pubs.acs.org/NanoLett
Highly Sensitive Surface-Enhanced Raman Scattering Substrate Made from Superaligned Carbon Nanotubes Yinghui Sun,† Kai Liu,† Jiao Miao,† Zheyao Wang,‡ Baozhong Tian,§ Lina Zhang,† Qunqing Li,† Shoushan Fan,† and Kaili Jiang*,† †
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center and ‡ Institute of Microelectronics, Tsinghua University, Beijing 100084, China and § Institute of Forensic Science, Ministry of Public Security, Beijing 100038, China ABSTRACT Surface-enhanced Raman scattering (SERS) has attracted wide attention because it can enhance normally weak Raman signal by several orders of magnitude and facilitate the sensitive detection of molecules. Conventional SERS substrates are constructed by placing metal nanoparticles on a planar surface. Here we show that, if the planar surface was substituted by a unique nanoporous surface, the enhancement effect can be dramatically improved. The nanoporous surface can be easily fabricated in batches and at low costs by cross stacking superaligned carbon nanotube films. The as-prepared transparent and freestanding SERS substrate is capable of detecting ambient trinitrotoluene vapor, showing much higher Raman enhancement than ordinary planar substrates because of the extremely large surface area and the unique zero-dimensional at one-dimensional nanostructure. These results not only provide a new approach to ultrasensitive SERS substrates, but also are helpful for improving the fundamental understanding of SERS phenomena. KEYWORDS Carbon nanotube, superaligned, SERS, surface plasmon, explosive detection
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iting rough films on a warm or cold (usually below 120 K) substrate,4 spraying metal colloids on a substrate,5,12 and fabricating nanostructures by lithography-based technique7,8,13 or porous anodic aluminum oxide (AAO) template.9 But the SERS substrates fabricated by these methods are nearly planar and thereby have limited surface area. If these planar surfaces were replaced by some nanoporous surfaces with huge surface area, more molecules would contribute to the SERS signal and the detection sensitivity should be further improved. Here we show that such a nanoporous surface can be easily fabricated in batches and at low costs by cross stacking superaligned carbon nanotube (SACNT) films. Zerodimensional (0D) metal nanoparticles can be automatically formed on the surface of one-dimensional (1D) CNTs by e-beam evaporation,14 leading to a unique 0D@1D structure which is beneficial to the enhancement of local E-field (Figure 1a). The as-prepared SERS substrate provides sensitive detection of different organic molecules, especially the detection of ambient explosive vapor with extremely low vapor pressure. This superior performance over ordinary planar SERS substrate is due to the unique nanoporous 0D@1D structure of which the huge surface area is favorable for the adsorption of molecules and the densely packed metal nanoparticles contribute abundant “hot” sites of SERS. To fabricate the unique nanoporous structure, we simply cross stacked several layers of SACNT films (Supporting Information Figure S1). The SACNT films can be directly drawn out in a dry state from superaligned carbon nanotube
aman scattering is an inelastic scattering process of photons with matters, accompanied by the absorption or emission of the quanta of a certain elementary excitation. Because of its second order dipole transition nature, the Raman signal is typically very weak. It is found that for molecules adsorbed onto roughened surfaces or nanoparticles of certain metals (e.g., Ag and Au), the Raman signal will be enhanced by several orders of magnitude, which is termed as surface-enhanced Raman scattering (SERS) effect.1-6 This enhancement effect is believed to be due to the excitation of the surface plasmon resonance (SPR) on the metal surface which greatly strengthens the local E-field near the surface. Many applications of SERS have been demonstrated in chemistry, biology, and material science of interfaces as an ultrasensitive detection technique with the capability of identifying trace molecules,7-9 in which a vital role is played by a SERS substrate. To achieve ultrasensitive detection of molecules by SERS, such as detection of TNT vapors for safety check, the SERS substrate should possess (1) huge surface area to adsorb more molecules to contribute to the Raman signal, and (2) abundant “hot” sites of metal nanostructures to enhance the local E-fields as well as the Raman signal.5,10,11 Conventional SERS substrate can be obtained by a variety of methods, such as electrochemically roughening metal foils,1-3 depos* To whom correspondence should be addressed. Tel.: +86 10 62796017. Fax: +86 10 62792457. E-mail:
[email protected]. Received for review: 01/18/2010 Published on Web: 04/13/2010 © 2010 American Chemical Society
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gaps.5,10,11 Thus it is desirable to load the CNT grids with densely packed metal nanoparticles so as to form abundant “hot” sites. There are many methods that can be used to load the nanoporous CNT grids with densely packed metal nanoparticles. For example, if the nanoporous CNT grids were soaked in a solution of monodispersed metal nanoparticles, densely packed metal nanoparticles would be self-assembled on the surface of CNTs because of the strong adsorbability of CNTs. Alternatively we can also electroplate metal nanoparticles on the CNTs’ surface by utilizing the good conductivity of CNTs. Here we use the physical vapor deposition method, which has the advantages of simple process, high purity, free of organic contaminations, and so forth. Figure 1c shows a TEM image of the CNT grid loaded with Ag nanoparticles by e-beam evaporation, which is termed as “Ag-CNT grid”. The nominal thickness of silver was monitored by a quartz crystal oscillator. Our studies indicated that the optimized nominal thickness was about 5 nm (see Supporting Information Section 2). To test the Ramanenhancing capability, an ethanol solution of Rhodamine 6G (R6G) (10-6 M) was applied to the SERS substrates, and Raman spectra were recorded after the evaporation of ethanol (see details in Supporting Information Section 4). As shown in Figure 1d, highly enhanced Raman peaks can be observed for R6G adsorbed on Ag-CNT grid, and the peak positions are similar to previous studies,5,12,20 while no visible Raman peaks of R6G can be observed on the pure CNT grid. We also compared the Raman-enhancing capability of Ag-CNT grid with a planar SERS substrate prepared by depositing 5 nm Ag onto a silicon wafer, termed as “Ag-Si wafer”. As shown in Figure 1d, the substrate of Ag-CNT grid provides higher SERS signal than Ag-Si wafer. These results indicate that Ag-CNT grid is an effective SERS substrate and has better performance than the planar SERS substrate. However, if we take a closer look at the microstructure of Ag-CNT grid shown in Figure 1c, the Ag nanoparticles usually form prolate ellipsoids along the axis of CNTs with nonuniform sizes, resulting in nonuniform and large interparticle gaps. Previous studies indicated that the interparticle gaps had a predominant influence on SERS signals.7,9-11,20,21 Usually reducing the gaps can lead to an increase of Raman enhancement according to some theoretical calculations10,11 and experimental observations.7,9,20 Therefore, it is still a challenge to optimize our SERS substrate for obtaining uniform nanoparticles with small gaps on the surfaces of CNTs. To fabricate densely packed Ag nanoparticles on the CNT frameworks, we tried to insert certain buffer layer to change the interface properties between Ag and CNTs. It is found that inserting a layer of amorphous silica can greatly improve the uniformity of Ag nanoparticles. In such “Ag-SiO2-CNT grid”, Ag tends to form quasi-uniform spheres on the amorphous silica layer around CNTs as shown in Figure 2a. Interparticle gaps can be tuned by adjusting the thickness of silver. Typically depositing 5 nm Ag will give rise to 2-5
FIGURE 1. (a) Schematic illustration of the nanoporous 0D@1D structure under a laser irradiation. (b) TEM image of a cross-stacking SACNT film. (c) TEM image of the Ag-CNT grid SERS substrate. (d) Raman spectra of R6G on Ag-CNT grid, Ag-Si wafer, and CNT grid. The laser power on samples is 470 µW and the exposure time is 20 s. Inset is the SEM image of Ag-Si wafer.
(SACNT) arrays.15-18 A SACNT array with an area of 0.01 m2 can be totally converted to a 6-10 square meter SACNT film, which is cost-effective for mass production of CNT films. The CNTs in SACNT films are parallel aligned to the drawing direction.15-18 When two layers of SACNT films are cross-stacked, large amounts of square nanoholes can be naturally formed, which is termed as “CNT grid” and shown in Figure 1b. The as-prepared cross-stacking CNT grid are robust, conductive, chemically inert, and nanoholey, which can serve as nanoporous frameworks for adsorbing nanoparticles or gaseous molecules.19 It has been reported that an effective SERS substrate requires the formation of abundant “hot” sites, that is, nanogaps between adjacent metal nanoparticles. These “hot” sites show tremendous enhancement effect on Raman signal due to the extremely strong local E-field excited in the © 2010 American Chemical Society
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FIGURE 2. (a) TEM image of the Ag-SiO2-CNT grid SERS substrate. The Ag nanoparticles form quasi-uniform spheres attached to the deposited amorphous silica layer. (b) High-resolution TEM image of densely packed Ag nanoparticles in the Ag-SiO2-CNT grid SERS substrate. (c) Raman spectra of R6G on Ag-SiO2-CNT grid, Ag-SiO2Si wafer, and pure CNT grid. The laser power on samples is 6 µW and the exposure time is 20 s. Inset is the SEM image of Ag-SiO2-Si wafer.
FIGURE 3. (a) Raman spectra of 10-3 M TNT methanol solution on the SACNT-based substrates. The exposure time is 30 s and the laser power on samples is very low (15 µW) to prevent any decomposition, local ignition, or desorption of TNT. (b) Raman spectra of 10-6 M BSA water solution on the SACNT-based substrates. The exposure time is 20 s and the laser power is 1.2 mW.
nm wide gaps between Ag nanoparticles, as shown in Figure 2b, which is believed to be beneficial to SERS enhancement.7,9-11,20,21 To validate the improvement of SERS effect after inserting a silica layer, Raman spectrum of R6G is measured on the SERS substrate of Ag-SiO2-CNT grid. Figure 2c shows that under a very weak laser irradiation (6 µW) Ag-SiO2-CNT grid can provide nearly the same Raman intensity as Ag-CNT grid under a 470 µW laser irradiation, which indicates that inserting a silica layer can dramatically improve the enhancement effect. We also compared the Raman-enhancing capability of Ag-SiO2-CNT grid with a planar SERS substrate, Ag-SiO2-Si wafer, which is prepared by depositing 20 nm amorphous silica and subsequently 5 nm Ag on a silicon wafer. As shown in Figure 2c, Ag-SiO2CNT grid provides higher SERS signal than the planar SERS substrate. We therefore can conclude that our SERS substrates based on SACNT films provide better Raman-enhancing capability than conventional planar substrates, and inserting a silica layer between Ag and CNTs can further improve the enhancing capability (Figure 1d and 2c). To demonstrate that the sensitive detection of the SACNTbased SERS substrates is widely applicable, we further measured the Raman spectra of two other kinds of organic molecules, trinitrotoluene (TNT) and bovine serum albumin (BSA), by using the SACNT-based substrates. The detection © 2010 American Chemical Society
and identification of trace TNT is a problem of great practical interest. SERS as a potential tool for trace analysis of explosives has been explored actively because of its sensitivity and reliable identification of molecular structures.22-25 TNT can be characterized by six dominant bands in its Raman spectrum, including a very strong band at 1360 cm-1 (NO2 symmetric stretching vibration), two bands at 1617 (CdC aromatic stretching vibration) and 1534 cm-1 (NO2 asymmetric stretching vibration), and the band at 1210 cm-1 (C6H2-C vibration). Out of plane vibrations can be seen at 792 and 822 cm-1 at a modest intensity.23,24 In our experiment, a methanol solution of TNT (10-3 M) is applied to the SACNT-based substrates, and Raman spectra were recorded after the evaporation of methanol. As shown in Figure 3a, highly enhanced Raman peaks can be observed for TNT adsorbed on the SERS substrate of Ag-SiO2-CNT grid. The positions of most of Raman peaks are similar to previous SERS studies,22-25 though some new peak positions and different relative intensities are observed. The sharp peak at 1428 cm-1 has not been reported before, but it together with the NO2 symmetric stretching vibration mode near 1360 cm-1 and the band near 1270 cm-1 could be used as 1749
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a “fingerprint” for the detection of TNT in our experiment. As a contrast, merely several Raman peaks of TNT are occasionally observed on the Ag-CNT grid, and no visible vibration modes of TNT can be detected on the pure CNT grid. BSA is a widely used water-soluble protein in cell medium, which can protect the embryo and increase the number of cells in the embryo. Many vibrational bands of BSA have been assigned to vibrations of secondary structure and functional groups.25-27 In our experiment, a water solution of BSA (10-3 M) was added to the SACNT-based substrates, and Raman spectra were recorded after the evaporation of water. As shown in Figure 3b, highly enhanced Raman peaks are observed for BSA adsorbed on the SERS substrate of Ag-SiO2-CNT grid. According to previous Raman scattering studies of BSA,25-27 the peak at 684 cm-1 belongs to C-S bond, 853 cm-1 to Tyrosine, 927 cm-1 to C-C, 1391 cm-1 to COO- group, those at 633 and 1001 cm-1 belong to Phenyl ring, 1335 and 1439 cm-1 to CH2 vibration, 1600 and 1623 cm-1 to Phenyl plus Tyrosine, 1244, 1287, and 1560 cm-1 to amide. Raman spectra collected on CNT grid and Ag-CNT grid did not present any visible vibration modes of BSA. These results demonstrate that our SERS substrates based on SACNT films are widely applicable to identify many kinds of molecules. It is found that the SACNT-based SERS substrates have much stronger enhancement effect than ordinary planar SERS substrates, and inserting a silica layer between Ag nanoparticles and CNTs can further improve the enhancing capability. To understand these phenomena and optimize the performance of our SERS substrates, we further carried out experimental and computational investigations on the possible mechanism of SERS effect. It is generally agreed that two fundamentally different mechanisms dominate in the SERS phenomenon: an electromagnetic (EM) effect associated with large local E-field due to the excitation of surface plasmon resonance (SPR) of metallic microstructures, and a chemical effect involving the electronic interaction between the molecule and metal surface. The EM contribution is believed to be several orders of magnitude more than the value for the chemical enhancement.4,11 Therefore the SPR of metallic microstructures plays a major role in the enhancement effect of SERS. In the case of EM description, the SPR is essentially localized surface plasmon, in contrast to the surface plasmons propagating along the metal surface. When a nanoparticle was irradiated by light, electrons on the metal surface will move with the oscillating E-field. Resonance will occur when the frequency of light equals to the Fro¨hlich frequency,28 resulting in an enhanced local surface E-field. When two nanoparticles are placed close to each other with a nanometer-sized gap and the polarization direction of the E-field is along the axis of the two particles, the local E-field in the gap can be further enhanced under the resonance condition, forming “hot” site for Raman scattering. The SERS © 2010 American Chemical Society
enhancement factor M originating from EM effect can be roughly expressed by M ) (Elocal/E0)4, where E0 and Elocal are the strength of the incident E-field E0 and the total local E-field Elocal at the presence of metal microstructures, respectively.11 It is worth noting that the Fro¨hlich frequency will decrease with increasing the size of a nanoparticle,28 and the resonant frequency of coupled nanoparticles often red shift with decreasing interparticle separation.29,30 Thus the guidelines for optimizing the SERS substrate are (1) varying the size of nanoparticles to tune the resonance frequency matching the frequency of the laser and (2) decreasing the gaps between nanoparticles to further enhance the local E-field inside the gaps. Following the aforementioned guidelines, we further tuned the nominal thickness of silver deposited on the CNT framework. Figure 4a shows the Ag thickness dependence of SERS signal of R6G at about 1650 cm-1 for Ag-SiO2-CNT nanoporous substrate (upper) and Ag-SiO2-Si planar substrate (lower), respectively. In the both cases, depositing 5 nm (nominal thickness) Ag gives rise to the highest enhancement of Raman signal (Supporting Information Figure S5). The nanoporous SACNT-based SERS substrates show much higher enhancement than the planar SERS substrates. We attribute this distinction to the huge surface area of the nanoporous structure, which can be easily understood, and the unique 0D@1D nanostructure. To demonstrate the enhancement effect contributed by the 0D@1D nanostructure, finite element analysis simulations (COMSOL Multiphysics) are used to calculate the spatial distribution of local E-fields for Ag nanoparticles sitting on 1D and 2D surfaces respectively (Supporting Information Figure S6 and S7). Calculation results indicate that the 0D@1D nanostructures give rise to 2-4 times field enhancement compared to 0D@2D structures (Supporting Information Table S1), corresponding to 16-256 times in the SERS enhancement factor. This calculated SERS enhancement factor is consistent with the experimental results shown in Figure 4a. For example, the Raman signal of R6G at about 1650 cm-1 is enhanced ∼43 times by Ag nanoparticles with nominal thickness of 5 nm. We therefore can conclude that, compared to the conventional planar SERS substrate, the extremely high enhancement effect of the SACNT-based SERS substrate is due to both the huge surface area of its nanoporous structure and high-field enhancement effect of the unique 0D@1D nanostructure. Now we turn to the question why 5 nm (nominal thickness) Ag deposited on SiO2/CNTs shows the best performance. Considering the localized SPR is essentially a light scattering and absorption process, the extinction spectra will reflect the SPR process. We then measured the optical transmittances of the SACNT-based SERS substrates by using a Perkin Elmer-Lambda950 spectrophotometer. Strictly speaking, the measured transmittance spectra are not the extinction spectra. Only part of the scattering loss and the absorption loss were measured by this machine. However, 1750
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fused silica substrates (Supporting Information Figure S8). This transmittance enhancement may be similar to the enhanced transmission through periodic arrays of subwavelength holes in optically thick metallic films, which has been indexed as bulk plasmon peak of silver.34,35 The dip next to it was formed due to the superposition of CNT’s π-plasmon dip and silver’s bulk plasmon peak. As shown in Figure 4b, an additional broad absorption band appears at about 450 nm when there is a silica layer between Ag and CNT. We attributed this absorption band to the localized SPR of Ag nanoparticles. When the nominal thickness of Ag increases from 3 to 5 nm, the absorption peak position (hereafter we use “peak” although it is a dip in our transmittance curve) change from 432 to 463 nm. From TEM images in Supporting Information Figure S2 and Figure 2a, we can see that the diameter of the Ag nanoparticles also increases with increasing the nominal thickness. Thus the absorption peak shift can be well explained by the relation between the Fro¨hlich frequency and the size of nanoparticles.28 It is worth noting here that the broad absorption band is partly due to the diameter distribution of the nanoparticles. When the nominal thickness of silver exceeds 5 nm, the absorption peak shifts to the opposite direction, and an absorption plateau appears at the near IR band. This can be understood from the SEM images shown in Supporting Information Figure S9, where the large particles merge into a continuous film which will reflect the near IR light evenly, and some small particles keep separate that still contribute to the localized SPR. Thus we can see that the nominal thickness of 5 nm gives rise to the localized SPR of which the resonant frequency is closest to the frequency of the excitation laser (514.5 nm). At the same time, from TEM images in Figure S2, Figure 2a, and SEM images in Supporting Information Figure S9 it is clear that the gaps between adjacent nanoparticles are decreasing with increasing the nominal thickness of silver, and then the nanoparticles become quasi-continuous when the nominal thickness is above 5 nm. Hence this 5 nm nominal thickness leads to the smallest gaps between Ag nanoparticles that are favorable for the field enhancement. We therefore conclude that both the nearest resonance condition and the smallest interparticle gaps result in the best Raman enhancement performance of the 5 nm Ag deposition. To further validate this conclusion, we also deposited Ag or Ag/SiO2 on unidirectional SACNT single-layer in which the CNTs are parallel aligned to the drawing direction. When the light polarization is parallel to the CNT aligning direction, the 5 nm Ag-SiO2-CNT layer has an absorption band at about 518 nm, which is very close to the wavelength of the excitation laser (Supporting Information Figure S11). When the light polarization is perpendicular to the CNT aligning direction, the absorption band is further away from the laser wavelength. According to our former discussion, the Raman signal should be higher when the laser polarization is parallel to the CNT aligning direction, than that when perpendicular
FIGURE 4. (a) Ag thickness dependence of SERS signal of R6G at ∼1650 cm-1. The solid line is a guide to the eye. (b) Optical transmittances of CNT grid and Ag-SiO2 (20 nm)-CNT grid with different Ag thicknesses. (c) Transmittances of CNT grid and Ag-CNT grid with different Ag thicknesses.
it still can partially reflect the SPR process. The measured transmittance spectra are shown in Figure 4b. There is always a broad absorption band near 275 nm (corresponding to energy of 4.5 eV) even without the presence of Ag, which should come from CNTs. Previous literatures attributed this strong absorption band to the collective excitation of the π-plasmon of CNT.31-33 After depositing Ag nanoparticles on CNTs, more peaks or dips emerge in the transmittance spectra. We identified the peak at 320 nm rather than the dip next to it as the result of a genuine physical process, because the peak can be unambiguously identified as a transmittance enhancement peak in the transmittance spectra of Ag films deposited on transparent © 2010 American Chemical Society
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excitation. We observed that the Raman signal of R6G of parallel excitation is almost twice of that of the perpendicular excitation (Supporting Information Figure S12), which is in consistent with our aforementioned discussion and further validates our viewpoints. However, optical transmittances of the cross-stacked SERS substrates under a polarized light did not show any distinct differences in their intrinsic peaks for both 0 and 90 degrees polarized light (Supporting Information Figure S13); the intensity of Raman signals is nearly the same for the two excitations. It indicates that there is no polarization effect on the cross-stacked SERS substrates. To understand the role played by the SiO2 layer, we further examined the transmittance spectra of the Ag-CNT grid SERS substrates as shown in Figure 4c. The CNT’s π-plasmon dip (∼275 nm) and silver’s bulk plasmon peak (∼320 nm) are still there, but the localized SPR of Ag nanoparticles disappears. We considered the absence of such a localized SPR as the result of intimate coupling between Ag nanoparticles and CNTs. Electrons may move freely across the boundary between them, so that the localized SPR of nanoparticles is greatly suppressed. To further validate this conjecture, we calculated the local E-field for two Ag nanoparticles sitting on identical substrate (1D or 2D) with or without a SiO2 layer respectively. The presence of a SiO2 layer leads to a field enhancement 2-3 times of that without a SiO2 layer (Supporting Information Table S1), corresponding to a 16-81 times enhancement in Raman signal. Thus, the role played by the SiO2 layer are (1) changing the interface properties between CNTs and Ag to benefit the formation of densely packed Ag nanoparticles, which give rise to abundant “hot” sites, and (2) isolating the Ag nanoparticles and CNTs to guarantee the excitation of localizedSPRwhichwillenhancethelocalE-fielddramatically. According to the aforementioned discussions, the extraordinary Raman-enhancing capability of our Ag-SiO2-CNT grid SERS substrate is due to (1) the nanoporous structure which provides huge surface area to adsorb more molecules; (2) the unique 0D@1D structure, which gives rise to a high field enhancement effect; and (3) the interfacial SiO2 layer which helps the formation of densely packed nanopartilces of which the localized SPR will be excited resonantly forming abundant “hot” sites. This extraordinary Raman-enhancing capability can be directly employed in ultrasensitive detection of trace molecules. For example, the detection of TNT vapor is very attractive in the trace analysis of explosives because of its extremely low vapor pressure (about 5 × 10-4 Pa at 25 °C).36 Although some effort has been put to detect 2,4dinitrotoluene (DNT) vapor by SERS technique,37,38 it is still a challenge to detect ambient TNT vapor by SERS technique, which requires a highly sensitive SERS substrate with a considerable adsorption area for TNT molecules. To prove the application of our SERS substrates in this field, we further carried out some preliminary experiments of detecting © 2010 American Chemical Society
FIGURE 5. Raman spectrum of TNT vapor adsorbed on Ag-SiO2-CNT grid. The laser power on sample is 47 µW and the exposure time is 10 s.
ambient TNT vapor. The SERS substrates were put above the TNT powder in a desiccator for several hours at room temperature before we took the Raman spectra. It is found that the adsorbed TNT molecules can be detected on the AgSiO2-CNT grid as shown in Figure 5. Because of different adsorption geometry of TNT molecules on SERS substrates, the details of Raman spectra for TNT solution and vapor are different, but the peak at about 1430 cm-1, the NO2 symmetric stretching vibration mode near 1360 cm-1, and the band near 1270 cm-1 could be used as a “fingerprint” for the detection of TNT as aforementioned. This result indicates that our SERS substrates may be potentially used for very sensitive detection of ambient TNT vapor. To make a comparison, identical experiments were also performed using conventional planar SERS substrate, pure CNT grid, and Ag-CNT grid, respectively, but no signals can be detected on these substrates. Despite these excellent performances, we have to admit that our SERS substrates need to be further optimized. The simple fabrication process of our SERS substrates cannot tune the nanoparticle size and the interparticle gap independently. If the two parameters can be adjusted independently, the SERS enhancement performance can be further improved. A possible approach is soaking the SACNT frameworks in a solution of presynthesized nanoparticles with desired diameters39 to form densely packed nanoparticles on the nanoporous SiO2-SACNT frameworks. The drawback of this approach might be the contamination from the solvents. It is worth noting that other 1D nano-objects can also be utilized to construct this nanoporous structure, such as Si nanowires, BN nanotubes, and single-walled CNTs. The prerequisites are (1) a scalable and easy process to batch fabricate such nanoporous structure and (2) the Raman signals of the material itself as low as possible (MWCNT is much better than SWCNT in this sense). In conclusion, we demonstrated an ultrasensitive SERS substrate that can be easily fabricated in batches and at low 1752
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costs by utilizing the dry-drawing SACNT technique. This SACNT-based SERS substrate is capable of sensitively detecting a variety of organic molecules, especially detecting ambient explosive vapor with extremely low vapor pressure, showing superior performance than ordinary planar SERS substrate due to the unique nanoporous and 0D@1D nanostructure. These results not only provide a new approach to ultrasensitive SERS substrates, but also are helpful for improving the fundamental understanding of SERS phenomena.
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Acknowledgment. We thank Yongchao Zhai, Changqing Yin, and Haitao Yang for their assistance in experiments, Professor Dapeng Yu and Professor Yumin Hou for their help in simulation analysis. This work was financially supported by the National Basic Research Program of China (2005CB623606,2007CB935301),NSFC(60871006,50825201, 10704044), Fok Ying Tung Education Foundation (111049), and China Postdoctoral Science Foundation (20090450349).
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Supporting Information Available. (1) Convenient cross stack of SACNT films. (2) Microstructures and Raman spectra of SACNT-based SERS substrates with different thicknesses of Ag. (3) X-ray energy dispersive spectroscopy of asprepared Ag-CNT grid. (4) Measurements of Raman spectra and transmittances. (5) Raman spectra of R6G on the AgSiO2-CNT grids with different thicknesses of silver. (6) Parameters and results for the finite element analysis simulation. (7) Optical transmittances of fused silica substrate and different thicknesses of Ag deposited on it. (8) SEM images of SACNT-based SERS substrates with Ag thickness above 5 nm. (9) SEM images of Ag-Si wafers with different thicknesses of Ag. (10) Optical transmittances of CNT single-layer deposited with Ag and Ag/SiO2 under perpendicularly polarized lights. (11) Optical transmittances of the cross-stackedSACNT-based SERS substrates under perpendicularly polarized lights. This material is available free of charge via the Internet at http://pubs.acs.org.
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DOI: 10.1021/nl100170j | Nano Lett. 2010, 10, 1747-–1753
