Article pubs.acs.org/Langmuir
Plasmonic Ag Core−Satellite Nanostructures with a Tunable SilicaSpaced Nanogap for Surface-Enhanced Raman Scattering Zhen Rong,† Rui Xiao,† Chongwen Wang,†,‡ Donggen Wang,*,§ and Shengqi Wang*,† †
Beijing Key Laboratory of New Molecular Diagnosis Technologies for Infectious Diseases, Beijing Institute of Radiation Medicine, Beijing 100850, PR China ‡ College of Life Sciences & Bio-Engineering, Beijing University of Technology, Beijing 100124, PR China § Institute of Transfusion Medicine, Beijing 100850, PR China S Supporting Information *
ABSTRACT: Plasmonic Ag core−satellite nanostructures were synthesized by utilizing the ultrathin silica shell as a spacer to generate a tunable nanogap between the Ag core and satellites. To synthesize the nanoparticles, Ag nanoparticles (Ag NPs) with a diameter of ∼60 nm were synthesized as cores, on which Raman dyes were adsorbed and then tunable ultrathin silica shells from 2.0 to 6.5 nm were coated, followed by the deposition of Ag NPs as satellites onto the silica surface. The relationships between the SERS signal and the important parameters, including the satellite diameter and the nanogap distance, were studied by experimental methods and theoretical calculations. The maximum SERS intensity of the core−satellite nanoparticles was over 14.6 times stronger than that of the isolated Raman-encoded Ag/PATP@SiO2 NP. The theoretical calculations indicated that the local maximum calculated enhancement factor (EF) of the hot spots with a 2.0 nm nanogap was 9.5 × 105. The well-defined Ag core− satellite nanostructures have a high structural uniformity and an anomalously strong electromagnetic enhancement for highly quantitative SERS, leading to a better understanding of hot spot formation and providing new insights into the optimal design and synthesis of the hot SERS nanostructures in a controlled manner.
1. INTRODUCTION Surface-enhanced Raman scattering (SERS), a powerful vibrational spectroscopic technique, can provide nondestructive and ultrasensitive characterization of molecules on or near the surface of plasmonic nanostructures. Because of its singlemolecule-level sensitivity and finger-printing capability,1 SERS has been widely applied in surface science,2 food safety,3 and the detection of chemicals,4 proteins,5 bacteria,6 and even cells.7 SERS theory has been studied by many researchers to unveil the mechanism of the huge SERS enhancement. It is commonly thought that SERS enhancement comes from two mechanisms: a short-range chemical mechanism (CM) and a long-range electromagnetic mechanism (EM). CM is based on the charge transfer between the adsorbed molecules and the substrate, which requires the molecules to be close enough to the substrate. EM makes a major contribution to the SERS phenomenon, and it relies on the great enhancement of the local electromagnetic near field around the surface of metallic nanoparticles (such as gold and silver nanoparticles) caused by localized surface plasmon resonances (LSPR) under the excitation of light with suitable wavelengths. The electric field is uniformly distributed around spherical nanoparticles, while anisotropic nanoparticles with sharp tips, such as rods,8 cubes,9 and stars,4,10 can significantly magnify the near-field enhancements. Another efficient means to produce an enormous electric field enhancement is the plasmonic coupling effect at © XXXX American Chemical Society
the nanometer gap junction between strongly coupled nanoparticles. The hot spots generated at the nanometer gap junction induce a redistribution of the local field and an enormous electromagnetic EF large enough for single-molecule SERS detection.1,11 Various synthesis and fabrication methods generate nanostructured materials with hot spots via salt-induced aggregation,12,13 the self-assembly of nanoparticles,14,15 dimers, or multimers,16−18 and various lithographic methods.19 In particular, plasmonic core−satellite nanostructures, which can control the plasmon coupling by varying the core and satellite size and shape, the satellite number, and the interparticle distance, have become of significant interest in SERS.20−25 Conventionally, core−satellite nanostructures have been achieved through the controlled assembly of Au/Ag nanoparticles via DNA hybridization,24,26 covalent forces,27,28 and electrostatic attraction.20 However, DNA sequences require a certain minimal number of base pairs for hybridization, thus the interparticle distances are limited by the relatively long DNA double strands. When molecular cross-linkers such as 1,10decanedithiol and p-aminothiophenol are used to arrange plasmonic nanostructures, the interparticle distances are Received: May 9, 2015 Revised: June 30, 2015
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DOI: 10.1021/acs.langmuir.5b01713 Langmuir XXXX, XXX, XXX−XXX
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min, and then the pH was increased to 11.5 by adding 0.5 M NaOH. Following this, the mixture was placed in a 100 °C oil bath and stirred for 90 min and then kept undisturbed at room temperature overnight to generate a thin, dense SiO2 layer. Eventually, the resulting mixture was centrifuged at 7000 rpm for 10 min and then washed with water three times to remove the suspension, followed by being redispersed in 50 mL of anhydrous ethanol. 2.4. Deposition of Ag NPs on Ag@SiO2 NPs. Recently, Kim et al. reported a procedure that involved the use of butylamine as the reductant to deposit Ag NPs onto the silica surface.34 Here, we employed the combination of a butylamine reduction system and sonochemical methods in the presence of PVP as a protective agent to explore the deposition of Ag NPs onto the Ag/PATP@SiO2 NPs. Before Ag deposition, the Ag/PATP@SiO2 NPs solution was centrifuged and washed thoroughly with anhydrous ethanol three times. In a typical sonochemical deposition, 1 mL of the Ag/PATP@ SiO2 NPs ethanol solution was mixed with 4 mM of AgNO3 in a polypropylene tube. The mixture was mechanically stirred at 30 °C for 30 min. Then, 0.1 mg/mL of PVP and 4 mM of butylamine were added to the solution. Subsequently, sonication of the mixture with ultrasonic irradiation was carried out by immersing the tube in an ultrasonic cleaning bath (40 kHz, 150 W) at 30 °C for 1 h. After ultrasonic irradiation, the resulting mixture was centrifuged at 7000 rpm for 10 min and washed with water, followed by being redispersed in 1 mL of water under sonication for 5 min. It should be noted that the concentrations of AgNO3 and butylamine were kept the same35 and that the concentration of PVP was 0.1 mg/mL in all experiments. 2.5. Instruments. Transmission electron microscopy (TEM) images were taken using a Hitachi H-7650 TEM with an acceleration voltage of 80 kV and a Hitachi H-9000 TEM with an acceleration voltage of 300 kV. The specimen was prepared by dropping the sample onto a carbon-coated copper grid, followed by drying at room temperature. The X-ray diffraction (XRD) patterns were obtained on a D8 Advance (Bruker, Germany) diffractometer with Cu Kα radiation at λ = 0.154 nm operating at 40 kV and 40 mA. UV−vis spectra were measured with a Shimadzu 2600 spectrometer. Samples were placed in a quartz cell of 1 cm optical path after dilution to 5% in Milli-Q water (v/v). The zeta potential measurements were conducted with a Nano ZS Zetasizer (model ZEN3600, Malvern Instruments) using a He−Ne laser at a wavelength of 632.8 nm. Raman spectra were collected from sample aqueous solutions in a glass cuvette on a B&W Tek, i-Raman Plus BWS465-785H portable Raman system spectrometer equipped with a back-illuminated CCD detector cooled to −2 °C. Samples were excited by using the 785 nm laser with a power of 25 mW. SERS spectra were obtained with a total acquisition of 10 s for each SERS spectrum.
difficult to define and vary. Besides, such methods suffer from low yield and instability due to the sensitivity of molecular cross-linkers to the pH and ionic strength of the solution. Therefore, despite these efforts, the control of well-defined, stable, and reproducible interparticle distances in core−satellite nanostructures remains challenging. Herein, for the first time to our knowledge, we demonstrated a novel synthesis method based on the in situ deposition of Ag satellites on tunable silica spacers to prepare plasmonic Ag core−satellite nanostructures with well-defined interparticle distances. The as-synthesized core−satellite nanostructures have Raman-dye-adsorbed Ag NPs cores, silica-spaced nanogaps, and in situ sonochemically deposited Ag NPs satellites, forming the technological basis of our strategy. Silica layers of 2.0−6.5 nm (or thicker) on Ag NPs can be precisely adjusted by carefully controlling parameters such as the concentration, reaction time, temperature, and pH; consequently, we are able to control the nanogap distances over a broad range to explore the relationships between the SERS enhancement factor and the nanogap distances. In the present study, the plasmonic core−satellite nanostructures with different satellite diameters and nanogap distances have been synthesized and characterized. The SERS intensity and the electromagnetic field distribution have been investigated with experiments and threedimensional finite-difference time domain (3D-FDTD) simulation. These studies were expected to unravel the important parameters for obtaining highly amplified and quantifiable SERS signals and to give insights into the design and synthesis of the SERS nanostructures in a controlled manner. These nanostructures, in the future, could be useful in various applications such as SERS tags for biological and medical imaging,29 catalysis,30 and plasmonic rulers.22
2. EXPERIMENTAL DETAILS 2.1. Chemicals. Sodium silicate stock solution (26.5% SiO2 in 10.6% Na2O), (3-aminopropyl)triethoxysilane (APTES), polyvinylpyrrolidone (PVP-40, average molecular weight of 40 kg/mol), and paminothiophenol (PATP) were purchased from Sigma-Aldrich. Other chemicals, unless specified, were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were analytical reagent grade and were used without further purification. Ultrapure water purified with a Millipore Milli-Q system (18.2 MΩ·cm−1) was used to prepare the aqueous solutions. All glassware was cleaned with aqua regia, followed by rinsing with water and drying. 2.2. Synthesis of Ag NPs. Ag NPs with an average diameter of 60 nm were synthesized according to the classical Lee and Meisel method.31 In a three-necked flask, 36 mg of silver nitrate was dissolved in 200 mL of water, and the solution was heated to boiling with magnetic stirring. Then, 4 mL of fresh 1% (w/v) sodium citrate was quickly added, and the solution was kept boiling for 1 h with continuous stirring. The color of the solution turned yellow and then greyish, indicating the formation of Ag NPs. Then, the solution was cooled and allowed to age overnight before further use. 2.3. Synthesis of Reporter-Molecule-Embedded Ag@SiO2 NPs. The reporter-molecule-embedded Ag@SiO2 NPs were synthesized according to the procedure described by Liz-Marzan et al. and slightly modified by Tian et al.32,33 Here, PATP was utilized as a model Raman molecule. First, 50 μL of PATP in ethanol (1 mM) was added dropwise to 50 mL of the Ag colloidal solution under rapid stirring for 30 min. For coating PATP-adsorbed Ag NPs with amorphous silica shells, APTES was utilized as a coupling agent to render the Ag NP surfaces vitreophilic for the coating of a complete silica shell. The pH of the mixture was decreased to 5 by adding 0.1 M H2SO4 before the dropwise addition of 500 μL of a 1 mM APTES aqueous solution and constantly stirred for an additional 15 min. Subsequently, 5 mL of fresh prepared sodium silicate (0.54 wt %) was added and stirred for 3
3. RESULTS 3.1. Characterization of the Ag Core−Satellite Nanostructures. Scheme 1 shows the major steps involved in the sonochemical synthesis of reporter-molecule-embedded Ag core−satellite nanostructures and the SERS measurements protocol. A typical synthesis process starts with the preparation of Ag NPs by the reduction of AgNO3 with sodium citrate acid under boiling conditions.31 Then, PATP is adsorbed onto the Ag NPs surfaces as Raman reporter molecules. A modified method based on that of Liz-Marzan et al. is employed to coat the PATP-adsorbed Ag NPs with a thin, dense SiO2 layer as a substrate for the sonochemical deposition of Ag satellites.32 The resulting Ag/PATP@SiO2 core−shell NPs are then dispersed in ethanol solution and mixed with AgNO3, leading to the adsorption of Ag+ ions on the SiO2 shell surfaces via an electrostatic attraction between the Ag+ ions and negatively charged surface OH− groups. The Ag satellites are achieved by the in situ reduction of Ag+ by butylamine with ultrasonic irradiation in the presence of PVP as a protective agent. Then, B
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surface coverage of Ag satellites on the SiO2 surface can be tuned by controlling the amount of AgNO3 and butylamine. When 0.5, 1, 2, 3, and 4 mM AgNO3 and butylamine are added to the Ag/PATP@SiO2 ethanol solution, Ag satellites with average diameters of 8, 10, 12, 16, and 22 nm are deposited on the SiO2 surfaces, respectively. Unfortunately, there is a slight difficulty in accurately counting the number of deposited satellites per core owing to the invisible satellites behind the core in the TEM images. The number appears to remain ca. 25 with the increase in AgNO3 concentration. This can be explained by the nucleation synthesis process. We assume that the number of satellites per core depends on the nucleation sites on the SiO2 surface and that these five concentrations of AgNO3 in the experiment are all adequate for the nucleation process, resulting in an invariable number of satellites per core. To verify the deposition of Ag satellites, Figure 2 shows the wide-angle XRD patterns of Ag NPs, Ag/PATP@SiO2 NPs, and Ag core−satellite nanostructures (synthesized with 4 mM AgNO3). As shown in Figure 2a, the specific XRD of Ag is characterized by five distinct peaks positioned at 2θ values of 38.2, 44.4, 64.6, 77.6, and 81.7°, corresponding to the [111], [200], [220], [311], and [222] crystalline planes of cubic Ag, respectively. The intensities of these five peaks of Ag/PATP@ SiO2 in Figure 2b are significantly weakened by the amorphous silica layer.43 It is found that the peaks of Ag core−satellite nanostructures in Figure 2c become much stronger, indicating the deposition of a large number of Ag NPs on the surface of Ag/PATP@SiO2. These XRD results well agree with that from TEM observations. It is known that the formation of a coupled plasmonic nanoparticle pair produces two notable optical signatures: (1) a strong red shift of the plasmon resonance seen in its UV−vis spectrum and (2) a greatly enhanced field in the interparticle junction formed when the interparticle gap distance decreases.44 The synthesized nanostructures were characterized via UV−visible spectroscopy, as shown in Figure 3. The position of the characteristic plasmon peak of Ag NPs is at ∼408 nm (Figure 3a). The coating of the ultrathin SiO2 shell induces a slight red shift of the plasmon band by ca. 3 nm (Figure 3b), which has already been observed by other groups.45 Figure 3c−g shows the UV−vis spectra of the Ag core−satellite nanostructures prepared with different concentrations of AgNO3. The inset of Figure 3 shows photographs of sample solutions corresponding to curves a−g. As the concentration of AgNO3 increases, the formation of much larger Ag satellites and higher coverage on Ag/PATP@SiO2 lead to a broadening and red shift of the plasmon resonance in accordance with the visible color change from yellow to dark brown, indicating the coupling of the surface plasmons between adjacent Ag satellites as the interparticle distance decreases and between the Ag core and satellites as the Ag satellites grow larger. It is worth mentioning that the colors of sample solutions corresponding to curves c−g also illustrate that the Ag core−satellite nanostructures are well dispersed in water. 3.2. Raman Activity of Ag Core−Satellite Nanostructures. To further investigate the Raman activity of Ag core− satellite nanostructures, the synthesized nanostructure aqueous solutions were added to a glass cuvette for Raman measurement. The Ag/PATP@SiO2 NPs with a 2.0 nm silica shell aqueous solution were used as a reference. As shown in Figure 4a, slight SERS peaks are observed for the Ag/PATP@SiO2 NPs due to the SERS response of Ag. In contrast, the Raman intensity of PATP, located in the junction of core−satellite
Scheme 1. Schematic Illustration of Major Steps Involved in the Synthesis of the Ag Core−Satellite Nanostructures for SERS
the Ag core−satellite nanostructures solutions are transferred to a glass cuvette to examine the SERS activity. It should be noted that we utilize PVP as a stabilizer to prohibit particle aggregation in this study. The optimum concentration of PVP is fixed at 0.1 mg/mL. We found that the supernatant obtained at a higher concentration of 0.2 mg/mL was yellow, indicating that Ag NPs were formed in the solution because PVP can also be a mild reducing agent for promoting silver nucleation in the solution.36,37 Meanwhile, the ultrasound-driven synthesis, which has been proven to be a useful technique for nanomaterial preparation, also plays an important role in Ag NP deposition.38−40 The chemical effect of ultrasound arises from acoustic cavitation, during which bubble collapse produces intense local heating, high pressures, and very short lifetimes, leading to transient temperatures of about 5000 K, a pressure of 1800 atm, and cooling rates in excess of 108 K/ s.41 The local heating can promote Ag satellite deposition onto the SiO2 surface. The ultrasonic irradiation also improves the dispersion of the NPs. Moreover, the microjets and shock waves created in the inner environment of the ultrasonic solution can push the Ag satellites toward the surface of the SiO2 shell at a high speed and adhere the Ag satellites to SiO2, which results in decreasing the nanogap distance between the Ag core and the satellites.36 Various noble metal nanoparticles can amplify the Raman signal of analytes due to the local optical fields at metal surfaces, in which silver is an efficient Raman-signal-enhancing material because the SERS enhancements of silver nanostructures are 10 to 100 times higher than those of similar gold nanostructures.42 A TEM image of the prepared spherical Ag NPs with an average diameter of 61 ± 8 nm is provided in Figure 1a. As shown in Figure 1b−d, the ultrathin SiO2 shells with a thickness of ∼6.5, 3.7, or 2.0 nm are seen as a bright region surrounding the Ag NPs. The SiO2 shell is compact and pinhole-free as described by Tian et al.2,33 The zeta potential of the Ag/ PATP@SiO2 NPs in ethanol is −35.6 ± 8.5 mV, suggesting that the surface hydroxyl groups are deprotonated in absolute ethanol. The negatively charged surfaces can adsorb Ag+ as nucleation sites for further Ag satellite deposition. Figure 1e−i shows the Ag core−satellite nanostructures synthesized with different concentrations of AgNO3. The particle size and C
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Figure 1. TEM images of (a) AgNPs with an average diameter of 61 ± 8 nm, Ag/PATP@SiO2 NPs with different shell thicknesses of (b) 6.5 nm, (c) 3.7 nm, and (d) 2.0 nm, and Ag core−satellite nanostructures synthesized with a 2.0 nm SiO2 spacer and different concentrations of AgNO3: (e) 0.5, (f) 1, (g) 2, (h) 3, and (i) 4 mM.
Figure 3. UV−vis spectra of (a) AgNPs and (b) Ag/PATP@SiO2 NPs and Ag core−satellite nanostructures synthesized with different concentrations of AgNO3: (c) 0.5, (d) 1, (e) 2, (f) 3, and (g) 4 mM. Inset: photographs of the above solutions arrayed from left to right corresponding to curves (a)−(g).
Figure 2. X-ray diffraction patterns of (a) AgNPs, (b) Ag/PATP@ SiO2 NPs, and (c) Ag core−satellite nanostructures. Curves are shifted vertically for clarity.
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respectively. This increasing enhancement could be attributed to the increasing size of Ag satellites as the AgNO 3 concentration increases from 0.5 to 4 mM. This experimental result indicates that the larger the size of the Ag satellites, the higher the SERS activity achieved. It is known that the electromagnetic field enhancements of metallic nanoparticles are highly correlated with the interparticle gap distance, in turn determining the SERS enhancement ability.2,47−49 Therefore, tuning a uniform and welldefined sub-10-nm plasmonic interparticle nanogap is the key to obtaining controllable plasmonic nanogap structures for the strong and quantifiable SERS signals. In this article, the gap distance is well defined by the ultrathin, highly uniform silica thickness. The silica thickness is easily controlled by adjusting the amount of precursors and the reaction time, which means that the gap distance can be well-defined and varied essentially at will. During the synthesis of the Ag core−satellite nanostructures, three silica gap distances of 2.0, 3.7, and 6.5 nm were formed to test the SERS signal dependence on the nanogap distance between the Ag core and satellites junctions. Figure 5a shows the comparison of SERS intensity of PATP-
Figure 4. (a) SERS spectra of PATP-embedded Ag core−satellite nanostructures synthesized with different AgNO3 concentrations. Curves are shifted vertically for clarity. (b) Relative SERS enhancement factor as the ratio between the intensity of Ag core−satellite nanostructures with different AgNO3 concentrations and the reference signal of Ag/PATP@SiO2 at the four selected Raman peaks.
particles, is significantly enhanced after the deposition of Ag satellites. To investigate the contribution of the enhancement from the local plasmon coupling at the hot spots between the Ag core and satellites, the SERS intensities of PATP at the four selected obvious Raman peaks, including the bands at ca. 1073, 1143, 1392, and 1440 cm−1, are shown in Figure 4b. Note that the observed characteristic SERS spectra of PATP are a major contribution from 4,4′-dimercaptoazobenzene (DMAB), an oxidative coupling product of PATP generated on the nanostructured metallic surfaces.46 The main bands in the SERS spectra of PATP on Ag surfaces include the bands at ca. 1077 and 1190 cm−1. On the other hand, the main bands in the SERS spectra of DMAB on Ag surfaces include the bands at ca. 1142, 1391, 1440, and 1573 cm−1. It is observed experimentally that the intensity of the characteristic SERS peaks is dependent on the size of Ag satellites, which is controlled by the AgNO3 concentration. The relative SERS enhancement is calculated as the ratio between the intensity of Ag core−satellite nanostructures obtained from different concentrations of AgNO3 and the reference signal at the four selected obvious Raman peaks. As shown in Figure 4b, the relative SERS enhancements of these four selected Raman peaks gradually increase with increasing AgNO3 concentration ranging from 0.5 to 4 mM, with maximum enhancements of ca. 9.7, 14.6, 12.2, and 13.6 times the peaks at 1073, 1143, 1392, and 1440 cm−1,
Figure 5. (a) Comparison of SERS intensity of PATP-embedded Ag core−satellite nanostructures with different gap distances. All of the core−satellite nanostructures were synthesized with the same AgNO3 concentration of 4 mM. Curves are shifted vertically for clarity. (b) Relative SERS enhancement factor as the ratio between the intensity of Ag core−satellite nanostructures with different gap distances and the reference signal of Ag/PATP@SiO2 at the four selected Raman peaks. E
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Figure 6. FDTD simulations showing the electric field distribution of the Ag core−satellite nanostructures with a 2.0 nm gap and different satellite diameters of (a) 8, (b) 12, (c) 16 and (d) 22 nm and nanostructures with 22 nm satellites and different gap distances of (e) 3.7 and (f) 6.5 nm. (g) Comparison of the line electrical field distribution profile along the center-horizontal line corresponding to (a−d). (h) Comparison of the line electrical field distribution profile along the center-horizontal line corresponding to (d−f). In this calculation, the gap is filled with SiO2, and the area surrounding the particle is filled with water. The incident light had a wavelength of 785 nm.
demonstrate that the electromagnetic field enhancement is inversely related to the interparticle gap distance, which is in good agreement with other reported results.2,48 3.3. Theoretical Calculation of Electromagnetic Interactions. To better understand the interactions of an electromagnetic wave with the Ag core−satellite nanostructures with different Ag satellite sizes and gap distances, 3D-FDTD simulations were performed using commercially obtained Lumerical FDTD software to visualize the EM field distribution, and the results were compared to those for Ag@ SiO2 core−shell NPs. The data of the dimensions of the nanoparticles were taken from the TEM images. The Ag core diameter was 60 nm, the silica spacer was 2.0, 3.7, or 6.5 nm, and the Ag satellite diameter was from 8 to 22 nm. Considering
embedded Ag core−satellite nanostructures synthesized with different gap distances. The intensity of the PATP is found to increase for decreasing interparticle gap distances. As shown in Figure 5b, the relative SERS enhancements of these four selected Raman peaks at 1073, 1143, 1392, and 1440 cm−1 for the 6.5 nm gap are ca. 2.1, 2.9, 2.4 and 2.7, respectively, which indicates rather small enhancements. The relative SERS enhancements of these four selected Raman peaks for the 3.7 nm gap are ca. 5.8, 8.8, 7.3 and 8.0, respectively. Furthermore, the relative SERS enhancements of these four selected Raman peaks for the 2.0 nm gap, as mentioned above, are ca. 9.7, 14.6, 12.2 and 13.6, respectively. The maximum relative SERS enhancements are similar to previous results of core−satellite nanostructures.27,50 As expected, the experimental results F
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calculations show that the field enhancement depends on the satellite size and gap distance, which is consistent with the experimental results. It may be possible to tune the SERS signal by the controllable introduction of a silica spacer and Ag satellites.
the geometry symmetry, we assumed the number of satellites per core to be 26 to simplify our model (Figure S1 in the Supporting Information). A linearly polarized plane wave at 785 nm was normally incident to the nanoparticles in the Z direction with the X-direction polarization. A background dielectric constant of 1.33 was used to replicate the liquid phase. A mesh size of 1 nm was used in all simulations. The boundary conditions were set as perfectly matched layers. As shown in Figure 6, the electric field distributions suggest that the electromagnetic enhancement is highly localized in the gap junction of the Ag core−satellite nanostructures where the Raman dye PATP would be located in the experiment. Importantly, the electric field distributions also reveal that the SERS signals taken from the hot spots are polarizationdependent. The hottest regions of core−satellite structures are found to be the gap sites along the polarization direction. The minimum electromagnetic enhancement occurs in the orthogonal direction of the incident polarization. This result is consistent with previous work.17,20,51 Owing to the hot spot regions between the Ag core and satellites in all directions, the Ag core−satellite nanostructures exhibit a large SERS enhancement of the reporter molecules located in the gap sites without adjusting the incident polarization direction. Moreover, it was found that the hottest SERS-active sites significantly determine the EF rather than other sites even though the hottest sites account for only a few sites in the total number of sites.52 Consequently, the narrowly distributed high EF values and the highly reproducible SERS enhancements can be obtained in the nanostructures. The satellite size and gap distance, two structural parameters defining this nanostructure geometry, were investigated by 3DFDTD simulation. We first focus on the effect of satellite size on the electric field distribution. As shown in Figure 6a−d,g, a larger Ag satellite leads to a progressively larger magnitude of the maximally enhanced electric field as the value of |Emax|/|Ein|, which could be attributed to a stronger plasmonic coupling effect. Moreover, Figure 6d−f,h shows the effect of gap distance on the electric field distribution. As expected, the 3D-FDTD simulation results demonstrate that a narrower gap distance generates a very high electromagnetic enhancement within the gap between the core and satellites. The maximum electric field enhancement is ∼31.2 times higher than that of the incident light in the gap region of the Ag core−satellite nanostructures with a 2.0 nm gap and 22 nm satellites. Thus, the local maximum calculated EF as the value of |Emax|4/|Ein|4 is ∼9.5 × 105. As a comparison, the local maximum calculated EF of Ag@ SiO2 NPs is only 5.5 × 102 (Figure S2 in the Supporting Information). To estimate the number of Raman dyes trapped in the hottest spot region of the core−satellite nanostructure, we assumed the hottest spot region to be a cap on the surface of the Ag core in the interparticle gap junction (Figure S3 in the Supporting Information). The radius of the hottest spot region is about 3 nm (red region in Figure 6d). The calculated results show that the number of molecules trapped in each hottest spot region is 142, and the total hottest spot region coverage is 6.5%. In this calculation, the PATP molecules are assumed to be adsorbed as a monolayer with a 0.2 nm2 molecular footprint onto the Ag core surface. As shown in Figure 6, the 3D-FDTD simulation results indicate that an increasing satellite size leads to a larger cap area, more dye molecules trapped in the hot spot regions, and a stronger field enhancement, thus producing a stronger SERS signal. Taken together, the theoretical
4. CONCLUSIONS We have demonstrated the design and synthesis of gap-tunable Raman-active plasmonic Ag core−satellite nanostructures. Raman-dye-adsorbed Ag cores can be easily coated with an ultrathin silica shell from 2.0 to 6.5 nm tuned by synthesis parameters, followed by the deposition of high-density Ag nanoparticles as satellites to generate highly quantitative and controllable hot spots for SERS. The SERS enhancement is directly related to the Ag satellite size and nanogap distance. It is found that the SERS signal of the core−satellite nanostructures can be up to ∼14.6 times higher than that of Ag/PATP@SiO2 NPs alone, with the largest Ag satellite size of 22 nm and the shortest nanogap distance of 2.0 nm, of which the local maximum calculated EF is ∼9.5 × 105. Through the appropriate introduction of Ag satellites and silica spacers, one may be able to control the field enhancement, the hot spot region coverage, and the SERS signal. Note that the SERS signals could be further improved by controlling the satellite deposition and using smaller gap distances (i.e.,
