Article pubs.acs.org/Langmuir
Boronic Acid-Protected Gold Clusters Capable of Asymmetric Induction: Spectral Deconvolution Analysis of Their Electronic Absorption and Magnetic Circular Dichroism Hiroshi Yao,*,† Masanori Saeki,† and Akito Sasaki‡ †
Graduate School of Material Science, University of Hyogo, 3-2-1 Koto, Kamigori-cho, Ako-gun, Hyogo 678-1297, Japan X-ray Research Laboratory, Rigaku Corporation, 3-9-12 Matsubara-cho, Akishima, Tokyo 169-8666, Japan
‡
S Supporting Information *
ABSTRACT: Gold clusters protected by 3-mercaptophenylboronic acid (3-MPB) with a mean core diameter of 1.1 nm are successfully isolated, and their absorption, magnetic circular dichroism (MCD), and chiroptical responses in metal-based electronic transition regions, which can be induced by surface D-/L-fructose complexation, are examined. It is well-known that MCD basically corresponds to electronic transitions in the absorption spectrum, so simultaneous deconvolution analysis of electronic absorption and MCD spectra of the gold cluster compound is conducted under the constrained requirement that a single set of Gaussian components be used for their fitting. We then find that fructose-induced chiroptical response is explained in terms of the deconvoluted spectra experimentally obtained. We believe this spectral analysis is expected to benefit better understanding of the electronic states and the origin of the optical activity in chiral metal clusters.
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INTRODUCTION Investigations of monolayer-protected metal clusters, possessing typically less than 100 atoms, are largely motivated in the past decade due to their intriguing size-dependent physicochemical properties.1,2 Recent advances in achieving atomic monodispersity in the clusters have particularly allowed detailed characterizations of their electronic, chemical, and spectral properties, such as highly structured (molecule-like) optical absorption and intense fluorescence.3 On the other hand, much attention is also paid on elucidating chirality in metal clusters owing to the widespread use of chirally modified metal surfaces for enantioselective catalysis and optoelectronics applications.4,5 Postsynthetic asymmetric transformation of metal clusters is one of the notable techniques for facile control of symmetrybreaking reactions that can lead to invariance of the cluster’s size and/or morphology. This method will be able to additionally give valuable insight into the relationship between the surface structure and chiroptical response of the clusters. According to this strategy, we have succeeded in achieving chiral induction of achirally-modified silver clusters based on the chemistry of boronic acid−saccharides; that is, we found that silver clusters bearing an achiral phenylboronic acid group responded (or bound) to chiral diols (fructose) to form optically active metal cluster species.6 Boronic acids are known to form complexes with cis-diols rapidly and strongly in basic aqueous media,7,8 so that saccharides, having prearranged cisdiols with multiple chiral centers, can form strong complexes with boronic acids to produce typically five-membered covalent esters with optical activity.8 Note that boronic acids have the © 2012 American Chemical Society
acidic character, and the trivalent neutral form is in equilibrium with the anionic tetrahedral boronate: For example, pKa of phenylboronic acid is 8.8 in water and its neutral−anionic equilibrium is expressed as shown in Scheme 1a.7b Moreover, it Scheme 1
is known that quantitative evaluation of phenylboronic acid− saccharide interaction provides the binding order of the phenylboronic acid as follows: D-fructose > D-galactose > Dmannose > D-glucose.8a,9 Received: December 1, 2011 Revised: January 20, 2012 Published: February 3, 2012 3995
dx.doi.org/10.1021/la204731a | Langmuir 2012, 28, 3995−4002
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fructose, was purchased from Cosmo Bio Co. Carbonate and oxalate standard buffer solutions (pH = 10.0 and 1.68, respectively) were obtained from Wako Pure Chemicals. Pure water was obtained by a water distillation supplier (Advantec GS-200). Preparation and Separation of 3-MPB-Protected Gold Clusters: Preparation. Under an argon atmosphere, 0.5 mmol of HAuCl4 dissolved in water (0.121 M) and 0.75 mmol of 3-MPB were at first mixed in methanol (100 mL), followed by rapid addition of a freshly prepared ice-cooled 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 darkbrown precipitate, while the supernatant was slightly white opaque. The precipitate was thoroughly washed with water/acetone (1/9), water/2-propanol (1/9), and water/acetonitrile (1/9) through redispersion−centrifugation processes. Finally, a cluster powder was obtained by a vacuum-drying procedure (as-prepared sample). Separation. The gold clusters can be negatively charged to form boronate species in basic solutions (Scheme 1), so to separate the clusters that differ in size and/or charge, we applied polyacrylamide gel electrophoresis (PAGE) using a slab gel unit (ATTO, AE-6200).16 The gels were comprised of 3% stacking gel (pH 9.0) and 20% separating gel (pH 9.0). The as-prepared product was first dissolved in a buffer solution (pH = 9.0), followed by loading onto the stacking gel top and eluted for ∼5 h at a constant voltage (150 V). It should be noted that three separable bands are typically assigned, but we focus on the most abundant fraction (band 2, see Figure 1b). To extract the gold cluster compound into aqueous solution, a part of the gel containing the fraction was cut out and homogenized, followed by the addition of distilled water. The gel was removed by a centrifuge. Addition of hydrochloric acid (6.0 M) into the supernatant solution (extract) allowed obtaining precipitates of the product (fractioned) due to the formation of neutral boronic acids from boronates, followed by washing with a common protocol for purification. We used methanol/aqueous buffer (1/1 v/v) mixtures for spectroscopic measurements.17 The pH of the buffer solution was 10.0 (carbonate; 0.5 M) or 1.68 (oxalate; 0.5 M) at room temperature. Coupling of Surface Protective 3-MPB with Chiral Fructose. The asymmetric transformation was conducted by mixing the fractioned gold cluster compound with enantiopure D- or L-fructose in methanol/aqueous buffer (1/1 v/v) mixtures.6a The absorbance at 400 nm was set around 1.0 (1 cm cuvette) and the concentration of fructose 10−3 M. Note that, at ambient temperatures, aqueous solutions of D-/L-fructose at equilibrium contain β-D-/L-pyranose anomer with the most abundant form, β-D-/L-furanose with the second most abundant form, α-D-/L-furanose, and a trace only of α-D-/Lfructopyranose, and this anomeric equilibrium is reached within tens of minutes.18 Hence, to attain the equilibrium, aqueous D- or L-fructose solutions were allowed to stand for about 1 day at room temperature prior to use. 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. Magnetic circular dichroism (MCD) measurements were made with the above spectropolarimeter equipped with a JASCO permanent magnet (PM491LB) of 1.6 T with parallel and antiparallel fields. Rectangular 1 cm or 5 mm cuvettes made of quartz were used for the CD or MCD measurements, respectively. In spectral shape analysis, peak fitting was performed using multipeak fitting packages included with Igor Pro ver.5 (WaveMetrics Inc.). The mean core diameter of the gold cluster sample was determined by a solution-phase small-angle X-ray scattering (SAXS) technique.16 The SAXS analyses are based on the assumption that spherical clusters are distributed with a simple Γdistribution function. FT-IR spectra were recorded with a Horiba FT720 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 (30 kV). Elemental analysis was conducted by energy dispersive X-ray (EDX)
Meanwhile, electronic and magnetic properties of some inorganic metal complexes have been studied by magnetic circular dichroism (MCD) in the UV−vis region.10 MCD spectroscopy measures the difference in absorption of left- and right-circularly polarized light in the presence of a magnetic field oriented parallel to the direction of light propagation.10,11 It is actually an application of the well-known Faraday effect, and the magnetic field induces apparent optical activity in any sort of species. In contrast to electronic CD spectroscopy, the species need not to be optically active and the analysis can provide information on electronic states as well as magnetic properties of the system because MCD is a consequence of the interaction of electronic energy levels with the magnetic field.11 The MCD signal then arises from the same transitions as those seen in the UV−vis absorption spectrum, but the selection rules are different. Hence MCD spectroscopy is complementary to UV−vis spectroscopy, yielding detailed information on the electronic structures of the species. For example, MCD spectra of the gold(I) complex Au9(PPh3)83+ (= Au(AuPPh3)83+) have been measured in a solution phase and provided better (higher) resolution and more individual features compared to the electronic absorption. For the Au9(PPh3)83+ complex ion, the spectra have been essentially interpreted in terms of the intramolecular transitions of the Au8 framework (σ Au−Au interaction from 6s orbitals) based on the structural assumption of D2h skeletal symmetry.12 In this study, we combine absorption and MCD spectroscopy for monolayer-protected gold clusters to gain a better understanding of their electronic structures as well as their chiroptical responses induced by their surface complexation with chiral compounds. Apart from stable, atomically monodisperse gold clusters such as Au 25 (SR) 18 − and Au38(SR)24,3a,b,13 UV−vis absorption spectra generally exhibit less pronounced structure even though they are size-selected14 and thus hard to isolate their electronic transitions without applying theoretical calculations based on the well-defined structural information. Almost no MCD investigation in the cluster size region of metals such as gold and silver (1−2 nm in diameter) has been reported so far,15 so we make an attempt to conduct spectral analysis of both the electronic absorption and MCD spectra of the size-fractioned gold cluster compound to obtain detailed information on their electronic states. A relationship between the MCD and normal (induced) CD responses is also examined, both of which distinctly stem from the cluster’s electronic transitions. We newly synthesize and utilize achiral 3-mercaptophenylboronic acid (3-MPB)-protected gold clusters (Scheme 1b) to evaluate their MCD responses,9 and comparison will be made between the chiroptical responses induced by postsynthetic asymmetric reaction using the chemistry of boronic acid−chiral fructose and the electronic transitions obtained by the spectral analysis of absorption and MCD.6a We also find that this asymmetric induction is pH-sensitive, additionally suggesting that the gold cluster−fructose complex formation has a great advantage for some biological applications.
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EXPERIMENTAL SECTION
Materials. HAuCl4·4H2O (99%), sodium borohydride (NaBH4, >90%), methanol (GR grade), 2-propanol (GR grade), acetonitrile (GR grade), and acetone (GR grade) were received from Wako Pure Chemical and used as received. 3-Mercaptophenylboronic acid (abbreviated as 3-MPB) and D-fructose (≥99%) were received from Aldrich and used as received. L-Fructose (≥98%), the enantiomer of D3996
dx.doi.org/10.1021/la204731a | Langmuir 2012, 28, 3995−4002
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IR spectral analyses elucidate the surface chemical properties of the as-prepared sample. First, the disappearance of the S−H vibrational band (2535−2564 cm−1) proved anchoring of 3MPB to the cluster surface through the sulfur atom. We next focused on vibrational information on the phenylboronic acid moiety that exhibits characteristic peaks in the 500−2000 cm−1 region (Figure 1a). In the assignments, we have to consider (i) whether the boron-containing moieties exist as free (monomeric) boronic acids or oligomeric boronic anhydrides and (ii) whether the tetrahedral boronate anions (−B(OH)3−) are produced or not in the cluster sample through NaBH4 reduction.6a,20 According to the literature,21 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) implies the formation of anhydrides. The observed spectrum (Figure 1a) exhibited a strong peak at around 1000 cm−1 (band A), so the existence of 3-MPB anhydride species is unlikely. The intense peaks observed at 1347 and 1445 cm−1 are then assigned to the B−O stretch and CC phenyl stretch modes of the free boronic acid, respectively. Note that anionic form of 3-MPB (that is, 3-MPB−OH− of a tetrahedral conformation with a sp3hybridized boron atom) does not exhibit a band at ∼1000 cm−1, but have a band at 1100−1130 cm−1, so the observed peak at 1130 cm−1 (band B) is likely due to the presence of the tetrahedral anionic species. Elemental analysis based on EDX spectroscopy gives us additional information on the chemical structure of the gold cluster compound. According to the EDX spectrum of the asprepared cluster sample (Supporting Information, Figure S2), the peak of Na was detected as the countercation for the anionic 3-MPB (boronate form); the Na element originates from the sodium borohydride reductant used.6a Moreover, we could determine the atomic ratios of S/Au and Na/S to be 0.60−0.66 and 0.30−0.33, respectively. Therefore, from both the EDX and IR spectral information, we may tentatively deduce a molar ratio between the neutral and anionic forms of surface 3-MPB as about 7:3. PAGE Separation. The 3-MPB-protected gold clusters will be fully negatively charged when dispersed in a strongly basic solution because pKa of phenylboronic acid is reported to be 8.8 in water (its neutral−anionic equilibrium is expressed in Scheme 1a).7b Hence, the clusters can be separated according to their size and charge by gel electrophoresis. A photograph of typical PAGE separation for the as-prepared gold cluster sample is shown in Figure 1b. We could observe three typical discrete bands in the gel under normal illumination, suggesting the appearance of magic number compounds. We refer to the separable species as compounds 1, 2, ..., with 1 being the most electrophoretically mobile species; but hereafter, we focus on the fraction denoted as 2 because this compound was the most abundant species and thus could be easily collected and analyzed. Finally, compound 2 was successfully isolated as solid upon addition of hydrochloric acid. The mean core size of the cluster compound 2 was determined by a solution-phase SAXS measurement. Figure 1c shows the experimental scattering profile of the compound along with the simulated curve; the simulated curve well reproduces the profile using the Γ-distribution function.16 The core diameter distribution determined is depicted in Figure 1d, and the average core diameter (dav) was 1.09 nm. This is roughly in good agreement with the average diameter of the asprepared gold clusters estimated by XRD.19b If we assume that
Figure 1. (a) FT-IR absorption spectrum of as-prepared 3-MPBprotected gold cluster sample. The 3-MPB moieties exist as both free phenylboronic acid and anionic phenylboronate, which is revealed by the appearance of the bands at ∼1000 cm−1 (labeled A) and ∼1130 cm−1 (labeled B), respectively. (b) Photograph of PAGE separation for the as-prepared gold cluster sample. Three representative bands are labeled in the order of cluster’s mobility (1 being the most mobile), and the most abundant species is compound 2. (c) Solution-phase small-angle X-ray scattering (SAXS) intensity profile of the gold cluster compound 2. The experimental and the simulated profiles are shown by dots and red curve, respectively. (d) Core diameter distribution of the cluster compound 2 obtained by the SAXS profile simulation. spectroscopy excited by an electron beam at 9.0 kV with an EDAX Genesis-2000 system attached to the S-4800 electron microscope.
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RESULTS AND DISCUSSION
Chemical Properties of As-Prepared Gold Cluster Compound. The X-ray diffractogram of the as-prepared powder sample of the gold clusters (see Supporting Information, Figure S1) exhibited a broad and intense (111) peak of gold at 2θ = ∼38°, suggesting the cluster core diameter of ∼0.9 nm, which was calculated from the Scherrer equation based on the width of the (111) reflection.13d,19a 3997
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∼280 nm are observed in the optical spectrum.14a This behavior would indicate some major structural differences between the Au40 and Au38 clusters with the same ligand. Our cluster compound 2 exhibits three steps at ∼590, 390, and 270 nm, relatively similar to the optical behavior of the Au40 cluster, and thus may have a different structural motif from the magic Au38 cluster having a face-fused bi-icosahedral Au23 core and that the core is further capped by three S−Au−S and six S− Au−S−Au−S staples.14 In any case, three ill-resolved transitions are only detectable in compound 2, so better resolution (if obtainable) will allow us to separate the absorption components with more accuracy and to give helpful information on its specific electronic structure. Magnetic Circular Dichroism (MCD). To obtain further insight into the cluster’s electronic states, magnetic circular dichroism (MCD) spectroscopy is quite effective because MCD gives valuable indications of the exact energy levels and degeneracies of the excited and ground states; in other words, MCD spectra in the UV−vis region assist the interpretation of electronic absorption spectra and provide experimentally based information on the electronic states involved in the observed transitions.10,11 A companion use of absorption spectra obtained simultaneously is required, and consequently, we can see more individual features through the MCD and electronic absorption spectra. The MCD spectra of the gold cluster compound 2 are shown in Figure 2b. The spectrum shows overall negative features in the metal-based electronic transition region at the magnetic field of +1.6 T, but the sign of the MCD signal is completely reversed when the field is switched (−1.6 T), confirming that signatures are not from an experimental artifact but originate from real MCD signals. Note that these MCD features must correspond to electronic transitions (even unresolved) in the absorption spectrum. The MCD signals were very weak at >∼500 nm, probably arising from transitions out of the HOMO into LUMO (essentially intraband transitions),13d,24 whereas relatively strong MCD signals were detected at higher energy transitions (
