Induced Optical Activity in Boronic-Acid-Protected Silver Nanoclusters

Feb 19, 2010 - Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, ... The Journal of Physical Chemistry C 2015 119 (30), 17111-1711...
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J. Phys. Chem. C 2010, 114, 15909–15915

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Induced Optical Activity in Boronic-Acid-Protected Silver Nanoclusters by Complexation with Chiral Fructose† Hiroshi Yao,* Masanori Saeki, and Keisaku Kimura Graduate School of Material Science, UniVersity of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo, 678-1297, Japan ReceiVed: NoVember 16, 2009; ReVised Manuscript ReceiVed: January 11, 2010

Silver nanoclusters protected by achiral 3-mercaptophenylboronic acid (3-MPB) with a mean core diameter of about 0.8 nm are successfully synthesized. Addition of chiral D- or L-fructose into the basic aqueous solution of the silver nanoclusters does not practically alter their absorption spectra, whereas it brings about appreciable Cotton effects or circular dichroism (CD) signals with an opposite sign (mirror-image relationship) in metalbased electronic transition regions. The induced CD response is controllable by a concentration of the chiral fructose added. This phenomenon is due to the complexation of surface 3-MPB ligands with chiral diols (D-/L-fructose) to form optically active boronate esters. The optical/chiroptical studies of the chirally transformed 3-MPB-protected silver nanoclusters find that the induced optical activity is most likely due to the dissymmetric field of the phenylboronic acid-fructose complexes. Introduction Investigations of monolayer-protected metal nanoclusters, possessing typically less than 100 atoms, are largely motivated in the past decade due to their intriguing size-dependent physicochemical properties.1,2 Very recently, much attention is paid on elucidating chirality in metal nanomaterials or nanoclusters owing to the widespread use of chirally modified metal surfaces for enantioselective catalysis3 and optoelectronics applications.4 Surface in situ functionalization by chiral molecules is an effective strategy for inducing chirality on metal nanoclusters.5 According to this strategy, we have reported the synthesis and chiroptical properties of optically active gold and silver nanoclusters protected by chiral penicillamine.6 The studies experimentally provided evidence of the importance of the ligand shell-metal core interactions that the ligand stereochemistry could transfer its chiral information to the metal core and influence the nanocluster’s chiroptical signals. So far, two major different mechanisms have been generally considered for the origins of the nanocluster’s chiroptical response on the basis of a combination of theoretical and experimental data.5-7 They are (i) formation of a intrinsically chiral metal core structure as a result of the deformation induced by the asymmetric adsorbate5a,6c,7c-e and (ii) influence of the dissymmetric fields originated from a chiral packing structure of adsorbate on the cluster surface7a,b and/or from asymmetry induced by the vicinal chiral adsorbate itself.6a,b,7f However, the origin of chirality or optical activity in metal nanoclusters is not still fully understood due to the lack of structural clarification. In our previous studies on chirally modified (penicillamineprotected) silver nanoclusters, we found that the intrinsically chiral metal core would be the main responsible for their very large optical activity, unlike the case of the analogous gold nanoclusters.6 To better understand the behaviors and to disentangle contributions from their mechanisms for more detail, † Part of the “Protected Metallic Clusters, Quantum Wells and MetalNanocrystal Molecules Symposium” special issue. * To whom correspondence should be addressed. Tel: +81-791-58-0160. Fax: +81-791-58-0161. E-mail: [email protected].

postsynthetic asymmetric reactions that do not involve place (or ligand) exchange modifications would give valuable insight into the relationship between the chiroptical responses and surface stereostructures of the nanoclusters. In particular, chiral induction strategy from achirally-modified (not racematemodified) silver nanoclusters is vital and indispensable to avoid diastereomeric interactions. The key to effective chiral induction lies in the utilization of strong intermolecular interactions between the achiral surface ligand and an external chiral molecule, yielding a new stereogenic center in the surface ligand shell. Our approach here is to use silver nanoclusters bearing an achiral boronic acid group that can respond (or bind) to chiral diol compounds in water (Figure 1b). Boronic acids are known to form complexes (boronate esters) with diols rapidly and strongly in basic aqueous media.8,9 Saccharides, having prearranged cis-diols with multiple chiral centers, then form strong complexes with boronic acids to produce typically fivemembered covalent esters and are very useful as an interface for artificial sugar receptors.9 Note that, in aqueous solution, boronic acids have the acidic character, and the trivalent neutral form is in equilibrium with the anionic tetrahedral boronate species: For example, pKa of phenylboronic acid is 8.8 in water and its neutral-anionic equilibrium is expressed as shown in Figure 1a.8b It is also well-known that the quantitative evaluation of phenylboronic acid-saccharide interaction provides the selectivity (or binding) order of the phenylboronic acid as follows: D-fructose > D-galactose > D-mannose > D-glucose.9a In this study, we contemplate transposing the chemistry of boronic acid-saccharides from common molecular systems to surfaces on monolayer-protected metal nanoclusters. D-/LFructose, possessing a strong affinity to phenylboronic acid, can be used as the chiral resource for breaking symmetry in achiral phenylboronic-acid-protected silver nanoclusters via complexation (Figure 1b).8,9 This complex formation reaction also has a great advantage in the use of aqueous media that are suitable for some biological applications. We here describe induced optical activity in boronic-acid-protected silver nanoclusters postsynthetically labeled with chiral fructose. The induced optical activity is then discussed on the basis of the dissymmetric

10.1021/jp910875s  2010 American Chemical Society Published on Web 02/19/2010

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Figure 1. (a) Formation of a tetrahedral phenylboronate anion in basic aqueous media. (b) Reaction scheme for postsynthetic binding between the surface phenylboronic acid and chiral fructose bearing diols in basic aqueous solution. (c) Chemical structures of 3-mercaptophenylboronic acid (3-MPB) and D-fructose.

field of the chirally transformed ligands with fructose, which have a strong influence on the electronic states of the silver core. Experimental Section Materials. Silver nitrate (AgNO3, 99.5%), sodium borohydride (NaBH4, > 90%), methanol (GR grade), 2-propanol (GR grade), acetonitrile (GR grade), acetone (GR grade), and ethylene glycol (GR grade) were received from Wako Pure Chemical and used as received. 3-Mercaptophenylboronic acid (abbreviated as 3-MPB) and D-fructose (g99%) were received from Aldrich and used as received. The chemical structures of 3-MPB and D-fructose are shown in Figure 1c. L-Fructose (g98%), the enantiomer of D-fructose, was purchased from Cosmo Bio Co. Pure water was obtained by a water-distillation supplier (Advantec GS-200). Preparation of 3-MPB-Protected Silver Nanoclusters. Under argon atmosphere, 0.5 mmol of AgNO3 and 0.75 mmol of 3-MPB were at first mixed in methanol (100 mL), followed by the rapid addition of a freshly prepared 0.2 M methanolic NaBH4 solution (25 mL) under vigorous stirring. After further stirring (1.5 h), the solution was stored overnight. Then the solvent was mostly evaporated under vacuum below 30 °C, followed by addition of a small amount of water to redissolve the crude product. The addition of acetone gave a dark-brown precipitate. The precipitate was thoroughly washed with water/ acetone (1/9), water/2-propanol (1/9), and water/2-propanol/ acetonitrile (1/1/8) through redispersion-centrifugation processes. Finally, a nanocluster powder was obtained by a vacuum-drying procedure. Coupling of 3-MPB on the Silver Nanocluster Surface with Chiral Fructose. The complexation reaction was conducted by mixing the as-prepared 3-MPB-protected silver nanocluster compound (0.32 mg/mL) with enantiopure D- or L-fructose at a different fructose concentration (from 10-4 to 10-2 M) in water. The pH of the solutions was about 9.8. Note that, at ambient

Yao et al. temperatures, aqueous solutions of D-fructose at equilibrium contain β-D-pyranose anomer with the most abundant form, β-Dfuranose with the second most abundant form, R-D-furanose, and a trace only of R-D-fructopyranose, and this anomeric equilibrium is reached within tens of minutes.10 Hence to attain the equilibrium, aqueous D- or L-fructose solutions were allowed to stand for about one day at room temperature prior to use. Computational Methods. The geometry optimizations of 3-MPB and the related compounds were carried out with the Gaussian 03 program at the density functional theory (DFT) level using B3LYP functional and a LanL2DZ basis set for a silver atom and a 6-31G* basis set for other atoms in gaseous phase, followed by calculations of their harmonic vibrational frequencies to verify their stability and to obtain IR spectra.11 Calculated frequencies were empirically scaled by a so-called scaling factor (0.9614 in this case).12 Instrumentation. UV-vis absorption spectra were recorded with a Hitachi U-4100 spectrophotometer. Circular dichroism (CD) spectra were recorded with a JASCO J-820 spectropolarimeter. Rectangular 1-cm cuvettes made of quartz were used for the measurements. The mean core size of the silver nanocluster sample was determined by a solution-phase smallangle X-ray scattering (SAXS) technique.6a The SAXS analyses are based on the assumption that spherical clusters are distributed with a simple Γ-distribution function.6a FT-IR spectra were measured with a Horiba FT-720 infrared spectrophotometer by the KBr disk pellet method. Field emission scanning transmission electron microscopy (FE-STEM) was conducted with a Hitachi S-4800 electron microscope operated at 30 kV. Elemental analysis was conducted by energy dispersive X-ray (EDX) spectroscopy excited by an electron beam at 10 kV with an EDAX Genesis-2000 system attached to the S-4800 electron microscope. Results and Discussion 3-MPB-Protected Silver Nanoclusters. a. Size Determination. The X-ray diffractogram of the nanocluster powder sample (Figure 2a) exhibits a broad and intense (111) peak of silver at 2θ ) ∼38°. A typical STEM micrograph (inset in Figure 2a) of the sample displays the presence of small nanoclusters with the sizes of 1-2 nm in diameter. Note that we could not determine the size distribution from the STEM observations because, under the influence of the STEM electron beam irradiation, they were observed to grow and coalesce into larger particles.6c The size distribution of the silver nanocluster compound was then determined by a solution-phase SAXS measurement. Figure 2b shows the experimental scattering profile along with the simulated curve obtained using the assumption that the spherical nanoclusters are distributed in Γ-distribution functions.13 The simulated curve well reproduces the observed profile. The determined core size distribution is depicted in Figure 2c, showing that most of the nanoclusters have their sizes within ∼2.0 nm. On this basis, the average core diameter (dav) of 0.77 nm was estimated. b. Surface Properties. IR spectral analyses of the sample elucidate the surface chemical properties of the 3-MPB-protected silver nanoclusters. We first confirmed the disappearance of the S-H vibrational band (2535-2564 cm-1) in the silver nanoclusters due to the anchoring of 3-MPB to the cluster surface through the sulfur atom. We next focused on vibrational information on the phenylboronic acid moiety that can be observed as characteristic peaks in the 500-2000 cm-1 region. Figure 3a shows the measured IR absorption spectra of pure 3-MPB used and 3-MPB-protected silver nanoclusters in that

Boronic-Acid-Protected Silver Nanoclusters

Figure 2. (a) Powder X-ray diffraction profile of the 3-MPB-protected silver nanocluster compound. The inset is a typical STEM image of the nanocluster sample. (b) Solution-phase small-angle X-ray scattering (SAXS) intensity profile of the 3-MPB-protected silver nanoclusters. The experimental and the simulated profiles are shown by dots and red curve, respectively. (c) Core size distribution of the silver nanocluster compound obtained by the SAXS profile simulation.

wavenumber region. Interestingly, a significant difference is evident between them. To understand the origin of these spectral differences, we have to consider (i) whether the boron-containing moieties exist as boronic acids or boronic anhydrides and (ii) whether the tetrahedral boronate anions (-B(OH)3-) are produced or not in the nanocluster compound through the NaBH4 reduction.14a-c One approach for the IR band assignments based on molecular structures is to optimize the molecular geometry for possible low energy conformations of the system using DFT theory, followed by the spectral simulations. Note here that comparison was made between the calculated (in the gas phase) and measured (in KBr matrices) spectra, allowing semiquantitative assignments of major features to be made with reasonable certainty.14d As a typical 3-MPB anhydride, we chose the trimer.14b In addition, for calculating the silver-thiolate complexes, we chose a simple configuration in which the thiolate anchors directly to one silver atom since IR observations should be a local property of the individual adsorbed molecules.15 The calculated IR spectra of 3-MPB and its anhydride, together with

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Figure 3. (a) Measured FT-IR absorption spectra of pure 3-MPB and 3-MPB-protected silver nanocluster compound. The arrows at around ∼1000 cm-1 stand for the characteristic B-O-H deformation mode of the neutral phenylboronic acid monomer. (b) Calculated IR absorption spectra of 3-MPB monomer and its anhydride (trimer). The stable geometries optimized by DFT calculations are also shown. (c) Calculated IR absorption spectra of Ag-(3-MPB) and anionic Ag-(3-MPB-OH-). The geometries optimized by DFT calculations are also shown.

those of neutral Ag-(3-MPB) and anionic Ag-(3-MPB-OH-) are shown in Figures 3b and 3c, respectively. The corresponding optimized geometries are also shown. In pure 3-MPB, a satisfactory agreement could be obtained between the IR spectra of experiment and calculation of the trimer anhydride (rather than the monomer). According to the literature,16 a characteristic band has been used as a criterion to determine the presence of anhydride: the disappearance (decrease) of a band at around 1000 cm-1 (B-O-H deformation mode; see the arrows labeled in Figure 3). The observed spectrum exhibited almost no peak at around 1000 cm-1, so we can conclude that pure 3-MPB we used contained a considerable amount of its anhydrides. The intense peaks observed at 1354 and 1412 cm-1 are then assigned to the B-O stretch and CdC phenyl stretch modes of the boronic anhydride, respectively. The experimental IR spectrum of the 3-MPB-protected silver nanocluster compound was compared with the calculated ones. It should be first noted that the nature of metal parts has a very minor influence on the vibrations of a ligand molecule,15 that is, calculations for 3-MPB and Ag-(3-MPB) provided very similar IR spectra between them (see Figure 3, panels b and c). A close inspection of the experimental spectrum indicates that the neutral boronic acid monomers are almost absent because

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Figure 4. Absorption spectra of 3-MPB-protected silver nanocluster compound, pure 3-MPB, and pure D-fructose in aqueous solution.

of the nearly absence of the band at around 1000 cm-1 (see the arrows in Figure 3, panels a and c).16,14c The existence of anhydrides would be also ruled out because of the disagreement between experiment and calculation of the anhydride. These are supported by the very weak appearance of the characteristic band observed at 1330 cm-1 (band d) that is ascribed to a B-O stretch mode of the neutral MPB species. A diminishment in this band, on the other hand, means the completion of the octet of the boron through complexation of a third oxygen atom.14b As a consequence, we can identify that bands observed at 935 (band a), 1085 (band b), and 1205 cm-1 (band c) are likely due to the anionic species of 3-MPB based on the calculation for Ag-(3-MPB-OH-). The band at 1375 cm-1 (band e) is primarily an aromatic CdC stretch mode. The peak centered at 1455 cm-1 (band f) would not be clearly assigned but probably due to a mixture of motions including CdC and B-O stretching.14a In any case, the surface chemistry of the present silver nanoclusters revealed that the 3-MPB ligands mostly exist as the anionic form in a tetrahedral boronate. High solubility of this nanocluster compound in water strongly implies the dominant presence of tetrahedral phenylboronate anions on the surface, whereas pure 3-MPB is hard to be dissolved in water. c. Probable Chemical Composition. Elemental analysis based on energy dispersive X-ray (EDX) spectroscopy gives us additional information on the chemical structure of the silver nanocluster compound. According to the EDX spectrum of the nanocluster (Figure S1 in the Supporting Information), the peak of Na was detected as the countercation for the anionic species of surface 3-MPB. Note that the Na element originates from the sodium borohydride reductant used. Moreover, we could determine the molar ratios of S/Ag and Na/S to be ∼1.0 and ∼1.2, respectively. From both the EDX and IR spectral information, we may tentatively deduce a mean chemical formula of the silver nanocluster compound; Ag14(3MPB-OH-)12 · Na12+, under the reasonable assumptions that (i) all surface ligands are anionic tetrahedral species of 3-MPB with sodium countercations; (ii) the silver core with the diameter of 0.77 nm is a sphere with the bulk density;17 (iii) the present cluster can be expressed within a “superatom” concept, where the exceptional stability of the metal cluster is associated with a total count of shell-closing n* ) 2, (8, 18, ...) electrons.18 Although the size distribution of the nanocluster compound is not taken into account, the derived mean chemical formula approximately satisfies the obtained molar ratios of S/Ag and Na/S within the accuracy of EDX quantification.19 See the Supporting Information for more details. Optical/Chiroptical Properties. Figure 4 shows the absorption spectra of 3-MPB-protected silver nanocluster compound

Figure 5. (a) CD spectra of 3-MPB-protected silver nanocluster compound in aqueous solution. No CD signals were detected indicating optically inactive. (b) CD spectra of D-fructose and L-fructose in aqueous solution. Mirror image relationship can be seen between them. The wavelength region painted out with pale yellow corresponds to those shown in Figure 6.

(0.053 mg/mL), pure 3-MPB, and D-fructose (10-2 M) in water. Pure 3-MPB and fructose contribute to the absorption signals only in the UV region (95%) phenylboronic acid moieties exist in a tetrahedral anionic form in the basic solution (by the Henderson-Hasselbalch equation). In addition, since the affinity of fructose with phenylboronic acid is very high in basis aqueous media, more quantitatively, the binding or stability constant between the tetrahedral phenylboronate anion and fructose is reported to be 4370 M-1 (several orders of magnitude larger than that for neutral phenylboronic acid-fructose complex),9a,24 sufficient binding between the surface 3-MPB of the nanoclusters and fructose is expected: When the chemical composition of the silver nanocluster sample is assumed to be expressed simply as Ag14(3-MPB-OH-)12 · Na12+, the above nanocluster

solution contains phenylboronic acid units of ∼8.4 × 10-5 M. Additionally, if each phenylboronic acid unit has the same binding constant with fructose, the concentration of unbound boronic acid moiety can be estimated to be 1.9 × 10-6, 1.5 × 10-5, or 5.1 × 10-5 M at [fructose] ) 10-2, 10-3, or 10-4 M, respectively, meaning that 98%, 82%, or 40% of the phenylboronic acid moieties is coupled with fructose to form a chiral boronate complex, respectively. Figure 6a shows the absorption spectra of the silver nanocluster compound as a function of the D-/L-fructose concentration. We found that fructose did not provide significant absorption changes of the silver nanoclusters, strongly indicating that complexation between the surface 3-MPB and chiral fructose hardly influences the electronic states of the silver nanoclusters, and consequently, the silver core rearrangement or size growth is unlikely to take place upon complexation. When we look more closely at their absorption profiles, the spectrum became slightly sharp with an increase in the concentration of D-/L-fructose (Figure 6a). The slight spectral width narrowing is not due to aggregation/disaggregation of the silver nanoclusters because a typical electron microscopy image for the nanoclusters after complexation with D-fructose was almost invariant with that obtained in the absence of fructose (see the Supporting Information). Hence this small perturbation in the spectra should be caused by the interactions between the surface phenylboronic acid moiety and fructose.

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To obtain an insight into the induced optical activity, we measured CD spectra of the silver nanoclusters in the presence of D- or L-fructose (Figure 6b). The CD spectrum of uncomplexed nanocluster species is also shown in the figure, exhibiting no CD signals. In stark contrast, the nanoclusters showed appreciable chiroptical responses with a positive (negative) peak at around 350 nm when complexed with D-fructose (L-fructose), respectively.25 Namely, an almost perfect mirror-image relationship could be detected in the region of metal-based electronic transitions, implying that enantiomeric binding can produce the corresponding enantiomeric silver nanoclusters. Note that chiral fructose has no CD signatures in the wavelength region longer than ∼300 nm. Furthermore, the CD signal intensity was increased with an increase in the fructose concentration and nearly saturated at between 10-3 and 10-2 M. This behavior is correlated with the estimated coupling yield of the boronic acid moiety with chiral fructose: 98, 82, or 40% at [fructose] ) 10-2, 10-3 or 10-4 M, respectively. Hence it is concluded that observed optical activity is induced by complexation of surface achiral 3-MPB ligands with chiral fructose. In addition, because of almost no change in the absorption property of the nanoclusters upon complexation, the dissymmetric field effect induced by the bound chiral fructose, which influences the electronic structure of the metal core, is essentially active.6a,7c,26 Chiroptical Origin: Effect of the Postsynthetic Complexation by Asymmetric Reaction. To sharply elucidate the effect of a postsynthetic linkage of chiral centers on chiroptical responses of the silver nanoclusters, we calculated the anisotropy factor (or g-factor), defined as g ) ∆/ () intensity of the molar dichroic absorption ∆ normalized to the extinction coefficient ), for the nanocluster compound, and compared with that of the silver nanoclusters directly covered with chiral penicillamine (Pen) with comparable size in the energy region of metal-based electronic transitions.6c From the optical/chiroptical responses of the present nanocluster-fructose complexes, a maximum g-factor of 3.2 × 10-5 was estimated. In the Penprotected silver nanocluster sample with its mean diameter of about 1.0 nm, the maximum g-factor was ∼1.0 × 10-3,6c about 30-fold larger than that of the chirally transformed 3-MPBprotected silver nanocluster by fructose.27 To discuss the difference in the maximum g-factors, we consider ligand chiral structures on the nanocluster surface. It is reported that phenylboronic acid and (D-)fructose in a ratio of 1:1, under aqueous alkaline conditions, form a 2,3,6-tridentate complex as the major product (>80%), with a small amount of 2,3-exo (>10%) and endo (∼5%) isomers.28 Based on this argument, we carried out simple quantum chemical calculations on the corresponding structures for 3-MPB/D-fructose complexes.11,29 In any structures, the obtained separation distance between sulfur and the nearest chiral carbon atom was in 6.2-6.7 Å, which is much longer than that in chiral penicillamine molecule (2.7-2.8 Å). Note that the distance between the metal surface and the nearest chiral carbon might be slightly longer (1-2 Å) than that obtained above in consideration of the Ag-S bond length of about 2.56 Å with a tetrahedral environment for the S atom.30 In the dissymmetric field origin on the metal nanocluster’s optical activity, the separation distance between the chiral point and the metal surface (referred to as DC-M) should have an influence on the chiroptical intensity due to a distance-dependent dissymmetric electrostatic perturbation.7a,b A simple calculation conducted by Beratan and co-workers demonstrated that the CD signal intensity decreased by a factor of ∼0.12 when DC-M increased from 2.41 to 9.46 Å.7a This model calculation is based

Yao et al. on the assumptions that (i) the metal core is modeled with noninteracting electrons confined in a cubic box and (ii) the number of electrons is directly related to that of metal atoms, and in the case of gold, each gold atom contributes only its 6s1 conducting electrons, implying that their results would be also applicable for silver that can contribute only its 5s1 electrons.7a Comparing the chiroptical response (or g-factor) of the fructosebound 3-MPB-protected silver nanoclusters with that of Penprotected nanoclusters, the decrease was much more remarkable (a factor of ∼0.03) even if the difference of DC-M was taken into account. The molecular differences may also affect the chiroptical signatures, but a very intense chiroptical response in chiral Pen-protected silver nanoclusters cannot be explained only in terms of the ligand dissymmetric field; that is, it is most likely due to contributions from both the chirally distorted core and dissymmetric field origins, which is consistent with what we have found previously.6c,d In summary, the present method using postcomplexation of achirally modified metal nanoclusters with chiral compounds has proven to be a powerful strategy to quantitatively induce optical activity in the metal nanoclusters in a controlled manner. In addition, this tactic was valuable for discriminating between origins of the nanocluster’s optical activity. Conclusion Boronic acid is known to strongly bind diols such as chiral saccharides in basic aqueous media with an inherent selectivity order. By using boronic acid as the chiral recognition motif, we have demonstrated induced optical activity in 3-mercaptophenylboronic-acid-protected silver nanoclusters (0.77 nm in diameter) by complexation with chiral D-/L-fructose. IR spectral observations and calculations as well as EDX spectroscopic analysis revealed that the surface phenylboronic acid ligands were present in the tetrahedral boronate form. Addition of chiral D-/L-fructose did not practically alter the absorption spectra of the silver nanoclusters, whereas the nanoclusters displayed appreciable CD signals with an opposite sign (mirror-image relationship) in metal-based electronic transition regions. The induced chiroptical response was controllable and thus increased with an increase in the concentration of the chiral fructose added. Comparison of the optical/chiroptical properties between the chirally transformed phenylboronic-acid-protected and penicillamine-protected silver nanocluster compounds with comparable size suggests that the induced optical activity observed in the present study is most likely due to the dissymmetric field of the boronate-fructose chiral complexes. Optically active nanomaterials will play an enormous role in future life and material sciences, so we believe this postsynthetic complexation methodology facilely allows to design chiral induction in various metal nanoclusters. Acknowledgment. The present work was financially supported by Grant-in-Aids for Scientific Research (B: 19310076 (H.Y.)) from Japan Society for the Promotion of Science (JSPS). We highly appreciate Dr. Akito Sasaki (Rigaku Co.) for the SAXS measurements. Supporting Information Available: Results on an EDX spectrum and a solvent effect on the absorption spectrum of the 3-MPB-protected silver nanocluster compound, together with a typical STEM image of the present nanoclusters after complexation with D-fructose and anisotropy factors of the nonsize-separated D-/L-Pen-protected silver nanoclusters synthesized in situ (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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