Molecular-Scale Ligand Effects in Small Gold–Thiolate Nanoclusters

Abstract

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, even though 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 experimentally in 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.

Ligand-protected gold/silver superatoms: current status and emerging trends

“…Zhang examined the interaction of solvent and superatoms, 144,145 which is often overlooked due to the difficulty of characterization in solution. A slight change of geometric and electronic structure of [Ag 44 (p-MBA) 30 ] 4in solution was monitored by XAS.…”

Single‐Atom Catalysts Supported by Crystalline Porous Materials: Views from the Inside

“…[40] Wavelet transformation (WT) provides a complementary method to FT-EXAFS by combining radial distance distribution and k-space in the form of a 2D-contour plot, where the k-space shows the oscillations caused by the interactions between a target atom and backscattering atoms. [41][42][43] As an example, Figure 3B,C show the FT-EXAFS and WT-EXAFS of silver nanoclusters protected Pd K-edge C) X-ray absorption near-edge structure (XANES) and D) Fourier-transformation-extended X-ray absorption fine structure (FT-EXAFS) spectra of 0.4 wt%-Pd(NH 3 ) 4 Cl 2 /USY measured every 0.6 min in a 5% H 2 flow at room temperature. Reproduced with permission.…”

“…[ 40 ] Wavelet transformation (WT) provides a complementary method to FT‐EXAFS by combining radial distance distribution and k‐space in the form of a 2D‐contour plot, where the k‐space shows the oscillations caused by the interactions between a target atom and backscattering atoms. [ 41–43 ] As an example, Figure 3B,C show the FT‐EXAFS and WT‐EXAFS of silver nanoclusters protected by tiopronin, respectively. [ 41 ] In the FT‐EXAFS spectrum, only two scattering paths were assigned corresponding to AgS and AgAg shells, while the peak above 3 Å could not be identified.…”

Elucidating ligand effects in thiolate-protected metal clusters using Au24Pt(TBBT)18 as a model cluster

“…Because these SRs have substantially different functional groups (Scheme S1 †), the resulting Au n (SR) m clusters have different chemical compositions and geometrical structures. 28,29 For example, when PET is used as the ligand, Au n (PET) m clusters, such as [Au 25 (PET) 18 ] − , [Au 38 (PET) 24 ] 0 , [Au 52 (PET) 32 ] 0 , [Au 144 (PET) 60 ] 0 , and [Au 329 (PET) 84 ] 0 , are formed, whereas, in the case of TBBT, Au n (TBBT) m clusters, such as [Au 28 (TBBT) 20 ] 0 , [Au 36 (TBBT) 24 ] 0 , [Au 52 (TBBT) 32 ] 0 , [Au 133 (TBBT) 52 ] 0 , and [Au 279 (TBBT) 84 ] 0 , are generated. The numbers of gold atoms and ligands are the same in [Au 52 (PET) 32 ] 0 and [Au 52 (TBBT) 32 ] 0 , but their framework structures are significantly different.…”

“…In addition to the chemical composition of the cluster, the stability, electronic structure, and functions of Au n (SR) m clusters change depending on the functional-group structure of the ligand. 9,[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] For example, it has been demonstrated that properties of Au 25 (SR) 18 , such as the stability in solution, reducibility, dissociation pattern under laser irradiation, quantum yield of photoluminescence, and catalytic activity, along with other properties, differ depending on the functional-group structure of the ligand. Therefore, the selection of the ligand functional group is essential in the functionalization of Au n (SR) m clusters.…”