The Origin of the Photoluminescence Enhancement of Gold‐Doped Silver Nanoclusters: The Importance of Relativistic Effects and Heteronuclear Gold–Silver Bonds

The weak photoluminescence of silver nanoclusters prevents their broad application as luminescent nanomaterials. Recent experiments, however, have shown that gold doping can significantly enhance the photoluminescence intensity of Ag₂₉ nanoclusters but the molecular and physical origins of this effect remain unknown. Therefore, we have computationally explored the geometric and electronic structures of Ag₂₉ and gold‐doped Ag_29?xAu_x (x=1–5) nanoclusters in the S₀ and S₁ states. We found that 1) relativistic effects that are mainly due to the Au atoms play an important role in enhancing the fluorescence intensity, especially for highly doped Ag₂₆Au₃, Ag₂₅Au₄, and Ag₂₄Au₅, and that 2) heteronuclear Au?Ag bonds can increase the stability and regulate the fluorescence intensity of isomers of these gold‐doped nanoclusters. These novel findings could help design doped silver nanoclusters with excellent luminescence properties.     

Homoleptic Alkynyl‐Protected Gold Nanoclusters: Au44(PhC≡C)28 and Au36(PhC≡C)24

For the first time total structure determination of homoleptic alkynyl‐protected gold nanoclusters is reported. The nanoclusters are synthesized by direct reduction of PhC≡CAu, to give Au₄₄(PhC≡C)₂₈ and Au₃₆(PhC≡C)₂₄. The Au₄₄ and Au₃₆ nanoclusters have fcc‐type Au₃₆ and Au₂₈ kernels, respectively, as well as surrounding PhC≡C‐Au‐C₂(Ph)Au‐C≡CPh dimeric “staples” and simple PhC≡C bridges. The structures of Au₄₄(PhC≡C)₂₈ and Au₃₆(PhC≡C)₂₄ are similar to Au₄₄(SR)₂₈ and Au₃₆(SR)₂₄, but the UV/Vis spectra are different. The protecting ligands influence the electronic structures of nanoclusters significantly. The synthesis of these two alkynyl‐protected gold nanoclusters indicates that a series of gold nanoclusters in the general formula Au_x (RC≡C)_y as counterparts to Au_x (SR)_y can be expected.     

Quasi‐Dual‐Packed‐Kerneled Au49(2,4‐DMBT)27 Nanoclusters and the Influence of Kernel Packing on the Electrochemical Gap

Although face‐centered cubic (fcc), body‐centered cubic (bcc), hexagonal close‐packed (hcp), and other structured gold nanoclusters have been reported, it was unclear whether gold nanoclusters with mix‐packed (fcc and non‐fcc) kernels exist, and the correlation between kernel packing and the properties of gold nanoclusters is unknown. A Au₄₉(2,4‐DMBT)₂₇ nanocluster with a shell electron count of 22 has now been been synthesized and structurally resolved by single‐crystal X‐ray crystallography, which revealed that Au₄₉(2,4‐DMBT)₂₇ contains a unique Au₃₄ kernel consisting of one quasi‐fcc‐structured Au₂₁ and one non‐fcc‐structured Au₁₃ unit (where 2,4‐DMBTH=2,4‐dimethylbenzenethiol). Further experiments revealed that the kernel packing greatly influences the electrochemical gap (EG) and the fcc structure has a larger EG than the investigated non‐fcc structure.     

Bimetallic Au2Cu6 Nanoclusters: Strong Luminescence Induced by the Aggregation of Copper(I) Complexes with Gold(0) Species

The concept of aggregation‐induced emission (AIE) has been exploited to render non‐luminescent Cu^ISR complexes strongly luminescent. The Cu^ISR complexes underwent controlled aggregation with Au⁰. Unlike previous AIE methods, our strategy does not require insoluble solutions or cations. X‐ray crystallography validated the structure of this highly fluorescent nanocluster: Six thiolated Cu atoms are aggregated by two Au atoms (Au₂Cu₆ nanoclusters). The quantum yield of this nanocluster is 11.7 %. DFT calculations imply that the fluorescence originates from ligand (aryl groups on the phosphine) to metal (Cu^I) charge transfer (LMCT). Furthermore, the aggregation is affected by the restriction of intramolecular rotation (RIR), and the high rigidity of the outer ligands enhances the fluorescence of the Au₂Cu₆ nanoclusters. This study thus presents a novel strategy for enhancing the luminescence of metal nanoclusters (by the aggregation of active metal complexes with inert metal atoms), and also provides fundamental insights into the controllable synthesis of highly luminescent metal nanoclusters.     

One‐Pot Synthesis of Phenylmethanethiolate‐Protected Au20(SR)16 and Au24(SR)20 Nanoclusters and Insight into the Kinetic Control

We report two synthetic routes for concurrent formation of phenylmethanethiolate (‐SCH₂Ph)‐protected Au₂₀(SR)₁₆ and Au₂₄(SR)₂₄ nanoclusters in one‐pot by kinetic control. Unlike the previously reported methods for thiolate‐protected gold nanoclusters, which typically involve rapid reduction of the gold precursor by excess NaBH₄ and subsequent size focusing into atomically monodisperse clusters of a specific size, the present work reveals some insight into the kinetic control in gold–thiolate cluster synthesis. We demonstrate that the synthesis of ‐SCH₂Ph‐protected Au₂₀ and Au₂₄ nanoclusters can be obtained through two different, kinetically controlled methods. Specifically, route 1 employs slow addition of a relatively large amount of NaBH₄ under slow stirring of the reaction mixture, while route 2 employs rapid addition of a small amount of NaBH₄ under rapid stirring of the reaction mixture. At first glance, these two methods apparently possess quite different reaction kinetics, but interestingly they give rise to exactly the same product (i.e., the coproduction of Au₂₀(SCH₂Ph)₁₆ and Au₂₄(SCH₂Ph)₂₀ clusters). Our results explicitly demonstrate the complex interplay between the kinetic factors that include the addition speed and amount of NaBH₄ solution as well as the stirring speed of the reaction mixture. Such insight is important for devising synthetic routes for different sized nanoclusters. We also compared the photoluminescence and electrochemical properties of PhCH₂S‐protected Au₂₀ and Au₂₄ nanoclusters with the PhC₂H₄S‐protected counterparts. A surprising 2.5 times photoluminescence enhancement was observed for the PhCH₂S‐capped nanoclusters when compared to the PhC₂H₄S‐capped analogues, thereby indicating a drastic effect of the ligand that is merely one carbon shorter.     

Plasmonic Enhancement of Two-Photon Excited Luminescence of Gold Nanoclusters

Plasmonic-enhanced luminescence of single molecules enables imaging and detection of low quantities of fluorophores, down to individual molecules. In this work, we present two-photon excited luminescence of single gold nanoclusters, Au₁₈(SG)₁₄, in close proximity to bare gold nanorods (AuNRs). We observed 25-times enhanced emission of gold nanoclusters (AuNCs) in near infrared region, which was mainly attributed to the resonant excitation of localized surface plasmon resonance (LSPR) of AuNRs and spectral overlap of LSPR band with photoluminescence of AuNCs. This work is an initial step in application of combined nanoparticles: gold nanorods and ultrasmall nanoclusters in a wide range of multiphoton imaging and biosensing applications.     

Aurophilic Interactions in the Self‐Assembly of Gold Nanoclusters into Nanoribbons with Enhanced Luminescence

Aurophilic interactions (Au^I???Au^I) are crucial in directing the supramolecular self‐assembly of many gold(I) compounds; however, this intriguing chemistry has been rarely explored for the self‐assembly of nanoscale building blocks. Herein, we report on studies on aurophilic interactions in the structure‐directed self‐assembly of ultrasmall gold nanoparticles or nanoclusters (NCs, <2 nm) using [Au₂₅(SR)₁₈]^? (SR=thiolate ligand) as a model cluster. The self‐assembly of NCs is initiated by surface‐motif reconstruction of [Au₂₅(SR)₁₈]^? from short SR‐[Au^I‐SR]₂ units to long SR‐[Au^I‐SR]_x (x>2) staples accompanied by structure modification of the intrinsic Au₁₃ kernel. Such motif reconstruction increases the content of Au^I species in the protecting shell of Au NCs, providing the structural basis for directed aurophilic interactions, which promote the self‐assembly of Au NCs into well‐defined nanoribbons in solution. More interestingly, the compact structure and effective aurophilic interactions in the nanoribbons significantly enhance the luminescence intensity of Au NCs with an absolute quantum yield of 6.2 % at room temperature.     

Fluorescent Gold Nanoclusters with Interlocked Staples and a Fully Thiolate‐Bound Kernel

The structural features that render gold nanoclusters intrinsically fluorescent are currently not well understood. To address this issue, highly fluorescent gold nanoclusters have to be synthesized, and their structures must be determined. We herein report the synthesis of three fluorescent Au₂₄(SR)₂₀ nanoclusters (R=C₂H₄Ph, CH₂Ph, or CH₂C₆H₄^tBu). According to UV/Vis/NIR, differential pulse voltammetry (DPV), and X‐ray absorption fine structure (XAFS) analysis, these three nanoclusters adopt similar structures that feature a bi‐tetrahedral Au₈ kernel protected by four tetrameric Au₄(SR)₅ motifs. At least two structural features are responsible for the unusual fluorescence of the Au₂₄(SR)₂₀ nanoclusters: Two pairs of interlocked Au₄(SR)₅ staples reduce the vibration loss, and the interactions between the kernel and the thiolate motifs enhance electron transfer from the ligand to the kernel moiety through the Au?S bonds, thereby enhancing the fluorescence. This work provides some clarification of the structure–fluorescence relationship of such clusters.     

An Unprecedented Kernel Growth Mode and Layer‐Number‐Odevity‐Dependent Properties in Gold Nanoclusters

Kernel atoms of Au nanoclusters are packed layer‐by‐layer along the [001] direction with every full (001) monolayer composed of 8 Au atoms (Au₈ unit) in nanoclusters with formula of Au_8n+4(TBBT)_4n+8 (n is the number of Au₈ units; TBBTH=4‐tert‐butylbenzenelthiol). It is unclear whether the kernel atoms can be stacked in a defective‐layer way along the [001] direction during growth of the series of nanoclusters and how the kernel layer number affects properties. Now, a nanocluster is synthesized that is precisely characterized by mass spectrometry and single‐crystal X‐ray crystallography, revealing a layer stacking mode in which a half monolayer composed of 4 atoms (Au₄ unit) is stacked on the full monolayer along the [001] direction. The size and the odevity of the kernel layer number influence the properties (polarity, photoluminescence) of gold nanoclusters. The obtained nanocluster extends the previous formula from Au_8n+4(TBBT)_4n+8 to Au_4n+4(TBBT)_2n+8 (n is the number of Au₄ units).     

An Atomic‐Level Strategy for Unraveling Gold Nanocatalysis from the Perspective of Aun(SR)m Nanoclusters

An atomic‐level strategy is devised to gain insight into the origin of nanogold catalysis by using atomically monodisperse Au_n(SR)_m nanoclusters as well‐defined catalysts for styrene oxidation. The Au_n(SR)_m nanoclusters are emerging as a new class of gold nanocatalyst to overcome the polydispersity of conventional nanoparticle catalysts. The unique atom‐packing structure and electronic properties of Au_n(SR)_m nanoclusters (<2 nm) are rationalized to be responsible for their extraordinary catalytic activity observed in styrene oxidation. An interesting finding is that quantum size effects of Au_n(SR)_m nanoclusters, rather than the higher specific surface area, play a major role in gold‐catalyzed selective oxidation of styrene. For example, Au₂₅(SR)₁₈ nanoclusters (≈1 nm) are found to be particularly efficient in activating O₂, which is a key step in styrene oxidation, and hence, the ultrasmall Au₂₅ catalyst exhibits higher activity than do larger sizes. This atomic‐level strategy has allowed us to obtain an important insight into some fundamental aspects of nanogold catalysis in styrene oxidation. The ultrasmall yet robust Au_n(SR)_m nanoclusters are particularly promising for studying the mechanistic aspects of nanogold catalysis and for future design of better catalysts with high activity and selectivity for certain chemical processes.     

Chiral Gold Nanoclusters: Atomic Level Origins of Chirality

Chiral nanomaterials have received wide interest in many areas, but the exact origin of chirality at the atomic level remains elusive in many cases. With recent significant progress in atomically precise gold nanoclusters (e.g., thiolate‐protected Au_n (SR)_m ), several origins of chirality have been unveiled based upon atomic structures determined by using single‐crystal X‐ray crystallography. The reported chiral Au_n (SR)_m structures explicitly reveal a predominant origin of chirality that arises from the Au–S chiral patterns at the metal–ligand interface, as opposed to the chiral arrangement of metal atoms in the inner core (i.e. kernel). In addition, chirality can also be introduced by a chiral ligand, manifested in the circular dichroism response from metal‐based electronic transitions other than the ligand's own transition(s). Lastly, the chiral arrangement of carbon tails of the ligands has also been discovered in a very recent work on chiral Au₁₃₃(SR)₅₂ and Au₂₄₆(SR)₈₀ nanoclusters. Overall, the origins of chirality discovered in Au_n (SR)_m nanoclusters may provide models for the understanding of chirality origins in other types of nanomaterials and also constitute the basis for the development of various applications of chiral nanoparticles.     

The Structure–Property Correlations in the Isomerism of Au21(SR)15 Nanoclusters by Density Functional Theory Study

Isomerism of atomically precise noble metal nanoclusters provides an excellent platform to investigate the structure–property correlations of metal nanomaterials. In this study, we performed density functional theory (DFT) and time‐dependent (TD‐DFT) calculations on two Au₂₁(SR)₁₅ nanoclusters, one with a hexagonal closed packed core (denoted as Au₂₁ ^hcp ), and the other one with a face‐centered cubic core (denoted as Au₂₁ ^fcc ). The structural and electronic analysis on the typical Au–Au and Au–S bond distances, bond orders, composition of the frontier orbitals and the origin of optical absorptions shed light on the inherent correlations between these two clusters.     

Template‐Free Supracolloidal Self‐Assembly of Atomically Precise Gold Nanoclusters: From 2D Colloidal Crystals to Spherical Capsids

We report supracolloidal self‐assembly of atomically precise and strictly monodisperse gold nanoclusters involving p‐mercaptobenzoic acid ligands (Au₁₀₂‐pMBA₄₄) under aqueous conditions into hexagonally packed monolayer‐thick two‐dimensional facetted colloidal crystals (thickness 2.7 nm) and their bending to closed shells leading to spherical capsids (d ca. 200 nm), as controlled by solvent conditions. The 2D colloidal assembly is driven in template‐free manner by the spontaneous patchiness of the pMBA ligands around the Au₁₀₂‐pMBA₄₄ nanoclusters preferably towards equatorial plane, thus promoting inter‐nanocluster hydrogen bonds and high packing to planar sheets. More generally, the findings encourage to explore atomically precise nanoclusters towards highly controlled colloidal self‐assemblies.     

Intermolecular interactions of a size‐expanded guanine analogue with gold nanoclusters

The interactions between a size‐expanded Guanine analogue x‐Guanine (xG) and gold nanoclusters, Au_n (n = 2, 4, 6, and 8), were studied theoretically using density functional theory. Geometries of neutral complexes were optimized using the B3LYP functional with the 6‐31+G(d,p) basis set for xG and the LANL2DZ basis set for gold clusters. The binding modes, interaction strength, and the charge‐transfer properties of different Au_n‐xG complexes were investigated. Natural population analysis was performed for natural bond order charges. It was found that gold nanoclusters form stable complexes with xG and these binding results in a substantial amount of electronic charge being transferred from xG to the gold clusters. The vertical first ionization potential, electron affinity, Fermi Level, and the HOMO–LUMO gap of xG and its complexes with gold nanoclusters were also analyzed. © 2013 Wiley Periodicals, Inc.     

Quantum-sized gold nanoclusters: bridging the gap between organometallics and nanocrystals

This Concept article provides an elementary discussion of a special class of large‐sized gold compounds, so‐called Au nanoclusters, which lies in between traditional organogold compounds (e.g., few‐atom complexes, <1 nm) and face‐centered cubic (fcc) crystalline Au nanoparticles (typically >2 nm). The discussion is focused on the relationship between them, including the evolution from the Au???Au aurophilic interaction in Au^I complexes to the direct Au? Au bond in clusters, and the structural transformation from the fcc structure in nanocrystals to non‐fcc structures in nanoclusters. Thiolate‐protected Au_n(SR)_m nanoclusters are used as a paradigm system. Research on such nanoclusters has achieved considerable advances in recent years and is expected to flourish in the near future, which will bring about exciting progress in both fundamental scientific research and technological applications of nanoclusters of gold and other metals.     

Dual Catalysis of Gold Nanoclusters: Photocatalytic Cross-Dehydrogenative Coupling by Cooperation of Superatomic Core and Molecularly Modified Staples**

Thiolate-protected gold nanoclusters (AuNCs) have attracted significant attention as nano-catalysts, revealing a superatomic core and gold-thiolate staples as distinct structural units. Here, we demonstrate the unprecedented dual catalytic activity of thiolate-protected [Au₂₅(SR)₁₈]⁻ nanoclusters, involving both photosensitized ¹O₂ generation by the Au₁₃ superatomic core and catalytic carbon-carbon bond formation facilitated by Au₂(SR)₃ staples. This synergistic combination of two different catalytic units enables efficient cross-dehydrogenative coupling of terminal alkynes and tertiary aliphatic amines to afford propargylamines in high yields of up to 93 %. Mixed-ligand AuNCs bearing both thiolate and alkynyl ligands revealed the intermediacy of the alkynyl-exchanged AuNCs toward both photosensitization and C−C bond-forming catalytic cycles. Density functional theory calculations also supported the intermediacy of the alkynyl-exchanged AuNCs. Thus, the use of ligand-protected metal nanoclusters has enabled the development of an exceptional multifunctional catalyst, wherein distinct nanocluster components facilitate cooperative photo- and chemo-catalysis.     

Combined synthesis of interconvertible Au11Cd and Au26Cd5 for photocatalytic oxidations involving singlet oxygen

A novel Au₁₁Cd nanocluster was synthesized by developing a combined method and controlling the kinetics, and another Au₂₆Cd₅ nanocluster was also obtained after the conditions were changed in the same reaction, which could transfer to Au₁₁Cd in a two-way style. Both alloy nanoclusters can photocatalyze the production of singlet oxygen (¹O₂) and exhibit enhanced efficiencies in photocatalyzing two kinds of organic oxidations involving singlet oxygen compared with their non-alloyed mother nanoclusters, indicating that the Cd-doping might be an efficient way to enhance the photocatalysis performance of gold nanoclusters and metal nanoclusters are promising photocatalysts for organic oxidation involving singlet oxygen.     

The Structure and Optical Properties of the [Au18(SR)14] Nanocluster

Decreasing the core size is one of the best ways to study the evolution from Au^I complexes into Au nanoclusters. Toward this goal, we successfully synthesized the [Au₁₈(SC₆H₁₁)₁₄] nanocluster using the [Au₁₈(SG)₁₄] (SG=L ‐glutathione) nanocluster as the starting material to react with cyclohexylthiol, and determined the X‐ray structure of the cyclohexylthiol‐protected [Au₁₈(C₆H₁₁S)₁₄] nanocluster. The [Au₁₈(SR)₁₄] cluster has a Au₉ bi‐octahedral kernel (or inner core). This Au₉ inner core is built by two octahedral Au₆ cores sharing one triangular face. One transitional gold atom is found in the Au₉ core, which can also be considered as part of the Au₄(SR)₅ staple motif. These findings offer new insight in terms of understanding the evolution from [Au^I(SR)] complexes into Au nanoclusters.     

Controlled growth of molecularly pure Au25(SR)18 and Au38(SR)24 nanoclusters from the same polydispersed crude product

We report the controlled growth of Au25(SR)18 and Au38 (SR)24 (where R = CH2CH2Ph) nanoclusters of molecular purity via size-focusing from the same crude product that contains a distribution of nanoclusters. In this method, gold salt was first mixed with tetraoctylammonium bromide (TOAB), and then reacted with excess thiol to form Au(I)-SR polymers in THF (as opposed to toluene in previous work), followed by NaBH 4 reduction. The resultant crude product contains polydisperse nanoclusters and was then used as the common starting material for controlled growth of Au25(SR)18 and Au38(SR)24 , respectively. In Route I, Au25(SR)18 nanoclusters of molecular purify were produced from the crude product after 6 h aging at room temperature. In Route II, the crude product was isolated and further subjected to thermal thiol etching in a toluene solution containing excess thiol, and one obtained pure Au38(SR)24 nanoclusters, instead of Au25(SR)18 . This work not only provides a robust and simple method to prepare both Au25(SR)18 and Au38(SR)24 nanoclusters, but also reveals that these two nanoclusters require different environments for the size-focusing growth process.     

Luminescent Quantum Clusters of Gold in Bulk by Albumin‐Induced Core Etching of Nanoparticles: Metal Ion Sensing,Metal‐Enhanced Luminescence,and Biolabeling

The synthesis of a luminescent quantum cluster (QC) of gold with a quantum yield of ～4 % is reported. It was synthesized in gram quantities by the core etching of mercaptosuccinic acid protected gold nanoparticles by bovine serum albumin (BSA), abbreviated as Au_QC@BSA. The cluster was characterized and a core of Au₃₈ was assigned tentatively from mass spectrometric analysis. Luminescence of the QC is exploited as a “turn‐off” sensor for Cu²⁺ ions and a “turn‐on” sensor for glutathione detection. Metal‐enhanced luminescence (MEL) of this QC in the presence of silver nanoparticles is demonstrated and a ninefold maximum enhancement is seen. This is the first report of the observation of MEL from QCs. Folic acid conjugated Au_QC@BSA was found to be internalized to a significant extent by oral carcinoma KB cells through folic acid mediated endocytosis. The inherent luminescence of the internalized Au_QC@BSA was used in cell imaging.