Controlling ultrasmall gold nanoparticles with atomic precision

Abstract

Gold nanoparticles are probably the nanoparticles that have been best studied for the longest time due to their stability, physicochemical properties and applications. Controlling gold nanoparticles with atomic precision is of significance for subsequent research on their structures, properties and applications, which is a dream that has been pursued for many years since ruby gold was first obtained by Faraday in 1857. Fortunately, this dream has recently been partially realized for some ultrasmall gold nanoparticles (nanoclusters). However, rationally designing and synthesizing gold nanoparticles with atomic precision are still distant goals, and this challenge might rely primarily on rich atomically precise gold nanoparticle libraries and the in-depth understanding of metal nanoparticle chemistry. Herein, we review general synthesis strategies and some facile synthesis methods, with an emphasis on the controlling parameters determined from well-documented results, which might have important implications for future nanoparticle synthesis with atomic precision and facilitate related research and applications.

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1. Introduction

Gold is a noble element with chemical inertness and enduring physical qualities, even after exposure to extreme conditions for tens or hundreds of years. Long after the discovery of gold in Bulgaria five thousand years ago, ancient Egypt and China started using colloidal gold for therapeutic and decorative purposes around the 5th or 4th century B.C.^1–3 The scientific realization of such colloidal gold can be traced back to Michael Faraday's research in 1857 when he successfully prepared a ruby-red gold colloid through the reduction method.⁴ After a great deal of experimental and theoretical research was performed in the following 150 years, gold colloids (gold nanoparticles) were found to not only be beautiful but also to exhibit rich properties such as optics, sensing, catalysis, and biomedicine properties.^1–3,5–7

The properties of gold nanoparticles are often related to a set of physical parameters that may include the size, shape, composition, and structure.^8,9 Over the past few decades, extensive efforts have been devoted to modulating gold nanoparticles to control their morphologies and compositions. Despite narrow dispersion in continuously improved synthesis, gold nanoparticles are still not atomically precise due to their polydisperse sizes and indefinable surface architecture. Controlling gold nanoparticles with atomic precision can be of great utility not only in tuning their properties accurately but also in obtaining an atomic-level understanding of their structure–property correlations (e.g. structure–catalytic activity relationships¹⁰). It has long been a major dream of nanochemists to prepare atomically precise gold nanoparticles, which was realized by the emergence of gold nanoclusters.

Gold nanoclusters (Au NCs), or ultrasmall gold nanoparticles, are aggregates of certain gold atoms protected by ligands with precise molecular formulae. Au NCs can be rigorously categorized as a branch of the broader class of gold nanoparticles and provide an ideal platform for the pursuit of atomically precise control of gold nanoparticles. In comparison with previously studied gold nanoparticles, Au NCs exhibit intriguing properties, such as discrete electronic energy levels, quantized charging, intrinsic chirality, strong luminescence, and excellent catalytic selectivity.^11–13 The architecture of Au NCs usually consists of various numbers of metal atoms, types of metal atoms and ligands, surface bonding (staples), packing styles of metal atoms, etc. Thus, unique molecular-like structures and properties render Au NCs sensitive to one or a few atomic alterations and accessible for control by methods such as chemists modifying organic molecules through various synthetic methods.^14–18 Nevertheless, rationally designing and synthesizing Au NCs are still challenging due to the limitation of synthesis methods and NC chemistry understanding.

This review aims to provide a comprehensive summary of the recent progress in precisely controlling Au NCs at the atomic level based on some existing synthesis methods, mainly including gold salt (complex) reduction, ligand induction, and antigalvanic reduction, among others. We will highlight the controlling parameters of how to influence the structures and properties of the resulting Au NCs. Finally, we conclude by presenting our perspective and suggestions for the further development of controlling Au NCs with atomic precision.

2. Synthesis strategy

Understanding the fundamentals of the underlying reaction process is extremely important to control Au NCs with atomic precision, which has attracted extensive attention in recent years. For example, Hainfeld et al. reported the preparation of size-controllable gold nanoparticles (2–6 nm) by tuning the polymeric Au(I) thiolate precursor size with pH control.¹⁹ Subsequently, Jin et al. achieved the controllable synthesis of NCs by tuning the kinetics of the formation of Au(I):SR intermediates, leading to the formation of Au₂₅ NCs with high yield.²⁰ They found that Au(I):SR intermediates formed at 0 °C, which, under very slow stirring conditions, were distributed in a narrow range of 100–400 nm, whereas those obtained at room temperature had a multimodal distribution with a broader range of 1–10 000 nm. Soon afterwards, an important process – “size focusing’‘ – was revealed in the growth of Au₂₅ by Jin and coworkers.^21,22 This aging stage involves a slow conversion process of the initially formed mixture of differently sized Au_n clusters into uniform Au₂₅. Then, motivated by the formation of Au₁₉ NCs using a weak reducing agent (borane tert-butylamine complex) instead of the strong reducing NaBH₄ agent, Wu and Jin proposed a general strategy for the synthesis of Au NCs, involving kinetic control and thermodynamic selection processes (Scheme 1),²³ which can be detailed as (i) the kinetic control of the initial size distribution of crude NCs by controlling the reduction speed, the ratio of Au : SR, and other parameters; and (ii) thermodynamic selection (or size focusing) during the aging process by controlling the aging time, temperature, and other parameters, leading to robust (thermodynamically favorable) cluster species. Xie et al. also demonstrated such a strategy in the synthesis of water-soluble Au₂₅ in aqueous solution by using electrospray ionization mass spectrometry (ESI-MS) to monitor the reaction process. They adopted the NaOH-mediated NaBH₄ reduction method to tune the formation kinetics of Au NCs and accelerate the thermodynamic selection of Au₂₅.²⁴ Another work by them is the usage of CO as a reducing agent that kinetically controls crude clusters, and Au₂₅ NCs were also obtained after thermodynamically controlled size focusing.²⁵ Although various synthetic methods have been developed in the last decade, the formation of atomically precise (narrow-sized) Au NCs can be mainly illustrated by kinetic control and thermodynamic selection (not limited to the gold salt (complex) reduction method (vide infra)).

Scheme 1 Illustration of the synthesis strategy “kinetic control and thermodynamic selection” for atomically monodisperse gold nanoclusters.

3. Synthesis method and controlling factors

There are two basic routes used for synthesizing atomically precise gold nanoparticles in terms of the charge state of the starting gold (Scheme 2): one route is the reduction of gold salt (complex), and the other is the transformation of gold nanoparticles (including nanoclusters).^18,26–28 For the first route, the synthesis method can be dubbed the “gold salt (complex) reduction method”; for the second route, there are two major synthesis methods, including the ligand induction method and anti-galvanic reduction method, in terms of inducing reagents. Below, we will specifically review the synthesis method and controlling parameters.

Scheme 2 Schematic illustration of the synthesis route classification in terms of the charge state of the starting gold.

3.1 Gold salt (complex) reduction method and controlling parameters

The gold salt (complex) reduction method is also known as a one-pot synthesis, and is one of the most versatile and best studied synthetic methods. This method is usually referred to as the direct reduction of a mixture of gold salt (complex) and other reactants, leading to final nanoparticles after essential purification but without any further disposal. The first gold colloid was synthesized by Faraday in 1857 from the aqueous reduction of gold chloride by phosphorus.⁴ An important advancement of this method was developed by Brust et al. (the Brust–Schiffrin method) in 1994.²⁹ They exploited thiols for gold nanoparticle synthesis via a two-phase system in which gold chloride was transferred in toluene using a phase-transfer reagent. Following the footsteps of Brust et al., many groups, including Whetten, Murray, Tsukuda, Jin and other groups, conducted tremendous research on the synthesis of gold thiolate nanoparticles and nanoclusters. Gold salt (complex) reduction (often modified from the Brust–Schiffrin method) has been an efficient approach used to control the synthesis of Au NCs. The controlling parameters comprise the ligand, temperature, solvent, and others.

3.1.1 Ligands. Ligands influence not only kinetics but also thermodynamics. Different ligands would lead to Au NCs with various sizes and structures due to the differences between their steric hindrance, electronic structure, bonding style and so on. Many Au NCs protected by different ligands, including thiols, phosphines, alkynes and mixed ligands, have been synthesized and structurally determined in recent years. Taking thiols an example, Au₁₈,³⁰ Au₂₃,³¹ Au₂₅,^32,33 Au₃₀,³⁴ Au₄₂,³⁵ Au₁₀₂,³⁶ and Au₁₄₄ (ref. 37 and 38) were obtained by using cyclohexanethiol, phenylethanethiol, adamantanethiol, 4-tert-butylbenzenelthiol, p-mercaptobenzoic acid, and phenylmethanethiol, respectively. Fig. 1 lists a few typical Au NCs protected by different ligands.^30–46

Fig. 1 Representative crystal structures of Au NCs capped by different ligands using the gold salt (complex) reduction method. CHT-H: cyclohexanethiol; PET-H: phenylethanethiol; H-S-Adm: adamantanethiol; p-MBA-H: p-mercaptobenzoic acid; BM-H: benzyl mercaptan; PPh₃: triphenylphosphine; PPhpy₂: bis(2-pyridyl)-phenylphosphine; L⁸: 1,8-bis(diphenylphosphino) octane; TBBT-H: 4-tert-butylbenzenelthiol; PA: phenylacetylene; H-SC₂H₅: ethanethiol.

Generally, smaller NCs are formed when using ligands with stronger steric repulsion, which can explain why the reported NCs protected by cyclohexanethiol, adamantanethiol and phosphines are often smaller than those protected by phenylethanethiol, 4-tert-butylbenzenelthiol and so on;^{30,31,34,36–38} Phosphine ligands (containing several benzene rings) often give rise to the formation of rod-shaped packing,^41,44,45 and conjugated thiophenols or alkynes favor the formation of NCs with FCC structures,¹¹ for example, most of the 4-tert-butylbenzenelthiolated gold nanoclusters obtained by reduction of gold salt (complex) are fcc-structured such as Au₃₆(TBBT)₂₄,^47,48 Au₄₄(TBBT)₂₈,^47,48 Au₅₂(TBBT)₃₂,⁴⁹ Au₅₆(TBBT)₃₄,⁵⁰ and Au₉₂(TBBT)₄₄ ⁵¹ (TBBT: 4-tert-butylbenzenelthiolate), while the phenylethanethiolated gold nanoclusters synthesized by reduction of gold salt (complex) generally adopt core–shell structures such as Au₂₅(PET)₁₈,^32,33 Au₃₈(PET)₂₄ ³⁷ and Au₁₄₄(PET)₆₀ ³⁸ except for Au₅₂(PET)₃₂ ⁵² (PET: phenylethanethiolate).

The ligand type not only influences the atomic packing mode, but also affects structure distortion. For example, on the basis of density functional theory calculations of a series of [Au₂₅(SR)₁₈]⁻ nanoclusters, Tlahuice-Flores et al. revealed that low-polarity R groups do not disturb the Au₂₅S₁₈ framework greatly, while in the case of p-thiolphenolate ligands (p-SPhX), a remarkable distortion of the Au₂₅S₁₈ framework destroys the inversion symmetry, the distortion increasing in the given order X = H, Cl, NO₂ and CO₂H. Similarly, the N-acetyl-cysteine ligand also notably distorts the framework.⁵³

3.1.2 Temperature. Temperature also influences reaction kinetics and thermodynamics. Among the early studies on Au₂₅, Jin et al. found that the products obtained at 0 °C showed characteristic absorption bands of Au₂₅ at 670, 450, and 400 nm.²⁰ However, a major product of larger Au clusters (Au∼140) was formed at room temperature. The state of the Au(I):SR aggregates was proven to account for such control via dynamic light scattering measurements. Regarding the synthesis of water-soluble Au₂₅ protected by mercaptohexanoic acid, Xie et al. reported that 40 °C is the optimal temperature to synthesize Au₂₅ in aqueous solution, while overheating (such as at 60 °C) induced the transformation of Au₂₅ into mixed clusters in the reaction.⁵⁴ Li et al. found that the high temperature treatment (60 °C) largely decreased the reaction time compared with the low temperature (0 °C) case used for synthesizing Au₂₅ NCs protected by glutathione.⁵⁵

3.1.3 Solvent. Solvents have a great impact on the aggregation of Au complex precursors and the dispersion of intermediate NCs in the reaction; thus, solvents were used for the selective control of the product clusters. Au₂₅(PET)₁₈ can be facilely prepared in tetrahydrofuran (THF).²¹ When the solvent was changed to methanol, the major product was proven to be Au₁₄₄(PET)₆₀,⁵⁶ the structure of which was recently revealed by using benzyl mercaptan as a ligand under similar conditions. Wu et al. reported the synthesis of a novel Au₃₈(PET)₂₆ (Au_38T) cluster in dichloromethane (Fig. 2).⁵⁷ Au_38T and the previously reported Au_38Q are structural isomers with the same formula but different atomic architectures (Fig. 2). The biicosahedral Au₂₃ core in Au_38Q is composed of two fused Au₁₃ icosahedra by sharing a Au₃ face, while the Au₂₃ core in Au_38T can be viewed as one Au₁₂ cap and one Au₁₃ icosahedron (Fig. 2A(a)) fused together by sharing two Au atoms. The surface staples of these two Au₃₈ isomers are also different from each other. More significantly, Au_38T exhibits remarkably higher catalytic activity than Au_38Q at low temperatures (for example, at 0 °C) in the 4-nitrophenol reduction reaction, indicating the structure–property correlation in Au NCs (Fig. 2C).

Fig. 2 (A) Structures of Au_38T (blue) and Au_38Q (black). (B) UV-vis spectra of Au_38T and Au_38Q in toluene. (C) Catalytic activities of Au₂₅, Au_38T and Au_38Q (reproduced from ref. 57 with permission from Springer Nature, Copyright (2015)).

Note that, except for the kinetic influence, solvents also have a thermodynamic influence on the reaction, since solvents provide the environment and influence the stability of the reaction species.

3.1.4 Reducing reagent. Based on the abovementioned synthesis strategy, kinetic control, such as the control of reduction kinetics, is quite important for governing the size of Au NCs. The widely used NaBH₄-reduction-based synthesis offers fast reaction kinetics and might lead to a relatively broad size distribution of intermediate Au_x (the ligands were omitted for clarification). Weak reducing agents are thus expected to slow down the reduction kinetics and finally produce different NCs. For example, Au₁₉(PET)₁₃ was obtained using a borane tert-butylamine complex as the reducing agent,²³ while Au₂₅ was often formed in this reaction system after NaBH₄ reduction. Carbon monoxide (CO) is a mild gaseous reducing agent that was also introduced in the synthesis of Au NCs by Xie and coworkers. As shown in Fig. 3A, they reported a versatile method to tailor the size of thiolated Au NCs by simple CO reduction as well as tuning the solution pH value.⁵⁸ A red-emitting Au₂₂(SG)₁₈ NC was also synthesized by this method (Fig. 3B).⁵⁹ The bright luminescence (QY ∼8%) of Au₂₂ at ∼665 nm was proposed to originate from long Au(I)-thiolate motifs via the aggregation-induced emission pathway. Pradeep et al. reported the synthesis of a highly fluorescent and biocompatible Au₁₈(SG)₁₄ NC using sodium cyanoborohydride (NaBH₃CN), a mild and selective reducing agent.⁶⁰ Antoine et al. synthesized Au₂₅ isomers using three isomers of mercaptobenzoic acids (p/m/o-MBA) as capping ligands by slow reduction of Me₃NBH.⁶¹

Fig. 3 (A) The synthesis of differently sized Au NCs using the CO-reduction method by adjusting pH. (B) UV-vis absorption and fluorescence spectra of Au₂₂(SG)₁₈ (reproduced from ref. 58 and 59 with permission from the American Chemical Society, Copyright (2013, 2014)).

Except for the type of reducing agent, the amount of the reducing agent could be considered for the control synthesis of gold nanoclusters.^62,63 For example, Xie et al. investigated the synthesis process of water-soluble Au₂₅ by using a stoichiometric amount of NaBH₄. Due to the precise control of reducing electrons from NaBH₄, a series of intermediate clusters (Au₄–Au₂₉) were confirmed with the help of the real-time mass spectrometry technique. Such stoichiometric⁶² or sub-stoichiometric⁶³ controls on the amount of the reducing agent not only enrich the library of gold nanoclusters, but also largely contribute to the mechanistic understanding of the reaction.

3.1.5 Assistant reagent. In addition to the selection of reducing agents, the addition of assistant reagents to the reaction, such as acids, bases, foreign metal ions, etc., was also proven to influence reaction kinetics and thermodynamics. For example, Xie et al. reported a NaOH-assisted reduction method for synthesizing mono-, bi, and tri-thiolate protected Au₂₅ with good capability and high yield.²⁴ The addition of NaOH can not only reduce the reduction ability of NaBH₄ by retarding the hydrolysis of NaBH₄ but can also accelerate the etching ability of free thiolate ligands. During the synthesis of Au NCs by CO reduction, the size of the obtained NCs increased with increasing pH.⁵⁸

Recently, the Wu group developed an acid-induction method via the addition of common acids (such as nitric acid, hydrochloric acid and acetic acid).⁵² Hydrogen ions that detached from the acid can accelerate the hydrolysis of NaBH₄ and strengthen the reactivity of NaBH₄. Meanwhile, they can interact with thiol groups, thus weakening the interaction between Au and thiolate and reducing the etching ability of thiolate. The influences of protons on the reaction kinetics and thermodynamics might be contrary to those of OH⁻ reported by Xie et al.²⁴ A novel Au₅₂(PET)₃₂ NC, whose kernel structure was revealed to be the isomer of the kernel structure of Au₅₂(TBBT)₃₂, was synthesized via this method. As shown in Fig. 4A–H, the gold atom fcc packing in Au₅₂(PET)₃₂ is a quadrangular prism-like core with defective top and bottom facets. The two defective facets consist of 6 gold atoms instead of 12 gold atoms for a complete facet. In Au₅₂(TBBT)₃₂, the 48-gold kernel comprises 4 × 4 × 6 Au atom layers along the X-, Y-, and Z-axes, respectively, and each layer has 8 gold atoms. The comparison of the UV/vis/NIR spectra indicated that such isomerization greatly influenced the electronic structure of the two Au₅₂ NCs (Fig. 4I).

Fig. 4 (A and E) The Au₅₀ kernel of Au₅₂(PET)₃₂ with defective facets; (C and G) Au₄₈ kernel of Au₅₂(TBBT)₃₂; (B, D, F, and H) aerial view of (A), (C), (E), and (G); (I) photon-energy plot of Au₅₂(PET)₃₂ and Au₅₂(TBBT)₃₂.

When adding acetic acid in the synthesis of Au NCs protected by TBBT, Au₄₂(TBBT)₂₆ instead of Au₃₆(TBBT)₂₄ and Au₄₄(TBBT)₂₈ formed after aging for two hours.³⁵ The Au₃₄ kernel of Au₄₂(TBBT)₂₆ is composed of four cuboctahedra, which is identical to that of Au₄₄(TBBT)₂₈. Of note, the surface staples are totally different between these two clusters. In Au₄₂(TBBT)₂₆, the bridging thiolates and dimeric staples anchor the [100] facets, and each of the monomer staples anchor each [110] facet, with no staples anchoring the [111] facet. However, in Au₄₄(TBBT)₂₈, the [100], [111] and [110] facets are protected by bridging thiolates, a dimeric staple and monomeric staple, respectively. The clusters only exist with different staple styles and share the same kernel and ligands, which are termed homoligand–homokernel–heterostaple clusters. Moreover, the reduction of the two Au–SR units led to decreased stability of Au₄₂(TBBT)₂₆ compared with that of Au₄₄(TBBT)₂₈ and a decrease in photoluminescence.

Later, Au₄₄(TBBT)₂₆ and Au₄₈(TBBT)₂₈ were prepared via the acid-induction method.⁶⁴ These two nanoclusters and Au₃₈(PET)₂₆ can be regarded as kernel homologues due to their similar kernel structures: one identical Au₂₃ only differs in its number of capping gold atoms: 0, 6, and 8 (see Fig. 5). It is found by DFT calculations that the HOMOs-LUMOs are mainly distributed on the Au₂₃ unit for every nanocluster, and the electrochemical properties of the three clusters are similar, both indicating that the Au₂₃ unit acts as a “functional group”. In this work, the concepts of homologue, homology and functional groups were introduced into nanochemistry to help understand the structure–property relationship for the first time.

Fig. 5 Comparison of the kernel structures of Au₃₈(PET)₂₄, Au₄₄(TBBT)₂₆ and Au₄₈(TBBT)₂₈ (reproduced from ref. 64 with permission from John Wiley and Sons, Copyright (2018)).

Another assistant reagent is foreign metal ions. The co-reduction of Au and other metal ions would produce alloy NC intermediates and, finally, monometallic cluster products. For example, a non-fcc-structured Au₄₂(TBBT)₂₆ (Au_42N) nanocluster was successfully synthesized in the presence of cadmium ions by Wu and coworkers.⁶⁵ The addition of Cd may influence kinetics and thermodynamics in the production of Au_42N nanoclusters by, for example, forming unstable Au/Cd intermediates, tuning the reducing ability of NaBH₄, or influencing the etching rate of thiol. Au_42N is the structural isomer of abovementioned Au₄₂(TBBT)₂₆ (Au_42F) with an fcc packing kernel produced in the acid-induction method. As shown in Fig. 6, Au_42N contains a non-fcc Au₂₆ kernel that can be split into three parts consisting of 9, 9, and 8 Au atoms. The Au₂₆ kernel is capped by several motifs, including four Au₃(TBBT)₄, four Au(TBBT)₂ and two TBBT units. Wu et al. found that the photoluminescence emission of Au_42F was approximately twice as extensive as that of Au_42N, indicating that the fcc structure does not actually inhibit the emission compared with the non-fcc structure, which was further demonstrated by DFT calculations.

Fig. 6 The kernel structure of Au_42N (reproduced from ref. 65 with permission from John Wiley and Sons, Copyright (2019)).

3.1.6 Others. Synthesis protocols also influence the size of the as-obtained NCs. For example, the stirring speed and the addition style of the reducing agent have been identified to be important for the selective synthesis of Au NCs.^66,67 The slow stirring and slow addition of NaBH₄ are favored by the formation of the series of Au₂₀, Au₂₄, and Au_39/40 NCs instead of Au₂₅.

3.2 Ligand induction method and controlling parameters

In early work, the ligand induction method typically involved the partial exchange of ligands for the functionalization and tailoring of the surface properties of nanoparticles, but changes in the nanoparticle size are often neglected due to the lack of structural determination.^68,69 Since the accessible structure of Au NCs was developed, great advances in ligand induction methods have been made mainly by Jin, Tsukuda, etc.^70,71 They introduced thermal conditions and the addition of a large excess of thiol into this method to control the size and structure of Au NCs. As one of the most important methods for synthesizing NCs, ligand induction often includes several step ligand exchange, structural transformation and size-focusing on the basis of the stability of differently sized NCs. Here, the precursor nanoclusters are not limited to monodispersed NCs and can be extended to mixed-sized NCs.

3.2.1 Ligands. Different ligands can lead to various products. In 2013, Jin et al. reported transformation from Au₃₈(PET)₂₄ to Au₃₆(TBBT)₂₄ by ligand induction and studied the reaction kinetics to unravel the total transformation process (see Fig. 7).⁷² The inner core of Au₃₆ is a truncated tetrahedral fcc Au₂₈ structure, which is quite different from the biicosahedral Au₂₃ core of Au₃₈. When using benzyl mercaptan as the incoming ligand, the parent Au₃₈ was transformed into a novel Au₆₀S₆(SCH₂Ph)₃₆ (Au₆₀S₆) NC.⁴³ It consists of an fcc Au₂₀ kernel and a pair of giant Au₂₀S₃(SCH₂Ph)₁₈ staple motifs, which are the largest staple motifs ever found in thiolated gold nanoclusters (Fig. 8A). More interestingly, Au₆₀S₆ NCs adopt a very special stacking sequence of ‘ABCDEF’ along the close-packed [001] direction with 60° rotation between two neighboring clusters (Fig. 8B). The new crystallographic stacking was named the 6H left-handed helical (6HLH) arrangement, and the fourth packing mode was revealed long after the third closest crystallographic packing (4H) was discovered in 1979. The fluorescence emission intensity of such crystallized Au₆₀S₆ was weaker than that of amorphous Au₆₀S₆, indicating possible energy transfer among the 6HLH arranged structures.

Fig. 7 Reaction pathway for the conversion of Au₃₈(PET)₂₄ to Au₃₆(TBBT)₂₄ (reproduced from ref. 72 with permission from the American Chemical Society, Copyright (2013)).

Fig. 8 (A) Anatomy of the atomic structure and (B) crystallographic arrangement of the Au₆₀S₆ nanocluster (reproduced from ref. 43 with permission from Springer Nature, Copyright (2015)).

Another example is the ligand exchange of Au₂₅(PET)₁₈. Au₂₈(TBBT)₂₀ nanoclusters can be synthesized by reacting Au₂₅ with excess TBBT thiol.⁷³ As shown in Fig. 9A, Au₂₈(TBBT)₂₀ is chiral and can be viewed as an fcc Au₂₀ kernel capped by four Au₂(SR)₃ dimeric staples and eight SR bridging thiolates. The Au₂₀ kernel is composed of two fused cuboctahedra sharing six common atoms. The Au₂₄(SCH₂Ph)₂₀ nanocluster was obtained after mixing Au₂₅ with benzyl mercaptan.⁷⁴ Au₂₄ consists of a bitetrahedral Au₈ kernel with an fcc-based antiprismatic shape and four Au₄(SR)₅ tetrameric staples (Fig. 9B). More significantly, the strong fluorescence of Au₂₄ NCs was rationally attributed to their particular structures compared with some other common nanoclusters. The interlocked Au₄(SR)₅ staples reduce emission loss by vibration, and the fully thiolate-bound kernel style strengthens electron transfer from the thiolates to the kernel through the Au–S bonds, thus leading to enhanced fluorescence.

Fig. 9 (A) The Au₂₀ kernel structure (a and b) and the thiolate-binding modes (c–h) in Au₂₈; (B) analysis of the Au₂₄(SCH₂Ph)₂₀ nanocluster structure (reproduced from ref. 73 with permission from the American Chemical Society, Copyright (2013) and ref. 74 with permission from John Wiley and Sons, Copyright (2016)).

Dass reported the synthesis of Au₉₉(SPh)₄₂ nanoclusters by etching Au₁₄₄(PET)₆₀ with benzenethiol.⁷⁵ They found the instability of Au₁₄₄ upon reaction with aromatic ligands, indicating that Au NCs protected by aromatic ligands favored different cores from those protected by aliphatic ligands. However, Jin and coworkers found that Au₁₄₄(PET)₆₀ converted to Au₁₃₃(TBBT)₅₂ after treatment with excess 4-tert-butylbenzenethiol at 80 °C for 4 days.⁷⁶ Au₁₃₃ has a four-shell structure. The central atom and the first shell form a 13-atom icosahedron that is wrapped by a 42-atom icosahedral shell, generating 55-atom Mackay icosahedra (MI). The third 52-atom shell is a transition layer between the Au₅₅ MI and the outermost protecting layer. The resultant Au₁₀₇ kernel is quasi-spherical and further protected by 26 monomeric staples. Notably, the surface gold-thiolate staples exhibit a self-organized helical “stripe” pattern. There are four such stripes that are bound with a curved surface clockwise or anticlockwise in the two chiral isomers.

An interesting study on the ligand effect was reported by Jin and coworkers in 2015.⁷⁷ They explored the control of the magic sizes of Au NCs by the subtle structural change of the surface protecting ligands. After etching the polydispersed Au_n clusters with isomeric methylbenzenethiols (MBT), Au₁₃₀(p-MBT)₅₀ (Au₁₃₀), Au₁₀₄(m-MBT)₄₁ (Au₁₀₄) and Au₄₀(o-MBT)₂₄ (Au₄₀) NCs are obtained using para-, meta- and ortho-methylbenzenethiol, respectively (Fig. 10). This work revealed that the magic size and stability of Au NCs were sensitive to the surface protecting thiols. Later, the structural determination of Au₁₃₀ and Au₄₀ was also achieved.^78,79 Au₁₃₀ is constructed in a four-shell manner with a barrel shape, while Au₄₀ exhibits a Kekulé-like ring shape composed of a series of four-atom tetrahedral units.

Fig. 10 The synthesis of differently magic-sized nanoclusters capped by three isomeric thiol ligands: p-MBT, m-MBT, and o-MBT (reproduced from ref. 77 with permission from the American Chemical Society, Copyright (2015)).

3.2.2 Temperature. The temperature used in the ligand induction method is crucial for the thermodynamic selection of the stable size of Au NCs under specific conditions. For example, Jin and coworkers successfully synthesized Au₄₄(TBBT)₂₈ (Au₄₄) and Au₅₂(TBBT)₃₂ (Au₅₂) by reacting mixed-sized Au_x(TBBT)_y clusters with excess TBBT thiol at 60 °C and 80 °C, respectively. From the structures of these two NCs as well as those of Au₂₈(TBBT)₂₀ (Au₂₈) and Au₃₆(TBBT)₂₄ (Au₃₆), Jin et al. found periodicities in the formula, growth pattern, and property evolution of Au NCs.⁴⁷ A unified formula of Au_8n+4(TBBT)_4n+8 (n = 3–6) is defined, and the four clusters all adopt a kernel evolution pattern with double-stranded growth at the bottom. Wu et al. obtained a novel Au₉₂(TBBT)₄₄ (Au₉₂) nanocluster while keeping the etching process at 65 °C.⁵¹ Au₉₂ is composed of an fcc-Au₈₄ kernel capped by an exterior shell including 28 TBBT thiolates and 8 Au(TBBT)₂ staples. Au₉₂ might be a transition-sized nanoparticle bridging the non-fcc-structured and fcc-structured gold nanoclusters. Interestingly, they found a linear relationship between the major maximum absorption wavelengths and the number of gold atoms in a series of TBBT-protected Au NCs (e.g., Au₂₈, Au₃₆, Au₄₄, Au₅₂ and Au₉₂). They used an extrapolation method to propose that a nanocluster with ∼155 gold atoms and an fcc structure may exhibit a typical surface plasmon resonance peak at ∼520 nm. Recently, the synthesis and structure of Au₅₆(TBBT)₃₄ were reported by Wu and coworkers through etching at 50 °C.⁵⁰ Compared with the six layers of the fcc Au₄₈ kernel in Au₅₂(TBBT)₃₂, the kernel atoms are packed in a similar manner, only with a defective layer, which is the seventh layer and consists of only 4 gold atoms (see Fig. 11A). In the same work, they found the size effect and the kernel-layer number odevity effect in both the polarity and photoluminescence of the series of TBBT-protected Au NCs. In detail, Au₃₆ and Au₅₂ with even number-layered kernels show weaker polarity and a higher emission intensity than those with odd number-layered kernels (e.g., Au₂₈, Au₄₄, and Au₅₆). When the odevity of the layer number is fixed, the NCs with large sizes exhibit stronger polarity and weaker emission intensity than those with small sizes (Fig. 11B and C).

Fig. 11 (A) The comparison of the kernels of Au₅₆(TBBT)₃₄ and Au₅₂(TBBT)₃₂; (B) structure evolution (a) and thin-layer chromatography (b); (C) the photoluminescence spectra of Au₂₈(TBBT)₂₀, Au₃₆(TBBT)₂₄, Au₄₄(TBBT)₂₈, Au₅₂(TBBT)₃₂, and Au₅₆(TBBT)₃₄ (reproduced from ref. 50 with permission from John Wiley and Sons, Copyright (2020)).

3.2.3 Etching time. With increasing the etching time, stable NCs with magic sizes were mainly formed during thermodynamic selection; thus, some novel NCs with low stability might be prepared by tuning the etching time. For example, in the synthesis of 2,4-dimethylbenzenethiolate (2,4-DMBT)-protected Au NCs reported by Wu's group, Au₄₄(2,4-DMBT)₂₆ was obtained after treating the precursors of the multisized Au_x(2,4-DMBT)_y clusters with excess DMBT-H thiol at 40 °C for 20 h.⁸⁰ The Au₄₄(2,4-DMBT)₂₆ structure comprises a Au₂₉ kernel and a surface protecting shell including two 2,4-DMBT thiolates, three Au(2,4-DMBT)₂, and six Au₂(2,4-DMBT)₃ staples (see Fig. 12A). The Au₂₉ kernel of Au₄₄(2,4-DMBT)₂₆ is composed of face-fused biicosahedral Au₂₃, as reported in Au_38Q, and a special Au₆ bottom cap. Au₄₄(2,4-DMBT)₂₆ bears an 18-electron shell closure structure, but it is less thermostable than Au₄₄(TBBT)₂₈, indicating that multiple factors contribute to the thermostability of Au NCs. Later, they found that a novel Au₄₉(2,4-DMBT)₂₇ nanocluster was formed when the etching time was reduced to 18 h.⁸¹ As shown in Fig. 12B, the Au₃₄ kernel of Au₄₉ is the combination of one non-fcc Au₁₃ unit and one quasi-fcc Au₂₁ unit despite some distortions. The exterior shell consists of six Au₂(2,4-DMBT)₃ dimers, three Au(2,4-DMBT)₂ monomers and three 2,4-DMBT ligands. The total structure of Au₄₉ is quite different from the Au₄₄(2,4-DMBT)₂₆ structure mentioned above. Wu et al. also investigated the influence of the kernel-packing mode on the electrochemical gap (EG) using DPV and found that the fcc-kerneled gold nanocluster has a larger EG than the investigated non-fcc-kerneled nanoclusters. The EG of Au₄₉ with mixed-packed kernels falls between that of the abovementioned two structures.

Fig. 12 (A) Anatomy of the structures of Au_38Q and Au₄₄(2,4-DMBT)₂₆ nanoclusters; (B) anatomy of the structure of the Au₄₉(2,4-DMBT)₂₇ nanocluster (reproduced from ref. 81 with permission from John Wiley and Sons, Copyright (2017)).

3.2.4 Others. Other parameters have also been explored in ligand induction methods; for example, the ratio of the thiol to the precursor might influence the attachment of the incoming ligands to the parent clusters, the extra catalyst might promote structural transformation, etc. After treating the precursors with 20 equivalents and 5 equivalents of cyclohexanethiol (CHT-H) at 40 °C, Wu and coworkers successfully synthesized Au₃₄(CHT)₂₂ and Au₄₂(CHT)₂₆, respectively.⁸² Au₃₄ can be viewed as a Au₂₀ kernel or six equivalent tetrahedral Au₄ units entangled together along two helixes, further capped by three Au₃(SR)₄ trimer staples and five Au(SR)₂ monomer staples. The structure of Au₄₂ consists of 8 tetrahedral units (Au₂₆) assembled along two helixes that are protected by six Au₂(SR)₃ dimer staples and four Au(SR)₂ monomer staples. Based on the resolved structures of Au₂₈(CHT)₂₀, Au₃₄(CHT)₂₂ and Au₄₂(CHT)₂₆, a novel kernel evolution pattern was demonstrated and depicted as alternate single-stranded growth at both ends (Fig. 13), which is remarkably different from the reported double-stranded growth at the bottom of the TBBT protected NC series.⁴⁷ Note that, compared with the one-dimensional growth mode, a two-dimensional one was recently proposed by Xu, et al.;⁸³ for the building block study, various polyhedral approaches have been proposed, see ref. 84–87. In 2016, Wu et al. reported an interesting phenomenon: ∼3 nm nanoparticles greatly promoted the transformation from Au₄₄ to Au₃₆ in the presence of TBBT thiol, whereas the transformation is extremely slow, even at higher temperatures without the existence of ∼3 nm nanoparticles.⁴⁸ The possible mechanism is that ∼3 nm particles can rivet Au₄₄ on the surface and activate the staple motif of Au₄₄ for subsequent attack by TBBT-H thiol.

Fig. 13 The kernel, total structure and growth mode of Au₂₈(CHT)₂₀, Au₃₄(CHT)₂₂ and Au₄₂(CHT)₂₆.

3.3 Anti-galvanic reduction (AGR) method and controlling parameters

Anti-galvanic reduction (AGR), which was first proposed in 2012 by Wu,⁸⁸ involves the reduction of metal ions by less reactive (more noble) metals, which is the opposite of the famous GR discovered by Italian scientist Luigi Galvani. The driving force of AGR is that the oxidation potential of ultrasmall metal NPs greatly decreases to a potential that is even lower than the reduction potential of some less noble metal ions. AGR has been used for metal ion sensing, antioxidation, synthesis of atomically precise alloy NCs or monometal NCs, etc. For more information, readers are recommended to read a recent review article, ref. 89. In this section, we mainly focus on the controlling parameters in the AGR synthesis method.

3.3.1 Gold nanocluster precursors. Gold nanocluster precursors are the main reactants in the reaction and thus certainly play a crucial role in AGR. It is revealed that not only the precursor size but also the ligand on the nanocluster surface influence product types. For example, Wu et al. demonstrated that PET-protected Au₂₅ can react with silver ions to produce several AuAg alloy NCs (such as Au₂₄Ag₁(PET)₁₈, Au₂₃Ag₂(PET)₁₈ and Au₂₅Ag₂(PET)₁₈), while glutathiolated Au₂₅ generated different products when it was reacted with the same type ions.^90,91 Wu and coworkers investigated the AGR of Au₂₃ and Au₃₄ that were both capped by CHT thiol and obtained different products after reacting with cadmium salts: Au₂₀Cd₄(SH)(SR)₁₉ for Au₂₃ and Au₂₆Cd₄(CHT)₂₂ for Au₃₄.^92,93 Au₂₀Cd₄(SH)(SR)₁₉ is composed of a Au₁₁Cd₂ kernel protected by two nonequivalent trimeric staples, one dimeric staple, two monomeric staples, four plain bridging thiolates, and one CdSH unit (Fig. 14A). The icosahedral Au₁₁Cd₂ kernel is distorted due to the introduction of two Cd atoms. Surprisingly, a unique CdSH motif has been observed for the first time in NCs, and this doping style was also different from previously reported modes. Au₂₆Cd₄(CHT)₂₂ can be viewed as a distorted face-centered cubic (fcc) Au₁₆ kernel capped by two “paw-like” Cd(S–Au–S)₃ staples, one pentameric Au₃Cd₂(SR)₈ staple and one monomer Au(SR)₂ staple (Fig. 14B). Compared to the structure of Au₃₄, the doping of four Cd atoms not only changed the surface structure but also induced a distinct distortion of the gold kernel.

Fig. 14 Anatomy of the structure of (A) Au₂₀Cd₄(SH)(SR)₁₉ and (B) Au₂₆Cd₄(CHT)₂₂ nanoclusters (reproduced from ref. 92 with permission from John Wiley and Sons, Copyright (2018) and ref. 93 with permission from the American Chemical Society, Copyright (2019)).

Recently, Zhu et al. carefully investigated the Ag doping position of TBBT-protected Au₂₈ and Au₃₆ NCs by reacting with the Ag(I)SR complex.⁹⁴ Compared with the general doping of Ag atoms in the isotropic nanocluster (e.g., Au₂₅(SR)₁₈), only specific Au atoms of Au₂₈ and Au₃₆ (e.g., Au atoms at the vertex sites in the metal core or in the staples) can be replaced by Ag atoms, which was attributed to the electrophilic effect revealed by DFT calculations.

3.3.2 Ion precursor. The type of ion precursor was extensively revealed to influence both AGR kinetics and thermodynamics. In 2015, Wu et al. studied the doping of Au₂₅(PET)₁₈ with different Ag(I) precursors, including Ag–EDTA, Ag–PET and Ag–DTZ complexes (EDTA: ethylenediamine tetraacetic acid disodium salt; DTZ: dithizone), which have different chelation abilities to Ag ions.⁹¹ They used mass spectrometry and preparative thin-layer chromatography (PTLC) and observed different products for different Ag precursors, such as Au₂₅(PET)₁₈⁰, Au₂₄Ag₁(PET)₁₈, Au₂₃Ag₂(PET)₁₈, and Au₂₅Ag₂(PET)₁₈. However, Au₂₅ was transformed into Au₄₄(PET)₃₂ when it was reacted with copper ions, and Au₄₄(SC₂H₄Ph)₃₂ exhibited the highest catalytic activity among the investigated nanoclusters for the reduction of nitrophenol at low temperatures.⁹⁵ Another example is the Cd doping of Au₂₅(PET)₁₈: Au₂₄Cd(PET)₁₈ (Au₂₄Cd) and [Au₁₃Cd₂(PPh₃)₆(PET)₆(NO₃)₂]₂Cd(NO₃)₄ (Au₂₆Cd₅) formed when the Cd–PET complex and Cd(PPh₃)₂(NO₃)₂ were used as the ion precursor, respectively.^96–98 The Cd atom replaced one Au atom of Au₂₅ in the former reaction, while the two Cd atoms capped on Au₂₅ with six dimer staples lost in the latter case (see Fig. 15). Au₂₄Cd can further react with Hg²⁺ to form Au₂₄Hg(PET)₁₈, and the doping of Cd or Hg in Au₂₅ can effectively tune the electronic structure of the parent Au₂₅. The doping of Cd can also greatly improve the parent nanocluster's catalytic activity; for example, the doped product Au₂₆Cd₅ exhibits high activity and substrate tolerance as well as good recyclability in catalyzing the A³-coupling reaction. In addition, the AGR product is ion dose dependent as well.⁹¹ For example, in the reaction of Au₂₅(PET)₁₈ with Ag–DTZ, as the Ag–DTZ dose increases, the content of Au₂₄Ag(PET)₁₈ in the product mixture increases while the content of Au₂₅Ag₂(PET)₁₈ decreases and is finally negligible when the Ag : Au atomic ratio reaches 2 : 1.

Fig. 15 (A) The doping position and UV-vis spectra of Au₂₄Cd and Au₂₄Hg. (B) Structural evolution from Au₂₅ to Au₂₆Cd₅ (reproduced from ref. 97 with permission from the American Chemical Society, Copyright (2019) ref. 98 with permission from John Wiley and Sons, Copyright (2017)).

When a special ion precursor–Au complex is introduced to react with Au NCs (such reactions were dubbed quasi- or pseudo-AGR), the parent NCs will convert into others by adding or losing Au atoms. Wu et al. found that fluorescent Au₂₄(PET)₂₀ was easily synthesized by reacting Au₂₅ with the Au–PET complex and revealed that the electrochemical gap was enlarged with decreasing size.⁹⁹ Very recently, the same group reported the simultaneous synthesis of two Au₂₈(CHT)₂₀ isomers by mixing Au₂₃(CHT)₁₆ with the Au–CHT complex.¹⁰⁰ The two isomers, consisting of the same Au₂₀(CHT)₈ kernel and different outer staples, can reversibly transform into each other through dissolution and crystallization processes with oscillating fluorescence. Wu et al. also revealed a module replacement (MR)-like process during the pseudo-AGR of Au₄₈(CHT)₂₆.¹⁰¹ The MR-like process involves the replacement of the local structural unit (including three or more Au atoms) by a new one. As-obtained Au₃₇(CHT)₂₃ and Au₄₈(CHT)₂₆ share the same Au₃₁(CHT)₁₂ section as the other section (Au₆(CHT)₁₁vs. Au₁₇(CHT)₁₄). Au₃₇(CHT)₂₃ exhibited stronger photoluminescence but weaker photothermy than Au₄₈(CHT)₂₆, indicating that photoluminescence and photothermy are balanced and can be at least partly converted into each other.

3.3.3 Phase number. AGR is initially performed in one phase by directly mixing the parent nanoclusters with foreign metal salts or complexes. Very recently, the Wu group designed a two-phase AGR method that consisted of Au₄₄(TBBT)₂₈ and TBBT thiol in an organic solvent and cadmium ions in aqueous solution, leading to the production of novel Au₄₇Cd₂(TBBT)₃₁ alloy nanoclusters, while only Au₃₆ was obtained in a common one-phase system.¹⁰² The introduction of the water phase may cause weakened interactions between Cd²⁺ and Au₄₄, the reduced oxidation ability of Cd²⁺, and the decreased etching ability of thiols, which influence reaction kinetics and thermodynamics and lead to different products. Au₄₇Cd₂(TBBT)₃₁ is composed of a Au₂₉ kernel and a protective shell that includes the following staple motifs: two paw-like Cd(S–Au–S)₃, three Au₂(TBBT)₃, one Au₃(TBBT)₄, and three Au(TBBT)₂ staples (Fig. 16B). As shown in Fig. 16A, the center of the Au₂₉ kernel is a Au₁₀ unit, which can be viewed as an Archimedean anti-prism capped with two half-octahedra. Two Au₉ units cap the front of the Au₁₀ unit by sharing two Au₃ facets. Another two octahedra cap the back of Au₁₀ by sharing two Au₃ facets, and a Au₄ tetrahedron is connected to the bottom of Au₁₀ by sharing one Au₃ facet. Notably, Au₄₇Cd₂ has higher faradaic efficiencies for electrocatalytically reducing CO₂ to CO (96% at −0.57 V) than the parent Au₄₄ NC (Fig. 16C–F).

Fig. 16 (A) Kernel structure and (B) anatomy of the structure of Au₄₇Cd₂(TBBT)₃₁. (C) LSV curves of Au₄₇Cd₂(TBBT)₃₁, Au₄₄(TBBT)₂₈ and the ca. 1.5 nm Au NPs in an Ar-saturated (dotted line) and a CO₂-saturated (full line) 0.5 M KHCO₃ solution. (D) CO faradaic efficiency of the catalysts examined with different applied potentials. (E) The corresponding CO partial current density. (F) The stability test (reproduced from ref. 102 with permission from John Wiley and Sons, Copyright (2020)).

3.3.4 Others. The choice of solvent can also affect the NC product in AGR. Taking the reaction of Au₂₅(PET)₁₈ with Ag⁺ as an example, polydisperse Au_24−xAg_x(PET)₁₈ NCs were obtained in dichloromethane or toluene, whereas dominant Au₂₅Ag₂(PET)₁₈ was produced in acetonitrile.¹⁰³ The two introduced Ag atoms do not replace the Au atoms in Au₂₅(PET)₁₈ but simply deposit on Au₂₅(PET)_18, as revealed by UV/vis/NIR spectra, mass spectra, and theoretical calculations (Fig. 17). Moreover, Au₂₅Ag₂(PET)₁₈ exhibited an enhanced catalytic activity in the hydrolysis of 1,3-diphenylprop-2-ynyl acetate.

Fig. 17 (A) MALDI-MS spectra of Au₂₅Ag₂ and Au₂₅. (B) Structure of Au₂₅ and Au₂₅Ag₂ and the comparison of the experimental and calculated spectra of Au₂₅Ag₂ (reproduced from ref. 103 with permission from the American Chemical Society, Copyright (2015)).

Since trimetallic nanoclusters can be achieved by successive AGR reactions, the sequence of the usage of metal complexes also affects the product type. For example, Au₂₅(PET)₁₈ first reacted with Hg²⁺ ions and then with the Ag(I)–PET complex, leading to the formation of Au_16.8Ag_7.2Hg₁(PET)₁₈ nanoclusters.¹⁰⁴ When the sequence of Hg²⁺ ions and the Ag(I)-PET complex was exchanged, Au_24−xAg_xHg₁(PET)₁₈ (x = 2–6) was obtained with a relatively low Ag content.¹⁰⁵

3.4 Other methods and controlling parameters

Except for the abovementioned methods, there are also some other efficient approaches for synthesizing and controlling Au NCs, including oxidation/reduction-driven transformation, intercluster reactions, acid etching, and solvent induction, to name a few.

In the oxidation/reduction-driven method, the charge or even the structure of the parent NCs was often altered, and some examples are shown below. Early in 2008, Jin et al. reported the transformation from the [Au₂₅(PET)₁₈]⁻ (Au₂₅⁻) anionic cluster into a charge neutral cluster [Au₂₅(PET)₁₈]⁰ (Au₂₅⁰) via air oxidation or H₂O₂ oxidation.¹⁰⁶ Despite the similar structures, the staples of Au₂₅⁰ have some apparent distortions; for instance, every two opposite Au₂(SR)₃ staples in Au₂₅⁰ are completely coplanar with the symmetry plane of the icosahedron kernel. Negishi et al. studied the oxidation of hexanethiol-capped Au₂₅ with the strong oxidizing agent Ce(SO₄)₂ and obtained positive Au₂₅.¹⁰⁷ In 2017, Jin and coworkers reported the structural transformation from another anionic Au NC (Au₂₃(CHT)₁₆) into Au₂₈(CHT)₂₀ by H₂O₂ oxidation.¹⁰⁸ Unlike the case of Au₂₅⁻ transformation to Au₂₅⁰, the metal core and surface staples of Au₂₈ are completely different from those of Au₂₃, indicating that oxidation-induced transformation is a potential method to access new structures of Au NCs. Wu et al. also found the conversion from Au₂₃(CHT)₁₆ to Au₂₈(CHT)₂₀ using Cd(NO₃)₂ as the oxidant.⁹² Reduction-induced transformation has also been reported. For example, Jin and Zhu reported the transformation from [Au₂₅(SePh)₁₈]⁻ to [Au₂₃(SePh)₁₆]⁻ in the presence of reductive NaBH₄.¹⁰⁹ The direct loss of two Au–SR units in the parent Au₂₅ was revealed by experiments and theoretical calculations.

Pradeep's group reported the first example of intercluster reactions between Au₂₅(SR)₁₈ and Ag₄₄(SR)₃₀ (SR: alkyl/aryl thiolate) (Fig. 18A).^110,111 The mass data demonstrated the exchange of the metal atoms as well as the metal–thiolate fragments. Of note, the number of Ag dopants added to Au₂₅ could reach up to 20. The thermodynamic feasibility of the reaction has also been revealed using DFT calculations. Almost at the same time, inspired by AGR,^88,89 Wu et al. reported the reaction between hydrophilic metal nanoparticles (including nanoclusters) (Fig. 18B).¹¹² Captopril-protecting Au₂₅(Capt)₁₈ (Capt: Captopril) was chosen to react with Ag₃₀(Capt)₁₈, and a novel alloy NC (Au₂₀Ag₅(Capt)₁₈) was demonstrated by mass spectrometry and X-ray photoelectron spectroscopy. Au₂₀Ag₅(Capt)₁₈ showed enhanced photoluminescence with a quantum yield of ca. 7.6% relative to rhodamine 6G, which is 130 times and 38 times higher than that of Au₂₅(Capt)₁₈ or Ag₃₀(Capt)₁₈, respectively. Wu et al. further investigated the reaction between Au₂₅(Capt)₁₈ and silver (copper) nanoparticles and observed the mass peaks of Ag- or Cu-doped Au₂₅, indicating the universality of the interparticle reaction of water-soluble nanoparticles. In another example, Tsukuda et al. reported that the reaction between an equal amount of [PtAu₂₄]⁰ and [PtAu₂₄]^2– could produce two equivalents of [PtAu₂₄]⁻, indicating possible electron transfer between differently valenced nanoclusters.¹¹³

Fig. 18 (A) Intercluster reactions between Au₂₅(SR)₁₈ and Ag₄₄(SR)₃₀ (SR: alkyl/aryl thiolate) (reproduced from ref. 110 with permission from the American Chemical Society, Copyright (2016)). (B) The reaction between Au₂₅ and Ag (Cu) nanoclusters or nanoparticles.

Bulk gold is inert in common acids; however, nanoscale gold is not the same as bulk gold. Utilizing the activity of gold toward acid, Wu and coworkers introduced the acid-etching synthesis method and revealed the size-dependent reactivity of PET- and TBBT-capped gold nanoparticles with acetic acid:¹¹⁴ large nanoparticles are still stable in acetic acid, while small nanoparticles (e.g., Au₂₅ and Au₃₈) show distinct reactivity with acetic acid. A new Au₃₈(PET)₂₆ nanocluster was obtained after etching Au₂₅ in acetic acid overnight. Au₃₈(PET)₂₆ exhibited great disparities from the reported Au₃₈(PET)₂₄ in terms of both optical and electrochemical properties. The etching mechanism was also studied, and Wu et al. observed some intermediate species, such as Au_n(PET)_n+1Cl₂(HAc) (n = 1–3, HAc: acetic acid), from mass spectra, indicating the possible attack of the staples by acetic acid.

More attention has been given to the structural transformation of Au NCs in solvents. Wei and coworkers reported an icosahedral-to-cuboctahedral structural transformation of Au₁₃ driven by hexane.¹¹⁵ The transformation process consists of the hexane-induced selective removal of the thiolate ligands from the Au₁₃ surface, and the subsequent structure conversion process was monitored by X-ray absorption fine structure (XAFS) analyses. Xie et al. also reported the size-conversion reaction from [Au₂₃(SR)₁₆]⁻ to [Au₂₅(SR)₁₈]⁻ (SR: para-mercaptobenzoic acid) by changing the solvent polarity (e.g., from water/ethanol to pure water).¹¹⁶ A surface-motif-exchange-induced mechanism was revealed by tandem mass analyses. The Wu group recently found that one of the Au₂₈(CHT)₂₀ isomers (Au_28i) could be transformed into another (Au_28ii) in diverse solvents.¹⁰⁰ The dielectric-constant effect and deuteration effect were observed for the first time in the solvent-induced conversion process of Au NCs, which was also confirmed by the adsorption energies and free energies obtained from DFT calculations.

4. Summary and future outlook

The past two decades have witnessed the rapid development of the synthesis of Au NCs, undergoing the stages of the acquisition of Au NCs, the pursuit of monodispersity, structural determination, and controlling the composition and structure with atomic precision. In this review, we have introduced important research on controlling the synthesis of Au NCs by categorizing the present synthetic methods into three main routes, (i) gold salt (complex) reduction, (ii) ligand induction, and (iii) antigalvanic reduction, noting the influences of various controlling parameters in the synthesis process. Although many types of Au NCs with diverse structures and compositions have been reported and explored, there are still several challenges for controlling Au NCs. At the end of this review, we attempt to propose several potential studies to address these issues:

(1) One to a few metal atom (ligand) manipulation. With the growing synthesis skill, we might expect the directed and precise removal, replacement or addition of one to a few metal atoms in NCs, with the remnant structures essentially being unchanged. Similarly, we also expect the manipulation of one to a few ligands (including reduction, addition and replacement) on the NC surface, with metal atom packing remaining untouched. Such a manipulation is of great significance not only for fundamental structure (composition)–property studies but also for improving the properties targeted for potential applications.

(2) Property-targeted control. Recently, Au NCs have attracted increasing attention due to their potential application in catalysis, sensing, and biology. Thus, the research community in this field is suggested to rationally control Au NCs with enhanced properties (e.g., catalytic activity and fluorescence) based on the existing structure–property correlation.

(3) Development of doping methods. The precisely controlled synthesis of multi-metal (ligand) Au NCs is crucial for obtaining unique properties and practical applications due to the lower cost or newly imparted properties generated by foreign metal atoms (ligands) or synergistic effects. More metal elements, especially relatively active metals,¹¹⁷ are expected to be doped into Au NCs. Controllable ligand doping has been rarely reported.

In brief, the future controlled synthesis of Au NCs will be developed not only with enriched sizes, compositions, and structures but also with attractive and selectable properties for applications.

Conflicts of interest

The authors have no conflicts of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21829501, 21701179, 21771186, 21925303, 21222301, 21603234, 21171170, and 21528303), Natural Science Foundation of Anhui Province (1708085QB36), CASHIPS Director's Fund (BJPY2019A02), Key Program of the 13th five year plan, CASHIPS (2020HSC-CIP005), and Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2020HSC-CIP005) and CAS/SAFEA International Partnership Program for Creative Research Teams.

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