Molecular-scale ligand effects in small gold-thiolate nanoclusters

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Molecular-scale ligand effects in small gold-thiolate nanoclusters Daniel M Chevrier, Lluís Raich, Carme Rovira, Anindita Das, Zhentao Luo, Qiaofeng Yao, Amares Chatt, Jianping Xie, Rongchao Jin, Jaakko Akola, and Peng Zhang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09440 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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Journal of the American Chemical Society

Molecular-scale ligand effects in small gold-thiolate nanoclusters Daniel M. Chevrier,1,‡ Lluís Raich,2,‡ Carme Rovira,2,3 Anindita Das,4 Zhentao Luo,5 Qiaofeng Yao,5 Amares Chatt,1 Jianping Xie,5 Rongchao Jin,4 Jaakko Akola,6,7 Peng Zhang1* 1Department

of Chemistry, Dalhousie University, Halifax, NS, B3H 4R2, Canada de Química Inorgànica i Orgànica (Secció Química Orgànica) & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain 3Institució Catalana de Recerca i Estudis Avançats (ICREA), Passeig Lluís Companys, 23, 08020 Barcelona, Spain 4Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA 15213, USA 5Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore, 119260 6Department of Physics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway 7Laboratory of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere, Finland 2Departament

KEYWORDS: Nanocluster structure, Gold-thiolate, X-ray absorption, Quantum/molecular mechanics, DFT ABSTRACT: Due to 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, although this information is critical for predicting the performance of AuNCs 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 nanoclusters. Two representative ligand types, smaller aliphatic cyclohexanethiolate and larger hydrophilic glutathione, are selected and their structures are followed 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 clusters. 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 Au18(SR)14 cluster surface demonstrates the ability of AuNCs to act as a catalyst for organic phase reactions. In contrast, glutathione ligands form very tight encapsulation of the AuNC core via intermolecular interactions, minimizing structural changes caused by interaction with aqueous solvent 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.

Introduction Thiolate(SR)-protected gold nanoclusters (AuNCs, nanoparticles with a diameter less than ~2-3 nm) with atomically-precise compositions (Aun(SR)m, where n and m have exact values) have generated considerable attention in the scientific community because of their remarkable stability, unique core and surface structures, and their robust optoelectronic and physicochemical properties.1,2 Due to the ultrasmall size of AuNCs, the protecting ligands play an even more significant role in directing the overall structure and properties compared to larger nanoparticles. For example, changing the protecting ligand type can modulate the electronic and optical properties of AuNCs based on the electron withdrawing/donating power of the ligand substituent (S“R”).3–8 Ligand substituent types can also influence the packing arrangement of Au atoms in the NC core during

synthesis or ligand-exchange processes based on the bulkiness, size or configuration of the SR ligand.9 Moreover, the selection of protecting SR ligand type and the compatible solvent can affect properties such as chemical reactivity, stability and optical properties,10–12 which are important considerations for their intended application as catalysts or as bioimaging agents.13–16 A series of stable glutathione(SG)-protected AuNCs were identified over one decade ago. They have been widely studied and implemented as water-soluble Aun(SR)m clusters.17 Switching out organo-soluble ligands for SG ligands can improve biocompatibility, enhance photoluminescence and provide additional functionality for biological-related applications.18,19 However, it is uncertain how SG ligands and an

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Figure 1. (a) Au18(SR)14 structure (Au atoms in Au9 core (large yellow), Au atoms in staple units (small yellow), and S atoms (red)) with ligand structure (R) shown below for glutathione (SG) and cyclohexanethiol (SC6H11). (b) UV-Vis absorption and (c) photoluminescence spectra (excitation @ 365 nm) of Au18(SG)14 in water and Au18(SC6H11)14 in toluene.

aqueous solvent environment alter the AuNC structure (and properties, in effect) since most of the Aun(SR)m systems have been crystallized from an organic solvent environment with aliphatic SR ligands. Nevertheless, SG-protected AuNCs (Aun(SG)m) have compositions identical to those found for some organothiolate-protected AuNCs, such as Au15(SR)13, Au18(SR)14 and Au25(SR)18,17,20 and hypothetically, have similar structural frameworks. One recurring distinction and benefit of SG ligands over organothiolates (e.g., phenylethanethiolate) is that Aun(SG)m clusters typically have higher emission yield.4,21 Wu et al.4 initially suggested that additional charge is donated to the Au core from amino acid residues adjacent to cysteine to produce higher emission than its phenylethanethiolate analog. More recent studies have reported some of the highest emission yields observed for Aun(SG)m clusters that are in an aggregated or rigidified state, either due to their ligand structure, added metal cations, or the particular solvent environment.22–25 In pursuit of understanding the impact of ligand type and solvation on Aun(SR)m clusters, this study utilizes X-ray absorption spectroscopy (XAS) and quantum mechanical simulations to understand the effect of ligand size and substituent type by comparing SG-protected (large and hydrophilic) and cyclohexane-protected (small and aliphatic) Au18(SR)14 NCs. Owing to the versatility of XAS measurements and the site-specific local structural information afforded through fitting analyses, the effect of protecting ligand type and solvent on AuNC structure can be examined.12,26,27 The Au18(SR)14 cluster was selected for this study because its small Au core size and high SR:Au ratio make it an ideal system for identifying the effect of ligand type and solvation on both surface and core environments. The crystal structure of cyclohexanethiolate-protected Au18(SC6H11)14 was reported simultaneously by two research groups.28,29 Both studies elucidated a hexagonal closed-packed (HCP) Au9 core (consisting of three staggered Au3 layers) and combination of -SR-(Au(I)-SR)x- oligomers or “staple-like” motifs (three monomeric, one dimeric and one tetrameric oligomers) protecting the Au core. Examining the change in ligand and solvent for the same AuNC composition also offers structural insights into the enhanced luminescence from SGprotected AuNCs, where protecting ligands encapsulate and protect the Au core from solvent molecules.

Results and Discussion The structural framework of Au18(SC6H11)14 is presented in Figure 1a with the protecting SR ligands studied herein (cyclohexanethiolate (SC6H11) and glutathione (SG)). The optical absorption profiles of Au18(SC6H11)14 and Au18(SG)14 clusters in Figure 1b have consistent spectral features, indicating a similar structural framework with either protecting ligand type. Subtle spectral differences for Au18(SG)14 include a small blue-shift in absorption features at 450 nm and at 680 nm, which should be caused by the difference of the two AuNCs in both ligand-Au and Au-Au bonding to be further discussed next. Both Au18(SR)14 clusters photoluminesce (Figure 1c) in the visible to near-infrared region (650 to 850 nm, excitation 365 nm) with the emission maximum red-shifted for Au18(SG)14 and almost two times as high as Au18(SC6H11)14. Comparatively, Au25(SR)18 clusters have an emission energy around 700 to 800 nm for phenylethanethiolate or SG ligands with a higher emission intensity for the latter clusters.4,7 X-ray absorption spectroscopy detects structural changes from ligand type and solvent Measuring the Au L3-edge EXAFS of Au18(SC6H11)14 and Au18(SG)14 clusters in solid-/solution-phase and fitting scattering shells that account for core and surface bonding revealed structural changes that would be difficult to discern with other characterization techniques. Au L3-edge FT-EXAFS of Au18(SC6H11)14 from solid-phase, low temperature measurement is presented in Figure 2a with EXAFS fitting for Au-S and Au-Au scattering environments.27,30 Specific EXAFS scattering shells (based on X-ray crystallographic structure of Au18(SC6H11)14 (Figure S1)) for Au-S bonding, Au-Au core bonding “Au-Aucore” (mainly within the Au3 layers, 2.65-2.85 Å), Au-Au surface bonding “Au-Ausurf” (between Au3 core layers or mainly related to Au9 core, 2.85-3.05 Å) and Au-Au aurophilic bonding “Au-Auauro” (between staple Au and the Au core, 3.05-3.50 Å) are shown in Figure 2b. Viewing from the end-on perspective, Au-Aucore and Au-Ausurf shells account for Au-Au interactions mainly within the Au9 core. The Au-Auauro shell relates to the interface between the Au9 core and Au(I) atoms in the metal-ligand


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shell. Bond distances refined from EXAFS fitting of Au18(SC6H11)14 (Table S1) correspond well with the average bond distances determined from the crystal structure (shown in parentheses in Table S1). Debye-Waller factors (DWFs (σ2)), which provide an indication of thermal and positional deviations in distance, are also within the expected range for small Aun(SR)m NCs, from short Au-S bonding (~0.002 Å2) to longer Au-Au interactions (~0.01 Å2), based on previous EXAFS studies.27,30–32

Figure 2. (a) Au L3-edge FT-EXAFS spectrum of solidphase Au18(SC6H11)14 at 90 K with EXAFS multi-shell fit and individual contributions from each scattering shell. (b) Side-on (left) and end-on (right) view of the Au18(SR)14 NC with distinct Au-Au scattering shells for EXAFS fitting. The refinement of EXAFS spectra for Au18(SC6H11)14 and Au18(SG)14 clusters in solid-phase with the described scattering shells show site-specific deviations in the average bond distances (Figure S3, Table 1, k-space in Figure S2). Distances for Au-Aucore and Au-Ausurf, representing the Au9 HCP core structure, show less divergence in distance (relative to the Xray crystallography data in Table S1) for Au18(SG)14 than for Au18(SC6H11)14. Furthermore, the average Au-Auauro distance for Au18(SG)14 is shorter than Au18(SC6H11)14. The shortening of Au-Au bonds in the Au-Auauro shells of Au18(SG)14 could occur from the higher spatial demand of SG ligands, which could also be the reason for the red-shifted emission of Au18(SG)14 observed in Figure 1. Since the Au-S bond length

does not increase from the bulkier SG, this result implies the Au-Au framework of Au18(SR)14 clusters can shrink or expand to accommodate surface-architectural changes such as ligand size and substituent type. Au18(SC6H11)14 clusters in toluene show pronounced structural changes in the Au core and Au-ligand bonding environments (Figure S3 and Table 1). Firstly, an increase in Au-S bond length and increase in DWF occurs. A similar solvationinduced increase in the Au-S bond length and DWF were previously observed for phenylethanethiolate-protected Au25(SR)18 and Au38(SR)24 clusters in toluene.26,27 Compared to the solid-phase, surface Au-Au interactions related to the Au9 core contract by almost 0.2 Å. Aurophilic interactions increase slightly in distance upon solvation, which could be coupled with the increase of Au-S bonding distance, moving the Au staple sites away from the core. Consistent solvation behavior has been reported for Au25(SR)18 clusters in toluene.27 EXAFS results for Au18(SC6H11)14 suggest that toluene molecules interact with cyclohexyl substituents, causing S and Au atoms in oligomer/staple structures to move away from the Au9 core along with an increase in surface structural disorder, reflected by the increase in DWFs for scattering shells representing the surface of the Au18(SR)14 (Table 1: Au-S, Au-Ausurf and Au-Auauro). The contraction of the Au9 core may stabilize the cluster, counteracting the toluene-induced disorder of the ligand shell. Remarkably, and on the contrary, EXAFS results for Au18(SG)14 in water indicate that there is hardly change in surface or core Au-Au bonding from the solid-phase measurement. Au-Au distances and DWFs are similar for both measurement conditions. The only consistent structural change observed for both ligand types from solid to solution is the lengthening of Au-S bonding. However, unlike Au18(SC6H11)14, there is no increase in disorder for Au-S interactions indicated by the DWF. We hypothesize that SG ligands efficiently encapsulate the AuNC, preventing water molecules from interacting with, or perturbing, the Au cluster surface and core. Thus, non-radiative energy release from the photo-excited cluster to the solvent would be less likely if SG ligands provide the described surface protection, leading to the higher emission observed for Au18(SG)14 clusters.

Table 1. Au L3-edge EXAFS fitting results for Au18(SC6H11)14 and Au18(SG)14 in solid and solution phases measured at 300K.

SC6H11

Au-S

SG

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Au-Aucore

Au-Ausurf

Au-Auauro

Phase

R (Å)

σ2 (Å2)

R (Å)

σ2 (Å2)

R (Å)

σ2 (Å2)

R (Å)

σ2 (Å2)

Solid

2.303(8)

0.0024(3)

2.65(5)

0.014(6)

3.11(3)

0.008(3)

3.34(4)

0.012(4)

Toluene

2.318(3)

0.0034(1)

2.717(7)

0.0073(5)

2.94(6)

0.03(1)

3.39(3)

0.021(5)

Solid

2.298(5)

0.0019(2)

2.74(3)

0.011(4)

3.00(4)

0.009(3)

3.23(5)

0.015(6)

Water

2.319(6)

0.0019(2)

2.73(4)

0.012(4)

2.99(4)

0.010(4)

3.24(3)

0.010(2)



A wavelet-transformed EXAFS (WT-EXAFS) analysis further supports the response of each Au18(SR)14 cluster to ligand and solvation (Figure 3 and S4). WT-EXAFS improves on conventional FT-EXAFS by adding resolution from k-space contributions, which provides a means to discern scattering features according to backscattering element type, and to separate multiple scattering features that coincide with single scattering. WT-EXAFS is used here to determine the stronger Au-Au scattering environments, which relate to a more rigid Au-Au framework having less structural disorder from solvation. As expected for small thiolate-protected AuNCs, the strongest feature (i) comes from Au-S scattering. Au-S multiple scattering (iii) is discernable in both spectra at 3.8 Å and centered around 6 Å-1 on the k axis. This multiple scattering peak is more intense for Au18(SG)14, indicating less structural disorder in the Au(I)-SR surface features. Comparing the Au-Au scattering region (ii) for both ligand types, the strongest Au-Au scattering peaks are located around 2.8 Å for Au18(SC6H11)14 and 3.2 Å for Au18(SG)14. Note that as a qualitative technique without quantitative fitting analysis, not all the Au-Au shells can be resolved in regions (ii) and only the most intense features (i.e., the 2.8 Å feature for Au18(SC6H11)14 and 3.2 Å feature for Au18(SG)14)) are distinguishable. The higher intensity and scattering position of these most intense features supports an overall contracted Au core for Au18(SC6H11)14 in toluene and a protected surface for Au18(SG)14 in water, from more rigid aurophilic interactions. Consistent with EXAFS fitting, the WT-EXAFS plot of Au18(SG)14 in solid-phase (Figure S4) is similar to the solution-phase.

Figure 3. Au L3-edge WT-EXAFS plots of Au18(SC6H11)14 in toluene and (b) Au18(SG)14 in water.

(a)

Toluene solvent environment contracts the gold core and creates accessible cavities on the surface QM/MM simulations were conducted (see Materials and Methods for details) to further elucidate the solvation-induced structural changes for Au18(SR)14 clusters, providing also, an additional perspective from the ligand shell. First, for the aliphatic system, interactions between solvent toluene molecules and Au18(SC6H11)14 were observed from the

simulation by the location of toluene molecules and the modulated arrangement of cyclohexanethiolate ligands on the surface, compared to the gas-phase simulation of Au18(SC6H11)14. Figure 4 shows from the front view that cyclohexyl groups rotate or bend at the α-C position in response to toluene molecules. The more significant effect of solvation can be observed more easily by inspection of the side view of Au18(SC6H11)14 in Figure 4 where open pockets in the ligand shell are visible on each side of the cluster. This ligand rearrangement results from toluene molecules occupying space between staple structures and interacting with cyclohexane rings (Figure 4, right), and could represent the type surface structural disorder detected from EXAFS. Toluene molecules form CH- interactions with the thiolate ligands via ring edge, which is pointed towards the middle of the cyclohexyl ring in a perpendicular conformation, and via ring facing ring (Figure S5). Figure 4 (bottom) shows the bond length distributions for the simulated gas-phase and toluenephase models of Au18(SC6H11)14. The Au-Au bonding distribution is narrower for the toluene-phase Au18(SC6H11)14, with Au-Au bond lengths ranging from 2.6 to 3.0 Å, compared to 2.6 to 3.2 Å for the gas-phase. This narrower distribution of bond lengths agrees well with the contraction of surface AuAu bonds found from EXAFS fitting (Table 1), and offers further evidence on the reliability of these findings. Glutathione ligands shield the gold core from water QM/MM simulations were performed in a similar manner for the Au18(SG)14 system (see Materials and Methods for details) by including water molecules to simulate the solution-phase. Figure 5 displays the simulated structural models and the bonding distribution for the gas-phase and water-phase calculations of Au18(SG)14. There is a small amount of twisting or bending of staple motifs on the surface of Au18(SR)14 from the larger SG ligands, but with no pockets formed in the ligand shell. SG ligands provide sufficient encapsulation of the cluster through intermolecular hydrogen bonds between ligands resulted from the interactions between the H-donors (– OH and –NH) and the H-acceptor (C=O). The conformation of the SG ligands from gas-phase to water-phase does however reveal an interesting behavior. Terminal carboxylic groups of SG ligands in the water-phase system extend outward from the cluster to form hydrogen bonding interactions with water molecules (Figure 5, right). On the contrary, in the gas-phase, amine or ammonium groups from glutamate residues are directed outwards (Figure 5, left). The bonding distribution shows a similar spread for Au-S and Au-Au distances between the gas-phase and the water-phase, with most Au-Au bond lengths occurring between 2.6 to 3.1 Å. The similarity of the Au-S and Au-Au bonding distributions in different environments supports the EXAFS findings of a surfaceprotected Au18(SG)14 cluster where minimal structural change is detected.


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Figure 4. Simulated AuNC structures and corresponding bond-distance distributions (below) from QM and QM/MM simulations of Au18(SC6H11)14 in gas-phase and in toluene-phase. In structural models: pink atoms are Au, yellow sticks are S and cyan sticks are C. The color code of bond distributions is: magenta, Au-S; and green, Au-Au).



Figure 5. Simulated AuNC structures and corresponding bond-distance distributions (below) from QM and QM/MM simulations of Au18(SG)14 in gas-phase and in aqueous-phase. In structural models: pink atoms are Au, yellow sticks are S and cyan sticks are C. The color code of bond distributions is: magenta, Au-S; and green, Au-Au).

Electronic properties change consistently with structural findings Figure 6 displays the Au L3-edge X-ray absorption near-edge structure (XANES) for each Au18(SR)14 cluster in respective solvents. The intensity of the first feature, the white-line, reflects the relative amount of unoccupied 5d valence levels for Au in the cluster. This decreases for Au18(SC6H11)14 when dissolved in toluene, shown in Figure 6a, along with a weaker near-edge resonance feature at higher energy (~11.930 keV). The lower white-line intensity indicates more localization of electron density in the 5d level for Au18(SC6H11)14 clusters in

toluene while the weaker resonance could be related to the toluene-induced structural disorder. Negligible differences in near-edge structure for Au18(SG)14 in water from the solidphase (Figure 6b) confirm the surface structural rigidity and ligand encapsulation of the cluster from the electronic perspective.


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Journal of the American Chemical Society and rigidified cluster. These structural-based findings, along with electronic structure analysis, provide a clear explanation for the enhanced photoluminescence, and demonstrate the suitability of glutathione-protected AuNCs as bio-sensing/imaging probes.

ASSOCIATED CONTENT Supporting Information includes Materials and Methods, additional EXAFS data, depictions of toluene-cyclohexane interactions from simulation, electronic calculations with detailed discussion and simulated 5d DOS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Email, [email protected]

Figure 6. Au L3-edge XANES of Au18(SC6H11)14 and Au18(SG)14 (offset by 0.1) in solid-phase and in solutionphase.

Present Addresses

The effective atomic charges (Hirshfeld type) and Au 5d-band density of states (Figure S6) have also been calculated using the QM/MM method. The Hirshfeld charge analysis shows that Au atoms have different charges depending on their location. For Au18(SC6H11)14 in the gas phase, the core atom values range between -0.27e and -0.12e (average -0.15e), while atoms in the ligand shell are in a different oxidized form with a charge of +0.35e. Sulfur is negatively charged with an average value of -0.35e. Upon placing the cluster in toluene solvent there is a slight charge transfer of -0.1e and -0.2e (in total) towards Au and S atoms, respectively, from the cyclohexane side groups. Overall, the effective charges are similar for the Au18(SG)14 cluster, but the ligand shell Au and S atoms are more polarized for SG (+0.38 and -0.38e, respectively). The water environment induces a charge transfer from the SG side groups towards the AuNC core (-0.1e and 0.3e to Au and S, respectively). The combined increased polarization and charge transfer direction from the ligand shell to the core provides evidence for the propensity of SGprotected AuNCs to have stronger emission properties compared to their aliphatic, organothiolate counterparts.

Author Contributions

Conclusion Significant ligand effects on the structural framework of small Au18(SR)14 clusters were elucidated by joint synchrotron X-ray experiments and QM/MM simulations. We found that smaller aliphatic cyclohexanethiolate ligands were susceptible to interactions with toluene molecules, causing a contraction of the core and increased disorder on the surface of Au18(SC6H11)14 clusters. Remarkably, toluene molecules create large cavities in the ligand shell on the AuNC surface. The identification of these surface accessible regions highlights the potential for AuNCs to serve as specific catalytic agents. Moreover, it implies that the influence of specific solventligand interactions could hamper or benefit AuNC catalytic activity. In contrast, larger hydrophilic glutathione ligands provide excellent encapsulation of the Au18(SR)14 cluster, minimizing structural changes when solved in water. The much better protection of the AuNC core by glutathione ligands via intermolecular interaction suggests a more stable

†If an author’s address is different than the one given in the affiliation line, this information may be included here. ‡These authors contributed equally.

Funding Sources N/A

Notes There are no additional notes

ACKNOWLEDGMENT D.M.C. was supported by the NSERC CGS-Alexander Graham Bell scholarship during this work. P.Z. and A.C. acknowledge NSERC Discovery Grants for funding. CLS@APS facilities (Sector 20-BM) at the Advanced Photon Source (APS) are supported by the U.S. Department of Energy (DOE), NSERC Canada, the University of Washington, the Canadian Light Source (CLS), and the APS. Use of the APS is supported by the DOE under Contract No. DE-AC02−06CH11357. R.J. acknowledges the Air Force Office of Scientific Research under AFOSR Award No. FA9550-15-1-9999 (FA9550-15-1-0154). J.X. acknowledge funding from the Ministry of Education, Singapore, under the grant R-279-000-481-112. C.R. acknowledges the Spanish Ministry of Economy and Competitiveness (MINECO) under grant CTQ2017-85496-P and Agència de Gestió d'Ajuts Universitaris i de Recerca (AGAUR), under grant SGR2017-1189. We acknowledge the computer resources and the technical support provided by the Barcelona Supercomputing Center (BSC-CNS) and CSC – IT Center for Science, Finland. Technical support at CLS@APS (Sector 20, APS) facilities from Dr. Robert Gordon and Dr. Zou Finfrock is acknowledged and greatly appreciated.

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TOC Graphic

Displaced Ligand

Au18(SC6H11)14

+ Gas / Solid

Solution-phase

Contracted Core

Surfaceprotected Custer

Au18(SG)14


Molecular-scale ligand effects in small gold-thiolate nanoclusters

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