ARTICLE pubs.acs.org/Langmuir
Preparation of Silica-Encapsulated Hollow Gold Nanosphere Tags Using Layer-by-Layer Method for Multiplex Surface-Enhanced Raman Scattering Detection Jianshe Huang, Ki Hyung Kim, Namhyun Choi, Hyangah Chon, Sangyeop Lee, and Jaebum Choo* Department of Bionano Engineering, Hanyang University, Ansan 426-791, South Korea
bS Supporting Information ABSTRACT: The use of silica shells offers many advantages in surface-enhanced Raman scattering (SERS)-based biological sensing applications due to their optical transparency, remarkable stability in environmental media, and improved biocompatibility. Here, we report a novel layer-by-layer method for the preparation of silica-hollow gold nanosphere (HGN) SERS tags. Poly(acrylic acid) was used to stabilize Raman reportertagged HGNs prior to the adsorption of a coupling agent, after which a silica shell was deposited onto the particle surface using St€ober’s method. Importantly, competitive adsorption of the Raman reporter molecules and coupling agents, which results in unbalanced loading of reporter molecules on individual nanoparticles, was avoided using this method. As a result, the loading density of reporter molecules could be maximized. In addition, HGNs exhibited strong enhancement effects from the individual particles because of their ability to localize the surface electromagnetic fields through pinholes in the hollow particle structures. The proposed layer-by-layer silica-encapsulated HGN tags showed strong SERS signals as well as excellent multiplexing capabilities.
’ INTRODUCTION Surface-enhanced Raman scattering (SERS) is a promising technique for the simultaneous detection of multiplex target molecules. In contrast to the broad and relatively featureless spectra of fluorescence labels, SERS labels produce signals with very narrow bandwidth and fingerprint-like information for analytes, which give them a high multiplexing capacity. Other advantages of SERS include lack of photobleaching and self-quenching, as well as the use of a single excitation source for multiple species.13 In particular, SERS nano tags are playing an increasingly important role in bioassays and biomedical diagnostics, including for detection of DNA and RNA,46 proteins,3,79 viral pathogens,10 and cellular imaging.1116 Specific nanostructures that can provide a high level of electromagnetic field enhancement are a prerequisite in order to obtain stable spectroscopic signatures and sensitive detection.17 With the exception of the commonly used Ag and Au nanoparticles, various nanostructures have been used to prepare SERS tags, such as Au nanorods,18 nanoshells,1921 hollow gold nanosphere,2224 AuAg nanocages,25 and Au nanostars.26 Most of the SERS nano tags developed to date have taken advantage of the high sensitivity offered by SERS and the specificity offered by surface probing molecules. However, a common drawback of these approaches is that the tags are not isolated from the environment and thus are easily influenced by surrounding conditions. For instance, signals from cell components, in addition to SERS signals generated by reporter molecules, have been observed from unprotected SERS tags used for pH sensing in living r 2011 American Chemical Society
cells.27 Therefore, it is necessary to prepare SERS nanoparticle tags with protective shells, which can prevent desorption of Raman reporters and adsorption of external species. To this end, several approaches have been developed to prepare polymer- or silica-encapsulated nanoparticle SERS tags. Polymer-encapsulated nanoparticle SERS tags have been prepared by using thiolmodified polyethylene glycol as a stabilizer and protective shell.9,14 Likewise, metal nanoparticles developed from polystyrene-block-poly(acrylic acid) (PS154-b-PAA60), an amphiphilic diblock copolymer, through self-assembly have also been reported.28 However, due to its optical transparency, easy surface functionalization, and biocompatibility, silica has been considered a good coating material for the synthesis of SERS nano tags. Mulvaney and co-workers29 were the first to prepare “glassencapsulated” nanoparticles for use in multiplex bioassays. In this method, the silane coupling agent 3-aminopropyltrimethoxysilane (APTMS) was used as a primer to render a gold surface vitreophilic, and a Raman reporter was coadsorbed on the surfaces of Au nanoparticles. The subsequent growth of the silica shell was obtained via St€ober’s method, which synthesizes spherical silica particles based on the hydrolysis of alkyl silicates and the condensation of silicic acid in alcohol solution.30 In addition, Doering and Nie31 prepared SERS nano tags using a similar Received: May 10, 2011 Revised: June 25, 2011 Published: June 26, 2011 10228
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Langmuir coadsorption method, although they used 3-mercaptopropyltrimethoxysilane (MPTMS) as a coupling agent. Methoxy-poly(ethylene glycol)-thiol (mPEG-SH) can also be used as a coupling agent to transfer Au nanoparticles into ethanol, where the silica shell can be grown directly on the nanoparticle surfaces through the St€ober process. Using this approach, SERS-coded nanoparticles were prepared by adding Raman reporters to the PEGstabilized Au colloids prior to the growth of the silica shell.32 Several drawbacks have been identified among the aforementioned methods. First, the competitive adsorption between Raman reporter molecules and coupling agents limits the binding amounts of Raman reporter molecules, leading to decreased SERS signal intensity. Second, while the silica encapsulation method is effective for maintaining the stability of nanoparticles, it limits amplification of SERS signals due to reduced particle aggregation. Specifically, the SERS signal of reporter molecules adsorbed on a metal surface is greatly amplified by hot spots such as aggregation junctions or gaps between metal nanoparticles. In the case of silica-encapsulated SERS nanotags, however, the SERS enhancement effect is reduced because the silica coating restricts the degree of nanoparticle aggregation. Thus, the issues with both competitive adsorption and limited localized field enhancement by silica encapsulation must be solved to obtain a stronger SERS enhancement as well as excellent multiplexing capability. In the present work, we report a novel layer-by-layer method for preparation of silica-encapsulated hollow gold nanosphere (HGN) SERS tags. Here, HGNs were used as single nanoparticle SERS agents. These particles show strong enhancement effects from individual particles because of their ability to localize surface electromagnetic fields through the pinholes in the hollow particle structures.2224 As a result, they exhibit high sensitivity and reproducible SERS signals regardless of silica encapsulation of individual particles. In addition, the layer-by-layer silica encapsulation process avoids the competitive adsorption between Raman reporters and coupling agents. In this approach, the presence of a poly(acrylic acid) (PAA) layer eliminated their competitive adsorption, allowing for more Raman reporters to be adsorbed on the HGN surface. Consequently, SERS intensity is greatly increased compared to the coadsorption of Raman reporters together with silica precursors. The synthesis process is highly controllable, producing SERS tags that contain a single nanosphere each with a controllable silica shell thickness. This property guarantees reproducible SERS signals on a particle-toparticle basis. Additionally, a wide range of Raman reporters can be used in our method. Multiplexing capability was demonstrated by using 30 -diethylthiadicarbocyanine (DTDC), 4,40 dipyridyl (DP) and thiophenol (TP) as Raman reporters.
’ EXPERIMENTAL SECTION Materials. Cobalt chloride hexahydrate, gold(III) chloride trihydrate (>99.9%), sodium borohydride (99%), sodium citrate dihydrate (99%), poly(acrylic acid) (PAA, Mw ≈ 1800), 3-aminopropyltrimethoxysilane (APTMS) (97%), and tetraethyl orthosilicate (TEOS) (98%) were obtained from Sigma-Aldrich. Rhodamine B isothiocyanate (RBITC) and malachite green isothiocyanate (MGITC) were purchased from Invitrogen Corporation. Additional Raman reporter molecules, including rhodamine 6G (R6G), crystal violet (CV), 30 diethylthiadicarbocyanine iodine (DTDC), 4,40 -dipyridyl (DP), and thiophenol (TP) were obtained from Aldrich. All reagents were used as received without further purification. Ultrapure water (18.2 MΩ) was used in all experimental processes as needed.
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Preparation of Hollow Gold Nanospheres (HGNs). HGNs were prepared by the sacrificial galvanic replacement of cobalt nanoparticles.37 Briefly, 50 mL of water was placed into a three-neck flask with 500 μL of sodium citrate solution (0.1 M) and 100 μL of cobalt chloride solution (0.4 M). This mixture solution was deoxygenated with ultrapure N2 for one hour to ensure an air-free condition. Next, 150 μL of a freshly prepared sodium borohydride solution (1 M) was added during rapid magnetic stirring. The resulting solution was allowed to react for 45 min under constant N2 flow, after which the gold precursor solution (0.1 M) was added via 10 additions of 50 μL aliquots. Upon completion of gold addition, N2 flow was stopped and the solution was exposed to ambient conditions to oxidize any remaining cobalt metal. At the end of the reaction, the color of the solution changed to deep purple. Preparation of Silica-Encapsulated HGN SERS Tags. Silicaencapsulated HGN SERS tags were synthesized by a layer-by-layer method. In a typical procedure, 6 μL of Raman reporter (1.0 104 M) solution was mixed with 3 mL of HGN solution under rapid stirring, and the mixture was allowed to equilibrate for 20 min. Next, 3 mL of the PAA solution (4 g L1), which had been adjusted to pH 7.0 with 0.1 M NaOH, was added dropwise with vigorous stirring and stirred for 3 h. The solution of polymer-encapsulated nanospheres was centrifuged twice to remove excess polymer molecules and redispersed into water. A coupling agent, 3-aminopropyltrimethoxysilane, was added to this solution up to a final concentration of 3 107 M and it was equilibrated for 20 min. Then the silica shell was grown up using the modified St€ober’s method, namely by the hydrolysis and condensation of TEOS using ammonia as a catalyst in alcohol solution. In brief, 17 mL of 2-propanol and 200 μL ammonia (25%) was successively added under gentle stirring, followed by addition of 8 μL of TEOS in four portions over a time interval of 1 h. After completion of TEOS addition, the mixture was allowed to react for 12 h, after which the mixture was centrifuged at 4500 rpm for 15 min. Finally, the precipitated dye-coded Raman tags were redispersed into water. For comparison, silica-encapsulated HGN SERS tags were also prepared by the coadsorption method with MPTMS as a coupling agent.31 SERS Detection. SERS measurements were performed using a Renishaw 2000 Raman microscope system (Renishaw, UK). A 632.8 nm (18 mW) laser was employed for excitation. Raman scattering was observed using a CCD camera at a spectral resolution of 4 cm1. All spectra were calibrated referring to the 520 cm1 line of silicon. WiRE 1.2 software based on GRAMS 32 (Thermo Galactic) was employed to control the instrument and for data acquisition. A Cary 100 spectrophotometer (Varian, USA) was used to acquire UVvisible absorption spectra. High-magnification transmission electron micrographs were taken using a JEOL JEM 2100F instrument at an accelerating voltage of 200 kV.
’ RESULTS AND DISCUSSION Figure 1 shows a schematic diagram of the (a) coadsorption and (b) layer-by-layer methods for preparation of silica-encapsulated HGN SERS tags. The Raman dye was first adsorbed on the surface of HGNs through AuS bond or electrostatic interaction. However, the adsorption of Raman dye can induce the aggregation of nanoparticles. Therefore, a polymer, poly(acrylic acid) (PAA), was used to protect HGNs from aggregation and at the same time to bind APTMS subsequently for the silica-layer growth. The main difference is that the surface coverage of Raman dye could be maximized (Figure 1b) using our method but that of coadsorption method would be half of the maximum coverage due to the unfavorable competition between Raman dye and APTMS toward the HGN surface (Figure 1a). PAA can be deposited on the dye-functionalized HGNs via 10229
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Figure 1. Schematic illustration of the preparation of silica-encapsulated hollow gold nanosphere SERS tags: (a) coadsorption, (b) layer-by-layer Siencapsulated method.
hydrophobic interaction between Raman dyes and its aliphatic chains.33,34 Once PAA was deposited onto the HGN surface, APTMS can be adsorbed through electrostatic interaction between amine group and carboxylic group.35 Then a vitreophilic surface was obtained via the growth of silica layer. Because PAAprotected nanoparticles exhibit pH-dependent precipitation-redispersion behavior,36,37 the PAA solution was adjusted to pH 7.0 in our system. Under this condition, PAA-protected HGNs were well redispersed after centrifugation without any aggregation. HGNs were selected as SERS substrates because strong, homogeneous Raman scattering can be obtained from individual particles.2224 The other attractive property of HGNs is that the surface plasmon band can be tuned for a wide region (550 820 nm) by controlling particle size and wall thickness.38 Figure 2a shows a transmission electron microscopy (TEM) image of the as-prepared HGNs. The average diameter and wall thickness of HGNs were estimated to be 47 ( 7.5 nm and 12 ( 3.2 nm, respectively. Growth of the silica shell was achieved by St€ober’s method with minor modifications.30 Briefly, the formation of silica shell involves base-catalyzed hydrolysis of TEOS to generate silica sols in the mixture solution of H2O/ammonia/2propanol, which is followed by nucleation and condensation of silica sols on the HGN surface. Figure 2b presents TEM images of RBITC-coded SERS tags. The thickness of the silica shell was 24 ( 2.3 nm according to our measurements. These HGN SERS tags containing single metal particle are known to provide reproducible SERS signals in comparison with the nanoaggregate-based SERS tags.3942 In addition, the silica shell thickness is easily controlled by simply changing the amount of TEOS. For example, SERS tags with different silica shell thickness of 20 ( 2.3 nm and 15 ( 1.7 nm were easily obtained by changing the concentration of TEOS (Figure 2c,d).
Figure 2. TEM images of (a) hollow gold nanospheres and RBITCcoded SERS tags with different silica shell thickness, (b) 24 ( 2.3 nm, (c) 20 ( 2.3 nm, and (d) 15 ( 1.7 nm.
The effect of silica encapsulation on the absorption spectra of HGNs is shown in Figure S1 of the Supporting Information, SI. A 24 nm-thickness silica encapsulation gave rise to a 12 nm (from 559 to 571 nm) red shift in the surface plasmon band of HGNs. This effect arose from the change of refractive index of the 10230
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Figure 3. Comparison of SERS spectra of RBITC-coded tags prepared by (a) coadsorption and (b) layer-by-layer methods as indicated in Figure 1. The y scale of each SERS spectrum ranges from 0 to 18000 Raman counts.
surrounding medium, where silica has a higher refractive index (1.57) as compared to that of water (1.33).31 Additionally, we observed an increase in absorbance, which has been ascribed to the stronger light scattering arising from the larger particles after silica encapsulation.32 An additional advantage of this layer-by-layer method is that it produces higher SERS intensity compared with the coadsorption method. Indeed, the avoidance of competitive adsorption allows higher surface coverage of Raman reporters, resulting in higher sensitivity. In order to exhibit this property, we prepared silicaencapsulated HGN SERS tags using the coadsorption method for comparison purposes; the ratio of Raman reporter to coupling agent (MPTMS) was maintained at 3: 10 as previously reported.31 As shown in Figure 3, the SERS signal for this layerby-layer method was approximately 9 times more intense than that of SERS tags prepared by the coadsorption method. Because Raman reporters can induce aggregation of HGNs, it is believed that obtaining complete monolayer coverage of Raman reporter without serious aggregation is impossible. Even so, this layer-bylayer method was able to provide coverage for as large an area as possible as long as no obvious aggregation was formed. Using the coadsorption method, some of the Raman reporter molecules were displaced by the coupling agent or silica layer because the Raman reporter molecule and coupling agent are competitively adsorbed on the nanoparticle surface. Consequently, this resulted in a serious decrease of SERS intensity of reporter molecules. For example, when rhodamine 6G and crystal violet were used as Raman reporters, no signal was observed after silica encapsulation, although intense SERS signals were obtained from the mixture solution of dye and Au nanoparticles.31 For the silica-encapsulated SERS probes, thus, only covalent bonding molecules, such as isothiolcyanate (NdCdS) or molecules including SH groups, could be used as Raman reporters. In our method, however, the competition of Raman reporter and coupling agent was successfully eliminated by the presence of the PAA layer. As shown in Figure 4, not only strongly binding reporters (such as RBITC, MGITC, DTDC, and TP), but also weakly binding reporters (e.g., DP), exhibited strong SERS signals. Even after silica encapsulation, we also observed strong SERS signals from all reporter-coded SERS tags. Such HGN SERS tags have similar coreshell structures as shown in Figure 2b but show different spectroscopic signatures from Figure 4. Therefore, the range of available reporter molecules was greatly extended by
Figure 4. SERS spectra of RBITC, MGITC, DP, TP, and DTDC-coded Raman tags and the molecular structure of these Raman reporters. The y axis of each Raman tag is ranged from 0 to 6000 Raman counts. Not only strongly binding dyes (such as RBITC, MGITC, DTDC, and TP), but also weakly binding dyes (e.g., DP), could be used as Raman reporters in this layer-by-layer method.
using this layer-by-layer method, which should facilitate the application of SERS tags in multiplex biosensing or labeling. There are several drawbacks to the use of unencapsulated SERS tags, namely (1) uncontrollable aggregation of nanoparticles, (2) leakage of Raman reporters, and (3) adsorption of additional compounds on the nanoparticle surface. These drawbacks result in poor reproducibility and reliability of analytical data. The use of silica-encapsulated SERS tags, however, can effectively resolve these problems. Specifically, the presence of a silica shell confers remarkable stability to SERS tags in comparison with unencapsulated SERS tags. As shown in Figure S2 of the SI, the absorbance of unencapsulated HGNs immediately decreases to a low value after the addition of a 0.1 M NaCl solution, indicating rapid aggregation and precipitation of HGNs. However, the absorption spectra of silica-encapsulated SERS tags show little or no change upon the addition of NaCl solution, indicating that these SERS tags can tolerate a high concentration of salt medium without aggregation. No change in SERS intensity was observed between the samples in either water or salt solution. This characteristic is expected to allow for the use of these SERS tags in various buffer solutions encountered in biological applications. Furthermore, we assessed the stability of silica-encapsulated HGN SERS tags in three different organic solvents, including acetone, methanol, and ethanol. As shown in Figure S3 of the SI, the adsorption spectra obtained in these three organic solvents were consistent with those obtained in aqueous solution, demonstrating that silicaencapsulated SERS tags possess good and high stability even in organic solvents. An additional function of the silica shell is the isolation of the nanoparticle core from the exterior environment; therefore, interference from external compounds can be avoided. Figure S4 of the SI shows the SERS spectra of RBITC-coded SERS tags with a 24-nm-thick silica shell and a mixture of SERS tags and crystal 10231
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Figure 5. (a) SERS spectra of a ternary mixture of DP, TP, and DTDC-coded SERS tags with different ratios (TP: DTDC: DP), and (b) the plots of Raman intensity as the function of SERS tags concentration. Error bars indicate the standard deviation from three measurements.
violet (CV) or R6G. As shown in the figure, the SERS signals originated exclusively from RBITC, while no signals were observed from CV or R6G. This result shows that the presence of the silica shell prevents the Raman reporter from leaking while simultaneously preventing inappropriate uptake of external molecules. In the case of SERS tags with a 15-nm-thick silica shell, after addition of CV or R6G, SERS signals from the external dyes were observed, with the exception of RBITC. In a previous report, it was shown that the silica shells prepared by St€ober’s method are usually porous, and that the resulting pore size tends to increase with increasing ammonia concentration,43 which can allow small molecules and ions to penetrate and react with the core.44 However, in our study, the rate of reaction was very slow for thicker silica shells. Specifically, SERS tags with thicker silica shells (e.g., 24 and 20 nm) can effectively block the penetration of external dye molecules. Conversely, when the silica shell was thinner (15 nm), external dyes were able to penetrate the shell and adsorb onto the HGN surface. The reason for this observed disparity was ascribed to the inhomogeneous thinner shell, as well as the numerous defects present in the silica shell, both of which promoted penetration of dye molecules. One of the most significant advantages of Raman labels is the capacity for multiplex coding for rapid identification of multiple targets. Recently, Lutz et al.45 reported a spectral fitting method for analysis of Raman probe signatures in multiplex detection. Importantly, despite the presence of overlapping Raman peaks, they were able to obtain the entire spectral fingerprint of individual Raman reporters. However, total spectral information of Raman reporters is not necessary when SERS tags are used for multiplex detection. Indeed, data analysis based on single characteristic peaks is more straightforward than the spectral fitting method, and considering the wide range of Raman reporters available for use with our method, individual SERS tags should be easily distinguished based on their specific Raman fingerprints. Figure 5a shows the SERS spectra of a series of tertiary SERS tag mixtures at different ratios. According to the well-separated Raman bands, quantitative determination of these SERS tags in the mixtures was made easily without the need for complex data processing. Specifically, the Raman bands located at 1073, 1131, and 1617 cm1 originated from TP, DTDC, and DP, respectively, and were used to evaluate quantitative spectral multiplexing of the SERS tags. The Raman intensity of the individual SERS tags was proportional to the concentration without any interference from the copresent SERS tags (Figure 5b). These results indicate that the combination of these three SERS tags can be used for quantitative
multiplex detection. Considering that there are hundreds of Raman reporters compatible with the layer-by-layer encapsulation method described in this study, the degree of multiplexing can be further improved to higher levels.
’ CONCLUSIONS A novel method for preparation of silica-encapsulated SERS tags was developed based on a layer-by-layer process. The presence of a PAA layer allowed for the use of Raman reporters with either strong or weak binding groups, both of which allowed for a maximum surface coverage of Raman reporters. These features extended the range of available Raman reporters and improved the sensitivity. High-quality SERS tags with single HGN core were obtained, producing highly reproducible SERS signals. The silica shell imparted greatly improved chemical and optical stability upon the SERS tags, allowing for their application in various bioassays. This method can be applied to prepare SERS tags with Au or Ag nanoparticle cores of various sizes and shapes. Furthermore, the capability of multiplex detection was demonstrated through the use of a combination of three SERS tags. Combined with the availability of hundreds of Raman reporters and easy functionalization of silica shell, more SERS tags with different signatures could be fabricated and used as platforms for multiplex biosensing and labeling. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figure S1, optical absorption spectra of aqueous suspensions of unencapsulated HGNs (black line) and 24-nm-thick silica-encapsulated HGN SERS tags (red line). A 24 nm-thick silica encapsulation gave rise to 12 nm (from 559 to 571 nm) red shift in the surface plasmon band of HGNs; Figure S2, optical absorption spectra of (a) unencapsulated HGNs and (b) silica-encapsulated HGN SERS tags before (black line) and after (red line) addition of a 0.1 M NaCl solution; Figure S3, stability comparison of silicaencapsulated HGN SERS tags dispersed in water and different organic solvents; Figure S4, SERS spectra of RBITC-coded SERS tags with a 24-nm-thick silica shell a) before and after the addition of 1.0 μM of (b) crystal violet and (c) rhodamine 6G. This material is available free of charge via the Internet at http://pubs.acs.org. 10232
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’ AUTHOR INFORMATION Corresponding Author
*Phone: +82-31-400-5201; Fax: +82-31-436-8188; E-mail: jbchoo@ hanyang.ac.kr.
’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (Grant Nos. R11-2009-044-1002-0 and K20904000004-09A0500-00410), and the KRIBB Research Initiative Program. This work was also partially supported by the Ministry of Knowledge Economy (MKE) and Korea Industrial Technology Foundation (KOTEF) through the Human Resource Training Project for Strategic Technology. ’ REFERENCES (1) Shen, A.; Chen, L.; Xie, W.; Hu, J.; Zeng, A.; Richards, R.; Hu, J. Adv. Funct. Mat. 2010, 20, 969–975. (2) Wang, Y.; Seebald, J. L.; Szeto, D. P.; Daniel, P.; Irudayaraj, J. ACS. Nano 2010, 4, 4039–4053. (3) Kang, T.; Yoo, S. M.; Yoon, I.; Lee, S. Y.; Kim, B. Nano Lett. 2010, 10, 1189–1193. (4) Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536–1540. (5) Wabuyele, M. B.; Vo-Dinh, T. Anal. Chem. 2005, 77, 7810–7815. (6) Graham, D.; Thompson, D. G.; Smith, W. E.; Faulds, K. Nat. Nanotechnol. 2008, 3, 548–551. (7) Grubisha, D. S.; Lipert, R. J.; Park, H. Y.; Driskell, J.; Porter, M. D. Anal. Chem. 2003, 75, 5936–5943. (8) Cao, Y. C.; Jin, R. C.; Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. J. Am. Chem. Soc. 2003, 125, 14676–14677. (9) Lee, S.; Kim, S.; Choo, J.; Shin, S. Y.; Lee, Y. H.; Choi, H. Y.; Ha, S.; Kang, K.; Oh, C. H. Anal. Chem. 2007, 79, 916–922. (10) Driskell, T. D.; Kwarta, K. M.; Lipert, R. J.; Porter, M. D. Anal. Chem. 2005, 77, 6147–6154. (11) Kneipp, J.; Kneipp, H.; Rice, W. L.; Kneipp, K. Anal. Chem. 2005, 77, 2381–2385. (12) Kim, J. H.; Kim, J. S.; Choi, H.; Lee, S. M.; Jun, B. H.; Yu, K. N.; Kuk, E.; Kim, Y. K.; Jeong, D. H.; Cho, M. H.; Lee, Y. S. Anal. Chem. 2006, 78, 6967–6973. (13) Hu, Q.; Tay, L. L.; Noesheden, M.; Pezacki, J. P. J. Am. Chem. Soc. 2007, 129, 14–15. (14) Qian, X.; Peng, X. H.; Ansari, D. O.; Yin-Goen, Q.; Chen, G. Z.; Shin, D. M.; Yang, L.; Young, A. N.; Wang, M. D.; Nie, S. Nat. Biotechnol. 2008, 26, 83–90. (15) Keren, S.; Zavaleta, C.; Cheng, Z.; de la Zerda, A.; Gheysens, O.; Gambhir, S. S. Pro. Natl. Acad. Sci. U.S.A. 2008, 105, 5844–5849. (16) Zavaleta, C. L.; Smith, B. R.; Walton, I.; Doering, W.; Davis, G.; Shojaei, B.; Natan, M. J.; Gambhir, S. S. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 13511–13516. (17) Kneipp, J.; Kneipp, H.; Witting, B.; Kneipp, K. Nanomed.Nanotechnol. Biol. Med. 2010, 6, 214–226. (18) von Maltzahn, G.; Centrone, A.; Park, J. H.; Ramanathan, R.; Sailor, M. J.; Hatton, T. A.; Bhatia, S. N. Adv. Mater. 2009, 21, 3175–3180. (19) Bishnoi, S. W.; Rozell, C. J.; Levin, C. S.; Gheith, M. K.; Johnson, B. R.; Johnson, D. H.; Halas, N. Nano Lett. 2006, 6, 1687–1692. (20) Zhang, P.; Guo, Y. J. Am. Chem. Soc. 2009, 131, 3808–2809. (21) Ochsenk€uhn, M. A.; Jess, P. R. T.; Stoquert, H.; Dholakia, K.; Campbell, C. J. ACS Nano 2009, 3, 3613–3621. (22) Schwartzberg, A. M.; Oshlro, T. Y.; Zhang, J. Z.; Huser, T.; Talley, C. E. Anal. Chem. 2006, 78, 4732–4736. (23) Chon, H.; Lee, S.; Son, S. W.; Oh, C. H.; Choo, J. Anal. Chem. 2009, 81, 3029–3034. (24) Lee, S.; Chon, H.; Lee, M.; Choo, J.; Shin, S. Y.; Lee, Y. H.; Rhyu, I. J.; Son, S. W.; Oh, C. H. Biosens. Bioelectron. 2009, 24, 2260–2263.
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(25) Rycenga, M.; Wang, Z.; Gordon, E.; Cobley, C. M.; Schwartz, A. G.; Lo, C. S.; Xia, Y. Angew. Chem., Int. Ed. 2009, 48, 9924–9927. (26) Allgeyer, E. S.; Pongan, A.; Browne, M.; Mason, M. D. Nano Lett. 2009, 9, 3816–3819. (27) Wang, Z.; Bonoiu, A.; Samoc, M.; Cui, Y.; Prasad, P. N. Biosens. Bioelectron. 2008, 23, 886–891. (28) Yang, M.; Chen, T.; Lau, W. S.; Wang, Y.; Tang, Q.; Yang, Y.; Chen, H. Small 2009, 5, 198–202. (29) Mulvaney, S. P.; Musick, M. D.; Keating, C. D.; Natan, M. J. Langmuir 2003, 19, 4784–4790. (30) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62–69. (31) Doering, W. E.; Nie, S. Anal. Chem. 2003, 75, 6171–6176. lvarez-Puebla, R. A.; (32) Fernandez-Lopez, C.; Mateo-Mateo, C.; A Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M. Langmuir 2009, 25, 13894–13899. (33) Kim, K.; Lee, H. B.; Shin, K. S. Langmuir 2008, 24, 5893–5898. (34) Kim, K.; Lee, H. S.; Kim, N. H. Anal. Bioanal. Chem. 2007, 388, 81–88. (35) Niu, D.; Li, Y.; Qiao, X.; Li, L.; Zhao, W.; Chen, H.; Zhao, Q.; Ma, Z.; Shi, J. Chem. Commun. 2008, 44, 4463–4465. (36) Sehgal, A.; Lalatonne, Y.; Berret, J.-F.; Morvan, M. Langmuir 2005, 21, 9359–9364. (37) Naka, K.; Tanaka, H.; Chujo, Y. Polym. J. 2007, 39, 1122–1127. (38) Schwartzberg, A. M.; Olson, T. Y.; Talley, C. E.; Zhang, J. Z. J. Phys. Chem. B 2006, 110, 19935–19944. (39) Su, X.; Zhang, J.; Sun, L.; Koo, T.-W.; Chan, S.; Sundararajan, N.; Yamakawa, M.; Berlin, A. A. Nano Lett. 2005, 5, 49–54. (40) Huang, P. J.; Chau, L. K.; Yang, T. S.; Tay, L. L.; Lin, T. T. Adv. Funct. Mater. 2009, 19, 242–248. (41) Brown, L. O.; Doorn, S. K. Langmuir 2008, 24, 2178–2185. (42) Brown, L. O.; Doorn, S. K. Langmuir 2008, 24, 2277–2280. (43) Deng, Z.; Chen, M.; Zhou, S.; You, B.; Wu, L. Langmuir 2006, 22, 6403–6407. (44) Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740–3748. (45) Lutz, B. R.; Dentinger, C. E.; Nguyen, L. N.; Sun, L.; Zhang, J.; Allen, A. N.; Chan, S.; Knudsen, B. S. ACS Nano 2008, 2, 2306–2314.
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