Detecting Trace Melamine in Solution by SERS Using Ag Nanoparticle

Oct 19, 2011 - The surface enhanced Raman spectroscopy (SERS) sensitivity was investigated using 4-hydroxythiophenol (4-HBT) as the model probe in the...
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Detecting Trace Melamine in Solution by SERS Using Ag Nanoparticle Coated Poly(styrene-co-acrylic acid) Nanospheres as Novel Active Substrates Ju-Mei Li,†,‡ Wan-Fu Ma,† Chuan Wei,† Li-Jun You,† Jia Guo,† Jun Hu,§ and Chang-Chun Wang*,† †

Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, and Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China ‡ School of Material Science and Engineering, Jingdezhen Ceramic Institute, Jingdezhen 333403, China § Department of Chemistry and Integrated Biosciences, The University of Akron, Akron, Ohio 44325-3601, United States

bS Supporting Information ABSTRACT: A systematic study for the preparation of Ag nanoparticle (Ag-NP) coated poly(styrene-co-acrylic acid) (PSA) composite nanospheres by in situ chemical reduction is reported. The experimental results showed that the reaction temperature and the surface coverage of the COOH determined the surface coverage and grain size of Ag nanoparticles on the PSA nanospheres. The surface enhanced Raman spectroscopy (SERS) sensitivity was investigated using 4-hydroxythiophenol (4-HBT) as the model probe in the solution of composite nanospheres stabilized by polyvinylpyrrolidone (PSA/Ag-NPs/PVP), with the detection limit of about 1  10 6 M. Potential application of the new SERS substrate was demonstrated with the detection of melamine, and the detection limit was about 1  10 3 M. Chemical noises from PVP and other impurities were observed and attributed mainly to the competitive adsorption of PVP on the surfaces of Ag-NPs. After tetrahydrofuran washing of the PSA/ Ag-NPs/PVP substrates that removed the PVP and other residuals, the signal/noise levels of SERS were greatly improved and the detection limit of melamine was determined to be 1  10 7 M. This result indicated that the new PSA/Ag-NPs system is highly effective and can be used as the SERS-active substrate for trace analysis of a variety of drugs and food additives.

’ INTRODUCTION Composite nanospheres consisting of dielectric polymer core and metallic shell have been a subject of intensive studies due to the unique fundamental mesoscopic properties and their applications in chemical and biological sensors, catalysis, photonic crystals, and so on.1,2 A great deal of efforts have been focused on preparing core shell composite nanospheres with noble metallic shells such as Au and Ag because they exhibit novel optical, electronic, and catalytic properties and, in particular, surface enhancement in the Raman spectroscopy.3 5 Various preparation methods have been developed, such as thermal evaporation,6 dispersion copolymerization,7 ultrasound irradiation,8 self-assembly,9 and solvent-assisted deposition methods.4 Recently, in situ reduction deposition method has been reported. In this process, various reducing agents, including NaBH4, N2H4, tannins, formaldehyde, butylamine, sodium citrate, glucose, ascorbic acid, and ethylene glycol, could be used for the preparation of desired metal nanoparticles and nanostructures.5,10 13 Polyvinylpyrrolidone (PVP) is a homopolymer with an imide repeating unit where the strongly dipolar CdO group solvates r 2011 American Chemical Society

metal salts and can also form surface dipolar self-assembly to the corresponding reduced metal surfaces. The polymer backbone serves as the steric stabilizer of the nanocrystal, and the CdO functional group serves as the capping agent to stabilize the incipient metal crystal surfaces during the nanoparticle formation, allowing the control of the size and shape of the colloidal nanoparticles. Thus, PVP was widely used in the preparation of stable colloidal solutions of metal nanoparticles (NPs) including Ag-NPs, Au-NPs, and Pt-NPs.14 17 In some cases, PVP was also reported as the reducing agent in the synthesis, for example, reported by Kan et al.18 In 2006, Xia et al.19 proposed that the reducing capacity of PVP arises from the hydroxyl end-group of the PVP polymer chain, formed in the polymerization process involving water and hydrogen peroxide. The polymer therefore acts as a long-chain alcohol which serves as a reductant. In addition, they successfully synthesized various noble metal nanoplates including Received: August 5, 2011 Revised: September 16, 2011 Published: October 19, 2011 14539

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Langmuir those of Ag, Pd, Au, and Pt in aqueous solutions using PVP as the reductant and the stabilizer.20 The supposition of reduction of metal salts by the hydroxyl end groups was verified by the fact that the conversion percentage from metal ions to metal nanoparticles was proportional to the amount of hydroxyl end-groups of PVP used in the reaction. PVP became the reagent of choice for preparing dielectric core/metallic shell composite materials because of its excellent performance in controlling size/size distribution and the above-mentioned reduction capability.21 23 For example, using [Ag(NH3)2]+ as the Ag source and PVP as the reducing and stabilizing agent, SiO2/Ag-NP21 and polystyrene/Ag-NP composite nanospheres were successfully prepared.22 Herein, we report the systematic study of the deposition of uniform Ag-NP shells on poly(styrene-co-acrylic acid) (PSA) nanospheres by in situ reduction of AgNO3 using PVP as a reducing agent in water. By changing the reaction conditions, we found that the size and coverage of the resultant silver nanoparticles on PSA nanospheres could vary drastically. Optimal reaction conditions were achieved to reproducibly yield uniform, high coverage Ag-NP layers on PSA nanospheres. Finally, the surface enhanced Raman spectroscopy (SERS) experimental results showed that these PSA/Ag-NP composite nanospheres have great potential for detecting trace of organic compounds with very high sensitivity in aqueous solutions.

’ EXPERIMENTAL METHODS Materials. Styrene (St) and acrylic acid (AA) were purchased from Aldrich and refined by distillation under reduced pressure. Potassium persulfate (KPS) was recrystallized from deionized water prior to use. PVP with an average molecular weight of 40 000 (PVP 40) was obtained from Amresco. Silver nitrate (AgNO3 > 99.8%), 4-hydroxythiophenol (4-HBT > 98%), melamine, tetrahydrofuran (THF), and anhydrous ethanol were purchased from Shanghai Chemical Reagent Co., Ltd. All chemicals were used as received unless specified. Highly pure water (Millipore) of resistivity greater than 18.0 MΩ 3 cm was used in all experiments. All glassware used in the experiments was cleaned with chromosulfuric acid before use. Synthesis of Poly(styrene-co-acrylic acid) (PSA) Nanospheres. The nanospheres of poly(styrene-co-acrylic acid) were synthesized via free radical copolymerization of St and AA in water according to the method described in our previous reports with some modifications.23 25 Briefly, A mixture of St and AA with different feed ratios (in all cases, the mass is 2.0 g in total) and 100 mL of H2O were charged into a three-necked flask, which was equipped with a mechanical stirrer and a condenser. The solution was purged with nitrogen to remove oxygen for 30 min and then heated to 70 °C while stirring. A solution of 0.06 g of KPS dissolved in 2 g of H2O was injected into the reaction mixture to initiate the polymerization, and reaction was allowed to continue for 6 h. The produced PSA nanospheres were purified by several cycles of centrifugation/redispersion in ultrapure water and then dispersed in water for further use. The polymer nanospheres were designated according to the feed ratio of AA to total monomer (AA + St) by weight as PSA-0 (AA% = 0 wt %), PSA-5 (AA% = 5 wt %), PSA-10 (AA% = 10 wt %), PSA-20 (AA% = 20 wt %), and PSA-30 (AA% = 30 wt %).

Preparation of PSA/Ag-NPs/PVP Composite Nanospheres. A typical procedure for fabricating PSA/Ag-NPs/PVP composite nanospheres is described as follows: A 50 mL one-necked flask equipped with a magnetic stirrer was charged with the aqueous PSA dispersion (10 mL, 5 mg/mL) and an aqueous solution of AgNO3 (10 mL, 0.2 M). The mixture was stirred for 4 h at room temperature to allow the ion exchanges on the surfaces of the nanospheres to reach equilibrium. After that, a PVP solution (10 mL, 10% weight in water) was added and the resulting mixture was

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allowed to react at 50 °C for 12 h while stirring. The product was collected by centrifugation and redispersion in ultrapure water for further examinations.

SERS Property of PSA/Ag-NPs/PVP and PSA/Ag-NPs Composite Nanospheres. The SERS activity of the PSA/Ag-NPs/PVP composite nanospheres was studied using 4-HBT and melamine as analytes. Specifically, 0.1 mg of PSA/Ag-NPs/PVP composite nanospheres was dispersed in 1 mL of the analyte solution and stirred for 0.5 h at room temperature. Then the dispersion was centrifuged several times with ethanol or water to remove excess analyte. Next, analyte-modified PSA/Ag-NPs/PVP composite nanospheres were dispersed in 1 mL of water and collected using a capillary tube for SERS measurement. The PSA/Ag-NPs composite nanospheres without stabilizer PVP were also used as SERS enhancer to detect trace melamine. The detailed procedure is described as follows: 0.1 mg of PSA/Ag-NPs/PVP composite nanospheres were dispersed in 1 mL of THF and stirred for 0.5 h at room temperature. After the dispersion was washed several times with water, the sediments were dispersed in 1 mL of melamine aqueous solution for 0.5 h and then were added into a glass cuvette for Raman measurement. Characterization. Transmission electron microscopy (TEM) images were obtained on a Hitachi H-600 transmission electron microscope at an accelerating voltage of 75 kV. All the specimens for TEM analysis were prepared by diluting samples with water. A drop of diluted solution was placed on the carbon-coated copper grid and was dried in air before observation. UV vis spectra were measured on a Shimadzu UV-3150 spectrometer. Powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation at λ = 0.154 nm operating at 40 kV and 40 mA. Raman spectra were recorded using the Invia Reflex by Renishaw with 632.8 nm laser excitation and a 50 objective. The data acquisition time was usually 30 s, and the peak intensities of samples were normalized with respect to that of the silicon wafer at 520 cm 1. Hydrodynamic diameter (Dh) measurements were conducted with a Nano ZS Zetasizer (model ZEN3600, Malvern Instruments) using a He Ne laser at a wavelength of 632.8 nm. Thermogravimetric analysis (TGA) was carried out on the Pyris-1 Series (Perkin-Elmer) under nitrogen atmosphere (flow rate 40 mL/min) at a heating rate of 20 °C/min.

’ RESULTS AND DISCUSSION Effect of the Carboxyl Group Amount of PSA on the Deposition of Ag Nanoparticles (Ag-NPs). Polymer matrices

with functional monomer units interacting with the metal precursors by ion pair or complex formation are well-known to play an important role in controlled synthesis of mesoscopic metal particles with defined size, shape, and distribution. For example, polymers bearing functional groups, such as hydroxyl ( OH),21 amino ( NH2),5 mercapto ( SH),26 and carboxyl ( COOH),10,22 have been used to prepare metallic nanoparticles. The polymers serve as template substrates for controlling the nucleation and growth of the particles as well as the supporting matrices to stabilize the incipient metal colloids that are otherwise thermodynamically metastable. Polymers with carboxyl groups have been used as nucleation sites for the deposition of metal nanoparticles previously. We have successfully attached Pt or Ni nanoparticles to MWNTs functionalized with COOH groups.27,28 In the present investigations, we used acrylic acid (AA) as functional monomer to prepare carboxyl groups decorated polymeric nanospheres via surfactant-free emulsion polymerization. In order to investigate the influence of carboxyl groups on the morphology of Ag-NPs on the nanospheres, we produced a series of PSA nanospheres with different ratios of AA and St. The detailed composition and dynamic light 14540

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Table 1. Recipes and Properties of PSA and PSA/Ag-NPs/PVP Nanospheres sample code

a

AA (g)

St (g)

Dh of PSA (nm)a

PDI of PSAa

Ag-NPs in PSA/Ag-NPs/PVP (wt %) b

PSA-0

0

2.0

222

0.007

7

PSA-5

0.1

1.9

277

0.061

13

PSA-10

0.2

1.8

268

0.005

25

PSA-20

0.4

1.6

202

0.024

39

PSA-30

0.6

1.4

140

0.021

Data were measured by dynamic light scattering (DLS). b Measurements were conducted via thermogravimetric analysis (TGA).

Figure 1. Typical TEM images of PSA nanospheres: (a) PSA-0, (b) PSA-5, (c) PSA-10, and (d) PSA-20.

Figure 2. Representative TEM images of PSA/Ag-NPs/PVP composite nanospheres prepared with (a) PSA-0, (b) PSA-5, (c) PSA-10, and (d) PSA-20.

scattering (DLS) characterizations of the nanospheres are summarized in Table 1. The DLS data indicated that all the PSA nanospheres are highly monodisperse and the sizes of PSA nanospheres decreased with the increase of the feeding amount of AA, which is in an excellent agreement with those from the TEM results (Figure 1). It is worth noting that the PSA-30 nanospheres were too small to be conveniently washed and collected by centrifugation, and thus the PSA-30 nanospheres were not used as the substrate of Ag-NPs, and the TEM image is not shown in Figure 1. Based on the method in the Experimental Methods section, a series of PSA/Ag-NPs/PVP composite nanospheres were prepared using various PSA nanospheres while keeping all other parameters unchanged, and their TEM images are shown in Figure 2. It can be observed that few Ag nanoparticles were immobilized onto the PSA-0 nanospheres due to the lack of the AA monomer in the synthesis and hence the lack of COOH groups in the nanospheres. As the AA monomer increases in the nanospheres, more Ag nanoparticles are immobilized onto the corresponding nanospheres. The coverage and uniformity of Ag nanoparticles on PSA nanospheres appears to increase accordingly. When 20 wt % AA was used, PSA-20/Ag-NPs/PVP composite nanospheres with almost complete and uniform AgNP coverage were obtained (Figure 2d and Figure S1 in the Supporting Information). These results suggest that the presence of surface carboxyl groups facilitated attachment of silver ions Ag+ onto the nanosphere surfaces by ion-pairing, which allowed, upon reduction, on the nanosphere surfaces, uniform initial formation of silver nuclei that served as seeds for further growth

to form the observed Ag-NP shells. The deposited Ag-NPs were rather robust and remained on the PSA-20 nanospheres after sonication by a bath sonicator (60 kHz) for 10 min. So we can conclude that the surface functionality of polymer substrates is a significant factor to control the nucleation, growth, and distribution of the metal nanoparticles supported on polymer cores. Thermogravimetric analysis (TGA) further provides quantitative results about Ag-NPs covered on composite nanospheres. TGA data (Figure S2 in the Supporting Information) obviously shows the Ag-NP content of composite nanospheres gradually increased from 7 to 13 to 25 to 39 wt % with increasing AA contents of nanospheres from 0 to 5 to 10 to 20 wt %. Powder XRD analysis (Figure S3 in the Supporting Information) was used to characterize the crystallinity of silver nanoparticles; it can be found that the diffraction peaks become increasingly stronger for PSA-0/Ag-NPs/PVP, PSA-5/Ag-NPs/PVP, PSA-10/AgNPs/PVP, and PSA-20/Ag-NPs/PVP composite nanospheres, which indicated the increased amount and crystallinity of the AgNPs accordingly. PSA-20/Ag-NPs/PVP composite nanospheres with highest Ag-NP coverage showed a well-defined diffraction pattern (Figure S3(iv) in the Supporting Information), and five diffraction peaks were observed at 2θ of 38°, 44.2°, 64.4°, 77.5°, and 81.5°, corresponding to diffractions from the (111), (200), (220), (311), and (222) planes of face centered cubic (fcc) phase of Ag (JCPDS card no. 04-0783), respectively. These results met well with the literature values,21 indicating that the Ag-NPs on PSA nanospheres are highly crystalline. In order to gain more insight into the microstructure of Ag-NPs, high-resolution TEM (HRTEM) imaging was further achieved for PSA-20/Ag-NPs/PVP composite 14541

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Langmuir nanospheres. The lattice fringes of the Ag nanoparticle with the lattice spacing of about 0.23 nm corresponded to the (111) plane of Ag crystal (Figure S3 inset in the Supporting Information). This confirmed the single-crystalline nature of silver nanoparticles to be highly faceted with the dominant (111) surfaces. Effect of the Temperature on the Deposition of Ag Nanoparticles. The size and coverage of the Ag nanoparticles deposited onto PSA nanospheres are expected to be also influenced by the temperature in the deposition process because the reductive power of the OH end-groups increases with the reaction temperature. In order to achieve optimal reaction temperature, we varied the temperature at 30, 40, 50, and 80 °C in the preparation of the PSA20/Ag-NPs/PVP composite nanospheres with other experimental parameters fixed. The resulting PSA-20/Ag-NPs/PVP nanospheres were examined with TEM as shown in Figure 3. Only a few Ag nanoparticles located on the surface of nanospheres when the reaction temperature was at 30 °C (Figure 3a), and the aqueous dispersion exhibited the intense yellow color due to the silver nanoparticles immobilized on the nanospheres being too far away from each other to give rise to the coupled plasmon resonance. When the reaction was carried out at 40 °C, more AgNPs immobilized on the nanospheres (Figure 3b) and the sample became dark yellow. When the reaction was carried out at 50 °C, uniform and high coverage Ag nanoparticles were obtained (Figure 3c). However, when the reaction temperature increased to 80 °C, not only a large number of small silver nanoparticles but also a few of silver agglomerates could be observed coexisting on the nanosphere surfaces (Figure 3d). Compared with low temperature, high temperature in deposition process leads to a faster reduction rate for Ag(0) forming in solution at a short time. The deposition became diffusion-controlled instead of the number of COOH on the surface of PSA-20 nanospheres. Some nuclei and silver nanoparticles were formed in the solution. Without enough PVP to stabilize the incipient colloidal Ag nanoparticles, the aggregation of Ag nanoparticles occurred at the 80 °C reduction condition. The formation of colloidal silver nanoparticles in the solution was identified by the yellow color of the supernatant obtained by centrifugation for 10 min at 14000 r/min (Figure S4 in the Supporting Information). The supernatant liquid samples obtained at 30, 40, and 50 °C were nearly colorless, and their UV vis spectra also hardly showed the corresponding absorption peak of the Ag-NPs plasmon resonance band. These experimental results indicated that the lower reduction temperature leads to slower reduction rate and a relatively small number of Ag nuclei and nanoparticles in the reaction mixtures. The Ag nanoparticles were most likely completely attached onto the polymer matrices of the PSA-20 nanospheres. SERS Property of PSA/Ag-NPs/PVP Nanospheres. Raman spectroscopy reveals the vibration hyperfine structures that are suitable for biological samples with the multiplexation advantages over most of other optical imaging and sensing methods. Recent advances in SERS allowed dramatic signal enhancement for analytes adsorbed on Au or Ag nanostructures.29 32 When fundamental understanding of the mechanism of the enhancement is emerging, the preparation of highly consistent SERS substrate with high enhancement factor remained an active research area. It has been observed that the aggregated Ag or Au particles give the high enhancement which is postulated to be the coupling of the plasmon resonance of the nearby particles with the strongest field enhancement between them.5,33,34 We thus chose the PSA-20/Ag-NPs/PVP composite nanospheres (Figure 3c) prepared at 50 °C as the model SERS substrates.

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Figure 3. TEM images of PSA-20/Ag-NPs/PVP composite nanospheres prepared at (a) 30 °C, (b) 40 °C, (c) 50 °C, and (d) 80 °C.

Figure 4. Raman spectra of (a) PSA with 10 3 mol/L 4-HBT and PSA20/Ag-NPs/PVP composite nanospheres with various 4-HBT concentrations of (b) 0 M, (c) 10 6 M, (d) 10 5 M, and (e) 10 4 M.

At first, their SERS activity was evaluated using 4-HBT as the reporting molecule. As shown in Figure 4, the SERS spectra of 4-HBT on PSA-20/Ag-NPs/PVP at varied concentrations are compared with the spectrum of 4-HBT on a PSA substrate without Ag-NPs. No Raman signals were observed at all for 10 3 mol/L 4-HBT in the absence of the Ag-NPs SERS substrates, as shown in Figure 4a. However, in the presence of Ag-NPs SERS substrates, Raman signals of 4-HBT appeared in SERS spectra at lower concentrations down to 10 6 M (Figure 4c e), with featured peaks at 629, 1073, 1167, and 1487 cm 1 of 4-HBT, which agreed well with the literature values.12 The results clearly indicated that the as-prepared PSA-20/Ag-NPs/PVP composite nanospheres are SERS-active substrates. It has to be noted that many other strong SERS bands appeared in the PSA-20/ Ag-NPs/PVP aqueous solution (Figure 4b). They should be assigned to the PVP signals according to the results of Xia et al.,35 which was also an indication that PVP was adsorbed on the surface of the Ag-NPs to stabilize the Ag colloidal solution. 14542

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slightly changed, and the density of Ag-NPs deposited on the PSA was not changed clearly. Then these PSA/Ag-NPs composite nanospheres were used as SERS substrates to detect melamine; the experimental results showed that the characteristic peak of melamine at 703 cm 1 (Figure 5B-b)40 is very clear even at the low level of 1  10 7 M. This result demonstrated that the as-prepared PSA/Ag-NPs composite nanospheres could serve as the SERS substrates to detect the targeting melamine molecules in solution. The PSA/Ag-NPs composite nanosphere based high performance SERS substrates will be of a great value for food safety and environment protection.

Figure 5. SERS spectra of melamine enhanced by (A) PSA-20/AgNPs/PVP at (a) 0 M, (b) 10 5 M, (c) 10 3 M; and by (B) PSA-20/AgNPs at (a) 0 M, (b) 10 7 M, (c) 10 6 M, (d) 10 5 M, and (e) 10 4 M.

Detecting Trace Melamine by SERS. Rapid detection and identification of poisonous substances in food or the environment is one of the most important tasks for SERS analysis. Melamine is an organic chemical material and is mainly used to produce melamine-formaldehyde resins for glues, adhesives, and plastics. However, due to its low cost and high nitrogen content (66% by mass), melamine has been illegally added to dairy products to boost the apparent protein contents. Although melamine is not inherently a toxic chemical, large-dose ingestion can result in kidney malfunction and even infant death. The safety limits of melamine in food are 2.5 ppm in the United States and 1 ppm for infant formula in China.36 Due to its ultrahigh sensitivity and “fingerprint-like” property, SERS exhibited a great advantage for the detection of melamine. Up to now, although a great deal of progress for detection of melamine has been achieved,13,37,38 the good SERS-active substrates usually used inorganic composite nanopsheres due to the synthesis procedure being based on the inclement hydrothermal process. In this paper, we developed a mild and facile route to prepare an organic/inorganic composite SERS-active substrate. We first used the as-prepared PSA-20/Ag-NPs/PVP as SERS substrates to detect melamine. The SERS spectra of melamine at 10 3 and 10 5 M are shown in Figure 5A. We can clearly observe the feature peak of 10 3 M melamine at 698 cm 1 (ring breathing II mode)39 (Figure 5A-c); unfortunately, the feature peak of melamine at 10 5 M is hardly visible (Figure 5A-b). The sensitivity for melamine is lower than that for 4-HBT; the possible reason may be due to the fact that the amino groups ( NH2) of melamine have a weaker affinity to the silver surface than the mercapto groups ( SH) of 4-HBT, then the melamine molecules are possibly disabled to tightly adsorb onto the silver surface stabilized by PVP. In order to overcome this drawback, we try to remove the adsorbed PVP on the surface of PSA-20/Ag-NPs/PVP composite nanospheres with THF. The Raman spectrum of PSA-20/Ag-NPs (Figure 5B-a) did not show any Raman signal of PVP, which indicated that the surface of PSA-20/Ag-NPs was clean and PVP was removed completely. The TEM image in Figure S5 in the Supporting Information shows that the morphology of these composite nanospheres was

’ CONCLUSION In summary, we demonstrated that the PSA/Ag-NPs/PVP composite nanospheres could be facilely prepared through the in situ reduction of silver nitrate by PVP in aqueous solution. The amount of the carboxyl group on the surfaces of PSA nanospheres and the reaction temperature were found to have major influences in controlling the size and coverage of Ag nanoparticles embedded in the PSA nanospheres. At the optimal preparation condition, the composite nanospheres showed excellent consistency and good activity in the SERS experiments and displayed broad scope in applications of microanalysis. The current work was focused on Ag-NPs deposition onto polymer substrates with carboxyl groups, and the synthetic approach described herein should also be adaptable to other metals and substrates with a variety of functional groups. It is expected that reduction of metal salts by the end groups of PVP may eventually provide an environmentally friendly and tunable route to well controlled synthesis of other kinds of metal-based composite materials. These experimental results also demonstrated that the as-prepared PSA/Ag-NPs composite nanospheres could serve as an excellent SERS substrate to detect melamine in solution. It will be of great value for food safety and environment protection. ’ ASSOCIATED CONTENT

bS

Supporting Information. Complementary TGA, XRD, UV vis, and TEM results of the composite nanospheres. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Telephone/Fax: +86-21-65640293. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by National Science Foundation of China (Grant Nos. 20974023, 21034003, and 51073040), National Science Foundation for Distinguished Young Scholars of China (Grant No. 50525310), and Shanghai Committee of Science and Technology, China (Grant No. 10XD1400500). ’ REFERENCES (1) Wen, F.; Zhang, W. Q.; Wei, G. W.; Wang, Y.; Zhang, J. Z.; Zhang, M. C.; Shi, L. Q. Chem. Mater. 2008, 20, 2144–2150. (2) Kim, J. H.; Bryan, W. W.; Lee, T. R. Langmuir 2008, 24, 11147– 11152. (3) Wang, W.; Asher, S. A. J. Am. Chem. Soc. 2001, 123, 12528– 12535. 14543

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Langmuir (4) Zhang, J. H.; Liu, J. B.; Wang, S. Z.; Zhan, P.; Wang, Z. L.; Min, N. B. Adv. Funct. Mater. 2004, 14, 1089–1096. (5) Piao, L. L.; Park, S.; Lee, H. B.; Kim, K.; Kim, J. W.; Chung, T. D. Anal. Chem. 2010, 82, 447–451. (6) Schueler, P. A.; Ives, J. T.; DeLaCroix, F.; Lacy, W. B.; Becker, P. A.; Li, J. M.; Caldwell, K. D.; Drake, B.; Harris, J. M. Anal. Chem. 1993, 65, 3177–3186. (7) Chen, C. W.; Serizawa, T.; Akashi, M. Langmuir 1999, 15, 7998–8006. (8) Pol, V. G.; Grisaru, H.; Gedanken, A. Langmuir 2005, 21, 3635– 3640. (9) Dokoutchaev, A.; James, J. T.; Koene, S. C.; Pathak, S.; Prakash, G. K. S.; Thompson, M. E. Chem. Mater. 1999, 11, 2389–2399. (10) Mayer, A. B. R.; Grebner, W.; Wannemacher, R. J. Phys. Chem. B 2000, 104, 7278–7285. (11) Kim, K.; Kim, H. S.; Park, H. K. Langmuir 2006, 22, 8083–8088. (12) Sanles-Sobrido, M.; Exner, W.; Rodríguez-Lorenzo, L.; CorreaDuarte, M. A.; Alvarez-Puebla, R. A.; Liz-Marzan, L. M. J. Am. Chem. Soc. 2009, 131, 2699–2705. (13) Hu, H. B.; Wang, Z. H.; Pan, L.; Zhao, S. P.; Zhu, S. Y. J. Phys. Chem. C 2010, 114, 7738–7742. (14) Schwartzberg, A. M.; Zhang, J. Z. J. Phys. Chem. C 2008, 112, 10323–10337. (15) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176–2179. (16) Kim, K. S.; Choi, S.; Cha, J. H.; Yeon, S. H.; Lee, H. J. Mater. Chem. 2006, 16, 1315–1317. (17) Lee, E. P.; Chen, J. Y.; Yin, Y. D.; Campbell, C. T.; Xia, Y. N. Adv. Mater. 2006, 18, 3271–3274. (18) Kan, C. X.; Cai, W. P.; Li, C. C.; Zhang, L. D. J. Mater. Res. 2005, 20, 320–324. (19) Xiong, Y. J.; Washio, I.; Chen, J. Y.; Cai, H. G.; Li, Z. Y.; Xia, Y. N. Langmuir 2006, 22, 8563–8570. (20) Yavuz, M. S.; Li, W. Y.; Xia, Y. N. Chem.—Eur. J. 2009, 15, 13181–13187. (21) Deng, Z. W.; Chen, M.; Wu, L. M. J. Phys. Chem. C 2007, 111, 11692–11698. (22) Cheng, X. J.; Tiong, S. C.; Zhao, Q.; Li, R. K. Y. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4547–4554. (23) Li, J. M.; Ma, W. F.; Wei, C.; Guo, J.; Wang, C. C. J. Mater. Chem. 2011, 21, 5992–5998. (24) Gong, T.; Wang, C. C. J. Mater. Sci. 2008, 43, 1926–1932. (25) Jin, L.; Liu, H. B.; Yang, W. L.; Wang, C. C.; Yu, K. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 2948–2959. (26) Shi, W. L.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610–1617. (27) Guo, G. Q.; Qin, F.; Yang, D.; Wang, C. C.; Xu, H. L.; Yang, S. Chem. Mater. 2008, 20, 2291–2297. (28) Tang, Y. C.; Yang, D.; Qin, F.; Hu, J. H.; Wang, C. C.; Xu, H. L. J. Solid State Chem. 2009, 182, 2279–2284. (29) Nie, S. M.; Zare, R. N. Annu. Rev. Biophys. Biomol. Struct. 1997, 26, 567–596. (30) Aroca, R. Surface-Enhanced Vibrational Spectroscopy; Wiley: 2006. (31) Kneipp, J.; Kneipp, H.; Kneipp, K. Chem. Soc. Rev. 2008, 37, 1052–1060. (32) Habouti, S.; Solterbeck, C. H.; Es-Souni, M. J. Mater.Chem. 2010, 20, 5215–5219. (33) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729–4735. (34) Yang, K. H.; Liu, Y. C.; Yu, C. C. J. Mater. Chem. 2008, 18, 4849–4855. (35) Zhang, Q.; Li, W. Y.; Moran, C.; Zeng, J.; Chen, J. Y.; Wen, L. P.; Xia, Y. N. J. Am. Chem. Soc. 2010, 132, 11372–11378. (36) Liang, X. S.; Wei, H. P.; Cui, Z. Q.; Deng, J. Y.; Zhang, Z. P.; You, X. Y.; Zhang, X. E. Analyst 2011, 136, 179–183. (37) Chen, L. M.; Luo, L. B.; Chen, Z. H.; Zhang, M. L.; Zapien, J. A.; Lee, C. S; Lee, S. T. J. Phys. Chem. C 2010, 114, 93–100. (38) Chen, L. M.; Liu, Y. N. ACS Appl. Mater. Interfaces 2011, 3, 3091–3096.

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(39) Zhang, X. F.; Zou, M. Q.; Qi, X. H.; Liu, F.; Zhu, X. H.; Zhao, B. H. J. Raman Spectrosc. 2010, 41, 1655–1660. (40) Wen, Z. Q.; Li, G. Y.; Ren, D. J. Appl. Spectrosc. 2011, 65, 514–521.

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