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A Novel Strategy for One-pot Synthesis of Gold Nanoplates on Carbon Nanotube Sheet as An Effective Flexible SERS Substrate Wenbo Xin, Jenn-Ming Yang, Chao Li, Mark S. Goorsky, Larry Carlson, and Igor De Rosa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10560 • Publication Date (Web): 20 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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ACS Applied Materials & Interfaces

A Novel Strategy for One-pot Synthesis of Gold Nanoplates on Carbon Nanotube Sheet as An Effective Flexible SERS Substrate

Wenbo Xin*a, Jenn-Ming Yanga, Chao Lia, Mark S. Goorskya, Larry Carlsonb, Igor M. De Rosa*b

a

Department of Materials Science and Engineering, University of California, Los Angeles,

410 Westwood Plaza, Los Angeles, CA 90095, USA E-mail: [email protected] b

Institute for Technology Advancement, University of California, Los Angeles, 410

Westwood Plaza, Los Angeles, CA 90095, USA E-mail: [email protected] * Corresponding authors

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Abstract In this work, we have demonstrated a novel route for one-pot synthesis of two dimensional gold nanoplates (2-D AuNPLs) on carbon nanotube (CNT) sheet. Welldefined AuNPLs are grafted onto CNT sheet via a facile hydrothermal reduction process, during which bromine ions are employed as the surfactant for gold anisotropic growth. Scanning electron microscopy (SEM) shows large-scale AuNPLs with micrometer-scaled length and sub-100 nm thickness are deposited uniformly on the CNT sheet. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) results confirm the synthesized AuNPLs are single-crystalline with preferential {111} orientation. Based on the CNT sheet/AuNPLs hybrid, we have fabricated a flexible surface enhanced Raman scattering (SERS) substrate, which can effectively detect the analyte Rhodamine 6G (Rh6G) at the concentration as low as 10-7 M. The excellent SERS performance of this novel flexible substrate is mainly attributed to nanoscaled gaps between the neighbors, large surface area with roughness, and their sharp edges and corners.

Keywords: One-pot synthesis; Gold nanoplates; Carbon nanotube sheet; Flexible SERS substrate.


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Introduction Surface enhanced Raman scattering (SERS) has been intensively considered as a powerful technique for label-free, non-destructive, and trace analysis of chemical and biological analytes at super-low concentration, even down to single molecule.1,2 It typically amplifies Raman signals of adsorbates on a rough surface of plasmonic metals by several orders of magnitude due to the localized electromagnetic field (electromagnetic mechanism, EM) and charge-transfer complexes (chemical mechanism, CM).3 The combination of the two mechanisms can contribute to an enhancement factor as high as ~1014-1015.3-6 Recently, a growing interest has been raised to develop flexible SERS substrates by integrating plasmonic nanocrystals, mostly Ag and Au nanoparticles with flexible matrices, such as cellulose paper,7 nanofiber assemblies,8 plastic PET (polyethylene terephthalate),9 carbon nanotube (CNT) templates

10-13

and so forth.

Flexible SERS substrates employed in the packaging can benefit their use in terms of special position requirements, for instance, wrapping around the non-planar substrates during testing.14,15 Among the aforementioned flexible substrates, CNT assemblies (yarns and sheets) are of particular interest due to the fact that they are highly flexible,16,17 mechanically strong,18 and chemically stable.19 In addition, the nanoporous structure of CNT network provides large surface area that is favorable for molecules adsorption and hence contributes to intense SERS “hot spots”.12 As a consequence, CNT templates decorated with Au nanoparticles (AuNPs) have been widely exploited as flexible SERS substrates. For instance, Zhang and co-workers have recently reported a flexible 3-D SERS membrane based on CNT/AuNPs, which identified the analyte melamine at the 3 ACS Paragon Plus Environment


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concentration down to 1 nM level.20 However, the reported substrates usually require a very dense loading of plasmonic particles in order to maintain sufficient hot spots. The detectable concentration of the analyte would be increased to 1 µM level when the particles are sparsely distributed on CNT substrate.21 Alternatively, Au nanocrystals with various shapes and dimensions like nanorods

22

and popcorn-like nanoparticles

23

have

been coupled with CNT assemblies in order to fabricate highly sensitive SERS substrates with excellent flexibility. However, there is much less research on SERS performance of the substrates made from CNT templates coupling with two dimensional Au nanoplates (2-D AuNPLs). It has been reported that 2-D AuNPLs can perform ultrasensitive detection of various analytes due to their extreme anisotropy and excellent localized surface plasmonic resonance (LSPR) property.24 Furthermore, 2-D AuNPLs possess large surface area, rough surfaces, sharp corners and nanoscale gaps between the neighboring accompanies.25-28 All of these unique characteristics can contribute to giant Raman scattering enhancement, which make them ideal to build high performance SERS substrates. As a matter of fact, AuNPLs based substrates have shown strong SERS activity, which can identify a number of analytes at the concentrations from 0.01 to 0.1 µM.24,27 Thus, assembling AuNPLs onto flexible CNT sheet is expected to be an effective strategy to fabricate desirable SERS substrates with high flexibility, extraordinary sensitivity, and excellent reproducibility. However, to our knowledge, one-pot synthesis of well-defined AuNPLs onto CNT substrate has not been reported yet. Herein, we demonstrate a facile one-pot method for the growth of large-scale AuNPLs on CNT sheet, which is based on the hydrothermal reduction of gold precursor in the solvent, followed


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by the gold nucleation and growth on CNT frameworks. In the process, moderate amount of bromide ions are used to promote the anisotropic growth and formation of AuNPLs.

Experimental Chemicals and Materials CNT sheet was received from Nanocomp Technologies Inc. (Merrimack, NH), which was fabricated by a continuous chemical vapor deposition (CVD) process. Nanotubes grown with iron catalysts in the CVD furnace was subsequently collected onto a drum to impart alignment in the drawing direction. CNT sheet used in this work is virtually scale-up production. Figure S 1 shows a spool of as-received CNT sheet. The width of the sheet is around 2.54 cm and the thickness varies from 30 µm to 40 µm. Gold chloride hydrate (HAuCl4 ·xH2O, 99.999%) and potassium bromide (KBr, ACS reagent, >99.0%) were purchased from Sigma-Aldrich, USA. L-ascorbic acid (>99.0%) and Rhodamine 6G (99%) were obtained from Fisher scientific, USA. Preparation of CNT sheet-AuNPLs hybrids CNT sheet was cut as 4 cm×1 cm (~4.5 mg) and then transferred to a glass vial that contained 18 mL deionized water. In a typical procedure, 80 µL HAuCl4 (28.5 mM) and 200 µL KBr (25 mM, in deionized water) were added to the vial. After the mixture was homogeneous mixed with gently shaking, 500 µL ascorbic acid (0.4 mM) was then added to the solution. The mixture was maintained at 90 oC for 8 hours for complete redox reaction. Subsequently, CNT sheet was taken out from the solution and washed thoroughly with deionized water. CNT sheet-AuNPLs hybrid was finally dried and kept in the vacuum oven at 100 oC overnight. In order to investigate the role of bromine ions 5 ACS Paragon Plus Environment


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in Au anisotropic growth, extra solutions with 0 µL, 50 µL and 1000 µL KBr (25 mM) were prepared with the same conditions as the typical procedure except for KBr concentrations. Characterizations Electron microscopy studies and X-ray diffraction (XRD) analysis were carried out in order to understand the morphologies and structures of CNT sheet-AuNPLs hybrid. Scanning

electron

microscopy

(SEM)

coupled

with energy

dispersive

X-ray

(SEM/EDX) analysis was obtained with FEI Nova NanoSEM 230. Transmission electron microscopy (TEM) images were captured with FEI Titan S/TEM system at the accelerating voltage of 80 keV and 300 keV. The X-ray diffraction (XRD) measurements were performed on a Jordan Valley D1 diffractometer using Cu Kα1 radiation. They were under double axis diffraction condition. The step size of the 2θ – ω scans was 0.04 degrees and the count time was 3 seconds. SERS performance Raman spectroscopy analysis was obtained with a Renishaw In-Via Raman system with the laser length of 785 nm and expose time of 10 s. To test surface-enhanced Raman scattering (SERS) performance, the prepared substrates were immersed into 10 mL Rh6G solution with various concentrations from 10-4 M to 10-8 M for 30 minutes, ensuring the analyte molecules sufficiently absorb on the substrate surface. The samples were then transferred to Raman equipment to make sure the substrates were not fully dried and corresponding Raman signals were recorded and analyzed.

Results and discussion 6 ACS Paragon Plus Environment

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Synthesis and characterizations of AuNPLs on CNT sheet Figure 1 depicts our one-pot synthesis route of AuNPLs on CNT sheet. During reaction, CNT sheet provides abundant immobilization sites for gold nuclei as well as spacious template for gold crystals growth. Chloroauric acid (HAuCl4) serves as the gold precursor and ascorbic acid (A.A) is the reductant. KBr functions as the surfactant for gold anisotropic growth. As demonstrated in the box in Fig. 1, after sufficient growth time at moderate high temperature (~8 hrs and 90 oC), AuNPLs can be deposited onto CNT sheet. Here, anisotropic growth of AuNPLs occurred due to the addition of KBr, from which Br- ions got liberated in the solution and played as surfactant. The role of Brions in guiding AuNPLs growth would be stressed in details in the following discussion. Detailed morphology study of synthesized CNT sheet-AuNPLs hybrid was carried out with SEM and the representative images are presented in Figure 2. For comparison, the image of pristine CNT sheet is shown Fig. 2(a). Carbon nanotube bundles are randomly entangled and cross-linked through the sheet surface, forming a porous network. Such porous CNT support provides large surface area, namely the abundant anchoring sites for gold nuclei. More SEM and TEM images of pristine CNT sheet at higher magnifications are listed in Figure S 2. Fig. 2(b) exhibits the morphology of CNT sheet surface after AuNPLs deposition, showing sufficient number of AuNPLs uniformly cover the sheet. A higher magnification image shown in Fig. 2(c) confirms well-defined 2-D AuNPLs are synthesized on CNT sheet substrate. It is noteworthy that the synthesized AuNPLs are identified as triangular and hexagonal plates and their sizes display a wide range from sub-micrometer to a few micrometers. Of particular interest is that not only the AuNPLs residing on top of CNT sheet are observed, but also the ones 7 ACS Paragon Plus Environment


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buried in CNT bundles are found, which suggests inside layers of CNT bundles also have immobilization sites for Au clusters at nucleation stage. Fig. 2(d) shows the side view of two overlapped AuNPLs. One CNT bundle is observed to cross the surface of a plate, which clearly justifies the position of the plate locating beneath CNT bundles. In addition, the measured thickness of two AuNPLs are 53.6 nm and 57.0 nm, respectively, showing their super-thin characteristic. In order to determine the composition of the plates, we performed the elemental analysis by using energy dispersive X-ray spectroscopy (EDX). Figure 3(a) shows the elemental mapping of a selected area, which contains one micrometer scaled hexagonal AuNPL and one nanoscaled triangular AuNPL. Three elements C (carbon), O (oxygen) and Au (gold) were captured and mapped. C and O originate from CNT sheet and Au comes from the synthesized plate. It is interesting to note that elemental mapping of Au domain corresponds to the hexagonal geometry of the plate shown in the SEM image. No more elements were detected from the hexagonal domain, which confirms the element of plate is pure gold. Moreover, we counted edge length and thickness distributions from SEM images of 80 random AuNPLs and the statistical result is presented in Fig. 3(b). The most frequently observed edge length of AuNPLs is in the range of 2.0-2.5 µm while a large variety can be seen from sub-1µm to larger than 3 µm. Meanwhile, their thickness is mostly less than 100 nm with the most frequently measured range from 50 nm to 70 nm. This result is consistent with the observation in Fig. 2(d). Such thin plates can be deformed and folded easily, as shown in Fig. S 3. The observed Au elemental map in Fig. 3(a) is not as intense as C and O due to their thin characteristic. In addition, the relative


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ratio of edge length over thickness can reach to 100, providing large surface areas compared to sphere-like particles at the same volume. The crystalline structure of AuNPLs was investigated with XRD and the corresponding spectra is shown in Fig. 4(a). In particular, we assigned all diffraction peaks to CNT {002} (26.0o) from carbon nanotube together with Au {111} (38.1o), Au {200} (44.3o), Au {220} (64.6o) and Au {311} (77.5o) planes, respectively. Au diffraction peaks correspond to the typical face-centered cubic (FCC) crystal structure. It is impressive that Au {111} peak displays strong intensity compared to those of Au {200}, Au {220} and Au {311} planes. The relative intensity of Au {200} /Au {111} is 0.34, lower than bulk gold value (0.52 from JCPDS #04-0784), suggesting the synthesized AuNPLs are {111} textured. However, the ratio 0.34 is much larger than that of pure AuNPLs reported somewhere else, which is 0.005.26 It results from two aspects (1) a large number of AuNPLs lie on porous CNT network with arbitrary angles instead of paralleling to the substrate and (2) Au nanoparticles, as the by-products, are synthesized associating with AuNPLs, which is confirmed by SEM images in Figure 2. We further carried out transmission electron microscopy (TEM) experiment for the purpose of studying AuNPLs structure in details. Fig. 4(b) demonstrates a typical TEM image of a hexagonal AuNPL on the sheet surface. The selected area electron diffraction pattern (SAED) of the AuNPL was obtained with incident electron beam normal to the plate surface, as shown in Fig. 4(c). The diffracted spots are well arranged in a hexagonal pattern with a six-fold symmetry, indicative of a typical {111} orientated single crystalline structure. We index the spots as {220} and 1/3 {422} Bragg reflections, where the presence of forbidden 1/3 {422} diffraction in FCC crystals is of particular interest. 9 ACS Paragon Plus Environment


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Many researchers have observed the identical result in the plate-like Au and Ag nanocrystals.26,29 Defects within the crystals such as stacking faults29 and/or twin boundaries30 parallel to {111} planes are supposed to result in the occurrence of 1/3 {422} diffraction. Thus, it is reasonable to speculate the as synthesized AuNPLs might contain structural defects across {111} planes. Furthermore, the atomic structure of AuNPL is demonstrated in Fig. 4(d). The measured fringe spacing is 2.50 Å, which equals to dspacing of 1/3{422} planes. The fast Fourier transformation (FFT) spots inserted in Fig. 4(d) are indexed as Au {220} and Au 1/3 {422}, which is consistent with the SAED in Fig. 4(c). Additionally, FFT patterns from the middle part and the edge of the plate are presented in Fig. 4(e) and (f), respectively. The identical arrangement of the spots confirms the structure uniformity throughout the entire plate. Because of sub-100 nm thickness, the plate shows high transparency under the electron beam at 300 keV, and thus the nanotubes beneath the plate are readily observable. Of particular interest is the integrity of nanotube walls was still preserved under 300 keV electron irradiation with the protection from the AuNPL (middle part, Fig. 4(e)). In contrast, nanotubes became amorphous rapidly without the coverage of the gold layer, as shown in Fig. 4(f). Function of bromine ions in AuNPLs growth In order to better understand the specific function of Br- ions in the synthesis of AuNPLs on CNT sheet, a series of experiments were carried out by varying KBr concentration. The representative SEM images of synthesized gold nanocrystals on CNT sheet with 0 µL, 50 µL, 200 µL and 1000 µL KBr (25 mM) are exhibited in Figure 5. With the solution free of KBr, as synthesized Au nanocrystals on CNT network are 10 ACS Paragon Plus Environment

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mostly nanoparticles with irregular shapes and large variety of sizes, as shown in Fig. 5(a) and (e). Gold deposition is non-uniform due to nanoparticles aggregation. In general, Au nanoparticles can be anchored onto CNT skeletons from the gold precursor through spontaneous

reduction,31

during

which

isotropic

growth

of

Au

crystals

is

thermodynamically favored in the environment lacking of capping agents.32 However, the addition of KBr in the solution, even at low concentration, i.e. 50 µL, leads to the presence of AuNPLs, notwithstanding a majority of Au nanoparticles are synthesized as shown in Fig. 5(b) and (f). It is worthy to mention that Au nanowires are occasionally observed in this case, identifying the anisotropic growth of gold nanocrystals are promoted by the introduction of KBr. Upon a gradual increase of KBr concentration, not only the percentage of Au nanoplates but also their size eventually increases. This tendency is well justified Fig. 5(c) and (h), suggesting the surfactant used in this report, namely Br- is of fundamental importance in the synthesis of AuNPLs. The phenomena of Br- ions favoring the growth of plate-like Au nanocrystals via hydrothermal reaction has been observed previously,33 however, the precise growth mechanism is still elusive. According to the previous reports32-34 and our observations, we speculate free Br- ions preferentially adsorb to Au {111} surfaces, protect Au {111} facets, and subsequently lead to a slower growth along direction. With sufficient growth time, Au nanoplates bounded with multiple {111} planes are finally synthesized. Whereas appropriate amount of Br- ions can induce the formation of 2-D AuNPLs, overdosed Br- (1000 µL) fails to produce higher yield AuNPLs but instead results in Au nanocrystals with polyhedral shapes, as shown in Fig. 5 (d) and (h). It is likely due to that excessive Br- ions might adsorb to different facets



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besides of Au {111}, and thus its preferential growth along direction would be suppressed. Unlike Au nanoparticles synthesized from the solution free of KBr, polyhedral Au nanocrystals have smaller variations on their sizes and deposit uniformly on CNT sheet. Application as flexible SERS substrate Prior to probe the analyte, we recorded the Raman response of pristine CNT sheet for comparison. In Fig. S 4 (a), three typical Raman peaks of CNT are observed, which are D band (1310 com-1), G band (1583 com-1) and G’ band (2605 com-1), respectively. Of particular interest are D and G bands shown in Fig. S 4(b). D band identifies the disorders of CNT that are usually attributed to amorphous carbon and defects. G band characterizes the crystallinity of graphitic CNT due to in-plane tangential vibration of carbon-carbon bonds.35 To evaluate SERS efficiency and sensitivity of CNT sheet-AuNPLs substrate, we employed Rhodamine 6G (Rh6G), a highly fluorescent dye as the analyte with a variety of concentrations from 10-4 to 10-8 M. The measured SERS spectra are shown in Fig. 6 (a), where Raman spectrum of solid Rh6G on silicon substrate is displayed as the reference. From solid Rh6G, five characteristic Raman peaks found at approximately 1182, 1306, 1362, 1506, and 1647 cm-1 are assigned to stretching modes of aromatic benzene rings.36 Apparently, all the representative peaks of Rh6G are dramatically enhanced and distinctly identified at the concentrations of 10-6 M (pink line) and 10-7 M (dark cyan line) when CNT sheet-AuNPLs is utilized as the substrate. Note peaks at around 1310 and 1583 cm-1 are attributed to CNT as aforementioned and D band overlaps with the peak of Rh6G at 1306 cm-1. Thus, the peak at around 1306-1310 cm-1 shows the 12 ACS Paragon Plus Environment

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highest intensity. Moreover, Raman signal intensity shows a gradual decrease with reduced concentration. When the concentration of Rh6G decreases to 10-8 M, the detection sensitivity becomes poor, where the signatures at 1362, 1506 and 1647 com-1 could be hardly justified. Thus, the as-fabricated SERS substrate shows high sensitivity, which is capable of probing the Rh6G at the concentration as low as 10-7 M. A control experiment using Au nanoplates assembly as the SERS substrate was carried out. The obtained results are summarized in Fig. S 5. SEM image of Au nanoplates in Fig. S 6(a) illustrates that well-defined Au nanoplates were synthesized. Fig. S 5(b) shows the enhanced Raman signals of the analyte from 10-5 to 10-7 M. Apparently, the analyte could be well identified at each level from 10-5 M to 10-7 M with Au nanoplates substrate. The function of CNT sheet is to provide flexible, stable and durable support with sufficient anchoring sites for Au nanoplates. It evidently shows that the integrity of SERS function from Au nanoplates is well maintained on the CNT support. In order to better demonstrate the advantages of CNT sheet-AuNPLs platform as SERS substrate, we further tested the performance of CNT sheet coupling with Au nanoparticles (AuNPs) as the substrate. CNT sheet-AuNPs hybrid was synthesized with the same quality of gold precursor and 0 µL KBr (Fig. 5(a) and (e)). The corresponding SERS spectra are shown in Fig. 6(b). In contrast to the excellent performance of CNT sheet-AuNPLs, it shows a much poorer sensitivity. Although five peaks of Rh6G at 10-4 M can be detected, the band at 1647 cm-1 is scarcely differentiated from the shoulder of CNT G-band (1583 cm-1). Furthermore, main peaks at 1362 and 1647 cm-1 almost disappear when the analyte concentration is reduced to 10-5 M. Eventually it fails to capture any of the signatures of the analyte at 10-6 M, whose Raman spectrum is identical 13 ACS Paragon Plus Environment


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to that of pristine CNT sheet shown in Fig. S 4 (b). A desirable SERS substrate should possess high sensitivity as well as good reproducibility. In order to justify the reproducibility of CNT sheet-AuNPLs as SERS substrate, we randomly selected and scanned 14 spots from the substrate deposited with 1 x 10-7 M Rh6G. The corresponding Raman spectra are illustrated in Fig.6 (c). All of these spectra present five predominant peaks of Rh6G, which are consistent with the result in Fig.6 (a). The relative standard deviation (RSD) of intensity of predominant peaks can be utilized to characterize reproducibility of SERS activity.7-9 The value of RSD is calculated from the equation: RSD=SD/Im, where SD is the standard deviation of the intensity and Im is the average Raman intensity of a predominant peak. For instance, the statistic calculation from 14 spots illustrates the RSD at the peak of 1306 cm-1 is 13.2%, as shown in Fig. 6(d). The RSD values of all five predominant peaks are summarized in Table S 1. These results confirm that SERS performance of CNT sheet-AuNPLs owns excellent reproducibility. Furthermore, CNT sheets loaded with different shapes of Au nanocrystals have also been employed as the SERS substrates and their performances are compared in order to determine the most suitable structures for SERS activity. The results of Rh6G at 10-7 M are shown in Fig. S 6. It apparently identifies that substrates with Au nanoparticles (with 0 µL and 1000 µL Br-) completely lost their sensitivity, since only two main peaks from CNT were observed from the corresponding Raman spectra. In addition, it is noteworthy that the presence of AuNPLs (with 50 µL Br-) on CNT sheet led to an enhanced SERS sensitivity with five predominant peaks (i.e. 1182, 1306, 1362, 1506, and 1647 cm-1) detectable. However, the signal intensity of Rh6G is relatively low compared


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to that of CNT, which could be attributed to the low ratio of AuNPLs over Au nanoparticles. When an increasing amount of AuNPLs is coupled with CNT sheet (with 200 µL Br-), the substrate offers further enhanced SERS sensitivity. Rh6G peaks are clearly identified with fairly high signals intensity. Therefore, 2-D Au nanoplate is a more suitable structure for SERS performance than that of Au nanoparticles, considering the same quality of gold precursor. The poor SERS performance of CNT sheet-AuNPs substrate is attributed to the sparsely distributed Au nanoparticles on the CNT support, which leads to large particle gaps and limit the enhancement of localized electrical field. Moreover, it has been proposed molecules adsorbed on spherical particles are randomly orientated in the nanoscale valley between adjacent particles, impeding the SERS sensitivity and uniformity.37 Hence, heavy loads of plasmonic nanoparticles on CNT scaffolds are of fundamental significance to build highly sensitive SERS substrate.20,21 As an alternative, the substrate based on CNT-AuNPLs hybrid presents enhanced SERS sensitivity with regard to the usage of the same quality of gold precursor. The reasons can be summarized in the following aspects. First, homogenous deposition of AuNPLs results in nanoscaled gaps between neighbors. The nanoscaled gaps are of high benefit for the significant enhancement of Raman signals.38 Second, AuNPLs have large surface area accompanying with rough wrinkles (Fig. 7(a)) and surface folds (Fig. S 3), which provide abundant active sites for plasmonic oscillation, namely hotspots. Third, Fig. 7(b) illustrates AuNPLs are arranged on CNT surface at arbitrary angles, which not only contributes to the aforementioned large Au {200}/ Au{111} ratio from XRD measurement but also more sharp edges and corners of the plates serving as extra



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hotspots. Sharp edges and corners of plasmonic crystals can enhance the coupling effect of electromagnetic field, contributing to the enhanced sensitivity.39 In other words, enhancement of Raman scattering from chemical mechanism (CM) is equivalent in AuNPLs and AuNPs, however, electromagnetic mechanism (EM) induced enhancement in AuNPLs is much larger compared to that of AuNPs from the same concentration of gold precursor. The scheme is illustrated in Fig. 7(c). Therefore, it is understandable that CNT sheet-AuNPLs as flexible SERS substrate showed higher sensitivity than that of CNT sheet-AuNPs platform, considering the same quality of gold precursor. Au/Ag nanoparticles have been widespreadly used as the plasmonic particles, which provide strong SERS activity. In order to achieve a super sensitive SERS performance, it normally requires very high load of plasmonic particles on the substrates. A large variation of detection limit of Rh6G from 10-5 to 10-10 M has been reported, crucially depending on how dense Au nanoparticles load on the substrates.20,21 Therefore, it is of importance to compare SERS sensitivity of each substrate with regard to the same load of plasmonic particles. Here, it aims to compare SERS sensitivity of CNT sheet-ANPLs with CNT sheet-Au nanoparticles at the same level of gold precursor. The results evidently confirm that AuNPLs on CNT sheet contribute to a significantly enhanced SERS sensitivity by two orders of magnitude, which is meaningful for the future design of efficient, adaptable, and particularly cost effective SERS substrates. In addition, the traditional flexible SERS substrates are usually fabricated using the two-step strategy, including (1) synthesis of plasmonic particles, and (2) assembling the particles onto.substrates.7,9 Such strategy often requires complex synthesis and process approaches. Alternatively, one-pot assembly of AuNPLs with CNT sheet are particularly 16 ACS Paragon Plus Environment

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adaptable for simple, fast and high-yield fabrication of flexible SERS substrates. Furthermore, nanoporous network of CNT sheet rather than ordinary planar substrates provides sufficient anchoring sites for Au nanocrystals, assuring long-term stability and reproducibility of SERS activity.13 CNT substrates also present high strength and excellent corrosive resistance, which could extend their application to the harsh environment, whereas traditional substrates such as paper/plastic invalid quickly.

Conclusions We have reported a simple one-pot method to synthesize large-scale 2-D AuNPLs on CNT sheet in the solution, where Br- ions serve as the surfactant. The synthesized AuNPLs have single-crystalline structure with the edge length in micro-meter scale and thickness at sub-100 nm level. It evidently shows the appropriate amount of Br- ions (200 µL) facilitate the anisotropic growth and formation of AuNPLs. In addition, we have achieved excellent SERS performance by employing CNT sheet-AuNPLs hybrid as the substrate. The detection limit for Rh6G is as low as 10-7 M, showing significant advantage in contrast to the substrate fabricated with CNT sheet and Au nanoparticles. The improved SERS sensitivity of AuNPLs can be attributed to their nanoscaled gaps, rough surface and sharp edges and corners. In one hand, by means of incorporating AuNPLs with commercially available CNT sheet, we show great potential for scale-up fabrication of this novel flexible SERS substrate in future. On the other hand, the final product still contains a portion of irregular Au nanoparticles, which requires further improvement in the future research. Acknowledgments 17 ACS Paragon Plus Environment


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We would like to recognize the generous support of the United States Department of Defense. We also would like to thank Nanocomp Technologies Inc. for supplying CNT materials and providing essential knowledge of the products. Fianlly, we acknowledge Electron Imaging Center for NanoMachines at California Nanosystem Institute for their advice and guidance on microscopy analysis. Supporting Information Digital photo of CNT sheet; microstructure of CNT; SEM image of wrinkled Au nanoplates; Raman spectra of CNT sheet; SERS activity of CNT sheet loaded with various Au crystals; Control experiment of SERS performance of Au nanoplates assembly as the substrate; RSD values of predominant peaks of Rh6G at 1x 10−7 M.

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Figure 1 Schematic illustration of one-pot synthesis of AuNPLs on CNT sheet


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Figure 2 Morphologies of CNT sheet and CNT sheet-AuNPLs hybrid. (a) SEM image of pristine CNT sheet. (b) representative SEM image of AuNPLs synthesized on CNT sheet. (c) high magnification image of AuNPLs deposited on CNT surface. (d) side view of two AuNPLs.



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Figure 3 (a) elemental mapping of AuNPLs on CNT sheet. (b) Edge length and thickness distribution of AuNPLs observed from SEM.


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Figure 4 Structure characterization of AuNPLs synthesized on CNT sheet (a) XRD spectrum of CNT sheet-AuNPLs hybrid. (b) representative TEM image of a AuNPL on CNT network at 80 keV (c) The corresponding SAED pattern of (b). (d) HRTEM image of selected area of (b) at atomic resolution at 300 keV. Insert is its corresponding FFT. And high resolution images of middle part (e) and the edge (f) of the plate at 300 keV.



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Figure 5 Representative SEM images of Au nanocrystals synthesized on CNT sheet with a variety of KBr concentrations. Low magnification (a) 0 µL (b) 50 µL (c) 200 µL and (d) 1000 µL KBr, and high magnification (e) 0 µL (f) 50 µL (g) 200 µL and (h) 1000 µL KBr.


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Figure 6 Performance of CNT sheet-AuNPLs hybrid utilized as flexible SERS substrate (a) SERS performance of CNT sheet-AuNPLs substrate for detection of Rh6G from 10-6 to 10-8 M and (b) SERS performance of CNT sheet-Au nanoparticles (AuNPs) substrate with Rh6G from 10-4 to 10-6 M. (c) reproducibility evaluation of CNT sheet-AuNPLs as SERS substrate (1 x 10-7 M Rh6G) with 14 randomly selected spots. (d) intensity distribution of 14 scans at 1306 com-1



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Figure 7 Characteristics of AuNPLs that favor the SERS performance (a) surface roughness. (b) AuNPLs with sharp edges and corners lie arbitrarily on CNT sheet. (c) schematic illustration of the mechanisms of Raman signal enhancement. EM represents electromagnetic mechanism and CM is chemical mechanism.


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