Molecular reactivity of thiolate-protected noble metal nanoclusters: synthesis, self-assembly, and applications

Abstract

Thiolate-protected noble metal (e.g., Au and Ag) nanoclusters (NCs) are ultra-small particles with a core size of less than 3 nm. Due to the strong quantum confinement effects and diverse atomic packing modes in this ultra-small size regime, noble metal NCs exhibit numerous molecule-like optical, magnetic, and electronic properties, making them an emerging family of "metallic molecules". Based on such molecule-like structures and properties, an individual noble metal NC behaves as a molecular entity in many chemical reactions, and exhibits structurally sensitive molecular reactivity to various ions, molecules, and other metal NCs. Although this molecular reactivity determines the application of NCs in various fields such as sensors, biomedicine, and catalysis, there is still a lack of systematic summary of the molecular interaction/reaction fundamentals of noble metal NCs at the molecular and atomic levels in the current literature. Here, we discuss the latest progress in understanding and exploiting the molecular interactions/reactions of noble metal NCs in their synthesis, self-assembly and application scenarios, based on the typical M(0)@M(i)-SR core-shell structure scheme, where M and SR are the metal atom and thiolate ligand, respectively. In particular, the continuous development of synthesis and characterization techniques has enabled noble metal NCs to be produced with molecular purity and atomically precise structural resolution. Such molecular purity and atomically precise structure, coupled with the great help of theoretical calculations, have revealed the active sites in various structural hierarchies of noble metal NCs (e.g., M(0) core, M-S interface, and SR ligand) for their molecular interactions/reactions. The anatomy of such molecular interactions/reactions of noble metal NCs in synthesis, self-assembly, and applications (e.g., sensors, biomedicine, and catalysis) constitutes another center of our discussion. The basis and practicality of the molecular interactions/reactions of noble metal NCs exemplified in this Review may increase the acceptance of metal NCs in various fields.

Fig. 1. Schematic illustration of the active sites of noble metal NCs for their interactions/reactions with ions, molecules, and other NCs. The crystal structure of [Au₂₅(SR)₁₈]⁻ (SR = thiolate ligand) was redrawn as a demonstrative example based on the reported crystallography data, in which only one SR–[Au(i)–SR]₂ motif is shown for clarity.

Fig. 2. Schematic illustration of adsorption and activation of (a) –CO, (b) O₂, and (c) solvated H⁺ on the Au₁₃ core of [Au₂₅(SR)₁₈]⁻ (a, b) or Au₁₂Pt core of [Au₂₄Pt(SR)₁₈]²⁻ (c). SR in (a) is –S-C₂H₄Ph (colour code: pink, core Au(0); blue, motif Au(i); golden, S; grey, C). SR in (b) is –S-CH₃; two SR–[Au(i)–SR]₂ motifs are removed for co-adsorption of O₂ and CO (colour code: golden, Au; yellow, S; grey, C; light grey, H; red, O). The solvent molecules in (c) are tetrahydrofuran (THF) (colour code: golden, Au; red, Pt; yellow, S; green, H; light grey, C). (a) is reproduced with permission from ref. 91. Copyright 2010, Elsevier. (b) is reproduced with permission from ref. 43. Copyright 2010, Springer Nature. (c) is reproduced with permission from ref. 97. Copyright 2017, the Authors, published by Springer Nature.

Fig. 3. Schematic illustration of (a) inter-cluster aurophilic interaction, (b) ligand exchange reaction, and (c) surface motif exchange reaction based on the Au–S interface of Au₂₅(SR)₁₈ (a), [Au₂₅(SR)₁₈]⁻ (b), and [Au₂₃(SR)₁₆]⁻ (c). (a) shows the formation of a 1D assembly by Au₂₅(SR)₁₈ based on the inter-cluster aurophilic interaction (colour code: golden, Au; red, S); reproduced with permission from ref. 106. Copyright 2014, American Chemical Society. (b) illustrates the preferential ligand exchange site on [Au₂₅(S-C₂H₄Ph)₁₈]⁻ by HS-Ph-p-Br based on its crystal structure (top panel) and solvent-accessibility surface model (bottom panel), where blue arrows indicate the solvent-exposed Au and red arrows indicate the preferential ligand exchange site (colour code: orange, Au; yellow, S; grey, C; red, Br); reproduced with permission from ref. 107. Copyright 2014, American Chemical Society. (c) shows surface motif exchange induced size conversion from [Au₂₃(SR)₁₆]⁻ to [Au₂₅(SR)₁₈]⁻; the glowing belts #1–#3 indicate SR–[Au(i)–SR]₂ motifs formed in the size conversion process; reproduced with permission from ref. 61. Copyright 2018, the Authors, published by Springer Nature.

Fig. 4. Schematic illustration of (a) 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) conjugation reaction, (b) ion-pairing reaction, and (c) CH–π interaction occurring on the SR ligands of metal NCs. (a) illustrates the EDC conjugation between folic acid (FA) and Au₂₂(SG)₁₈ (SG = glutathione ligand), where the –NH₂ groups of SG ligands are protected by benzyl chloroformate (CBz) prior to the EDC reaction; reproduced with permission from ref. 122. Copyright 2017, Wiley-VCH. (b) illustrates the production of amphiphilic Au₂₅(MHA)₁₈@xCTA NCs (x = 6–9) by the phase-transfer-driven ion-pairing reaction, where MHA denotes 6-mercaptohexanoic acid and CTA denotes cetyltrimethylammonium; reproduced with permission from ref. 129. Copyright 2015, American Chemical Society. (c) exhibits inter-cluster CH–π interactions (top panel) and the “herringbone pattern” of –S-Nap ligands in the supercrystals of Au₁₀₃S₂(S-Nap)₄₁ NCs, where HS-Nap denotes 2-naphthalenethiol (colour code: light blue, motif Au(i); yellow, S; light grey, H; blue/magenta, C); reproduced with permission from ref. 55. Copyright 2017, American Chemical Society.

Fig. 5. (a) Schematic illustration (top panel) and UV-vis absorption spectra (bottom panel) of size growth from [Au₂₅(SR)₁₈]⁻ to [Au₄₄(SR)₂₆]²⁻via the CO-reduction method, where magenta arrows indicate the characteristic absorption of [Au₂₅(SR)₁₈]⁻ and [Au₄₄(SR)₂₆]²⁻ (colour code: golden, Au; purple, S). (b) ESI-MS spectra of [Au₂₅(SR)₁₈CO]⁻ adducts identified in the [Au₂₅(SR)₁₈]⁻ solution saturated by CO. (a) and (b) are reproduced with permission from ref. 49. Copyright 2017, the Authors, published by Springer Nature. (c) Schematic illustration of the formation of Au₃₈(SR)₂₄ by the fusion of two Au₂₅(SR)₁₈ NCs. (d) Time-dependent UV-vis absorption and the corresponding absorption-derivative spectra of the size growth of Au₃₈(SR)₂₄ by the fusion of two Au₂₅(SR)₁₈ NCs. (c) and (d) are reproduced with permission from ref. 148. Copyright 2018, American Chemical Society.

Fig. 6. (a) Schematic illustration of the formation of Au₂₅Ag₂(SR)₁₈ by adding two additional Ag atoms to [Au₂₅(SR)₁₈]⁻; reproduced with permission from ref. 156. Copyright 2015, American Chemical Society. (b) ESI-MS spectra of Au_25−xM_x(SR)₁₈ formed by reacting [Au₂₅(SR)₁₈]⁻ with the corresponding M(i)–SR/M(ii)–SR complexes (M = Ni, Pd, Pt, Cu, Ag, Cd, and Hg); reproduced with permission from ref. 58. Copyright 2015, American Chemical Society. (c) ESI-MS spectra of [Ag₄₄(p-MBA)₃₀]⁴⁻ reacted with Au(i)-(p-MBA) at different feeding ratios of Ag₄₄ : Au(i); L denotes the protecting ligand; reproduced with permission from ref. 60. Copyright 2017, the Authors, published by Springer Nature. (d) HPLC spectra of Au_38−xAg_x(SR)₂₄ formed by cluster-metal complex reaction (CMCR) and co-reduction of metal ions (CRMI) at different reaction times; reproduced with permission from ref. 59. Copyright 2019, American Chemical Society.

Fig. 7. (a) Schematic illustration of the dimerization of [Au₂₅(SR)₁₈]⁻ through bridging Ag atoms to form Ag₂Au₅₀(SR)₃₆ (colour code: green, core Au(0); cyan, motif Au(i); yellow, S; red, Ag); reproduced with permission from ref. 187. Copyright 2020, Wiley-VCH. (b) Schematic illustration of the formation of nanoribbons by inter-cluster aurophilic interactions between Au NCs: structural anatomy of [Au₂₅(SR)₁₈]⁻ (i), smaller-sized Au NCs formed by surface rearrangement of [Au₂₅(SR)₁₈]⁻ (ii) and their self-assembly into nanoribbons (iii) (colour code: golden, core Au(0); black, motif Au(i); light grey, S); reproduced with permission from ref. 108. Copyright 2019, Wiley-VCH. (c) Structural model of partially protonated Au₁₀₂(p-MBA)₄₄ (i); TEM image (ii), electron tomographic model (iii), and cartoon structural model (iv) of spherical capsids formed by Au₁₀₂(p-MBA)₄₄; TEM image of hexagonal close packed (HCP) layered architectures formed by Au₁₀₂(p-MBA)₄₄ (v); the arrows in (i) indicate the orientations of p-MBA ligands; the insets of (v) are the high resolution TEM image (top panel) and packing model (bottom panel) of HCP layered architectures; reproduced with permission from ref. 128 and 189. Copyright 2016 and 2017, respectively, Wiley-VCH.

Fig. 8. (a) Packing structure, (b) H-bonds, and (c) mechanical response of protonated [Ag₄₄(p-MBA)₃₀]⁴⁻ in its triclinic supercrystals; the top panel in (b) illustrates H-bonds in a typical double-bundle of p-MBA ligands, while the bottom panel exhibits H-bonds within (horizontal direction) and between (vertical direction) the crystalline layers; V and V₀ in (c) are the volume and initial volume of supercrystals, respectively; (a)–(c) are reproduced with permission from ref. 190. Copyright 2014, Springer Nature. (d) Formation diagram of the octahedral and concave-octahedral supercrystals by deprotonated [Ag₄₄(p-MBA)₃₀]⁴⁻ in a range of Cs⁺ concentrations and dimethyl sulfoxide (DMSO) fractions in the crystallization solution; reproduced with permission from ref. 191. Copyright 2015, Wiley-VCH. (e) Schematic illustration of intra- (blue dashed lines) and inter-cluster (yellow dashed lines) CH–π interactions between two [Au₅₂Cu₇₂(S-Ph-p-CH₃)₅₅]⁺ NCs approaching in an edge-to-edge fashion in their supercrystals, where a triangular mosaic pattern of –S-Ph-p-CH₃ ligands can be identified (colour code: pink/golden, Au/Cu; yellow, S; grey/magenta/orange, C; light grey, H); reproduced with permission from ref. 17. Copyright 2020, the Authors, published by Springer Nature.

Fig. 9. (a) UV-vis absorption (dotted lines) and emission (solid lines, λ_ex = 350 nm) spectra of Ag₁₂(SG)₁₀ in the presence (black lines) and absence (blue lines) of Hg²⁺; insets are digital photos of Ag₁₂(SG)₁₀ in the presence (#2) and absence (#1) of Hg²⁺ under UV light; (b) schematic illustration of the metallophilic interaction between Hg²⁺ and Ag₁₂(SG)₁₀ that can quench cluster luminescence; (c) matrix-assisted laser desorption/ionization (MALDI) mass spectra of Ag₁₂(SG)₁₀ in the presence (black line) and absence (blue line) of Hg²⁺; (d) digital photos of Ag₁₂(SG)₁₀ in the presence of varied metal cations under UV light (365 nm); (a)–(d) are reproduced with permission from ref. 47. Copyright 2012, Royal Society of Chemistry. (e) Schematic illustration of the detection mechanism of cysteine (Cys) by Ag₁₆(SG)₉ NCs; (f) relative emission intensity and digital photos of Ag₁₆(SG)₉ NCs in the presence of 20 natural amino acids; reproduced with permission from ref. 216. Copyright 2012, American Chemical Society.

Fig. 10. Proposed catalytic mechanism of styrene oxidation by [Au₂₅(SR)₁₈]⁻ in the presence of varied oxidants: BuOOH, O₂, and a mixture of them. Colour code: grey, core Au(0); light grey, motif Au(i); SR ligands are omitted for clarity. Reproduced with permission from ref. 244. Copyright 2010, Wiley-VCH.

Qiaofeng Yao

Zhennan Wu

Zhihe Liu

Yingzheng Lin

Xun Yuan

Jianping Xie