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Improve the surface-enhanced Raman scattering from rhodamine 6G adsorbed gold nanostars with vimineous branches
Accepted Manuscript Title: Improve the surface-enhanced Raman scattering from rhodamine 6G adsorbed gold nanostars with vimineous branches Author: Jian Zhu Jie Gao Jian-Jun Li Jun-Wu Zhao PII: DOI: Reference:
S0169-4332(14)02335-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.095 APSUSC 28957
To appear in:
APSUSC
Received date: Revised date: Accepted date:
16-8-2014 14-10-2014 16-10-2014
Please cite this article as: J. Zhu, J. Gao, J.-J. Li, J.-W. Zhao, Improve the surface-enhanced Raman scattering from rhodamine 6G adsorbed gold nanostars with vimineous branches, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript. The changes are marked with underline.
Improve the surface-enhanced Raman scattering from rhodamine 6G
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adsorbed gold nanostars with vimineous branches
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Jian Zhu*, Jie Gao, Jian-Jun Li, Jun-Wu Zhao*
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The Key Laboratory of Biomedical Information Engineering of Ministry of Education,
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School of Life Science and Technology, Xi’an Jiaotong University,
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Xi’an 710049, China
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Telephone: 86-29-82664224
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* Corresponding author
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Fax numbers: 86-29-82664224
Email: [email protected]; [email protected] Address: School of Life Science and Technology, Xi’an Jiaotong University, Xi’an, 710049,
Peoples Republic of China
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Abstract:
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The surface-enhanced Raman scattering (SERS) activity of the gold nanostars with vimineous branches has been investigated by using rhodamine 6G (R6G) as the Raman active probe. The colloidal gold nanostars have two
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intense localized surface plasmon resonance (LSPR) peaks in the visible and infrared ranges respectively. Besides the visible LSPR dependent local field effect induced Raman signal enhancement, the SERS ability also greatly
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depends on the infrared absorption from the plasmon resonance along the aligned branches. Whether increasing the peak intensity or wavelength of the infrared absorption leads to the efficient improvement of SERS. These
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correlations between plasmonic absorption and SERS indicate that the lightning rod effect and creation of hot
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spots have been enhanced with the length and number of gold branches.
Keywords:
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absorption spectrum
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gold nanostars; surface-enhanced Raman scattering (SERS); localized surface plasmon resonance (LSPR);
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1. Introduction Surface-enhanced Raman scattering (SERS) of molecules attached on the surface of metallic nanostructures render them appropriate candidates for applications in chemical and biological sensing. Because of the coherent oscillation of conduction band electrons, gold and silver nanoparticles exhibit a localized surface plasmon
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resonance (LSPR) in the visible and infrared regions. The LSPR can also generates an intense local electric field, and subsequently enhance the Raman scattering signal of nearby molecules[1,2]. Therefore, SERS becomes a
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useful technique in sensing and analysis, which could be used in single-molecule detection and molecular structure investigation[3]. For example, Guo et al. reported an ultra-sensitive SERS-based detection for
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trinitrotoluene by using the gold nano-dumbbell structures[4]. Detection of α-fetoprotein (AFP) by using SERS-based immunoassay has been studied by Wang et al.[5]. They reported a sensitive and highly specific
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immunoassay system by utilizing gold nanoparticles and SERS. By using their method, AFP with a very low concentration of 100 pg/ml has been detected. In the study of Seo et al., SERS-based detection of cancer cells
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using dye molecules-embedded gold-silica core-shell nanorods has been investigated[2]. The excellent SERS performance is enough to detect both agglomerated and single cancer cells. A Au-Si core-shell nanoparticle-based
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SERS biosensor for label-free glucose detection has been reported by Al-Ogaidi et al.[6]. For this gold nanostar-silica core-shell nanostructure conjugated with glucose oxidase, the SERS signal shows the response to
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the concentration range of glucose from 25 μM to 25 mM.
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In the application of SERS-based biologic and chemical sensing, fabricating plasmonic nanostructures with hot spots is requisite. The hot spots of local electric field are resulted from the LSPR modes resonantly excited by the incident light [7]. Thus many efforts have been developed to optimize the properties of LSPR and improve the resonant SERS signal by designing and fabricating the metallic nanoparticles with different shape, structure and arrangement[3,4,8-10]. For example, the longitudinal LSPR of gold nanorods could be tuned into the infrared region by increasing the aspect ratio. By using the excellent LSPR properties of gold nanorods, the embedded methylene blue molecules present near-infrared light-induced intense SERS, which is strong enough for the single cancer cell detection [2]. Liao et al. studied the LSPR and SERS biosensing of Au-Ag-Au double nanoshells[11]. The binding of target molecule at the surface of Au-Ag-Au double nanoshells was detected based on both plasmonic absorption and SERS spectra. Besides non-aggregated SERS, aggregation of metallic nanoparticles can also provide excellent SERS. Because of the plasmon coupling between two adjacent particles, the decreasing inter-particle distance results in distinct red shift of the LSPR band and intense local field enhancement in the gaps between the particles[12]. Delange et al. reported the plasmon focusing in short nanochains of gold nanosphere for 3
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SERS[13]. Due to the efficient plasmonic energy trapping and focusing, stronger field enhancement could be created in the short chains of gold nanospheres, which provides the need of SERS. Recently, Pilo-Pais et al. used DNA origami to organize the gold nanoparticles and form the tetramers structure[7]. The resulting assemblies exhibit hot spots of enhanced local field in the gaps between the particles. And a significant SERS signal from the
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molecules attached to the assemblies has been observed.
In recent years, star-shaped gold nanoparticles are particularly interesting because their tunable plasmonic
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properties result from tip effect, large aspect ratio of the branches, and plasmon hybridization between core and the tips[14-17]. The using of gold nanostars for SERS has also been studied[18]. In the report of Fales et al., gold
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nanostars have been demonstrated as one of the best nanostructures for producing SERS in a non-aggregated state [19]. Raman scattering from individual gold nanostars had been observed by Hrelescu et al.[20]. The SERS signal
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could be detected without the aggregations of nanoparticles. By using the finite-difference time-domainy simulation, great local field enhancement and excellent SERS response of the 4-pointed gold nanostar array have
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been demonstrated[21]. By using the seed-mediated growth method, gold nanostars were prepared in aqueous solutions[22]. It has been found that the SERS activity of the star-shaped gold nanoparticles was much stronger
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than that of the spherical gold nanoparticles with similar size. In the report of Su et al., the SERS efficiency as a function of morphology of gold nanostar has been studied[16]. They found that the gold nanostars with longest
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branches could generate the best SERS efficiency.
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However, in these previous reports, the branches or tips of the nanostars are relative short, and the corresponding LSPRs are shorter than 900 nm. Recently, by using Triton X-100 in the seed-growth synthesis, five-branched gold nanostars were prepared, and the corresponding LSPR could be tuned into the wavelength ranges of 1100-1600 nm[17]. How about the SERS ability of the gold nanostars with vimineous branches? In this letter, the SERS activity of the gold nanostars has been evaluated by using rhodamine 6G (R6G) as the Raman active probe. It has been found that the SERS activity of gold nanostars greatly depends on the infrared absorption induced from the LSPR of aligned branches.
2. Experimental 2.1 Synthesis of gold nanostars The colloidal gold nanostars in this study were synthesized according to the Triton X-100 participant seed-growth method developed by Pallavicini et al. with slight modification[17]. Initially, the Au seed solution was prepared by mixing the aqueous HAuCl4 (5×10-4M, 5mL) with TritonX-100 (0.2M, 5mL) solution in a test tube of 15mL. 4
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After gentle hand-shaking, a pale yellow colour of the solution is obtained. Secondly, the previously prepared ice-cold aqueous solution of NaBH4 (0.01M, 0.6mL) was added. After gentle hand-shaking, a reddish-brown colour appeared. Then the seed solution was kept in ice for next use. The growth solution was prepared in a 10mL test tube. Aqueous AgNO3 (0.004M, 125µL), HAuCl4 (0.001M, 2.2mL) are added in this order to aqueous
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TritonX-100 (0.2M, 2.5mL) solution. In order to obtain gold nanostars with different branch lengths, the HAuCl4 amount in the growth solution has been increased to 2.3mL, 2.4mL, 2.5mL and 2.6mL in our experiment. Then, an
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aqueous solution of ascorbic acid (0.0788M, 85µL) was added. After gentle hand-shaking, the mixture was gradually changed from yellow to colorless transparent liquid. At last, the seed solution (5µL) were added, and the
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mixture gradually changed from colorless to pink and then quickly became a turquoise liquid, finally appeared as
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dark green solution. The samples are allowed to equilibrate at 27℃ for 1h.
2.2 Preparation of SERS samples
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2.2.1 The surfactant replacement
In order to increase the particle concentration of gold nanostars, the surfactant replacement is processed at
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first. Aqueous CTAB (0.1M, 1mL) solution was mixed with gold nanostar colloid (4ml). After intense
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hand-shaking for one minute, keep the samples equilibrate at room temperature and the surfactant replacement will finish in 2h. Then, the centrifugation (12000rpm,15min) was carried out and removed the supernate, finally
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the samples were resuspended into ultrapure water (4.5mL) respectively.
2.2.2 R6G-adsorbed gold nanostars with different HAuCl4 volumes in the growth solution Gold nanostar colloid (2mL) with different branch lengths (the HAuCl4 amounts in the growth solution for each sample were 2.2mL, 2.3mL, 2.4mL, 2.5mL, 2.6mL respectively) were taken for centrifugal (12000rpm, 25℃, 15min) firstly. Then, remove the supernatant carefully, and the remaining is 0.45mL. Secondly, aqueous R6G solutions (1×10-6M, 50uL) were added in the remaining, so the final concentration of R6G is 1×10-7M. At last, the samples were sonicated for 10 min (45KHz, 25℃) prior to the measurements.
2.2.3 R6G-adsorbed gold nanostars with different particle concentrations Took five centrifuge tubes (10mL) and then added 5.5mL gold nanostar colloid (the HAuCl4 amounts in the
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growth solution was 2.2mL) respectively. Then, the centrifugation (12000rpm, 25℃,15min) was carried out for two times. After getting rid of the supernate, the samples were resuspended into 0.8mL, 1.0mL, 1.2mL, 1.4mL and 1.6mL ultrapure water respectively. Thus the gold nanostar colloids with different particle concentrations have
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been obtained. At last, different amount of aqueous R6G was added into each samples, and then the volume of each sample finally reached 1.0mL, 1.25mL, 1.5 mL, 1.75mL and 2.0mL respectively. So that the R6G in each
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sample finally reached the same concentration of 2×10-7M. The specific content has been reported in Table 1.
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2.2.4 R6G-adsorbed gold nanostars with different plasmonic resonance wavelengths
Gold nanostar colloid (2mL) with different branch lengths (the HAuCl4 amounts in the growth solution for
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each sample were 2.2mL, 2.3mL, 2.4mL, 2.5mL, 2.6mL respectively) were injected into five test tubes, and then different amounts of ultrapure water were added in each tube to make the third absorption peak reached the same intensity. After centrifugation (12000rpm, 25℃, 15min), the supernatants of each sample were removed, and the
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volume of the remaining is 0.27mL. At last, aqueous R6G (1×10-6M, 30uL) were added into the remaining, so the final concentration of R6G in each sample is 1×10-7M. In these samples, the R6G-adsorbed gold nanostars with
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2.2.5 Equipment
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gold nanorods.
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2.2mL HAuCl4 in the growth solution was also used in the experiment of comparison between gold nanostars and
The Raman spectra were collected in a back scattering geometry through a 50× (NA=0.75) objective HORIBA JOBIN YVON Raman spectrometer (HORIBA, France). The wavelength of laser excitation was 785nm, the integration time was 30s, and the radius of the laser spot was 1.28μm. The laser power focused on samples was 5mW. Absorption spectra were measured by a UV-3600UV-VIS-NIR spectrophotometer (Shimadzu, Japan). Transmissionelectron microscopy (TEM) image of gold nanostars was taken with a JEM-200CXinstrument (JEOL, Japan).
3. Results and discussion 3.1 LSPR properties of gold nanostars From the TEM image in the bottom right inset of Figure 1, one can find these star-shaped gold nanoparticles have 5-6 vimineous branches. The gold cores at the particle centre have a diameter of about 10 nm, and the 6
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average aspect ratio of the branches is about 5.0. What's more, in each nanostar at least two branches are co-linear. Thus in these aligned branches, the aspect ratio has been greatly increased. These special geometrical features leads to the corresponding LSPR become abundant and tunable. Figure 1 also shows that the corresponding absorption spectrum (the solid line) of gold nanostars has three absorption peaks. The first peak with shorter
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wavelength is very weak, whereas the other two peaks with longer wavelength are intense. These absorption spectral properties are similar to the results of Pallavicini et al.[17]. From short to long wavelength, the first
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absorption peak at 515 nm could be attributed to the LSPR of the nanocore; the second absorption peak at 735 nm could be attributed to the LSPR of the gold nanobranches; and the third absorption peak with the largest intensity
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at 1258 nm could be attributed to the LSPR of the aligned branches. By changing the concentration of ascorbic acid or chloroauric acid tetrahydrate in the growth solution, the branch length of the gold nanostars could be tuned.
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The TEM image of the gold nanostar with short branches is shown in the top left inset of Figure 1. One can find the average aspect ratio of the branches is decreased to 3.0, and the two intense LSPR peaks in the absorption
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spectrum (the dash line) blue shift to 665 and 1058 nm respectively.
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3.2 Improve the SERS activity of the gold nanostars by tuning the plasmonic absorption In order to investigate the effect of the third LSPR peak at infrared band on the SERS activity of the gold
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nanostars, the branch length of the nanostars has been tuned by changing the amount of HAuCl4 in the growth
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solution. Figure 2 shows the absorption spectra of gold nanostars with different volumes of the HAuCl4 solution. As the HAuCl4 amount in the growth solution is decreased from 2.6 to 2.2 mL, the third absorption peak in the infrared region red shifts from 1060 to 1240 nm, and the second absorption peak in the visible region red shifts from 640 to 710 nm. The red shift of LSPR bands indicates that too large amount of HAuCl4 in the growth solution may prevent the longitudinal growth of the branches. In this study, the SERS activity of the gold nanostars has been evaluated with a laser line of 785 nm. By using R6G as the Raman active probe, the SERS spectra of R6G adsorbed on the gold nanostars have been measured in Figure 3(a). The SERS spectra show the characteristic SERS bands of R6G. The band at about 613 cm-1 is assigned to the C-C-C ring in-plane vibration mode, the bands at about 1312 cm-1 is assigned to the N-H in-plane bend mode, the bands at about 1362 and 1510 cm-1 are assigned to the C-C stretching modes[23]. As the amount of HAuCl4 is decreased, i.e. the intensity and wavelength of the third absorption peak is increased, the SERS signal gets intense obviously. In Figure 3(b), Raman intensity at 1510 cm-1 is used to test the effect of the plasmonic absorption properties on the SERS ability. One can find that the SERS is sensitive to both the intensity and wavelength of the absorption peak. The nonlinear 7
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enhancement of SERS could always be observed as the wavelength or absorption intensity of the third LSPR peak in the infrared region is increased. In order to distinguish the effect between plasmonic absorption intensity and wavelength on the SERS ability of gold nanostars, 5 samples of colloidal gold nanostars with the same geometry but different particle
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concentrations are used to test the SERS spectra. In this experiment, gold nanostars are re-suspended in the ultra-pure water with different volumes. Thus the corresponding absorption spectra have the same peak
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wavelengths but different peak intensities, as shown in Figure 4(a). The corresponding SERS spectra of R6G-adsorbed gold nanostars are compared in Figure 4(b). One can observed that the Raman intensities get
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intense rapidly as the particle concentration of gold nanostars (i.e. the peak intensity of the LSPR absorption) is increased. What's more, the SERS band at 613 cm-1 is more sensitive to the peak intensity of the LSPR absorption.
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Next, the effect of plasmonic resonance wavelength on the SERS ability is studied. In this experiment, gold nanostars with different LSPR absorption wavelengths (i.e. with different volumes of the HAuCl4 in the growth
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solution) are re-suspended in the ultra-pure water with different volumes. Thus the third absorption peaks in the infrared region have the same peak intensity but different peak wavelengths, as shown in Figure 5(a). From left to
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right, the 5 samples have the resonance wavelengths of 1060, 1122, 1175, 1202 and 1235 nm, respectively. The corresponding SERS spectra of R6G-adsorbed gold nanostars are compared in Figure 5(b). One can observed that
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the Raman intensities get intense as the branch length of gold nanostars (i.e. the peak wavelength of the LSPR
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absorption) is increased. What's more, the SERS band at 1510 cm-1 is more sensitive to the peak wavelength of the LSPR absorption.
In Figure 6, the SERS ability of gold nanostars has been compared with gold nanorods. In this experiment, the gold nanorods were synthesized according to the seed-mediated method[24]. By changing the concentration of silver ions in the growth solution, the longitudinal plasmonic absorption peak of gold nanorods has been tuned to take place at 705 nm, which is equal to the wavelength of the second plasmonic absorption peak of gold nanostars, as shown in Figure 6(a). On the other hand, by re-suspending the gold nanorods in the ultra-pure water, the longitudinal absorption peak of the gold nanorods has a relative intensity of 1.2, which is equal to the intensity of the third absorption peak of gold nanostars. The corresponding SERS spectra of R6G-adsorbed gold nanoparticles are compared in Figure 6(b). One can find that both gold nanorods and nanostars could improve the Raman signals. However, the SERS ability of gold nanostars is much stronger than that of gold nanorods. The absorption spectra in Figure 6(a) indicate that the major difference between nanorods and nanostars is that the gold nanostars has the third LSPR band at infrared region, which is absent from the gold nanorods. Thus we believe that the 8
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SERS ability of gold nanostars is greatly dependent on the third plasmonic absorption band.
3.3 The physical mechanism of the infrared LSPR dependent SERS from gold nanostars In this section, we discuss the physical origin of the improved SERS from gold nanostars. It is well known
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that the local electromagnetic-field enhancement is the major contribution to the SERS. Many previous studies indicated that the strong enhancement of local field in the metallic nanoparticles could be attributed to
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LSPR[25,26,27]. When the surface plasmon has been excited by the incident light, the collective excitation of surface electrons in a nanoparticle results in a strong local electric field and SERS[25,28,29]. In this study, the
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second LSPR absorption peak at 600-800 nm is near to the exciting wavelength of 785 nm. Thus the observed SERS could be partly attributed to the plasmon resonance induced local field enhancement. However, it is
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necessary to note that the surface plasmon resonance is not the only origin of local field enhancement[30]. By using the classical electrodynamics, Gersten and Nitzan obtained the analytical expression of Raman scattering
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enhancement factor[31]. In their expression, three sources of Raman scattering enhancement are noted. Besides resonant excitation of surface plasmons and image dipole enhancement effect, the lightning rod effect also plays
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an important role for the enhanced Raman scattering. In the lightning rod effect, because of the large surface curvature, the surface charges and local fields could be greatly enhanced and concentrate near the particle tips
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with sharp surface features[32,33]. What’s more, it has been interesting to find that the lightning rod effect is
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strongly dependent on the particle shape[31,34]. Especially, in the case of highly prolate ellipsoid described by Gersten[35], the enhancement factor could be reduced to a function of semimajor and semiminor axis of the ellipsoid under the certain condition. As a consequence, the enhancement from the lightning rod effect is a purely geometric effect. Thus the local field strength increases distinctly with the aspect ratio for the rod-like particles. In deed, many experimental reports show that the SERS is greatly affected by the particle shape, whereas the exciting wavelength does not match the resonance wavelength[18,31,34,36,37]. In this study, the observed SERS ability of gold nanostars is also attributed to the lightning rod effect from vimineous branches of gold nanostars. Because of the mismatching between the exciting wavelength of 785 nm and the plasmon resonance wavelength, the plasmon absorption at infrared region is not directly related to the local field enhancement and then to SERS. However, the intense absorption at infrared region is attributed to the LSPR of the aligned branches, and red shifts as the aspect ratio of the branches is increased. This result could be supported by the TEM images comparison of gold nanostars with different LSPR wavelengths, as shown in Figure 1. According to the shape-dependent lightning rod effect, the local field enhancement increases greatly as the 9
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aspect ratio of the branches is increased. Therefore, the gold nanostars with longer and thinner branches are more effective in SERS. Because the length of the aligned branches could be monitored by the infrared LSPR absorption, the Raman intensity has been improved distinctly when the plasmonic absorption band at infrared region has longer peak wavelength. On the other hand, the intensity increase of the plasmonic absorption at
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infrared region indicates that the number of the aligned branches is increased. Therefore, more hot spots for local field enhancement have been created near the branch tips and tiny cavities, and consequently, the SERS ability has
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been greatly enhanced[38]. This mechanism could be further supported by the comparison of absorption and SERS spectra between gold nanostars and gold nanorods, as shown in Figure 6. The gold nanorods have no LSPR
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absorption in the infrared region, and the longitudinal LSPR takes place at 705 nm, which is near to the exciting wavelength. Thus the enhanced Raman signals from gold nanorods could be attributed to the LSPR induced local
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field enhancement. However, the SERS ability of gold nanostars is much stronger than that of gold nanorods. One can find the plasmon peak at 705 nm is weaker than that of gold nanorods. Therefore, the SERS ability of gold
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nanostars is greatly dependent on the third plasmonic absorption band in the infrared region. This infrared plasmonic absorption indicates that the nanostars have longer and thinner branches and more intense lightning rod
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4. Conclusions
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effect.
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In summary, we report the observation of Raman scattering from R6G-adsorbed gold nanostars with vimineous branches. It has been found that the SERS ability of gold nanostars greatly depends on the infrared absorption induced from the LSPR of aligned branches. The Raman intensity could be improved distinctly as the third plasmonic absorption band at infrared region has stronger intensity or longer peak wavelength. The corresponding physical origin could be attributed to the lightning rod effect from the branches of gold nanostars. The increase of the branch number and aspect ratio creates more hot spots of electric field and provides the condition of SERS. These experimental results demonstrated that gold nanostars with a stronger plasmonic absorption at longer infrared wavelength usually have longer and thinner branches, which leads to the improvement of SERS activity.
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities under grant No. 10
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2011jdgz17 and the National Natural Science Foundation of China under grant No. 11174232, 61178075, 81101122.
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25. L. Smitha, K.G. Gopchandran, T.R. Ravindran and V.S. Prasad, Gold nanorods with finely tunable longitudinal
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surface plasmon resonance as SERS substrates, Nanotechnology 22 (2011) 265705.
26. M. Li, S.K. Cushing, J. Zhang, J. Lankford, Z.P. Aguilar, D. Ma and N. Wu, Shape-dependent
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surface-enhanced Raman scattering in gold-Ramanprobe-silica sandwiched nanoparticles for biocompatible applications, Nanotechnology 23 (2012)115501.
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27. A. Lee, G.F.S. Andrade, A. Ahmed, M.L. Souza, N. Coombs, E. Tumarkin, K. Liu, R. Gordon, A.G. Brolo and E. Kumacheva, Probing Dynamic Generation of Hot-Spots in Self-Assembled Chains of Gold Nanorods by
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Surface-Enhanced Raman Scattering, J. Am. Chem. Soc. 133 (2011)7563-7570. 28. Z.D. Schultz, J.M. Marr and H. Wang, Tip enhanced Raman scattering: plasmonic enhancements for nanoscale
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chemical analysis, Nanophotonics 3(2014) 91-104.
29. X.L. Liu, J.H. Wang, S. Liang, D.J. Yang, F. Nan, S.J. Ding, L. Zhou, Z.H. Hao and Q.Q. Wang, Tuning
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Plasmon Resonance of Gold Nanostars for Enhancements of Nonlinear Optical Response and Raman Scattering, J.
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Phys. Chem. C 118 (2014) 9659-9664.
30. J.L. Yao, J. Tang, D.Y. Wu, D.M. Sun, K.H. Xue, B. Ren, B.W. Mao and Z.Q. Tian, Surface enhanced Raman scattering from transition metal nano-wire array and the theoretical consideration, Surf. Sci. 514 (2002) 108-116. 31. J.I. Gersten and A. Nitzan, Electromagnetic theory of enhanced Raman scattering by molecules adsorbed on rough surfaces, J. Chem. Phys. 73 (1980) 3023-3037. 32. F. Le, D.W. Brandl, Y.A. Urzhumov, H. Wang, J. Kundu, N.J. Halas, J. Aizpurua and P. Nordlander, Metallic nanoparticle arrays: A common substrate for both surface-enhanced Raman scattering and surface-enhanced infrared absorption, ACS Nano 4 (2008)707-718. 33. Y. You, N.A. Purnawirman, H. Hu, J. Kasim, H.Yang, C. Du, T.Yu, and Z. Shen, Tip-enhanced Raman spectroscopy using single-crystalline Ag nanowire as tip, J. Raman Spectrosc. 41 (2010)1156-1162. 34. R. Boyack and E.C. Le Ru, Investigation of particle shape and size effects in SERS using T-matrix calculations, Phys. Chem. Chem. Phys. 11 (2009) 7398-7405. 35. J.I. Gersten, The effect of surface roughness on surface enhanced Raman scattering, J. Chem. Phys. 72 13
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(1980)5779-5780. 36. J. Grand, M. Lamy de la Chapelle, J.L. Bijeon, P.M. Adam, A. Vial and P. Royer, Role of localized surface plasmons in surface-enhanced Raman scattering of shape-controlled metallic particles in regular arrays, Phys. Rev. B 72 (2005) 033407.
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37. X. Dong, J. Zhou, X. Liu, D. Lin and L. Zha, Preparation of monodisperse bimetallic nanorods with gold nanorod core and silver shell and their plasmonic property and SERS efficiency, J. Raman Spectrosc. 45
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(2014)431-437.
38. J. Xie, Q.B. Zhang, J.Y. Lee and D.I.C. Wang, The Synthesis of SERS-Active Gold Nanoflower Tags for In
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Vivo Applications, ACS Nano 2 (2008)2473-2480.
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Figure 1
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List of figures:
Absorption spectra of gold nanostars with different branch lengths, the insets show the TEM images of
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the gold nanostars.
Absorption spectrum of gold nanostars with different volumes of the HAuCl4 in the growth solution.
Figure 3
(a) SERS spectra of R6G-adsorbed gold nanostars with different volumes of the HAuCl4 in the growth
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Figure 2
(a) Absorption spectra and (b) SERS spectra of R6G-adsorbed gold nanostars with different particle
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concentrations.
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Figure 4
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solution; (b) Raman intensity at 1510 cm-1 as a function of wavelength and absorbance of the third LSPR band.
Figure 5
(a) Absorption spectra and (b) SERS spectra of R6G-adsorbed gold nanostars with different plasmonic
resonance wavelengths.
Figure 6
The comparison of (a) Absorption spectra and (b) SERS spectra between gold nanostars and gold
nanorods.
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Preparation of R6G-adsorbed gold nanostars with different particle concentrations.
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Table 1
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List of table captions:
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Table 1 Preparation of R6G-adsorbed gold nanostars with different particle concentrations
Gold Nanostar colloid(mL)
R6G 10-6M (uL)
Sample Volume(mL)
Final concentration of R6G(M)
A
0.8
200
1.0
2×10-7
B
1.0
250
1.25
2×10-7
C
1.2
300
1.5
D
1.4
350
1.75
E
1.6
400
2.0
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Sample Number
2×10-7 2×10-7
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2×10-7
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Highlight 1. The SERS ability greatly depends on the infrared plasmonic absorption of Au nanostars.
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3. The lightning rod effect from the branches of Au nanostars improves the SERS.
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2. The infrared absorption indicates the increase of the branch number and aspect ratio.
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S0169-4332(14)02335-6 http://dx.doi.org/doi:10.1016/j.apsusc.2014.10.095 APSUSC 28957
To appear in:
APSUSC
Received date: Revised date: Accepted date:
16-8-2014 14-10-2014 16-10-2014
Please cite this article as: J. Zhu, J. Gao, J.-J. Li, J.-W. Zhao, Improve the surface-enhanced Raman scattering from rhodamine 6G adsorbed gold nanostars with vimineous branches, Applied Surface Science (2014), http://dx.doi.org/10.1016/j.apsusc.2014.10.095 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript. The changes are marked with underline.
Improve the surface-enhanced Raman scattering from rhodamine 6G
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adsorbed gold nanostars with vimineous branches
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Jian Zhu*, Jie Gao, Jian-Jun Li, Jun-Wu Zhao*
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The Key Laboratory of Biomedical Information Engineering of Ministry of Education,
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School of Life Science and Technology, Xi’an Jiaotong University,
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Xi’an 710049, China
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Telephone: 86-29-82664224
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* Corresponding author
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Fax numbers: 86-29-82664224
Email: [email protected]; [email protected] Address: School of Life Science and Technology, Xi’an Jiaotong University, Xi’an, 710049,
Peoples Republic of China
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Abstract:
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The surface-enhanced Raman scattering (SERS) activity of the gold nanostars with vimineous branches has been investigated by using rhodamine 6G (R6G) as the Raman active probe. The colloidal gold nanostars have two
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intense localized surface plasmon resonance (LSPR) peaks in the visible and infrared ranges respectively. Besides the visible LSPR dependent local field effect induced Raman signal enhancement, the SERS ability also greatly
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depends on the infrared absorption from the plasmon resonance along the aligned branches. Whether increasing the peak intensity or wavelength of the infrared absorption leads to the efficient improvement of SERS. These
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correlations between plasmonic absorption and SERS indicate that the lightning rod effect and creation of hot
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spots have been enhanced with the length and number of gold branches.
Keywords:
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absorption spectrum
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gold nanostars; surface-enhanced Raman scattering (SERS); localized surface plasmon resonance (LSPR);
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1. Introduction Surface-enhanced Raman scattering (SERS) of molecules attached on the surface of metallic nanostructures render them appropriate candidates for applications in chemical and biological sensing. Because of the coherent oscillation of conduction band electrons, gold and silver nanoparticles exhibit a localized surface plasmon
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resonance (LSPR) in the visible and infrared regions. The LSPR can also generates an intense local electric field, and subsequently enhance the Raman scattering signal of nearby molecules[1,2]. Therefore, SERS becomes a
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useful technique in sensing and analysis, which could be used in single-molecule detection and molecular structure investigation[3]. For example, Guo et al. reported an ultra-sensitive SERS-based detection for
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trinitrotoluene by using the gold nano-dumbbell structures[4]. Detection of α-fetoprotein (AFP) by using SERS-based immunoassay has been studied by Wang et al.[5]. They reported a sensitive and highly specific
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immunoassay system by utilizing gold nanoparticles and SERS. By using their method, AFP with a very low concentration of 100 pg/ml has been detected. In the study of Seo et al., SERS-based detection of cancer cells
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using dye molecules-embedded gold-silica core-shell nanorods has been investigated[2]. The excellent SERS performance is enough to detect both agglomerated and single cancer cells. A Au-Si core-shell nanoparticle-based
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SERS biosensor for label-free glucose detection has been reported by Al-Ogaidi et al.[6]. For this gold nanostar-silica core-shell nanostructure conjugated with glucose oxidase, the SERS signal shows the response to
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the concentration range of glucose from 25 μM to 25 mM.
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In the application of SERS-based biologic and chemical sensing, fabricating plasmonic nanostructures with hot spots is requisite. The hot spots of local electric field are resulted from the LSPR modes resonantly excited by the incident light [7]. Thus many efforts have been developed to optimize the properties of LSPR and improve the resonant SERS signal by designing and fabricating the metallic nanoparticles with different shape, structure and arrangement[3,4,8-10]. For example, the longitudinal LSPR of gold nanorods could be tuned into the infrared region by increasing the aspect ratio. By using the excellent LSPR properties of gold nanorods, the embedded methylene blue molecules present near-infrared light-induced intense SERS, which is strong enough for the single cancer cell detection [2]. Liao et al. studied the LSPR and SERS biosensing of Au-Ag-Au double nanoshells[11]. The binding of target molecule at the surface of Au-Ag-Au double nanoshells was detected based on both plasmonic absorption and SERS spectra. Besides non-aggregated SERS, aggregation of metallic nanoparticles can also provide excellent SERS. Because of the plasmon coupling between two adjacent particles, the decreasing inter-particle distance results in distinct red shift of the LSPR band and intense local field enhancement in the gaps between the particles[12]. Delange et al. reported the plasmon focusing in short nanochains of gold nanosphere for 3
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SERS[13]. Due to the efficient plasmonic energy trapping and focusing, stronger field enhancement could be created in the short chains of gold nanospheres, which provides the need of SERS. Recently, Pilo-Pais et al. used DNA origami to organize the gold nanoparticles and form the tetramers structure[7]. The resulting assemblies exhibit hot spots of enhanced local field in the gaps between the particles. And a significant SERS signal from the
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molecules attached to the assemblies has been observed.
In recent years, star-shaped gold nanoparticles are particularly interesting because their tunable plasmonic
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properties result from tip effect, large aspect ratio of the branches, and plasmon hybridization between core and the tips[14-17]. The using of gold nanostars for SERS has also been studied[18]. In the report of Fales et al., gold
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nanostars have been demonstrated as one of the best nanostructures for producing SERS in a non-aggregated state [19]. Raman scattering from individual gold nanostars had been observed by Hrelescu et al.[20]. The SERS signal
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could be detected without the aggregations of nanoparticles. By using the finite-difference time-domainy simulation, great local field enhancement and excellent SERS response of the 4-pointed gold nanostar array have
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been demonstrated[21]. By using the seed-mediated growth method, gold nanostars were prepared in aqueous solutions[22]. It has been found that the SERS activity of the star-shaped gold nanoparticles was much stronger
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than that of the spherical gold nanoparticles with similar size. In the report of Su et al., the SERS efficiency as a function of morphology of gold nanostar has been studied[16]. They found that the gold nanostars with longest
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branches could generate the best SERS efficiency.
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However, in these previous reports, the branches or tips of the nanostars are relative short, and the corresponding LSPRs are shorter than 900 nm. Recently, by using Triton X-100 in the seed-growth synthesis, five-branched gold nanostars were prepared, and the corresponding LSPR could be tuned into the wavelength ranges of 1100-1600 nm[17]. How about the SERS ability of the gold nanostars with vimineous branches? In this letter, the SERS activity of the gold nanostars has been evaluated by using rhodamine 6G (R6G) as the Raman active probe. It has been found that the SERS activity of gold nanostars greatly depends on the infrared absorption induced from the LSPR of aligned branches.
2. Experimental 2.1 Synthesis of gold nanostars The colloidal gold nanostars in this study were synthesized according to the Triton X-100 participant seed-growth method developed by Pallavicini et al. with slight modification[17]. Initially, the Au seed solution was prepared by mixing the aqueous HAuCl4 (5×10-4M, 5mL) with TritonX-100 (0.2M, 5mL) solution in a test tube of 15mL. 4
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After gentle hand-shaking, a pale yellow colour of the solution is obtained. Secondly, the previously prepared ice-cold aqueous solution of NaBH4 (0.01M, 0.6mL) was added. After gentle hand-shaking, a reddish-brown colour appeared. Then the seed solution was kept in ice for next use. The growth solution was prepared in a 10mL test tube. Aqueous AgNO3 (0.004M, 125µL), HAuCl4 (0.001M, 2.2mL) are added in this order to aqueous
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TritonX-100 (0.2M, 2.5mL) solution. In order to obtain gold nanostars with different branch lengths, the HAuCl4 amount in the growth solution has been increased to 2.3mL, 2.4mL, 2.5mL and 2.6mL in our experiment. Then, an
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aqueous solution of ascorbic acid (0.0788M, 85µL) was added. After gentle hand-shaking, the mixture was gradually changed from yellow to colorless transparent liquid. At last, the seed solution (5µL) were added, and the
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mixture gradually changed from colorless to pink and then quickly became a turquoise liquid, finally appeared as
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dark green solution. The samples are allowed to equilibrate at 27℃ for 1h.
2.2 Preparation of SERS samples
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2.2.1 The surfactant replacement
In order to increase the particle concentration of gold nanostars, the surfactant replacement is processed at
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first. Aqueous CTAB (0.1M, 1mL) solution was mixed with gold nanostar colloid (4ml). After intense
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hand-shaking for one minute, keep the samples equilibrate at room temperature and the surfactant replacement will finish in 2h. Then, the centrifugation (12000rpm,15min) was carried out and removed the supernate, finally
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the samples were resuspended into ultrapure water (4.5mL) respectively.
2.2.2 R6G-adsorbed gold nanostars with different HAuCl4 volumes in the growth solution Gold nanostar colloid (2mL) with different branch lengths (the HAuCl4 amounts in the growth solution for each sample were 2.2mL, 2.3mL, 2.4mL, 2.5mL, 2.6mL respectively) were taken for centrifugal (12000rpm, 25℃, 15min) firstly. Then, remove the supernatant carefully, and the remaining is 0.45mL. Secondly, aqueous R6G solutions (1×10-6M, 50uL) were added in the remaining, so the final concentration of R6G is 1×10-7M. At last, the samples were sonicated for 10 min (45KHz, 25℃) prior to the measurements.
2.2.3 R6G-adsorbed gold nanostars with different particle concentrations Took five centrifuge tubes (10mL) and then added 5.5mL gold nanostar colloid (the HAuCl4 amounts in the
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growth solution was 2.2mL) respectively. Then, the centrifugation (12000rpm, 25℃,15min) was carried out for two times. After getting rid of the supernate, the samples were resuspended into 0.8mL, 1.0mL, 1.2mL, 1.4mL and 1.6mL ultrapure water respectively. Thus the gold nanostar colloids with different particle concentrations have
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been obtained. At last, different amount of aqueous R6G was added into each samples, and then the volume of each sample finally reached 1.0mL, 1.25mL, 1.5 mL, 1.75mL and 2.0mL respectively. So that the R6G in each
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sample finally reached the same concentration of 2×10-7M. The specific content has been reported in Table 1.
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2.2.4 R6G-adsorbed gold nanostars with different plasmonic resonance wavelengths
Gold nanostar colloid (2mL) with different branch lengths (the HAuCl4 amounts in the growth solution for
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each sample were 2.2mL, 2.3mL, 2.4mL, 2.5mL, 2.6mL respectively) were injected into five test tubes, and then different amounts of ultrapure water were added in each tube to make the third absorption peak reached the same intensity. After centrifugation (12000rpm, 25℃, 15min), the supernatants of each sample were removed, and the
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volume of the remaining is 0.27mL. At last, aqueous R6G (1×10-6M, 30uL) were added into the remaining, so the final concentration of R6G in each sample is 1×10-7M. In these samples, the R6G-adsorbed gold nanostars with
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2.2.5 Equipment
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gold nanorods.
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2.2mL HAuCl4 in the growth solution was also used in the experiment of comparison between gold nanostars and
The Raman spectra were collected in a back scattering geometry through a 50× (NA=0.75) objective HORIBA JOBIN YVON Raman spectrometer (HORIBA, France). The wavelength of laser excitation was 785nm, the integration time was 30s, and the radius of the laser spot was 1.28μm. The laser power focused on samples was 5mW. Absorption spectra were measured by a UV-3600UV-VIS-NIR spectrophotometer (Shimadzu, Japan). Transmissionelectron microscopy (TEM) image of gold nanostars was taken with a JEM-200CXinstrument (JEOL, Japan).
3. Results and discussion 3.1 LSPR properties of gold nanostars From the TEM image in the bottom right inset of Figure 1, one can find these star-shaped gold nanoparticles have 5-6 vimineous branches. The gold cores at the particle centre have a diameter of about 10 nm, and the 6
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average aspect ratio of the branches is about 5.0. What's more, in each nanostar at least two branches are co-linear. Thus in these aligned branches, the aspect ratio has been greatly increased. These special geometrical features leads to the corresponding LSPR become abundant and tunable. Figure 1 also shows that the corresponding absorption spectrum (the solid line) of gold nanostars has three absorption peaks. The first peak with shorter
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wavelength is very weak, whereas the other two peaks with longer wavelength are intense. These absorption spectral properties are similar to the results of Pallavicini et al.[17]. From short to long wavelength, the first
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absorption peak at 515 nm could be attributed to the LSPR of the nanocore; the second absorption peak at 735 nm could be attributed to the LSPR of the gold nanobranches; and the third absorption peak with the largest intensity
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at 1258 nm could be attributed to the LSPR of the aligned branches. By changing the concentration of ascorbic acid or chloroauric acid tetrahydrate in the growth solution, the branch length of the gold nanostars could be tuned.
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The TEM image of the gold nanostar with short branches is shown in the top left inset of Figure 1. One can find the average aspect ratio of the branches is decreased to 3.0, and the two intense LSPR peaks in the absorption
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spectrum (the dash line) blue shift to 665 and 1058 nm respectively.
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3.2 Improve the SERS activity of the gold nanostars by tuning the plasmonic absorption In order to investigate the effect of the third LSPR peak at infrared band on the SERS activity of the gold
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nanostars, the branch length of the nanostars has been tuned by changing the amount of HAuCl4 in the growth
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solution. Figure 2 shows the absorption spectra of gold nanostars with different volumes of the HAuCl4 solution. As the HAuCl4 amount in the growth solution is decreased from 2.6 to 2.2 mL, the third absorption peak in the infrared region red shifts from 1060 to 1240 nm, and the second absorption peak in the visible region red shifts from 640 to 710 nm. The red shift of LSPR bands indicates that too large amount of HAuCl4 in the growth solution may prevent the longitudinal growth of the branches. In this study, the SERS activity of the gold nanostars has been evaluated with a laser line of 785 nm. By using R6G as the Raman active probe, the SERS spectra of R6G adsorbed on the gold nanostars have been measured in Figure 3(a). The SERS spectra show the characteristic SERS bands of R6G. The band at about 613 cm-1 is assigned to the C-C-C ring in-plane vibration mode, the bands at about 1312 cm-1 is assigned to the N-H in-plane bend mode, the bands at about 1362 and 1510 cm-1 are assigned to the C-C stretching modes[23]. As the amount of HAuCl4 is decreased, i.e. the intensity and wavelength of the third absorption peak is increased, the SERS signal gets intense obviously. In Figure 3(b), Raman intensity at 1510 cm-1 is used to test the effect of the plasmonic absorption properties on the SERS ability. One can find that the SERS is sensitive to both the intensity and wavelength of the absorption peak. The nonlinear 7
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enhancement of SERS could always be observed as the wavelength or absorption intensity of the third LSPR peak in the infrared region is increased. In order to distinguish the effect between plasmonic absorption intensity and wavelength on the SERS ability of gold nanostars, 5 samples of colloidal gold nanostars with the same geometry but different particle
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concentrations are used to test the SERS spectra. In this experiment, gold nanostars are re-suspended in the ultra-pure water with different volumes. Thus the corresponding absorption spectra have the same peak
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wavelengths but different peak intensities, as shown in Figure 4(a). The corresponding SERS spectra of R6G-adsorbed gold nanostars are compared in Figure 4(b). One can observed that the Raman intensities get
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intense rapidly as the particle concentration of gold nanostars (i.e. the peak intensity of the LSPR absorption) is increased. What's more, the SERS band at 613 cm-1 is more sensitive to the peak intensity of the LSPR absorption.
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Next, the effect of plasmonic resonance wavelength on the SERS ability is studied. In this experiment, gold nanostars with different LSPR absorption wavelengths (i.e. with different volumes of the HAuCl4 in the growth
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solution) are re-suspended in the ultra-pure water with different volumes. Thus the third absorption peaks in the infrared region have the same peak intensity but different peak wavelengths, as shown in Figure 5(a). From left to
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right, the 5 samples have the resonance wavelengths of 1060, 1122, 1175, 1202 and 1235 nm, respectively. The corresponding SERS spectra of R6G-adsorbed gold nanostars are compared in Figure 5(b). One can observed that
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the Raman intensities get intense as the branch length of gold nanostars (i.e. the peak wavelength of the LSPR
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absorption) is increased. What's more, the SERS band at 1510 cm-1 is more sensitive to the peak wavelength of the LSPR absorption.
In Figure 6, the SERS ability of gold nanostars has been compared with gold nanorods. In this experiment, the gold nanorods were synthesized according to the seed-mediated method[24]. By changing the concentration of silver ions in the growth solution, the longitudinal plasmonic absorption peak of gold nanorods has been tuned to take place at 705 nm, which is equal to the wavelength of the second plasmonic absorption peak of gold nanostars, as shown in Figure 6(a). On the other hand, by re-suspending the gold nanorods in the ultra-pure water, the longitudinal absorption peak of the gold nanorods has a relative intensity of 1.2, which is equal to the intensity of the third absorption peak of gold nanostars. The corresponding SERS spectra of R6G-adsorbed gold nanoparticles are compared in Figure 6(b). One can find that both gold nanorods and nanostars could improve the Raman signals. However, the SERS ability of gold nanostars is much stronger than that of gold nanorods. The absorption spectra in Figure 6(a) indicate that the major difference between nanorods and nanostars is that the gold nanostars has the third LSPR band at infrared region, which is absent from the gold nanorods. Thus we believe that the 8
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SERS ability of gold nanostars is greatly dependent on the third plasmonic absorption band.
3.3 The physical mechanism of the infrared LSPR dependent SERS from gold nanostars In this section, we discuss the physical origin of the improved SERS from gold nanostars. It is well known
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that the local electromagnetic-field enhancement is the major contribution to the SERS. Many previous studies indicated that the strong enhancement of local field in the metallic nanoparticles could be attributed to
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LSPR[25,26,27]. When the surface plasmon has been excited by the incident light, the collective excitation of surface electrons in a nanoparticle results in a strong local electric field and SERS[25,28,29]. In this study, the
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second LSPR absorption peak at 600-800 nm is near to the exciting wavelength of 785 nm. Thus the observed SERS could be partly attributed to the plasmon resonance induced local field enhancement. However, it is
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necessary to note that the surface plasmon resonance is not the only origin of local field enhancement[30]. By using the classical electrodynamics, Gersten and Nitzan obtained the analytical expression of Raman scattering
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enhancement factor[31]. In their expression, three sources of Raman scattering enhancement are noted. Besides resonant excitation of surface plasmons and image dipole enhancement effect, the lightning rod effect also plays
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an important role for the enhanced Raman scattering. In the lightning rod effect, because of the large surface curvature, the surface charges and local fields could be greatly enhanced and concentrate near the particle tips
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with sharp surface features[32,33]. What’s more, it has been interesting to find that the lightning rod effect is
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strongly dependent on the particle shape[31,34]. Especially, in the case of highly prolate ellipsoid described by Gersten[35], the enhancement factor could be reduced to a function of semimajor and semiminor axis of the ellipsoid under the certain condition. As a consequence, the enhancement from the lightning rod effect is a purely geometric effect. Thus the local field strength increases distinctly with the aspect ratio for the rod-like particles. In deed, many experimental reports show that the SERS is greatly affected by the particle shape, whereas the exciting wavelength does not match the resonance wavelength[18,31,34,36,37]. In this study, the observed SERS ability of gold nanostars is also attributed to the lightning rod effect from vimineous branches of gold nanostars. Because of the mismatching between the exciting wavelength of 785 nm and the plasmon resonance wavelength, the plasmon absorption at infrared region is not directly related to the local field enhancement and then to SERS. However, the intense absorption at infrared region is attributed to the LSPR of the aligned branches, and red shifts as the aspect ratio of the branches is increased. This result could be supported by the TEM images comparison of gold nanostars with different LSPR wavelengths, as shown in Figure 1. According to the shape-dependent lightning rod effect, the local field enhancement increases greatly as the 9
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aspect ratio of the branches is increased. Therefore, the gold nanostars with longer and thinner branches are more effective in SERS. Because the length of the aligned branches could be monitored by the infrared LSPR absorption, the Raman intensity has been improved distinctly when the plasmonic absorption band at infrared region has longer peak wavelength. On the other hand, the intensity increase of the plasmonic absorption at
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infrared region indicates that the number of the aligned branches is increased. Therefore, more hot spots for local field enhancement have been created near the branch tips and tiny cavities, and consequently, the SERS ability has
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been greatly enhanced[38]. This mechanism could be further supported by the comparison of absorption and SERS spectra between gold nanostars and gold nanorods, as shown in Figure 6. The gold nanorods have no LSPR
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absorption in the infrared region, and the longitudinal LSPR takes place at 705 nm, which is near to the exciting wavelength. Thus the enhanced Raman signals from gold nanorods could be attributed to the LSPR induced local
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field enhancement. However, the SERS ability of gold nanostars is much stronger than that of gold nanorods. One can find the plasmon peak at 705 nm is weaker than that of gold nanorods. Therefore, the SERS ability of gold
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nanostars is greatly dependent on the third plasmonic absorption band in the infrared region. This infrared plasmonic absorption indicates that the nanostars have longer and thinner branches and more intense lightning rod
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4. Conclusions
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effect.
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In summary, we report the observation of Raman scattering from R6G-adsorbed gold nanostars with vimineous branches. It has been found that the SERS ability of gold nanostars greatly depends on the infrared absorption induced from the LSPR of aligned branches. The Raman intensity could be improved distinctly as the third plasmonic absorption band at infrared region has stronger intensity or longer peak wavelength. The corresponding physical origin could be attributed to the lightning rod effect from the branches of gold nanostars. The increase of the branch number and aspect ratio creates more hot spots of electric field and provides the condition of SERS. These experimental results demonstrated that gold nanostars with a stronger plasmonic absorption at longer infrared wavelength usually have longer and thinner branches, which leads to the improvement of SERS activity.
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities under grant No. 10
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2011jdgz17 and the National Natural Science Foundation of China under grant No. 11174232, 61178075, 81101122.
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Figure 1
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List of figures:
Absorption spectra of gold nanostars with different branch lengths, the insets show the TEM images of
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the gold nanostars.
Absorption spectrum of gold nanostars with different volumes of the HAuCl4 in the growth solution.
Figure 3
(a) SERS spectra of R6G-adsorbed gold nanostars with different volumes of the HAuCl4 in the growth
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Figure 2
(a) Absorption spectra and (b) SERS spectra of R6G-adsorbed gold nanostars with different particle
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concentrations.
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Figure 4
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solution; (b) Raman intensity at 1510 cm-1 as a function of wavelength and absorbance of the third LSPR band.
Figure 5
(a) Absorption spectra and (b) SERS spectra of R6G-adsorbed gold nanostars with different plasmonic
resonance wavelengths.
Figure 6
The comparison of (a) Absorption spectra and (b) SERS spectra between gold nanostars and gold
nanorods.
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Preparation of R6G-adsorbed gold nanostars with different particle concentrations.
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Table 1
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List of table captions:
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Table 1 Preparation of R6G-adsorbed gold nanostars with different particle concentrations
Gold Nanostar colloid(mL)
R6G 10-6M (uL)
Sample Volume(mL)
Final concentration of R6G(M)
A
0.8
200
1.0
2×10-7
B
1.0
250
1.25
2×10-7
C
1.2
300
1.5
D
1.4
350
1.75
E
1.6
400
2.0
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Sample Number
2×10-7 2×10-7
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2×10-7
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Highlight 1. The SERS ability greatly depends on the infrared plasmonic absorption of Au nanostars.
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3. The lightning rod effect from the branches of Au nanostars improves the SERS.
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2. The infrared absorption indicates the increase of the branch number and aspect ratio.
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