Enhanced Chiroptical Activity in Glutathione-Protected Bimetallic

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Enhanced Chiroptical Activity in Glutathione-Protected Bimetallic (AuAg) Nanoclusters with Almost Intact Core-Shell Configuration 18

Hiroshi Yao, Ryota Kobayashi, and Yoshiyuki Nonoguchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b09429 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on January 4, 2016

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The Journal of Physical Chemistry

Enhanced Chiroptical Activity in Glutathione-Protected Bimetallic (AuAg)18 Nanoclusters with Almost Intact Core-Shell Configuration 1

Hiroshi Yao,1,* Ryota Kobayashi, Yoshiyuki Nonoguchi

2

1: Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan 2: Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan

* To whom correspondence should be addressed. Fax: +81-791-58-0161. E–mail: [email protected] (H. Yao). ABSTRACT

We here focus on the chiroptical response of a glutathione (GS)-protected bimetallic (AuAg) 18 nanocluster containing comparable amounts of Au and Ag atoms. X-ray photoelectron spectroscopy (XPS) as well as the electrophoretic pattern allow to determine the chemical composition of the nanocluster as Au8.5 Ag9.5 (SG) 14 . Chiroptical activity of the Au 8.5 Ag 9.5 nanocluster is enhanced as compared to that of the corresponding monometallic Au counterpart, whereas that of bimetallic (AuAg) 18 nanocluster with the atomic ratio of Au/Ag = 4.0 (Au 14.4 Ag3.6 ) decreases. Since thiolate-protected Au18 nanocluster has recently been revealed to consist of a face-fused Au 9 bi-octahedral kernel protected by one Au 4 (SR)5 tetramer, one Au2 (SR)3 dimer and three Au(SR)2 monomers, we use density functional theory (DFT) calculations to predict the lowest-energy structure of a model cluster compound Au 9 Ag 9 (SH)14 on the basis of this geometry, yielding an Agcore–Austaple intact core–shell configuration. This structure is in good agreement with the experimental XPS data of Au 8.5 Ag 9.5 (SG)14 . Additionally, calculations give an overall increase in the chiroptical response as compared to the monometallic Au 18 compound when nine Ag atoms are incorporated in the 18-metal-atom bimetallic nanocluster. From a structural viewpoint, in reality, a limited number of possible configurational isomers with similar (but slightly higher) energies may lower the response 1 ACS Paragon Plus Environment


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magnitude, but the overwhelming presence of the nanocluster with almost intact core–shell configuration prevents positional fluctuation of Ag atoms in the cluster, suppressing the averaging of circular dichroism (CD) responses with positive and negative signs of different (possible) geometrical isomers, which should be conclusively responsible for enhancing the chiroptical activity in this bimetallic nanocluster system with heavy Ag-doping.

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INTRODUCTION

Alloy nanoclusters often exhibit unique physicochemical properties compared to the single component systems.1 In particular, silver-gold alloy nanoclusters are of interest because the lattice constants (or atomic size) of silver and gold are nearly identical so the two noble metals can be mixed in various proportions, leading to significant electronic structure perturbations under similar valence electron characters.2–4 In cluster chemistry, on the other hand, recent advances in the selective synthesis of atomically precise, thiolate-protected gold nanoclusters with molecular purity, Au m (SR) n where SR denotes thiolates, has motivated researchers to investigate their peculiar structures and interesting properties.5,6 In the Au m (SR) n nanocluster system, it is now widely accepted that they are composed of a metal core surrounded by oligomeric –RS-(Au-SR) x – motifs often called “staples” or “semi-rings”;7–9 for example, a well-known Au 25 (SR) 18 nanocluster has a 13-atom icosahedral core surrounded by six –RS-(Au-SR) 2 – dimer units. Hence the heteroatom doping in such Au nanoclusters allows to probe the core/staple sensitivity of the nanocluster’s physical and chemical properties. In well-studied thiolate-protected Au 25 nanoclusters as described above, a previous theoretical work on Ag doping into Au 25 (SH) 18 (that is, Au 25−n Ag n (SH) 18 nanoclusters) indicated that, for n = 1, Ag doping of the icosahedral shell of the metal core is energetically more favorable than doping of the metal-thiolate units or the center of the core.10 For 2 ≤ n ≤ 12, arrangements where the Ag atoms are close to each other tend to be less favorable but all isomers should be accessible under experimental conditions.10 Moreover, an increase in the Ag doping level increases the contribution of Ag atoms to the HOMO and LUMO level, but percentage of contribution of S atom is maintained.11 Experimentally, it has been shown that eleven silver atoms can be doped into Au 25 (SC 12 H 25 ) 18 ,12 and the optical absorption spectrum of such highly doped system is close to the theoretical



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time-dependent density functional theory (DFT) calculations made by Aikens on a related Au 12 Ag 13 (SH) 18 nanocluster.13 Obviously, the effect of Ag doping provides an opportunity to investigate whether traditional alloy concepts can adequately describe very small mixed-metal nanocluster systems.1 In our previous study on bimetallic alloy nanoclusters, we found that chiral glutathione (GS)-protected Au 17.6 Ag 7.4 (SG) 18 and Au 14.4 Ag 3.6 (SG) 14 nanoclusters (that is, 25-metal-atom (M 25 ) and 18-metal-atom (M 18 ) AuAg nanoclusters) exhibited quite different Cotton effects from those of the monometallic counterparts.14 Influence of geometrical nanocluster isomers, which can be produced by statistical Ag atom distribution in the core of the cluster, on their chiroptical responses has been also discussed.14 In the bimetallic system, the atomic ratio of Au/Ag was much larger than unity; that is, Au atoms were considerably enriched in the nanocluster compounds. In atomically precise M 25 nanocluster compounds, for example, surface of the 13-metal-atom core (core-surface) is the preferred position for silver doping, so the heavier doping of Ag atoms in such cluster systems may strongly perturb the core or semi-ring configuration and thus bring about unique chiroptical properties when covered with chiral thiolates. In the present study, the effect of comparable Ag-atom doping to Au atoms on the chiroptical responses of optically active GS-protected nanoclusters is investigated. In particular, we focus on the 18-metal atom (M 18 ) nanocluster compounds that have been less studied compared to the M 25 nanoclusters, and find that heavy doping of Ag atoms into the Au 18 nanocluster leads to an increase in its chiroptical activity, in sharp contrast to the case of light doping of Ag atoms.14 Comparison of chiroptical properties between the M 18 and M 25 nanoclusters is also made. Recently, the precise structure of thiolate-protected Au 18 nanocluster has been resolved using X-ray crystallography,15,16 and it exhibits a face-fused bi-octahedral (or hcp) Au 9 kernel protected by staple-like motifs including one tetramer, one dimer and three monomers.15,16 With the aid of theoretical considerations assuming this unraveled structure, we find that intact Agcore–Austaple core-shell geometry can be the 4 ACS Paragon Plus Environment

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most probable configuration, which corroborates our observed data based on X-ray photoelectron spectroscopy (XPS). We then propose that the exclusive (or overwhelming) presence of the lowest-energy core-shell configuration would make the different statistical isomers much less possible, which avoids canceling out the chiroptical responses with positive and negative bands of the different isomers. We believe this structural specificity will involve a unique approach to the finding of new chiral alloy nanomaterials.

EEPERIMENTAL SECTION

Materials. HAuCl 4 • 4H 2 O (99%), AgNO 3 , sodium borohydride (NaBH 4 , > 90%), glutathione in the reduced form (abbreviated as GSH, GR grade; chemical structure is shown in Figure 1), methanol (GR grade), and ethanol (GR grade) were received from Wako Pure Chemical and used as received. All gel electrophoresis reagents were received from Nacalai Tesque. Pure water was obtained by a water-distillation supplier (Advantec GS-200). Nanocluster Synthesis and Size Fractionation. GS-protected bimetallic AuAg nanoclusters containing comparable amounts of Au and Ag atoms were prepared by a similar method to that for GS-protected Au nanoclusters.14,17,18 Typically, mixtures of HAuCl 4 (0.25 mmol) and AgNO 3 (0.25 mmol), that is, the feed mole ratio of Au/Ag = 1, and 1.0 mmol of GSH were at first mixed in methanol (100 mL) under an argon atmosphere, followed by rapid addition of freshly prepared ice-cooled aqueous NaBH 4 solution (0.5 M, 25 mL) under vigorous stirring. After further stirring (1.5 h), the solution was stored overnight. Addition of methanol/ethanol (1/1) yielded a crude precipitate, which was thoroughly washed with methanol/ethanol/water (5/4/1) using a common purification protocol. Finally, an as-prepared powder sample was obtained by vacuum-drying. GS-protected monometallic Au or Ag nanoclusters, abbreviated as Au(SG) or Ag(SG), respectively,



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were also prepared in a similar manner, but the starting solution to be reduced contained pure HAuCl 4 (0.5 mmol) or AgNO 3 (0.5 mmol), respectively. The raw product was separated using polyacrylamide gel electrophoresis (PAGE). The separating gel concentration was 28% (pH 8.8). The sample solution was loaded onto a gel top and eluted for ~6 h at a constant voltage (150 V) to achieve separation. A part of the gel containing the magic-number fraction was cut out, followed by the addition of distilled water. Then the gel lumps were removed by using a centrifuge and filter with 0.2 µm pores. In addition, we succeeded in isolating the fractionated nanocluster compounds as solid by adding methanolic acetic acid (2%) solution, so characterizations were exclusively conducted for the purified solid species. Nanocluster Characterization. UV-vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Circular dichroism (CD) spectra were recorded with a JASCO J-820 spectropolarimeter. The mean core size of the metal nanocluster sample was examined by a solution-phase small angle X-ray scattering (SAXS) technique.17 X-ray photoelectron spectroscopy (XPS) measurements were carried out on an XPS system (Shimadzu/Kratos AXIS-165) using a monochromatic Al Kα X-ray source (1486.6 eV). Binding energies (BE) were referenced to the C 1s BE at 284.8 eV. Computation. M 18 (SH) 14 (M = AuAg) nanocluster geometries were constructed by use of the crystallographically determined Au 18 (SR) 14 structure as a starting point.15 On this basis, we carried out DFT calculations (optimizations) using PBE functional for the exchange and correlation terms,11,19,20 the LanL2DZ basis set for Au and Ag, and the 6-31G** basis set for S and H, as implemented in the Gaussian 09 program package.21 In the model calculations, nine Au atoms in various sites of the nanocluster were replaced by Ag, giving rise to structures with a 9/9 Au/Ag atomic ratio, since this Au/Ag ratio was very close to that of the size-fractioned M 18 (SG) 14 nanocluster species experimentally obtained. Time-dependent DFT (TD-DFT), as also implemented in Gaussian 09, was used to calculate optical absorption according to the literature.15 A Gaussian 6 ACS Paragon Plus Environment

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spectral smearing of 2500 cm–1 was used. We focused on calculating the spectrum in a low energy region, so the lowest 53 excited singlet states were considered.

RESULTS AND DISCUSSION

Isolation of GS-Protected M 18 and M 25 Nanoclusters. The as-prepared nanocluster samples could be separated using PAGE (Figure 1). The images (i) and (ii) represent those of the electrophoretic separation for the bimetallic AuAg and monometallic Au samples, respectively. Note that, in the case of monometallic silver nanocluster Ag(SG) sample, no bands (species) could be seen in the same mobility region of the gel, suggesting formation of Ag nanoclusters with different sizes.14 In Figure 1, several separable bands are observable in the gel under normal illumination, which means the presence of magic-numbered compounds. Two pairs of distinct bands (compounds 2 AuAg and 4 AuAg indexed in image (i), and compounds Au 18 and Au 25 indexed in image (ii)) were regularly observed at the same positions with each other, so we fractionated these four bands. There is a general consensus that nanocluster fractions in the same mobility have similar size and chemical components with each other.17,22 In addition, some bands clearly seen (and indexed) in image (ii) have been chemically identified as magic-numbered compounds Au 15 (SG) 13 , Au 18 (SG) 14 , and Au 25 (SG) 18 ,22,23 which have been confirmed by their optical absorption properties displaying structured features.14 All separated compounds were successfully isolated as solids upon addition of methanolic acetic acid. In comparison between Au and Ag atoms, the atomic diameters and thiol packing densities are known to be very similar in bulk Au and Ag;24–26 moreover, both metal atoms contribute one free electron per atom to the clusters so the same pattern of atomic shell closings is expected. We found that Au 18 (SG) 14 and 2 AuAg , or, Au 25 (SG) 18 and 4 AuAg , appeared at the identical position with each other in the PAGE separation, implying that their charges are also identical when they have the same 7 ACS Paragon Plus Environment


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core size, so we can safely conclude that bimetallic compound 2 AuAg or 4 AuAg is a 18-metal-atom or 25-metal-atom bimetallic nanocluster formulated as M 18 (SG) 14 or M 25 (SG) 18 , where M = AuAg, respectively.25 We also confirmed that a solution-phase SAXS yielded the mean core diameter of 0.68 nm for 2 AuAg and 0.80 nm for 4 AuAg (see the Supporting Information). Underestimation of the mean core size of tiny thiolate-protected nanocluster has been verified in the current SAXS analysis,14 but the estimated diameters of 2 AuAg and 4 AuAg are in reasonable agreement with those of the M 18 and M 25 nanocluster compounds, respectively. X-ray Photoelectron Spectroscopy (XPS). To investigate the bimetallic composition and chemical information on the valence state of metals in the separated nanocluster compounds, X-ray photoelectron spectroscopic (XPS) measurements were carried out. The XPS spectra of Au 4f and Ag 3d regions for Au 18 (SG) 14 , Ag(SG), 2 AuAg or 4 AuAg are shown in Figures 2a and 2b, respectively. Quantitative compositional analysis was done by measuring the photoemission peak areas involving their relative elemental sensitivity factors, and gave an average Au/Ag atomic ratio of 47/53 (= ~0.89) or 44/56 (= ~0.79) for compound 2 AuAg or 4 AuAg , respectively. Note that XPS data may provide more information about surface composition since X-ray can normally penetrate a depth of about several nanometers into the sample surface; however, in the present case, sizes of the metal nanocluster compounds including the glutathione ligand shells are still smaller than ~3 nm, so the obtained Au/Ag compositional ratio can be that of the whole nanoclusters.14 On this basis, we identified the compounds 2 AuAg and 4 AuAg as Au 8.5 Ag 9.5 (SG) 14 and Au 11 Ag 14 (SG) 18 , respectively. The actual Au/Ag atomic ratios obtained are slightly smaller than that of the feeding solutions (Au/Ag = 1.0), but particularly for the M 18 nanocluster, the numbers of Au and Ag atoms are very close to each other. We further analyzed the XPS data to deduce the locations of Ag (or Au) atoms. In Figure 2a, the Au 18 compound exhibited two peaks at binding energies of 88.1 and 84.5 eV that were close to peak positions of bulk Au metal (for example, 84.0 eV for Au 4f 7/2 ) but slightly shifted to a blue region, 8 ACS Paragon Plus Environment

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which can be assigned to Au 4f 5/2 and Au 4f 7/2 spin states of zero-valent gold, respectively. The increased Au 4f binding energies can be attributed to the thiolate ligands withdrawing electronic density from Au atoms.27,28 In the Ag(SG) sample, binding energies of 368.0 eV (Ag 3d 5/2 ) and 373.9 eV (Ag 3d 3/2 ), which correspond to zero-valent silver, are similar to those of other thiolate-protected silver clusters,29 but a very slight negative shift compared to the bulk Ag (for example, 368.2 eV for Ag 3d 5/2 ) is also found.30 In the bimetallic nanoclusters 2 AuAg and 4 AuAg (lower row in Figure 2), they displayed overall similar Au 4f and Ag 3d binding energies to those of the monometallic Au and Ag counterparts, respectively; however, with a close inspection of the Au 4f XPS spectrum of 2 AuAg , we found a slight but distinct positive binding-energy shift compared to the pure Au species, suggesting a large abundance of Au-thiolate species in the M 18 compound.28 For compound 4 AuAg , on the other hand, the Au 4f XPS spectrum was almost identical with that of the pure Au 18 species, indicative of a wide range of distribution of Au atoms including the core and staples. Note that electron donation from the less electronegative Ag heteroatoms (χ = 1.9) to the more electronegative Au atoms (χ = 2.4)31 is possible and thus may cause a negative shift in the Au 4f peaks, but bear in mind that compound 2 AuAg showed the opposite trend. Meanwhile, the Ag 3d XPS spectrum of 2 AuAg is almost identical with other nanocluster compounds, and the absence of further negative shift will rules out the predominance of Ag(I)-thiolate bonding, because for silver, oxidation-based changes to the lattice potential, work function, and/or extra-atomic relaxation energies are thought to negatively shift the Ag 3d peaks.32 Therefore, in consideration with the fact that the 18-metal-atom nanocluster protected by 14 thiolates has 9 metal atoms in the core site (including core-surface) and 9 metal atoms in the staple site,8,9,11,15 so an almost intact core–shell type (= core–staple) configuration can be expected in the M 18 nanocluster (2 AuAg ). Absorption and Circular Dichroism (CD) Spectroscopy. In our previous study,14 we found that substituting a few Au atoms with foreign Ag atoms provided interesting opportunities to tune the 9 ACS Paragon Plus Environment


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electronic and chiroptical properties of chiral GS-protected nanoclusters. In detail, under the condition of the atomic ratio of Au/Ag = ~3, bimetallic 18- and 25-metal-atom nanoclusters exhibited weaker circular dichroism (CD) responses than those of the corresponding Au counterparts. Such a decrease in the chiroptical activity could be explained in terms of many geometrical isomers to be produced by statistical distribution (incorporation) of Ag heteroatoms in the nanocluster,14 since an increased number of configurations can give the average in the CD response with positive and negative bands of different optical isomers. In the present study, Ag heteroatoms are heavily (commensurately) doped in well-defined, magic-numbered Au nanoclusters, so more exciting optical/chiroptical characters should be emerged in these compounds. Figure 3a or 3b displays UV-vis absorption spectra of 2 AuAg (= Au 8.5 Ag 9.5 (SG) 14 ) and 4 AuAg (= Au 11 Ag 14 (SG) 18 ) or Au 18 (SG) 14 and Au 25 (SG) 18 in aqueous solution, respectively. We first confirmed that absorption features at around 620 nm and 500–550 nm are characteristic absorptions of Au 18 (SG) 14 ,16,33 and those at distinct peaks at 780, 670, and 450 nm are of Au 25 (SG) 18 .27 From these spectra, as expected, mixing of Au and Ag perturbs discrete energy levels within the nanocluster’s electronic structure; for example, (i) in comparison between the Au 18 and Au 8.5 Ag 9.5 nanoclusters, commensurate doping of Ag in the M 18 nanocluster seemingly made the first absorption peak to lower energy, but actually, the first peak intensity in the Au 18 compound was rather weak (~620 nm) and strictly shifted to blue upon Ag doping; (ii) heavy Ag atom doping in the M 25 nanocluster made both the absorption peak and its onset blue-shift. In the latter (M 25 ) case, this behavior is in good agreement with previous theoretical results showing that the Ag heteroatoms increase the LUMO energy while leaving the HOMO energy unaffected, and this perturbation shifts characteristic optical absorption to higher energy up to the comparable amount of Ag doping to Au.10,11 In the former (M 18 ) case, we will revisit this issue later. As for chirality, the well-investigated monometallic Au 25 (SR) 18 nanocluster having a structure with a symmetrical Au 13 core surrounded by RS–(Au-SR) 2 staples is not essentially chiral, but it 10 ACS Paragon Plus Environment

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becomes chiroptically active when the protecting thiolate is chiral.9,14 The Au 38 (SR) 24 nanocluster is, on the other hand, chiral owing to the asymmetric arrangement of similar staples, although it has a high symmetric gold kernel.34 To obtain information on the chiroptical activity of the present bimetallic M 18 (SG) 14 and M 25 (SG) 18 nanoclusters, CD spectroscopy was performed. Figure 3c displays CD spectra of aqueous 2 AuAg and 4 AuAg , and Figure 3d the calculated profiles of the anisotropy factor (g-factor) defined as the molar dichroic absorption ∆ε normalized to the extinction coefficient ε. Note that, for better comparison, g-factors of 2 AuAg , 4 AuAg , Au 18 , and Au 25 compounds together with those of Au 14.4 Ag 3.6 (SG) 14 (18 metal atoms, Au/Ag = 4.0) and Au 17.6 Ag 7.4 (SG) 18 (25 metal atoms, Au/Ag = 2.4) previously reported are all summarized in the Supporting Information.14 Irrespective of the doping level (heavy or light) of Ag atoms, we found significant spectroscopic modulations in the Cotton effects for metal-based electronic transition regions. Strong emphasis is here placed on the finding that the maximum anisotropy factor of ~4.0 × 10–4 is typically observed for the bimetallic 18-metal-atom compound 2 AuAg and is larger than those (at most 2.0 × 10–4) of other M 18 nanoclusters including both Au 14.4 Ag 3.6 and monometallic Au 18 . On the other hand, among the M 25 nanocluster compounds, the monometallic Au 25 has the largest g-factor. It is worth noting that, in the extension of a molecule-like Au cluster model, the metal-based electronic transitions with higher energy than the HOMO–LUMO transitions are more or less imparted with ligand character,35 so the observed chiroptical responses would involve electronic transitions with a character of both the metal (pure Au or AuAg alloy) and surface ligands (that is, electronic state mixing of the ligands and metal atoms). Intact Core–Shell Type Bimetallic M 18 (SH) 14 Nanocluster Formation. In what follows, we try to reveal why the comparable amount of mixing between Au and Ag in the bimetallic M 18 (SG) 14 system did not cause a decrease (or enhanced) in the chiroptical activity from a structural point of view. Hence the structural prediction of 2 AuAg is the major issue here. Our computational efforts were focused on the Au 9 Ag 9 (SH) 14 nanoclusters since the atomic ratio of Au/Ag in compound 2 AuAg 11 ACS Paragon Plus Environment


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experimentally obtained was 8.5/9.5. We simply choose SH as ligands instead of glutathione since the assignment in multiple electronic transitions observed in the experiments for Au 25 (SR) 18 nanocluster has been very successful using a model of Au 25 (SH) 18 system.13,36 In addition, the present calculation is for gas-phase environment and does not include solvent effect, since it is known that solvents (or solvent polarity) have negligible effects on the core structure of the thiolate-protected nanoclusters and thus on their optical excitation properties.36 The key concept for the geometrical prediction and calculations of the thiolate-protected AuAg nanoclusters is that an M m (SR) n cluster has a metal core protected by RS–(M–SR) x motifs, which is essentially identical with the corresponding pure Au nanocluster species.37 In Au 18 (SR) 14 , it should have a face-fused Au 9 bi-octahedral kernel protected by one Au 4 (SR) 5 tetramer, one Au 2 (SR) 3 dimer and three Au(SR) 2 monomers as determined by Jin and co-workers.15 In addition, Au and Ag share equivalent atomic radii and valence electron counts,11 so the Ag substitution is not expected to significantly change the overall structure of the Au 18 nanocluster. On the basis of the above argument, we first constructed a geometry of Au 18 (SH) 14 nanocluster (precisely, Au 9 core–Au 9 staple(SH) 14 ; see the most left image in Figure 5), and structurally optimized.15 We next modeled (and optimized) a structure in which nine Au atoms in the staples were fully substituted by Ag, that is, Au 9 core–Ag 9 staple(SH) 14 as shown in Figure 4 (the most left image). Then, we sequentially replaced the metal atoms in the staples as, Ag 9 → Ag 8 Au 1 → Ag 7 Au 2 → Ag 6 Au 3 →Ag 5 Au 4 → … → Au 9 to keep the overall geometries unchanged. Accordingly, the 9-metal-atom core configuration changed as Au 9 → Au 8 Ag 1 → Au 7 Ag 2 → Au 6 Ag 3 →Au 5 Ag 4 → … → Ag 9 . Through the assessment, we found a systematic trend showing a monotonic decrease in the total energy of the isomers with an increase in the Au atoms in the staple regions (some typical results are shown in the Supporting Information). Then, in the low-energy staple configurations selected (such 12 ACS Paragon Plus Environment

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as Ag 2 Au 7 ), we intensively examined different nanocluster isomers to obtain their relative stabilities. The optimized geometries of various Au 9 Ag 9 (SH) 14 isomers (I-V) are representatively shown in Figure 4a. The relative total energies (in kJ•mol–1) for these isomers are also depicted in the figure. Figure 4b displays those for a family of the isomer-IV (isomers-IV, IV a , IV b , and IV c ; that is, nanoclusters with (Ag 7 Au 2 )core(Ag 2 Au 7 )staple configurations). From the figures, we can see that (i) incorporation of Ag atoms into the tetrameric staple unit makes the total energy decrease, but that into the monomeric staples is not effective to stabilize its structures; (ii) the total energy difference becomes rather small between the isomers when more than five Au atoms are located in the staples. Nonetheless, within our examination, we can conclude that the Ag 9 core (Ag 9 staple) isomer, that is, isomer-V (isomer-I) in the figure, is the most stable (unstable) configuration, respectively. These results indicate that thiolate-protected Au 9 Ag 9 nanoclusters likely contain Ag atoms on the bi-octahedral core region, whereas Ag occupation in the staples is considerably unfavorable, yielding an intact core–shell (core–staple) geometry in the nanocluster, and thus there is little possibility to have different isomers whose cores (and staples) are compositionally disordered. The energetically favorable intact core–shell configuration obtained here is also in good agreement with the recently predicted structure showing that, in Au 6 Ag 7 (SR) 10 nanocluster, likely to our case with comparable amounts of Au and Ag, the Ag atoms prefer the inner (core) positions while Au atoms are located on surface staple-like motifs.38 In reality, some isomers with Ag-enriched AuAg-core configurations (for example, those having one or two Au atom(s) in the core and thus one or two Ag atom(s) in the tetrameric staple site) will be possible. In addition, the ratio Au/Ag = 8.5/9.5 experimentally obtained is an average value for the M 18 nanocluster compound 2 AuAg so the compositional fluctuations would also exist. Despite the simplicity of these model nanoclusters, the structural prediction on bimetallic (Au 9 Ag 9 ) nanoclusters is satisfied with the Au 4f peaks in the XPS data of 2 AuAg , so we can reasonably conclude that Au 8.5 Ag 9.5 (SG) 14 nanocluster is likely to have an almost intact Agcore–Aushell type configuration. 13 ACS Paragon Plus Environment


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Revisit to the Optical and Chiroptical Properties of Bimetallic M 18 Nanoclusters. To better understand the optical/chiroptical properties of the bimetallic M 18 (SG) 14 nanocluster containing comparable amounts of Au and Ag atoms, we discuss the following two points on the basis of the DFT results: (i) Can the observed spectroscopic differences between Au 8.5 Ag 9.5 and Au 18 nanoclusters be verified from a theoretical aspect based on the core–shell morphology?16 (ii) How does the intact Agcore–Austaple geometry influence its chiroptical activity? Figure 5 shows calculated absorption and CD spectra of the most stable isomer-V of Au 9 Ag 9 (SH) 14 together with those of Au 18 (SH) 14 and some other higher-energy isomers. In the monometallic Au 18 species, strength in the first transition (677 nm; labeled as a in Figure 5a) was small and an intense peak was prominent at 621 nm (labeled as b). This trend is in agreement with the experimental one, but actual (observed) peak positions were more blue-shifted than those of the calculation. In the intact core–shell Au 9 Ag 9 nanocluster (isomer-V in Figure 5e), on the other hand, calculated absorption was similar to that of the monometallic Au 18 species but the first transition showed a well-defined intense peak at around 604 nm (labeled as a in Figure 5e), which is blue-shifted as compared to that of Au 18 , also in reasonable agreement with the experimental observation. The calculated absorption spectrum of isomer-I exhibiting a significant red shift of the absorption onset did not match the experimental. Statistical contribution of slightly higher-energy isomers such as isomer-IV or isomer-IV a could involve some complications with a small red shift in the first peak as shown in Figures 5c and 5d (also labeled as a). In the case of chiroptical activity, we found that, according to calculations, arbitrary incorporation of nine Ag atoms in the bimetallic M 18 nanoclusters yielded an overall increase in the chiroptical response as compared to the monometallic Au 18 compound, although the influence of ligand chirality is not involved. Hence substitution of nine Ag atoms in the thiolate-protected M 18 nanocluster system is expected to enhance the chiroptical responses as compared to the monometallic Au 18 nanocluster. In the case of the experimental compound 2 AuAg , the exclusive existence of single 14 ACS Paragon Plus Environment

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nanocluster isomers is unrealistic,14,39,40 so the observable CD response would be more or less influenced by some low-energy geometrical isomers, bringing about an isomer-averaging effect.14 Here, although the energy difference between the isomers-IV, IV a and isomer-V was larger than that of room temperature (= 2.48 kJ•mol–1), we simply averaged the CD spectra of these three isomers without considering a Boltzmann distribution on a trial basis, and compare it with that of monometallic Au 18 (SH) 14 shown in Figure 5a (see the Supporting Information). The averaged CD signal was overall still larger than the chiroptical response of the monometallic Au 18 nanocluster compound, indicating that the existence of a limited number of possible configurational isomers still prevents a significant reduction in the CD response.41 The most importantly, under the condition of the almost intact core–shell morphology such as compound 2 AuAg , most of the Ag heteroatoms would preferentially occupy at the 9-metal-atom core positions, so the structural diversity (or positional fluctuation of Ag atoms) in the nanocluster can be energetically restricted (or contribution from other possible isomers having different configurations with similar (but slightly higher) energies can be very small), giving rise to a reduction in the degree of isomer-averaging effect for the CD responses. In the 25-metal-atom nanocluster, interestingly, theoretical calculations have also predicted that a stable structure is the fully doped core Ag 13 coreAu 12 staple system.10,38 Here the M 25 nanocluster possesses one icosahedral metal core and six equivalent staples of RS–(M–SR) 2 motifs.10 In compound 4 AuAg (Au 11 Ag 14 ), our XPS data exhibited that Au atoms were located in the whole cluster regions, suggesting that it did not strictly have the intact core-shell configuration. Hence the energetically possible geometrical isomers or fluctuations are readily to influence the chiroptical activity of the bimetallic nanocluster system, leading to a CD response reduction. Consequently, configurational restriction of the magic-numbered nanoclusters plays a significant role in suppressing the isomer-averaging for the chiroptical responses of the alloy nanocluster systems.



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CONCLUSIONS

In summary, we have demonstrated that comparable amount of mixing between Au and Ag atoms in bimetallic M 18 (SG) 14 nanocluster enhanced its chiroptical activity in metal-based electronic transition region as compared to that of the corresponding monometallic Au counterpart, whereas it caused a decrease in the chiroptical responses of 25-metal-atom nanocluster compounds M 25 (SG) 18 . X-ray photoelectron spectroscopy (XPS) as well as the electrophoretic pattern made these nanoclusters formulate as Au 8.5 Ag 9.5 (SG) 14 and Au 11 Ag 14 (SG) 18 . To particularly understand the chiroptical behavior of the M 18 nanocluster system from a structural viewpoint, DFT calculations were used to obtain the lowest-energy configurations. Au 9 Ag 9 (SH) 14 was chosen as a model (simplified) cluster compound, and the structures were optimized on the basis of the crystallographic geometry consisting of a face-fused Au 9 bi-octahedral kernel protected by one Au 4 (SR) 5 , one Au 2 (SR) 3 and three Au(SR) 2 .15,16 We then found that Agcore–Austaple intact core–shell configuration would be the most probable, lowest-energy structure for Au 9 Ag 9 (SH) 14 , which was in good agreement with the experimental XPS data of Au 8.5 Ag 9.5 (SG) 14 , Consequently, almost intact core–shell structures of Au 8.5 Ag 9.5 (SG) 14 would reduce a probability of geometrical isomer formation with various (statistical) bimetallic mixing patterns, and thus suppress the average (that is, reduction) in the CD responses with positive and negative signs of different isomers, even though a limited small number of possible configurational isomers may give a weak isomer-averaging effect in reality. We then believe that specific alloy configurations such as the intact core–shell structure will play an interesting role in developing a new class of chiral nanoalloys in the future.


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ASSOCIATED CONTENT

Supporting Information SAXS analysis for 2 AuAg and 4 AuAg nanocluster compounds. Summary of anisotropy factors (g-factors) of 2 AuAg , 4 AuAg , monometallic Au 18 and Au 25 compounds along with those of Au 14.4 Ag 3.6 (SG) 14 and Au 17.6 Ag 7.4 (SG) 18 . Optimized geometries of some Au 9 Ag 9 (SH) 14 isomers with staple configurations of Ag 8 Au 1 , Ag 7 Au 2 , Ag 6 Au 3 , Ag 5 Au 4 , and Ag 4 Au 5 . The simple average in the CD responses of isomers-IV, IV a and V obtained using TD-DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS We thank Mr. Yasuo Okajima (NAIST) for helping in the XPS, Dr. Akito Sasaki (Rigaku Corporation) for the SAXS measurements. We also thank Dr. Yoshihiro Todokoro and Dr. Tsuyoshi Kawai (NAIST) for helpful discussion. The present work was in part supported by “Nanotechnology Platform” program from MEXT, and Grant-in-Aids for Scientific Research (C: 15K04593 (H. Y.)) from Japan Society for the Promotion of Science (JSPS).

References and Notes

(1) Ferrando, R.; Jellinek, J.; Johnston, R. L. Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles. Chem. Rev. 2008, 108, 845–910.



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(13) Aikens, C. M. Origin of Discrete Optical Absorption Spectra of M 25 (SH) 18 – Nanoparticles (M = Au, Ag). J. Phys. Chem. C 2008, 112, 19797–19800. (14) Kobayashi, R.; Nonoguchi, Y.; Sasaki, A.; Yao, H. Chiral Monolayer-Protected Bimetallic Au–Ag Nanoclusters: Alloying Effect on Their Electronic Structure and Chiroptical Activity. J. Phys. Chem. C 2014, 118, 15506–15515. (15) Das, A.; Liu, C.; Byun, H. Y.; Nobusada, K.; Zhao, S.; Rosi, N.; Jin, R. Structure Determination of [Au 18 (SR) 14 ]. Angew. Chem. 2015, 127, 3183−3187. (16) Chen, S.; Wang, S.; Zhong, J.; Song, Y.; Zhang, J.; Sheng, H.; Pei; Y.; Zhu, M. The Structure and Optical Properties of the [Au 18 (SR) 14 ] Cluster. Angew. Chem. Int. Ed. 2015, 54, 3145− 3149. (17) Yao, H.; Miki, K.; Nishida, N.; Sasaki, A.; Kimura, K. Large Optical Activity of Gold Nanocluster Enantiomers induced by a Pair of Optically Active Penicillamines. J. Am. Chem. Soc. 2005, 127, 15536–15543. (18) Yao, H. Optically Active Gold Nanoclusters. Curr. Nanosci. 2008, 4, 92–97. (19) Hidalgo, F.; Noguez, C.; de la Cruz, M. O. Metallic Influence on the Atomic Structure and Optical Activity of Ligand-Protected Nanoparticles: A Comparison Between Ag and Au. Nanoscale 2014, 6, 3325–3334. (20) (a) Rappoport, D.; Crawford, N. R. M.; Furche, F.; Burke, K. Which functional should I choose?, in Computational Inorganic and Bioinorganic Chemistry 2009. (b) The exchange-correlation functional GGA (generalized gradient approximation) is typically more accurate than LDA (local density approximation). The most universal GGA is PBE, and has been successfully applied to both molecules and solids, including metals and their clusters such as refs. 11 and 14. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. et al. Gaussian 09, Gaussian, Inc., Wallingford CT, 2010. 19 ACS Paragon Plus Environment


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(22) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. Magic-Numbered Au n Clusters Protected by Glutathione Monolayers (n = 18, 21, 25, 28, 32, 39): Isolation and Spectroscopic Characterization. J. Am. Chem. Soc., 2004, 126, 6518–6519. (23) Yu, Y.; Luo, Z.; Chevrier, D. M.; Leong, D. T.; Zhang, P.; Jiang, D.; Xie, J. Identification of a Highly Luminescent Au 22 (SG) 18 Nanocluster. J. Am. Chem. Soc. 2014, 136, 1246−1249. (24) Sellers, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. Structure and Binding of Alkanethiolates on Gold and Silver Surfaces: Implications for Self-Assembled Monolayers. J. Am. Chem. Soc. 1993, 115, 9389–9401. (25) Kumar, S.; Bolan, M. D.; Bigioni, T. P. Glutathione-Stabilized Magic-Number Silver Cluster Compounds. J. Am. Chem. Soc. 2010, 132, 13141–13143. (26) Guo, J.; Kumar, S.; Bolan, M.; Desireddy, A.; Bigioni, T. P.; Griffith, W. P. Mass Spectrometric Identification of Silver Nanoparticles: The Case of Ag 32 (SG) 19 . Anal. Chem. 2012, 84, 5304−5308. (27) Negishi, Y.; Nobusada, K.; Tsukuda, T. Glutathione-Protected Gold Clusters Revisited: Bridging the Gap Between Gold(I) − Thiolate Complexes and Thiolate-Protected Gold Nanocrystals. J. Am. Chem. Soc. 2005, 127, 5261−5270. (28) Christensen, S. L.; MacDonald, M. A.; Chatt, A.; Zhang, P.; Qian H.; Jin, R. Dopant Location, Local Structure, and Electronic Properties of Au 24 Pt(SR) 18 Nanoclusters. J. Phys. Chem. C 2012, 116, 26932−26937. (29) Rao, T. U. B.; Pradeep, T. Luminescent Ag 7 and Ag 8 Clusters by Interfacial Synthesis. Angew. Chem., Int. Ed. 2010, 49, 3925–3929. (30) Udayabhaskararao, T.; Sun, Y.; Goswami, N.; Pal, S. K.; Balasubramanian, K.; Pradeep, T. Ag 7 Au 6 : A 13-Atom Alloy Quantum Cluster. Angew. Chem., Int. Ed. 2012, 51, 2155−2159. (31) CRC Handbook of Chemistry and Physics. 77th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1996−1997. 20 ACS Paragon Plus Environment

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(32) Weaver, J. F.; Hoflund, G. B. Surface Characterization Study of the Thermal Decomposition of AgO. J. Phys. Chem. 1994, 98, 8519−8524. (33) Our Au 18 nanocluster exhibited the HOMO-LUMO transition at 620–630 nm as a shoulder probably due to the sample quality issue. (34) H. Qian, W. T. Eckenhoff, Y. Zhu, T. Pintauer, R. Jin, Total Structure Determination of Thiolate-Protected Au 38 Nanoparticles. J. Am. Chem. Soc. 2010, 132, 8280 – 8281. (35) Zhu, M.; Qian, H.; Meng, X.; Jin, S.; Wu, Z.; Jin, R. Chiral Au 25 Nanospheres and Nanorods: Synthesis and Insight into the Origin of Chirality. Nano Lett. 2011, 11, 3963–3969. (36) Aikens, C. M. Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au 25 Nanoparticle. J. Phys. Chem. A 2009, 113, 10811–10817. (37) Jiang, D. E.; Tiago, M. L.; Luo, W.; Dai, S. The “Staple” Motif: A Key to Stability of Thiolate-Protected Gold Nanoclusters. J. Am. Chem. Soc. 2008, 130, 2777–2779. (38) Tlahuice, A. On the structure of the thiolated Au 6 Ag 7 cluster. Phys Chem. Chem. Phys. 2014, 16, 18083–18087. (39) Kauffman, D. R.; Alfonso, D.; Matranga, C.; Qian, H.; Jin, R. A Quantum Alloy: The Ligand-Protected Au 25–x Ag x (SR) 18 Cluster. J. Phys. Chem. C 2013, 117, 7914–7923. (40) C. Noguez, A. Sanchez-Castillo, F. Hidalgo, Role of Morphology in the Enhanced Optical Activity of Ligand-Protected Metal Nanoparticles. J. Phys. Chem. Lett. 2011, 2, 1038–1044. (41) (a) Matsuoka, N.; Hidaka, J.; Shimura, Y. Absorption and Circular Dichroism Spectra of Oxalatobis(aminoacidato)cobaltate(III) Complexes. Inorg. Chem. 1970, 9, 719−723. (b) Mixing of many kinds of chiral isomers definitely makes their chiroptical signals smear out, so the number of possible isomers related to the observed CD responses should be limited.



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Figure Captions

Figure 1. Photographs of PAGE separation for glutathione-protected (i) bimetallic AuAg and (ii) pure Au nanoclusters. In image (i), four bands are labeled in the order of mobility. In the present study, we focused on 2 AuAg and 4 AuAg , whose positions were identical with the well-known nanocluster compounds Au 18 (SG) 14 and Au 25 (SG) 18 (labeled as Au 18 and Au 25 in image (ii)), respectively. No fractions were found for the Ag(SG) nanocluster compound in the corresponding mobility region. Chemical structure of glutathione (GSH) is also shown.

Figure 2. XPS spectra of (a) Au 4f and (b) Ag 3d regions for the pure Au 18 (SG) 14 and Ag(SG) nanoclusters as well as the fractioned AuAg nanocluster compounds 2 AuAg and 4 AuAg .

Figure 3. Absorption spectra of a family of discrete (a) bimetallic nanocluster compounds (2 AuAg and 4 AuAg ) and (b) Au 18 and Au 25 . Spectra of 4 AuAg and Au 25 are offset by a constant for clarity. (c) CD spectra of the bimetallic nanocluster compounds 2 AuAg and 4 AuAg . (d) The corresponding anisotropy factors (g-factors) for 2 AuAg and 4 AuAg .

Figure 4. (a) Various isomers of Au 9 Ag 9 (SH) 14 nanocluster compounds optimized on the basis of DFT calculations. Here the superscript “c” or “s” of Au (or Ag) means that the metal atom is located at the “core” or “staple” position, respectively. In addition, relative energies with respect to the most stable isomer-V are also indicated. (b) Optimized geometries and relative energies for a family of the isomer-IV (isomers-IV, IV a , IV b , and IV c ) having (Ag 7 Au 2 )core(Ag 2 Au 7 )staple configurations. The energy represented in the upper or lower row indicates its relative value with respect to the isomer-IV or the most stable isomer-V, respectively. Consequently, core–shell (or core–staple) type configuration of the bimetallic AuAg nanocluster is found to be energetically favorable.

Figure 5. (a)–(e) Calculated absorption and CD spectra for Au 18 (SH) 14 and geometrical isomers of Au 9 Ag 9 (SH) 14 nanocluster species (isomers-I, IV, IV a , and V). In optical absorption spectra of (a), and (c)–(e), the first transition is labeled as a.


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Table of Contents Graphic


Enhanced Chiroptical Activity in Glutathione-Protected Bimetallic

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