Fabrication of Au@Ag Core–Shell Nanoparticles Using Polyelectrolyte

Article pubs.acs.org/Langmuir

Fabrication of Au@Ag Core−Shell Nanoparticles Using Polyelectrolyte Multilayers as Nanoreactors Xin Zhang,† Hui Wang,‡ and Zhaohui Su*,† †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States ABSTRACT: A new synthetic strategy has been developed for the fabrication of Au−Ag bimetallic core−shell nanoparticles (NPs) using polyelectrolyte multilayers (PEMs) as unique nanoreactors. Bimetallic NPs composed of Au core and Ag shell were successively incorporated into PEMs by repeating anion/cation exchange/ reduction cycle multiple times in a stepwise manner. The strategy described here allows for the facile preparation of Au@Ag core− shell NPs with well-controlled core and shell dimensions and geometrically tunable optical properties by simply varying the number of ion-exchange/reduction cycles in the PEM matrix. The strategy can be extended to synthesize in situ other core−shell NPs in polymer matrix.

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poly(vinylpyrrolidone) as a surfactant, Tsuji and co-workers prepared Au@Ag core−shell particles with different shapes including octahedron, cube, decahedron, rod, wire, and icosahedrons.25−27 Park and co-workers, on the other hand, directed the growth of an Ag shell with iodide ion over Au nanodisks to yield Au@Ag core−shell NPs with different morphologies.28 Mirkin and co-workers synthesized Au@Ag core−shell triangular nanoprisms via plasmon-mediated growth of an Ag shell on spherical and triangular Au cores, resepctively.29,30 Xia and co-workers synthesized Au@Ag core−shell nanocubes with well-controlled sizes using Au single crystals as the seeds and cetyltrimethylammonium chloride as the capping agent.31 Crooks and co-workers fabricated Au−Ag bimetallic NPs of 1−3 nm sizes encapsulated in a dendrimer via a sequential loading method.32 Chen and coworkers reported facile one-step synthesis of concentric and eccentric polymer-coated Au@Ag core−shell NPs.33,34 Xu and co-workers prepared Au@Ag core−shell NPs restricted and stabilized on a metal organic framework.17 In most studies to date, synthesis of Au−Ag bimetallic NPs has been carried out in aqueous or organic medium, with the resultant NPs dispersed in the liquid medium and stabilized by a surfactant. However, for many practical applications, such as catalysis and optoelectronic and sensor devices, the NPs often need to be immobilized on a substrate or embedded in a matrix. In many cases, it is challenging to retain the intrinsic structure, size, and morphology of the NPs when the solution-dispersed NPs are deposited on a support or embedded in a matrix.35 Therefore,

imetallic nanoparticles (NPs) have attracted growing interest due to their fascinating optical, electronic, catalytic, and magnetic properties that can be dramatically different from those of corresponding monometallic counterparts.1−4 Properties of monometallic NPs are dependent on their size and shape, and consequently extensive studies have focused on various NP morphologies including spheres,5 triangles,6,7 cubes,8 wires,9,10 rods,11 and flowers.12 Besides the overall particle size and shape, bimetallic NPs possess two additional geometrical parameters, composition and interior structures, which can be rationally adjusted to further fine-tune the properties of the NPs.13,14 Bimetallic NPs are classified into core−shell, heterostructure, and intermetallic or alloyed nanostructures.2,3,13,15 Among them NPs with core−shell structures are of particular interest because their physicochemical properties, which are distinct from those of individual metallic counterparts and alloys, strongly depend on the core and shell dimensions and compositions.3 Au@Ag core−shell NPs (NPs with Au cores and Ag shells) have been the most studied bimetallic core−shell particles due to their interesting optical properties and potential application in various fields ranging from antibacterial materials,16 catalysts,17 sensors,18 surface-enhanced Raman scattering19 to Fano resonance generation.20 Many groups have reported the fabrication of Au@Ag core− shell NPs using various synthetic routes.21−34 Successive reduction of metallic precursor salts is an effective and the most widely used approach to fabricating Au@Ag core−shell NPs. For example, Zhang and co-workers reported the synthesis of Au@Ag core−shell NPs by a seed-mediated growth method.22 Nam and co-workers synthesized DNAembedded Au@Ag core−shell NPs with high stability by growing a silver shell with controllable thickness on DNAmodified Au seeds.23 Using a microwave-polyol method24 with © 2012 American Chemical Society

Received: August 16, 2012 Revised: September 14, 2012 Published: October 17, 2012 15705

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Scheme 1. Process of Fabrication of Au@Ag Core−Shell NPs in the PEM

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in situ synthesis of bimetallic NPs within solid films is highly desirable. The layer-by-layer (LbL) self-assembly technique offers a simple but versatile strategy for the fabrication of thin films on solid substrates and allows a wide range of functional species to be incorporated within the film structure. Polyelectrolyte multilayers (PEMs) have been employed as nanoreactors for synthesizing a broad range of NPs because their porous nature allows particles to nucleate and grow inside the pores while polymer chains can limit particle size and prevent aggregation.36 Synthesis of monometallic NPs within PEMs has been extensively explored,37−44 and more recently, fabrication of Au−Ag bimetallic NPs in PEMs was reported by several groups.45−47 Dong and co-workers reported that the loading of AuCl4− and Ag+ into PEMs assembled from two weak polyelectrolytes can be facilitated by manipulating the pH and subsequent thermal reduction would give rise to the formation of Au−Ag alloy NPs.45 A similar approach was adopted by Choi and co-workers to fabricate Au−Ag bimetallic NPs in much thicker and freestanding PEMs assembled from polyelectrolyte complex. 46 Xie and co-workers directly assembled a polyethylenimine−Ag+ complex with AuCl4− ion and obtained a composite film by chemical reduction of the incorporated ions.47 Recently, we demonstrated that counterions are universally present in PEMs and ionic species can be facilely introduced into PEMs via ion exchange, the charge type depending on the identity of the terminating polyelectrolyte, and the ions can be subsequently reduced to yield metal NPs.42,43 The method was extended to prepare core−shell NPs which exhibit improved catalytic performance compared to the monometallic counterparts.48 In the present work, we demonstrate synthesis of Au@ Ag core−shell NPs with controlled core−shell dimensions and compositions. The strategy of our approach is illustrated in Scheme 1 using a typical PEM assembled from poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) as an example. First, the PEM terminated with PDDA, the polycation, is loaded with AuCl4− ions via ion exchange, which are reduced to generate Au NPs, and then a layer of PSS is deposited to convert the cap layer into a polyanion, so that Ag+ ions can be introduced into the PEM by countercation exchange. Then the Ag+ ions are reduced with a weak reducing agent with Au NPs as seeds to yield Au@Ag core−shell NPs. Both ion-exchange/reduction cycles can be repeated multiple times so that the size of Au core and thickness of Ag shell can be controlled independently.

EXPERIMENTAL SECTION

Materials. Chloroauric acid tetrahydrate (HAuCl4·4H2O) and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium chloride (NaCl) was purchased from Beijing Chemical Reagents Co. Ascorbic acid was purchased from Huishi Biochemical Co., Ltd. (Shanghai, China). PDDA (20 wt % in water, MW ∼ 200K−350K), PSS (MW ∼ 70K), and sodium boronhydride (NaBH4) were purchased from Aldrich. All chemicals were used as received without further purification. Ultrapure water (18.2 MΩ cm at 25 °C) was purified with a PGeneral GWA-UN4 system and used in all experiments. Preparation of (PDDA/PSS)n Film. Quartz and glass slides were cleaned in a boiling piranha solution (H2SO4/H2O2 = 70:30 v/v) and subsequently rinsed with copious amounts of water. Following a previously reported procedure,43 a (PDDA/PSS)n film was assembled by sequential dipping of the substrate into PDDA (1.0 mg/mL) and PSS (1.0 mg/mL) aqueous solutions for 30 min each until the desired number of bilayers, n, was obtained. The (PDDA/PSS)n film with an additional PDDA capping layer is denoted (PDDA/PSS)nPDDA. Every dipping was followed by sufficient water rinse. NaCl of 1.5 M concentration was maintained in all polyelectrolyte solutions. Synthesis of aAu@bAg NPs. A (PDDA/PSS)nPDDA film was dipped into a HAuCl4 solution (10 mL, 1 mM) for 5 min, removed and rinsed with water, and then treated with a freshly prepared aqueous solution of NaBH4 (10 mL, 0.1 M, for the first reduction only) or ascorbic acid (10 mL, 0.1 M, for all subsequent reduction) for 5 min. This exchange/reduction reaction cycle was repeated for certain times to produce a PEM loaded with Au NPs. PEMs loaded with Ag NPs were synthesized from (PDDA/PSS)n films and AgNO3 solution (10 mL, 10 mM) using the same protocol. Then a layer of PSS was deposited following the assembly procedure described above onto the (PDDA/PSS)nPDDA film loaded with Au NPs. Next this (PDDA/ PSS)n+1 film was dipped into the AgNO3 solution for 5 min, removed and rinsed with water, and then treated with a freshly prepared aqueous solution of ascorbic acid for 5 min. This exchange/reduction reaction cycle was repeated for certain times. The monometallic and core−shell NPs prepared are denoted aAu, bAg, and aAu@bAg for short, a and b being the number of exchange/reduction cycles carried out for Au and Ag, respectively. Characterization. UV−vis spectra of the PEMs containing metallic NPs on quartz slides were acquired on a TU-1901 spectrometer (Beijing Purkinje General Instrument Co., Ltd.). Transmission electron microscopy (TEM) measurements were carried out on a JEM-1011 microscope operated at an accelerating voltage of 100 kV. A small piece of PEM film containing metallic NPs was peeled off from substrate in hydrofluoric acid, floated in water, and transferred to a carbon-coated copper grid for TEM characterization. Highresolution TEM (HRTEM) images, high-angle annular dark-field scanning transmission electron micrographs (HAADF-STEM), and energy dispersive X-ray elemental mapping were acquired on a Tecnai F20 microscope (Philips, FEI, TECNAI) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a ThermoElectron ESCALAB 250 spectrometer, using monochromatic Al Kα radiation as the X-ray source for excitation. The spectra were recorded 15706

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Figure 1. UV−vis spectra of (a) (PDDA/PSS)4PDDA containing Au NPs and (b) (PDDA/PSS)5 containing Ag NPs with different number of exchange/reduction cycles.

Figure 2. Calculated extinction spectra of (a) Au and (b) Ag nanospheres with varying particle radius as indicated in each panel. at 90° takeoff angle and 20 eV pass energy. The binding energies of all peaks were referenced to a C 1s value of 284.6 eV. The composition of Au−Ag bimetallic NPs in the PEMs was measured by inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP 6300, Thermo Scientific).

emerges at near 420 nm, indicating the formation of Ag NPs (Figure 1b). The overall size of Au or Ag NPs can be controlled by varying the number of reaction cycles. More reactions cycles generally give rise to increased particle sizes. The intensity of the plasmon peaks increases with the number of exchange/ reduction cycles, which can be interpreted as a result of increase in extinction cross sections of the particles at the plasmon resonance wavelengths as the NPs become larger. No obvious shift of the extinction peak is observed as the size of Au NPs increases because the plasmon resonance of Au NPs in the size range from 3 to 40 nm occurs at essentially fixed wavelength (∼535 nm), whereas the position of the plasmon resonance peak for Ag NPs exhibits a slight, progressive red-shift from 414 to 430 nm as the number of reaction cycles increases. We have also used Mie scattering theory to calculate the extinction spectra of individual spherical Au or Ag NPs dispersed in water (permittivity of 1.77) using the dielectric functions of bulk Au and Ag. The calculated spectra were expressed as extinction cross sections, which is the sum of absorption and scattering cross sections of the NPs, as a function of wavelength. The experimentally observed evolution of extinction spectral features as the size of Au and Ag NPs progressively increases is in very good agreement with the results of Mie scattering theory calculations, as shown in Figure 2. The experimental extinction spectra show broader plasmon band widths than the calculated curves largely due to the size distribution and structural nonideality of the experimentally fabricated NP samples. This PEM-based approach can be readily extended to the controllable fabrication of Au−Ag bimetallic core−shell NPs. A PSS layer was assembled onto a (PDDA/PSS)4PDDA film loaded with Au NPs, such that the PEM, now (PDDA/PSS)5,

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RESULTS AND DISCUSSION In this work, PEM films were prepared via alternating depositions of PDDA and PSS, which are typical strong polyelectrolytes, on clean substrates from solutions containing 1.5 M NaCl. When the outmost layer of the PEM is PDDA, the counterions existing in the film are mainly anions, Cl− in this work, which can then be exchanged by AuCl4−, a precursor for Au NPs. Whereas for the PEMs capped with PSS, most of the small ions present within the PEM are cations (Na+), which can be replaced with Ag+ to synthesize Ag NPs by chemical reduction. In either case, the Na+ or Cl− counterions are regenerated in the reduction reaction; therefore, the ionexchange/reduction cycles can be repeated multiple times to improve the loading and/or size of the metallic NPs in the PEM.43,44 In this process, a strong reducing agent, NaBH4, is used in the first reduction cycle to generate metal NPs as seeds, and in all subsequent cycles the metal precursor is reduced with ascorbic acid, a weak reducing agent, so that the reduction only occurs at the surface of the metal NPs.32 First we prepared PEM-supported monometallic NPs. Figure 1 shows UV−vis extinction spectra of the PEMs loaded with Au and Ag NPs respectively with increasing number of ionexchange/reduction cycles. For (PDDA/PSS)4PDDA, as seen in Figure 1a, an extinction peak corresponding to characteristic plasmon resonance of Au NPs at near 535 nm is clearly identified, whereas for (PDDA/PSS)5, the plasmon peak 15707

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Figure 3. UV−vis spectra of (PDDA/PSS)5 supported aAu@bAg bimetallic NPs with (a) b = 2 and a varied from 1 to 4, and (b) a = 2 and b varied from 0 to 4 (a and b are the number of exchange/reduction cycles carried out for Au and Ag, respectively).

Figure 4. Calculated extinction spectra of Au@Ag core−shell nanospheres with (a) fixed shell thickness of 2 nm and varying core radius and (b) fixed core radius of 4 nm and varying shell thickness as indicated in each panel.

contained Au NPs as well as Na+ counter cations, and Ag+ can be further incorporated via the ion-exchange/reduction protocol as discussed above. UV−vis extinction spectra of the bimetallic NPs prepared with different Au loading cycles (Figure 3a) show the evolution of plasmonic features of the NPs with fixed Ag shell thickness and varying Au core sizes. A double-peak spectral line shape, which is the characteristic plasmonic feature of Au−Ag core−shell NPs,49 is observed in the extinction spectra. As the size of the Au core increases relative to the Ag shell thickness, the high-energy plasmon resonance progressively blue-shifts while the low-energy plasmon band red-shifts and becomes increasingly intensified. Figure 3b shows the UV−vis extinction spectra of the Au−Ag bimetallic NPs prepared with different number of Ag loading cycles, which reveal how the variation in Ag shell thickness modifies the overall plasmonic features of the bimetallic core− shell NPs. The effect of wrapping Ag nanoshells surrounding Au cores on the plasmon spectrum is manifested by a blue-shift of the low-energy plasmon resonance and the emergence of a high-energy resonance peak near 420 nm, which becomes increasingly pronounced as the thickness of the Ag shells increases and eventually converges to the resonance of monometallic Ag NPs as the plasmon excitation of the Au core is completely screened by the Ag shell when a thick Ag shell regime is reached. A similar phenomenon has been previously reported for Au@Ag core−shell nanocubes synthesized in solution.31 The high-energy and low-energy plasmon modes observed here are the synergistic optical features of the bimetallic core−shell NPs and thus cannot be simply regarded as a linear combination of plasmon modes of the Au core and Ag shell. The synergistic plasmonic properties of the bimetallic

NPs, which are observed to be geometrically tunable, essentially arise from the interactions between the core and the shell. The spectral evolution as the core and shell dimensions vary provides strong evidence that the as-fabricated bimetallic NPs are not mixtures of monometallic Au and Ag NPs. It has been known that deposition of a Ag layer results in dramatic blueshifts of the plasmon band of the substrate Au NPs,50 whereas for Au−Ag alloy NPs there is only one surface plasmon band, the position of which falling in between that of monometallic Au and Ag depending on the alloy composition.51 In addition, the low-energy plasmon peak of bimetallic NPs shown in Figure 3a emerges at shorter wavelength than that for monometallic Au NPs, also in line with the Au@Ag core− shell structure. Furthermore, the color of the PEM loaded with the bimetallic NPs changes from purple to dark pink to yellow brown (Figure 3b inset) with increasing number of Ag exchange/reduction cycles, consistent with variation of the UV−vis extinction profile. All these phenomena suggest that the bimetallic NPs in the PEM are Au@Ag core−shell heterostructured NPs rather than alloy NPs. We have further calculated the extinction spectra of individual concentric core− shell nanospheres with varying core and shell dimensions using the Mie scattering theory applied to concentric multilayer spherical particles.52 As shown in Figure 4, the calculated extinction spectra qualitatively match the experimental data in terms of both spectral line shapes and the geometry dependence of optical features, further verifying the Au@Ag core−shell structures of the NPs. The deviation between calculated and measured extinction spectral line shapes is largely due to the structural nonideality and polydispersity of the experimentally fabricated NPs. 15708

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Figure 5. (a) HRTEM image and (b) a magnified HRTEM image of Au−Ag bimetallic NPs dispersed in the (PDDA/PSS)3 film. (c) HADDFSTEM image of Au−Ag bimetallic NPs, and (d) an enlarged view of the area marked by the square in (c). (e, f) Energy-dispersive X-ray elemental maps of (e) gold and (f) silver corresponding to image (d).

Figure 6. TEM images and size distribution histograms for (a) 2Au, (b) 4Au, (c) 6Au, (d) 2Au@2Ag, (e) 2Au@4Ag, and (f) 2Au@6Ag NPs in the (PDDA/PSS)3 film. More than 200 NPs were measured for each histogram.

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Figure 7. (a) Au 4f region and (b) Ag 3d region of XPS spectra for PEMs containing 2Au@bAg core−shell NPs (b = 0−4). (c) Ag/Au molar ratio for the NPs in the PEM measured by ICP-OES as a function of the ratio between the Ag and Au exchange/reduction cycles.

show 2Au@bAg NPs with the same Au cores (a = 2) encapsulated by Ag shells prepared with different Ag loading cycles (b), where the variation in the particle size must arise from the Ag shells, and it can be derived that the thickness of the Ag shell grows almost linearly with the Ag loading cycle, with an increment of ∼1.2 nm per cycle. These results demonstrate that the size of the cores and the thickness of the shells of the bimetallic core−shell NPs synthesized in the PEM can be readily controlled by independently varying the number of exchange/reduction cycles for each component. Furthermore, the diameter increase per loading cycle should depend on the concentration of the counterions in the PEM and therefore is a function of the ionic strength in the polyelectrolyte solutions used to assemble the PEM. Of course, the size increase with loading cycle for either component (core or shell) is expected to slow down when the particles become bigger, since the surface area of the particles expands with the particle size while the amount of the counterions (hence that of metal atoms to be deposited) remains the same throughout.43 In addition, due to geometric constrain imposed by the PEM matrix, there is a upper limit for particle size above which the NPs would impinge into one another, a number depending on PEM assembly conditions (film thickness) and reduction conditions (number of NPs). Under suitable conditions NPs up to 100 nm size are possible. Finally, we investigated the overall composition of the bimetallic NPs in the PEM. In our previous work it has been shown that the content of monometallic NPs in the PEM rises linearly with the number of the exchange/reduction cycle.43 Thus, one can expect that the contents of Au and Ag in the composite film can be manipulated in the same way. The composite films were assessed by XPS. Figure 7a shows the Au 4f region of XPS spectra of the composite films containing 2Au@bAg, with b = 0−4. The 4f7/2 and 4f5/2 doublet located at 84.1 and 87.9 eV corresponds to Au in zerovalent state. Figure 7b shows the Ag 3d region, where the Ag 3d5/2 and 3d3/2 peaks

TEM was then utilized to further character the size, morphology, and structure of the Au−Ag bimetallic NPs formed in the PEM. As seen in Figure 5a, the NPs are largely spherical, with diameters of ∼15 nm. Each NP exhibits inhomogenous electron density with a dark core coated by a lighter shell. A typical HRTEM image is included in Figure 5b, showing a single spherical NP with contrast difference. The darker core of ∼9 nm size should be Au, the element of higher atomic number, whereas the lighter shell of ∼4 nm thickness is Ag. The high-angle annular dark field scanning TEM (HAADFSTEM) image in Figure 5c again shows different contrast between the bright cores and darker shells. To further assess the structure of bimetallic NPs, energy-dispersive X-ray elemental analysis was used to map the element distribution in the particles. The individual maps of the two elements (Au and Ag) within two selected NPs (shown in Figure 5, e and f) clearly indicate the presence of both Au and Ag in the same NPs. Furthermore, while the Ag distributes throughout the entire particles, Au is found in smaller and central areas that overlap well with the bright cores of the particles in Figure 5c. All these results confirm that the NPs in the PEM are bimetallic particles with Au cores and Ag shells. Particle size is the most important parameter for monometallic NPs, and previously we have demonstrated that for NPs synthesized in PEMs, the particle size can be readily controlled by varying the number of the exchange/reduction cycle.43 For bimetallic core−shell NPs, the core size and the shell thickness are also structural parameters crucial to their properties.31 Figure 6 displays the TEM images and the corresponding particle size distribution for the Au cores and Au@Ag core−shell NPs synthesized in the PEM with different Au and Ag loading cycles (i.e., aAu@bAg with various a and b). From panels a−c it can be seen that the average size of the Au particles increases from 9.2 nm for 2Au to 14.3 nm for 6Au, with an increment of ∼1.3 nm per Au loading cycle after the initial reduction. On the other hand, Figure 6 panels a and d−f 15710

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exchange/reduction cycles for the core and the shell, respectively. This work provides a novel method for preparing Au@Ag core−shell NPs in solid supports and demonstrates a general strategy to synthesize other bimetallic core−shell NPs in polymer thin films in situ.

occur at 368.4 and 374.4 eV, indicative of the presence of metallic Ag.45 It can also be seen from Figure 7a,b that for these films embedded with 2Au@bAg NPs the intensity of the Ag peaks rises with b, the Ag loading cycle, whereas the Au peaks decline at the same time. This result is consistent with the core−shell structure of the NPs: more Ag loading cycles leads to greater Ag contents (hence stronger Ag peaks) as well as thicker Ag shells, which in turn more effectively decay photoelectrons from the shielded Au cores, reducing the Au peaks in the XPS spectra. Because XPS can only assess the top several nanometers of surfaces, the shielding effect of the Ag shells (several nanometers thick as discussed above) on the photoelectron generated in the Au cores is significant, as seen in Figure 7a; therefore, atomic composition data derived from XPS do not reflect the actual Au and Ag contents in the composite films. Consequently, ICP-OES was used to quantify the composition of the Au@Ag core−shell NPs in the composite films, where the specimens were completely digested and ions in solutions were quantified. Figure 7c plots the molar ratio of Ag and Au against the ratio between the exchange/ reduction cycles for Ag-shell and Au-core (b/a), which exhibits a good linear relationship. This indicates that varying the number of reaction cycles for Au-core and Ag-shell is an effective way to control the composition of the Au@Ag core− shell NPs in the PEM. Although the present study is focused on Au@Ag core−shell NPs, the strategy is general and can be utilized to synthesize bimetallic core−shell NPs of other elements in multilayer thin films, as long as the standard reduction potential of the core element is higher than that of the shell one. The exact procedure depends on the charge types of the two metal precursor ions. First, a PEM is assembled and terminated with the appropriate polyelectrolyte so that the dominating counterion in the PEM is of the same charge type as the precursor ion for the core. Then both core and shell are synthesized sequentially to yield the core−shell NPs, each via multiple exchange/reduction cycles, if their precursor ions are of the same charge type;53 however, if their charges are opposite, after the core is synthesized an additional polyelectrolyte layer, opposite to the terminating layer, needs to be deposited onto the PEM to switch the type of the dominating counterion before the shell is synthesized via multiple exchange/reduction cycles. This strategy can be further extended to the controllable fabrication of onion-like, multilayer nanostructures.

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AUTHOR INFORMATION

Corresponding Author

*Phone (+86)431-85262854; Fax (+86)431-85262126; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21174145). Z.S. thanks the NSFC Fund for Creative Research Groups (50921062) for support. H.W. acknowledges the Start-up support provided by the College of Arts and Sciences of University of South Carolina.

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REFERENCES

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CONCLUSIONS We have demonstrated a facile and effective strategy to synthesize bimetallic Au@Ag core−shell NPs with well-defined structure and composition in situ in polymer thin films. This approach takes advantage of the counterions existing in PEMs. First a precursor ion is introduced into the PEM via ionexchange and then reduced in situ to synthesize the cores. Then by deposition of an additional polyelectrolyte layer the dominating counterion is switched from anionic to cationic (or vise versa), and then the shells are synthesized via ionexchange/reduction of the precursor ion for the shell, of opposite charge to that for the core. Weak reducing agent is used so that subsequent reductions occur on the seed NPs formed in the first cycle to ensure formation of core−shell structure and more uniform particle size. The core size, shell thickness, and the composition of the NPs in the polymer matrix can be well controlled by varying the number of 15711

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on October 17, 2012. Reference 53 has been modified. The correct version was published on October 23, 2012.

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dx.doi.org/10.1021/la303320z | Langmuir 2012, 28, 15705−15712