A concise guide to chemical reactions of atomically precise noble metal nanoclusters

Abstract

Nanoparticles (NPs) with atomic precision, known as nanoclusters (NCs), are an emerging field in materials science in view of their fascinating structure–property relationships. Ultrasmall noble metal NPs have molecule-like properties that make them fundamentally unique compared with their plasmonic counterparts and bulk materials. In this review, we present a comprehensive account of the chemistry of monolayer-protected atomically precise noble metal nanoclusters with a focus on the chemical reactions, their diversity, associated kinetics, and implications. To begin with, we briefly review the history of the evolution of such precision materials. Then the review explores the diverse chemistry of noble metal nanoclusters, including ligand exchange reactions, ligand-induced structural transformations, and reactions with metal ions, metal thiolates, and halocarbons. Just as molecules do, these precision materials also undergo intercluster reactions in solution. Supramolecular forces between these systems facilitate the creation of well-defined hierarchical assemblies, composites, and hybrid materials. We conclude the review with a future perspective and scope of such chemistry.

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1. Introduction

Richard Feynman's historic Caltech address, There's Plenty of Room at the Bottom, in 1959, discussed the concept of nanotechnology which envisioned “maneuvering things atom by atom”.¹ Development of atomically precise metal nanoclusters may be viewed as a direction to create materials atom by atom. The term ‘metal cluster’ was originally defined by Cotton in 1964 as “a finite group of metal atoms which are held together mainly or at least to a significant extent, by bonds directly between the metal atoms, even though some nonmetal atoms may also be intimately associated with the cluster” to refer to coordination compounds.^2,3 This term was also used in the early literature to refer to plasmonic noble metal particles consisting of several hundreds or a few thousands of atoms, although the term ‘metal nanocluster’ is nowadays used more appropriately to refer to atomically precise, bare, or ligand-protected particles with a precise, molecule-like composition (M_xL_y; M = metal atom, L = ligands such as thiolate, phosphine, etc.) and well-defined properties. Precision refers to structure as well, both in the molecular form in the gaseous, solution and solid states. Therefore, clusters in the context of this review may be defined as “a finite group of atoms with precise composition and structure, composed of two or more metal atoms held together by chemical bonds between them, and the structure formed stands protected or unprotected with ligands, with well-defined properties.”

Electronic confinement of noble metals has been an important subject matter of research in the past few decades.^4–6 As the particle size shrinks below ∼3 nm to an intermediate size regime, bridging the dimensions of molecules and condensed matter, molecule-like properties arise in such materials.^4,7 Such molecular materials are called atomically precise noble metal nanoclusters (NCs), which have precise composition, structure, and unique properties.^7–11 Au₁₀₂(p-MBA)₄₄ nanocluster (p-MBA=para-mercaptobenzoic acid) was the first reported single-crystal structure in the family of thiolate-protected atomically precise nanoclusters,¹² although clusters such as Au₁₁I₃[P(C₆H₄-p-Cl)₃]₇ (ref. 13) and [Au₁₃(PPhMe₂)₁₀C₁₂]³⁺ (ref. 14) were known since 1970, with other ligands. Since then, more than 250 nanocluster crystal structures have been published.^7,11,15 Many more noble metal nanoclusters with proteins,^16–21 DNA,^22–27 poly(amidoamine)-based dendrimers,^28–31 and cyclodextrins^32–34 are now known.¹¹ Today, atomic precision in assemblies is attainable in several materials, such as metals, metal oxides, semiconductors, ionic compounds, and even rare gases.³⁵ Carbon clusters, such as Buckminsterfullerene or C₆₀, are some of the most popular clusters investigated so far.^36,37 Molecular clusters such as (H₂O)_2,3,4… and (CH₃OH)_n(H₂O)_m, where n, m = 1,2,3,…, and zero-dimensional (0D) particles of perovskites, graphene, etc., are also gaining interest.^38,39 In our latest book we presented a comprehensive overview of noble metal nanoclusters and their properties, with a compendium of all reported clusters.¹⁵ However, in this review, we will be focusing on the thiolate and phosphine-protected nanoclusters in the context of their chemical reactions.

Noble metal nanoclusters exhibit well-defined physical, chemical, and electronic properties.^7,11,12 Unique characteristic properties of nanoclusters include discrete electronic structures,^40–47 corresponding HOMO–LUMO transitions,^48–53 chemical reactivity,^{10,54–56,57–59} photoluminescence^60–63 and intrinsic magnetism.^64–69 Metal nanoclusters have attracted tremendous interest from the scientific community due to their potential applications in optoelectronics,^70,71 sensing,^{63,72–74,75} bioimaging,^{63,76–79,80} catalysis,^8,81–83,84 and others.^7,11

Today, the chemistry of well-defined monolayer-protected nanoclusters is an active area of research. We present this review as a mini guide to ligand-protected atomically precise metal nanoclusters and their diverse chemistry (schematically represented in Fig. 1). To begin with, we trace the origin and landmark developments in nanocluster science. The article presents the recent research on ligand-induced chemistry, intercluster & interparticle reactions and their mechanism, thermodynamics, kinetics, and implications. Knowledge of the precise chemical reactivity of such nanoclusters gives a way to control their composition to form alloys and hybrid materials, and also for engineering their properties. Such new materials may find suitable roles in photophysical, catalytic, and optoelectronic applications. Chemical reactions between nanomaterials of various types provide new insights into the dynamics at the nanoscale. Reactivity at the nanoscale is of importance to chemistry in general, and to catalysis, functional materials, photophysics, nanomedicines, sensors, and clean water, in particular.

Fig. 1 Schematic illustration of the diverse chemistry of the ligand-protected atomically precise noble metal nanoclusters. Images under assemblies are adapted with permission from ref. 275, 281, 278 and 285. Copyright 2014 and 2018 John Wiley and Sons. Copyright 2014 American Chemical Society. Copyright 2020 The Royal Society of Chemistry. Images under halocarbons are adapted from ref. 258 and 257. Copyright 2013 Royal Society of Chemistry. Images under supramolecular are adapted from ref. 259 and 260. Copyright 2014 and 2018 American Chemical Society. Images under intercluster are adapted from ref. 55. Copyright 2016 Springer Nature Group. Images under LEIST are adapted from ref. 218 and 176. Copyright 2018 Royal Society of Chemistry. Copyright 2020 American Chemical Society.

2. Atomically precise noble metal nanoclusters: the evolution

Size-dependent studies of colloidal silver particles in solution using radiolytic and electrochemical methods by Henglein et al. in the 1980s were among the earliest experiments using nanoscale noble metals.^85–88 Haruta's discovery, in 1989, of the catalytic activity of finely divided, nanosized gold particles supported on oxide surfaces boosted research on noble metal NPs.⁸⁹ However, most of the early insights into atomically precise metal nanoclusters were derived from gas-phase investigations.

2.1. Insights from gas-phase studies

Gas-phase studies provided the first glimpses into the characteristics of metal nanoclusters. Nanoclusters of alkali, alkaline earth, and noble metals have been studied extensively since the 1980s. Gas-phase investigations of metal nanoclusters made extensive use of techniques like mass spectrometry, ion mobility spectrometry, photoelectron spectroscopy, vibrational spectroscopy, etc.^90,91 Using mass spectrometry, Knight et al. characterized sodium Na_n (n = 4–100) metal nanoclusters in the gas phase.⁹² The most abundant peaks observed correspond to the nanoclusters with n = 8, 20, 40, 58, and 92. According to the jellium model, the electrons in these nanoclusters are distributed into discrete electronic shells, just as in atoms. The numbers 8, 20, 40, 58, and 92 correspond to the total number of valence electrons (3s¹) in these nanoclusters, analogous to the valence shell electron configurations of noble gases. The nanoclusters whose valence shell electron counts fall in this series of ‘magic’ numbers are referred to as ‘magic clusters’. The abundance of these nanoclusters in the mass spectra is attributed to their stability gained from the completion of electronic shell structures, just like noble gases. This is one of the reasons for the fact that ‘every atom counts’ in the case of atomically precise metal nanoclusters. Magic nanoclusters of metals such as aluminum were also observed. For example, Khanna et al., showed that Al₁₃⁻, which has a magic number of 40 electrons, exhibits special inertness towards gas-phase etching reactions.^93,94 Such nanoclusters are also called ‘superatoms’, a term coined by Khanna et al. in 1995.⁹⁵ They also introduced the idea of using superatomic nanoclusters as building blocks for nanocluster-assembled materials. The stability of gas-phase metal nanoclusters, especially larger ones, depends on closed geometric shells as well as electronic shells. In the gas phase, the geometry of the nanocluster, rather than the electronic structure, determines the stability. Due to the overlap of electronic bands, there is a negligible change in electronic energy with the addition of each new atom to the nanocluster. For example, positively charged calcium nanoclusters in the gas phase such as Ca₅₆₁, Ca₁₄₁₂, Ca₂₈₆₅, etc., exhibit mass spectral abundance which is ascribed to the successive addition of layers of atoms to form stable geometries.⁹⁶ Bare gold nanoclusters in the gas phase have been investigated since the 1980s. Kappes et al. used ion mobility (IM) measurements and trapped ion electron diffraction^97–99 in conjunction with density functional theory (DFT) calculations in order to assign structures of Au_n⁻ (n < 13) nanoclusters⁹⁸ and also suggested that planar to three-dimensional transition in these nanoclusters occurs at n = 11.⁹⁸ The structure of unprotected noble metal nanoclusters deposited on surfaces has also been probed using techniques such as scanning tunneling microscopy.^100,101

2.2. Atomically precise metal nanoclusters in solution: phosphine- and thiolate-protected metal nanoclusters

Solution-phase nanochemistry of noble metals was accelerated after the discovery of the Brust–Schiffrin method reported in 1994,¹⁰² wherein thiolates were used as protecting ligands with limited information on the structure and composition of these particles.

The earliest examples of atomically precise metal cluster compounds studied in the solution phase were gold–phosphine coordination complexes, like Au₁₁(PPh₃)₇(SCN)₃ and Au₁₁I₃(P(C₆H₄-p-Cl)₃)₇. These compounds were synthesized in 1969 and 1970, respectively.^13,103 Au₁₁X₃[PR₃]₇ is the first known crystal structure with an incomplete icosahedral core.¹³ In 1981, Briant et al. reported the first[Au₁₃(PPhMe₂)₁₀C₁₂]³⁺ nanocluster consisting of a perfect icosahedral core.¹⁴ Bigger nanoclusters such as [Au₃₉(PPh₃)₁₄Cl₆]Cl₂ consisting of larger, atomically precise Au cores were reported in 1992.¹⁰⁴ Schmid et al. synthesized the well-known molecule, Au₅₅[P(C₆H₅)₃]₁₂CI₆, in 1981, which attracted significant attention in the community.¹⁰⁵ The crystal structure of this nanocluster remained elusive; however, insights into its structure came from later studies.¹⁰⁶ Though techniques such as fast atom bombardment mass spectrometry (FABMS) were used to analyze compounds such as [Pt₂(AuPPh₃)₁₀Ag₁₃Cl₇], etc.,¹⁰⁷ single-crystal X-ray diffraction was the major tool for probing their compositions and structures. Extensive reviews are available on the single-crystal structure of atomically precise nanoclusters.^7,11,108

Even though phosphine-protected noble metal nanoclusters have been known since the 1960s as mentioned above, the big leap in the field of solution-phase nanochemistry of noble metals occurred only after the pioneering efforts of the Whetten and Murray research groups on thiolate-protected noble metal particles. In 1996, Whetten et al. were the first to observe atomically precise compositions for thiolate-protected gold particles using mass spectrometry. Particles with such compositions were referred to as ‘nanocrystal gold molecules’.¹⁰⁹ Murray et al. electrochemically observed a molecule-like electronic structure for many such particles. In 2005, Shichibu et al. synthesized glutathione (SG)-protected Au₂₅(SG)₁₈ nanocluster via ligand exchange reaction of preformed Au-phosphine nanoclusters.^110–112 In the same year, Tsukuda et al. mass spectrometrically observed a series of glutathione-protected gold nanoclusters with precise and molecule-like compositions, such as Au₁₀(SG)₁₀, Au₁₅(SG)₁₃, Au₁₈(SG)₁₄, Au₂₂(SG)₁₆, Au₂₂(SG)₁₇, Au₂₅(SG)₁₈, Au₂₉(SG)₂₀, Au₃₃(SG)₂₂, and Au₃₉(SG)₂₄ which were separated by polyacrylamide gel electrophoresis (PAGE).¹¹³ In 2006, Häkkinen et al. proposed the ‘divide and protect’ structural model wherein these nanoclusters were viewed as consisting of a discrete metal core, protected by well-defined metal–ligand oligomeric units.¹¹⁴ In 2007, Whetten et al. unambiguously assigned the composition of the most popular nanocluster in this family, Au₂₅(PET)₁₈ (PET = 2-phenylethanethiolate) (which was wrongly assigned as Au₃₈(PET)₂₄ in earlier investigations) through electrospray ionization mass spectrometry (ESI MS). In 2007, Shichibu et al. synthesized a biicosahderal nanocluster, [Au₂₅(PPh₃)₁₀(SC_nH_2n+1)₅Cl₂]²⁺ (n = 2–18) by the chemical reaction between [Au₁₁(PPh₃)₈Cl₂]⁺ and n-alkanethiol (C_nH_2n+1SH, n = 2, 8, 10, 12, 14, 16, and 18).¹¹⁵ This was the first Au₂₅ nanocluster compound whose crystal structure was resolved. Apart from thiolate and phosphinate monolayers, anion templates, such as halides,¹¹⁶ sulfides,¹¹⁷ chalcogenides,¹¹⁸ and polyoxometalates,^119,120 are becoming increasingly prominent in the preparation of high-nuclearity atomically precise Ag nanoclusters.¹²¹ Mass spectrometry, separation techniques such as electrophoresis, size-exclusion chromatography (SEC), and single-crystal X-ray diffraction have made tremendous contributions to the science of ligand-protected noble metal nanoclusters.^91,122,123 Recently, microelectron diffraction has been used to resolve the structures of those nanoclusters for which crystallization was difficult.^124,125 Computational methods have made significant contributions to our understanding of the structures and properties of these nanoclusters, complementing the experimental approaches.^{9,41,47,50,126–129}

2.3. Early insights into structures

The identification of atomically precise nanoclusters such as Au₂₅(PET)₁₈ sparked extensive research into numerous nanoclusters and their properties; however, the structures of these early nanoclusters remained unknown for quite some time. Therefore, these particles were called ‘monolayer-protected nanoclusters’ (MPCs); the term was originally used to refer to their larger (consisting of a few hundred metal atoms), plasmonic counterparts wherein the protecting ligands were assumed to be arranged in a uniform, 2D fashion, as in the case of self-assembled monolayers (SAMs)¹³⁰ of ligands on metals. In 2007, Kornberg et al. were the first to resolve the crystal structure of a thiolate-protected gold nanocluster, namely Au₁₀₂(MBA)₄₄ (MBA = p-mercaptobenzoic acid).¹³¹ Au₁₀₂(MBA)₄₄ provided significant new insights into the structure of the ligands on the NPs. Au₁₀₂(MBA)₄₄ consists of an Au₇₉ core protected by nineteen Au(SR)₂ and two Au₂(SR)₃ units, often referred to as staple units. In 2008, the crystal structure of Au₂₅(SR)₁₈, one of the most popular members of this family of nanoclusters,¹³² was resolved independently by Akola et al. and Heaven et al., which showed that it consisted of an Au₁₃ icosahedron protected by six Au₂(SR)₃ oligomeric staples.^133,134 These findings proved that the structural arrangement of protecting ligands on metal NPs can be completely different from that of SAMs. Recently, atomic precision has been achieved in a larger size regime of nanoclusters wherein plasmonic features start appearing. For example, structures of Au₂₇₉(SPh-tBu)₈₄,^135,136 Au₃₂₉(SR)₈₄,^137,138 [Ag₃₇₄(SR)₁₁₃Br₂Cl₂],¹³⁹ Au₃₃₃(SR)₇₉,¹⁴⁰ and Au₂₄₆(SR)₈₀ ¹⁴¹ were reported by different groups. There are also attempts to accurately probe the composition of plasmonic NPs using mass spectrometry.^142,143

In the few past years, there have been immense advancements in analytic instrumentation, making it possible to study these precision materials in detail.^{26,91,144–146,147} High-resolution mass spectrometry (HR MS) coupled with soft ionization can precisely determine the composition of the core and ligands as well as the charge states of the nanocluster.^55,148 Other advanced mass spectrometric (MS) techniques, like ion mobility MS (IM-MS)^97,149–153 and tandem MS (MS/MS),^19,154,155 are becoming increasingly powerful to understand the size, shape, and structural evolution. Single-crystal X-ray crystallography has made it possible to resolve the structures of several thiol (–SR)-capped nanoclusters, like Au₂₅(SR)₁₈,¹³⁴ Au₂₈(SR)₂₀,¹⁵⁶ Au₃₈(SR)₂₄,¹⁵⁷ Au₄₀(SR)₂₄,¹⁵⁸ Au₅₂(SR)₃₂,¹⁵⁹ Au₄₀(SR)₂₄,¹⁶⁰ Au₉₂(SR)₄₄,¹⁵⁹ Au₁₀₂(SR)₄₄,¹³¹ Au₁₃₃(SR)₅₂,¹⁶¹ Ag₄₄(SR)₃₀,¹⁶² Ag₂₅(SR)₁₈,¹⁶³ Ag₂₉(SR)₁₂,¹⁶⁴ and more. Furthermore, with the development of hyphenated techniques, other inherent nanocluster properties, such as electron affinity (EA), ionization energy (IE), electronic transitions, etc., are being studied in greater detail.⁹¹ A few of the milestones in the development of noble metal cluster chemistry are briefly presented in Fig. 2.

Fig. 2 (A) A representative timeline of the evolution of nanocluster science. (B) A few notable mentions from the nanocluster timeline, namely (a) mass spectra of Au_n(SR)_m, (b) mass spectra of Au_n(SG)_m series, (c) divide and protect concept as visualized from the difference in electron density due to bonds, (d) Kohn–Sham (KS) orbital energy level diagram of Au₂₅(SH)₁₈⁻, crystal structures of (e) Au₁₀₂(p-MBA)₄₄, (f) [N(C₈H₁₇)₄][Au₂₅(SH)₁₈], (g) Ag₄₄(4-FTP)₃₀(PPh₄)₄, (h) Au₃₈(SR)₂₄, (i) Na₄Ag₄₄(MBA)₃₀, and (j) Ag₂₅(DMBT)₁₈, and (k) Borromean rings diagram of Au₂₅(SMe)₁₈ nanocluster. Adapted from ref. a,¹⁰⁹ b,¹¹³ c,¹¹⁴ d^50,126 e,¹³¹ f,¹³⁴ g,²⁹¹ h,²⁹² i,¹⁶² j,¹⁶³ and k.¹⁶⁶ Copyright 1996 John Wiley and Sons. Copyright 2005, 2006, 2008, 2010, and 2015 American Chemical Society. Copyright 2007 American Association for the Advancement of Science. Copyright 2013 Springer Nature Group.

2.4. Structural models

Various models have been proposed to understand the structure and stability of these nanoclusters.^9,41,129 When the ‘divide and protect model’,¹¹⁴ which was one of the earliest, was put forward, no crystal structures were known for thiolate-protected noble metal nanoclusters. In 2008, Häkkinen et al. proposed the superatom complex (SAC), which was an extension of the superatom theory and was used in the case of gas-phase metal nanoclusters, to understand the stability of ligand-protected noble metal nanoclusters.¹³⁰ In 2013, Cheng et al. proposed the superatom network (SAN) model for some of the thiolate-protected noble metal nanoclusters.¹⁶⁵ They also proposed the superatom valence bond (SVB) model for non-spherical nanoclusters such as Au₃₈(SR)₂₄ for which the ordinary SAC model cannot be used for explaining the stability. In 2015, Natarajan et al. proposed a new structural model, namely the Borromean ring model,¹⁶⁶ for thiolate-protected noble metal nanoclusters wherein these nanoclusters are viewed as a single structural unit, i.e., interlocked oligomeric metal–ligand rings in contrast to the ‘divide and protect model’ wherein nanoclusters possess a discrete core and staple units. According to this model, Au₂₅(SR)₁₈, for example, is viewed as three interlocked Au₈(SR)₆ rings surrounding the central Au atom. For the first time, a systematic method of precise naming of alloy nanoclusters and mixed-ligand nanoclusters based on this model was proposed. In 2016, Xu et al. proposed another structural model, namely the grand unified model (GUM) that successfully comprehended the structures of all the gold nanoclusters known by then.¹⁶⁷ Here, these gold nanoclusters are viewed as built up from triangular and tetrahedral elementary building blocks. An interesting suggestion based on this model is that the evolution of the gold cores in these nanoclusters cannot be viewed simply as the addition of an Au atom alone, but rather as built from these elementary building blocks.

2.5. Properties of metal nanoclusters

Soon after the crystal structures of Au₁₀₂(SR)₄₄ and Au₂₅(SR)₁₈ were resolved, attempts to understand the structure–property relations in these nanoclusters also commenced. For example, distinct electronic absorption bands of Au₂₅(SR)₁₈ were assigned to various electronic transitions within the molecular orbitals derived from the metal atoms and the ligands.^50,126 Several groups have studied the electrochemistry of various metal nanoclusters¹⁶⁸ and their alloys,^169–171 such as Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au₆₇(SR)₃₅, Au₁₀₂(SR)₄₄, Au₁₄₄(SR)₆₀, Au₃₃₃(SR)₇₉ Ag_xAu_25−x(SR)₁₈ (x = 1–5), M₂Au₃₆(SR)₂₄ (M = Pd, Pt), etc., further establishing the molecule-like electronic structures of these nanoclusters.^172,173 In 2008, distinct charge states of Au₂₅(SR)₁₈ were observed by a few research groups.^66,130 Electron paramagnetic resonance (EPR) spectroscopy of Au₂₅(SR)₁₈ was also reported subsequently.⁶⁶ Photoluminescence in gold nanoclusters was first reported by Wilcoxon et al. in 1998 ¹⁷⁴ and subsequently by many other groups.^{50,111,126,128} Photoluminescence was observed from protein-protected noble metal nanoclusters as well.¹⁹ Nuclear magnetic resonance (NMR) spectra of Au₂₅(SR)₁₈ were reported by different groups.^67,175,176 In 2010, the structure of Au₃₈(SR)₂₄, another popular member of the thiolate-protected gold nanoclusters, was theoretically predicted by Lopez-Acevedo et al.,⁴⁴ and its crystal structure was revealed in the same year by Qian et al.¹⁵⁷ Chirality in thiolate-protected noble metal nanoclusters was first reported by Schaaff and Whetten in 2000.¹⁷⁷ In 2012, the first separation of enantiomers of Au₃₈(SR)₂₄ was achieved using chiral HPLC by Dolamic et al.,¹⁷⁸ which was a significant step toward understanding the chirality of nanoclusters. In 2015, the structural isomerism in Au₃₈(SR)₂₄ was observed by Tian et al.¹⁷⁹ Infrared and Raman spectroscopy of these nanoclusters were reported by Dolamic et al. and Varnholt et al., in 2013 and 2014, respectively, revealing the distinct vibrational features of the staples of these nanoclusters.^180,181 Whereas most of these advancements were centered around gold nanoclusters, the search for an atomically precise silver nanocluster was fruitful only in 2013 when Desireddy et al. reported the structure of Ag₄₄(SR)₃₀.¹⁶² Ag₂₅(SR)₁₈, which is structurally and compositionally analogous to Au₂₅(SR)₁₈, was discovered in 2015 by Joshi et al.¹⁶³ Clusters of other metals and alloy nanoclusters composed of two, three and four elements have been reported.^10,182 Apart from thiolates, a wide variety of ligands, such as selenolates,^183–187 tellurolates, alkynes,^188,189 carbenes,¹⁹⁰etc., have also been used as protecting ligands for noble metal nanoclusters. For a comprehensive summary of the advancements in the field of thiolate-protected noble metal nanoclusters, please consult several additional references.^{7,8,10–12,41,57,91,173,191–194}

3. Chemical reactivity of ligand-protected atomically precise metal clusters

The molecule-like nature of the physical properties of these nanoclusters is evident from the structural and spectroscopic studies discussed above. Recently, we have shown that these nanoclusters exhibit molecule-like chemical reactivity as well. In the following sections, we discuss different aspects of their chemistry in detail.

3.1. Ligand exchange and ligand-induced transformations

Post-synthetic modification in particles is a versatile approach for the transformation in atomically precise nanoclusters in terms of compositional, morphological, and structural changes. Reactions of nanoclusters with structurally different ligands are another way to synthesize new nanoclusters. Substitution or exchange of ligands is one of the earliest reactions of such nanoclusters.¹⁹⁵

Over the past two decades, monolayer-protected metal nanoclusters reacting with various ligands have produced nanoclusters with novel physical and chemical properties.¹⁹⁶ Murray et al. performed the first of such ligand-exchange reactions with thiol-protected gold nanoclusters.¹⁹⁷ Murray et al. also studied the mechanism of these ligand-exchange reactions in detail using mass spectrometry, NMR spectroscopy, and electrochemistry.^195,198 The rate of such reactions depends on the concentrations of both the nanocluster and the foreign ligand. The electron-donating and withdrawing nature of the functional groups on the ligands also governs the reaction rates. Using electrochemistry, Parker et al. showed that the electron-withdrawing ligands accelerate the exchange rate relative to electron-donating ligands.¹⁹⁹ Murray et al. observed that the ligand exchange is a second-order reaction.²⁰⁰ Rate of reaction is determined by the bonding of incoming and outgoing ligands with metal, much like an associative mechanism. However, the rate is independent of the size of the nanoclusters. For example, both Au₃₈(PET)₂₄ and Au₁₄₀(PET)₅₃ showed similar rate constants during ligand exchange using various p-substituted aryl thiols.²⁰¹ The understanding of site selectivity and specificity of ligands in exchange reactions significantly improved with the availability of single-crystal structures of nanoclusters.

When noble metal nanoclusters react with foreign ligands, they undergo a transformation that leads to three types of ligand-exchange products. These products can result in the nanocluster with (i) retention of its structure and composition upon exchange, (ii) alteration in geometry while retaining its composition, or (iii) alteration in both structure and composition.

The initial reports on ligand-exchange reactions indicate that structure and composition of the nanocluster remain unaltered in the process. Murray et al. extensively studied the ligand-exchange reactions on Au₂₅(PET)₁₈ with different SR (where, R = Ph–CH₃, Ph–F, etc.), which resulted in Au₂₅(SR)_18−x(SR′)_x series (x = 1–12).^200,202,203 Partial ligand exchange was observed during the reaction of p-BBT (BBT = bromobenzenethiol) with Au₁₀₂(p-MBA)₄₄ and Au₂₅(PET)₁₈ which led to the formation of Au₁₀₂(p-MBA)₄₀(p-BBT)₄ and Au₂₅(PET)₁₆(p-BBT)₂, respectively.^204,205 In 2014, Abdulhalim et al. reported the ligand exchange on Ag₄₄(4-FTP)₃₀ (FTP = fluorothiophenol) with various other aryl thiols such as MNBA (5-mercapto-2-nitrobenzoic acid), 4-NTP (NTP = nitrothiophenol), and 2-NT (NT = naphthalenethiol) which resulted in the formation of Ag₄₄(SR)₃₀ (where SR = MNBA/4-NTP/2-NT).²⁰⁶ A second type of ligand exchange, referred to as ligand-induced isomerization, was reported in 2016 by Jin et al. upon the reaction of Au₂₈(CHT)₂₀ (CHT = cyclohexanethiol) with TBBT ligand (4-tert-butylbenzenethiol), in which the structure of the nanocluster changed while the composition remained unchanged.²⁰⁷

In 2008, Shibu et al. came up with the first report on post-synthetic modification of atomically precise Au₂₅ nanoclusters via ligand exchange reaction.¹⁹⁵ Performing a ligand exchange with functionalized glutathione on the Au₂₅(SG)₁₈ nanocluster altered its optical and photoluminescence properties. These reactions have since found widespread application for modifying the chemical and other properties of nanoclusters through the introduction of new ligands to the parent clusters. Recently, ligand-exchange-induced structure transformation (LEIST) has become a rapidly developing technique in nanoclusters. In such reactions, when a foreign ligand is introduced, it can cause significant distortion of the core, resulting in both structural and compositional changes within the nanocluster. In 2013, Jin et al. introduced the term LEIST when they observed the transformation of Au₃₈(SR)₂₄ to Au₃₆(SR′)₂₄ nanocluster through a ligand-exchange reaction. Jin et al. performed the ligand-exchange reaction on Au₃₈(PET)₂₄ with excess TBBT (TBBT = 4-tert-butylbenzenethiol) under thermal conditions, resulting in molecularly pure Au₃₆(TBBT)₂₄ in excellent yield.²⁰⁸ The process of ligand exchange brings about a change in the structure of biicosahedral Au₃₈(PET)₂₄, transforming it into a truncated tetrahedral Au₃₆(TBBT)₂₄ with an FCC kernel. Interestingly, one of the first examples of an FCC-structured Au_n(SR)_m nanocluster is Au₃₆(TBBT)₂₄. Zeng et al. established the universality of the LEIST method by reacting Au₂₅(PET)₁₈ with TBBT under thermal conditions, which led to the formation of Au₂₈(TBBT)₂₀.¹⁵⁶ The Jin group's discovery of the LEIST method led to the synthesis of numerous new nanoclusters.²⁰⁹ Following this work, a large number of transformations was studied by different groups, such as conversions of Au₁₁(PPh₃)₇Cl₃ to [Au₂₅(SR)₅(PPh₃)₁₀X₂]²⁺,²¹⁰ Au₁₅(SG)₁₃ to Au₁₆(S-Adm)₁₂,²¹¹ Au₁₈(S-c-C₆H₁₁)₁₄ to Au₂₁(S-Adm)₁₅,²¹²etc., which proved the method to be versatile for making new structures. Furthermore, similar structural changes were observed in silver nanoclusters, and the mechanisms underlying these changes were investigated in detail. Bakr et al. showed the reversible conversion between Ag₂₅(2,4-DMBT)₁₈ (DMBT = dimethylbenzenethiol) and Ag₄₄(4-FTP)₃₀ (FTP = fluorothiophenol).²¹³ Upon reaction with 2,4-DMBT, Ag₄₄(4-FTP)₃₀ underwent a disproportionation reaction to form smaller sized Ag₂₅(4-FTP)₁(2,4-DMBT)₁₇ and bigger sized Ag_46–50(4-FTP)_4–9(2,4-DMBT)_21–26. After complete ligand exchange, other less stable nanoclusters transformed to more stable Ag₂₅(2,4-DMBT)₁₈. On the other hand, the conversion of Ag₂₅(2,4-DMBT)₁₈ to Ag₄₄(4-FTP)₃₀ occurred via dimerization of Ag₂₅(2,4-DMBT)₁₈ followed by a rearrangement pathway. A similar mechanism was observed during the conversion of Ag₃₅(SG)₁₈ to Ag₄₄(4-FTP)₃₀.²¹⁴ Khatun et al. showed a distinctly different mechanistic pathway during the transformation of Ag₅₉(2,5-DCBT)₃₂ (DCBT = dichlorobenzenethiol) to Ag₄₄(2,4-DCBT)₃₀, Ag₂₅(2,4-DMBT)₁₈ and Ag₂₉(1,3-BDT)₁₂(PPh₃)₄ (BDT = benzenedithiol) upon reaction with 2,4-DCBT, 2,4-DMBT and 1,3-BDT/PPh₃, respectively (Fig. 3).²¹⁵ In the presence of incoming thiol ligands, Ag₅₉(2,5-DCBT)₃₂ dissociated completely into smaller nanoclusters and thiolates instead of ligand exchange. Then, these smaller nanoclusters and thiolates recombined and rearranged to form the final product. The nature of the thiolate ligand plays an important role in determining the structure and composition of the product nanoclusters. Khatun et al. also reported the synthesis of MAg₂₈(1,3-BDT)₁₂(PPh₃)₄ from MAg₂₄(2,4-DMBT)₁₈via the LEIST method. Recently, a phosphine-protected nanocluster Ag₁₈(PPh₃)₁₀H₁₆ synthesized by Bakr et al. was observed as a very good precursor for the LEIST reaction (Fig. 4).²¹⁶ Bodiuzzaman et al. synthesized two new nanoclusters, Ag₄₆(DMBT)₂₄(PPh₃)₈ and Ag₄₀(DMBT)₂₄(PPh₃)₈via the LEIST method, using Ag₁₈(PPh₃)₁₀H₁₆.²¹⁷ Manju et al. synthesized NIR-emitting [Ag₃₄S₃(SBB)₂₀(CF₃COO)₆]²⁺ nanocluster from Ag₁₈(PPh₃)₁₀H₁₆ upon reacting it with tertiary-butylbenzylthiol (SBB).²¹⁸ Upon reacting Ag₁₈(PPh₃)₁₀H₁₆ with 2-pyrene imine thiol (2-PIT), Jana et al. synthesized a new dual-emitting nanocluster, [Ag₃₅(2-PIT)₇(PPh₃)₇@(H₂O)]³⁺.²¹⁹ Kang and Zhu published an extensive review on the evolution of the LEIST methodology and its application.²²⁰

Fig. 3 Schematic representation of ligand exchange-induced conversion of Ag₅₉(2,5-DCBT)₃₂ to Ag₄₄(2,4-DCBT)₃₀, Ag₂₅(2,4-DMBT)₁₈ and Ag₂₉(1,3-BDT)₁₂(PPh₃)₄ after the reaction with 2,4-DCBT, 2,4-DMBT and 1,3-BDT/PPh₃, respectively, under ambient conditions. Adapted from ref. 215. Copyright 2017 Royal Society of Chemistry.

Fig. 4 Formation of [Ag₃₄S₃(SBB)₂₀(CF₃COO)₆]²⁺, [Ag₃₅(2-PIT)₇(PPh₃)₇@(H₂O)]³⁺, Ag₄₆(DMBT)₂₄(PPh₃)₈ and Ag₄₀(DMBT)₂₄(PPh₃)₈ from Ag₁₈(PPh₃)₁₀H₁₆via LEIST method. Adapted from ref. 217. Copyright 2019 John Wiley and Sons.

Ligand exchange is an effective strategy to improve the physical and chemical properties of nanoclusters. The method has largely been used to enhance the emission quantum yield (QY) of several non-luminescent or feebly luminescent nanoclusters. Jin et al. showed an improvement in photoluminescence (PL) intensity while preserving the composition of Au₂₅(SR)₁₈.²²¹ They found higher QY for Au₂₅(PET)₁₈ (PET = 2-phenylethanethiol) than for Au₂₅(DDT)₁₈ (DDT = dodecanethiol) and Au₂₅(HT)₁₈ (HT = hexanethiol). Later on, the PL intensity of Au₂₅(PET)₁₈ enhanced 6.5 fold on using NAP (NAP = 2-(naphthalen-2-yl)ethanethiolate) ligand instead of PET.²²² Kim et al. showed enhancement in PL intensity of Au₃₆(TBBT)₂₄ (TBBT = tert-butylbenzenethiol) by partial ligand exchange using CPT (CPT = cyclopentanethiol) ligand.²²³ Similar to gold nanoclusters, ligand engineering in silver nanoclusters also led to the enhancement of the PL QY. Khatun et al. found structure-conserved ligand exchange in Ag₂₉(BDT)₁₂(PPh₃)₄ (BDT = 1,3-benzenedithiol) using various diphosphine ligands such as DPPM (1,1-bis(diphosphino)methane), DPPE (1,2-bis(diphosphino)ethane) and DPPP (1,3-bis(diphosphino)propane) which resulted in the increment of PL QY, as shown in Fig. 5.²²⁴ Among these nanoclusters, Ag₂₉(BDT)₁₂(DPPP)₄ exhibited highest PL QY which is 30 fold higher than that of Ag₂₉(BDT)₁₂(PPh₃)₄. The PL intensity can also be modified by the structure, transformed due to ligands. Such an example is the conversion of Pt₁Ag₂₄(SR)₁₈ to Pt₁Ag₂₈(SAdm)₁₈(PPh₃)₄ (SAdm = adamentanethiol) which displayed 50-fold higher PL QY than the parent nanocluster.²²⁵ Other properties such as chirality is introduced in nanoclusters using the ligand exchange method.^226,227 Bürgi et al. introduced chirality in Au₃₈(PET)₂₄ nanocluster by partial ligand exchange using chiral bidentate thiol, BINAS (BINAS = 1,1′-binaphthyl-2,2-dithiol).²²⁸

Fig. 5 Excitation and emission spectra of Ag₂₉(BDT)₁₂ and Ag₂₉(BDT)₁₂(P)₄ where P = PPh₃, DPPM, DPPE, DPPP. A systematic enhancement of PL intensity is observed. Photographs of nanoclusters under UV light are also shown. Reproduced from ref. 224. Copyright 2018 Royal Society of Chemistry.

3.2. Reactions with metal ions

The interaction of a metal ion with the noble metal nanocluster is reflected in the changes observed in the absorption and emission spectra of the nanocluster. Some metal nanoclusters are known to be highly fluorescent compared to their bulk counterparts. Also, the post-synthetic metal-exchange reactions with noble metal nanoclusters are an important method for the preparation of alloy nanoclusters. Of all the known metal-exchange methods, galvanic reduction is one of the most efficient approaches for the preparation of multimetallic alloy nanoclusters.

Metal ion-induced alteration in the fluorescence of nanoclusters is one of the most used strategies for sensing heavy metal ions, like Hg²⁺, Cu²⁺, As³⁺, Cr³⁺, Pb²⁺, etc.⁷⁵ In 2007, Habeeb et al. were the first to report the reactivity of Au₂₅SG₁₈ nanocluster to AuCl₄⁻.²²⁹ The Au₂₅SG₁₈ nanocluster underwent an instantaneous decomposition in the presence of AuCl₄⁻ ions to form an insoluble gold–glutathione coordination polymer (Au_nSG_m). The characteristic absorption features of Au₂₅SG₁₈ nanocluster get quenched immediately after addition AuCl₄⁻ ions due to the formation of Au(I) – glutathione complex. Upon reaction with other metal ions, such as Ag⁺, Fe³⁺, Cu²⁺, Ni²⁺, Cd²⁺, Zn²⁺ and Sr²⁺, the Au₂₅SG₁₈ nanocluster decomposes but at a slower rate. The net reaction of Au₂₅SG₁₈ nanocluster – Au³⁺ ion can be represented as an electron-transfer process where the electrons from the nanocluster core reduce AuCl₄⁻ to AuCl₂⁻ ions.

Mercury (Hg) is one of the most toxic heavy metals. Bootharaju and Pradeep reported that the Ag_7,8(MSA)_7,8 nanocluster (MSA = mercaptosuccinic acid) can act as a Hg and other heavy metal scavenger.²³⁰ In the reaction medium, the silver nanocluster interacts preferentially through its core and the carboxylate group of mercaptosuccinic acid (MSA) ligand depending upon the concentration of Hg²⁺ ion. The Ag_7,8(MSA)_7,8 nanocluster undergoes luminescence quenching as it interacts with Hg²⁺. To understand the scavenging property of the Ag_7,8(MSA)_7,8 nanocluster, it is important to note that the redox potentials of Ag¹⁺/Ag⁰ decrease with the particle size compared with the bulk metal, which is +0.79 V, whereas it is +0.8 V for the interacting metal ion (Hg²⁺/Hg⁰). The net cell electromotive force (emf) for the reduction of Hg²⁺ by the silver nanocluster is positive. The alumina-loaded Ag_7,8(MSA)_7,8@Al₂O₃ nanocluster can be used quantitatively for Hg²⁺ removal from contaminated water. Later on, from the same group, Chakraborty et al. went ahead to report the selective reaction of Ag₂₅SG₁₈ nanocluster with Hg²⁺ ion. The chemical interaction of Ag₂₅SG₁₈ nanocluster and Hg²⁺ ion resulted in the formation of Ag₃Hg₂ alloy (paraschachnerite with an orthorhombic crystal structure), as observed with the appearance of new blue-shifted features in the optical absorption spectra (Fig. 6). XPS studies show that the Ag₂₅SG₁₈ nanocluster – Hg²⁺ ion reaction is a redox process which involves oxidation of Ag⁰ to Ag⁺ and reduction of Hg²⁺ to Hg⁰. The luminescent Ag₂₅SG₁₈ nanocluster can act as a sensing material, as it undergoes fluorescence quenching upon interaction with Hg²⁺ ions with a limit of detection of 1 ppb.

Fig. 6 Optical absorption spectra of the Ag₂₅SG₁₈ nanocluster solution on addition of different metal ions. Under UV excitation, images corresponding to fluorescence quenching of (a) phase transferred nanocluster and on the addition of (b) 1 ppm, (c) 2 ppm, and (d) 10 ppm of Hg²⁺. Adapted from ref. 290. Copyright 2012 Elsevier.

Another heavy metal present in drinking water is Cu²⁺, which has a permissible limit of 1.3 ppm in drinking water as set by the U.S. Environmental Protection Agency (EPA). Glutathione (GSH) is a natural tripeptide in amino acids, such as cysteine, glutamic acid, and glycine. GS-protected gold nanoclusters are biocompatible and have strong near-IR fluorescence emission compared with other non-aqueous Au_n(SR)_m, making them a popular candidate for developing biological and heavy metal sensors. Back in 2009, fluorescent Au@GS NPs made a debut as a highly selective Cu²⁺ sensor, which is a classic example of aggregation-induced fluorescence quenching.²³¹ Later on, efforts were undertaken by George et al. for sensing Cu²⁺ ions using thiolate-protected gold nanoclusters.³⁴ This work involved a GS-protected Au₁₅ nanocluster encapsulated in cyclodextrin (CD) cavities (denoted by Au₁₅@CD). The sensor material is prepared by loading the Au₁₅@CD nanocluster onto a freestanding film of chitosan. This nanocluster composite material is a bright luminescent film under UV light. Upon exposing the nanocluster composite material to Cu²⁺ ions, luminescence quenching happens which is selective to Cu²⁺ ion concentration. A change in emission maximum was observed in the PL spectra of the material before and after its exposure to Cu²⁺ ions. The sensing specificity of the nanocluster composite material towards Cu²⁺ ion was studied using XPS analysis, which suggested a reduction of Cu²⁺ to Cu¹⁺/Cu⁰ by the glutathione ligand or the Au₁₅ core. The reported limit of detection of the GS-protected-Au₁₅@CD nanocluster composite material is 1 ppm of Cu²⁺ ion present in the medium. Zhang et al. reported the application of water-soluble GS-protected gold nanoclusters in Cu²⁺ sensing with a limit of detection of 86 nM.⁷² The quenching of the fluorescence is attributed to the carboxylic group in GSH-ligand, which is a chelating agent with a high affinity and selectively towards Cu²⁺ ion over other metal ions, like Hg²⁺ and Pb²⁺, present in the medium. Krishnadas et al. reported a highly luminescent MSA-protected Ag–Au bimetallic nanocluster (denoted as AgAu@MSA) material as a Cu²⁺ sensor. Initially, the preparation of the AgAu@MSA nanocluster involved a galvanic reduction of polydispersed Ag NPs by Au^I-MSA thiolates. A methanolic solution of the AgAu@MSA nanocluster undergoes immediate luminescence quenching selectively upon interaction with Cu²⁺ even in the presence of other metal ions. The mechanism of metal-induced fluorescence quenching of the nanocluster was investigated using XPS, and it was concluded that the Cu²⁺ ion interacts with the AgAu metal core of the nanocluster. The nanocluster–metal ion interaction is a redox process; the AgAu metal core reduces the Cu²⁺ ion to Cu¹⁺/Cu⁰, while it gets oxidized in the process.

In the following part, we will focus on the metal-exchange reactions of noble metal nanoclusters resulting in alloy nanoclusters. As per the galvanic theory, the metal ion with a higher reduction potential replaces another metal with a lower reduction potential, and subsequently, it gets reduced (Fig. 7A). The reduction potential of metals in decreasing order is Fe²⁺ > Cd²⁺ > Co²⁺ > Ni²⁺ > [Au NCs] > Cu²⁺ > Hg²⁺ > Ag¹⁺ > Pd²⁺ > Pt²⁺ > Au¹⁺.^232,233 Using the galvanic replacement method, various multimetallic noble metal alloy nanoclusters are formed. One such example is the Au-incorporated Ag₂₄Au(DMBT)₁₈ nanocluster derived from Ag₂₅(DMBT)₁₈ studied by Bootharaju et al., where the reduction potentials of Ag⁺/Ag and Au⁺/Au are 0.080 and 1.69 V, respectively (Fig. 7C(a)).²³⁴ The structure of the bimetallic Ag₂₄Au(DMBT)₁₈ nanocluster is analogous to its monometallic counterpart with enhanced stability and photoluminescence. The reaction of PdAg₂₄(SR)₁₈ and Au-salt was found to proceed via trimetallic PdAuAg₂₃(SR)₁₈ nanocluster intermediate and finally resulting in a replacement product, the AuAg₂₄(SR)₁₈ nanocluster. But the Au ionic reaction with the structurally equivalent PtAg₂₄(SR)₁₈ nanocluster led to the formation of Au₂PdAg₂₂(SR)₁₈ nanocluster. Later, Kang et al. reported dopant-dependent shape-controlled galvanic exchange reactions.²³⁵ The Au-doping in a PtAg₂₄(DMBT)₁₈ nanocluster with the precursors Au-DMBT and AuBrPPh₃ resulted in the formation of trimetallic nanoclusters with shape-unaltered PtAu_xAg_24−x(DMBT)₁₈ and altered Pt₂Au₁₀Ag₁₃(PPh₃)₁₀Br₇ nanoclusters, respectively (Fig. 7C(b)). Similarly, multimetallic PtCu_xAg_28−x(BDT)₁₂(PPh₃)₄ and Pt₁Ag₁₂Cu₁₂Au₄(S-Adm)₁₈(PPh₃)₄ nanoclusters were prepared using the galvanic exchange method.²³⁶ Bootharaju et al. studied the metal-exchange reaction between MAg₂₄(SR)₁₈ (M = Pd/Pt) and AuPPh₃Cl salt, using mass spectrometry (Fig. 7C(c)).²³⁷ Likewise, using the templated galvanic metal-exchange route, a highly stable Au_5.34Ag_44.66(Dppm)₆(TBBM)₃₀ (Dppm = bis(diphenylphosphino)methane and TBBM = 4-tert-butylbenzyl mercaptan) nanocluster was prepared from Ag₅₀(Dppm)₆(TBBM)₃₀ (Fig. 7C(d)).²³⁸

Fig. 7 Schematic representation of (A) galvanic replacement and (B) anti-galvanic replacement reaction from the context of doping in Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈, respectively. Examples showing the preparation of alloy nanoclusters using (C) galvanic and (D) anti-galvanic reaction routes. Reproduced with permission from ref. C-a,²³⁴ b,²³⁶ c,²³⁷ and d,²³⁸ and D-a,²⁴⁶ b,²⁴⁸ c,²⁵¹ and d.²⁴⁹ Copyright 2015 and 2017 American Chemical Society. Copyright 2016 and 2018 John Wiley and Sons. Copyright 2016 Royal Society of Chemistry. Copyright 2017, Springer Nature Group. Copyright 2019 National Academy of Sciences.

An anti-galvanic reaction (AGR) defies the classical galvanic reduction (GR) as the metal ions get reduced by the less reactive (or more noble) metal (Fig. 7B). The driving force for an anti-galvanic reaction can be explained in terms of the difference in the redox potential of the participating entities. In 1985, Plieth proposed a theory on the relationship between the electrode potential of metal nanoparticles and their particle diameter.²³⁹ Theoretically, bulk metals (M_bulk) can be transformed into small metal particles (M_NP) by dissolving into metal ions (M⁺) and then redepositing as particles. The reduction of bulk metal (M_bulk) into metal nanoparticles (M_NP) can be described by the electrochemical cell reaction,²³⁹

M_bulk → M⁺(aq) + ze⁻

M⁺(aq) + ze⁻ → M_NP(s)

Plieth states that the standard reduction potential of a small metal particle undergoes a negative shift, as expressed in the following equation,²³⁹

where, μ_d and μ_b are the reduction potentials of metal NP and electrode of the same metal, respectively. The other terms are molar mass (M), specific mass (ρ), number of electrons (z), Faraday's constant (F), surface free energy (γ), and NP radius (r). Later, Zamborini et al. and Henglein et al. experimentally demonstrated a significant negative shift in the electrode potentials of Ag⁸⁷ and Au NPs²⁴⁰ having sizes below 4 nm.

The ultrasmall-sized noble metals nanoclusters have been recently emerging as interesting candidates for anti-galvanic reactions.^232,241 The anti-galvanic reaction route provides a more facile and milder method towards alloying with a better control over the composition, structure, and properties of the nanoclusters. Choi et al. reported the first anti-galvanic reaction in nanoclusters with the alloying of [Au₂₅(PET)₁₈] nanocluster.²⁴² The [Au₂₅(PET)₁₈]⁻ nanocluster upon reaction with Ag⁺ ion resulted in a bimetallic [Au₂₄Ag(PET)₁₈]⁻ nanocluster. From the electrochemical series, it is known that Au is less reactive than Ag, and it was anticipated that the reduction of Au(III) by Ag metal is facile in the ambient conditions but the opposite is not. In 2012, Wu studied the reaction of Au₂₅(PET)₁₈ with Ag⁺ and Cu²⁺ ions resulting in bimetallic products as characterized using matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS) and XPS.²³² Ag⁺ ions failed to react with 2–3 nm Au NPs, thereby establishing Au₂₅(PET)₁₈ nanoclusters as a unique candidate towards anti-galvanic reaction. The thiolate (–SR) ligand coverage on the nanocluster surface plays a pivotal role in the anti-galvanic reactions. As the ligand attaches to the metal surface, it gains a partial negative charge which further assists in the reduction of less noble ions, like, Ag⁺ and Cu²⁺. Here, the anti-galvanic reaction is catalyzed by the highly reactive metal atom on the nanocluster surface. The AGR products are influenced by monolayers on the nanocluster surface. For instance, Wu et al. demonstrated that when exposed to silver ion precursors (such as AgNO₃, Ag-PET, Ag-EDTA, and Ag-DTZ; where EDTA stands for ethylenediamine tetraacetic acid disodium salt and DTZ for dithiazone), Au₂₅(PET)₁₈ reacts and forms various Ag–Au alloy nanoclusters.²⁴³ However, Au₂₅(SG)₁₈ does not exhibit any reaction with Ag⁺ ions.²⁴⁴

Using anti-galvanic reaction routes, gold nanoclusters can be alloyed by a heteroatom addition or replacement that also involves a retention or an alteration of the structural framework.²⁴¹

Here are a few examples of heteroatom addition with retention of the initial nanocluster structural framework. The reaction between Au₂₅(PET)₁₈ and AgNO₃ in acetonitrile results in the formation of Au₂₅Ag₂(PET)₁₈ nanocluster as a major product.²⁴⁵ The incoming Ag-atoms, instead of replacing Au-atoms, get added to the Au₂₅(PET)₁₈ nanocluster structure. Upon comparison with the Au₂₅(PET)₁₈ nanocluster, the Ag-added species, Au₂₅Ag₂(PET)₁₈, has a ∼3.5-fold enhancement in QY while no change was seen in the Ag-replaced species, Au_25−xAg_x(PET)₁₈ (x = ∼3). Wang et al. observed that a foreign Ag-atom can squeeze into the hollow site of the Au₂₄(PPh₃)₁₀(PET)₅Cl₂ nanocluster without altering its composition or structure (Fig. 7D(a)).²⁴⁶ Other popular examples of metallic replacement with the retention of starting nanocluster structure are Ag, Pt, and Pd-doped Au₃₈(PET)₂₄, Ag-doped Au₃₆(TBBT)₂₄, Cu and Ag-doped Au₁₄₄(PET)₆₀.²⁴⁷ In 2015, Liao et al. synthesized Au₂₅Hg₁(PET)₁₈ and Au₂₅Cd₁(PET)₁₈ from Au₂₅(PET)₁₈ using an anti-galvanic reaction (Fig. 7D(b)).²⁴⁸ The single-crystal X-ray diffraction of Au₂₅Hg₁(PET)₁₈ revealed the structural similarity with Au₂₅(PET)₁₈ where one of the outer-shell Au-atoms is replaced by a Hg-atom, while for the Au₂₅Cd₁(PET)₁₈ structure, the Cd-atom replaces the Au-atom to occupy the central position (Fig. 7B).

Jin et al. pioneered alloying methods where the heteroatomic replacement initiates a structural transformation. Li et al. studied the Ag-doping-induced transformation of the Au₂₃(CHT)₁₆ (CHT = cyclohexanethiolate) nanocluster into Au_25−xAg_x(CHT)₁₈ (Fig. 7D(d)).²⁴⁹ The alloying reaction between Au₂₃(CHT)₁₆ and Ag(I)-CHT proceeds through a two-step metal-exchange route: (i) Au₂₃(CHT)₁₆ is initially converted to a Au_23−xAg_x(CHT)₁₆ (x ∼ 1) intermediate, and (ii) then it is allowed to grow into Au_25−xAg_x(CHT)₁₆ (x ∼ 4), with Ag sitting at the icosahedral inner shell.²⁵⁰ Zhu et al. reported another method of synthesis of the bimetallic nanocluster involving a non-replacement of the heteroatom along with a structural transformation. The Au₂₀Cd₄(SH)(CHT)₁₉ nanocluster was prepared from Au₂₃(SR)₁₆ using the anti-galvanic reaction (Fig. 7D(c)).²⁵¹ The starting Au₂₃(SR)₁₆ nanocluster is known to have an Au₁₅ bi-capped cuboctahedron-based kernel, protected by two Au₃(SR)₄ trimeric and Au(SR)₂ monomeric staples along with four simple bridge –SR– ligands.²⁵² Structural similarity of Au₂₅ and Au₂₀Cd₄(SH)(CHT)₁₉ nanocluster system was found to be composed of a centered icosahedral Au₁₃ and Au₁₁Cd₂ kernel, respectively. The introduction of two Cd-atoms distorts the Au₁₃ kernel of Au₂₅. The resulting nanocluster structure consists of a distorted central icosahedral Au₁₁Cd₂ kernel with the capping of two non-equivalent trimeric staples, one dimeric staple, two monomeric staples, four bridging thiolates (–SR–), and one CdSH unit. Li et al. reported Cd-addition to the Au₂₂(SAdm)₁₆ (SAdm = 1-adamantanethiol) nanocluster (bioctahedral Au₁₀ kernel), which resulted in a structurally transformed Au₂₂Cd₁(SAdm)₁₆ nanocluster (cuboctahedral Au₁₂Cd₁ kernel).²⁵³ Using the reaction between Cl@Ag₁₄ and AgClO, Hau et al. introduced the metal core enlargement in nanoclusters, leading to the formation of bigger Cl₆Ag₈@Ag₃₀.¹¹⁶ Recent investigations have shown that carboxylate ligands on the surface of Ag nanoclusters provide ligand–shell flexibility, inducing structural modifications in the NCs due to differential coordination of Ag between carboxylate and thiol/alkyl moieties.^254,255 For example, monocarboxylate and dicarboxylate ligand triggers structural transformations Mo₆O₂₂@Ag₄₄ → Mo₈O₂₈@Ag₅₀ (ref. 254) and Ag₅₄ → Ag₂₈, (ref. 255) respectively.

3.3. Reactions with halocarbons

The studies of the reaction of halocarbons with noble metal NPs opened up a new direction in nanocluster chemistry. Nair and Pradeep reported that citrate-capped Ag and Au NPs possess catalytic property for the destruction of halocarbons, resulting in the formation of metal halides and amorphous carbon. This was the first time such properties had been reported.²⁵⁶ The halocarbon, CCl₄, upon reaction with an alcoholic solution of Ag and Au NPs resulted in the formation of AgCl and AuCl₃, respectively, along with an amorphous carbon residue. This reaction was able to completely mineralize halocarbons like choloro, fluoro, and bromocarbons. Later, Bootharaju and Pradeep identified a pesticide degradation pathway using the citrate-capped Ag and Au NPs.²⁵⁷ Here, chlorpyrifos (CP), an organophosphorothioate pesticide, was used as the model pesticide. After reacting with CP, optical absorption spectroscopy showed a red shift in the surface plasmon of Ag NP, and transmission electron microscopic (TEM) analysis revealed aggregation. Upon reaction with the unsupported and alumina-supported Ag and Au NP, the CP degrades into less toxic by-products like 3,5,6-trichloro-2-pyridinol (TCP) and diethyl thiophosphate (DETP), which was established using mass spectrometric studies. The proposed mechanisms involve steps like: (i) first, the CP binds to the NP surface through an Ag_n⁺ ← S bond, (ii) P–O cleavage, (iii) nucleophilic H₂O attack at the electrophilic P site, and (iv) finally, electron withdrawal from Ag_n⁺ ← N and Ag_n⁺ ← S bonds resulting the formation of stable TCP and DETP compounds. The Ag NPs were found to have better catalytic performance over Au NPs. When reacting with CP, the unsupported Ag@citrate NPs tend to aggregate, while the alumina-supported Ag NPs do not. This makes the latter more efficient for water purification, and reusable.

In 2013, Bootharaju et al. reported the degradation of halocarbons like CCl₄, C₆H₅CH₂Cl, and CHCl₃, using atomically precise Ag₉MSA₇ nanoclusters.²⁵⁸ The reaction products, AgCl, CCl₃COOH, amorphous carbon, and acetone, were characterized using XRD, Raman, infrared, optical absorption, X-ray photoelectron spectroscopy, and mass spectrometry. To increase the miscibility of halocarbons like CCl₄ in the reaction mixture, isopropyl alcohol (IPA) was used. The precipitate is amorphous carbonaceous material with a graphitic structure, while the supernatant contains acetone from oxidation of IPA. The proposed mechanism (Fig. 8) involves an initial adsorption of IPA and CCl₄ on the nanocluster surface, which in turn catalyses the oxidation of IPA into acetone and activation of the C–Cl bond of the CCl₄. The surface activities initiate a series of electron transfer reactions like release of H⁺ and Cl⁻ ions to the medium, making it acidic, Cl⁻ ions replacing the MSA ligands, oxidation of Ag⁰ to Ag⁺, and finally, mineralization of CCl₄.

Fig. 8 Schematic representation of the proposed mechanism for degradation of halocarbons, like CCl₄, by Ag nanocluster. Adapted from ref. 258. Copyright 2013 Royal Society of Chemistry.

3.4 Supramolecular chemistry of metal nanoclusters

The supramolecular chemistry of nanoclusters is an emerging area of research which also highlights the molecular nature of atomically precise nanoclusters. The organic ligands protecting the metal core of the nanocluster can interact with suitable molecules by weak supramolecular interactions. Such interactions include π–π, C–H⋯π, van der Waals and electrostatic interactions. Moreover, metallophilic interactions of the nanocluster core also play an active role in controlling the interactions. Such interactions have been the major driving force in controlling the crystal packing of the nanoclusters. Recently, such interactions have also been explored with other molecules. Mathew et al. reported interaction between Au₂₅SBB₁₈ (SBB = 4-(t-butyl)benzyl mercaptan) and cyclodextrins (CDs), which showed that the SBB ligands were encapsulated in the cavity of CDs, forming inclusion complexes of the nanocluster with CDs.²⁵⁹ Chakraborty et al. reported host–guest complexes of nanoclusters and fullerenes.²⁶⁰ Such interactions were largely dependent on the geometrical compatibility of the two molecules for forming the adducts and further assisted by weak supramolecular interactions. A range of such complexes can be made depending on the structure of the nanoclusters. Ag₂₉(BDT)₁₂ nanocluster can capture C₆₀ molecules on its surfaces, forming adducts such as [Ag₂₉(BDT)₁₂(C60)_n]³⁻ (n = 1–9). Structures of [Ag₂₉(BDT)₁₂(C₆₀)₄]³⁻ and [Ag₂₉(BDT)₁₂(C₆₀)₈]³⁻ are presented in Fig. 9A(a), which reveals that C₆₀ molecules are captured on tetrahedral sites on the nanocluster surface, assisted by interactions with the BDT ligands. Similarly, Ag₂₅(DMBT)₁₈⁻ and Au₂₅(PET)₁₈⁻ nanoclusters also formed adducts with fullerenes.²⁶¹ Due to a different geometrical structure of M₂₅(SR)₁₈⁻ nanoclusters compared with that of [Ag₂₉(BDT)₁₂]³⁻ nanocluster, the nature of the host–guest adducts with fullerenes were also different in the two cases. Ag₂₅(DMBT)₁₈⁻ and Au₂₅(PET)₁₈⁻ nanoclusters formed aggregates with fullerenes as shown in Fig. 9A(b), and these aggregates were actually dimeric, trimeric, or polymeric adducts of the nanoclusters. Supramolecular interactions of nanoclusters with crown ethers have also been observed, and such complexes were crystallized.²⁶² Crown ethers were captured in the crystal lattice of Ag₂₉ nanoclusters, forming lattice inclusion compounds, as shown in Fig. 9B. Such interactions also resulted in a change in the emission properties compared to the crystals of parent nanocluster. The chemical reactivity of the nanoclusters with other molecules also leads to the emergence of new properties in the host–guest complexes. Nag et al. reported inclusion complexes of Ag₂₉(BDT)₁₂³⁻ nanocluster with CDs.²⁶³ Such complexes showed isomerism due to the different binding possibilities of CDs on the nanocluster surface, as presented in Fig. 9C. About six CD attachments to the nanocluster were observed and the geometry of the supramolecular adducts resulted in isomerism similar to the octahedral coordination complexes of metals. Water-soluble and red luminescent Ag₂₉(LA)₁₂ (LA is lipoic acid) nanocluster also formed host–guest complexes with cucurbiturils and CDs which resulted in an enhancement in luminescence of the nanoclusters, as shown in Fig. 9D.²⁶⁴ Such luminescent complexes were used for dopamine sensing. Pillar[5]arene-protected nanoclusters, Ag₂₉(LA–P5)₁₂(TPP)₂, were reported by Muhammed et al. which formed spherical assemblies with enhanced luminescence.²⁶⁵ The reactivity of the nanoclusters was also reflected in their interaction with nanostructures like Au nanorods and Te nanorods to form a variety of self-assembled hybrid nanostructures (to be discussed later in detail). Recently, Sheng et al. reported the first supramolecular polymorphs of high-nuclearity Ag₄₈ NCs encapsulated in an anionic template via solvent mediation.²⁶⁶

Fig. 9 (A) Host–guest complexes of (a) Ag₂₉(BDT)₁₂³⁻ and (b) Ag₂₅(DMBT)₁₈⁻ nanocluster with C₆₀, (B) crystal packing of Ag₂₉(BDT)₁₂(TPP)₄³⁻ nanocluster with 18-crown-6-ether, (C) separation of isomers of the inclusion complex, [Ag₂₉(BDT)₁₂(β-CD)₂]³⁻ by ion mobility mass spectrometry, (D) enhancement in emission of Ag₂₉LA₁₂@CB complexes compared with that of Ag₂₉LA₁₂ nanocluster alone. Reproduced with permission from ref. A-a,²⁶⁰ b,²⁶¹ B,²⁶² C,²⁶³ and D.²⁶⁴ Copyright 2018, 2019, and 2020 American Chemical Society.

3.5 Intercluster reactions

Nanoclusters that undergo chemical reactions with one another, also known as intercluster reactions, are a rapidly developing field in nanoscience. Intercluster reactions are now utilized as tools to generate novel hybrid nanoclusters. It is crucial to have an atomic-level understanding of the chemical transformations of nanoclusters in such reactions. In this section, we will be discussing a few such examples of intercluster reactions and their associated mechanisms.

3.5.1. Diversity of reactions. In 2016, Krishnadas et al. reported the reaction between structurally and compositionally different Au₂₅(PET)₁₈ and Ag₄₄(FTP)₃₀ nanoclusters.⁵⁴ The intercluster reactions proceed through multiple intersystem exchanges involving both the metal and ligand to form alloy nanoclusters as the reaction product. The ESI mass spectra of the Au₂₅(PET)₁₈ and Ag₄₄(FTP)₃₀, and the reaction products, are shown in Fig. 10A. The mass spectrum shows a series of intercluster reaction products, like alloy nanoclusters formed from Au₂₅(PET)₁₈ by exchange of metal atoms (Au–Ag exchange), ligands (PET-FTP exchange) and metal–ligand fragments (Au-PET with Ag-FTP exchange). Similarly, the Au atoms, PET ligands and Au-PET fragments are also exchanged with Ag₄₄(FTP)₃₀, resulting in the formation of Ag-rich alloy nanoclusters. The total number of metal atoms, ligands, the overall structural features and the charge states of the nanoclusters are preserved in this reaction. Intercluster reaction was then studied for two structurally and compositionally analogous nanoclusters, Au₂₅(PET)₁₈ and Ag₂₅(DMBT)₁₈.⁵⁵ In this case, both interacting nanoclusters possess common structural features, like M₁₃ (Ag/Au) icosahedral core and M₂(SR)₃ staple motifs. As shown in Fig. 10B, the ESI mass spectra of these two nanoclusters and the alloy nanoclusters formed as a reaction product were observed. Similarly, these nanoclusters also exchange their metal atoms, ligands and metal–ligand fragments (Au-PET with Ag-DMBT exchange) to form alloy nanoclusters. Unlike the previous example, metal exchange (Ag–Au exchange) was only detected, and other exchanges, such as ligand (PET-DMBT exchange) and metal–ligand fragment (PET-DMBT exchange) were not detected in mass spectral measurements as the molecular masses of PET and DMBT ligands are equal. The mass spectrum collected after 2 min of reaction shows an entire range of alloy nanoclusters of M₂₅(SR)₁₈ composition as formed in the solution. Also, most importantly, in the course of reaction, the overall structure and charge of the nanocluster are conserved.

Fig. 10 Intercluster reaction of structurally and compositionally (A) different, Au₂₅(PET)₁₈ and Ag₄₄(FTP)₃₀, and (B) analogous Au₂₅(PET)₁₈ and Ag₂₅(DMBT)₁₈ nanoclusters. Adapted from ref. A,⁵⁴ and B.⁵⁵ Copyright 2016 American Chemical Society and Springer Nature Group.

Several questions arise, such as (i) how two negatively charged nanoclusters interact despite the electrostatic repulsion and steric hindrance offered by the ligands, (ii) whether the reaction is driven by the entire nanocluster entity or any metal–thiolate fragments, and (iii) whether the reaction involves any intermediate or adduct species. In the next section, we will be discussing answers to a few of the questions, while a few remain unanswered.

3.5.2. Mechanism of intercluster reactions. To address the dynamics involved in the intercluster reactions in terms of atomic events, a systematic structural model is needed. Recent experiments suggest that the origin of any intercluster reactivity is the dynamic structure of nanoclusters in the solution. Of all the reported models, the Borromean ring model or aspicule (Greek meaning of “aspis” is the shield, with “molecule”) model, wherein these nanoclusters are viewed as interlocked rings of metal thiolates (Fig. 11A), explains the intercluster reaction better.¹⁶⁶ As per this model, an M₂₅(SR)₁₈ (M = Ag/Au) is composed of three interlocked M₈(SR)₆ rings around the central metal atom, M, where, the M₂₅(SR)₁₈ can be represented as M@[M₈(SR)₆]₃. The most important aspect of this aspicule model lies in the fact that the metal atoms (excluding the central metal) belong to a single structural unit, that is, metal-thiolate oligomeric rings, contrary to the divide and protect model, where the metal atoms belong to two distinct structural units, namely the innermost M₁₃ icosahedron and the six outer M₂(SR)₃ staple units. The Borromean ring construction of the nanocluster suggests that in the event of metal–sulfur bond cleavage from any of the rings, the nanocluster can be reorganised as the entire cluster can be separated. This makes it possible for rapid metal exchange, as observed experimentally. In short, the Borromean ring model addresses the intercluster reactions in terms of the structural dynamics of the interlocked rings.

Fig. 11 (A) Borromean rings diagram of Au₂₅(SMe)₁₈, (B) DFT-optimization of the structure of [Ag₂₅Au₂₅(DMBT)₁₈(PET)₁₈]²⁻ adduct (with Ag₂₅ on the left and Au₂₅ on the right) as obtained from a force-field-based molecular docking simulation, (C) energy profiles of the metal exchange reaction between the [Au₂₅(PET)₁₈]⁻ and [Ag₂₅(DMBT)₁₈]⁻ nanoclusters; (a) path 1 initiated by Au–S_PET bond breaking, (b) DFT optimized metal–metal and metal–S bond lengths (in Angstrom) (c) path 2 initiated by Ag–SDMBT bond breaking, and (D) time-dependent mass spectra of a 1 : 1 mixture of [Au₂₅(PET)₁₈]⁻ and [Ag₂₅(DMBT)₁₈]⁻. Reproduced with permissions from ref. A–B,¹⁶⁶ C,²⁷⁰ and D.²⁷¹ Copyright 2015 and 2021 American Chemical Society. Copyright 2016, The Royal Chemical Society.

To study the mechanism of intercluster reactions, one needs to understand the role played by the metal–ligand interface in such reactions. The intercluster reaction is a redox-like reaction triggered by the difference in oxidation states of the metal atoms present in the core and staple. For example, let us consider the intercluster reaction between Ag₂₅(SR)₁₈ and Au₂₅(SR)₁₈.⁵⁵ Here, an Ag₂₅(SR)₁₈ molecule reacts with the Au₂(SR)₃ staples of Au₂₅(SR)₁₈, wherein Au in the Au₂(SR)₃ staples is in the +1 oxidation state. Similarly, an Au₂₅(SR)₁₈ molecule reacts with the Ag₂(SR)₃ staples of Ag₂₅(SR)₁₈, wherein Ag from the Ag₂(SR)₃ staple is in the +1 oxidation state. Such redox reactions between M₂₅(SR)₁₈ and M(I) thiolates, where M = Ag/Au, are well studied,^267,268 although it is still unclear how this difference in oxidation states contributes to the chemical reaction. Next, the intercluster reaction was studied for two entirely different nanoclusters, Au₂₅(SR)₁₈ and Ag₄₄(SR)₃₀, that resulted in the formation of reactive fragments like Ag(SR)₂⁻, which further reacts with the Au₂(SR)₃ staples of Au₂₅(SR)₁₈, resulting in an exchange of metal atoms, ligands, and metal–ligand fragments.⁵⁴ In conclusion, the stability and chemical reactivity of these nanoclusters are characteristics of the nature of the ligand and the bonding present in the metal–ligand oligomeric units.⁵⁸

New insights into the intercluster reactions of Au₂₅(PET)₁₈ and Ag₂₅(DMBT)₁₈ came up with the detection of [Ag₂₅Au₂₅(DMBT)₁₈(PET)₁₈]²⁻ adduct species.⁵⁵ Detection of such species indicates a possible pathway involving the formation of an adduct intermediate when two intact nanoclusters participate in these ‘bimolecular’ reactions. Using density functional theory (DFT), the Ag–S bond between the staples of the nanoclusters is observed in an optimized adduct structure (Fig. 11B). Computational studies suggest an interaction at the metal–ligand interface for the reacting nanoclusters at the early stages of the reaction. Furthermore, no metallic exchange was detected when nanoclusters, Ag_xAu_38−x(SR)₂₄ and Au₃₈(SR)₂₄, were separated by a dialysis membrane, and that suggests an intercluster collision as the origin of such reactions.²⁶⁹

In this section, some more mechanistic insights into the intercluster reactions are presented. More studies are needed to understand how two negatively charged nanoclusters collide, overcoming the electrostatic repulsion. One possible explanation for this could be coming from the fact that the anionic nanoclusters are not point charges; the overall negative charge is diffused over the entire nanocluster entity. At the early stages of the reaction, the intercluster interaction could lead to collisions, electron transfer, etc., resulting in nanocluster destabilization. The destabilization of the nanocluster eventually may lead to ring opening (refer to the Borromean ring model) followed by it taking up a flexible elongated conformation, allowing atoms to interact freely with other nanoclusters. At this stage, the nanoclusters with open rings can interact more easily and undergo metal, ligand, and metal–ligand fragment exchanges. The Borromean ring model of Au₂₅(SR)₁₈ suggests the spontaneous inclusion of Ag-atom in the nanocluster core as it is not sterically hindered. Theoretical calculations were performed by Huang et al. to understand the intercluster exchange reaction mechanism between Au₂₅(SR)₁₈ and Ag₂₅(SR)₁₈ (Fig. 11C).²⁷⁰ As per calculations, the intercluster reactions are a two-step mechanism: (i) dianionic adduct [Au₂₅Ag₂₅(PET)₁₈(DMBT)₁₈]²⁻ intermediate formation followed by metal–ligand staple rearranges to facilitate metal exchange, and (ii) then the heterometal atom in the staple swaps with the metal atom present in the icosahedral M₁₃-kernel. Recently, Neumaier et al. reported a detailed experimental and computational study of the intercluster reaction kinetics of Au₂₅(PET)₁₈ and Ag₂₅(DMBT)₁₈ at room temperature.²⁷¹ During the reaction, the participation of both nanocluster monomer and dimers were observed in mass spectral and collision-induced dissociation (CID) measurements. For an equimolar concentration of nanoclusters, time-dependent mass spectra show a sufficient abundance of both monomers and dimers along with continuous change in overall Ag : Au compositions until a dynamic equilibrium is achieved (Fig. 11D). The kinetic model suggested a three-step reaction route involving dimerization of monomers, metal atom exchange in the transient dimer, and dimer dissociation.

Chakraborty et al. reported isotopic metal exchanges in nanoclusters, which provided further insights into the mechanism of atom transfer in NPs. Two isotopic Ag₂₅(DMBT)₁₈⁻ nanoclusters, made from ¹⁰⁷Ag and ¹⁰⁹Ag, reacted spontaneously in solution to form an isotopically mixed nanocluster (Fig. 12A).²⁷² Such isotopic exchanges were similar to isotopic exchanges in H₂O/D₂O, further supporting the molecular nature of the nanoclusters. The exchange was rapid and occurred within seconds after mixing the solutions at room temperature. The exchange could be controlled by controlling the temperature. The exchange kinetics was better studied using a more rigid nanocluster system, Ag₂₉(BDT)₁₂(TPP)₄³⁻, which exhibited slower exchange rates. Similar isotopic ¹⁰⁷Ag/¹⁰⁹Ag atom exchange was also observed in this case, and a time-resolved study revealed that the process involved an initial fast reaction rate, probably arising from atom exchanges in the staples, followed by a slow reaction rate arising from the diffusion of atoms from staple to core and a final state where the exchanged atoms rearrange until they attain the thermodynamic equilibrium state (Fig. 12B). Such an exchange process was driven by the entropy of mixing. These results suggested the dynamic nature of the metal atom transfer in nanoclusters in solution. The dynamic nature of the ligand monolayers was also reported in a study by Salassa et al.²⁷³ The metal–ligand interface also plays an important role in controlling atom transfer and intercluster reactions of nanoclusters.

Fig. 12 (A) ESI MS of (a) [¹⁰⁷Ag₂₅(DMBT)₁₈]⁻, (b) [¹⁰⁹Ag₂₅(DMBT)₁₈]⁻, and (c) isotopically mixed [Ag₂₅(DMBT)₁₈]⁻ nanocluster with ¹⁰⁷Ag : ¹⁰⁹Ag ratio of 1 : 1. (B) Kinetics of isotopic Ag atom exchange in [Ag₂₉(BDT)₁₂(TPP)₄]³⁻ nanocluster. Adapted from ref. 272. Copyright 2019 American Association for the Advancement of Science.

3.6. Interparticle reactions: reactions with higher dimension materials

In 2014, Ghosh et al. made the first attempt to study the chemical interactions between atomically precise nanoclusters with other nanomaterials.²⁷⁴ In this study, graphene reacted with Au₂₅(PET)₁₈, resulting in the formation of a larger nanocluster, Au₁₃₅(PET)₅₇. The conversion of Au₂₅(PET)₁₈ nanocluster was monitored using time-dependent matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) (Fig. 13A). As the reaction progressed, the nanocluster peak at m/z 7391 due to Au₂₅(PET)₁₈ gradually disappeared with the simultaneous evolution of the peak at m/z 34.4 kDa corresponding to Au₁₃₅(PET)₅₇. This conversion process is driven by an overall energy gain of the system due to the entrapment of smaller nanoclusters at the local valleys of the graphenic surface resulting in the reduction in surface curvature and, finally, leading to coalescence. This study gave insights into the utility of surface as a reactive substrate for the chemical transformations of ligand-protected metal nanoclusters.

Fig. 13 (A) MALDI MS study for the conversion of Au₂₅(PET)₁₈ to Au₁₃₅(PET)₅₇ entrapped on the on the graphene surface upon reaction with graphene. (B) HRTEM images and corresponding schematic representation of pure Te NW upon reaction with Ag₃₂SG₁₉ and upon further heating yielding Ag nodule-decorated Te NW and dumbbell-shaped Ag–Te–Ag NWs, respectively. Adapted from ref. 274 and 275 for A and B, respectively. Copyright 2014 American Chemical Society.

Unique reactivity of water-soluble Au₃₂SG₁₉ nanocluster to Te nanowires (NWs) was reported by Som et al.²⁷⁵ This reaction results in the formation Ag–Te hybrid NWs with the growth of nodule-shaped Ag NPs on the NW surface (Fig. 13B). Structural analysis of the modified Te NWs using HRTEM-EDS and XRD confirmed the presence of Ag as nodules with Te NW retaining its inherent (001) hexagonal structure. Furthermore, on heating the Ag-decorated Te NWs, the morphology evolves into nano dumbbell-shaped Ag–Te–Ag NWs with Ag NPs specifically located at the tips (Fig. 13B). The ultrasmall size of the Ag₃₂SG₁₉ nanocluster provides an increased surface free energy, thereby inducing a tendency of intercluster coalescence, resulting in bigger particles.

Ligand-protected metal nanoclusters make excellent building blocks to create self-assembled hierarchical frameworks.^192,276,277 An interesting phenomenon of self-assembly arises in Te NWs when the surface is modified with Ag₄₄(p-MBA)₃₀ nanocluster. The p-MBA ligand shells initiate a H-bonding interaction among themselves, leading to the formation of a bilayer assembly of NWs oriented at an angle of 81° w.r.t. each other (Fig. 14A).²⁷⁸ Nonappa et al. reported the unique ability of p-MBA-protected gold nanoclusters to undergo intercluster H-bonding resulting in monolayer-thick 2D nanosheets and spherical capsids.^276,279 Som et al. showed that the Na₄Ag₄₄-pMBA₃₀ nanoclusters can be self-assembled into a large-area freestanding elastic membrane via entrapping them in a transient solvent layer at the air–water interface.²⁸⁰ The patchy distribution of ligands around the metal core facilitates symmetry breaking and eventually directs a preferential interlayer H-bonding between the carboxylic acid groups of the p-MBA ligands.^276,277,279 Chakraborty et al. showed hydrogen bonding-induced chemical interaction between the Ag₄₄(p-MBA)₃₀ nanocluster and a plasmonic gold nanorod (GNR), leading to the encapsulation of the latter (Fig. 14B), here denoted as GNR@Ag₄₄.²⁸¹ The nanocage-like hybrid material was found to have an octahedral morphology as studied using a series of highly sophisticated microscopes, like transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), and 3D reconstruction using electron tomography. The anisotropic growth was credited to the preferential anchoring of the nanoclusters to Au 〈110〉 over Au 〈100〉 facet of GNR@p-MBA. Ag₂₅(DMBT)₁₈ nanocluster-mediated site-selective etching of anisotropic planar gold nanotriangles (NTs) was reported.²⁸² Due to differential surface energies, the Au NTs interacting with the nanocluster underwent metallic etching at the edges and doping at the tips while the core remained unaltered. Roy et al. reported a polydispersed CuO NP and Au₂₅(PET)₁₈ reaction-mediated formation of spherical-shaped nanoaggregates of Cu-doped-Au-nanoclusters (Fig. 14C).²⁸³ The reaction involved an NP–NC atom transfer reaction route. Rival et al. recently demonstrated that the thiolated azobenzene-protected Au₂₅ nanoclusters form reversible assemblies triggered by light.²⁸⁴ With the right choice of nanocluster system, such hybrid nanomaterials having extraordinary stability at room temperature can significantly improve the limit of detection of nanocluster-based sensors.

Fig. 14 (A) Composite Ag₄₄@Te NW bilayer oriented at an 81° angle w.r.t to each other and (B) its corresponding packing driven by the intercluster H-bonding. (C) Schematic representation and HRTEM micrographs illustrating the assembly of Ag₄₄ on the GNR. Adapted from ref. A,²⁷⁸ B,²⁸¹ and C.²⁸³ Copyright 2014 and 2018, John Wiley and Sons. Copyright 2023, The Royal Society of Chemistry.

Bose et al. extended the interparticle chemistry to the reactions involving isotropic Ag NPs.²⁸⁵ A spontaneous reaction between Au₂₅(PET)₁₈ nanocluster and polydispersed Ag NPs with a core diameter of 4.4 ± 2.3 nm, protected with 2-phenylethanethiol (PET), resulted in monodispersed alloy NPs with a core diameter of 3.4 ± 1.2 nm under ambient conditions. The resulting NPs also underwent a spontaneous self-assembly to form a 2D superlattice which was analysed using HRTEM. Using STEM-EDS analysis, the reacted NPs were found to be Ag–Au alloy NPs. A 3D reconstruction of the 2D assembly using electron tomography further revealed that the assembly was composed of reacted alloy NPs arranged in a hexagonal close-packed (hcp) lattice with an interparticle distance of ∼4.5 nm (Fig. 15A). The mechanism involved an interparticle atomic exchange (metal–ligand species), and the metal–ligand interface was found to be crucial in controlling the reaction. Systematic analysis using time-dependent ESI MS showed that the reaction proceeds through a transient Au_25−xAg_x(PET)₁₈ (x = 1, 2, 3,…,) species along with other alloy nanocluster intermediates. Similar reactions were performed for Au₂₅(PET)₁₈ nanocluster with differently sized Ag@PET NPs, in which the interparticle reactivity was enhanced upon decreasing the size of NPs. The nanocluster–NP reactions can thereby open up an entirely new way of generating alloy NPs in the solution phase with better control over the NP size distribution.

Fig. 15 (A) TEM micrograph, its corresponding 3D reconstruction, and inverse fast Fourier transform image of 2D superlattice assembly (hcp) of the Au–Ag alloy NP resulting from the NP–nanocluster reaction. (B)The number of Ag-doped Au nanocluster versus time plot for the nanocluster–Ag surface reaction and the schematic representation metallic exchange at the metal–ligand interface on the bulk surface. Reproduced with permission A²⁸⁵ and B.²⁸⁸ Copyright 2020 Royal Society of Chemistry. Copyright 2008 American Chemical Society.

As we move from noble metal nanosystems to their bulk counterparts, chemical properties are altered due to changes in energy levels. Baksi et al. explored the glucose-mediated extraction of Ag from bulk silver surfaces otherwise considered inert.²⁸⁶ Later, Nag et al. reported the solution phase synthesis of Ag nanoclusters from bulk metallic silver in the presence of carbohydrate and glutathione followed by chemical reduction.²⁸⁷ Kazan et al. utilized the bulk thiolated silver surface to understand intercluster reaction as an interfacial phenomenon of metal–ligand exchange.²⁸⁸ The study involved Au₂₅(PET)₁₈ and Au₃₈(PET)₂₄ nanoclusters, and pure silver foil as model systems. MALDI MS and XPS characterization techniques were used to study the effect of reaction on the nanocluster and foil, respectively. Upon time-dependent monitoring of the Au₂₅(PET)₁₈ and PET-monolayered Ag foil, Au_25−xAg_x(PET)₁₈ (x = 1–4) appeared just after 2 min and a higher Ag-doping was detected for a longer reaction time. With the neat Ag foil, the reaction was found to be slow, as Au₂₄Ag(PET)₁₈ appeared only after 3 h. Hence, the kinetics of pre-adsorbed and neat silver foils are very different. The average number of doped Ag plotted as a function of time shows that the substitution follows a 2-phase kinetics (initially fast followed by slower exchange) and a sigmoidal trend (initially delayed followed by faster exchange) in preadsorbed and neat foils, respectively. This sigmoidal kinetics can be related to the autocatalytic reactions where the starting 3 h is an induction period for the thiolate, here acting as a catalyst, to deposit on the surface. As the time progressed, thiolate deposition happened, and this is reflected with an increased reaction rate. The mechanism behind the 2-phase kinetics in an atomic exchange reaction was explained using the scheme in which thiolated-Ag on the surface exchanges faster in the early stages of the reaction (Fig. 15B). At the later stages of the reaction, kinetics become slower due to less availability of exchangeable sites on the Ag foil surface. XPS measurements of the reacted Ag foil surface showed the presence of the metallic Au. Similar trends were observed when the Au₃₈(PET)₁₈ nanocluster reacts with both the pre-adsorbed and neat Ag surfaces. XPS measurements of the reacted pre-adsorbed thiolated and neat foils also confirmed the presence of metallic Au. The Au₂₅(PET)₁₈ and Au₃₈(PET)₁₈ reaction was also explored with pre-adsorbed and free surface of other metals like Cd and Cu. This study concluded that for a feasible Au doping in the Ag nanocluster, thiol plays a key role. Recently, Chakraborty et al. reported the dynamics of isotopic exchange reactions of isotopically pure Ag nanocluster with different dimensions of metallic Ag, like nanoclusters, plasmonic NPs, and bulk.²⁸⁹ Isotopically pure ¹⁰⁷Ag₂₅(DMBT)₁₈ and ¹⁰⁸Ag₂₅(DMBT)₁₈ was reacted with different sizes of Ag@DMBT NPs (naturally abundant Ag). With the increase in the NP size, the rate of atomic exchange was reduced. The exchange rate further decreased when the nanocluster was reacted with bulk Ag samples, such as foil and micron-sized powder. The kinetics of isotopic exchange, i.e., reaction timescales, was analyzed by fitting the reactant concentration as a function of time to a reaction model. Under similar reaction conditions, the reaction timescale was longer for the nanocluster-NP reactions compared with intercluster reactions. This suggests that suchreactions can be controlled by careful engineering the reacting nanostructures.

4. Insights from nanocluster chemistry

From the results presented, we list below the factors determining the feasibility of reactions of nanoclusters.

(1) The metal–ligand interface primarily controls the atomic exchange reactions and their feasibility.

(2) The geometry and stereochemistry of the protecting groups play crucial roles in the intercluster and interparticle reactions.

(3) Thermodynamics drives the interparticle and isotopic exchange reactions.

(4) Entropy of mixing drives the isotopic metal exchange in intercluster reactions.

(5) Nanoclusters exhibit redox-like reactions triggered by the difference in oxidation states of the metal atoms present in the core and staple.

(6) The core–shell geometry of a nanocluster influences the nanocluster–analyte reaction primarily via weak interactions, such as metallophilic and supramolecular interactions.

(7) Supramolecular interactions like C–H⋯π, π⋯π, van der Waals, hydrogen bonding, and electrostatic interactions guide the formation of the nanocluster assemblies.

(8) Reactive intermediates, such as adducts and fragments, vary depending on the reacting species.

(9) LEIST, a method to prepare new nanoclusters, is a ligand-controlled phenomenon. The rate of ligand exchange depends also on the electron-donating or withdrawing nature of the ligand.

(10) Chemical reactions occur between atomically precise nanoclusters and a range of systems such as ions, clusters, NPs, and bulk metals, and therefore we may suggest that nanoclusters are reactive to the whole range of chemical systems.

(11) The chemistry is highly sensitive to reaction conditions such as concentration, time, and temperature, and is expected to be influenced by other factors such as the medium, ionic strength, etc.

5. Conclusions and future perspectives

Nanoclusters, with their inherent molecule-like properties, show a wide range of chemical reactions with counterparts such as metal ions, clusters, NPs and bulk metals. These chemical reactions yield well-defined alloys which may be nanoclusters, NPs or bulk materials, depending on the systems involved. Such reactions also lead to ligand exchange. The processes yield supramolecular interactions forming assemblies and superstructures. Thermodynamics and kinetics govern the chemistry, and the underlying processes can be modelled with greater accuracy. The chemistry is sensitive to various process conditions such as concentration, temperature, solvent, etc., as typical of molecular events. The products formed and their kinetics and dynamics confirm their molecular nature. In the coming years, such cluster chemistry of atomically precise metal nanoclusters will be explored with oxides, sulfides, and others in the form of clusters, NPs, and bulk materials, yielding new materials. Similar science will be possible between materials of different size regimes of oxides and sulfides themselves, expanding the diversity of the area. The science presented will be greatly influenced by the experimental and computational methodologies used, which can reveal the intricate details of the processes involved. The science at this stage has provided us with many insights into the phenomena at the nanoscale as revealed by the fast isotopic exchange between NPs. Applications of such science are still at infancy.

Author contributions

All authors contributed equally.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the Department of Science and Technology (DST), Government of India, and Centre of Excellence program of the Indian Institute of Technology Madras, on the theme of Molecular Materials and Functions, under the Institutions of Eminence of Ministry of Education, India, for supporting our research.

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Footnotes

† Present address: School of Chemistry, Indian Institute of Science Education and Research Thiruvananthapuram, Vithura, Thiruvananthapuram-695551, Kerala, India.

‡ Present address: Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, and Institute of Physical Chemistry, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.

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