Bimetallic Multifunctional Core@Shell Plasmonic Nanoparticles for

Article pubs.acs.org/ac

Bimetallic Multifunctional Core@Shell Plasmonic Nanoparticles for Localized Surface Plasmon Resonance Based Sensing and Electrocatalysis Ji-Eun Lee, Kyungwha Chung, Yoon Hee Jang, Yu Jin Jang, Saji Thomas Kochuveedu, DongXiang Li,† and Dong Ha Kim* Department of Chemistry and Nano Science, Division of Molecular and Life Sciences, College of Natural Sciences, Ewha Womans University, 52, Ewhayeodae-Gil, Seodaemun-Gu, Seoul 120-750, Korea S Supporting Information *

ABSTRACT: Smart bimetallic core@shell nanoparticles were fabricated based on gold nanoparticles (AuNPs) decorated with pH-sensitive polymer shell. Concretely, AuNPs having poly(4-vinylpyridine) (P4VP) on the surface were first fabricated through surface-initiated atom transfer radical polymerization (SI-ATRP). Then, they were mixed with selected metal precursor solutions followed by reduction using reducing agent. The metal NPs thus introduced were uniformly distributed in P4VP polymer shells. In order to explore the diversity and viable function of the resultant nanostructures, we controlled the size of AuNP, pH, selectivity of metal precursors, etc. We investigated the structural alteration during the sequential synthetic process. The bimetallic nanostructures of AuNP@P4VP nanocomposites containing another type of metal NP at the P4VP periphery exhibit a controlled sensing property in terms of the change in the refractive index of surrounding media and a typical electrocatalytic activity for methanol oxidation reaction.

N

responsive polymer brushes, because the pyridyl groups of P4VP can be protonated at low-pH condition, leading to extended chain configuration.17,18 It has been reported that polymer-functionalized AuNPs can be prepared using surfaceinitiated atom transfer radical polymerization (SI-ATRP).19 It suggests a facile pathway for the combination of multifunctional polymers. Especially, AuNPs with pH-sensitive polymer ligand are reported to exhibit LSPR properties in response to the external stimuli. Core@shell-type nanospheres have also been utilized as a platform for the integration of extra functionalities into both the core and shell. The interaction between two metal NPs selectively located in the core and shell may alter the LSPR property, depending on the relative amount of the two metal components, the relative distance between them, and the shell thickness, etc. In this paper, we report the fabrication of bimetallic core@ shell nanoarchitectures utilizing AuNPs decorated with pHresponsive P4VP mediated by SI-ATRP as templates for tailored nanocomposites. The typical experimental procedure is

oble metal nanoparticles have demonstrated distinctly different properties and potential uses in electronics, magnetics, catalysis, optics, and sensing due to the localized surface plasmon resonance (LSPR) phenomenon, the coupling of light into the resonant oscillation of charge density on the noble metal surface. Their versatility in a wide range of applications stems from their unique physical and chemical properties directly related to particle size, shape, interparticle distance, and dielectric environment.1−5 Recently, it has been recognized that multilayered core@shell nanoarchitectures consisting of noble metals are expected to be promising candidates in catalysis6−8 and surface-enhanced Raman scattering (SERS)9,10 studies. In previous studies, various core@shell nanoparticles have been synthesized on the basis of a seed growth mechanism.11−13 However, these shells possess no nanoporosity, providing very low surface area. Also, it is impossible for guest species to reach the inner metal cores. These facts seriously devalue the advantages of the core− shell structures. The functionalized AuNPs, particularly polymer-functionalized ones, have been attracting increased attention. The surface properties of AuNPs depend on the functionality and architecture of the polymer blocks.14−16 For example, poly(4-vinylpyridine) (P4VP) can be used as pH© 2012 American Chemical Society

Received: March 6, 2012 Accepted: July 5, 2012 Published: July 5, 2012 6494

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

Article

Scheme 1. Schematic Diagram for the Fabrication of AuNP@P4VP−Metal Nanocomposites by Sequential SI-ATRP and Metal Incorporation

conditions. In detail, a round-bottom flask was added with CuBr (57.3 mg) and CuBr2 (0.4 mg) degassed by three freeze− pump−thaw cycles under N2 atmosphere. A degassed mixture of AuNP@initiator (1 mg) dissolved in DMF (3.5 mL), PMDETA (0.416 mL), H2O (0.5 mL), and a certain amount of free initiator (10 μL) were injected into the flask through a syringe, followed by degassed 4VP (2.10 g) with stirring. The reaction was performed for 48 h at 40 °C and terminated by opening the system to air. The AuNP@P4VP nanocomposites were purified by more than three cycles of centrifugation and DMF washing. Nanocomposites of AuNP@P4VP and Metal. The coordinated complex of AuNP@P4VP nanocomposites and metal NPs was prepared by adding HAuCl4, AgNO3, Na2PdCl4, H2PtCl6 (4VP/metal molar ratio of 1:1, 1.0 mg/mL) into AuNP@P4VP NPs (2 mL, 0.1 nM) in a pH 3 HCl solution, followed by continuous stirring for 3 h. The red loose precipitate was redispersed by ultrasonication and washed with water three times by centrifugation. Second, the metal ions attached on the redispersed AuNP@P4VP nanocomposites were reduced by a sodium borohydride (50 μL, 0.01 M) solution with stirring, and then the mixture was centrifuged to separate and purify the composite bimetallic NPs. Refractive Index Sensing. Glycerol (2 mL) was added into AuNP@P4VP, AuNP@P4VP−Au colloids (100 μL, 0.1 nM) in pH 3.2 HCl solution to make final concentrations ranging from 10% to 50%. The mixtures were incubated for 1 h, and their UV−vis spectra were measured. Glycerol concentrations of 0%, 10%, 20%, 30%, 40%, and 50% correspond, respectively, to an index of refraction of 1.330, 1.345, 1.357, 1.373, 1.384, and 1.401. Electrochemistry. The electo-oxidation of sulfuric acid and potassium hydroxide was performed in a three-electrode system using a potentiostat with a platinum foil and Ag/AgCl (in saturated KCl) electrode as the counter and reference electrode, respectively, in N2-saturated aqueous 0.5 M H2SO4, 0.1 M KOH, or 1 M methanol + 0.5 M H2SO4, 1 M methanol + 0.1 M KOH solution. The working electrodes (glassy carbon electrode, GCE) were prepared by drop-casting a AuNP@ P4VP−Pt, AuNP@P4VP−Pd NPs containing suspension and air-dried. A GC disk electrode with diameter of 3 mm was used as the support. CVs (cyclic voltammograms) were recorded from −0.2 to 1.0 V and −1.0 to 0.4 V versus Ag/AgCl at scan rate of 50 mV/s. Instruments. Transmission electron microscope (TEM) measurements were carried out using a JEOL JSM2100-F

schematically illustrated in Scheme1. First, the disulfide initiator was immobilized on the surface of gold nanoparticles (AuNP@ initiator). Subsequently, ATRP of 4-vinylpyridine (4VP) was performed on AuNPs catalyzed by N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) and CuBr. The obtained AuNP@ P4VP nanocomposites are pH-responsive at a critical point of pH 3.2.20 Such environmentally responsive nanocomposites with effective coordinating pyridyl segments provide a smart supporter or carrier to transition metal ions and NPs to construct novel bimetallic nanocomposites, especially in LSPRbased sensing or catalytic applications. For instance, bimetallic nanostructures containing a second noble metal moiety at the periphery can find use in refractive index sensing, whereas the ones containing Pd or Pt may exhibit an enhanced catalytic activity for oxidation/reduction in fuel cell electrodes.

■

EXPERIMENTAL SECTION Materials. 11-Mercapto-1-undecanol, 2-bromo-2-methylpropionyl bromide, 4VP, CuBr, CuBr2, HAuCl4, AgNO3, Na2PdCl4, H2PtCl6, sodium citrate, and PMDETA were purchased from Sigma-Aldrich. 4VP was purified to remove the inhibitor. The disulfide initiator [S−(CH2)11OCOC(CH3)2Br]2 for SI-ATRP was synthesized from 11-mercapto1-undecanol and 2-bromo-2-methylpropionyl bromide through a modified procedure according to ref 21. Synthesis of Gold Nanoparticles (AuNPs). Amounts of 150 mL of distilled water and 1.5 mL of HAuCl4 solution (25.4 mM) were added into a 250 mL round-bottom flask, and the solution was brought to boil on a hot plate with vigorous stirring. Then 0.9 mL of sodium citrate (170 mM) solution was added rapidly, and the solution was boiled for another 20 min while stirring. The color of the solution finally turned to wine red. The AuNPs synthesized from this method give a size in the range of 15 ± 2 nm. The concentration of AuNPs used in this study is 3.1 × 10−9 M. Preparation of SI-ATRP Initiator. The disulfide initiator was immobilized on the surface of gold nanoparticles by ligand exchange with citrate. A certain volume of the 15 nm AuNP suspension (300 mL) was slowly added to the same volume of disulfide initiator (150 μL, 1.0 mM) in tetrahydrofuran (THF) (200 mL) with stirring for 24 h. Then the AuNP@initiator samples were collected and washed with N,N-dimethylformamide (DMF) and deionized water by centrifugation. The final samples were dispersed in DMF and were stored at −20 °C. SI-ATRP Process of 4VP. Polymerization of 4VP on a Au surface was performed in a mixed solvent at ambient 6495

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

Article

Figure 1. TEM images of (A) AuNP@citrate, (B) AuNP@initiator, and (C) AuNP@P4VP nanostructure. The inset in panel C shows a magnified image.

Figure 2. TEM images of AuNP@P4VP−metal nanocomposites with a different kind of second metal nanoparticle in the periphery: (A) AuNPs, (B) AgNPs, (C) PtNPs, and (D) PdNPs.

microscope operated at 100 kV. UV−vis absorption spectra were recorded on a Varian Technologies Cary 5000. The electrochemical study was performed on an Autolab ECO Chemie PGSTAT302N potentiostat at room temperature.

AuNP@initiator exhibited an LSPR band centered around 528 nm, with a red-shift of 8−10 nm as compared to the initial AuNP@citrate. The obtained AuNP@P4VP can accommodate various moieties through electrostatic interaction since the pyridine group can be protonated. Such AuNP@P4VP nanocomposites can be considered as coordinative supports for transition metal ion or metal nanoparticles due to the presence of the pyridyl-containing shells. Moreover, it is advantageous that the SI-ATRP is possible in any pH range. The size of AuNPs and the P4VP thickness were measured to be ∼15 and ∼10 nm, respectively, as confirmed by the TEM image in Figure 1C. The average hydrodynamic diameter of the

■

RESULTS AND DISCUSSION Characterization of AuNP@P4VP Nanocomposites. Figure 1A is a representative TEM image of the citrate-capped AuNPs (AuNP@citrate) with an average size of 15 nm. The AuNP@citrate exhibited an LSPR band around 518 nm. Figure 1B shows the TEM image of the AuNP after disulfide initiator was immobilized by ligand exchange reaction. The resulting 6496

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

Article

Figure 3. UV−vis absorption spectra of (A) AuNP@P4VP−Au, (B) AuNP@P4VP−Ag, (C) AuNP@P4VP−Pt, and (D) AuNP@P4VP−Pd nanocomposites.

Figure 4. (A) UV−vis absorption spectra of AuNP@P4VP colloid solutions with different glycerol concentrations. (B) Optical response of AuNP@ P4VP colloids to the variation of glycerol concentration in terms of λmax vs refractive index. TEM images of AuNP@P4VP colloid solutions in glycerol with different concentrations: (C) 10%, (D) 30%, and (E) 50%.

Information). The transformation of polymer chain from expanded state to collapsed one can be induced by the environmental pH values so that the pH-sensitive nanocomposites are of significance in a pH-variable biological system. The pure AuNP@P4VP showed swelling−shrinking behavior around pH 3−4.20 Next, metal precursors were incubated with the as-obtained AuNP@P4VP nanocomposites during which the metal ions were segregated into the P4VP shells. After subsequent reduction with sodium borohydride the bimetallic AuNPs-,

AuNP@citrate and AuNP@P4VP was found to be 15 and 37 nm as determined by the dynamic light scattering (DLS) measurements (see Figure S-1 in the Supporting Information). These observations demonstrate the successful coating of a polymer layer around AuNPs. Gel permeation chromatography (GPC) measurements were carried out to measure the Mn and the polydispersity of surface-grafted P4VP. Concretely, free polymers were generated by adding free initiators during the surface-initiated ATRP, and the obtained values were 11 700 g mol−1 and 1.06, respectively (see Figure S-2 in the Supporting 6497

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

Article

Figure 5. TEM images of AuNP@P4VP−Au colloid solutions with different glycerol concentrations: (A) glycerol 10%, (B) glycerol 30%, and (C) glycerol 50% (images in lower magnification are displayed at the lower panel). (D) UV−vis absorption spectra of AuNP@P4VP−Au colloid solutions with different glycerol concentrations. (E) Optical response of AuNP@P4VP−Au colloids to the variation of glycerol concentration in terms of λmax vs refractive index.

band in the 400−480 nm region arises from the Ag component in Au/Ag shells, whereas the main broad peak above ca. 500 nm is ascribed to the Au. The AuNP@P4VP−Pt and AuNP@ P4VP−Pd nanocomposites exhibited LSPR bands around 530 nm. Note that, generally, the shells of Pt or Pd can strongly damp out the dipolar oscillations of Au cores because Pd or Pt has significantly lower conductivity at the optical frequency than Au.6,22,23 Refractive Index Sensing. The LSPR sensing property of AuNP@P4VP colloids is demonstrated in terms of their response to the change in the surrounding media, i.e., refractive index sensing, which is realized by varying glycerol concentration from 10% to 50%. UV−vis absorbance spectra of AuNP@P4VP colloids in glycerol solution with different concentrations are displayed in Figure 4A, and the wavelength of each maximum LSPR band (λmax) is summarized in Figure 4B as a function of the refractive index of glycerol solutions. The λmax value was centered at 549, 538, 536, 534, and 533 nm, for the 10%, 20%, 30%, 40%, and 50% solution, respectively. Once the AuNP@P4VP is mixed with 10% glycerol solution, a red-shift of LSPR band is first observed with respect to the pure aqueous solution. Then, with increasing the glycerol concentration up to 50%, a blue-shift behavior is observed. The mechanism of the phenomena needs to be further investigated

AgNPs-, PtNPs-, PdNPs-loaded AuNP@P4VP nanocomposites (AuNP@P4VP−metal) were fabricated. The metal NPs with a size of 2−3 nm are densely decorated around the AuNPs (Figure 2). In Figure 3, the UV−vis spectra of the AuNP@ P4VP−Au (Figure 3A), AuNP@P4VP−Ag (Figure 3B), AuNP@P4VP−Pt (Figure 3C), and AuNP@P4VP−Pd (Figure 3D) samples are displayed, respectively. Characteristic LSPR bands at ca. 530 nm are observed from the AuNP@P4VP nanoparticles as shown in Figure 3. After the preparation of P4VP−metal shells, the spectrum of AuNP@P4VP−Au nanoparticles becomes very broad with a peak at ca. 550 nm. It is commonly known that typical LSPR bands of spherical NPs appear between the 400 and 600 nm region, depending on the type of metal and molar ratios of particles. In case of AgNP, the appearance of a long tail band in the visible region arises from the formation of a core−shell structure. In the AuNP@ P4VP−Ag nanocomposites, the increase of Ag composition leads to a blue-shift in the absorption band of bimetal nanoparticles with respect to neat AuNPs. Distinguished absorption bands for both Ag and Au in the bimetal nanocomposites were observed at increased Ag composition (Figure 3B), suggesting the presence of both the Ag and Au components in the nanoparticles. On the basis of these observations, it is reasonable to assume that the weak shoulder 6498

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

Article

Figure 6. CVs obtained from AuNP@P4VP−Pt nanocomposites in (A) 0.5 M H2SO4 and (B) 0.5 M H2SO4 + 0.1 M methanol. CVs obtained from AuNP@P4VP−Pd nanocomposites in (C) 0.1 M KOH and (D) 0.1 M KOH + 0.1 M methanol.

use in fuel cell electrodes. We prepared composite NPs by mixing the AuNP@P4VP with the aqueous solutions of metal precursors (H2PtCl6, Na2PdCl4) with the P4VP/precursor molar ratio of 1:1 and 1:2. The electrocatalytic performance was evaluated using thus-obtained AuNP@P4VP nanocomposites loaded with PtNPs and PdNPs at the P4VP periphery (AuNP@P4VP−metal) (Figure S-3 in the Supporting Information). The metal NPs with a size of 2−3 nm are densely decorated on the AuNPs surfaces as shown in Figure 2, parts C and D. The surface properties of AuNP@P4VP−metal hybrid nanocomposites were characterized by cyclic voltammetry (CV) measurement (Figure 6). The direct methanol fuel cells (DMFCs) have been intensely studied because of their numerous advantages, such as highenergy density, the ease of handling the liquid, low operating temperatures, and their possible applications to microfuel cells. The performance of DMFCs is known to be strongly dependent on the electrocatalytic materials used. Remarkably, bimetallic Au−Pt or Au−Pd nanoparticles were reported to have a great potential as electrocatalyst. The disappearance of the Au oxide reduction peak observed at AuNP@P4VP−metal nanocomposites suggests that the Au nanoparticles surfaces are almost completely covered by small metal NPs. From the cyclic voltammograms in Figure 6, it is found that the AuNP@P4VP− metal nanocomposites show conspicuous catalytic behavior for the electro-oxidation of methanol as confirmed by the appearance of an oxidation current in the positive potential region, whereas the conventional metal/carbon does not exhibit noticeable electrocatalytic activity for the same reaction. The CVs for methanol oxidation obtained from AuNP@P4VP−Pt and AuNP@P4VP−Pd nanocomposites in 0.5 M H2SO4 and 0.1 M KOH solution containing 0.1 M methanol at a scan rate of 50 mV/s are displayed in Figure 6, parts B and D, respectively. The onset potentials for the electrocatalytic oxidation are around 0.3 and 0.2 V (vs Ag/AgCl) for the AuNP@P4VP−Pt nanocomposites with 4VP/Pt molar ratio of 1:1 and 1:2, respectively, indicating that the system with higher Pt amount shows better performance. The current peak at about 0.68 V in the forward scan is attributed to the

by correlating with TEM images. The AuNP@P4VP in 10% glycerol solution shows aggregated nanocomposites as shown in Figure 4C. Then, the assembly of AuNP aggregates in 30% glycerol solution is partially dissociated (Figure 4D) and more randomly dispersed at 50% solution. On the basis of the UV− vis and TEM results, one can interpret the behavior of the colloidal assembly as follows: adding a small amount of glycerol into the aqueous AuNP@P4VP solution, the LSPR band is redshifted due to the strong assembly and enhanced LSPR coupling, reflecting that the glycerol acts as linker molecule between neighboring AuNPs. If the concentration of glycerol solution is increased further, the extra glycerol molecules are segregated into the existing glycerol domains, leading to an increase in the average distance between them as evidenced by the TEM images in Figure 4, parts B and C, and the blue-shift of LSPR bands. The overall assembly of AuNP@P4VP−Au nanocomposites solution shows a similar trend as AuNP@P4VP. At 10% glycerol concentration, the majority of the NPs were aggregated (Figure 5A) with local dense clusters. At a higher concentration up to 50% glycerol, well-dispersed AuNP assembly was observed as shown in Figure 5, parts B and C. The λmax value of the LSPR bands was centered at 548, 546, 545, 544, and 544 nm for the AuNP@P4VP−Au at 10%, 20%, 30%, 40%, and 50% solution, respectively. Again it showed a red-shift upon adding glycerol into the aqueous solution and a gradual blueshift with increasing the concentration. It is noteworthy that the degree of LSPR band shift for the AuNP@P4VP loaded with AuNPs at the periphery (AuNP@P4VP−metal) is smaller than that observed at AuNP@P4VP colloids system. This indicates that the presence of AuNPs in the periphery reduces the flexibility of the P4VP domains, which in turn leads to less sensitive response of the AuNP@P4VP−metal system upon increasing the glycerol concentration compared with AuNP@ P4VP. Catalytic Activity of AuNP@P4VP−Metal Nanocomposites. Platinum (Pt) is recognized as the most popular electrocatalyst while palladium (Pd) catalyst is attracting an attention as a relatively new and interesting alternative to Pt for 6499

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500

Analytical Chemistry

■

electrocatalytic methanol oxidation on the surface of AuNP@ P4VP−Pt nanocomposites. In the reverse scan, an oxidation peak is also observed around 0.5 and 0.54 V for the AuNP@ P4VP−Pt with 4VP/Pt molar ratio of 1:1 and 1:2, respectively, which is probably associated with the removal of the residual carbon species formed in the forward scan. The ratio of the forward oxidation current peak (If) to the reverse one (Ib), If/Ib, is an index of the catalyst tolerance to the poisoning species, PtCO. A higher If/Ib ratio generally indicates more effective removal of the poisoning species on the catalyst surface. The calculated ratio for the AuNP@P4VP−Pt nanocomposites with 4VP/Pt molar ratio of 1:2 is 2.5 (Figure 6B). The electrocatalytic property of AuNP@P4VP−Pd nanocomposites for methanol oxidation is also performed and compared. As shown in Figure 6D, the oxidation current peak was observed at about −0.1 and −0.33 V in the forward and reverse scans, respectively. The calculated If/Ib ratio for the AuNP@P4VP−Pd nanocomposites with 4VP/Pd molar ratio of 1:2 is 2.2, which turns out to be markedly higher than that of the metal/carbon catalyst. In brief, the AuNP@P4VP−metal nanocomposites deduced in this study were proven to be very efficient electrocatalysts potentially useful in fuel cell electrodes and optical sensors with controlled LSPR property.

REFERENCES

(1) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338. (2) Sun, Y. G.; Xia, Y. Analyst 2003, 128, 686. (3) Murphy, C. J.; San, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (4) Maier, S. A.; Atwater, H. A. J. Appl. Phys. 2005, 98, 011101. (5) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685. (6) Guo, S. J.; Li, J.; Dong, S. J.; Wang, E. K. J. Phys. Chem. C 2010, 114, 15337. (7) Lee, Y. W.; Kim, M.; Kim, Y.; Kang, S. W.; Lee, J. H.; Han, S. W. J. Phys. Chem. C 2010, 114, 7689. (8) Wang, S. Y.; Kristian, N.; Jiang, S. P.; Wang, X. Electrochem. Commun. 2008, 10, 961. (9) Liu, X. J.; Knauer, M.; Ivleva, N. P.; Niessner, R.; Haisch, C. Anal. Chem. 2010, 82, 441. (10) Xie, W.; Herrmann, C.; Kompe, K.; Haase, M.; Schlucker, S. J. Am. Chem. Soc. 2011, 133, 19302. (11) Wang, L.; Yamauchi, Y. J. Am. Chem. Soc. 2009, 131, 9152. (12) Zhao, D.; Xu, B. Q. Angew. Chem., Int. Ed. 2006, 45, 4955. (13) Tang, X. L.; Tsuji, M. CrystEngComm 2011, 13, 72. (14) Shou, Q. H.; Guo, C.; Yang, L. R.; Jia, L. W.; Liu, C. Z.; Liu, H. Z. J. Colloid Interface Sci. 2011, 363, 481. (15) Li, D. X.; He, Q. A.; Li, J. B. Adv. Colloid Interface Sci. 2009, 149, 28. (16) Song, J. B.; Cheng, L.; Liu, A. P.; Yin, J.; Kuang, M.; Duan, H. W. J. Am. Chem. Soc. 2011, 133, 10760. (17) Akamatsu, K.; Shimada, M.; Tsuruoka, T.; Nawafune, H.; Fujii, S.; Nakamura, Y. Langmuir 2010, 26, 1254. (18) Li, D.; Jang, Y. J.; Lee, J.; Lee, J. E.; Kochuveedu, S. T.; Kim, D. H. J. Mater. Chem. 2011, 21, 16453. (19) Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921. (20) Li, D. X.; He, Q.; Yang, Y.; Mohwald, H.; Li, J. B. Macromolecules 2008, 41, 7254. (21) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (22) Mandal, S.; Mandale, A. B.; Sastry, M. J. Mater. Chem. 2004, 14, 2868. (23) Sadeghmoghaddam, E.; Lam, C.; Choi, D.; Shon, Y. S. J. Mater. Chem. 2011, 21, 307.

■

CONCLUSIONS In summary, we suggest a fabrication route to AuNP@P4VP NPs loaded with metal NPs at the P4VP periphery mediated by SI-ATRP and subsequent reduction of precursors. We first demonstrated that the AuNP@P4VP hybrid NPs can be used as efficient nanosupports for transition metal catalysts due to the presence of the coordinative pyridyl groups. The AuNP@ P4VP and AuNP@P4VP−Au nanocomposites showed interesting LSPR sensing property to the surrounding media with different amounts of glycerol. The AuNP@P4VP−Pt and AuNP@P4VP−Pd bimetallic nanocomposites showed viable electrocatalytic activity for methanol oxidation, providing their potential chance to be integrated into fuel cell electrodes. Because the synthesized nanoparticles have unique structural, compositional, and pH-dependent aspects, they may find extended multifunction in numerous applications.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +82-2-3277-3419. Present Address †

College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Research Foundation (NRF) of Korea Grant funded by the Korean Government (20110029409, 20110030255, 2012K001319). 6500

dx.doi.org/10.1021/ac300654k | Anal. Chem. 2012, 84, 6494−6500