Synthesis of Normal and Inverted Gold− Silver Core− Shell

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Synthesis of Normal and Inverted Gold-Silver Core-Shell Architectures in β-Cyclodextrin and Their Applications in SERS Surojit Pande,† Sujit Kumar Ghosh,† Snigdhamayee Praharaj,† Sudipa Panigrahi,† Soumen Basu,† Subhra Jana,† Anjali Pal,† Tatsuya Tsukuda,‡ and Tarasankar Pal*,§ Departments of CiVil Engineering and Chemistry, Indian Institute of Technology, Kharagpur 721302, India, and Institute for Molecular Science (IMS), Myodaiji, Okazaki 444-8585, Japan ReceiVed: January 11, 2007; In Final Form: May 21, 2007

Beta-cyclodextrin (β-CD) in alkaline solution has been observed to produce mono- and bimetallic nanoparticles of silver and gold and to provide in-house stability to both types of particles. Thus, the weak reducing capability of the β-CD molecule (oxidation occurs at +1.33 V vs Ag/AgCl) and its unique kinetic control over the evolution of both normal and inverted core-shell bimetallic architectures have been established. The structure and composition of the bimetallic particles were characterized by UV-visible spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, electron dispersive spectroscopy, and X-ray photoelectron spectroscopy. Bimetallic core-shell particles containing silver shells have been shown to provide an elegant SERS-active substrate compared to the corresponding monometallic nanoparticles, and therefore, they highlight the importance of electronic ligand effects on the enhancement of the Raman signals of molecular probes on nanostructured metallic surfaces.

1. Introduction In recent years, bimetallic nanoparticles have fascinated scientists because of their superior optical,1 electronic,2 catalytic,3 and magnetic4 properties compared to their monometallic counterparts. These interesting physicochemical properties result from the combination of two kinds of metals and their fine structures, evolving new surface characteristics.5 Thus, bimetallic nanoparticles, composed of two different metal elements, are of greater interest than monometallic nanoparticles, from both the scientific and technological points of view.6-10 The structure of bimetallic nanoparticles is defined by the distribution modes of the two elements and can be oriented in random alloy, alloy with an intermetallic compound, cluster-in-cluster, and coreshell structures. A variety of synthetic strategies (chemical, radiolytic, and photolytic) have been adopted during the past few years to prepare stable bimetallic particles in both aqueous11-13 and nonaqueous14 media. In addition, different capping agents have been used to stabilize the resulting particles.15,16 Methods for the preparation of bimetallic nanoparticles from metal salts can be divided into two groups: coreduction and successive reduction of two metal salts. Coreduction is the simplest preparative method whereby the simultaneous reduction of two metal precursors is achieved. Successive reduction is usually carried out for the preparation of core-shell or structured bimetallic nanoparticles. Therefore, synthesis of stable bimetallic particles with sizes in the nanometer range requires a great deal of control over the synthetic technique. Colloidal solutions of noble metals, especially those of gold and silver, have been studied extensively because of their intense absorption band in the visible region, often called surface * To whom correspondence should be addressed. E-mail: tpal@chem. iitkgp.ernet.in. † Department of Civil Engineering, Indian Institute of Technology. ‡ Institute for Molecular Science (IMS). § Department of Chemistry, Indian Institute of Technology.

plasmon absorption. This band is attributed to the collective oscillation of the electron gas in the particles with a change in the electron density at the surface.17 Because of these attractive plasmon absorption features, the optical properties of bimetallic nanoparticles composed of gold and silver are the subject of considerable interest in the fields of nanoscience and nanotechnology. The comparison of calculated and measured surface plasmon (SP) extinction spectra has frequently been employed as one of the criteria for distinguishing between alloyed and layered core-shell structures of bimetallic Au-Ag/Ag-Au nanoparticles. Since its discovery in 1974, the single-molecule sensitivity of Raman scattering enhanced by resonantly excited metal nanoparticles has caused a renewed interest in surfaceenhanced Raman scattering (SERS).18 However, the mechanism for SERS has been a matter of considerable debate.19-24 The two phenomena responsible for SERS are a large increase in the electric field near the surface (electromagnetic effect) and specific adsorbate-surface interactions (chemical effect). Modified surfaces of silver and gold nanoparticles have attracted wide interest because of their potential applications in sensors,25 catalysis,26 nonlinear optical materials,27 etc. Modification of silver and gold nanoparticle surfaces with different organic receptors is important for the development of biological tracers as well as optoelectronic nanodevices. Cyclodextrin (CD), a soluble nontoxic molecule, is an important receptor and becomes a unique choice as it can form both channel and cage complexes incorporating nanosize metal guests with its cyclic oligosaccharide containing internal cavities. The host molecules are made up of six, seven, or eight glucose units connected in a large ring, called R-, β-, or γ-CD, respectively. Among the three CDs, β-CD is the most widely used because its internal cavity diameter ranges from 6 to 6.5 Å.28 The “mod of fit” between host and guest and the hydrophobic effect are probably the most important. The use of β-CD as a complexing agent represents a useful strategy for minimizing the photoinduced biological damage associated with many nonsteroidal anti-

10.1021/jp0702393 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007

Gold-Silver Core-Shell Architectures in β-Cyclodextrin inflammatory drugs (NSAIDs) as well as a tool for increasing drug photostability. Moreover, the weak binding forces responsible for association with the β-CD cavity provide a useful model for mimicking the interactions of drugs with hydrophobic pockets of biological substrates. To exploit the host-guest interactions of β-CD, different modified29 and unmodified β-CD molecules have been chemisorbed onto gold films, gold electrodes, and gold electrodes of a quartz microbalance.30-32 Recently, perthiolated β-CD, i.e., per-6-thio-β-CD and mono6-lipoyl-amido-2,3,6-O-permethyl-β-CD, has been used to modify gold nanoparticle surfaces upon direct addition to HAuCl4 solution. Unmodified β-CD has also been used to prepare gold and silver nanoparticles in the presence of different reducing agents such as dimethyl formamide, ethanol, methanol, ethylene glycol, and sodium citrate.33-35 We recently reported the reducing and capping properties of glucose for preparing size- and shape-selective metal nanoparticles in alkaline medium.36 As β-CD is the higher homologue of glucose, we tried to examine its glucose-like properties for the reduction of metallic precursor salts to well stabilize metal nanoparticles. With this idea in mind, for the first time, we have employed unmodified β-CD in aqueous solution, which acts as both a reducing and stabilizing agent in alkaline medium. In this article, we report a general method for the synthesis of gold and silver nanoparticles and their bimetallic conjugates, i.e., Aucore-Agshell and Agcore-Aushell, using unmodified β-CD in alkaline medium. The structure and composition of the bimetallic particles were characterized by UV-visible spectroscopy, transmission electron microscopy (TEM), highresolution transmission electron microscopy (HRTEM), electron dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). Here, β-CD plays a dual role: it acts as both a reducing and capping agent. β-CD becomes kinetically capable of reducing Au(III) ions on silver seed particles to obtain an inverted core-shell architecture. Finally, we used 1,10-phenanthroline as a molecular probe to compare the SERS activity of the core-shell bimetallic nanoparticles with that of the corresponding monometallic nanoparticles and found that the coreshell structured bimetallic nanoclusters exhibit a higher SERS activity than the corresponding monometallic nanoclusters. 2. Experimental Section 2.1. Reagents and Instruments. All reagents were of analytical reagent grade. Chloroauric acid (HAuCl4), silver nitrate (AgNO3), β-CD, and 1,10-phenanthroline were obtained from Sigma-Aldrich and were used as received. Pyrene, mercuric chloride (HgCl2), sodium hydroxide (NaOH), and potassium cyanide (KCN) were obtained from S. D. Fine Chemicals. Dopamine, aminopyrene, crystal violet, and rhodamine 6G were obtained from Sigma-Aldrich. Doubly distilled water was used throughout the course of this investigation. All absorption spectra were recorded on a Shimadzu UV160 spectrophotometer (Kyoto, Japan) using solutions in a 1-cm quartz cuvette. TEM, HRTEM, and EDS measurements of the metal sols were performed with a Hitachi H-9000 NAR instrument on samples prepared by placing a drop of fresh metal sols on copper grids precoated with carbon films and then evaporating the solvent under a vacuum. XPS analysis was performed on an ESCA LAB MK II instrument using Mg as the exciting source. The samples were prepared by placing one drop of the prepared mono- and bimetallic nanoparticles on a clean glass slide and then allowing them to dry in air. SERS spectra of 1,10-phenanthroline were obtained with a Renishaw Raman microscope, equipped with a He-Ne laser excitation

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10807 source emitting at a wavelength of 633 nm and a Peltier cooled (-70 °C) charge coupled device (CCD) camera. A Leica microscope was attached and was fitted with three objectives (5×, 20×, 50×). For these experiments, the 20× objective was used. Laser power at the sample was 20 mW, and the data acquisition time was usually 30 s. The holographic grating (1800 grooves/mm) and the slit provided a spectral resolution of 1 cm-1. 2.2. Synthesis of Monometallic Gold and Silver Colloids. The metal nanoparticles (gold and silver) were synthesized by dissolving 0.0396 g of β-CD in 4.93 mL of water and shaken well. After dissolving the β-CD, 0.02 mL of the corresponding metal salt solution HAuCl4 (10 mM) and AgNO3 (10 mM) were added to it. After 2 min, 0.05 mL of NaOH (1.0 M) was added to the solution so that the pH of the solution became ∼10-12. Then, the reaction mixtures were shaken well and heated on a water bath. After ∼20 min, the solution turned pink for gold and yellow for silver, indicating the formation of the corresponding metal nanoparticles in the solution. 2.3. Synthesis of Aucore-Agshell and Agcore-Aushell Bimetallic Nanoparticles. Aucore-Agshell nanoparticles were synthesized using the following procedure: An aliquot of 0.04 mL of AgNO3 (10 mM) was added dropwise to 5 mL of the preformed gold colloidal solution. The solution was allowed to incubate for 5 min and then heated on a water bath. After about 30 min, the pink color of the solution turned to reddish yellow, indicating the formation of a silver shell on the gold particles, i.e., the formation of Aucore-Agshell particles. During the formation of the particles, the initial pink color of the solution gradually deceased, and a yellow color developed. The final concentrations of the gold and silver colloids in the bimetallic nanoparticles were adjusted to 0.05 and 0.1 mM, respectively. No extra β-CD or NaOH was added to induce the chemical reduction of silver ions and their deposition on the gold seed particles. The alkalinity of the gold colloidal solution helps to induce the chemical reduction of Ag(I) ions and their deposition on the gold particles. For the synthesis of inverted core-shell nanoparticles, an aliquot of 0.04 mL HAuCl4 (10 mM) was added dropwise to 5 mL of preformed silver colloids with vigorous stirring. During the progress of the reaction, the yellow color of the silver colloid gradually diminished, and the pink color of the gold colloid evolved; after ∼20 min, the pink color of the solution was found to be persistent, indicating the formation of Agcore-Aushell nanoparticles. Here, also, no extra β-CD or NaOH was added to induce the formation of core-shell particles. 3. Results and Discussion 3.1. Evolution of Normal and Inverted Core-Shell Bimetallic Nanoparticles. The interaction of small metal particles with an external electromagnetic field induced by light results in coherent oscillations of the conduction (free) electrons (mainly within the surface), called surface plasmon oscillations. This surface plasmon resonance largely depends on the particle size and shape and, of course, on the metallic material and its surrounding environment. The optical absorption spectra of gold and silver colloids have been a subject of study for decades. Because of the different plasmonic excitation resonances of the two metals, these colloids exhibit absorption maxima at about 520 and 402 nm for gold and silver, respectively. Figure 1 shows UV-visible spectra of gold and silver colloids in their final stage of preparation using β-CD at an alkaline pH of ∼10-12 by reduction of HAuCl4 and AgNO3, respectively. Interestingly, it was seen that the absorption profile shows a high degree of

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Figure 1. Absorption spectra of (a) Au(0) and (b) Ag(0) seed particles.

Pande et al. particles gradually becomes blue-shifted, and the absorption spectrum of silver emerges. Within ∼20 min, the two bands merge together into a single band at 402 nm. Finally, the AucoreAgshell bimetallic colloids show only one plasmon resonance at 402 nm, which can be attributed to the plasmon resonance of silver particles alone. The appearance of only one absorption band corresponding to silver indicates that homogeneous mixed colloidal particles of the two metals are formed without significant formation of independent particles. The presence of only one plasmon band indicates the core-shell structure of the particles that reflects the true silver plasmon band as if there were no monometallic gold in the medium. Therefore, it is presumed that the gold nanoparticles are now completely covered by silver. The evolution of the surface plasmon band for the coreshell bimetallic nanoparticles can be described by considering the polarizability of a core-shell particle of radius R as follows39 (s - m)(c + 2s) + (1 - g)(c - s)(m + 2s) R ) R3 (s + 2m)(c + 2s) + (1 - g)(2s - 2m)(c - s)

Figure 2. Time-dependent absorption spectra of (A) Aucore-Agshell and (B) Agcore-Aushell bimetallic nanoparticles prepared from β-CD (7 mM) capped Au and Ag seed at pH ∼ 10-12.

well-imposed absorption profiles, indicating the reproducibility of nanoparticle synthesis during β-CD-mediated reduction for the metal salts. During the evolution of the bimetallic nanoparticles from the monometallic particles, we recorded their UV-visible spectra at different stages. The modification of the plasmon absorption band in successive stages of formation of the core-shell structure from gold seeds is shown in Figure 2A. Initially, gold particles were synthesized separately by the β-CD reduction method and were used as seed for the evolution of AucoreAgshell bimetallic nanoparticles. The colloidal solution of the seed particles shows a pink color with an absorption band at 520 nm. Upon addition of Ag(I) ions to the gold seed particles, silver ions are adsorbed onto the surface of the gold seed particles and thereby reduced37,38 in successive stages to form a shell layer over the gold particles, as seen from the absorbance measurements. At the intermediate stages of particle evolution, two plasmon bands can be observed at 508 and 438 nm, which indicates the partial coverage of gold by silver nanoparticles. With progressive heating, the absorption band of the gold

(i)

where m is the dielectric constant of the ambient; c and s are the dielectric functions of the core and shell materials, respectively; and g is the volume fraction of the shell layer (i.e., if the outer radii of the composite particle and its core are R2 and R1, respectively, then g ) 1 - R13/R23). This equation indicates that, in the absorption spectrum, which initially looks like that of gold, the surface plasmon peak observed for silver colloidal particles progressively gets more intense as the silver shell increases in thickness. Thus, it can be concluded that the optical properties differ from those of both silver and gold and the AuAg composition varies smoothly from Ag-lean to Ag-rich material as the concentration of silver allowed to react with the colloidal gold seed particles increases. Figure 2B shows the emergence in the absorption spectra of gold-coated silver colloids, i.e., Agcore-Aushell bimetallic nanoparticles from silver seeds. When HAuCl4 is added, the color of the silver colloids changes to reddish pink through orange within a few minutes. In the colloid having a lower amount of gold deposited on silver, a unique, broad absorption band appears in an intermediate wavelength region (within 405515 nm) because of the partial coverage of the seed particles. The peak maximum shifts to longer wavelength compared to that of silver nanoparticles. Moreover, there is no separate plasmon resonance of silver in the gold colloids, which indicates that the nanoparticles are not a simple physical mixture of two different kinds of metal particles. Finally, the peak becomes shifted to 523 nm, which can be attributed to the complete encapsulation of silver nanoparticles by gold, i.e., Agcore-Aushell particles are formed. The changes in the surface plasmon absorption of the Agcore-Aushell particles can be explained by considering the polarizability in eq i in a fashion similar to that used for the Aucore-Agshell particles. From these surface plasmon resonance spectra, we conclude that the monometallic gold and silver colloids and bimetallic Aucore-Agshell and Agcore-Aushell particles are formed by using β-CD at alkaline pH of ∼10-12. Thus, this phenomenon indicates that β-CD acts as a reducing agent under alkaline conditions. 3.2. Characterization of the Bimetallic Nanoparticles. 3.2.1. Transmission Electron Microscopy (TEM) Study. Clear confirmation of the formation of normal and inverted coreshell structures emerged from TEM, HRTEM, EDS, and XPS studies. The TEM images of the mono- (gold and silver) and

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Figure 3. TEM images of (A) Au(0), (B) Ag(0), (C) Aucore-Agshell, and (D) Agcore-Aushell nanoparticles.

bimetallic particles (Aucore-Agshell and Agcore-Aushell) are shown in Figure 3A-D. From the TEM images, it can be seen that the monometallic particles are ∼10 nm, which is consistent with the results obtained by UV-visible spectroscopy. From TEM images of bimetallic particles, it can be observed that the particle sizes are 13 ( 2 and 11 ( 3 nm for the Aucore-Agshell and Agcore-Aushell particles, respectively. To examine whether a core-shell structure was obtained in the β-CD reduction reaction, HRTEM images of the as-prepared materials were obtained. The HRTEM images support the formation of normal and inverted core-shell structures rather than hollow-core structure. Figure 4A,B shows HRTEM images of Aucore-Agshell and Agcore-Aushell nanoparticles, respectively. The inverted core-shell structure shows a less dense interior core of silver surrounded by a thicker shell. Here, the less dense interior core and thicker shell were produced because of presence of silver in the core and gold in the shell. EDS measurements indicate the organization of the particles or the presence of the metals employed in the preparation procedure. Here, the EDS spectra of the Agcore-Aushell structure show the presence of silver and gold atoms (see Supporting Information). HRTEM observations together with the EDS measurements, conducted with electron beam irradiation clearly indicate the formation of the Agcore-Aushell structure. Prolonged electron beam irradiation (∼10 min) during EDS studies not only destroys the core-shell morphology but also brings the core material, Ag(0), to the surface (see Supporting Information). Thus, both silver and gold become obvious surface atoms, and the bimetallic particles take on a new shape. From this

observation, we conclude that, in the case of Agcore-Aushell nanoparticles, silver remains in the core as Ag(0). 3.2.2. X-ray Photoelectron Spectroscopy (XPS). XPS was employed as a surface monitoring technique to further support the above results. In the case of the Aucore-Agshell nanoparticles (Figure 5A,B), XPS clearly shows that the shell is composed of silver. The binding energies of Ag 3d5/2 and Ag 3d3/2 measured for the two peaks are 373 and 379 eV, respectively. The binding energies of Au 4f7/2 and Au 4f5/2 were measured at approximately 88.2 and 92 eV, respectively. Similarly, in the case of the Agcore-Aushell nanoparticles (Figure 6A,B), the shell is encapsulated with gold. The binding energies of Au 4f7/2 and Au 4f5/2 measured for the two peaks are 88.2 and 92 eV, and those for Ag 3d5/2 and Ag 3d3/2 are 373 and 379 eV, respectively. The energies for gold and silver are in good agreement with the literature values for the binding energies of gold and silver.40 These data confirm the formation of an inverted core-shell structure rather than formation of a hollow core inside the gold shell. 3.2.3. Cyanide Dissolution Reaction. To confirm the formation of the core-shell architectures, i.e., Aucore-Agshell and Agcore-Aushell structures, typical complexation reactions with cyanide were carried out under ambient conditions.12,36 The successive changes of the surface plasmon absorption of the bimetallic colloids upon dissolution with cyanide clearly support the formation of core-shell structures. An aliquot of KCN (10 mM) was added to 5 mL of bimetallic colloids separately, and the changes in absorbance values were measured spectrophotometrically. In the case of Aucore-Agshell particles, it was

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Figure 5. XPS spectra of (A) Aucore and (B) Agshell in Aucore-Agshell bimetallic nanoparticles.

Figure 4. HRTEM images of (A) Aucore-Agshell and (B) Agcore-Aushell bimetallic nanoparticles.

found that the shell first dissolves slowly and the peak due to the Aucore-Agshell nanoparticles gradually red shifts. On continuous shaking in the presence of oxygen, the yellow-colored solution (Aucore-Agshell nanoparticles) turned pink because of the stepwise removal of silver layers. Finally, bare gold nanoparticles, i.e., seeds, reappeared with the plasmon peak for gold nanoparticles at 526 nm via a double-hump curve. Addition of excess cyanide ions then dissolved the gold nanoparticles, and finally, the solution became colorless. This study confirms that the gold layer was completely covered by silver nanoparticles, i.e., Aucore-Agshell bimetallic nanoparticles formed in solution. Similarly, in the case of Agcore-Aushell nanoparticles, the Au shell first dissolved, and the color changed from pink to yellow. Because of the dissolution of the Au shell, the peak was blue-shifted to 412 nm for silver, and finally, the yellow color vanished because of the dissolution of the Ag core. The kinetics of the dissolution reaction has been described elsewhere.41 Thus, the stepwise vanishing of the surface plasmon oscillation demonstrates that the core-shell structures are dissolved in the presence of cyanide ions under ambient conditions. 3.3. Role of β-CD. 3.3.1. Kinetic EVolution and Stabilization of the Particles: Ordinary Role of β-CD. Through the manipulation of the kinetics of particle growth, β-CD is introduced to the reaction medium at a particular concentration that preferentially allows the adsorption, reduction, and growth of a metal on preformed gold or silver particles, resulting in bimetallic structures. The layer-by-layer wrapping of one metallic constituent onto the surface of the other is observed because of the similar lattice parameters of the constituents.42 Thus, hierarchical

Figure 6. XPS spectra of (A) Agcore and (B) Aushell in Agcore-Aushell bimetallic nanoparticles.

nanostructured core-shell structures of both normal and inverted architectures are generated. As these metallic particles shrink into the nanoscale regime, either electrostatic43 or steric stabilization44 for the particles must occur to account for the very high surface energies of the particles in the nanometer size range. β-CD molecules serve this purpose well. Alkaline

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conditions facilitate the deprotonation of the alcohol functionalities of β-CD molecules (the stretching frequency due to -OH shifts from 3368 to 3390 cm-1), which promotes the kinetic evolution and stabilization of nanoparticles. Interestingly enough, the evolution of mono- or bimetallic nanoparticles of gold and silver from the corresponding precursor salt solutions was not observed when individual PACs (polyaromatic compounds) such as pyrene or 1,10-phenanthroline or HgCl2-saturated β-CD were employed in lieu of unsaturated β-CD. These guest molecules individually can saturate the cavity of β-CD, forming host-guest-type inclusion complexes.45 In such cases, β-CD binds the guest, the alcohol functionalities of β-CD remain completely engaged with stoichiometric amounts of individual guest molecules, and cavity saturation takes place through hydrogen bonding. Under these circumstances, β-CD fails to reduce metallic precursor salts because its reducing functionalities (-OH groups) remain occupied by the formation of inclusion complexes with the guest. However, conventional reducing agents can produce metallic particles from HAuCl4 or AgNO3 solution in the presence of guest-saturated β-CD, but the metallic particles precipitate from alkaline β-CD solutions. This precipitation is observed because the guest-saturated β-CD cannot offer stability to the otherwise produced metal particles. During the reduction of metal salts to the corresponding metal nanoparticles, the β-CD molecules are oxidized. The presence of oxidized and unoxidized β-CD was confirmed by ESI mass spectra measured in anionic mode. From the ESI mass spectra, the peaks are obtained in the 1134 and 1232 mass regions due to unoxidized β-CD and the oxidized primary alcohol functionality of β-CD, i.e., the -COOH group, respectively. It has been reported that, at pH ∼10-12, the secondary alcoholic group of β-CD is not oxidized and also no dioxirane formation is formed.46,47 To offset the van der Waals forces responsible for particle coalescence, carboxylic acids (produced as the oxidation products of β-CD molecules) bearing a negative surface charge provide a dense coating on the surfaces of the metal particles and provide stabilization. 3.3.2. Probing Normal and InVerted Core-Shell Structures Obtained by a Similar Synthetic Strategy: Extraordinary Role of β-CD. It is now well-established in the literature that one can easily obtain a normal core-shell structure from the constituents where the more noble metal constitutes the core while the less noble metal forms the shell of the bimetallic cluster.37 However, the synthesis of an inverted core-shell architecture is unfamiliar without the use of any extra reagent. It is almost impossible because of the inevitable redox reaction, i.e., Ag(0) oxidation by Au(III) ions.39,48-50 The successful synthesis of inverted core-shell architectures thus addresses several challenges. In the present experiments, the preformed Au(0) or Ag(0) nanoparticles act as seeds and β-CD becomes kinetically capable of reducing the incoming Ag(I) or Au(III) ions onto the seed particles to create the core-shell morphology as represented in the following reaction β-CD

8 Aucore-Agshell Aucore + Ag(I) 9 pH ∼ 10-12 β-CD

Agcore + Au(III) 9 8 Agcore-Aushell pH ∼ 10-12

(1) (2)

Reaction 1 is favorable, as the standard reduction potential of Ag(I)/Ag(0) is lower than that of Au(III)/Au(0). In the case of reaction 2, however, β-CD helps to produce the inverted coreshell architecture through the reduction of Au(III) ions in the presence of preformed Ag(0). Here, the Au(III) reduction rate

is possibly higher than the galvanic replacement reaction rate under the experimental conditions. Mechanistically, it can be specified that the oxidation resistance of silver presumably increases and this inhibits the normal oxidation of Ag(0) in the presence of Au(III) ions. The diffusion resistance (DR) and charge-transfer resistance (CTR) should also have a bearing on the oxidation of Ag(0), as has been described only recently.13 Presumably, both the DR and CTR of Au(III) are increased in β-CD, so that the electron transfer from Ag(0) to Au(III) ions is inhibited. Furthermore, it can also be suggested that the selforganization and hydrophobic interaction of β-CD with Ag(0) makes the latter nobler than Au(0). This results in the formation of the Agcore-Aushell structure instead of the formation of a hollow core inside the gold shell. 3.4. Application of Normal and Inverted Core-Shell Bimetallic Nanoparticles in SERS Spectroscopy: Electronic Ligand Effect. SERS spectroscopy has been producing enlightening results since its discovery in 1974.18 Noble-metal nanoparticles, usually silver and gold, are well-known for their strong interactions with visible light through the resonant excitations of the collective oscillations of the conduction electrons within the particles. Creighton et al.51 and Jeanmaire and Van Duyne52 independently confirmed the findings on the enhancement aspect of Raman signals, and silver is the bestsuited choice for such studies.45,49,50 However, the mechanism of SERS has been a matter of considerable debate.19,53-55 It has been widely accepted that there are two mechanisms for the observed and, at times, huge enhancement in SERS: electromagnetic and chemical. The electromagnetic mechanism is based on the interaction of the electric field of the surface plasmons with the transition moment of the adsorbed molecule, whereas the chemical mechanism is based on the idea that mixing of molecular and metal states occurs. The chemical mechanism might arise from the mixing of metal orbitals with orbitals on a molecule, providing charge transfer that results in a resonant Raman mechanism at much lower energies than those available in the free molecule. Both mechanisms and even others can contribute simultaneously to the SERS enhancement to a certain extent, which is dependent on the experimental conditions, the nature and morphology of the metallic nanoparticles, and other factors. The present report of SERS measurements probes the influence of the shell material of the as-prepared particles in solution using 1,10-phenanthroline as a molecular probe. The molecule 1,10-phenanthroline has been used in SERS as a probe by Li et al. and Miranda in the cases of gold electrode and silver sols.56,57 However, there have been no reports of SERS studies in a particular dielectric medium involving individual gold, silver, and their bimetallic conjugates as substrates for examining the electromagnetic (EM) mechanism. This has not been possible, and it has remained a challenge to produce all of these particles in the same dielectric environment. Here, the probe molecules induce particle aggregation without the need of any extra electrolyte for generating a “hot junction”. The junction can therefore function as an electromagnetic “hot spot” analogous to those predicted to exist in large fractal aggregates. Thus, absolute EM effects can be confirmed from the SERS signal enhancement. In the present experiments, bimetallic nanoparticles were employeing the d at the same concentration as monometallic particles of silver and gold of the same size range (∼12 nm) to investigate the electronic ligand effect4,5,8,9 (the so-called electronic ligand effect in the case of core-shell particles) on the SERS spectra of 1,10-phenanthroline. It can be seen that the probe molecules show distinctly different SERS

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Figure 7. SERS spectra of 1,10-phenanthroline (0.5 mM) using (a) Aucore-Agshell nanoparticles, (b) Agcore-Aushell nanoparticles, (c) Ag(0), and (d) Au(0) (0.15 mM).

SCHEME 1: Schematic Representation of the Formation of Mono- and Bimetallic Nanoparticles and their Corresponding SERS Spectra

Pande et al. or the work function. Because the Fermi level is lower for gold (about -5.0 eV) than for silver (about -4.6 eV), charge transfer occurs along a direction from silver to gold. Thus, the plasmon band of silver will be red-shifted, and blue shifting will be observed when the gold core is taken into consideration. The spectra of Aucore-Agshell bimetallic nanoparticles with higher proportions of Ag are the same as those of monometallic silver particles (Figure 2A). On the other hand, in the case of AgcoreAushell nanoparticles, the gold shell will show the opposite effect. Thus, Aucore-Agshell nanoparticles should welcome probes such as 1,10-phenanthroline, dopamine, aminopyrene, crystal violet, and rhodamine 6G for attachment to show a marked SERS effect. Therefore, the core-shell bimetallic architectures show higher SERS activities than the corresponding monometallic structures. Hence, one can probe the shell material beyond doubt. 4. Conclusion In summary, the present synthetic method is of three-fold interest. First, the results reported herein describe particle evolution in alkaline β-CD solution without the need of any other reducing agent. Second, this “green chemistry” approach has the potential to produce both normal and inverted coreshell morphologies in bimetallic counterparts without changing the medium. Third, leaving aside the usual redox reaction, β-CD supports the kinetic evolution of a desired shell on preformed seed particles. Finally, the present work substantiates the fact that Aucore-Agshell nanoparticles provide an elegant substrate for SERS studies and offers the possibility of examining probe molecules down to the single-molecule level. Thus, the practical application of unmodified β-CD under alkaline conditions should gain further momentum in nanoparticle chemistry. Acknowledgment. The authors are thankful to the UGC, DST, NST, and CSIR, New Delhi, India, and the IIT Kharagpur for financial assistance. We are also thankful to IMS, Japan, for the ESI mass spectroscopy.

signals in the presence of different sets of colloidal solutions. The salient feature of physical significance is that we observe better enhancement of the SERS signal of the probe molecule with Aucore-Agshell particles than with bimetallic Agcore-Aushell or monometallic gold or even silver particles, as shown in Figure 7 (Scheme 1). Similar results were also obtained with other probe molecules, e.g., dopamine, aminopyrene, crystal violet, rhodamine 6G, etc. It is known that, in general, silver monometallic nanoclusters are good SERS-active substrates, whereas gold monometallic nanoclusters are less active. Thus, one can conclusively characterize the constituents of a metallic substrate. The enhancement of Raman signals with Aucore-Agshell nanoparticles in comparison to monometallic silver particles can be explained in light of the electronic ligand effect in bimetallic nanoparticles. The gold atoms in the core can have a strong electronic effect on the surface silver atoms by charge transfer, and thus, the surface silver atoms neighboring the gold core become more active than the silver atoms in monometallic silver nanoclusters.8-11 Charge-transfer interactions in Aucore-Agshell bimetallic nanoparticles can be explained in terms of the electronegativity values of the constituents. Given that gold is more electronegative than silver, gold will withdraw electron density from the silver shell in the case of the Aucore-Agshell nanostructure. As a result, the silver shell will experience the effect of electrophiles, and the core will experience the effect of nucleophiles.58,59 Again, this can be explained on the basis of the concept of the Fermi level

Supporting Information Available: EDS of spectra of Agcore-Aushell bimetallic nanoparticles and HRTEM images of Agcore-Aushell bimetallic nanoparticles (a) before and (b) after prolonged (∼10 min) electron beam irradiation. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Henglein, A.; Brancewicz, C. Chem. Mater. 1997, 9, 2164. (2) Hostetler, M. J.; Zhong, C. J.; Yen, B. K. H.; Anderegg, J.; Gross, S. M.; Evans, N. D.; Porter, M.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 9396. (3) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (4) Toshima, N.; Yonezawa, T.; Harada, M.; Asakura, K.; Iwasawa, Y. Chem. Lett. 1990, 815. (5) Sinfelt, J. H. Bimetallic Catalyst: DiscoVeries Concepts and Application; John Wiley & Sons: New York, 1983. (6) Treguer, M.; de Cointet, C.; Remita, H.; Khatouri, J.; Mostafavi, M.; Amblard, J.; Belloni, J.; de Keyzer, R. J. Phys. Chem. B 1998, 102, 4310. (7) Zaera, F. J. Phys. Chem. B 2002, 106, 4043. (8) Sinfelt, J. H. J. Catal. 1973, 29, 308. (9) Sinfelt, J. H. Acc. Chem. Res. 1987, 20, 134. (10) Toshima, N. J. Macromol. Sci. Chem. 1990, A27, 1225. (11) Shiraishi, Y.; Ikenaga, D.; Toshima, N. Aust. J. Chem. 2003, 56, 1025. (12) Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Nano Lett. 2001, 1, 319. (13) Srnova-Sloufova, I.; Vlckova, B.; Bastl, Z.; Hasslett, L. T. Langmuir 2004, 20, 3407. (14) Nath, S.; Praharaj, S.; Panigrahi, S.; Ghosh, S. K.; Kundu, S.; Basu, S.; Pal, T. Langmuir 2005, 21, 23.

Gold-Silver Core-Shell Architectures in β-Cyclodextrin (15) Mandal, M.; Ghosh, S. K.; Kundu, S.; Esumi, K.; Pal, T. Langmuir 2002, 18, 7792. (16) Kundu, S.; Pal, A.; Ghosh, S. K.; Nath, S.; Panigrahi, S.; Praharaj, S.; Pal, T. Inorg. Chem. 2004, 43, 5489. (17) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (18) Fleischmannn, M.; Hendra, P. J.; MacQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (19) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783. (20) Weitz, D. A.; Garoff, S.; Gersten, J. I.; Nitzan, A. J. Phys. Chem. 1983, 78, 5324. (21) Creighton, J. A. Surf. Sci. 1983, 124, 209. (22) Campion, A.; Kambhampati, P. Chem. Soc. ReV. 1998, 27, 24. (23) Voisin, C.; Fatti, N. D.; Christofilos, D.; Vallee, F. J. Phys. Chem. B 2001, 105, 2264. (24) Kerker, M. Selected Papers on Surface-Enhanced Raman Scattering; SPIE, The International Society for Optical Engineering: Bellingham, MA, 1990. (25) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (26) Sau, T. K.; Pal, A.; Pal, T. J. Phys. Chem. B 2001, 105, 9266. (27) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (28) March, J. AdVanced Organic Chemistry, 4th ed.; John Wiley and Sons: New York, 1992. (29) (a) Rojas, M. T.; Koeniger, R.; Stoddart, F. J.; Kaifer, E. A. J. Am. Chem. Soc. 1995, 117, 336. (b) Liu, J.; Alvarez, J.; Ong, W.; Kaifer, E. A. Nano Lett. 2001, 1, 57. (c) Liu, J.; Alvarez, J.; Kaifer, E. A. AdV. Mater. 2000, 12, 1381. (30) Suzuki, I.; Murakami, K.; Anzai, J.-I. Mater. Sci. Eng. C 2001, 17, 149. (31) Velic, D.; Knapp, M.; Kohler, G. J. Mol. Struct. 2001, 598, 49. (32) Ng, S.-C.; Sun, T.; Chan, H. S. O. Tetrahedron Lett. 2002, 43, 2863. (33) Liu, Y.; Male, K. B.; Bouvrette, P.; Luong, J. H. T. Chem. Mater. 2003, 15, 4172. (34) Bonacchi, D.; Caneschi, A.; Gatteschi, D.; Sangregorio, C.; Sessoli, R.; Falqui, A. J. Phys. Chem. Solids 2004, 65, 719. (35) He, B.; Tan, J. J.; Liew, K. Y.; Liu, H. J. Mol. Catal. A 2004, 221, 121.

J. Phys. Chem. C, Vol. 111, No. 29, 2007 10813 (36) Panigrahi, S.; Nath, S.; Praharaj, S.; Ghosh, S. K.; Kundu, S.; Basu, S.; Pal, T. Colloids Surf. A: Physicochem. Eng. Aspects 2005, 264, 133. (37) de Cointet, C.; Khatouri, J.; Mostafavi, M.; Belloni, J. J. Phys. Chem. B 1997, 101, 3517. (38) Ghosh, S. K.; Kundu, S.; Mandal, M.; Nath, S.; Pal, T. J. Nanopart. Res. 2003, 5, 577. (39) Moskovits, M.; Sloufova, I. S.; Vlckova, B. J. Chem. Phys. 2002, 116, 10435. (40) NIST X-ray Photoelectron Spectroscopy Database, ver. 2.0; National Institute of Standards and Technology (NIST): Gaithersburg, MD, 1997. (41) Pal, T.; Ganguly, A.; Maity, D. Anal. Chem. 1986, 58, 564. (42) Lee, I.; Han, W. S.; Kim, K. Chem. Commun. 2001, 1782. (43) Kalsin, A. M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J. M.; Grzybowski, B. A. Science 2006, 312, 420. (44) Praharaj, S.; Ghosh, S. K.; Nath, S.; Kundu, S.; Panigrahi, S.; Basu, S.; Pal, T. J. Phys. Chem. B 2005, 109, 13166. (45) Belloni, J.; Treguer, M.; Remita, H.; de Keyzer, R. Nature 1999, 402, 865. (46) Fraschini, C.; Vignon, M. R. Carbohydr. Res. 2000, 328, 585. (47) Deary, M. E.; Davies, D. M. Carbohydr. Res. 1998, 309, 17. (48) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (49) Cui, Y.; Ren, B.; Yao, J. L.; Gu, R. A.; Tian, Z. Q. J. Phys. Chem. B 2006, 110, 4002. (50) Li, Z. Y.; Chen, Y. Y.; Palmer, R. E.; Wilcoxon, J. P. Appl. Phys. Lett. 2005, 87, 243103. (51) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (52) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (53) Chaumet, P. C.; Rahmani, A.; Nieto-Vesperinas, M. Phys. ReV. Lett. 2002, 88, 13601. (54) Xu, H. X.; Ka¨ll, M. Phys. ReV. Lett. 2002, 89, 246802. (55) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. ReV. Lett. 1997, 19, 1667. (56) Peng, Y.; Niu, Z.; Huang, W.; Chen, S.; Li, Z. J. Phys. Chem. B 2005, 109, 10880. (57) Miranda, M. M. J. Phys. Chem. A 2000, 104, 7803. (58) Henglein, A. J. Phys. Chem. B 1993, 97, 5457. (59) Pal, T.; Sau, T. K.; Jana, N. R. Langmuir 1997, 13, 1481.