郑有坤，姜晖，王雪梅

东南大学生物科学与医学工程学院，生物电子学国家重点实验室，生物医学工程国家级实验教学示范中心，南京 210096

1 Introduction

Ultrasmall metal nanoclusters (NCs), such as gold, silver,platinum, and palladium, usually comprising of several to a few hundred metal atoms, represent a class of emerging nanomaterials that contain a metal core with sizes less than 2 nm and a ligand shell1–5. When particles reach this size scale it will show the quantum confinement effect, which result in discrete energy levels and molecular-like properties, such as the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) transitions6, quantized charging7, magnetism8, and strong photoluminescence9–12.With such excellent physicochemical properties, ultrasmall metal NCs show diversified applications potential in sensing,catalysis, imaging, and biomedicine13–20.

Meanwhile, along with the rapid development of monometallic NCs, the research of mixing two or more different metal species into one NC to obtain novel geometries and tune fundamental properties has also made great progress recently21–25. Compared to monometallic NC analogues, alloy NCs have more attractive advantages, such as the physicochemical properties of different metals can be integrated into one NC, the electronic structure of NC could be precisely tailored by adjusting their compositions and structures, the strong photoluminescence and excellent catalytic performance could be obtained, and so on26–28. These alloying metal NCs have also widely been used in the fields of catalysis,biosensor and biomedicine29–32.

Nanomaterial research highly depends on synthetic methodological breakthroughs. For a long time, it is a major challenge in nanoscience to accomplish the controllable synthesis and structural control of NCs with atomic precision.Only by realizing the controllable synthesis and structural regulation at the atomic scale, we can better understand and tuned fundamental properties of NCs, and ultimately achieve efficient use. Fortunately, with the advancement of technology,this challenge is being overcome in recent years18,32. A large number of groundbreaking studies on this field has been achieved, including those towards alloy NCs18,27–34. Therefore,it is very important to summarize the research advances on the controllable synthesis and structural characterization of metal NCs with the atomic scale, which may shed some light on the future studies of metal NCs. Several excellent reviews have focus on the synthesis, physicochemical properties and applications of the atom-precise gold NCs18,29,32,35,36. In this review, we mainly highlighted recent advances in methodology involved atom-precise alloy NCs, including one-pot synthesis,metal exchange, ligand exchange, chemical etching, intercluster reactions, surface motif exchange reaction and in situ two-phase ligand exchange strategy. The relevant advantages and disadvantages of each method were also discussed in details.

2 One-pot synthesis

One-pot synthesis is rather simple and straightforward to fabricate alloy NCs and a number of atomically precise alloy nanoclusters have been successfully prepared by using this strategy37-41. In general, two (or three) different metal precursors first simultaneously react with excess thiolate ligand in a suitable solvent to generate metal-thiolate complexes.Then, a certain amount of sodium borohydride (NaBH4) is mixed to reduce the complexes, leading to the formation of ligand-protected alloy NCs. Similar to the monometallic NCs,where thiolate-protected Au25, Au38, and Au144NCs are the most popular NC types due to their easily synthesis, good stability, well-defined cluster architectures, and intriguing physicochemical properties, alloy NCs even for trimetallic comprising of 25, 38, and 144 metal atoms are such three most popular types that have been fabricated by using the one-pot synthesis method22,28,42–48. Ag, Pt, Pd, Cu, and Ni are most common metal species that can be incorporated in Au NCs for the generation of alloy NCs. Moreover, non-Au alloy NCs such as Ag-Pt, Ag-Pd, and Ag-Cu have been also fabricated recently.The representative crystal structure of thiolate-protected alloy NCs was presented in Fig. 125,40–44,49–57.

2.1 Au-Ag alloy NCs

Au and Ag are two most common noble metal elements, both of which have many similar physicochemical properties such as nearly identical atomic radius, same valence electron, and strong metallophilic interaction58. The strong Au-Ag metallophilic interaction can facilitate the synthesis of Au-Ag alloy NCs, with a negligible distortion in their architecture. For example, using one-pot method, Negishi and coworkers successfully synthesized a series of Au25-nAgn(SC12H25)18alloy NCs with different compositions (n = 0–11) by adjusting the HAuCl4and AgNO3rations (Fig. 2a)59. The electronic structures of Au25-nAgn(SC12H25)18 NCs can be modulated by incorporation different number of Ag atoms in the Au-Ag alloy NCs, which were also reflected in optical spectra (Fig. 2b,c). A theoretical study has predicted that Ag heteroatoms will preferentially occupy sites on the surface of the cluster core. Up to 13 Ag atoms can be incorporated in the Au25-nAgnNCs.Similar results have recently been observed from other thiolates-protected Au25-nAgnNCs, such as Au25-nAgn(SC2H4Ph)1821,60, suggesting that the generation of Au25-nAgnalloy NCs is not solely dependent on the type of thiolates. In another study, Xie and coworkers developed a NaOH-mediated one-pot method for the preparation of high-quality Au25-nAgnalloy NCs protected by mono- or bi-thiolate ligands in water61.These Au-Ag alloy NCs feature with rich surface chemistry could be easily functional for practical applications62.

Except for Au25-nAgn NCs, thiolate-protected Au38-nAgn, and Au144-nAgnNCs have also been successfully synthesized46-48.The maximum doping of Ag atoms in the thiolated Au38-nAgnand Au144-nAgnNCs was 12 and 60, respectively. Similar to Au25-nAgnNCs, the 60 Ag atoms of the Au144-nAgnNCs were suggested to be in the third shell of M60(M = metal) core,while the 12 Ag atoms of the Au38-nAgn NCs were selectively incorporated in the M2348.

In addition to three categories described above, several constructions for novel Au-Ag alloy NCs, such as Au4Ag13(DPPM)3(SR)9(where DPPM = bis(diphenylphosphino)methane; SR = 2,5-dimethylbenzenethiol)63,[Au3Ag38(SCH2Ph)24X5]2-(X = Cl or Br)64,Au34Ag28(PhC≡C)3465, [Ag46Au24(SR)32](BPh4)2(where SR =tertiary butylmercaptan)66, Au24Ag20(2-SPy)4(PhC≡C)20Cl2(where 2-Spy = 2-pyridylthiolate)51, and[Au80Ag30(PhC≡C)42Cl9]Cl54, are also fabricated recently. In an excellent study, Zheng and coworkers reported a typical synthesis of three [Au12Ag32(SR)30]4-intermetallic NCs protected with fluorinated arylthiols (SR = SPhF, SPhF2or SPhCF3) via one-pot reduction of AgBF4and ClAuPPh3precursors by using NaBH4in the presence of fluorinated arylthiol and PPh4Br34. The NCs form a Keplerate solid of concentric icosahedral and dodecahedral atom shells, protected by 6 Ag2(SR)5 units (Fig. 3). The Au12Ag32(SR)30 NC carried 4 negative charges per cluster and fulfilled the 18-electron superatom criteria, and thus explained their excellent thermal stability.

Fig. 1 Crystal structures of selected alloy NCs 25,40–44,49–57.The colors of the letters reflect those of the respective metal atoms in the structure. Color online.

Fig. 2 MALDI-TOF mass spectra of Au25-nAgn(SC12H25)18 NCs at different HAuCl4/AgNO3 ratios (a). Optical absorption spectra (b), and optical absorption (blue), photoemission (red), and photoexcitation (green) spectra (c) of Au25(SC12H25)18 and Au25-nAgn(SC12H25)18 NCs 59.

Fig. 3 Representative crystal structure of the [Au12Ag32(SR)30]4- cluster 34.(a) The overall structure of the [Au12Ag32(SR)30]4- cluster. All hydrogen and fluorine atoms are omitted for clarity. (b) The two-shell Au12@Ag20 core of the cluster.(c) The arrangement of six [Ag2(SPhF2)5] motif units on the surface of the cluster. (d) The structure of the surface [Ag2(SPhF2)5] motif. Color legend: orange sphere,Au; green sphere, Ag; yellow sphere, S; cyan sphere, F; grey stick/sphere, C.

2.2 Au-Pt alloy NCs

Pt is a bioinert noble metal element that is of importance in catalytic applications, and many studies have shown that Au-Pt alloy NCs has better catalytic performance than pure Pt NCs67–69,so the fabrication of Au-Pt alloy NCs is very attractive in this perspective. Unlike Ag, Pt has a relatively smaller atomic radius than Au. Compared to Au-Ag alloy NCs, therefore,different doping patterns may exist in Au-Pt alloy NCs. Several incompatibility phenomenons, such as lattice mismatch, have been found to exist during the doping of Pt in Au NCs. In 2012,Jin and coworkers have successfully produced a mixture of Au25(SC2H4Ph)18 and Pt1Au24(SC2H4Ph)18 NCs via the size-focusing one-pot process30. They observed that only one Pt atom can be incorporated in Au25NCs regardless of the HAuCl4and H2PtCl6ratios. Interestingly, Pt1Au24(SC2H4Ph)18NCs showed different optical properties and catalytic activity compared to that of Au25(SC2H4Ph)18 NCs, although there was only one Pt atom difference in both NCs (Fig. 4). The Pt atom exists in the center of Au13icosahedral core has been found via both the experimental and theoretical calculations30,70,71. With this approach, very recently, a molecule-like PtAu24(SC6H13)18NC has been obtained, which can act as a high-efficiency electrocatalyst for hydrogen production44. In another study,Zhao and coworkers successfully synthesized a series of Au-Pt alloy NCs with a total 100 atomic number (i.e., Au100-nPtn NCs,n = 0–100) by the one-pot reduction of HAuCl4 and K2PtCl4 with NaBH4using Tween 80 as a stabilizer in water72. These Au-Pt NCs are reported with the highest atomic number of alloy NCs so far, and which have very unusual biological activity. This study opens up promising chances for synthesis and application other sized high-quality Au-Pt alloy NCs.

2.3 Au-Pd alloy NCs

Pd also is of importance in catalytic applications. Similar to the Pt-doped Au NCs, only one Pd atom can be incorporated in Au25(SC2H4Ph)18NCs73. The Pd1Au24(SC2H4Ph)18NCs showed different optical absorption and electrochemical properties compared to that of Au25(SC2H4Ph)18 NCs.Moreover, the Pd1Au24 NCs have superior stability than Au25 NCs due to their unique core-shell Pd@Au24(SR)18architecture, where the central Au atom in the Au13icosahedral core was replaced by one Pd atom (Fig. 1)39,57,74. This unique architecture was further confirmed via employing197Au Mössbauer and Pd K-edge EXAFS spectroscopy75. Then,two-Pd atom doped cluster, Pd2Au36(SC2H4Ph)24 NC, were synthesized in high purity by reacting HAuCl4and Na2PdCl4with PhC2H4SH in tetrahydrofuran76. Similar to Pd1Au24(SC2H4Ph)18NC, the Pd2Au36(SC2H4Ph)24NCs are more stable than their Au NC analogues (i.e., Au38(SC2H4Ph)24NCs)against degradation and core etching by small molecular thiols in solution. Very recently, Kang and coworkers demonstrated an example of a self-assembled-based one-pot synthetic strategy for Pd2Au23(PPh3)10Br7NCs77. The doped Pd atoms all locate in the center of M13icosahedral kernel (Fig. 5a–c).During the self-assembled process, two Pd1Au12 icosahedrons were bonded together through the vertex-sharing, forming the bi-icosahedron Pd2Au23alloy NCs (Fig. 5d,e). Moreover, the halogen atoms (such as Br or Cl) are first successfully arranged as binding bridges in rod-like M25clusters, that is Pd2Au23NCs(Fig. 5e). Besides the thiolate-protected Au-Pd alloy NCs,phosphine-protected (in solution) and naked Au-Pd alloy NCs(in gas) have also been investigated78,79.

Fig. 4 Mono-Pt doping of gold NCs and catalytic application 30.(a) Procedure for Pt1Au24(SR)18 NCs synthesis. (b) UV-vis optical spectrum and (c) ESI mass spectrum of Pt1Au24(SR)18 NCs. (d) Crystal structure of [Pt1Au24(SR)18]0 and[Au25(SR)18]- and photograph of [Pt1Au24(SR)18]0, [Au25(SR)18]0, and [Au25(SR)18]- NCs solutions under room light. (d) Catalytic performance of Pt1Au24(SR)18/TiO2 and Au25(SR)18/TiO2 for styrene oxidation (R = C2H4Ph).

2.4 Au-Cu alloy NCs

Fig. 5 Structural anatomy of Pd2Au23(PPh3)10Br7 alloy NC 77.(a) the single-atom Pd core, (b) the Au12 shell, (c) the icosahedral Pd1Au12 kernel,(d) the bi-icosahedral Pd2Au23 metallic structure, (e) total structure of Pd2Au23(PPh3)10Br7 NC. The colors of the letters reflect those of the respective atoms in the structure.

The same as Au and Ag, Cu also belongs to the IB group in the periodic table. However, Cu has a smaller atomic radius than Au and the Au-Cu interaction is stronger than that of Au-Ag, resulting in the doping of Cu in Au NCs maybe cause a severe distortion in their architecture, which could reduce the stability of Au-Cu alloy NCs. Fortunately, one efficient way is that addition of thiolate ligands can address this issue because of thiolate ligands can reduce difference the redox potential of Au and Cu80, leading to a better control in the synthesis of Au-Cu alloy NCs. Fox example, Dass’s group adapted an efficient strategy for Au144 NCs to fabricate Au-Cu alloy NCs and have successfully synthesized Au144-nCun(SC6H13)60alloy nanomolecules by one-pot reducing HAuCl4and CuCl2in ethanol and in the presence of excess HSC6H1381. The number of Cu atoms can be modulated by changing the Au3+/Cu2+ratio from 1 : 0 to 1 : 0.5 and a maximum of 23 Cu atoms can be doped in the Au144NCs. Interestingly, a surface plasmon-like peak appears at ～520 nm can be observed when the number of Cu atoms is higher than 8, which indicating the Cu atom incorporation may be significantly influence the electronic structures of Au144NCs. Similar to the synthesis strategy of Au144–nCun(SC6H13)60alloy NCs, Au25-nCun(SC2H4Ph)18(n =1–5) alloy NCs have also successfully obtained by reducing gold salt and copper salt in methanol and in the presence of HSC2H4Ph21,82.

Recently, the cluster structures of two Au-Cu NC species(Au13Cun (n = 2, 4, 8), Au12+nCu32(SR)30+n (SR = thiolates; n = 0,2, 4, 6) NCs) have been successfully presented by using X-ray crystallography83,84. In one study, Zheng’s group have successfully resolved the crystal structures of three Au-Cu alloy NCs, Au13Cu8(PPh2Py)12, Au13Cu4(PPh2Py)4(SC6H4-tert-C4H9)8,Au13Cu2(PPh3)6(SPy)6NCs, and stabilized by a mixed layer of thiolate and phosphine ligands bearing pyridyl groups83. These alloy NCs have an icosahedral Au13 core face-capped by 2, 4,and 8 Cu atoms, respectively (Fig. 6). All face-capping Cu atoms in the alloy NCs are triply coordinated by thiolate or pyridyl groups. Furthermore, in another study, they fabricated a series of all-thiol-protected Au-Cu alloy NCs,Au12+nCu32(SR)30+n(n = 0, 2, 4, 6 and SR = SPhCF3) NCs and characterized their crystal structures (Fig. 7)84. In particular,they found that each cluster has a Keplerate two-shell Au12@Cu20core protected by (6-n) units of Cu2(SR)5and n units of Cu2Au(SR)6 (n = 0, 2, 4, 6) motifs (Fig. 7e–i). More interestingly, when n = 0, the Au-Cu alloy NC has a very similar geometric architecture to that of Au12Ag32(SR)30NC34,suggesting that Ag and Cu have the same binding motifs with thiolates in the Au12M32(M = Ag or Cu) NCs (Figs. 3 and 7a–d).

Fig. 6 Crystal structures of Au13Cu2(PPh3)6(SPy)6 (a, b),Au13Cu4(PPh2Py)4(SC6H4-tert-C4H9)8 (c, d), and Au13Cu8(PPh2Py)12 (e, f) NCs 83.Color legend: golden sphere, Au; green sphere, Cu; yellow sphere, S; pink sphere, P;gray stick, C; blue stick, N. All H atoms in both clusters and tert-butyl groups in Au13Cu4(PPh2Py)4(SC6H4-tert-C4H9)8 NCs are omitted.

2.5 Others

Besides those described above, several others alloy NC species, such as Ag-Pt41,50,55,85, Ag-Pd50,55, Ag-Cu25, Ag-Ni55,86,Au-Cu-Pd49(Fig. 1), can be also fabricated via one-pot synthesis. For example, a charge-neutral, rod-shaped, di-Ptdoped Ag NC of [Ag23Pt2Cl7(PPh3)10] was obtained by using NaBH4 co-reduction of AgNO3 and Na2PtCl6 in the presence of PPh3 in methanol and dichloromethane as cosolvents41. The crystal structure of the alloy NC revealed the self-assembly of two Pt-centered Ag icosahedra via vertex sharing (Fig. 8). In addition, a facile ion pairing strategy has been developed for asymmetric synthesis of new chiral [Ag28Cu12(SR)24]4-(where SR is 2,4-dichlorobenzenethiol) alloy NCs using chiral ammonium cations25. Nickel nitrate and silver nitrate can be simultaneously reduced with NaBH4to fabricate ultrasmall and atom-precise bimetallic NC, Ag4Ni2(DMSA)4, in the presence of meso-2,3-dimercapto- succinic acid (DMSA) in deoxygenated ethanol86. Moreover, trimetallic NCs, Au24-nCunPd(SC12H25)18(n = 0–3), have also been synthesized by using a consistently one-pot synthetic procedure49.

In summary, one-pot synthesis is an effective and versatile in the synthesis of atomically precise alloy NCs. It is the most used synthetic strategy, and a great many atomically precise alloy NCs, such as Au-Ag, Au-Pt, Au-Pd, Au-Cu, Ag-Pt,Ag-Pd, Ag-Cu, Ag-Ni, and trimetallic Au-Cu-Pd, have been successfully fabricated. However, several key issues need to be overcome in further research. First, the precise control of the metallic composition and architectural feature in the alloy NCs also remains as a great challenge. Moreover, the as-prepared alloy NCs through the one-pot synthetic strategy often appear very somber fluorescence, which limit their use in the case of fluorescence detection and bioimaging. In addition to the above-mentioned several metal species, other interesting metals such as Zn and Fe should be also considered for use replenish in the family of atomically precise alloy NCs.

Fig. 7 Crystal structure of the [Au12Cu32(SR)30]4- NC (a–d) and surface structures of Au12Cu32@Aun (n = 0, 2, 4, 6) NCs (e–i) 84.(a) Overall structure of the [Au12Cu32(SR)30]4- NC. (b) Two-shell Au12@Cu20 core of the cluster. (c) Structure of the cluster with the Au12@Cu20 two-shell cores in the space-filling style. (d) Structure of the surface [Cu2(SPhCF3)5] motif. (e)[Au12Cu32(SR)30]4-. (f) [Au12Cu32Au2(SR)32]4-. (g) Schematic diagram of the formation of the Cu2Au(SR)6 unit from Cu2(SR)5 unit. (h) [Au12Cu32Au4(SR)34]4-. (i)[Au12Cu32Au6(SR)36]4-. Color legend: orange/golden sphere, Au; blue sphere, Cu; yellow spheres, S; gray sticks/spheres, C. All hydrogen, fluorine, and carbon (e–i) atoms are omitted for clarity.

Fig. 8 (A) Vertex Ag atoms of two Ag12Pt icosahedra being shared and connected are shown with a dotted ellipse and dotted lines, respectively.(B) Biicosahedral Ag23Pt2 rod. (C) Bridging and terminal chlorides bind with the structure shown in (B) to form Ag23Pt2Cl7. (D) Capping with the 10 P atoms of 10 PPh3 ligands gives Ag23Pt2Cl5P10 41.Color legend: cyan, Ag; blue, Pt; yellow, P; green, Cl; gray, C.The H atoms of the ligands have been omitted.

3 Metal exchange

Metal exchange is another efficiently method for synthesizing alloy NCs. Typical, the ligand-protected monometallic NCs (e.g., Au25, Ag25) were first synthesized by reduction using NaBH4, and then another metal precursors(e.g., HAuCl4) or metal-thiolate (e.g., AgI-thiolate) complexes were added, leading to the formation of ligand-protected alloy NCs. According to the metal activity (i.e., galvanic sequence),the metal exchange strategy can be divided into galvanic metal exchange and anti-galvanic metal exchange.

3.1 Galvanic metal exchange

Galvanic metal exchange reaction is a redox process, which provided a privileged tool for fabrication of alloy NCs with well-defined compositions and structures87–89. The driving force for the galvanic metal exchange reaction is the redox potential difference between the two metals (e.g., Ag NCs can be oxidized by HAuCl4in the reaction solution according to the following redox reaction 3Ag0+ Au3+→ Au0+ 3Ag+due to the fact that the redox potential of Au3+/Au (～1 V) is higher than that of Ag+/Ag (0.8 V)) and this method can use a particular metal to reduce a more noble (higher inert) metal ions to form alloy NCs. For example, Zhang and coworkers prepared a crown jewel structured Au-Pd alloy NCs by the galvanic metal exchange reaction33. A size of 1.8 nm Pd147NC was served as the “crown”, and the Au atoms formed by the spontaneous metal exchange reaction embedded in the corner of Au-Pd NCs were decorated as “jewels” (Fig .9a). Considering the surface energy difference of the top, edge and face Pd atoms of the Pd147NCs, the metal exchange reaction first occurs from the top Pd atoms, leading to a preferential exchange reaction between Au3+and Pd atoms on the corner to form the top Au atoms in situ. Interestingly, only these top-replaced Pd-Au alloy NCs have highly catalytic activity toward the aerobic glucose oxidation. Another interesting example is the simple mortar grinding for the synthesis of biopolymer-protected and catalytically Au-Ag alloy NCs (Fig. 9b)90. In this study, the chitosan was employed as both a stabilizing and reducing agent.

Fig. 9 (a) Schematic illustration of the deposition of top Au atoms on Pd mother clusters by the galvanic metal exchange reaction method 33.(b) Photographic indication of color changes during the synthesis of Au-Ag alloy NCs 90.

With this approach, several studies have reported the preparation of luminescent Au-Ag alloy NC91. For instance,Bootharaju and coworkers demonstrated the fabrication of the atom-precise [Ag24Au(SPhMe2)18]-NCs by this approach92.The galvanic metal exchange reactions were performed by mixing pure [Ag25(SPhMe2)18]-NCs templates with chloro(triphenylphosphine) Au(I) in dichloromethane solvent at room temperature. The as-fabricated [Ag24Au(SPhMe2)18]-NCs showed strong luminescence. Similar, a superatom cluster with strong red luminescence (i.e., Ag7Au6(MSA)10(MSA =mercaptosuccinic acid)) has been successfully synthesized by introducing HAuCl4to a MSA-protected Ag7,8NC via galvanic metal exchange reaction93. In another excellent study, Zhu’s group have successfully synthesized a series of thiolate/phosphine co-protected Au-Ag alloy NCs that are AunAg50-n(DPPM)6(SR)30by adding Au(I)-SR into pure Ag50(DPPM)6(SR)30via galvanic metal exchange strategy (where SR =4-tert-butylbenzyl mercaptan)94. The X-ray crystallography revealed that Au atoms exchanged the Ag atoms in the hollow M12metal core (Fig. 10a). Further studies have found that the doping of Au into the Ag core can mainly affect HOMO-LUMO orbitals and UV-Vis spectrum and increase the photoluminescence and thermal stability of homologue Ag NCs, that is Ag50(DPPM)6(SR)30NCs (Fig. 10b–d). In another study, they successfully realized the synthesis of trimetallic NCs of different shapes by changing the dopants95. The spherical [Pt1Ag24(SPhMe2)18]2-NCs could be converted to spherical or rod-like trimetallic NCs by a controllably galvanic metal exchange strategy. The [Pt1Ag24(SPhMe2)18]2-NCs with Au complexes coordinated by the same thiolate ligands (i.e.,HSPhMe2) keeps up the spherical geometric architecture. In contrast, alloying with [AuI(PPh3)Br] transforms the shape to a rod-like, owing to the different ligands (Fig. 11).

Fig. 10 (a) Exchange from Ag50(Dppm)6(SR)30 to AunAg50-n(Dppm)6(SR)30 by galvanic metal exchange method.Comparison of Ag50(Dppm)6(SR)30 and AunAg50-n(Dppm)6(SR)30(n = 5.34) for (b) UV-Vis, (c) photoluminescence spectras, and(d) differential pulse voltammograms 94.

It is evident that the galvanic metal exchange is a straightforward, effective, and versatile strategy for the synthesis of atomically precise alloy NCs. We believe that it will continue attracting much attention in the relevant research areas, and a variety of novel alloy NCs species with well-defined compositions, architectures, and surface chemistries could be synthesized, which may further promote their applications in the field of catalysis, bio-analysis and biomedicine.

3.2 Anti-galvanic metal exchange

Fig. 11 Illustrative scheme of the transformations from Pt1Ag24 to Pt1AunAg24-n and Pt2Au10Ag13 NCs 95.

Very recently, the surprising anti-galvanic metal exchange reaction (opposite to galvanic metal exchange) has been serendipitously characterized in Au-Ag alloy NCs chemistry.Noble metal ions are reduced by more noble metals, which is not thermodynamically favorable and totally against classic galvanic theory. This reaction was first revealed by Choi and coworkers96in 2010 and general concept was first proposed by Wu97in two years later. Indeed, Choi and coworkers had first revealed that the [Au25(SC2H4Ph)18]-NC can react with Ag+forming [Au24Ag(SC2H4Ph)18]2+, [Au23Ag2(SC2H4Ph)18]2+, and[Au22Ag3(SC2H4Ph)18]2+alloy NCs by regulating the[Au25(SC2H4Ph)18]-/Ag+ratios, thus suggesting that anti-galvanic reaction could occur96. Wu proposed a general anti-galvanic reaction concept97. He found that when the Au or Ag NCs’ size decreased to less than approximately 3 nm, the reducing activity is significantly increased and they can reduce some more reactive noble metal ions (for example, reduce Cu2+to Cu0). Since then began the new era of anti-galvanic metal exchange reaction appeared that have been regarded as a new method of alloy NCs synthesis. For example, Ag atoms(Ag(I)-cyclohexanethiolate) have been shown to replace Au atoms by the anti-galvanic metal exchange reaction in Au23(SC6H11)16NCs to form heavily Ag-doped Au25-nAgn(SC6H11)16NCs (where n up to 19)22. The reaction induces the structural transformation of Au23(SC6H11)16NCs to Au25-nAgn(SC6H11)16NCs (Fig. 12a). Wu and coworkers have successfully achieved mono-Cd and mono-Hg doping of Au25(SC2H4Ph)18NCs through anti-galvanic metal exchange reaction56. X-ray crystallography and theoretical calculations revealed that one of the inner-shell Au atoms of Au25(SC2H4Ph)18NC was replaced by a Cd atom. However, the doping mode is distinctly different from that of mono-Hg doping, where one of the outer-shell Au atoms was replaced by a Hg atom. Very interestingly, Au24Cd (SC2H4Ph)18NC is readily transformed to Au24Hg(SC2H4Ph)18NC, while the reverse endergonic reaction of Au24Hg (SC2H4Ph)18NC with Cd2+to form Au24Cd(SC2H4Ph)18NC is forbidden, which can be used to evaluate the HOMO-LUMO energies of Au24Hg(SC2H4Ph)18 NC. Zhu and coworkers demonstrated the first synthesis of the thiolate-protected trimetallic M1AgnAu24-n(SC2H4Ph)18(M = Cd/Hg) NC by replacement of the central Au atom in AgnAu25-n(SC2H4Ph)18by a Cd2+or Hg2+thiolate complex43. X-ray crystallography revealed that the core of the trimetallic NC is a Cd or Hg atom, and inner-shell of the trimetallic NC is a Ag-Au bimetallic M12 unit, and the icosahedral Au13core surrounded by two Au2(SC2H4Ph)3units(Fig. 12b). Meanwhile, anti-galvanic metal exchange method based upon Au25(SC2H4Ph)18NCs is designed to fabricate alloy NCs including AgnAu25-n(SC2H4Ph)18, CunAu25-n(SC2H4Ph)18,Cd1Au24(SC2H4Ph)18, and Hg1Au24(SC2H4Ph)18NCs through reaction of the template with metal-thiolate (i.e., SC2H4Ph)complexes of Ag+, Cu2+, Cd2+, and Hg2+98. Their results reconfirm that the exchange between Au atoms in NCs and those of the other metal in the thiolated complex not always observe the galvanic sequence. Moreover, Jin and coworkers demonstrated a site-specific “surgery” on the surface motif of an atom-precise [Au23(SC6H11)16]-NC by a two-step antigalvanic metal exchange method between [Au23(SC6H11)16]-and AgI(Ph2PCH2PPh2), which results in the “resection” of two surface Au atoms and the formation of a new 21-gold-atom NC,[Au21(SC6H11)12(Ph2PCH2PPh2)2]+, without changing the other parts of the starting NC structure (Fig. 13)99. This breakthrough constitutes a major step toward the development of atom-precise, versatile nanochemistry for the precise tailoring of the NC architecture. In addition, a series of alloy NCs with unanticipated structures and properties such as Au15Ag3(SPhMe2)14, Au22Ir3(SC2H4Ph)18, CunAu25-n(PPh3)10(SC2H4Ph)5Cl22+, Cu3Au34(PPh3)13(tBuPhCH2S)6S23+, and Au24-n AgnHg1(SC2H4Ph)18were also synthesized recently52,53,100,101.These breakthroughs suggest that the anti-galvanic metal exchange method has great potential for precise control of alloy NCs synthesis.

Fig. 12 (a) Ag(I)–cyclohexanethiolate induced transformation of Au23(SC6H11)16 NCs to Au25-nAgn(SC6H11)16 NCs with Ag doping on the icosahedral shell and the exterior staple motifs 22. (b) Two-step metal exchange method for the synthesis of M1AgnAu24-n(SC2H4Ph)18 NC. Only part of the Au25(SC2H4Ph)18 structure is shown: the icosahedral Au13 core surrounded by two Au2(SC2H4Ph)3 motifs 43.Color labels: (a) magenta, Au; yellow, S; gray, Ag or Au; blue, N; all C and H atoms not shown. R = c-C6H11. (b) Red, S; yellow, Au; gray, Au/Ag; green, Cd/Hg.

Fig. 13 Molecular surgery on the atomically precise 23-gold-atom NC by a two-step metal-exchange method: peeling off two parts of the cluster wrapper and closing the gaps with two Ph2PCH2PPh2 plasters 99.(A) Schematic of the molecular surgery on [Au23(SR)16]-; all carbon tails are omitted for clarity. (B) Site-specific surface motif tailoring with a two-step metal-exchange method.The transformation from [Au23(SR)16]- through [Au23-nAgn(SR)16]- (n ～ 1) to[Au21(SR)12(Ph2PCH2PPh2)2]+ is revealed by single-crystal x-ray analysis.Magenta and blue, Au; gray, Ag; yellow, S; orange, P; green, C; light green, Cl.Other C and all H atoms are omitted.

The mechanism of anti-galvanic metal exchange reaction has also been studied in several recent reports. Originally, when Wu proposed the general concept of the anti-galvanic reaction, he speculated that the thiolate ligands of Au25NCs may play a vital role in assisting the anti-galvanic metal exchange reduction of less noble metal ions because theses thiolates provide some partial negative charge to the Au or Ag NCs upon binging to the surfaces of these NCs97. However, in their later study rejected this hypothesis and confirmed that the anti-galvanic metal exchange reaction does not require the assistance of reductive ligands, and that it is an intrinsic property of the ultrasmall NCs102. In this study, they found that ligand-free noble metal NPs such as Au NPs, Pt NPs, and Pd NPs have a distinct reducibility with regard to less noble metal ions such as Ag+and Cu2+when the size of noble NPs less than 3 nm. Later, the anti-galvanic reaction is ion-precursor and ion-dose dependent was also found by Wu’s group103.Moreover, Zhu and coworkers indicated that the anti-galvanic reaction of Au25(SC2H4Ph)18 NCs with Ag+, Cu2+, Cd2+, and Hg2+to form stable MnAu25-n(SC2H4Ph)18(M = Ag, Cu, Cd,Hg) NCs are due to their 8e-shell closing structure98. In general,the mechanism of the anti-galvanic reaction is still unclear.Therefore, more systematically experimental and theoretical studies are required to further understand the driving force and mechanism of the anti-galvanic metal exchange reaction for alloy NC formation.

4 Ligand exchange

Similar to the metal exchange method, the ligand exchange approach has been demonstrated to be an effective strategy to produce diverse monometallic NCs especially Au NCs and Ag NCs with different core sizes and architectures104–106. As a versatile synthetic strategy, this approach has also been used for synthesis of atom-precise alloy NCs. This approach calls for parent alloy NCs to react with other ligands under the two phase conditions. Xiang and coworkers used a glutathione(GSH)-capped, water soluble [(AuAg)n(GSH)m] cluster as the starting material107. Then, a two-phase ligand exchange experiment was performed by mixing this [(AuAg)n(GSH)m]aqueous solution with a solution of cyclohexanethiol and dichloromethane under vigorous stirring at room temperature.The phase transfer was completely finished after 6 h and lead to solvent soluble [Au15Ag3(SC6H11)14] alloy NCs with high purity. Similarly, when cyclohexanethiol is replaced by tert-butyl thiophenol (SPh-tBu), a series of new clusters that is Au36-nAgn(SPh-tBu)24(n = 1–8) alloy NCs with high purity will generate108. These two types of Au-Ag alloy NC have completely different electronic structures and optical properties, indicating that the choice of ligands significantly affects the formation of NCs in ligand exchange process, which leads to different properties and functions107,108. A site-selective ligand exchange process is also demonstrated recently109.

Fig. 14 (a) Comparison of HPLC chromatograms of exchanged PdAu24 (18 h), PtAu24 (27 h), and Au25 (6 h) to reference PdAu24(2-PET)18(un-exchanged). (b) HPLC chromatogram of the PdAu24(2-PET)18-2n(BINAS)n exchanged product. Kinetic fitting of the integrated HPLCnchromatograms after exchange of PdAu24 (c) and PtAu24 (d) 110.The larger dots are the values as determined from HPLC experiments, while the small dots represent the corresponding fits. Green dots refer to the initial 2-PET protected cluster, whereas the red, blue, and yellow dots respectively represent the first, second, and third exchange products.

The reaction kinetics of ligand exchange between Pt-/or Pd-doped Au25(2-PET)18 (2-PET = 2-phenylethylthiolate) NCs and enantiopure 1,1′-binaphthyl-2,2′-dithiol (BINAS) were monitored in situ by using chiral high performance liquid chromatography (HPLC)110. BINAS ligand exchange was performed on all three NCs (Au25(2-PET)18, PdAu24(2-PET)18,PtAu24(2-PET)18) at different reaction times to allow the formation of similar exchanged products. The reaction outputs where then injected in chiral HPLC and the result shows that for the doped clusters a better separation of the different exchanged product was achieved (Fig. 14a). During the ligand exchange reactions, replacement of two protecting thiols(2-PET) with one new entering BINAS ligand on the cluster surface occurs (Fig. 14b). In addition, it can be observed how the different exchange products are formed one after the other in a consecutive manner (Fig. 14c,d). Using Au24Pd(SC2H4Ph)18as a model cluster, the mechanism of ligand exchange reactions on the clusters were demonstrated by probing isomer distributions using reversed-phase HPLC111.The results revealed that the exchange reaction starts to occur preferentially at thiolates that are bound directly to the metal core (thiolates of a core site) in all reactions. Further study on the isomer-separated Au24Pd(SC2H4Ph)17(SC12H25) revealed that clusters vary the coordination isomer distribution in solution by the ligand exchange reaction between clusters and that control of the coordination isomer distribution of the starting clusters enables control of the coordination isomer distribution of the products generated by ligand exchange reactions between clusters.

5 Chemical etching

As an alternative strategy, thiols-induced chemical etching also was used for fabricating alloy NCs through top-down etching the relatively large-sized particles by using excess suitable thiol etching agents. The strategy depends on the strong interaction between sulfur and metal atoms and has been widely used to prepare atomically precise monometallic and alloy NCs. For example, in 2001, Dass’s group successfully synthesized (AuAg)144(SR)60 by using thiol-etching strategy47.They first synthesized a series of phenylethane thiol(HSC2H4Ph)-protected AuAg nanoparticles (NPs) with adjustable Au/Ag ratios. Thereafter, excess phenylethane thiol was added to etch the as-synthesized AuAg NPs, resulting in the formation of (AuAg)144(SC2H4Ph)60. Using the same method, the Au144-nPdn(SC2H4Ph)60(n = 0–7) and Au130-nAgn(SC2H4Ph)50 (n = 0–20) alloy NCs have also been fabricated112,113. Very recently, bi-ligands-protected Pt1Ag28(S-Adm)18(PPh3)4NCs have also been synthesized by etching Pt1Ag24(SPhMe2)18NCs simultaneously with Adm-SH(1-adamantanethiol) and PPh3 ligands23. A tetrahedral structure is found in the metal framework of Pt1Ag28NC and an overall surface shell (Ag16S18P4), as well as discrete Ag4S6P1motifs(Fig. 15). Very interestingly, the photoluminescence and the thermal stability of Pt1Ag28(S-Adm)18(PPh3)4 NCs were drastically enhanced when compared to that of their precursor,Pt1Ag24(SPhMe2)18NCs. It is observed that thiol-induced chemical etching presents an efficient strategy to fabricate fluorescent alloy NCs. Further study requires more efficient procedures to fabricate the parental alloy NPs with well-defined composition and structure, which could be subsequently etched to form high-quality alloy NCs.

6 Intercluster reactions

Fig. 15 Ball-and-stick views of (A) the Pt1Ag12 kernel, outer-motif and the overall structure; (B) the Ag4S6P1 motif in the tetrahedral shape and (C) the total structure in the tetrahedral shape. (D) A space-filling view of the nanocluster 23.Color codes: green spheres, Pt; cerulean sphere, Ag on the shell; violet sphere,Ag in the kernel; orange sphere, S bonding the kernel; red sphere,S linking the motifs; purple sphere, P. The hydrogen atoms are not shown.

Another emerging and efficient strategy similar to reactions between molecules which were defined as “intercluster reactions” has been exploited to the fabrication of atom-precise alloy NCs. Pradeep’s group demonstrated the first exemplification of intercluster reactions between two atomically precise metal NCs, [Au25(PET)18]-and[Ag44(FTP)30]4-(FTP = 4-fluorothiophenol) (Fig. 16)114. These clusters could spontaneously exchange the metal atoms and ligands as well as the metal-thiol fragments in solution at ambient conditions. The number of exchanged species could be controlled by varying the initial compositions of the reactant clusters. In addition, a reaction of [Au25(PET)18]-with[Ag25(DMBT)18]-(DMBT = 2,4-dimethylbenzenethiol) has also been reported to demonstrate that atom-exchange reactions happen with structures conserved115. A transient dianionic adduct, [Ag25Au25(DMBT)18(PET)18]2-formed between the two NCs could be readily detected, indicating that this adduct is a possible intermediate of the reaction. Similarly, two Au25-nAgn alloy NC species, protected by both PPh3 and HSC2H4Ph ligands, were successfully synthesized by introducing Ag(I)-SC2H4Ph complexes into large-sized PPh3-protected Au NPs and small-sized PPh3-protected Au11NCs, respectively28. The introduction of Ag(I)-SC2H4Ph complexes could remarkable affect the stability of both the large-sized Au NPs and small-sized Au11 NCs due to the competitive effects between Au species, Ag species, phosphate ligands, and thiolate ligands(i.e., HSC2H4Ph), leading to the lysis of Au NPs and Au11NCs,and followed by recombination to generate thermodynamically stable Au25-nAgnNCs. Interestingly, the two Au25-nAgnalloy NC species have remarkable difference of fluorescence and the strong fluorescence was mainly attributed from the thirteenth Ag atom in the NCs28. With this approach, a series of thiol-protected Au-Ag alloy NCs that are Ag25-nAun(SR)18(n =1–6), Ag25-nAun(SR)30(n = 15–22), Au25-nAgn(SR)18(n = 1–7),Au25-nAgn(SR)18(n = 9–16), Ag44-nAun(FTP)30(n = 4–9), and Ag44-nAun(SR)30(n ＞ 12) (where SR = alkyl/aryl thiolate) NCs were also synthesized recently116–118.

Fig. 16 Schematic illustration of the intercluster reactions between Au25(PET)18 and Ag44(FTP)30 114.

7 In situ two-phase ligand exchange

Inspired by the simplicity of the one-pot synthetic strategy and the effectiveness of the two-phase ligand-exchange approach, more recently, Zhu’s group developed a combinative synthetic strategy, in situ two-phase ligand-exchange method(Fig. 17)119. This synthesis process is distinctively different from the one-pot synthetic strategy and two-phase ligandexchange approach. The precursor of in situ two-phase ligand exchange has high structure dispersion, and selective reduction and ligand exchange simultaneously. Therefore, the in situ two-phase ligand exchange has the advantages of the reaction environment and simple operation time. With this strategy, they have successfully synthesized a series of alloy NCs with six free electrons, including Au20Ag1(SR)15, Au21-nAgn(SR)15(n =4–8), Au21-nCun(SR)15(n = 0, 1), and Au21-nCun(SR)15(n = 2–5) (where SR = tert-butyl thiolate). X-ray crystallography shows that these alloy NCs exist in two configurations,Au20Ag1(SR)15, Au21-nAgn (SR)15 (n = 4–8), Au21-nCun(SR)15 (n =0, 1), which have similar configurations, and the doping of multiple Cu atoms significantly changes the configuration of the alloy NCs, resulting in the Au21-nCun(SR)15(n = 2–5)exhibits distinctly different geometric architecture (Fig. 17).The optical absorption of these alloy NCs show distinct differences, suggesting that the metal-incorporating can observably affect the electronic structure of the NCs. This in situ two-phase ligand-exchange method might potentially act as an excellent alternative for synthesizing alloy NCs and even homothallic ones with unprecedented atomic structures119,120.

8 Surface motif exchange reaction

Fig. 17 Schematic illustration of the in situ two-phase ligand-exchange method 119.

Fig. 18 Schematic illustration of surface motif exchange reaction on Ag44 NCs 121.(a) Pristine [Ag44(SR)30]4-; (b) association of [Au2(SR′)2Cl]- to [Ag44(SR)30]4-;(c) Ag―S bond deformation in [Au2(SR’)2Cl]- associated [Ag44(SR)30]4-;(d) dissociation of AgCl and [Au(SR)2]-; (e) [Ag43Au(SR)28(SR′)2]4- formed by exchange of one SR-Ag(I)-SR motif and (f) [Ag32Au12(SR)6(SR’)24]4- produced by complete exchange of twelve SR-Ag(I)-SR motifs, where SR and SR’ denote pristine and foreign thiolate ligands, respectively. For an easy and clear presentation, the Ag atoms in Ag12 inner core and Ag20 external core are shown as gray and light blue large balls, respectively; while the other atoms are shown as small dots (color legend: Ag(gray/light blue/purple), Au (yellow), S (orange/magenta), and Cl (light green)).The hydrocarbon tails and carboxylic groups of the protecting ligands are omitted.

As described above, a large number of Au-Ag alloy NCs have been successfully prepared based on the one-pot synthesis and the metal exchange reaction58–66,91–94. In these attempts, Au atoms were assigned in the core of alloy NCs due to ubiquitous involvement of the reduction reaction of Au(III)/Au(I) to Au(0).As a result, Au-core/Ag-shell NCs could be generally obtained rather than Ag-core/Au-shell NCs because they are thermodynamically less favorable. To obtain stable Ag-core/Au-shell NCs, very recently, Xie and coworkers present a delicate surface motif exchange reaction, which could use the Au(I)-SR complexes (SR = thiolate) in solution to precisely replace Ag(I)-SR-protecting motifs on the Ag NC surface, and assign Au heteroatoms in the protecting shell of alloy NCs121.By a delicate exchange reaction between the [Ag44(SR)30]4-NCs and the introducing Au(I)-SR complexes,[Ag44-nAun(SR)30]4-(n = 0–12) alloy NCs could be readily fabricated (Fig. 18). This well-controlled reaction could largely stabilize Au(I) and prohibit the galvanic reduction of Au(III)Au(I) through the Ag(0) core of [Ag44(SR)30]4-, which could prevent the formation of Au-core/Ag-shell NCs.Meanwhile, theoretical calculations also suggest that the formation of such thermodynamically less favorable Ag@Au core-shell structure could be attributed to the diffusion barrier established by the Ag20external core of [Ag44(SR)30]4-. The surface motif exchange reaction is expected to extend to the fabrication of binary new alloy NCs with precise alloy sites to broaden the physicochemical properties of the NCs, which has far-reaching significance for both basic and applied explorations.

9 Conclusions

In summary, a number of effective synthetic strategies have been developed for obtaining high-quality alloy NCs with well-defined compositions, sizes, and architectures in recent years. In this review, we summarized the recent advances in the controllable synthesis of atomically precise alloy NCs and classified these synthetic strategies for alloy NCs into several routes, which are one-pot synthesis, metal exchange, ligand exchange, chemical etching, intercluster reactions, surface motif exchange reaction and in situ two-phase ligand exchange strategy. Among them, one-pot synthesis is facile and the most used synthetic strategy for monodisperse alloy NCs with well-defined compositions, sizes, architectures, and surface chemistries. However, the alloys NCs through the one-pot synthetic strategy often appear very somber fluorescence. In contrast, the other strategies often fabricates alloy NCs with strong photoluminescence, though their processes are relatively complex. The two or multiple metal species incorporated in one alloy NC show some unexpected synergistic properties, such as 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, biosensor and biomedicine.