Chiral Monolayer-Protected Bimetallic AuâAg Nanoclusters: Alloying

Article pubs.acs.org/JPCC

Chiral Monolayer-Protected Bimetallic Au−Ag Nanoclusters: Alloying Effect on Their Electronic Structure and Chiroptical Activity Ryota Kobayashi,† Yoshiyuki Nonoguchi,‡ Akito Sasaki,§ and Hiroshi Yao*,† †

Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan § X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 169-8666, Japan ‡

S Supporting Information *

ABSTRACT: Chiral glutathione (GSH)-protected bimetallic Au−Ag nanoclusters (AuAg(SG)) are synthesized by the reduction of mixtures of Au and Ag salts at the feed mole ratio of Au/Ag = 3/1 in the presence of GSH and isolated by gel electrophoretic separation, yielding a family of magic-numbered bimetallic Au−Ag nanoclusters. The PAGE pattern similarity between Au(SG) and AuAg(SG) as well as the analyses based on small-angle X-ray scattering and X-ray photoelectron spectroscopy (XPS) allow comparisons of their chemical compositions for each family of nanoclusters, and the fractioned nanocluster compounds (three samples) are identified as Au12.2Ag2.8(SG)13, Au14.4Ag3.6(SG)14, and Au17.6Ag7.4(SG)18. The XPS study also suggests that (i) incorporation of Ag heteroatoms into smaller-sized Au nanoclusters is less favorable and (ii) Ag heteroatoms preferentially occupy sites on the cluster’s core (including the core-surface and center) rather than the peripheral semiring shell. Optical spectroscopy shows that the electronic structures of Au nanoclusters are significantly modulated by incorporation of Ag atoms. Interestingly, the bimetallic Au−Ag nanocluster compounds exhibit weaker circular dichroism (CD) responses than those of the corresponding Au counterparts. Such a decrease in chiroptical responses can be explained in terms of the increased geometrical isomers that are formed by statistical distribution of Ag heteroatoms in the nanocluster because an increased number of possible configurations is expected to give the average in the CD response with positive and negative bands of different optical isomers.

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nanostructures.8,9 In Aum(SR)n systems, it is now accepted that they are composed of a metal core surrounded by oligomeric −S−Au−S− or −S−Au−S−Au−S− motifs often called “staples” or “semi-rings”;10−12 for example, a well-known Au25(SR)18 nanocluster has a 13-atom icosahedral core surrounded by six −S−Au−S−Au−S− units. Therefore, the heteroatom doping (or alloying) in gold nanoclusters allows us to probe the atomic sensitivity of the nanocluster’s physical and chemical properties as well as imparting the nanocluster with new properties. In previous works, density functional theory (DFT) for Au25−nAgn(SH)18 nanoclusters predicted 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.13 For n ≥ 2, arrangements where the Ag

INTRODUCTION Alloy metal nanoparticles often demonstrate unique characteristics compared with single-component systems.1 Interactions between the constituent atoms can modify the alloy’s electronic structure and surface composition, leading to enhanced chemical, catalytic, and optical properties.1−4 In particular, the gold−silver (Au−Ag) system in very small (< ∼2 nm) metal nanoclusters that only contain a few dozen atoms has received much attention because complete miscibility of that pair of metals has been found in the bulk at any composition with no change in lattice parameters,5,6 and such alloying (or heteroatom doping) can lead to significant electronic structure perturbations under similar valence electron characters and atomic sizes between gold and silver.5−7 From a viewpoint of cluster chemistry, recent significant development of selective synthesis of atomically precise, thiolate-protected gold nanoclusters with molecular purity, Aum(SR)n, where SR denotes thiolates, has motivated researchers to investigate curious structures and physicochemical properties of very small © 2014 American Chemical Society

Received: April 14, 2014 Revised: June 25, 2014 Published: June 30, 2014 15506

dx.doi.org/10.1021/jp503676b | J. Phys. Chem. C 2014, 118, 15506−15515

The Journal of Physical Chemistry C

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atoms are close to each other tend to be less favorable, but all isomers should be accessible under experimental conditions.13 Moreover, triply Ag-doped Au22Ag3(SCH3)18 found that the structure with Ag atom(s) located in the icosahedral shell is more energetically stable than other locations.14 Experimentally, some Au−Ag bimetallic nanoclusters have been synthesized by substituting known numbers of Ag heteroatom into atomically precise Au nanoclusters.14−18 For example, a distribution of Ag dopants in Au25−nAgn(SC12H25)18 (n ≤ ∼11) has been found, and the electronic structure of Au25(SC12H25)18 was sensitive to Ag doping and continuously modulated by incorporation of the Ag atoms. Similar results are also found on Ag doping in Au25(SC2H4Ph)18.18 In any case, the effect of Ag doping provides an exciting opportunity to investigate whether traditional alloy concepts can explain very small mixed-metal nanocluster systems.1 In this study, the main objective is to gain insight into the electronic structures and chiroptical responses of bimetallic Au−Ag nanoclusters protected by chiral thiolate ligand, glutathione (GSH), and compare them with monometallic counterparts (mainly Au nanoclusters) with comparable size. In previous works on chirally modified Au and bimetallic nanoclusters, large chiroptical activities have been found.19−24 The studies experimentally provided evidence of the importance of the outermost ligand shell−metal core interactions as well as the bimetallic core configurations, which influence the nanoclusters’ chiroptical responses. We here present an effect of Ag doping on the chiroptical responses of optically active Au nanoclusters. Upon Ag doping, modulation of the electronic structures should be expected and thus influence the chiroptical behaviors of the bimetallic Au−Ag nanocluster compounds when the surface is protected by chiral ligands. We find in the present study that GSHprotected Au−Ag nanoclusters exhibit quite different Cotton effects from those of the monometallic Au nanoclusters. We also discuss the difference in term of the geometrical nanocluster isomers that can be produced by statistical Ag atom distribution in the cluster. We believe this study involves a high-throughput approach to the finding of new catalytic and optically active nanoalloys.

Figure 1. (a) Photographs of PAGE separation for glutathioneprotected (i) Au, (ii) bimetallic Au−Ag, and (iii) Ag nanoclusters. Three bands are labeled in the order of mobility and focused in the present study (1 being the most mobile). When distinguishing whether the compound is monometallic or bimetallic, a suffix Au or AuAg is added at the end the compound number. No fractions are found for the Ag(SG) nanocluster compound in the corresponding mobility region, which can be shown by a red rectangle (dash line) in the image. Chemical structure of glutathione (GSH) is also shown.

evaporated under vacuum below 30 °C, followed by the addition of methanol/ethanol (1/1) to obtain a brown crude precipitate. The precipitate was thoroughly washed with methanol/ethanol/water (5/4/1) through redispersion−centrifugation processes. Finally, an as-prepared powder sample was obtained by a vacuum-drying procedure. GSH-protected monometallic Au or Ag nanoclusters, abbreviated as Au(SG) or Ag(SG), respectively, were also prepared in a similar manner, but the starting solution to be reduced contained pure HAuCl4 (0.5 mmol) or AgNO3 (0.5 mmol), respectively. Polyacrylamide Gel Electrophoresis. The raw product was separated using polyacrylamide gel electrophoresis (PAGE) according to the size or charge. 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. To extract the metallic nanocluster compound into aqueous solution, a part of the gel containing the 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. Note that solution-phase stability of the fractioned Au−Ag bimetallic nanoclusters was also analyzed by taking into account the time evolution of their absorption spectra. See the Supporting Information.

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EXPERIMENTAL SECTION Materials. HAuCl4·4H2O (99%), AgNO3, sodium borohydride (NaBH4, > 90%), GSH in the reduced form (γ-glu-cysgly; 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). Syntheses of Glutathione-Protected Bimetallic Au− Ag Nanoclusters and Monometallic Counterparts. GSHprotected bimetallic Au−Ag nanoclusters, referred to as AuAg(SG), were prepared by a similar method to that for penicillamine- or GSH-protected Au nanoclusters.19,21 Typically, in a 3/1 Au/Ag feed mole ratio, mixtures of HAuCl4 (0.375 mmol) and AgNO3 (0.125 mmol) and 1.0 mmol of GSH were at first mixed in methanol (100 mL) under an inert argon atmosphere, followed by rapid addition of a freshly prepared ice-cooled 0.2 M aqueous NaBH4 solution (25 mL) under vigorous stirring. In the reaction, the mole ratio of thiol to (total) metal was two. After further stirring (1.5 h), the solution was stored overnight. Nearly half of the solvent was 15507



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Figure 2. (a,b) Typical small-angle X-ray scattering (SAXS) intensity profiles of bimetallic Au−Ag nanocluster compound 3AuAg and the corresponding counterpart 3Au, Au25(SG)18. The experimental and the simulated profiles are shown by dots and curves, respectively. (c,d) Obtained cluster size distributions of each numbered compound.

Instrumentation. UV−vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Fluorescence spectra were obtained with a Hitachi F-4500 spectrofluorometer. Circular dichroism (CD) spectra were recorded with a JASCO J-820 spectropolarimeter. Field-emission scanning transmission electron microscopy (FE-STEM) was conducted with a Hitachi S-4800 electron microscope. The mean core size of the metal nanocluster sample was examined by a solution-phase small-angle X-ray scattering (SAXS) technique.19 The SAXS analyses are based on the assumption that spherical clusters are distributed with a simple Γdistribution function. X-ray photoelectron spectroscopy (XPS) measurements were carried out on an XPS system (Shimadzu AXIS-165) using a monochromatic Al Kα X-ray source (1486.6 eV). Binding energies (BEs) were referenced to the C 1s BE at 284.8 eV.

separated compounds are referred to as compounds 1−3, with 1 being the most mobile yellowish species. When distinguishing whether the compound is monometallic or bimetallic, a suffix Au or AuAg is added at the end the compound number, for example, 1Au, ... for the Au(SG) and 1AuAg, ... for the AuAg(SG) sample. It should be emphasized here that nanocluster compounds 1Au, 2Au, and 3Au have been chemically identified as the magic-numbered compounds Au15(SG)13, Au18(SG)14, and Au25(SG)18, respectively,25 which will be also supported by the optical properties displaying structured features in the absorption spectra. The separated compounds were successfully isolated as solids upon the addition of methanolic acetic acid. In the case of the Ag(SG) nanocluster sample, no separable bands could be seen in the same mobility region of the gel. (See Figure 1iii surrounded by a dashed rectangle; mobility Rf ≈ 0.55 to 0.75.) SAXS Analysis. The mean core sizes of the cluster compounds 1AuAg−3AuAg were evaluated by a solution-phase SAXS measurement. Note that the present SAXS analysis can only give information on the size distribution expressed by a Γdistribution function, even if the nanocluster species have a uniform size.19 To validate the current size analysis, we also carried out the SAXS measurement for compound 3Au that corresponds to a well-known Au25(SG)18 molecular cluster and compared the data with that of compound 3AuAg. Figure 2a,b shows the experimental scattering profile of the compound 3Au or 3AuAg along with the simulated curve, respectively; the simulated curve well reproduces the profile using the Γdistribution function.19 Importantly, the scattering profiles of 3Au and 3AuAg were almost identical with each other, indicating that these nanocluster compounds have very similar core size (and distribution). The core diameter distributions determined on the basis of the SAXS analysis are depicted in Figure 2c,d,

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RESULTS AND DISCUSSION PAGE Separation. The GSH-protected Au, Ag, and bimetallic Au−Ag nanoclusters can be negatively charged,25 so the as-prepared products were separated using PAGE. Photographs of typical PAGE separation are shown in Figure 1. The images (i)−(iii) represent those of separation for the Au(SG), AuAg(SG), and Ag(SG) nanocluster samples, respectively. In Au(SG) and AuAg(SG) nanocluster samples, several bands are observable in the gel under normal illumination, and the appearance of discrete bands suggests the presence of magic-numbered compounds. Three highmobility bands were regularly observed at the same positions with each other, so we separated (and hereafter focused on) these three bands. Note that nanocluster fractions in the same mobility have similar size and chemical components with each other.19,25 On the basis of the electrophoretic mobility, the 15508



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Figure 3. XPS spectra of (a) Au 4f and (b) Ag 3d regions for the pure Au(SG) and Ag(SG) nanoclusters as well as the fractioned Au−Ag nanocluster compounds 1AuAg−3AuAg. Each arrow in panel a or b indicates the binding energy of bulk Au 4f or Ag 3d, respectively.

and the average core diameters (dav) of 1AuAg−3AuAg are 0.43, 0.65, and 0.76 nm, respectively. The dav of compound 3Au was also 0.76 nm, being reasonably identical with that of compound 3AuAg. Note that the present SAXS analysis would cause underestimation of the mean core size of the nanocluster compounds when compared with that reported for the Au25 compound (0.98 nm).9 It assumes both a spherical shape of the nanocluster and a size distribution with Γ-distribution function.19 In a very small thiolate-protected Au nanocluster possessing a staple surface, its shape is not a sphere in the strict mathematical sense. Under the sphere-based analysis, therefore, the estimated size would become smaller than expected because a sphere has the smallest surface-to-volume ratio. In addition, an asymmetric character of the Γ-distribution function may contribute to the underestimation of the cluster mean core size. In comparison between Au and Ag atoms, (i) the atomic sizes and thiol packing densities are known to be very similar in

bulk Au and Ag;26−28 (ii) both metal atoms contribute one free electron per atom to the clusters so the same pattern of electronic shell closings is expected. Additionally, in consideration with the fact that compounds 3Au and 3AuAg appeared at the identical position with each other in the PAGE separation, which implies that their charges are also identical when they have the same core size, we can safely conclude that bimetallic compound 3AuAg has the chemical formula of Au25−nAgn(SG)18. Because compounds 1Au and 1AuAg or 2Au and 2AuAg also displayed the same PAGE mobility between them, we can assign that 1AuAg or 2AuAg should be a 15- or 18-metal atom nanocluster, respectively. The SAXS analysis surely tells us that metal nanoclusters with a large mobility should have a small mean-core diameter, but in the present case, scattering profile comparisons played a significant role in determining the precise size (or chemical structure) of the nanocluster. 15509



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X-ray Photoelectron Spectroscopy. It is now wellestablished that for the thiolate-protected Au nanocluster system sulfur atoms are incorporated into the outermost surface of the nanocluster, distancing surface Au atoms from the metal core, producing a staple-like layer (or semiring). In Au25(SR)18, for example, three coordination environments are formed: a center Au site, 12 surface (or core-surface) Au sites, and 12 Au atoms in the staple sites.10−12 Hence, to further investigate the composition distribution or heteroatom location, the nature of the monolayer binding, and chemical information on the valence state of metals in the nanoclusters, X-ray photoelectron spectroscopic (XPS) measurements were carried out. The XPS spectra of Au 4f and Ag 3d regions for the fractioned 1AuAg− 3AuAg together with those of Au(SG) and Ag(SG) are shown in Figure 3a,b, respectively. a. Compositional Analysis. We first made compositional analysis, enabling the comparison of relative elemental distribution in the samples. Normalization of X-ray photoemission peaks by their relative elemental sensitivity factors yielded an average Au/Ag atomic ratio of 81.5/18.5 (= 4.4), 80.1/19.9 (= 4.0), or 70.6/29.4 (= 2.4) for compounds 1AuAg, 2AuAg, or 3AuAg, respectively. In other words, the average doping level of each compound was 0.185, 0.199, or 0.294, respectively. Note that XPS data may provide more information about surface composition because 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 GSH ligand shells are still smaller than ∼3 nm, so the obtained Au/Ag compositional ratio can be that of the whole nanoclusters. On this basis, we then identified the compounds 1AuAg, 2AuAg, and 3AuAg as Au12.2Ag2.8(SG)13, Au14.4Ag3.6(SG)14, and Au17.6Ag7.4(SG)18, respectively. The result means that the actual Au/Ag atomic ratios of the fractioned nanocluster compounds are slightly different from that of the feeding solutions (= 3.0); that is, the smaller the bimetallic Au−Ag nanocluster becomes, the larger the Au/Ag atomic ratio is contained in the nanocluster (under the constant synthetic feeding molar ratio), indicating that incorporation of Ag heteroatoms in the thiolate-protected Au nanocluster system is less favorable when the cluster size is small. Theoretical calculations suggest that under the assumption that only doping of the core-surface is considered, arrangements where the silver dopants are in close proximity tend to be slightly less favorable than other isomers.13 In bimetallic Au−Ag nanoclusters, encounter probability between two (or more) Ag atoms on the core-surface sites increases with a decrease in the size of the nanocluster because of an increase in the large surface-to-volume ratio, so the observed sizedependent incorporation ratio of Ag heteroatoms is in reasonable agreement with the theoretical consideration. b. Ag Dopant Location. We also used XPS to deduce the location of the Ag dopant. The Au(SG) sample 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 4f7/2) but slightly shifted to blue regions, which can be assigned to Au 4f5/2 and Au 4f7/2 spin states of zerovalent gold, respectively. Similar phenomenon has been observed for other thiolate-protected Au nanoclusters, and increased Au 4f binding energies can be attributed to the thiolate ligands withdrawing electronic density from Au atoms.29,30 In the Ag(SG) sample, binding energies of 368.0 and 373.9 eV for Ag 3d5/2 and 3d3/2, respectively, which correspond to zerovalent silver, are similar to those of other thiolate-protected silver clusters,31 and we

found a very slight negative shift compared with the bulk Ag (for example, 368.2 eV for Ag 3d5/2).17 For silver, oxidationbased changes to the lattice potential, work function, or extraatomic relaxation energies are thought to negatively shift the Ag 3d peaks,32 so the negative shift supports an inclusion of the state of Ag(I)−thiolate. In the bimetallic AuAg(SG) nanoclusters, interestingly, all fractioned compounds displayed similar Au 4f and Ag 3d binding energies to those of the monometallic Au and Ag counterparts, respectively. These spectral similarities give the following indications for the nanoclusters’ structure: (i) All compounds 1AuAg−3AuAg have similar metal (Au and Ag) atom distributions in the nanoclusters (including the core and staples) with each other. (ii) A positive binding-energy shift for the Au 4f peaks suggests the presence of Au-thiolate species in all compounds. (iii) A negative binding-energy shift for the Ag 3d peaks can be due either to the valence state of Ag atoms (that is, Ag-thiolate formation) or to electron donation from the less electronegative Ag heteroatoms (χ = 1.9) to the more electronegative Au atoms (χ = 2.4)14,33 in all compounds; note that solely from the Au 4f and Ag 3d XPS spectra we cannot determine which one is the essential origin. Significant information on the core/ligand-shell binding properties could be obtained by an analysis of the S 2p binding energy for the bimetallic nanoclusters and their counterparts. A set of XPS spectra in the S 2p region is shown in Figure 4. We here chose compound 3AuAg as a representative measurement.

Figure 4. XPS spectra of S 2p region for the Au(SG), Ag(SG), and 3AuAg nanocluster compounds. 15510



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Figure 5. Absorption spectra of a family of discrete (a) Au(SG) and (b) AuAg(SG) nanocluster compounds. The spectra in the right-handed side show magnified absorption spectra around the HOMO−LUMO transition regions for all separated compounds.

those on a wide range of nanoclusters with different size and composition would give a deep understanding of their effects. Figure 5a provides UV−vis absorption spectra for the sizefractioned compounds 1Au−3Au and 1AuAg−3AuAg. It is again emphasized that compounds 1Au−3Au have been chemically identified as Au 15 (SG)13, Au18(SG) 14, and Au 25 (SG)18 , respectively.25 Additionally, all ligand-protected nanoclusters had molecule-like optical transitions. The absorbance onset was determined by linearly extrapolating the low-energy absorbance tail to the spectral baseline39 and depicted in each spectrum. From Figure 5, we found some complicated features in the spectral behaviors: (i) In comparison, between 1Au and 1AuAg (or Au15 and Au12.2Ag2.8 nanoclusters), doping of Ag heteroatoms made the absorption spectra rather featureless, and the absorption onset slightly shift to red. (ii) In comparison, between 2Au and 2AuAg (or Au18 and Au14.4Ag3.6 nanoclusters), Ag doping made the first absorption peak shift to lower energy, but a close inspection revealed that the onset for 2Au appeared at lower energy than that for 2AuAg. (iii) Ag atom doping in the well-known Au25(SG)18 nanocluster made the absorption onset blue shift, in good agreement with previous results showing that the Ag heteroatoms increase the LUMO energy while leaving the HOMO energy unaffected, and this perturbation shifts characteristic absorption to higher energy.14 Moreover, heteroatom-induced splitting of discrete energy levels also introduce new features in the Au17.6Ag7.4 absorption spectrum. In any case, Ag heteroatom doping definitely modifies the electronic structures of thiolate-protected Au cluster compounds, but the modulation in the HOMO− LUMO absorption is not simple; that is, a mode of spectral shift upon Ag doping is strongly size-dependent under the present conditions. This phenomenon is in contrast with the fact that Ag heteroatom doping continuously modulates the electronic structure of Au25(SC 12 H25 )18, which implies continuous doping into the M13 core.15 To fully understand the details, we have to wait for completion of related theoretical studies.

We found a doublet feature for all S 2p XPS spectra (binding energy of 161−163 eV for S 2p3/2 and that of 163 to 164 eV for S 2p1/2), which is characteristic of the thiolate species adsorbed on metals such as Au and Ag.34 In addition, from the spectra of monometallic counterparts, the lower binding characteristics of S 2p in the pure Ag(SG) nanocluster sample are evident, whereas pure Au(SG) compound exhibited higher binding energies. Of importance here is the striking similarity between the spectra of 3AuAg and Au(SG) rather than that of Ag(SG). A previous work has revealed that two sulfur species with different binding energies provide a way to determine the relative concentration of two different metals on the surface of the nanoclusters.34 Additionally, in Au−Ag bimetallic nanoparticles with a high Au atom content the S 2p spectrum is found to show an additional set of S 2p components at a higher binding energy (162.6 and 163.9 eV) in comparison with that of nanoparticles with a low Au content.35 Hence a higher bindingenergy component for S 2p detected predominantly in the present bimetallic AuAg(SG) nanocluster can be attributed to the possibility of Au−S sites or staples preferentially on the outermost surface of the clusters. In other words, we can conclude that Ag atoms are selectively incorporated in the “core” region (including the core surface or center), not in the “staple” region, resulting in the preferential formation of surface −GS−Au−SG− oligomer units in the bimetallic Au−Ag nanoclusters. This is also in excellent agreement with the recent crystallographic data on Au18.3Ag6.7(SCH2CH2Ph)18 nanoclusters, which expresses that the center atom is exclusively Au with 100% occupancy, while the 12-atom icosahedral vertex atoms are partially occupied by Ag or Au atoms and the six staples are exclusively occupied by Au.36 Absorption and Fluorescence Spectroscopy. Substituting one or more Au atoms with foreign atoms of other metal elements such as Pd, Pt, or Ag should provide exciting opportunities to tune the electronic and optical properties of atomically precise ligand-protected nanoclusters.13−15,32,37,38 Most of the studies are currently limited to Au25 clusters, so 15511



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As for the fluorescence (or photoluminescence) of thiolateprotected Au or Ag nanoclusters, the exact mechanism is still under debate. Among ligand-protected metal nanoclusters, fluorescence from Au25(SG)18 nanoclusters has been the most widely studied.40−42 Typically, it is reported that Au25 exhibits a broad band at near-IR (NIR) 715 nm in the emission spectrum, in good agreement with our data shown in Figure 6a,

structures (and the related relaxation processes) of compounds 1 and 2 (that is, 15-mer and 18-mer) are expected to be similar but significantly different from that of compound 3 (25-mer). The Ag heteroatom doping would alter the intra/inter-staple interaction, which is probably size-dependent, resulting in strong modulation in the emission properties.44 Chiroptical Responses. Metal nanoclusters with chirality are particularly attractive for enantioselective and stereoselective catalysis.45,46 In general, there are two methods to impart chirality to ligand-protected metal nanoclusters:20,22−24 (i) One is to passivate metal nanoclusters with ligands possessing chirality, or chiral patterns by ligands, in which the ligands can induce optical activity in metal-based electronic transitions. (ii) The other is to synthesize intrinsically chiral metal nanoclusters. If a heteroatom doping in such metal nanoclusters can be achieved, metal core or surface (staple) layer modifications should influence the chiroptical responses of the nanoclusters. To obtain information on the chiroptical activity of the present nanoclusters, we measured the CD spectrum of each numbered compound. We can then examine the effect of Ag doping on their chiroptical responses with comparable core sizes. Figure 7a,c displays CD spectra for the sequential sample of GSH-protected monometallic Au or bimetallic Au−Ag nanoclusters, respectively.47 Interestingly, CD responses of the bimetallic nanocluster compounds exhibited quite different Cotton effects from the Au counterparts in metal-based electronic transition regions; for example, comparing 3Au and 3AuAg, the M25 clusters, the phases of their CD signals are nearly reversed. For a better comparison, the anisotropy factor (or g factor) is calculated. Anisotropy factors are defined as the difference in absorption of left- and right-handed circularly polarized light divided by the absorption; that is, g = Δε/ε, where the intensity of the molar dichroic absorption Δε is normalized to the extinction coefficient ε. The results are shown in Figure 7b,d for the sequential sample of Au(SG) and AuAg(SG) nanoclusters, respectively. Note that scattered gfactor signals in the long wavelength region (> ∼500 nm) for compounds 1Au and 1AuAg are due to the fact that they are obtained by the division of a very weak CD signal by almost zero absorption intensity. The maximum anisotropy factor of ∼4.0 × 10−4 is typically found for the fractioned compounds of Au(SG) sample, whereas that of all fractioned compounds of AuAg(SG) has a smaller value of at most ∼2.0 × 10−4, suggesting that the Ag doping make the chiroptical signal decrease in metal-based electronic transition regions. A possible mechanism (origin) for the observed optical activity can be seen in the chiroptical response of the Au25 nanocluster (3Au). Structurally, the thiolate-protected Au25 nanocluster consists of an icosahedral Au13 core with high Ih symmetry plus six exterior S−Au−S−Au−S staples.9 Hence the Au13 core should not be chiral.48 In the extension of a molecule-like Au cluster model, the metal-based electronic transitions with higher energy than the HOMO−LUMO (metal core) transitions are more or less imparted with ligand character,48 so it is reasonable to consider that the observed CD signals involve electronic transitions with a character of both the metal (Au) and surface ligands and therefore arise from the electronic-state mixing of the ligands with dissymmetric fields and surface-metal atoms.48 The Au17.6Ag7.4 nanocluster compound (3AuAg) exhibited a very similar chiroptical behavior, so the same mechanism is expected for compound 3Au. Compounds 2Au and 2AuAg also displayed similar behaviors as those of compounds 3, suggesting the

Figure 6. Emission spectra of the separated nanocluster compounds (a) 1Au−3Au and (b) 1AuAg−3AuAg.

compound 3Au (λex = 520 nm). For small Au nanoclusters, excitation occurs from the levels of d band into those of the sp band.42 Then, in their excited-state dynamics, two relaxation processes are generally considered:43 The first process resulting in the emission with higher energy (normally visible region) was assigned to a recombination process almost entirely in the Au13 core with just a little perturbation from the surface ligands. In the second process resulting in the band in the NIR region, a decay mechanism via the semiring states (midgap states) of the outer part of the metal core is suggested.43 On this basis, NIR emissions of the monometallic Au nanocluster compounds 1Au−3Au can all arise from a state of the semirings or staples (Figure 6a). Note that compounds 1Au and 2Au had smaller core sizes than compound 3Au (Au25) but exhibited a more redshifted emission as compared with 3Au, suggesting that strong intra/inter-staple interaction (or distortion) in compound 1Au or 2Au may produce a deeper level in the semiring states. In the GSH-protected bimetallic Au−Ag nanoclusters, modulation of the emission properties caused by the Ag heteroatom doping is directionless and size-dependent (Figure 6b). For example, compounds 1AuAg, 2AuAg, and 3AuAg (the core size increases in this order) could luminesce with a maximum emission from 680, 710, and ∼815 nm, respectively, exhibiting a blue shift modulation for compounds 1 and 2 and a significant red shift for compound 3 upon Ag doping. It is difficult to identify which transitions exactly correspond to these emission peaks because the detailed electronic structures of these nanocluster compounds are unclear; however, electronic 15512



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Figure 7. (a,c) CD spectra of fractioned Au(SG) and AuAg(SG) nanocluster compounds. (b,d) Corresponding anisotropy factors (g factors) for all fractioned Au(SG) and AuAg(SG) samples calculated out of the CD and absorption spectra.

similar origin for their chiroptical activity. For the smallest fractions 1Au and 1AuAg, the whole cluster’s chirality might also be involved in their chiroptical responses.49 Isomer-Averaging Effect on the Chiroptical Activity. A CD signal of the chiral monolayer-protected metal (or alloy) nanocluster compound should be correlated with its overall

structure involving the core geometry and surface ligand configurations or conformations,50 so it is here worthwhile to discuss the reduction in the CD response of the GSH-protected bimetallic Au−Ag nanoclusters in comparison with that of the pure Au counterparts in terms of a geometrical isomer effect. 15513



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Such a decrease in the chiroptical activity could be explained in terms of the increased geometrical isomers, which were produced by statistical distribution of Ag heteroatoms in the nanocluster, because an increased number of configurations can give the average in the CD response with positive and negative bands of different optical isomers. We then believe that such nanoengineering methodology will play a fundamental role in an entire new class of chiral materials in the future.

In a bare bimetallic alloy nanocluster XmYn, where X or Y denotes a certain metal atom, the number of possible structural (or geometrical) isomers grows exponentially with the size (m + n) of the cluster even when the composition (m/n ratio) is defined. The presence of two different metals in the system introduces complexity due to all possible permutations of the different atom types in the structure.51 Similarly, in a thiolateprotected Au22Ag3(SH)18 nanocluster, for example, Kauffman and coworkers have identified 176 symmetry-inequavalent geometrical isomers within the defined cluster geometry, and an analysis based on DFT calculations yielded stable configurations, in which Ag heteroatoms preferentially occupy equivalent sites on the surface of the cluster’s core (or core surface) rather than the peripheral (−S−Au−S−Au−S−)6 ligand shell or the central atomic position;14 as a consequence, the most stable structures of the Au22Ag3(SH)18 cluster have the Agcore−Agcore−Agcore configurations in the nanocluster, but calculated narrow energy distribution within the core−core− core family implies that the specific location of Ag atoms on the core surface may vary from cluster to cluster.32 That is, many geometrical isomers are energetically possible in thiolateprotected bimetallic Au−Ag nanocluster systems. Meanwhile, the presence of various geometrical isomers has been reasonably confirmed in some metal complexes. For example, in mixed amino-acid complexes of cobalt(III) ions [Co(ox)(L-ser)2]−, where ox or ser denotes the oxalato or serinato ligand, six geometrically chiral isomers (Δ-/Λtrans(N), Δ-/Λ-C1-cis(N), and Δ-/Λ-C2-cis(N) forms) have been identified, and their chiroptical responses have been evaluated.52 The results showed that UV−vis absorption spectra were similar to each other, but the CD signals were quite different between the isomers, even though they had similar chiral configurations. This means that when these chiral isomers are mixed together CD signals should be averaged out, making the observed chiroptical signals smaller than the pure optical isomers. Using this argument, observation of decreasing optical activity upon Ag heteroatom doping in the Au nanocluster can be explained by an increased configurational space and thus an increased probability for producing geometrical isomers with various bimetallic mixing patterns. An increasing number of possible configurations with different (positive and negative) CD bands, which is caused by statistical Ag heteroatom distribution in the nanocluster, would make the CD responses average out and thus reduce down.

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ASSOCIATED CONTENT

S Supporting Information *

Time evolution of the solution-phase absorption spectra for the fractioned bimetallic Au−Ag nanocluster compounds and their monometallic counterparts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81-791-58-0161. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Yasuo Okajima and Dr. Yoshihiro Todokoro (NAIST) for helping in the XPS measurements. The present work was partially supported by the “Nanotechnology Network” program from MEXT.

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REFERENCES

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CONCLUSIONS In summary, we have demonstrated the existence of a family of magic-numbered chiral GSH-protected bimetallic Au−Ag nanoclusters. The PAGE pattern similarities between the Au(SG) and AuAg(SG) as well as the SAXS and XPS analyses allow comparisons of their average chemical compositions for each family of nanoclusters. The nanocluster compounds isolated were assigned as Au12.2Ag2.8(SG)13, Au14.4Ag3.6(SG)14, and Au17.6Ag7.4(SG)18. The XPS data also revealed that (i) incorporation of Ag heteroatoms into the Au nanocluster system was less favorable when the cluster size was small and (ii) Ag heteroatoms preferentially occupied equivalent sites on the cluster’s core including the core-surface and center rather than the peripheral staples. Optical spectroscopy showed that the electronic structure of the Au nanocluster was largely modulated by incorporation of Ag atoms. Importantly, the bimetallic Au−Ag nanocluster compounds exhibited weaker CD responses than those of the monometallic Au counterpart. 15514



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Chiral Monolayer-Protected Bimetallic AuâAg Nanoclusters: Alloying

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