Thiolate-protected Au Nanoclusters Research Articles

Directing Intrinsic Chirality in Gold Nanoclusters: Preferential Formation of Stable Enantiopure Clusters in High Yield and Experimentally Unveiling the "Super" Chirality of Au144.

Chiral gold nanoclusters offer significant potential for exploring chirality at a fundamental level and for exploiting their applications in sensing and catalysis. However, their widespread use is impeded by low yields in synthesis, tedious separation procedures of their enantiomeric forms, and limited thermal stability. In this study, we investigated the direct synthesis of enantiopure chiral nanoclusters using the chiral ligand 2-MeBuSH in the fabrication of Au25, Au38, and Au144 nanoclusters. Notably, this approach leads to the unexpected formation of intrinsically chiral clusters with high yields for chiral Au38 and Au144 nanoclusters. Experimental evaluation of chiral activity by circular dichroism (CD) spectroscopy corroborates previous theoretical calculations, highlighting the stronger CD signal exhibited by Au144 compared to Au38 or Au25. Furthermore, the formation of a single enantiomeric form is experimentally confirmed by comparing it with intrinsically chiral Au38(2-PET)24 (2-PET: 2-phenylethanethiol) and is supported theoretically for both Au38 and Au144. Moreover, the prepared chiral clusters show stability against diastereoisomerization, up to temperatures of 80 °C. Thus, our findings not only demonstrate the selective preparation of enantiopure, intrinsically chiral, and highly stable thiolate-protected Au nanoclusters through careful ligand design but also support the predicted "super" chirality in the Au144 cluster, encompassing hierarchical chirality in ligands, staple configuration, and core structure.

Photoluminescence of the Au38(SR)26 nanocluster comprises three radiative processes

Photoluminescence of ultrasmall, atomically precise gold nanoclusters constitutes an area of significant interest in recent years for both fundamental research and biological applications. However, the exploration of near-infrared photoluminescence of gold nanoclusters is still in its infancy due to the limitations of synthetic methods and characterization techniques. Herein, the photoluminescence properties of an Au38(PET)26 (PET = 2-phenylethanethiolate) nanocluster are investigated in detail. The Au38(PET)26 exhibits an emission peak at 865 nm, which is revealed to be a mix of fluorescence, thermally activated delayed fluorescence, and phosphorescence via the combined analyses of time-resolved and temperature-dependent photoluminescence measurements. The quantum yield of Au38(PET)26 is determined to be 1.8% at room temperature under ambient conditions, which increases to above 90% by suppressing the non-radiative relaxation pathway at a cryogenic temperature (80 K). Overall, the results of this work discover the coexistence of three radiative processes in thiolate-protected Au nanoclusters and will pave the way for understanding the intriguing photoluminescence properties of gold nanoclusters in future studies.

Elucidating the stabilities and properties of the thiolate-protected Au nanoclusters with detaching the staple motifs.

Thiolate-protected Au nanoclusters (AuNCs) have been widely studied in areas of catalysis, biosensors, and bioengineering. In real applications, e.g., catalytic reactions, the thiolate groups are normally partially detached. However, which of the thiolate groups are easily detached and how the detachment of the ligands affects the geometries and electronic structures of the Au nanoclusters have been rarely studied. In this work, we employed the density functional theory calculations as well as the molecular orbital analysis to explore the detachment effect of the ligands using nine thiolate-protected AuNCs as examples. Our results showed that there existed a nearly linear relationship between the averaged detachment energies and the numbers of Au atoms in the motifs. Detaching longer motifs normally required more energies owing to the stronger aurophilic effects. For detaching a full motif, based on the structure decomposition via the grand unified model, analysis on the inner Au core indicated that the change in Au-Au bond length was more sensitive for the inter-block compared to the intra-block. The detachment of the -SH fragment generally needs less energy and brings less structural deformations when compared to the removal of a full motif. Molecular orbital analysis showed that the relative energies of the HOMO orbitals were elevated, which led to the narrow down of the HOMO-LUMO gap. This work provides a primary description of the correlation of the ligands' detachment with the relative stabilities and structures of the AuNCs, which would be beneficial for establishing the structure-property relationship of AuNCs in real applications.

Molecular-Scale Ligand Effects in Small Gold–Thiolate Nanoclusters

Because of the small size and large surface area of thiolate-protected Au nanoclusters (NCs), the protecting ligands are expected to play a substantial role in modulating the structure and properties, particularly in the solution phase. However, little is known on how thiolate ligands explicitly modulate the structural properties of the NCs at atomic level, eventhough this information is critical for predicting the performance of Au NCs in application settings including as a catalyst interacting with small molecules and as a sensor interacting with biomolecular systems. Here, we report a combined experimental and theoretical study, using synchrotron X-ray spectroscopy and quantum mechanics/molecular mechanics simulations, that investigates how the protecting ligands impact the structure and properties of small Au18(SR)14 NCs. Two representative ligand types, smaller aliphatic cyclohexanethiolate and larger hydrophilic glutathione, are selected, and their structures are followed experimentallyin both solid and solution phases. It was found that cyclohexanethiolate ligands are significantly perturbed by toluene solvent molecules, resulting in structural changes that cause disorder on the surface of Au18(SR)14 NCs. In particular, large surface cavities in the ligand shell are created by interactions between toluene and cyclohexanethiolate. The appearance of these small molecule-accessible sites on the NC surface demonstrates the ability of Au NCs to act as a catalyst for organic phase reactions. In contrast, glutathione ligands encapsulate the Au NC core via intermolecular interactions, minimizing structural changes caused by interactions with water molecules. The much better protection from glutathione ligands imparts a rigidified surface and ligand structure, making the NCs desirable for biomedical applications due to the high stability and also offering a structural-based explanation for the enhanced photoluminescence often reported for glutathione-protected Au NCs.

Synthesis of Water-Soluble [Au25(SR)18]- Using a Stoichiometric Amount of NaBH4.

Determination of the stoichiometry of reactions is a pivotal step for any chemical reactions toward a desirable product, which has been successfully achieved in organic synthesis. Here, we present the first precise determination of the stoichiometry for the reactions toward gold nanoparticle formation in the sodium borohydride reduction method. Leveraging on the real-time mass spectrometry technique, we have determined a precise balanced reaction, 32/ x [Au(SR)] x + 8 e- = [Au25(SR)18]- + 7 [Au(SR)2]- (here SR denotes a thiolate ligand), toward a stoichiometric synthesis of water-soluble [Au25(SR)18]-, where 8 electrons (from reducing agents) are sufficient to react with every 32 Au atoms, leading to the formation of high-purity [Au25(SR)18]-. More interestingly, by real-time monitoring of the growth process of thiolate-protected Au nanoclusters, we have successfully identified an important yet missing byproduct, [Au(SR)2]-. This study not only provides a new method for Au nanocluster synthesis using only a stoichiometric amount of reducing agent in aqueous solutions (although the synthesis of organic-soluble Au nanoclusters might require a more delicate design of synthetic chemistry) but also promotes the mechanistic understandings of the Au nanocluster growth process.

Spontaneous Breaking and Remaking of the RS–Au–SR Staple in Self-assembled Ethylthiolate/Au(111) Interface

The stability of the self-assembled RS–Au–SR (R = CH2CH3)/Au(111) interface at room temperature has been investigated using scanning tunneling microscopy (STM) in conjunction with density functional theory (DFT) and MD calculations. The RS–Au–SR staple, also known as Au-adatom-dithiolate, assembles into staple rows along the [112̅] direction. STM imaging reveals that while the staple rows are able to maintain a static global structure, individual staples within the row are subjected to constant breaking and remaking of the Au–SR bond. The C2S–Au–SC2/Au(111) interface is under a dynamic equilibrium and it is far from rigid. DFT/MD calculations show that a transient RS–Au–Au–SR complex can be formed when a free Au atom is added to the RS–Au–SR staple. The relatively high reactivity of the RS–Au–SR staple at room temperature could explain the reactivity of thiolate-protected Au nanoclusters, such as their ability to participate in ligand exchange and intercluster reactions.

The structural isomerism in gold nanoclusters.

The isomerism in thiolate-protected gold (Au) nanoclusters is important for the understanding of structure-property correlations and the design of Au nanoclusters with specific structures and properties. Although recent studies have identified stereoisomerism, the understanding of structural isomerism is still lacking. Herein, we identified three distinct mechanisms of structural isomerism (i.e., core isomerism, staple isomerism, and complex isomerism) based on the crystallized isomers of thiolate-protected Au nanoclusters, and these mechanisms can be viewed as analogous to those of the structural isomerism in organic molecules (i.e., chain isomerism, point isomerism, and functional isomerism). Using the discovered core isomerism and staple isomerism, two Au28(SR)20 isomers are predicted and their synthesis feasibilities are illuminated. These new insights into the structural isomerism can facilitate rational design of new isomers of thiolate-protected Au nanoclusters and guide future experimental synthesis.

The Nucleation and Growth Mechanism of Thiolate-Protected Au Nanoclusters.

The understanding of the evolution mechanism of thiolate-protected Au nanoclusters from the homoleptic Au(I)-SR clusters to core-stacked ones is crucial for the synthesis of novel thiolated Au clusters. In this work, the global search for a series of "intermediate" Aum(SR)n clusters with m and n ranging from 5 to 12 was implemented by combining basin hopping algorithm, genetic algorithm, and density functional theory (DFT) calculations. Most of Aum(SR)n clusters possess the core-shell structure. Specifically, some typical topologies, such as tetrahedral Au4, triangular bipyramid Au5, octahedral Au6, and vertex-shared Au7, are found to be dominant within the inner core of various clusters. Along with the increase in the number of gold atoms and thiolates, the preliminary nucleation and growth processes of both inner-core and staple-motif protecting units are grouped into three kinds of size evolution routes, i.e., core growth, core dissolution, and staple-motif growth, respectively. Some metastable isomers may also play an important role in the evolution of clusters. The core structures in the lowest-lying isomers and some metastable isomers are similar to the intact or part of the cores found in experimentally detected species. Both the lowest-lying and metastable intermediate clusters may serve as the building block for the further growth. These results rationalize the preliminary nucleation in the "reduction growth" stage, shedding light on the size-evolution mechanism of RS-AuNPs.

Enhancing stability through ligand-shell engineering: A case study with Au25(SR)18 nanoclusters

While thiolate-protected Au nanoclusters (NCs) have drawn considerable interest in various fields, their poor stability in aqueous solution remains a major hurdle for practical applications. Here, we report a unique strategy based on ligand-shell engineering to improve the stability of thiolated Au NCs in solution. By employing two thiol-terminated ligands having oppositely charged functional groups on the surface of the NCs, we demonstrate that the electrostatic attraction between the oppositely charged functional groups of neighboring ligands could amplify the coordination among surface ligands, leading to the formation of pseudo-cage-like structures on the NC surface that could offer higher protection to the Au core in aqueous solution. The strategy developed in this study could be extended to other metal NCs, further paving the way toward practical applications.

Ligand-Conformation Energy Landscape of Thiolate-Protected Gold Nanoclusters

Although several thiolate-protected Au nanoclusters have yielded to total-structure determination, the ligand-conformation energy landscapes and how they affect the relative stability of the whole clusters are not well understood. In this work, we employ a force-field-based approach to perform the ligand-conformation search for isolated thiolate-protected Au nanoclusters using Au25(SR)18 (R = C2H4Ph) as an example. We find that the ligand-conformation energy landscape of Au25(SC2H4Ph)18 comprises multiple low-energy funnels of similar stability instead of a single global minimum. In fact, we find slightly more stable conformations of isolated Au25(SC2H4Ph)18 than those observed in the experiment from a crystalline state, indicating that specific environments such as crystal packing and solvents may all affect the ligand conformation. This work reveals the role of ligand conformation in the cluster energy landscape.

Density Functional Theory Studies on Structure, Ligand Exchange, and Optical Properties of Ligand-Protected Gold Nanoclusters: Thiolate versus Selenolate

Atomically precise thiolate-protected Au nanoclusters (NCs), i.e. Aum(SR)n, have attracted intensive research interest during the past few years. Recently, the synthesis and isolation of selenolate-protected gold clusters (Aum(SeR)n) via the ligand exchange of thiolate with selenol were achieved, which demonstrated identical compositions to those of thiolate-protected Au NCs. In this study, we perform a comprehensive theoretical study on the structure, electronic structure, and electronic optical absorption properties of 11 selenolate-protected gold clusters on the basis of density functional theory (DFT) calculations. Our results propose that the selenolate-protected Au NCs with framework structure identical to the thiolated ones are stable local minima. The ligand effect is proposed to understand the distinct geometrical structures of Au24(SeR)20 and Au24(SR)20 NCs. In addition, the optical absorption properties of thiolate- and selenolate-protected Au NCs are compared via the time-dependent density fun...

New Structure Model of Au22(SR)18: Bitetrahederon Golden Kernel Enclosed by [Au6(SR)6] Au(I) Complex.

The study of atomic structure of thiolate-protected gold with decreased core size is important to explore the structural evolution from Au(I) complex to Au nanoclusters. In this work, we theoretically predicted the structure of recently synthesized four valence electron (4e) Au22(SR)18 cluster. The Au22(SR)18 cluster is proposed to possess a bitetrahedron Au7 kernel that is surrounded by a unique [Au6(SR)6] Au(I) complex and three Au3(SR)4 staple motifs. More interestingly, the Au22(SR)18 exhibits structural connections with Au24(SR)20 and Au20(SR)16. The stability of Au22(SR)18 can be understood from the superatom electronic configuration of the Au kernel as well as the formation of superatomic network. The present study can offer new insight into the structural evolution as well as electronic structure of thiolate-protected Au nanoclusters.

Solvent Controls the Formation of Au29(SR)20 Nanoclusters in the CO‐Reduction Method

This paper reports an efficient synthetic protocol to produce Au29(SR)20 in a CO-reduction method by adjusting the dielectric constant of mixed solvent to influence the formation kinetics of thiolate-protected Au nanoclusters.