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Zhen Liu, Zhongbo Yang, Bo Peng, Cuong Cao, Chao Zhang, Hongjun You, Qihua Xiong,* Zhiyuan Li,* and Jixiang Fang* Surface-enhanced Raman scattering (SERS) spectroscopy is one of a few techniques that are capable of ultimately detecting matter at single molecule scale.[1] As a powerful analysis technique, SERS-based signal detection and molecular identification will provide an unprecedented opportunity for investigations in biomedicine, life science, analytical chemistry, etc.[2] Since the first demonstration of single molecule detection via SERS, great efforts have been devoted to the fabrication of a variety of SERS substrates. Ideally, a SERS substrate should possess a super signal amplification, high uniformity and reproducibility. However, up to now, quite few techniques can simultaneously achieve the aforementioned requirements, hence impede the use of SERS sensors in both laboratories and mobile applications. To date, exciting successes have been achieved using colloid chemistry strategy to well define nanogaps and thus achieve super sensitivity of SERS enhancement.[3] However, preventing the formation of large conglomerations of nanoparticles before the analyte is introduced, at the same time controlling the gap size via random aggregation of colloid nanoparticles thus achieving reproducible and uniform hot spots remains challenging.[4] Patterning techniques, e.g.,electron-beam lithography, nanosphere lithography, or nanoimprinting, indeed can improve the uniformity and reproducibility of the SERS signal at a certain molecule concentration.[5] However, creating a high density of hotspots via the aforementioned lithographic protocols is rather difficult. Therefore, probe molecules might not land inside hot spot regions even if they could be precisely prefabricated particularly at an ultralow molecule concentration. The current dilemma is that Z. Liu, Z. B. Yang, Dr. H. J. You, Prof. J. X. Fang State Key Laboratory for Mechanical Behavior of Materials School of Science, Xi’an Jiaotong University Shann Xi, 710049, P. R. China E-mail: [email protected] Dr. B. Peng, Dr. C. Cao, Prof. Q. H. Xiong Division of Physics and Applied Physics, School of Physical and Mathematical Sciences Nanyang Technological University Singapore, 637371 E-mail:[email protected] C. Zhang, Prof. Z. Y. Li Institute of Physics Chinese Academy of Sciences Beijing, 100080, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201305106
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Highly Sensitive, Uniform, and Reproducible SurfaceEnhanced Raman Spectroscopy from Hollow Au-Ag Alloy Nanourchins
the more uniform enhancement across a SERS substrate is, i.e., the better the reproducibility, the lower the enhancement tends to be and vice versa.[6] In order to maximize the sensitivity, uniformity and reproducibility of the SERS signal, some SERS studies have been pioneered, such as single-particle SERS (sp-SERS) [7–10] and 3D SERS substrates. [11–14] By introducing abundant nanogaps or nanobridges, sharp protrusions or crevices, into the mesoscale particles, a high enhancement factor (EF) at 107–108 and an improved uniformity and reproducibility of SERS signal have been obtained. The architecture of 3D hierarchical arrays may create densely packed metal nanostructures with plentiful hot spots, thus may maximally meet the above requirements of an ideal SERS substrate. In our previous studies on sp-SERS of gold urchin-like mesoparticles,[9] owing to the anisotropic nature of the multiple tips, the uniformity of the sp-SERS signal shows not so high. Furthermore, the particle size is around ∼1µm, which is an embarrassed situation because the laser spot is generally comparable to this size. This makes it difficult to obtain the real signal from the whole mesoparticles and hence also restricts the uniformity. Based on our previous studies on mesocrystals,[15–17] here, a strategy using highly roughened hierarchical nanoparticles is suggested to both increase the sensitivity and improve the uniformity and reproducibility of the SERS substrate by means of simply dropping the hierarchical nanoparticles onto a substrate. Differing from the conventional colloid nanoparticles, which show smooth surface and low plasmonic effect, the hierarchical nanoparticles should be synthesized as small as possible in size, but with complex structure combined by enhanced nanogaps or nanotips hence increased hot spot density for individual nanoparticle. Herein, we show a highly sensitive, uniform and reproducible SERS performance obtained from the randomly aggregated hollow Au-Ag alloy nanourchins (HAAA-NUs). The individual HAAA-NU nanoparticle is composed of many sharp protrusions and nanogaps, e.g., around 70–100 tips on 100 nm HAAA-NUs. These nanotips, serving as nanoantenna, may dramatically increase the excitation cross section and the electromagnetic field enhancement. The individual HAAA-NU nanoparticle demonstrates ultrahigh density of hot spots at the level of 1500 µm–2, hence shows a super enhancement capability at ∼109 magnitude. Importantly, the overlapping between individual HAAA-NUs in the assembly may create additional hot spots. Moreover, around 30-50 HAAA-NU nanoparticles can be included within a laser spot, i.e., ∼1 µm in diameter. Thus, the assembly of HAAA-NUs displays a high uniformity and
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Figure 1. UV-Vis spectra, SEM, and TEM images of HAAA-NUs. a) UV-Vis spectra of silver seeds with various particle sizes (20–50 nm) measured in DI water. b) UV-Vis spectra of HAAA-NUs with various particle sizes (80 – 300 nm) measured in DI water. c) A scheme of the HAAA-NU particle growth. d) TEM images of silver seed and HAAA-NU correlated with structural evolution. The scale bar for Ag seed is 10 nm, and the scale bars for others are 50 nm, respectively. e) SEM image of HAAA-NUs. f) Magnified TEM image of HAAA-NUs. g) TEM image of HAAA-NUs. The white areas marked by arrows in the center of each particle imply the hollow interior.
reproducibility, e.g., 100 tips within individual ∼100 nm nanourchins (Figure 1e). The tips serving as nanoantennas and the root of tips as nanogaps (Figure 1e, 1f) may display ultrahigh density of hot spots, e.g.,1500 µm–2 (|E|4 >108, ignoring the contribution of chemical effect and Ag element), thus present a high SERS sensitivity (see below). 2) The formation of Au-Ag alloy (Figure S4-S6), compared with pure
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Au nanostructures,[29,30] may increase the SERS sensitivity. Moreover, the Au-Ag nanourchins present the better chemical stability than the pure Ag nanostructures, thus improving the reliability of the SERS substrate. 3) There is wide tunable capability of localized surface plasmon resonances (LSPRs). In this synthesis, we can also change sizes of the Ag seeds to tune the hollow interior size of the HAAA-NUs (Figure 1g), which can be used to not only tune the LSPRs but also exploit, for example, the drug carrier and delivery in cancer therapeutics. To prove that both high sensitivity and high reproducibility of SERS spectroscopy could be achieved via the current strategy, a systematic SERS evaluation of the HAAA-NUs were carried out using the crystal violet (CV) molecule as Raman probe. Firstly, the sp-SERS performance was analyzed using 532 nm and 633 nm incident wavelengths. Figure 2a shows the UV spectra of the HAAA-NUs measured in air with particle sizes of 80, 100, 150, 200 and 300 nm, respectively. As compared with the results measured in DI water, the UV spectra obtained in air show obvious blue shifts. Figure 2b presents the AFM-correlated nano-Raman images measured with a 532 nm excitation laser for various types of aggregations of the HAAA-NUs including single particles (1, 2, 3 in Figure 2b), trimer (7 in Figure 2b), tetrad (5 in Figure 2b) and several particles (4, 6 in Figure 2b). The corresponding SERS map shows good correlation with the AFM image, where only two particles around particles 2# were not recognized by the incident laser. The unidentified particles could be dusts or small nanoparticles because the particle height from the AFM image was around ∼50 nm, significantly smaller than the size of HAAA-NUs (Figure 2c).
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As a comparison, single particle of HAAA-NUs (particle 1#) was measured and displayed in Figure 2c, which was around ∼100 nm in height. Figure 2d illustrates the SERS spectra taken from the particles Nos. 1 to 7 in Figure 2b, which are highly consistent with the aggregated states of HAAA-NUs. The single particles show relatively weak Raman signals (∼50 counts) and the aggregation consisting of several HAAA-NUs presents relatively strong signals with around ∼200 counts, while the trimer and tetrad indicate a middle enhancement with ∼100 counts. From the UV spectra of HAAA-NUs measured in both DI water and air, a long incident wavelength is helpful to achieve a large enhancement of Raman signals. Thus, we have also measured the sp-SERS by using a 633 nm incident laser with CV molecule concentrations ranging from 10−7 to 10−11 M. From Figure 2e, the SERS spectra can still be identified at the CV concentration of 10−10 M for individual HAAA-NUs. According to the procedure and assumptions used in our previous experiments[9,31,32] and also in Ref [33] for the CV molecules, the average enhancement factor (EF) for the individual HAAANUs is estimated about ∼1.6 × 109 under the 633 nm excitation wavelength (see details in S2.4). According to the statistic of hot spots obtained via finite difference time domain (FDTD) simulations, an ultrahigh density of hot spots, i.e., 1500 µm-2, has been obtained at the situation of only considering electromagnetic field enhancement, |E|4 >108. In fact, if combining the contributions of chemical effect and Ag element, we may obtain the density of hot spots, 1500 µm-2 with larger than ∼109 even ∼1010 SERS enhancement (see details in S2.5). Up to now, a great amount of well-controlled colloid nanostructures have been synthesized,[34–38] for example, nanoparticle dimer,[34] trimer,[35] self-orienting nanocubes,[36] planet-satellite structures[37] even lace capsule.[38] These nanostructures present EFs on the order ∼108. However, only few novel nanostructures demonstrate EFs higher than 109, such as nanostars on the molecule-decorated Au substrate,[39,40] nanodumbbells[41] and nanobridged nanogap particles.[10] It is noted that the measured EF value in this study is two or three orders of magnitude higher than the calculated one (see below). This is reasonable, because chemical enhancement and the contribution from the Ag composition in Au-Ag alloy can be involved in actual experiments. It is worth noting that the Raman signals obtained from 30 individual HAAA-NUs demonstrate a relatively large dispersion with standard deviation of 14.3% for 10−7 M and 61.8% for 10−8 M, respectively. This large dispersion could be partially from the unidentifiable aggregation of individual HAAA-NUs from optical microscopy or from the relatively small amount of molecule absorbed on individual small HAAA-NU (e.g., around hundreds CV molecules under 10−8 M concentration). To understand the interactions of an electromagnetic wave with the HAAA-NUs, the finite difference time-domain (FDTD) method was applied for calculations and the results were compared with those for larger-size mesoparticle in our previous studies.[9] The shape and configuration of the HAAA-NUs and tips have the same dimensions to match the physical measurement of the structures in the SEM images. In the present calculations, various hollow interior sizes and incident wavelengths were considered. Meanwhile, the chemical enhancement and the influence of Ag in Au-Ag alloy were not considered, and both factors are expected to increase the total SERS
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enhancement.[42 ] Figure 3a is the calculated electromagnetic field distribution of the individual HAAA-NUs at the 633 nm incident laser wavelengths. Clearly, the hot spots or maxima in |E| are located in the sharp tips and the nanogaps between adjacent tips of the HAAA-NUs. The calculated UV spectra of the HAAA-NUs according to the models in S2.6 and Figures S14S17 are shown in Figure 3b, where the UV peaks red-shift with the increase of interior hollow sizes. One can notice that the calculated UV spectra display two plasmon bands around 680 nm and 780 nm, which may be assigned to the plasmon resonance of short tips (i.e., 15 nm) and long tips (i.e., 25 nm), respectively. However, in the true picture of the HAAA-NUs, the lengths of nanotips gradually vary from 10 nm to 30 nm even longer, thus the UV spectra exhibit a broad peak as shown by the dot curve in Figure 3b. We further calculated the incident wavelength dependence of the electromagnetic field magnification for HAAA-NUs under three different wavelengths (532, 633 and 785 nm, Figure 3c). One can see that, the 785 nm incident wavelength produced the highest signal intensity with (|E|/|E0|∼140), which corresponds to a SERS enhancement (proportional to ∼|E|4) of ∼108. The small change of |E|/|E0| for different hollow interior sizes at a certain wavelength is due to the plasmon band shift as clarified above. In order to analyze the underlying physics of the observed super enhancements of HAAA-NUs relative to our previous gold sea urchin mesoparticles,[9] a FDTD model for the sea urchin-like mesoparticles was calculated (Figure 3d). According to the FDTD calculations, the sea urchin-like mesoparticles demonstrate an electromagnetic field enhancement |E|/|E0| of 8.4 at 633 nm and 15.1 at 785 nm incident wavelengths (S2.6 and Figure S16), which is much smaller than that of HAAANUs. The plasmon resonance peaks locate at a long wavelength range, i.e., >800 nm (Figure 3f). From the electromagnetic field distribution, it seems that the hot spots are located in the nanogaps between adjacent tips under excitation with a laser of wavelength less than 785 nm, implying that a small dimension of the sharp nanotips and a high nanotip density for HAAA-NUs are crucial to obtain a super SERS sensitivity. In this study, we have also investigated the interparticle effect between HAAA-NUs (Figure 3e), where the HAAA-NUs have a distance of 2 nm between tips. From the electromagnetic field distribution (Figure S17), one can find that the plasmon coupling hence additional hot spots can be created via the interparticle assembly, which is greatly beneficial to the improvement of the uniformity and reproducibility of SERS signals. From the calculated UV spectra, compared with the individual HAAANUs, the interparticle assembly may result in a red shift of the plasmon resonance peaks. The uniformity of SERS signals was evaluated via point by point scanning Raman spectra with a 633 nm excitation laser and AFM-correlated nano-Raman images (Figure 4). The SERS substrate was prepared by facilely dropping and evaporating the HAAA-NUs onto a Si surface without special treatment. The surface topography of the obtained randomly aggregated assembly is shown in Figure 4a, displaying thick multiple layers with some pin holes of several tens to ∼0.5 µm in size. From the SEM image, it is likely that the depth of the pin holes is less than ∼0.5 µm, which is still within the z resolution of the Raman spectroscopy (∼0.7 µm) hence resulting in
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COMMUNICATION Figure 3. The finite difference time domain calculations of HAAA-NUs under various incident laser wavelengths, particle sizes and configurations of interparticles. a) Calculated electromagnetic field distribution and intensity of the individual HAAA-NUs for 20 nm hollow interior at 633 nm wavelength. b) Calculated ultraviolet-visible spectra of HAAA-NUs with various hollow interior sizes (from left to right, 0, 20, 30, 40 nm respectively) in air. c) Histograms of field enhancements calculated at excitation wavelengths of 532, 633 and 785 nm, respectively. d) Calculated electromagnetic field distribution of the individual mesoparticles with 400 nm in size[9]. e) electromagnetic field distribution of two HAAA-NUs. f) Calculated extinction spectra for individual HAAA-NUs (red), two HAAA-NUs (yellow) and mesoparticle (blue) shown in d. The scale bars of electromagnetic field intensity |E|2 have been plotted on a log-scale for clarity.
small influence on Raman signal. The point by point scanning mode was carried out using a laser spot diameter of 0.8 µm (×100 objective lens with NA = 0.9) and a step-size of 1 µm (e.g., in Figure 4b) under various CV concentrations. The amount of nanoparticles irradiated for each SERS collection was around 13∼17 (Figure S18), which can be also beneficial to achieve a uniform SERS signal. Figure 4c demonstrates the 3D waterfall plot of the enhanced Raman spectra obtained from the point by point scanning of the area in Figure 4b and CV concentration of 10−8 M, displaying very uniform Raman intensity of CV peaks at 1172, 1371 and 1619 cm−1. The calculated standard deviations of the Raman intensity at CV concentrations of 10−7 M and 10−8 M are 7.8% and 9.7%, respectively, according to statistic of CV peak at 1172 cm−1 (Figure 4d). The above remarkable uniformity of SERS signals can be further supported by the AFM-correlated nano-Raman images shown in Figures 4e and 4f. As compared with the SEM image, the AFM image shown in Figure 4e displays a rough topography with more agglomerates and valleys, which could be attributed to the AFM tip effect. This argument could be reasonable because, from the nano-Raman map (Figure 4f) that excludes the influence of AFM tip, the surface shows relatively smooth and homogeneous SERS signals. The above deviation values obtained from randomly aggregated assembly are much smaller than those from individual HAAA-NUs, i.e., in Figure 2f. This result is in good agreement with our protocol suggested here, and we may achieve
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high sensitivity, uniformity and reproducibility of SERS signals, simultaneously. The graceful uniformity and reproducibility of SERS signals can be attributed to the small size of the HAAA-NUs as mentioned above as well as the interparticle and/or intraparticle effect as indicated by FDTD simulations. The great number of multiple tips within individual HAAANUs results in strong plasmonic coupling between tips. Furthermore, interparticle interactions via abundant tips may also contribute to additional and/or more hot spots, which compensate the disadvantages from the structural inhomogeneity, i.e., the pin-holes, thus a sensitive, uniform and reproducible signal can be obtained. It is notable that both the sensitivity and the reproducibility by means of HAAA-NUs, are quite unusual in contrast with some lithographic platforms such as the glass nanopillar arrays with nanogap-rich silver nanoislands,[43] silver capped nanopillar arrays,[11] hierarchical electrohydrodynamic structures,[12] Au nanofiner arrays,[13] vertically oriented sub-10 nm plasmonic nanogap arrays,[44] and so on. Even compared with recent demonstrated novel structures, for example, optical antennae,[45] gold nanofingers[14] as well as polygonal nanofiner assemblies,[46] the current results are still remarkable. This is due to the fact that HAAA-NUs can be used not only as “array-type” SERS substrate by simply dropping and evaporating HAAA-NUs onto a surface (Figure 5a-b) but also as “colloid suspension-type” SERS substrate which occupies practical applications for example in the field of in vivo and in situ biodetection and bioimaging.
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Figure 4. The SEM image of HAAA-NUs, Raman spectra and corresponding Raman intensity deviation, AFM-correlated nano-Raman image. a,b) SEM and optical images of HAAA-NUs obtained from randomly distributed assembly made via facilely dropping and evaporating HAAA-NUs on a Si substrate. The green nets represent the step size (1 µm) and region of the point by point scanning of Raman spectra. c) Raman spectra of the area as shown in b) measured by a 633 nm excitation laser using a laser spot diameter of 0.8 µm and a step-size of 1 µm with CV concentration of 10−8 M. d) the Raman intensity deviations at CV concentrations of 10−7 M (7.8%) and 10−8 M (9.7%) for randomly distributed assembly of HAAA-NUs. e,f) Atomic force micrograph and corresponding nano-Raman image of HAAA-NU assembly obtained from 532 nm excitation laser.
In the present study, by means of the hierarchical nanoparticle strategy, we have achieved a graceful uniformity, reproducibility as well as a super sensitivity of SERS signals. The remarkable uniformity and reproducibility of SERS signals can significantly depend on the particle size of the HAAA-NUs as shown in Figure 5c and Figure S11. As mentioned above, at the particle size of HAAA-NUs around 100 nm, standard deviations of the Raman intensity at CV concentrations of 10−7 M and 10−8 M are 7.8% and 9.7%, respectively. The deviations of the Raman signal are seen to increase with increasing the particle size of HAAA-NUs. At around 200 nm, the deviation values are 15% and 20% for 10−7 M and 10−8 M, respectively (Figure S19a-b). These values can be further increased to 26% and 30% for 10−7 M and 10−8 M at particle size around 350 nm, respectively (Figure S19c-d). Therefore, the key factor to simultaneously achieve super signal amplification, high uniformity and reproducibility is synthesizing hierarchical structure as small as possible in size. The current strategy demonstrates a very simple and facile process. By means of dropping and evaporating the dispersion of HAAA-NUs onto a substrate, the particles may self-assemble and become an aggregate which shows very good uniformity and reproducibility of SERS enhancement (Figure 5a-b). Furthermore, owing to the novel structures, The HAAA-NUs would have extraordinary potentials for diverse applications in various fields such as the drug carrier and delivery in cancer therapeutics, in vivo and in situ biodetection and bioimaging, ultralow concentration detection in food safety, drug security, 2436
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environment protection and so on. To examine the capability of HAAA-NUs for inspecting food and environment safety, bis (2-ethylhexyl) phthalate (DEHP), one of the most frequently used plasticizers, which pose many risks in landfills, household items, paints, drinking water, children’s toys, perfume, and so on,[47] was detected at ultralow concentrations. Figure 5d shows the Raman spectra of DEHP molecule obtained from a 785 nm excitation laser at concentrations of 10−13 M, 10−14 M and 10−15 M, respectively. The vibrational signatures of the DEHP molecule are well assigned in literatures (see details in Figure S20).[48] The spectra as shown in Figure 5d exhibit well-resolved, sharp, and enhanced Raman bands characteristic of DEHP molecule. The feature spectral peaks (652, 1000 and 1500 cm−1) are still observed even down to the concentration of 10−15 M. This demonstrates that HAAA-NUs could have tremendous scope as a simple-to-use, super sensitivity and costeffective SERS substrate. Compared with some novel structures those enable sensitivity down to fM,[49] such as the vertically aligned Au nanorod monolayer, the current dropcast multilayer may be facile to fabricate. For diverse molecule detections, the detection limit may be influenced by the absorption feature of molecules on the surface of nanostructures and various experimental conditions, such as laser wavelength, laser power and spot size, signal acquisition time, and so on. In addition, although many sensors have been exploited for the applications such as in food safety or environment protection, their transformation into robust practical tools is, to date, a challenge. An essential feature of a chemical sensor is its selectivity not only
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COMMUNICATION Figure 5. The particle size-dependent uniformity of the SERS substrate aggregated by HAAA-NUs and SERS spectra of DEHP molecules at different concentrations. a,b) The schematic representations of current strategy, the inset shows the specimens of silver seeds and HAAA-NUs. c) The particle size-dependent Raman intensity deviation of the HAAA-NUs SERS substrate at CV concentrations of 10−7 and 10−8 M. d) Raman spectra of DEHP molecules obtained from 785 nm excitation laser at concentrations of 10−13 M, 10−14 M and 10−15 M, respectively.
to isolated targets but also to mixtures that are actual analogous to the real life samples. Therefore, the multiplex detection needs a combined multidisciplinary research effort to extract and gather all these features to develop highly sensitive, quantitative and reliable platforms that could compete with current commercially available sensor devices and applications.[50] In summary, we have developed a hierarchical nanoparticles strategy to simultaneously gain super Raman signal amplification, high uniformity and reproducibility. A new type of hierarchical nanoparticles, i.e., hollow Au-Ag alloy nanourchins, was synthesized via a seed-mediated growth by means of a galvanic reaction followed by chemical reduction. The HAAA-NUs display ultrahigh density of hot spots owing to a great number of sharp tips and roots, e.g., more than 100 tips within individual ∼100 nm nanourchins, thus show a super Raman signal amplification with an enhancement factor up to 109 in magnitude for individual HAAA-NUs under 633 nm excitation wavelength. Importantly, a graceful uniformity and reproducibility of SERS signal may be gained when the HAAA-NUs are randomly distributed via facilely dropping and evaporating HAAA-NUs onto a substrate. The SERS signal dispersion highly depends on the particle size of HAAA-NUs, where a small standard deviation, e.g., 7.8%, has been obtained at the particle size around 100 nm via point by point scanning HAAA-NUs-aggregated SERS substrate. By increasing the particle size of HAAA-NUs, e.g., to 200 or >300 nm, the deviation value becomes large. Therefore, the key to success of the current strategy in gaining an ultrahigh
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sensitivity, good uniformity and reproducibility is the synthesis of hierarchical nanostructures with high hot spots density, meanwhile, as small as possible in size. The FDTD calculations indicate that the intraparticle effect, i.e., ultrahigh density sharp tips, contribute to the super electromagnetic field enhancement. In the meantime, the interparticle effect owing to the electromagnetic field coupling is very useful to improve the uniformity and reproducibility of SERS signals. As an example, the HAAA-NUs-aggregated SERS substrate was utilized to detect DEHP, a high risk plasticizer in food and environment fields, at very low target concentration down to 1 fM. Finally, the current strategy offers a robust avenue to generate a new class of SERS substrate with super sensitivity, improved uniformity and reproducibility, simultaneously. The HAAA-NUs, owing to the novel structures, may open extraordinary potentials, for example, in drug carrier, in vitro bioassay, in situ probe tracking in cells, in vivo/ex vivo Raman imaging, particle-based drug carrier and photothermal therapeutics.
Experimental Section Synthesis of HAAA-NUs: The HAAA-NUs were synthesized via a seedmediated growth route using L-Dopa as reduction agent (see details in Methods and Supporting Infromation, S1-S2). During the synthesis of HAAA-NUs, galvanic reaction was firstly happened between HAuCl4 and Ag seeds, followed by a reduction reaction between L-Dopa and HAuCl4 and Ag+ (the by-product of the galvanic reaction). As a result,
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www.MaterialsViews.com HAAA-NUs were synthesized. Briefly, for the synthesis of sub-100nm HAAA-NUs, HAuCl4 aqueous solution (2.4 mL, 10 mM) was mixed with DI water (4.3 mL) in a glass vial (25 mL). Then, the vial was put into a water bath of around 15 °C under magnetic stirring of 300 r.p.m. After 10 min, 0.9 mL of the as-prepared silver seeds (25 nm) were fed into the system, followed by the addition of L-DOPE aqueous solution (2.4 mL, 10 mM). After 1 min, the spin rate was slowed down to 100 r.p.m. As L-Dopa was added, the transparent yellow solution immediately turned opaque black-green, followed by deep black color. After 10 min, the product was collected by centrifugation (6000 rpm for 1 min) and washed with HCOOH (500 mM) once, ammonia solution once and DI water twice. Characterization: The products were characterized via various routes, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) as well as the ultraviolet-visible spectra. To investigate Raman features, the HAAA-AUs were measured with confocal microprobe Raman spectrometer with various excitation laser wavelengths including 532 nm, 633 nm and 785 nm, respectively. Crystal violet (CV) and bis (2-ethylhexyl) phthalate (DEHP) dye molecules were used to evaluate the sensitivity and uniformity of SERS signal. AFM-correlated Nano-Raman spectroscopy was used to study the aggregate dependent of Raman enhancement. The three-dimensional finite difference time-domain (FDTD) calculation was also used to study the interaction between light and nanostructures.
Supporting Information Detailed synthesis methods, FDTD calculation and characterization data from SEM, TEM, XPS, and SERS measurements. Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements J. X. Fang was supported by National Natural Science Foundation of China (No 51171139), Tengfei Talent Project of Xi’an Jiaotong University, the New Century Excellent Talents in University (NCET), Scientific New Star Program in Shann Xi Province (No.2012KJXX-03), Doctoral Fund of Ministry of Education of China (Nos.20110201120039, 20130201110032) and the Fundamental Research Funds for the Central Universities (No. 08142008). Z. Y. Li was supported by the 973 Program of China (No. 2013CB632704). Q.H. Xiong gratefully acknowledges the strong support from Singapore National Research Foundation via a fellowship grant (NRF-RF2009-06) and Ministry of Education via two Tier 2 grants (MOE2011-T2-2-051 and MOE2011-T2-2-085)
Received: October 14, 2013 Revised: November 14, 2013 Published online: January 21, 2014
[1] J. F. Li, X. D. Tian, S. B. Li, J. R. Anema, Z. L. Yang, Y. Ding, Y. F. Wu, Y. M. Zeng, Q. Z. Chen, B. Ren, Z. L. Wang, Z. Q. Tian, Nature Protocols 2013, 8, 52–65. [2] T. Y. Liu, K. T. Tsai, H. H. Wang, Y. Chen, Y. H. Chen, Y. C. Chao, H. H. Chang, C. H. Lin, J. K. Wang, Y. L. Wang, Nature Communications 2011, 2, 538–545. [3] P. H. C. Camargo, M. Rycenga, L. Au, Y. N. Xia, Angew. Chem. Int. Ed. 2009, 48, 2180–2184. [4] L. Guerrini, D. Graham, Chem. Soc. Rev. 2012, 41, 7085–7107. [5] H. Lee, S. You, P. V. Pikhitsa, J. Kim, S. Kwon, C. G. Woo, M. Choi, Nano Lett. 2011, 11, 119–124.
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[6] E. C. L. Ru, P. Etchegoin, Principles of surface enhanced Raman spectroscopy and related plasmonic effects, Elsevier publishing, 2008, page 22. [7] H. Wang, N. J. Halas, Adv. Mater. 2008, 20, 820–825. [8] H. Y. Liang, Z. P.Li, W. Z. Wang, Y. S. Wu, H. X. Xu, Adv. Mater. 2009, 21, 4614–4618. [9] J. X. Fang, S. Y. Du, S. Lebedkin, Z. Y. Li, R. Kruk, M. Kappes, H. Hahn, Nano Lett. 2010, 10, 5006–5013. [10] D. K. Lim, K. S. Jeon, J. H. Hwang, H. Kim, S. Kwon, Y. D. Suh, J. M. Nam, Nature Nanotechnology 2011, 6, 452–460. [11] M. S. Schmidt, J. Hübner, A. Boisen, Adv. Mater. 2012, 24, OP11–OP18. [12] P. Goldberg-Oppenheimer, S. Mahajan, U. Steiner, Adv. Mater. 2012, 24, OP175–OP180. [13] A. Kim, F. S. Ou, D. A. A. Ohlberg, M. Hu, R. S. Williams, Z. Y. Li, J. Am. Chem. Soc. 2011, 133, 8234–8239. [14] M. Hu, F. S. Ou, W. Wu, I. Naumov, X. M. Li, A. M. Bratkovsky, R. S. Williams, Z. Y. Li, J. Am. Chem. Soc. 2010, 132, 12820–12822. [15] J. X. Fang, B. J. Ding, H. Gleiter, Chem. Soc. Rev. 2011, 40, 5347–5360. [16] J. X. Fang, P. M. Leufke, R. Kruk, D. Wang, T. Scherer, H. Hahn, Nanotoday 2010, 5, 175. [17] J. X. Fang, H. J. You, P. Kong, Y. Yi, X. P. Song, B. J. Ding, Cryst. Growth Des. 2007, 7, 864–867. [18] B. Wiley, T. Herricks, Y. Sun, Y. N. Xia, Nano Lett. 2004, 4, 1733–1739. [19] C. L. Lu, K. S. Prasad, H. L. Wu, J. A. Ho, M. H. Huang, J. Am. Chem. Soc. 2010, 132, 14546–14553. [20] D. Ferrer, A. Torres-Castro, X. Gao, S. Sepulveda-Guzman, U. Ortiz-Mendez, M. Jose-Yacaman, Nano Lett. 2007, 7, 1701–1705. [21] X. M. Lu, H. Y. Tuan, J. Y. Chen, Z. Y. Li, B. A. Korgel, Y. N. Xia, J. Am. Chem. Soc. 2007, 129, 1733–1742. [22] K. G. M. Chow, J. Nanopart. Res. 2012, 14, 1186. [23] Z. Liu, F. L. Zhang, Z. B. Yang, H. J. You, C. F. Tian, Z. Y. Li, J. X. Fang, J. Mater. Chem. C 2013, 1, 5567–5576. [24] L. Rodríguez-Lorenzo, R. D. L. Rica, R. A. Álvarez-Puebla, L. M. Liz-Marzán, M. M. Stevens, Nature Materials 2012, 11, 604–607. [25] H. Yuan, C. G. Khoury, H. Hwang, C. M. Wilson, G. A. Grant, T. Vo-Dinh, Nanotechnology 2012, 23, 075102–075111. [26] T. K. Sau, A. L. Rogach, M. Döblinger, J. Feldmann, Small 2011, 7, 2188–2194. [27] D. Y. Kim, T. Yu, E. C. Cho, Y. Y. Ma, O. O. Park, Y. N. Xia, Angew. Chem. Int. Ed. 2011, 50, 6328–6331. [28] Z. D. Wang, J. Q. Zhang, J. Ekman, P. J. Kenis, Y. Lu, Nano Lett. 2010, 10, 1886–1891. [29] M. Rycenga, C. M. Cobley, J. Zeng, W. Y. Li, C. H. Moran, Q. Zhang, D. Qin, Y. N. Xia, Chem. Rev. 2011, 111, 3669–3712. [30] F. G. Xu, K. Cui, Y. J. Sun, C. L. Guo, Z. L. Liu, Y. Zhang, Y. Shi, Z. Li, Talanta 2010, 82, 1845–1852. [31] J. X. Fang, S. Y. Liu, Z. Y. Li, Biomaterials 2011, 32, 4877–4884. [32] C. F. Tian, C. H. Ding, S. Y. Liu, S. C. Yang, X. P. Song, B. J. Ding, Z. Y. Li, J. X. Fang, ACS Nano. 2011, 5, 9442–9449. [33] W. B. Cai, B. Ren, X. Q. Li, C. X. She, F. M. Liu, X. W. Cai, Z. Q. Tian, Surface Science 1998, 406, 9. [34] G. Chen, Y. Wang, M. X. Yang, J. Xu, S. J. Goh, M. Pan, H. Y. Chen, J. Am. Chem. Soc. 2010, 132, 3644–3645. [35] K. L. Wustholz, A. I. Henry, J. M. Mcmahon, R. G. Freeman, N. Valley, M. E. Piotti, M. J. Natan, G. C. Schatz, R. P. Van Duyne, J. Am. Chem. Soc. 2010, 132, 10903–10910. [36] B. Gao, G. Arya, A. R. Tao, Nature Nanotech. 2012, 7, 433–437. [37] N. Gandra, A. Abbas, L. M. Tian, S. Singamaneni, Nano Lett. 2012, 12, 2645–2651.
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[45] D. X. Wang, W. Q. Zhu, Y. Z. Chu, K. B. Crozier, Adv. Mater. 2012, 24, 4376–4380. [46] F. S. Ou, M. Hu, I. Naumov, A. Kim, W. Wu, A. M. Bratkovsky, X. M. Li, R. S. Williams, Z. Y. Li, Nano Lett. 2011, 11, 2538– 2542. [47] J. M. Hankett, C. Zhang, Z. Chen, Langmuir 2012, 28, 4654– 4662. [48] J. R. Ferraro, K. Nakamoto, C. W. Brown, Journal of Molecular Structure 1989, 212, 169–186. [49] B. Peng, G. Y Li, D. H. Li, S. Dodson, Q. Zhang, J. Zhang, Y. H. Lee, H. V. Demir, X. Y. Ling, Q. H. Xiong, ACS Nano 2013, 7, 5993–6000. [50] R. A. Alvarez-Puebla, L. M. Liz-Marzan, Angew. Chem. Int. Ed. 2012, 51, 11214–11223.
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[38] M. Yang, R. Alvarez-Puebla, H. S. Kim, P. Aldeanueva-Potel, L. M. Liz-Marzan, N. A. Kotov, Nano Lett. 2010, 10, 4013–4019. [39] L. Rodrı´guez-Lorenzo, R. A. Alvarez-Puebla, I. Pastoriza-Santos, S. Mazzucco, O. Stephan, M. Koviak, L. M. Liz-Marzan, F. J. Garcia de Abajo, J. Am. Chem. Soc. 2009, 131, 4616–4618. [40] N. Yamamoto, S. Ohtani, F. J. García de Abajo, Nano Lett. 2011, 11, 91–95. [41] D. K. Lim, K. S. Jeon, H. M. Kim, J. M. Nam, Y. D. Suh, Nature Materials 2010, 9, 60–67. [42] J. F. Li, Y. F. Huang, Y. Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y. Zhou, D. Y. Wu, B. Ren, Z. L. Wang, Z. Q. Tian, Nature 2010, 464, 392–395. [43] Y. J. Oh, K. H. Jeong, Adv. Mater. 2012, 24, 2234–2237. [44] H. Im, K. C. Bantz, N. C. Lindquist, C. L. Haynes, S. H. Oh, Nano Lett. 2010, 10, 2231–2236.
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Zhen Liu, Zhongbo Yang, Bo Peng, Cuong Cao, Chao Zhang, Hongjun You, Qihua Xiong,* Zhiyuan Li,* and Jixiang Fang* Surface-enhanced Raman scattering (SERS) spectroscopy is one of a few techniques that are capable of ultimately detecting matter at single molecule scale.[1] As a powerful analysis technique, SERS-based signal detection and molecular identification will provide an unprecedented opportunity for investigations in biomedicine, life science, analytical chemistry, etc.[2] Since the first demonstration of single molecule detection via SERS, great efforts have been devoted to the fabrication of a variety of SERS substrates. Ideally, a SERS substrate should possess a super signal amplification, high uniformity and reproducibility. However, up to now, quite few techniques can simultaneously achieve the aforementioned requirements, hence impede the use of SERS sensors in both laboratories and mobile applications. To date, exciting successes have been achieved using colloid chemistry strategy to well define nanogaps and thus achieve super sensitivity of SERS enhancement.[3] However, preventing the formation of large conglomerations of nanoparticles before the analyte is introduced, at the same time controlling the gap size via random aggregation of colloid nanoparticles thus achieving reproducible and uniform hot spots remains challenging.[4] Patterning techniques, e.g.,electron-beam lithography, nanosphere lithography, or nanoimprinting, indeed can improve the uniformity and reproducibility of the SERS signal at a certain molecule concentration.[5] However, creating a high density of hotspots via the aforementioned lithographic protocols is rather difficult. Therefore, probe molecules might not land inside hot spot regions even if they could be precisely prefabricated particularly at an ultralow molecule concentration. The current dilemma is that Z. Liu, Z. B. Yang, Dr. H. J. You, Prof. J. X. Fang State Key Laboratory for Mechanical Behavior of Materials School of Science, Xi’an Jiaotong University Shann Xi, 710049, P. R. China E-mail: [email protected] Dr. B. Peng, Dr. C. Cao, Prof. Q. H. Xiong Division of Physics and Applied Physics, School of Physical and Mathematical Sciences Nanyang Technological University Singapore, 637371 E-mail:[email protected] C. Zhang, Prof. Z. Y. Li Institute of Physics Chinese Academy of Sciences Beijing, 100080, P. R. China E-mail: [email protected]
DOI: 10.1002/adma.201305106
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Highly Sensitive, Uniform, and Reproducible SurfaceEnhanced Raman Spectroscopy from Hollow Au-Ag Alloy Nanourchins
the more uniform enhancement across a SERS substrate is, i.e., the better the reproducibility, the lower the enhancement tends to be and vice versa.[6] In order to maximize the sensitivity, uniformity and reproducibility of the SERS signal, some SERS studies have been pioneered, such as single-particle SERS (sp-SERS) [7–10] and 3D SERS substrates. [11–14] By introducing abundant nanogaps or nanobridges, sharp protrusions or crevices, into the mesoscale particles, a high enhancement factor (EF) at 107–108 and an improved uniformity and reproducibility of SERS signal have been obtained. The architecture of 3D hierarchical arrays may create densely packed metal nanostructures with plentiful hot spots, thus may maximally meet the above requirements of an ideal SERS substrate. In our previous studies on sp-SERS of gold urchin-like mesoparticles,[9] owing to the anisotropic nature of the multiple tips, the uniformity of the sp-SERS signal shows not so high. Furthermore, the particle size is around ∼1µm, which is an embarrassed situation because the laser spot is generally comparable to this size. This makes it difficult to obtain the real signal from the whole mesoparticles and hence also restricts the uniformity. Based on our previous studies on mesocrystals,[15–17] here, a strategy using highly roughened hierarchical nanoparticles is suggested to both increase the sensitivity and improve the uniformity and reproducibility of the SERS substrate by means of simply dropping the hierarchical nanoparticles onto a substrate. Differing from the conventional colloid nanoparticles, which show smooth surface and low plasmonic effect, the hierarchical nanoparticles should be synthesized as small as possible in size, but with complex structure combined by enhanced nanogaps or nanotips hence increased hot spot density for individual nanoparticle. Herein, we show a highly sensitive, uniform and reproducible SERS performance obtained from the randomly aggregated hollow Au-Ag alloy nanourchins (HAAA-NUs). The individual HAAA-NU nanoparticle is composed of many sharp protrusions and nanogaps, e.g., around 70–100 tips on 100 nm HAAA-NUs. These nanotips, serving as nanoantenna, may dramatically increase the excitation cross section and the electromagnetic field enhancement. The individual HAAA-NU nanoparticle demonstrates ultrahigh density of hot spots at the level of 1500 µm–2, hence shows a super enhancement capability at ∼109 magnitude. Importantly, the overlapping between individual HAAA-NUs in the assembly may create additional hot spots. Moreover, around 30-50 HAAA-NU nanoparticles can be included within a laser spot, i.e., ∼1 µm in diameter. Thus, the assembly of HAAA-NUs displays a high uniformity and
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Figure 1. UV-Vis spectra, SEM, and TEM images of HAAA-NUs. a) UV-Vis spectra of silver seeds with various particle sizes (20–50 nm) measured in DI water. b) UV-Vis spectra of HAAA-NUs with various particle sizes (80 – 300 nm) measured in DI water. c) A scheme of the HAAA-NU particle growth. d) TEM images of silver seed and HAAA-NU correlated with structural evolution. The scale bar for Ag seed is 10 nm, and the scale bars for others are 50 nm, respectively. e) SEM image of HAAA-NUs. f) Magnified TEM image of HAAA-NUs. g) TEM image of HAAA-NUs. The white areas marked by arrows in the center of each particle imply the hollow interior.
reproducibility, e.g., 100 tips within individual ∼100 nm nanourchins (Figure 1e). The tips serving as nanoantennas and the root of tips as nanogaps (Figure 1e, 1f) may display ultrahigh density of hot spots, e.g.,1500 µm–2 (|E|4 >108, ignoring the contribution of chemical effect and Ag element), thus present a high SERS sensitivity (see below). 2) The formation of Au-Ag alloy (Figure S4-S6), compared with pure
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Au nanostructures,[29,30] may increase the SERS sensitivity. Moreover, the Au-Ag nanourchins present the better chemical stability than the pure Ag nanostructures, thus improving the reliability of the SERS substrate. 3) There is wide tunable capability of localized surface plasmon resonances (LSPRs). In this synthesis, we can also change sizes of the Ag seeds to tune the hollow interior size of the HAAA-NUs (Figure 1g), which can be used to not only tune the LSPRs but also exploit, for example, the drug carrier and delivery in cancer therapeutics. To prove that both high sensitivity and high reproducibility of SERS spectroscopy could be achieved via the current strategy, a systematic SERS evaluation of the HAAA-NUs were carried out using the crystal violet (CV) molecule as Raman probe. Firstly, the sp-SERS performance was analyzed using 532 nm and 633 nm incident wavelengths. Figure 2a shows the UV spectra of the HAAA-NUs measured in air with particle sizes of 80, 100, 150, 200 and 300 nm, respectively. As compared with the results measured in DI water, the UV spectra obtained in air show obvious blue shifts. Figure 2b presents the AFM-correlated nano-Raman images measured with a 532 nm excitation laser for various types of aggregations of the HAAA-NUs including single particles (1, 2, 3 in Figure 2b), trimer (7 in Figure 2b), tetrad (5 in Figure 2b) and several particles (4, 6 in Figure 2b). The corresponding SERS map shows good correlation with the AFM image, where only two particles around particles 2# were not recognized by the incident laser. The unidentified particles could be dusts or small nanoparticles because the particle height from the AFM image was around ∼50 nm, significantly smaller than the size of HAAA-NUs (Figure 2c).
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As a comparison, single particle of HAAA-NUs (particle 1#) was measured and displayed in Figure 2c, which was around ∼100 nm in height. Figure 2d illustrates the SERS spectra taken from the particles Nos. 1 to 7 in Figure 2b, which are highly consistent with the aggregated states of HAAA-NUs. The single particles show relatively weak Raman signals (∼50 counts) and the aggregation consisting of several HAAA-NUs presents relatively strong signals with around ∼200 counts, while the trimer and tetrad indicate a middle enhancement with ∼100 counts. From the UV spectra of HAAA-NUs measured in both DI water and air, a long incident wavelength is helpful to achieve a large enhancement of Raman signals. Thus, we have also measured the sp-SERS by using a 633 nm incident laser with CV molecule concentrations ranging from 10−7 to 10−11 M. From Figure 2e, the SERS spectra can still be identified at the CV concentration of 10−10 M for individual HAAA-NUs. According to the procedure and assumptions used in our previous experiments[9,31,32] and also in Ref [33] for the CV molecules, the average enhancement factor (EF) for the individual HAAANUs is estimated about ∼1.6 × 109 under the 633 nm excitation wavelength (see details in S2.4). According to the statistic of hot spots obtained via finite difference time domain (FDTD) simulations, an ultrahigh density of hot spots, i.e., 1500 µm-2, has been obtained at the situation of only considering electromagnetic field enhancement, |E|4 >108. In fact, if combining the contributions of chemical effect and Ag element, we may obtain the density of hot spots, 1500 µm-2 with larger than ∼109 even ∼1010 SERS enhancement (see details in S2.5). Up to now, a great amount of well-controlled colloid nanostructures have been synthesized,[34–38] for example, nanoparticle dimer,[34] trimer,[35] self-orienting nanocubes,[36] planet-satellite structures[37] even lace capsule.[38] These nanostructures present EFs on the order ∼108. However, only few novel nanostructures demonstrate EFs higher than 109, such as nanostars on the molecule-decorated Au substrate,[39,40] nanodumbbells[41] and nanobridged nanogap particles.[10] It is noted that the measured EF value in this study is two or three orders of magnitude higher than the calculated one (see below). This is reasonable, because chemical enhancement and the contribution from the Ag composition in Au-Ag alloy can be involved in actual experiments. It is worth noting that the Raman signals obtained from 30 individual HAAA-NUs demonstrate a relatively large dispersion with standard deviation of 14.3% for 10−7 M and 61.8% for 10−8 M, respectively. This large dispersion could be partially from the unidentifiable aggregation of individual HAAA-NUs from optical microscopy or from the relatively small amount of molecule absorbed on individual small HAAA-NU (e.g., around hundreds CV molecules under 10−8 M concentration). To understand the interactions of an electromagnetic wave with the HAAA-NUs, the finite difference time-domain (FDTD) method was applied for calculations and the results were compared with those for larger-size mesoparticle in our previous studies.[9] The shape and configuration of the HAAA-NUs and tips have the same dimensions to match the physical measurement of the structures in the SEM images. In the present calculations, various hollow interior sizes and incident wavelengths were considered. Meanwhile, the chemical enhancement and the influence of Ag in Au-Ag alloy were not considered, and both factors are expected to increase the total SERS
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enhancement.[42 ] Figure 3a is the calculated electromagnetic field distribution of the individual HAAA-NUs at the 633 nm incident laser wavelengths. Clearly, the hot spots or maxima in |E| are located in the sharp tips and the nanogaps between adjacent tips of the HAAA-NUs. The calculated UV spectra of the HAAA-NUs according to the models in S2.6 and Figures S14S17 are shown in Figure 3b, where the UV peaks red-shift with the increase of interior hollow sizes. One can notice that the calculated UV spectra display two plasmon bands around 680 nm and 780 nm, which may be assigned to the plasmon resonance of short tips (i.e., 15 nm) and long tips (i.e., 25 nm), respectively. However, in the true picture of the HAAA-NUs, the lengths of nanotips gradually vary from 10 nm to 30 nm even longer, thus the UV spectra exhibit a broad peak as shown by the dot curve in Figure 3b. We further calculated the incident wavelength dependence of the electromagnetic field magnification for HAAA-NUs under three different wavelengths (532, 633 and 785 nm, Figure 3c). One can see that, the 785 nm incident wavelength produced the highest signal intensity with (|E|/|E0|∼140), which corresponds to a SERS enhancement (proportional to ∼|E|4) of ∼108. The small change of |E|/|E0| for different hollow interior sizes at a certain wavelength is due to the plasmon band shift as clarified above. In order to analyze the underlying physics of the observed super enhancements of HAAA-NUs relative to our previous gold sea urchin mesoparticles,[9] a FDTD model for the sea urchin-like mesoparticles was calculated (Figure 3d). According to the FDTD calculations, the sea urchin-like mesoparticles demonstrate an electromagnetic field enhancement |E|/|E0| of 8.4 at 633 nm and 15.1 at 785 nm incident wavelengths (S2.6 and Figure S16), which is much smaller than that of HAAANUs. The plasmon resonance peaks locate at a long wavelength range, i.e., >800 nm (Figure 3f). From the electromagnetic field distribution, it seems that the hot spots are located in the nanogaps between adjacent tips under excitation with a laser of wavelength less than 785 nm, implying that a small dimension of the sharp nanotips and a high nanotip density for HAAA-NUs are crucial to obtain a super SERS sensitivity. In this study, we have also investigated the interparticle effect between HAAA-NUs (Figure 3e), where the HAAA-NUs have a distance of 2 nm between tips. From the electromagnetic field distribution (Figure S17), one can find that the plasmon coupling hence additional hot spots can be created via the interparticle assembly, which is greatly beneficial to the improvement of the uniformity and reproducibility of SERS signals. From the calculated UV spectra, compared with the individual HAAANUs, the interparticle assembly may result in a red shift of the plasmon resonance peaks. The uniformity of SERS signals was evaluated via point by point scanning Raman spectra with a 633 nm excitation laser and AFM-correlated nano-Raman images (Figure 4). The SERS substrate was prepared by facilely dropping and evaporating the HAAA-NUs onto a Si surface without special treatment. The surface topography of the obtained randomly aggregated assembly is shown in Figure 4a, displaying thick multiple layers with some pin holes of several tens to ∼0.5 µm in size. From the SEM image, it is likely that the depth of the pin holes is less than ∼0.5 µm, which is still within the z resolution of the Raman spectroscopy (∼0.7 µm) hence resulting in
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COMMUNICATION Figure 3. The finite difference time domain calculations of HAAA-NUs under various incident laser wavelengths, particle sizes and configurations of interparticles. a) Calculated electromagnetic field distribution and intensity of the individual HAAA-NUs for 20 nm hollow interior at 633 nm wavelength. b) Calculated ultraviolet-visible spectra of HAAA-NUs with various hollow interior sizes (from left to right, 0, 20, 30, 40 nm respectively) in air. c) Histograms of field enhancements calculated at excitation wavelengths of 532, 633 and 785 nm, respectively. d) Calculated electromagnetic field distribution of the individual mesoparticles with 400 nm in size[9]. e) electromagnetic field distribution of two HAAA-NUs. f) Calculated extinction spectra for individual HAAA-NUs (red), two HAAA-NUs (yellow) and mesoparticle (blue) shown in d. The scale bars of electromagnetic field intensity |E|2 have been plotted on a log-scale for clarity.
small influence on Raman signal. The point by point scanning mode was carried out using a laser spot diameter of 0.8 µm (×100 objective lens with NA = 0.9) and a step-size of 1 µm (e.g., in Figure 4b) under various CV concentrations. The amount of nanoparticles irradiated for each SERS collection was around 13∼17 (Figure S18), which can be also beneficial to achieve a uniform SERS signal. Figure 4c demonstrates the 3D waterfall plot of the enhanced Raman spectra obtained from the point by point scanning of the area in Figure 4b and CV concentration of 10−8 M, displaying very uniform Raman intensity of CV peaks at 1172, 1371 and 1619 cm−1. The calculated standard deviations of the Raman intensity at CV concentrations of 10−7 M and 10−8 M are 7.8% and 9.7%, respectively, according to statistic of CV peak at 1172 cm−1 (Figure 4d). The above remarkable uniformity of SERS signals can be further supported by the AFM-correlated nano-Raman images shown in Figures 4e and 4f. As compared with the SEM image, the AFM image shown in Figure 4e displays a rough topography with more agglomerates and valleys, which could be attributed to the AFM tip effect. This argument could be reasonable because, from the nano-Raman map (Figure 4f) that excludes the influence of AFM tip, the surface shows relatively smooth and homogeneous SERS signals. The above deviation values obtained from randomly aggregated assembly are much smaller than those from individual HAAA-NUs, i.e., in Figure 2f. This result is in good agreement with our protocol suggested here, and we may achieve
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high sensitivity, uniformity and reproducibility of SERS signals, simultaneously. The graceful uniformity and reproducibility of SERS signals can be attributed to the small size of the HAAA-NUs as mentioned above as well as the interparticle and/or intraparticle effect as indicated by FDTD simulations. The great number of multiple tips within individual HAAANUs results in strong plasmonic coupling between tips. Furthermore, interparticle interactions via abundant tips may also contribute to additional and/or more hot spots, which compensate the disadvantages from the structural inhomogeneity, i.e., the pin-holes, thus a sensitive, uniform and reproducible signal can be obtained. It is notable that both the sensitivity and the reproducibility by means of HAAA-NUs, are quite unusual in contrast with some lithographic platforms such as the glass nanopillar arrays with nanogap-rich silver nanoislands,[43] silver capped nanopillar arrays,[11] hierarchical electrohydrodynamic structures,[12] Au nanofiner arrays,[13] vertically oriented sub-10 nm plasmonic nanogap arrays,[44] and so on. Even compared with recent demonstrated novel structures, for example, optical antennae,[45] gold nanofingers[14] as well as polygonal nanofiner assemblies,[46] the current results are still remarkable. This is due to the fact that HAAA-NUs can be used not only as “array-type” SERS substrate by simply dropping and evaporating HAAA-NUs onto a surface (Figure 5a-b) but also as “colloid suspension-type” SERS substrate which occupies practical applications for example in the field of in vivo and in situ biodetection and bioimaging.
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Figure 4. The SEM image of HAAA-NUs, Raman spectra and corresponding Raman intensity deviation, AFM-correlated nano-Raman image. a,b) SEM and optical images of HAAA-NUs obtained from randomly distributed assembly made via facilely dropping and evaporating HAAA-NUs on a Si substrate. The green nets represent the step size (1 µm) and region of the point by point scanning of Raman spectra. c) Raman spectra of the area as shown in b) measured by a 633 nm excitation laser using a laser spot diameter of 0.8 µm and a step-size of 1 µm with CV concentration of 10−8 M. d) the Raman intensity deviations at CV concentrations of 10−7 M (7.8%) and 10−8 M (9.7%) for randomly distributed assembly of HAAA-NUs. e,f) Atomic force micrograph and corresponding nano-Raman image of HAAA-NU assembly obtained from 532 nm excitation laser.
In the present study, by means of the hierarchical nanoparticle strategy, we have achieved a graceful uniformity, reproducibility as well as a super sensitivity of SERS signals. The remarkable uniformity and reproducibility of SERS signals can significantly depend on the particle size of the HAAA-NUs as shown in Figure 5c and Figure S11. As mentioned above, at the particle size of HAAA-NUs around 100 nm, standard deviations of the Raman intensity at CV concentrations of 10−7 M and 10−8 M are 7.8% and 9.7%, respectively. The deviations of the Raman signal are seen to increase with increasing the particle size of HAAA-NUs. At around 200 nm, the deviation values are 15% and 20% for 10−7 M and 10−8 M, respectively (Figure S19a-b). These values can be further increased to 26% and 30% for 10−7 M and 10−8 M at particle size around 350 nm, respectively (Figure S19c-d). Therefore, the key factor to simultaneously achieve super signal amplification, high uniformity and reproducibility is synthesizing hierarchical structure as small as possible in size. The current strategy demonstrates a very simple and facile process. By means of dropping and evaporating the dispersion of HAAA-NUs onto a substrate, the particles may self-assemble and become an aggregate which shows very good uniformity and reproducibility of SERS enhancement (Figure 5a-b). Furthermore, owing to the novel structures, The HAAA-NUs would have extraordinary potentials for diverse applications in various fields such as the drug carrier and delivery in cancer therapeutics, in vivo and in situ biodetection and bioimaging, ultralow concentration detection in food safety, drug security, 2436
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environment protection and so on. To examine the capability of HAAA-NUs for inspecting food and environment safety, bis (2-ethylhexyl) phthalate (DEHP), one of the most frequently used plasticizers, which pose many risks in landfills, household items, paints, drinking water, children’s toys, perfume, and so on,[47] was detected at ultralow concentrations. Figure 5d shows the Raman spectra of DEHP molecule obtained from a 785 nm excitation laser at concentrations of 10−13 M, 10−14 M and 10−15 M, respectively. The vibrational signatures of the DEHP molecule are well assigned in literatures (see details in Figure S20).[48] The spectra as shown in Figure 5d exhibit well-resolved, sharp, and enhanced Raman bands characteristic of DEHP molecule. The feature spectral peaks (652, 1000 and 1500 cm−1) are still observed even down to the concentration of 10−15 M. This demonstrates that HAAA-NUs could have tremendous scope as a simple-to-use, super sensitivity and costeffective SERS substrate. Compared with some novel structures those enable sensitivity down to fM,[49] such as the vertically aligned Au nanorod monolayer, the current dropcast multilayer may be facile to fabricate. For diverse molecule detections, the detection limit may be influenced by the absorption feature of molecules on the surface of nanostructures and various experimental conditions, such as laser wavelength, laser power and spot size, signal acquisition time, and so on. In addition, although many sensors have been exploited for the applications such as in food safety or environment protection, their transformation into robust practical tools is, to date, a challenge. An essential feature of a chemical sensor is its selectivity not only
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COMMUNICATION Figure 5. The particle size-dependent uniformity of the SERS substrate aggregated by HAAA-NUs and SERS spectra of DEHP molecules at different concentrations. a,b) The schematic representations of current strategy, the inset shows the specimens of silver seeds and HAAA-NUs. c) The particle size-dependent Raman intensity deviation of the HAAA-NUs SERS substrate at CV concentrations of 10−7 and 10−8 M. d) Raman spectra of DEHP molecules obtained from 785 nm excitation laser at concentrations of 10−13 M, 10−14 M and 10−15 M, respectively.
to isolated targets but also to mixtures that are actual analogous to the real life samples. Therefore, the multiplex detection needs a combined multidisciplinary research effort to extract and gather all these features to develop highly sensitive, quantitative and reliable platforms that could compete with current commercially available sensor devices and applications.[50] In summary, we have developed a hierarchical nanoparticles strategy to simultaneously gain super Raman signal amplification, high uniformity and reproducibility. A new type of hierarchical nanoparticles, i.e., hollow Au-Ag alloy nanourchins, was synthesized via a seed-mediated growth by means of a galvanic reaction followed by chemical reduction. The HAAA-NUs display ultrahigh density of hot spots owing to a great number of sharp tips and roots, e.g., more than 100 tips within individual ∼100 nm nanourchins, thus show a super Raman signal amplification with an enhancement factor up to 109 in magnitude for individual HAAA-NUs under 633 nm excitation wavelength. Importantly, a graceful uniformity and reproducibility of SERS signal may be gained when the HAAA-NUs are randomly distributed via facilely dropping and evaporating HAAA-NUs onto a substrate. The SERS signal dispersion highly depends on the particle size of HAAA-NUs, where a small standard deviation, e.g., 7.8%, has been obtained at the particle size around 100 nm via point by point scanning HAAA-NUs-aggregated SERS substrate. By increasing the particle size of HAAA-NUs, e.g., to 200 or >300 nm, the deviation value becomes large. Therefore, the key to success of the current strategy in gaining an ultrahigh
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sensitivity, good uniformity and reproducibility is the synthesis of hierarchical nanostructures with high hot spots density, meanwhile, as small as possible in size. The FDTD calculations indicate that the intraparticle effect, i.e., ultrahigh density sharp tips, contribute to the super electromagnetic field enhancement. In the meantime, the interparticle effect owing to the electromagnetic field coupling is very useful to improve the uniformity and reproducibility of SERS signals. As an example, the HAAA-NUs-aggregated SERS substrate was utilized to detect DEHP, a high risk plasticizer in food and environment fields, at very low target concentration down to 1 fM. Finally, the current strategy offers a robust avenue to generate a new class of SERS substrate with super sensitivity, improved uniformity and reproducibility, simultaneously. The HAAA-NUs, owing to the novel structures, may open extraordinary potentials, for example, in drug carrier, in vitro bioassay, in situ probe tracking in cells, in vivo/ex vivo Raman imaging, particle-based drug carrier and photothermal therapeutics.
Experimental Section Synthesis of HAAA-NUs: The HAAA-NUs were synthesized via a seedmediated growth route using L-Dopa as reduction agent (see details in Methods and Supporting Infromation, S1-S2). During the synthesis of HAAA-NUs, galvanic reaction was firstly happened between HAuCl4 and Ag seeds, followed by a reduction reaction between L-Dopa and HAuCl4 and Ag+ (the by-product of the galvanic reaction). As a result,
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www.MaterialsViews.com HAAA-NUs were synthesized. Briefly, for the synthesis of sub-100nm HAAA-NUs, HAuCl4 aqueous solution (2.4 mL, 10 mM) was mixed with DI water (4.3 mL) in a glass vial (25 mL). Then, the vial was put into a water bath of around 15 °C under magnetic stirring of 300 r.p.m. After 10 min, 0.9 mL of the as-prepared silver seeds (25 nm) were fed into the system, followed by the addition of L-DOPE aqueous solution (2.4 mL, 10 mM). After 1 min, the spin rate was slowed down to 100 r.p.m. As L-Dopa was added, the transparent yellow solution immediately turned opaque black-green, followed by deep black color. After 10 min, the product was collected by centrifugation (6000 rpm for 1 min) and washed with HCOOH (500 mM) once, ammonia solution once and DI water twice. Characterization: The products were characterized via various routes, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) as well as the ultraviolet-visible spectra. To investigate Raman features, the HAAA-AUs were measured with confocal microprobe Raman spectrometer with various excitation laser wavelengths including 532 nm, 633 nm and 785 nm, respectively. Crystal violet (CV) and bis (2-ethylhexyl) phthalate (DEHP) dye molecules were used to evaluate the sensitivity and uniformity of SERS signal. AFM-correlated Nano-Raman spectroscopy was used to study the aggregate dependent of Raman enhancement. The three-dimensional finite difference time-domain (FDTD) calculation was also used to study the interaction between light and nanostructures.
Supporting Information Detailed synthesis methods, FDTD calculation and characterization data from SEM, TEM, XPS, and SERS measurements. Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements J. X. Fang was supported by National Natural Science Foundation of China (No 51171139), Tengfei Talent Project of Xi’an Jiaotong University, the New Century Excellent Talents in University (NCET), Scientific New Star Program in Shann Xi Province (No.2012KJXX-03), Doctoral Fund of Ministry of Education of China (Nos.20110201120039, 20130201110032) and the Fundamental Research Funds for the Central Universities (No. 08142008). Z. Y. Li was supported by the 973 Program of China (No. 2013CB632704). Q.H. Xiong gratefully acknowledges the strong support from Singapore National Research Foundation via a fellowship grant (NRF-RF2009-06) and Ministry of Education via two Tier 2 grants (MOE2011-T2-2-051 and MOE2011-T2-2-085)
Received: October 14, 2013 Revised: November 14, 2013 Published online: January 21, 2014
[1] J. F. Li, X. D. Tian, S. B. Li, J. R. Anema, Z. L. Yang, Y. Ding, Y. F. Wu, Y. M. Zeng, Q. Z. Chen, B. Ren, Z. L. Wang, Z. Q. Tian, Nature Protocols 2013, 8, 52–65. [2] T. Y. Liu, K. T. Tsai, H. H. Wang, Y. Chen, Y. H. Chen, Y. C. Chao, H. H. Chang, C. H. Lin, J. K. Wang, Y. L. Wang, Nature Communications 2011, 2, 538–545. [3] P. H. C. Camargo, M. Rycenga, L. Au, Y. N. Xia, Angew. Chem. Int. Ed. 2009, 48, 2180–2184. [4] L. Guerrini, D. Graham, Chem. Soc. Rev. 2012, 41, 7085–7107. [5] H. Lee, S. You, P. V. Pikhitsa, J. Kim, S. Kwon, C. G. Woo, M. Choi, Nano Lett. 2011, 11, 119–124.
2438
wileyonlinelibrary.com
[6] E. C. L. Ru, P. Etchegoin, Principles of surface enhanced Raman spectroscopy and related plasmonic effects, Elsevier publishing, 2008, page 22. [7] H. Wang, N. J. Halas, Adv. Mater. 2008, 20, 820–825. [8] H. Y. Liang, Z. P.Li, W. Z. Wang, Y. S. Wu, H. X. Xu, Adv. Mater. 2009, 21, 4614–4618. [9] J. X. Fang, S. Y. Du, S. Lebedkin, Z. Y. Li, R. Kruk, M. Kappes, H. Hahn, Nano Lett. 2010, 10, 5006–5013. [10] D. K. Lim, K. S. Jeon, J. H. Hwang, H. Kim, S. Kwon, Y. D. Suh, J. M. Nam, Nature Nanotechnology 2011, 6, 452–460. [11] M. S. Schmidt, J. Hübner, A. Boisen, Adv. Mater. 2012, 24, OP11–OP18. [12] P. Goldberg-Oppenheimer, S. Mahajan, U. Steiner, Adv. Mater. 2012, 24, OP175–OP180. [13] A. Kim, F. S. Ou, D. A. A. Ohlberg, M. Hu, R. S. Williams, Z. Y. Li, J. Am. Chem. Soc. 2011, 133, 8234–8239. [14] M. Hu, F. S. Ou, W. Wu, I. Naumov, X. M. Li, A. M. Bratkovsky, R. S. Williams, Z. Y. Li, J. Am. Chem. Soc. 2010, 132, 12820–12822. [15] J. X. Fang, B. J. Ding, H. Gleiter, Chem. Soc. Rev. 2011, 40, 5347–5360. [16] J. X. Fang, P. M. Leufke, R. Kruk, D. Wang, T. Scherer, H. Hahn, Nanotoday 2010, 5, 175. [17] J. X. Fang, H. J. You, P. Kong, Y. Yi, X. P. Song, B. J. Ding, Cryst. Growth Des. 2007, 7, 864–867. [18] B. Wiley, T. Herricks, Y. Sun, Y. N. Xia, Nano Lett. 2004, 4, 1733–1739. [19] C. L. Lu, K. S. Prasad, H. L. Wu, J. A. Ho, M. H. Huang, J. Am. Chem. Soc. 2010, 132, 14546–14553. [20] D. Ferrer, A. Torres-Castro, X. Gao, S. Sepulveda-Guzman, U. Ortiz-Mendez, M. Jose-Yacaman, Nano Lett. 2007, 7, 1701–1705. [21] X. M. Lu, H. Y. Tuan, J. Y. Chen, Z. Y. Li, B. A. Korgel, Y. N. Xia, J. Am. Chem. Soc. 2007, 129, 1733–1742. [22] K. G. M. Chow, J. Nanopart. Res. 2012, 14, 1186. [23] Z. Liu, F. L. Zhang, Z. B. Yang, H. J. You, C. F. Tian, Z. Y. Li, J. X. Fang, J. Mater. Chem. C 2013, 1, 5567–5576. [24] L. Rodríguez-Lorenzo, R. D. L. Rica, R. A. Álvarez-Puebla, L. M. Liz-Marzán, M. M. Stevens, Nature Materials 2012, 11, 604–607. [25] H. Yuan, C. G. Khoury, H. Hwang, C. M. Wilson, G. A. Grant, T. Vo-Dinh, Nanotechnology 2012, 23, 075102–075111. [26] T. K. Sau, A. L. Rogach, M. Döblinger, J. Feldmann, Small 2011, 7, 2188–2194. [27] D. Y. Kim, T. Yu, E. C. Cho, Y. Y. Ma, O. O. Park, Y. N. Xia, Angew. Chem. Int. Ed. 2011, 50, 6328–6331. [28] Z. D. Wang, J. Q. Zhang, J. Ekman, P. J. Kenis, Y. Lu, Nano Lett. 2010, 10, 1886–1891. [29] M. Rycenga, C. M. Cobley, J. Zeng, W. Y. Li, C. H. Moran, Q. Zhang, D. Qin, Y. N. Xia, Chem. Rev. 2011, 111, 3669–3712. [30] F. G. Xu, K. Cui, Y. J. Sun, C. L. Guo, Z. L. Liu, Y. Zhang, Y. Shi, Z. Li, Talanta 2010, 82, 1845–1852. [31] J. X. Fang, S. Y. Liu, Z. Y. Li, Biomaterials 2011, 32, 4877–4884. [32] C. F. Tian, C. H. Ding, S. Y. Liu, S. C. Yang, X. P. Song, B. J. Ding, Z. Y. Li, J. X. Fang, ACS Nano. 2011, 5, 9442–9449. [33] W. B. Cai, B. Ren, X. Q. Li, C. X. She, F. M. Liu, X. W. Cai, Z. Q. Tian, Surface Science 1998, 406, 9. [34] G. Chen, Y. Wang, M. X. Yang, J. Xu, S. J. Goh, M. Pan, H. Y. Chen, J. Am. Chem. Soc. 2010, 132, 3644–3645. [35] K. L. Wustholz, A. I. Henry, J. M. Mcmahon, R. G. Freeman, N. Valley, M. E. Piotti, M. J. Natan, G. C. Schatz, R. P. Van Duyne, J. Am. Chem. Soc. 2010, 132, 10903–10910. [36] B. Gao, G. Arya, A. R. Tao, Nature Nanotech. 2012, 7, 433–437. [37] N. Gandra, A. Abbas, L. M. Tian, S. Singamaneni, Nano Lett. 2012, 12, 2645–2651.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, 26, 2431–2439
www.advmat.de www.MaterialsViews.com
Adv. Mater. 2014, 26, 2431–2439
[45] D. X. Wang, W. Q. Zhu, Y. Z. Chu, K. B. Crozier, Adv. Mater. 2012, 24, 4376–4380. [46] F. S. Ou, M. Hu, I. Naumov, A. Kim, W. Wu, A. M. Bratkovsky, X. M. Li, R. S. Williams, Z. Y. Li, Nano Lett. 2011, 11, 2538– 2542. [47] J. M. Hankett, C. Zhang, Z. Chen, Langmuir 2012, 28, 4654– 4662. [48] J. R. Ferraro, K. Nakamoto, C. W. Brown, Journal of Molecular Structure 1989, 212, 169–186. [49] B. Peng, G. Y Li, D. H. Li, S. Dodson, Q. Zhang, J. Zhang, Y. H. Lee, H. V. Demir, X. Y. Ling, Q. H. Xiong, ACS Nano 2013, 7, 5993–6000. [50] R. A. Alvarez-Puebla, L. M. Liz-Marzan, Angew. Chem. Int. Ed. 2012, 51, 11214–11223.
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
COMMUNICATION
[38] M. Yang, R. Alvarez-Puebla, H. S. Kim, P. Aldeanueva-Potel, L. M. Liz-Marzan, N. A. Kotov, Nano Lett. 2010, 10, 4013–4019. [39] L. Rodrı´guez-Lorenzo, R. A. Alvarez-Puebla, I. Pastoriza-Santos, S. Mazzucco, O. Stephan, M. Koviak, L. M. Liz-Marzan, F. J. Garcia de Abajo, J. Am. Chem. Soc. 2009, 131, 4616–4618. [40] N. Yamamoto, S. Ohtani, F. J. García de Abajo, Nano Lett. 2011, 11, 91–95. [41] D. K. Lim, K. S. Jeon, H. M. Kim, J. M. Nam, Y. D. Suh, Nature Materials 2010, 9, 60–67. [42] J. F. Li, Y. F. Huang, Y. Ding, Z. L. Yang, S. B. Li, X. S. Zhou, F. R. Fan, W. Zhang, Z. Y. Zhou, D. Y. Wu, B. Ren, Z. L. Wang, Z. Q. Tian, Nature 2010, 464, 392–395. [43] Y. J. Oh, K. H. Jeong, Adv. Mater. 2012, 24, 2234–2237. [44] H. Im, K. C. Bantz, N. C. Lindquist, C. L. Haynes, S. H. Oh, Nano Lett. 2010, 10, 2231–2236.
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