂₅(SPhMe₂)₁₈ nanoclusters (Fig. 2(d)).¹⁷³ Taking the transformation of Ag₄₄ to Ag₂₅ as an example to illustrate the mechanism during the reaction progress, this total conversion procedure was summarized into four parts: (step 1) up to 18 SPhF on the surface of the parent Ag₄₄(SPhF)₃₀ nanoclusters were replaced by the new SPhMe₂ ligands and the main product Ag₄₄(SPhF)₁₂(SPhMe₂)₁₈ was produced. (Step 2) Ag₄₄(SPhF)₁(SPhMe₂)₂₉ became the most predominant product. (Step 3) the distorted Ag₄₄(SPhF)₁(SPhMe₂)₂₉ was dissociated disproportionately to form mixed ligand protected Ag₂₅(SPhF)₁(SPhMe₂)₁₇. And (step 4) all of the intermediates, including Ag₄₆, Ag₄₈ and Ag₅₀, were decomposed to Ag₂₅(SPhF)₁(SPhMe₂)₁₇, and finally Ag₂₅(SPhMe₂)₁₈ was obtained. Besides, structural transformation has also been achieved from thiol protected silver nanoclusters to Se-donor ligand protected silver nanoclusters. Liu's group exchanged Ag₂₀{S₂P(OⁱPr)₂}₁₂ and Ag₂₁{S₂P(OⁱPr)₂}₁₂ with Se-donor ligands,¹⁹⁰ giving rise to Ag₂₀{Se₂P(OⁱPr)₂}₁₂ and Ag₂₁{Se₂P(OEt)₂}₁₂, respectively. A THF suspension of Ag₂₀{S₂P(OⁱPr)₂}₁₂ was reacted with an excess of NH₄[Se₂P(OⁱPr)₂] for less than 1 minute and then the target nanoclusters were separated from the mixtures. Structurally, Ag₂₀{Se₂P(OⁱPr)₂}₁₂ consisted of a centered icosahedral Ag₁₃ kernel and 7 capping Ag atoms and 12 dsep ligands. However, cluster Ag₂₁{Se₂P(OEt)₂}₁₂ with four three-fold rotational axes contained an Ag₂₁ core and 12 ligands and the packing of these clusters displayed the same bridging pattern of trimetallic tri-connectivity with one core Ag atom and two ligand Ag atoms. In general, using the ligand exchange, the structural change between the gold/silver nanoclusters can be implemented under the optimal conditions. Numerous excellent cases have been reported (Fig. 2), including the size (structure) transformation of Au₃₂₉(SC₂H₄Ph)₈₄ to Au₂₇₉(SPh^tBu)₈₄,¹⁸³ Au₁₄₄(SC₂H₄Ph)₆₀ to Au₉₉(SPh)₄₂,¹⁹¹ Au₁₄₄(SC₂H₄Ph)₆₀ to Au₁₃₃(SPh^tBu)₅₂,¹⁹² Au₂₅(SC₂H₄Ph)₁₈ to Au₂₀(SPh^tBu)₁₆,¹⁹³ Au₂₅(PET)₁₈ to Au₂₈(TBBT)₂₀,¹⁹⁴ Au₂₅(SC₂H₄Ph)₁₈ to Au₂₄(SC₂H₄Ph)₂₀,¹⁹⁵ Au₂₃(S-c-C₆H₁₁)₁₆ to Au₂₄(SCH₂Ph^tBu)₂₀,¹⁹⁶ Au₃₈(SC₂H₄Ph)₂₄ to Au₃₀(S^tBu)₁₈,¹⁹⁷ Au₆₀S₆(SCH₂Ph)₃₆ to Au₆₀S₇(SCH₂Ph)₃₆,¹⁹⁸ Au₃₀(S^tBu)₁₈ to Au₃₆(SPhX)₂₄,¹⁹⁹ Ag₄₄(p-MBA)₃₀ to Ag₅₀(dppm)₆(SCH₂Ph^tBu)₃₀,⁴⁹ Ag₅₁(SSR)₁₉(PPh₃)₃ to Ag₂₉(SSR)₁₂(PPh₃)₄,²⁰⁰ Ag₅₉(SPhCl₂)₃₂ to Ag₄₄(SPhF)₃₀,²⁰¹ Ag₅₉(SPhCl₂)₃₂ to Ag₂₅(SPhMe₂)₁₈,²⁰¹ and Ag₄₄(SPhF)₃₀ to Ag₃₇(SG)₂₁.²⁰²

Fig. 2 (a) Nanocluster transformation from Au₃₈ to Au₃₆. Reproduced with permission from ref. 180. Copyright 2013, American Chemical Society. (b) Nanocluster transformation from Au₃₈ to Au₃₀. Reproduced with permission from ref. 197. Copyright 2017, American Chemical Society. (c) Selective partial substitution of thiolate on the surface of Au₂₅ nanoclusters by the selenolate ligand. Reproduced with permission from ref. 186. Copyright 2019, American Chemical Society. (d) Reversible transformation of the structure between Ag₄₄ and Ag₂₅. Reproduced with permission from ref. 173. Copyright 2016, American Chemical Society.

(ii) Induced phase transfer of nanoclusters. Ligand exchange has also been utilized as an important strategy for the phase transfer. For example, by ligand exchanging phosphine-stabilized Au₁₁ nanoclusters with a water-soluble thiol (GSH), Tsukuda and coworkers synthesized Au₂₅(SG)₁₈ nanoclusters with well-defined compositions on a large scale under the optimized conditions.¹⁰³ One reason why GSH protected metal nanoclusters could react with the free thiol might be that the hydrophobic ligand could accelerate the decomposition of the GSH ligand capped multiple-disperse nanoclusters in the two-phase system under the optimal conditions (e.g. at an elevated temperature or pressure), and various intermediate clusters were formed during the ligand exchange process, and finally the most stable metal nanoclusters survived after a spontaneous “size-focusing” procedure. Qian et al. reported a facile and large-scale synthesis of hydrophobic ligand stabilized monodisperse Au₃₈ nanoclusters in high purity through the ligand exchange reaction of glutathione-protected Au_n nanoclusters with the dodecanethiol ligand.²⁰³ After the reactions finished, no GSH ligands were located on the surface of the obtained Au₃₈ clusters, suggesting that the transformation between the two clusters was complete. Using the same method, Jin et al. also prepared monodisperse hydrophobic Ag₆₂S₁₂(SBu^t)₃₂ nanoclusters through exchanging the hydrophilic Ag_n(SG)_m clusters with the tert-butyl mercaptan capping agent in a biphase system at ambient temperature.²⁰⁴

(iii) Induced functionalization of nanoclusters. The ligand exchange approach is also an efficient method to prepare multifunctional metal nanoclusters (fluorescence and catalysis). An excellent case in nanocluster functionalization was provided by Pradeep's group.²⁰⁵ In 2008, Shibu from this group exchanged the ligand of glutathione thiolate protected Au₂₅ with two kinds of ligands which included 3-mercapto-2-butanol (MB) and N-acetyl- and N-formyl-glutathione (NAGSH and NFGSH, respectively) to tune the optical and photoluminescence properties of the clusters. Compared to the parent Au₂₅(SG)₁₈ nanoclusters, the optical absorption properties of all the nanoclusters were similar and there was no change in the chemical nature of the metal core. Although the inherent fluorescence and solid-state emission of these clusters were observed, the excitation spectra of the MB-exchanged clusters and the acetyl- and formyl-glutathione exchanged products were remarkably different. Additionally, Zhao and coworkers prepared novel adamantanethiolate-protected Au₄₀(S-Adm)₂₂ nanoclusters and introduced a water-soluble component, γ-CD-MOF, into the hydrophobic ligand to endow the thiol protected gold nanoclusters with excellent aqueous phase catalytic activity in a horseradish peroxidase-mimicking reaction system.²⁰⁶

In 2017, Zhu's group proposed an in situ two-phase exchange method, which combined the merits of the simple one-pot approach and the efficient two-phase ligand-exchange method.²⁰⁷ This developed method is also very suitable for the preparation of alloy nanoclusters with well-defined structures. In detail, various ingredients, including metal salts (AgNO₃ and HauCl₄) and the water-soluble GSH ligand, were mixed and stirred in a heated aqueous solution, forming GSH-capped bimetal alloy nanoclusters in a short amount of time. Subsequently, a toluene solution containing a dose of tert-butylamine complex and tert-butyl mercaptan was added into the above water phase reaction system, which could facilitate the in situ two-phase exchange from the GSH-capped AgAu alloy nanoclusters to a series of thiolate-capped alloy nanoclusters, such as Au₂₀Ag₁(SR)₁₅ (NC-1), Au_21−xAg_x(SR)₁₅ (x = 4–8), Au_21−xCu_x(SR)₁₅ (x = 0 or 1), and Au_21−xCu_x(SR)₁₅ (x = 2–5). Although the structure information of the GSH-protected AgAu alloy nanoclusters was unavailable, the frameworks of the thiolated alloy nanoclusters have been determined. More importantly, the authors pointed out that neither the typical one-pot nor the conventional ligand-exchange method could be used to synthesize the target alloy nanoclusters obtained by this newly proposed method, implying that the in situ two-phase exchange method could be used as a powerful alternative for preparing precise alloy nanoclusters. Another representative alloy nanocluster (Au₂₄Cu₆(SPh^tBu)₂₂) was also synthesized by Zhu's group using this method.²⁰⁸

4. Galvanic/anti-galvanic reaction

The galvanic reaction is a redox process, in which a noble metal ion can be spontaneously reduced by a less noble metal in solution owing to the difference in electrochemical potential.²⁰⁹ According to the classical metal activity sequence (Zn > Fe > Cd > Co > Ni > Pb > Cu > Ru > Hg > Ag > Pd > Ir > Pt > Au), the most common example of a galvanic reaction in clusters is that Ag(0) clusters can be oxidized by the Au(III) ion in the gold salt (HAuCl₄) through the 3Ag(0) + Au(III) → Au(0) + 3Ag(I) redox reaction process based on the fact that the redox potential of Au(III)/Au(0) is approximately 1 V, which is obviously higher than that of Ag(I)/Ag(0) (Fig. 3(a)).^210,211 Thus, the difference in electrochemical potential acts as the driving force. This method is widely used to prepare alloy nanoclusters by introducing a noble metal ion into a less noble metal cluster. For example, Bootharaju et al. demonstrated the atomically precise doping of atom-accurate silver nanoclusters by gold atoms via the novel galvanic reaction strategy.⁵⁹ In the galvanic reaction process, reacting the pure Ag₂₅ cluster with an Au salt precursor (AuClPPh₃) gave birth to the formation of the monogold-doped Ag₂₅ cluster, and the entire galvanic reaction involving the formation of AuAg₂₄(SR)₁₈ (SR = 2,4-dimethylbenzenethiol; 2,4-DMB for short) is shown as follows: Ag₂₅(SR)₁₈⁻ + Au⁺ → AuAg₂₄(SR)₁₈⁻ + Ag⁻. Compared to the structure of the famous Ag₂₅ cluster, the alloy cluster retained the total structure except for the inner core. Based on the results of ESI-MS and X-ray single crystallography, the inserted gold atom was located in the center of the icosahedral Ag₁₂ to form the Au@Ag₁₃ core, forming the multiple core–shell structure of the Au-doped Ag₂₅ cluster. Interestingly, by directly reducing the gold and silver precursors with a reductant in the mixed solvent of DCM and methanol containing the capping agent, a mixture of compositions Ag_25−xAu_x(2,4-DMB)₁₈⁻ (x = 1–8) with the same nominal structural framework was produced. Although the Au⁺ feed was varied during the synthetic process, a pure product with monodispersity was not obtained, suggesting the importance of selecting a suitable synthesis approach in preparing the target clusters. The influence of heteroatom doping on the parent cluster was manifested in two aspects: surface structure and properties. On the surface of the mono-doped Ag₂₅ cluster, only eight out of eighteen ligands were found to participate in parallel-displaced π–π interactions (distance between both phenyl rings, 3.706–3.725 Å), which was obviously different from the twelve ligands in the pure Ag₂₅ cluster. Aside from the structural difference, the photoluminescence of the gold-doped silver nanocluster was improved by a factor of 25 times compared to the pure Ag₂₅ cluster, and the ambient stability of the former was also better than the latter, suggesting that the heteroatom doping of monodisperse Ag₂₅ could alter the electronic structure and increase the stability of the target cluster. Under the guidance of the galvanic reaction, by introducing the heteroatom (gold atom) into the parent silver cluster, a bimetallic thirteen-atom alloy quantum cluster Ag₇Au₆, which was capped by the mercaptosuccinic acid (H₂MSA) ligand, was obtained by Pradeep and coworkers in 2012.²¹² The monodispersity of the cluster was proved by PAGE and its properties were further characterized by multiple techniques including UV/Vis, FTIR, luminescence, TEM, XPS, SEM/EDAX, and ESI MS. Theoretically, one of the possible crystal structures was proposed, and it had a spherical structure in which a distorted icosahedral core with C_2v symmetry was capped by a bridged staple of –Au/Ag–SR–Au/Ag–. According to the synthetic procedure, the most crucial step in the synthesis process was that addition of an appropriate amount of gold salt to the parent Ag_7,8 cluster in the last step produced the final alloy Ag₇Au₆ cluster. Recently, starting from phosphine and thiolate ligand co-protected pure Ag₅₀(SR)₃₀(DPPM)₆ (SR = 4-tert-butylbenzyl mercaptan), Zhu's group successfully synthesized a medium-sized AgAu alloy cluster (Ag_50−nAu_n(SR)₃₀(DPPM)₆) via the galvanic metal exchange reaction.⁴⁹ The X-ray crystallography results showed that both clusters shared a similar structure, and the only difference was that the hollow Ag₁₂ core was partially replaced by the gold atoms. Besides, further investigation showed that exchanging the silver atoms by the gold atoms in the core could affect the HOMO–LUMO orbitals and optical absorption properties, as well as stability. In another excellent case, Xie's group reported the surface silver atom exchange between the well-known Ag₄₄(p-MBA)₃₀⁴⁻ cluster and the preformed Au-pMBA precursor to produce the isomeric Au₁₂Ag₃₂(p-MBA)₃₀⁴⁻ cluster.²¹³ Using mass spectrometry to track the surface motif exchange (SME) reaction, the results showed that the original surface structure of –SR–Ag–SR– protecting modules of the parent cluster was replaced by the incoming –SR–Au–SR– motifs, leading to the formation of a core–shell alloy nanocluster. Theoretically, the construction of the thermodynamically less favorable core–shell Ag@Au nanocluster could be attributed to the intermediate Ag₂₀ shell, which provided a diffusion barrier to prevent surface gold atom inward diffusion. Similarly, an alternating array stacking structure of Ag₂₄Au and Ag₂₆Au clusters, named as a double nanocluster ionic compound (DNIC), was successfully assembled by adding an Au(PPh₃)Cl complex and an auxiliary ligand (PPh₃) into DCM containing multi-sized Ag_x(SR)_y (SR = 2-ethylbenzenethiol) via the galvanic metal exchange method (Fig. 3(b)).²¹⁴ In the unique crystal structure, the cationic Ag₂₆Au(SR)₁₈(PPh₃)₆⁺ and anionic Ag₂₄Au (SR)₁₈⁻ gathered and assembled along the [001] direction using electrostatic interactions with weak interactions (e.g., π–π and C–H–π interactions). The overall geometric structure of Ag₂₆Au was similar to that of the analogue Ag₂₆Pt(SR)₁₈(PPh₃)₆ (SR = 2-ethylbenzenethiol) cluster,⁴⁵ in which a bipyramid AuAg₁₄ core was surrounded by two identical ring-like Ag₆(SR)₉(PPh₃)₃ staple motifs, and the framework of Ag₂₄Au(SR)₁₈ was identical to that of Ag₂₄Au(SR)₁₈ in the single nanocluster ion compound (SNIC).^{59,66,215,216} Besides, owing to the multiple non-covalent interactions in the crystal, the fluorescence intensity of the crystalline state of DNIC was higher than that of the amorphous state. Reacting Au–SR complexes with spherical Ag₂₄Pt(SR)₁₈ (SR = 2,4-dimethylbenzenethiol) with a precise molecular structure generated the shape-maintained Au_xAg_24−xPt(SR)₁₈ ternary alloy cluster.²¹⁷ In contrast, by replacing the Au–SR complexes with an Au(PPh₃)Br precursor, the formed ternary alloy cluster was determined to be the rod-like Au₁₀Ag₁₃Pt₂(PPh₃)₁₀Br₇ cluster.²¹⁸ Based on the structural features of the two ternary alloy nanoclusters, as well as the parent bimetal nanocluster, the authors believed that: (i) the Pt atom in the icosahedron was stable enough to hold the central position; (ii) the phosphine ligand could peel the outer shell of the spherical parent cluster to form the building blocks of rod-like nanoclusters; (iii) the anionic Br⁻ played a crucial role in assembling the final target nanocluster.

Fig. 3 (a) Schematic illustration of the GR and AGR processes. Reproduced with permission from ref. 210. Copyright 2018, American Chemical Society. (b) Route for preparation of DNIC (Ag₂₆Au@Ag₂₄Au). Reproduced with permission from ref. 214. Copyright 2019, John Wiley and Sons. (c) “Molecular surgery” on the Au₂₃ cluster via the AGR. Reproduced with permission from ref. 51. Copyright 2017, American Chemical Society. (d) Cd²⁺ induced transformation of Au₂₃ to Au₂₈vs. Cd(SR)₂ induced transformation of Au₂₃ to Au₂₀Cd₄. Reproduced with permission from ref. 219. Copyright 2018, John Wiley and Sons.

Recently, the anti-galvanic reaction, which is opposite to the classic galvanic reaction, has been regarded as a new and efficient approach to prepare atomically precise alloy nanoclusters.²⁰⁹ As early as 2010, Murray's group reported that phenylethanethiolate (PET) protected negative Au₂₅ clusters could react with silver ions, producing Au₂₄Ag(PET)₁₈²⁺ and Au₂₃Ag₂(PET)₁₈²⁺ alloy nanoclusters, by regulating the ratio of the parent cluster to silver ions, showing that the opposite of the classical galvanic reaction could occur.²²⁰ Later, Wu's group first proposed a general concept of the anti-galvanic reaction (AGR) and confirmed that the AGR is an intrinsic nature of ultrasmall nanoclusters and is associated with several factors including the size of a metal, the type of surface ligand and the ion precursor as well as ion dose, and revealed that, aside from negative Au₂₅, neutral Au₂₅ and ultra-small thiolated metal (e.g., silver and gold) nanoparticles with less than 3 nm in size could react with silver or copper ions.^221,222 Since then, the new era of AGRs has begun, which have been considered a new and efficient way for the preparation of alloy nanoclusters.^209,210 For example, Wu's group successfully achieved mono-Hg and mono-Cd atom doping of the well-studied Au₂₅(PET)₁₈ cluster by means of the AGR (Fig. 3(d)).^{54,123,219,223} One structural feature to be clear for the Cd-doped cluster was that one gold atom on the surface of the icosahedral core of homo-Au₂₅ was replaced by the incoming Cd atom, which was obviously different from that of the Hg-doped cluster where one gold atom in the staple motif was replaced by a foreign Hg atom, which was also different from that of the Cd-doped Au₂₅ cluster reported by Zhu's group. Interestingly, the Au₂₄Cd cluster could be transformed into the Au₂₄Hg cluster, but the reverse process did not occur owing to the different stability.²²⁴ Zhu's group also prepared thiolated trimetallic MAg_nAu_24−n(PET)₁₈ (M = Cd or Hg) by replacing the central gold atom with Cd– or Hg–PET complexes. The crystal structure showed that the Cd or Hg atom was located in the central position of the core, and the icosahedral M₁₂ consisted of silver–gold bimetallic metals, which was further capped by the staple motifs.²²⁵ Negishi and coworkers synthesized two trimetallic nanoclusters Au_∼20Ag_∼4Pd(SC₂H₄Ph)₁₈ and Au_∼20Ag_∼4Pt(SC₂H₄Ph)₁₈ by means of the AGR between the neutral Au₂₄Pd(SC₂H₄Ph)₁₈ or Au₂₄Pt(SC₂H₄Ph)₁₈ and Ag–SC₂H₄Ph.²²⁶ The geometries of the trimetallic nanoclusters were similar to those of the spherical M₂₅ model such as Au₂₅(PET)₁₈^−/0, Au₂₄Pt(PET)₁₈⁰, Au₂₄Pd(PET)₁₈⁰, Au_24−xAg_xHg(PET)₁₈⁰, Au_25−xAg_x(PET)₁₈⁻, and Au_24−xAg_xCd(PET)₁₈⁰. In both trimetallic clusters, the Pd or Pt atom was situated in the center of the icosahedral core. By combination of an AGR and a quasi-AGR strategy, Jin and coworkers reported a site-specific “surgery” on the surface of atomically precise Au₂₃(S-c-C₆H₁₁)₁₆ to generate a new Au₂₁(SR)₁₂(P–C–P)₂⁺ (P–C–P = Ph₂PCH₂PPh₂) cluster.⁵¹ In the interesting “surgery” reaction, the Au₂₃ cluster was sequentially reacted with Ag(I)-SR and Au₂Cl₂(P–C–P) complexes, giving birth to the final target Au₂₁ cluster through an intermediate product silver-doped Au_23−xAg_x(SR)₁₆ (x ∼ 1), without changing the structure of the remaining part of the parent cluster. Similarly, Wang et al. successfully achieved single metal atom shuttling into and out of the rod-like Au₂₄(PET)₅(PPh₃)₁₀Cl₂ cluster by adding the MCl salt (M = Cu/Ag) into DCM containing the parent system.²²⁷ Based on the experimental and theoretical results, they found that the foreign atom could squeeze the pre-existing gold atom of the cluster into the hollow site to transform it into a non-hollow MAu₂₄ cluster. In particular, the incoming silver atom was located in the icosahedral vertex of M₁₃, while the copper atom could occupy either the icosahedral vertex (30% population) or waist (70% population) positions of M₁₃. Interestingly, the AgAu₂₃ cluster transformed from the centrally hollow Au₂₄ could be further converted into the Ag₂Au₂₃ cluster in which both silver atoms occupied the apex positions of the cluster and connected with Cl atoms. Besides, a few cases of Cd atom doping of gold nanoclusters were also achieved by means of the AGR, including Au₂₀Cd₄(SH)(CHT)₁₉,²¹⁹ Au₁₉Cd₃(SR)₁₈ (ref. 228) and Au₂₆Cd₄ (CHT)₂₂.²²³

5. Etching

Generally, the etching approach can be divided into two categories, physical and chemical etching. Belonging to the regime of “top down” synthetic routes, chemical etching, which consists of ligand etching, acid etching and so on, is more favorable for metal cluster synthesis due to many controllable factors.²²⁹ As early as 2007, using multivalent coordinating polymers to etch preformed high-quality gold nanocrystals (7 nm in size), Nie and coworkers reported an etching process for synthesizing highly fluorescent and water-soluble gold nanoclusters.²³⁰ Although the crystal structure of the target nanocluster was not obtained at that time, the composition was determined to be Au₈ by electrospray ionization (ESI) mass spectrometry. Based on a series of analyses, two possible etching synthesis mechanisms in this process were proposed. One mechanism was that Au₈ clusters were liberated from the nanocrystal surface and immediately protected by a multivalent and hyperbranched PEI ligand; the other was that Au(I) ions were generated during the ligand exchange process and the released Au(I) species were aggregated by the PET ligand. An interesting contribution to preparation of highly fluorescent metal nanoclusters in a mild etching environment was provided by Yuan et al. who synthesized stable and monodisperse Au, Ag, Pt and Cu nanoclusters through mild etching of initially unstable, multi-scaled and nonfluorescent parent nanoclusters.⁵⁵ The water soluble non-fluorescent parent metal nanoclusters protected by the water soluble glutathione ligand were first prepared using the traditional co-reduction approach under designed conditions, and then the as-obtained parent metal nanoclusters were transferred to a toluene solution in the presence of cetyltrimethylammonium bromide via electrostatic interaction between negatively charged carboxyl groups from the GSH ligand on the surface of the metal nanoclusters and positively charged cations of the hydrophobic salt. Finally, the most important step, mild etching, was achieved in the organic phase (toluene solution), giving birth to the formation of highly stable and fluorescent metal nanoclusters. Interestingly, the produced metal nanoclusters could be transferred back to the aqueous phase after removing the cation CAT⁺. In addition, Xia et al. successfully synthesized novel phenylethanethiolate protected Au₃₈(PET)₂₆ nanoclusters by an acid etching synthesis method.²³¹ In this work, a series of phenylethanethiolate protected nanoparticles in different sizes (including nanoparticles in sizes ∼4 nm and ∼2 nm, Au₁₄₄, Au₃₈ and Au₂₅ nanoclusters) were etched with acetic acid at elevated temperature (80°). Interestingly, Au₁₄₄ nanoclusters and large size Au nanoparticles could not react with the etchant, while smaller sized Au₃₈ and Au₂₅ could react with the etchant under the same conditions, indicating a size-dependent reactivity of gold nanoparticles with the etchant. Jin et al. also reported the synthesis of monodisperse Au₁₄₄(SCH₂CH₂Ph)₆₀ nanoclusters via one-phase thiol etching of Au nanocrystals protected by –SCH₂CH₂Ph thiolates (Fig. 4).²³² Although the detailed chemical etching mechanism of the foreign ligand induced medium or larger metal nanoparticle etching has not yet been particularly clear, this method has great potential for the synthesis of small size metal nanoclusters.

Fig. 4 (A) MALDI mass spectra of Au nanoparticles before (black profile) and after thiol etching and (B). UV-vis absorption spectra of Au nanoparticles before and after thiol etching (inset: optical absorbance vs. photon energy). Reproduced with permission from ref. 232. Copyright 2009, American Chemical Society.

6. Solid phase synthesis

It is important to note as well that almost all methods of synthesizing ultra-small metal nanoclusters capped with hydrophobic and hydrophilic ligands are carried out in solution (aqueous or organic phase) but not in the solid phase because interactions between the chemical reactants are more active and more complete. In contrast to the methods mentioned above that are related to solutions, the synthetic method reviewed in this subsection is the solid phase synthesis which contains many merits such as simple operation and large-scale, and this method is widely used to prepare atomically precise highly stable metal nanoclusters.² In 2010, Pradeep and coworkers developed a new method (later named the solid state route) to prepare mercaptosuccinic acid (H₂MSA) protected Ag nanoclusters containing nine atoms.²³³ The reported synthetic route in this work includes three steps: first of all, by grinding a mixture of silver salt (AgNO₃) and a capping agent (H₂MSA) in the solid state in a mortar, polymeric silver–thiolates were prepared using the strong affinity of the sulfur from the capping agent to the metal atom. Second, sodium borohydride (powder state) as a strong reducing agent was added and the polymeric precursor was reduced in the grinding process under a strongly reducing environment, giving birth to the formation of a brownish-black powder. In the last step, a certain amount of pure water was added to complete the reaction, resulting in the formation of strong photoluminescent Ag₉ nanoclusters. Multiple techniques such as XPS, MS and TEM were performed to characterize the as-obtained nanoclusters after their purification using ethanol precipitation, centrifugation and re-extraction processes. Chakraborty et al. from the same group prepared 2-phenylethanthiol (PET) protected Ag₁₅₂ nanoclusters using a derivative method of the solid state synthesis (Fig. 5).²³⁴ In this synthesis process, ethanol was added in the last step rather than pure water. This small change will benefit the control of the target nanocluster size and cleaning of the final products. Additionally, many atomically precise metal nanoclusters were prepared using this method.^235,236

Fig. 5 MALDI mass spectra and optimized structure of the Ag₁₅₂ nanoclusters prepared by a solid state route. Reproduced with permission from ref. 234. Copyright 2012, American Chemical Society.

7. Intercluster reaction

The reaction between different clusters is proved to be an efficient strategy to prepare multi-metallic nanoclusters. As an emerging method, it is defined as the intercluster reaction. Pradeep's group systemically demonstrated the intercluster reaction between mono-gold and mono-silver clusters, Au₂₅(PET)₁₈⁻ (PET = phenylethanethiolate)¹¹³ and Ag₄₄(FTP)₃₀⁴⁻ (FTP = 4-fluorothiophenol),^237,238 which gave birth to the formation of bimetallic nanoclusters including silver-doped Au₂₅ (Ag_xAu_25−x(SR)₁₈) and gold-doped Ag₄₄ (Au_xAg_25−x(SR)₁₈) bimetallic nanoclusters (Fig. 6).²³⁹ Interestingly, both parent clusters could spontaneously react in the solvent to exchange the metal atoms as well as the metal–thiolate fragments under ambient conditions. The number of exchanged intermediate species could be controlled by varying the Ag₄₄ : Au₂₅ molar ratios. In particular, the number of doped silver and gold atoms in the parent clusters could be varied from one atom to 24. In the case of gold-doped Ag₄₄ clusters, DFT simulations showed that the gold atom preferred to occupy the icosahedral kernel followed by the outer staple motifs and last the second inner dodecahedron, which was confirmed by the bimetallic Ag₃₂Au₁₂(SR)₃₀ cluster formed by the substitution of all of the silver atoms in the icosahedral kernel of the Ag₄₄ cluster. Besides, using three model monolayer-capped bimetallic nanoclusters, Ag_25−xAu_x(SR)₁₈, Au_25−xAg_x(SR)₁₈, and Au_xAg_44−x(SR)₃₀, the structure–reactivity correlation in metal atom substitutions was demonstrated and the results showed that although the alloy clusters possessed certain common structural features, they exhibited distinctly different reactivities in the substitution reactions. In detail, the metal atoms at the outermost shells (gold atoms in the M₂(SR)₃ motifs of Ag_25−xAu_x(SR)₁₈ and Au_xAg_44−x(SR)₃₀, and silver atoms in the M₂(SR)₅ motifs of Au_25−xAg_x(SR)₁₈) were substituted more easily compared to those at the inner icosahedral sites, and the gold atoms in the second shells of the Ag_25−xAu_x nanocluster could be completely replaced by silver atoms, but such complete substitution was not possible for the Au_25−xAg_x cluster. Interestingly, a dianionic adduct, formed in the spontaneous alloying between the two clusters (Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈), was detected to be one of the earliest intermediates in the intercluster reaction, suggesting that aside from the fragments, clusters themselves might be involved in the inter-cluster reactions. Coupled with molecular docking simulations, DFT calculations showed that van der Waals forces and bonding between the staple motifs on the periphery of the clusters could be the key factors to form these unique adducts. In addition, a novel Au₂₂Ir₃(PET)₁₈ cluster, as the first case of an Ir atom doped monolayer-capped bimetallic cluster, was prepared by Bhat et al. via the reaction between Au₂₅(PET)₁₈ and Ir₉(PET)₆ clusters.²⁴⁰ Compared to the intermediate products of other intercluster reactions, this reaction formed only one single product, which could be further purified and separated using the thin-layer chromatography (TLC) method and characterized by the ESI-MS and UV-vis techniques. The most favorable geometric structure was calculated with aid of DFT simulations, and the results showed that the alloy cluster possessed a similar skeleton to the Au₂₅ cluster, in which one Ir atom was located at the center of the icosahedral kernel and the remaining two Ir atoms were positioned on the icosahedral shell, forming a triangular structure via strong Ir–Ir interactions. Khatun and coworkers discussed the intercluster reaction between the bimetallic alloy cluster MAg₂₈(PPh₃)₄(BDT)₁₂ (M = Ni, Pt, Pd) and Au₂₅(PET)₁₈ cluster, leading to the formation of trimetallic MAu_xAg_28−x(PPh₃)₄(BDT)₁₂ (x = 1–12).²⁴¹ In this reaction, only a maximum of twelve gold doped nanoclusters were formed for the bimetallic cluster as a major product, which was obviously different from previous reports of intercluster reactions. The central atom (Ni, Pd, or Pt) could not be transferred from the center position of the bimetallic cluster to the mono-thiolate capped Au₂₅ cluster, suggesting that the central metal atom did not participate in the interaction reaction. Xia and coworkers reported the synthesis of a novel Au₂₀Ag₅(Capt)₁₈ cluster by the reaction between the Au₂₅ cluster and ultrasmall silver nanoclusters (Ag₃₀(Capt)₁₈).²⁴² By replacing the Ag₃₀ cluster with ultrasmall silver nanoparticles, polydisperse products (Au₂₂Ag₃S₁₂, Au₂₁Ag₄S₁₂, and Au₂₀Ag₅S₁₂) were obtained. Besides, copper atoms could be doped into the Au₂₅ cluster by means of the intercluster reaction.

Fig. 6 (a) Schematic illustration of intercluster reactions of Au₂₅ and Ag₄₄. Reproduced with permission from ref. 239. Copyright 2016, American Chemical Society.

8. Summary and outlook

In this review, we summarized the recent progress in the controllable synthesis of ultra-small metal nanoclusters stabilized by surface protecting ligands and systemically classified these methods into several types, including the Brust–Schiffrin method, ligand-exchange, galvanic/anti-galvanic reaction, etching, solid phase synthesis and intercluster reaction. Among them, both the Brust–Schiffrin and ligand exchange methods are the facile and most widely used methods to prepare both single-component and multi-component metal nanoclusters, and the galvanic/anti-galvanic reaction and intercluster reaction methods are suitable for the synthesis of alloy nanoclusters including bimetallic and trimetallic and even multimetallic nanoclusters. Yet, despite substantial progress in thiolated metal nanoclusters (including diverse structures, compositions and applications), some suggestions are outlined in future work on this fascinating field: (i) the expansion of the types of metal clusters. At present, two types of typical metal nanoclusters (mainly gold and silver nanoclusters) are emerging and promising nanomaterials, whose geometric and electronic structures, sizes, surface capping agents and compositions have been extensively studied. Although the development of copper clusters obviously lags behind the gold and silver nanoclusters in terms of the number, structural diversity and the types of synthesis methods, copper clusters have also attracted considerable attention owing to their novel size-dependent properties. To the best of our knowledge, aside from the coinage clusters, other types of clusters, such as Pt, Pd and Ir clusters, also have great potential application in the energy and catalytic fields. Thus, it is greatly important to fully explore the other types of metal nanoclusters with different sizes, structures and compositions. (ii) In-depth understanding of the details of the cluster synthesis process. In many cases, chemists only focus on the final target cluster and the physicochemical properties and structures associated with it, and lose sight of the important formation mechanism and process. So it is no exaggeration to say that the partly meaningful details of the process of cluster synthesis have been a mystery. However, the mechanism of cluster synthesis or assembly is conducive to understanding the structural transformation of metal nanoclusters in specific experiments. Thus, mass spectroscopy techniques (ESI-MS and MALDI-MS) should be carried out to map out the intermediate mixture species before obtaining the final product to gain more information about the assembly or conversion mechanism of metal nanoclusters in solution. (iii) Exploring the cluster-based potential applications. To be honest, the performance of metal nanoclusters is still far from the actual application. Thus, more work should be devoted to developing new and high-performance cluster-based nanomaterials and revealing their structure–application relationships.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22201225), the China Postdoctoral Science Foundation (2023M733539), the XiangJiang Scholars Scheme Program (XJ2023006), the Postdoctoral Researcher Scientific Activity Funding Project in Anhui Province (2023A660), the Scientific Program Funded by the Shaanxi Provincial Education Department (23JK0456), the Scientific and Technological Plan Project of Xi'an Beilin District (GX2310) and the Scientific and Technological Plan Project of Xi'an Science and Technology Bureau (24GXFW0017).

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Footnote

† L. H. and T. D. contributed equally to this work.

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