Engineering Nanostructures of Inorganic Materials for Optical and Chemical ApplicationsView this Special Issue
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Yanqin Cao, Dan Li, Fang Jiang, Yong Yang, Zhengren Huang, "Engineering Metal Nanostructure for SERS Application", Journal of Nanomaterials, vol. 2013, Article ID 123812, 12 pages, 2013. https://doi.org/10.1155/2013/123812
Engineering Metal Nanostructure for SERS Application
Surface-enhanced Raman scattering (SERS) has attracted great attention due to its remarkable enhancement and excellent selectivity in the detection of various molecules. Noble metal nanomaterials have usually been employed for producing substrates that can be used in SERS because of their unique local plasma resonance. As the SERS enhancement of signals depends on parameters such as size, shape, morphology, arrangement, and dielectric environment of the nanostructure, there have been a number of studies on tunable nanofabrication and synthesis of noble metals. In this work, we will illustrate progress in engineering metallic nanostructures with various morphologies using versatile methods. We also discuss their SERS applications in different fields and the challenges.
The physical phenomenon behind Raman spectroscopy is an inelastic scattering of a photon from a molecule in which the frequency changes precisely matching the difference in vibrational energy levels . Raman spectroscopy offers rich information of molecules, such as molecular structures, surface information, interface reactions, and so forth . The Raman signal is several orders of magnitude weaker than the fluorescence emission in most cases because of its very small scattering cross-section , thus preventing its applications in many fields. However, the discovery of the surface-enhanced Raman scattering (SERS) phenomenon renews interests in Raman scattering due to the improved signal intensity.
The first measurement of SERS was reported by Fleischmann et al. in 1974, who observed intense Raman signal from pyridine adsorbed onto a roughened silver electrode surface . In 1977, Van Duyne and Jeanmaire reported that the enhanced Raman signal intensity was due to an increase of 105-106 times from the scattering cross-section compared to the signal intensity of the bulk pyridine . The effect was later called surface-enhanced Raman scattering (SERS).
Generally, noble metals and some oxides  are used as SERS substrates, but the SERS enhancement originated from metal oxides is not strong . In this paper, we mainly discuss noble metal nanoparticles and their SERS applications.
Following the discovery of SERS, there have been an extensive amount of fundamental researches and theoretical studies on the SERS effect. After much debate, the general consensus is achieved that there are two important mechanisms underlying SERS: the electromagnetic enhancement and chemical enhancement, and the former is responsible for the major portion of SERS enhancement . We will discuss the theories in detail in the following part.
There have emerged a number of reports focusing on metal substrates with various morphologies for SERS applications, such as nanospheres, nanocubes, aggregate , as well as well-designed one-dimensional (1D), two-dimensional (2D), three-dimensional (3D) arrays and patterns [8, 10]. With those special structures, SERS has been used in various fields, such as trace detection of chemical and biological molecules and fast and effective detection of food additives, illicitly sold narcotics, and explosives. Although there are so many methods and different designs, it is still a challenge to produce highly sensitive, greatly selective, stable, reproducible substrates using a facile, robust, low-cost, and high-yield method.
In this paper, we will briefly review the theoretical background of SERS firstly. Then some typical methods for metallic SERS substrates will be demonstrated. At last, we will report popular applications of SERS in different practical fields.
2. Theoretical Background
Since the discovery of SERS, researchers have devoted much effort to understand the mechanism for the enhancement. At present, the electromagnetic enhancement and chemical enhancement are regarded as the two predominant mechanisms. While electromagnetic enhancement arises from increasing the applied electric field, chemical enhancement derives from amplifying the molecular polarizability. For electromagnetic enhancement, the intensity of Raman spectrum is proportional to the square of the induced dipole moment, which is a product of the molecular polarizability and the applied electric field .
Electromagnetic (EM) enhancement is due to that the optical field is facilitated by localized surface plasmon resonance (LSPR) of metallic nanostructure, which results in the enhanced Raman signal intensity . At a certain excitation frequency, collective oscillation of electrons in the conduction band of the metal will resonate with the incident light, resulting in a strong oscillation of the surface electrons, which commonly known as LSPR . LSPR may appear only when dimensions of the structure are much smaller than the wavelength of the light. Besides, the shape, architecture, composition, and surrounding environment of metal nanomaterials also affect the LSPR; thus, it is of great importance to design these parameters of nanomaterials for high SERS enhancement [1, 8, 12].
Chemical enhancement is considered to be relative to the direct interaction between the adsorbed molecule and the metal surface, which lead to an increased Raman cross-section for the adsorbed molecule . The most attractive way is charge transfer between the adsorbed molecule and the metal surface . Because of the chemisorptions of the molecules on the surface, new electronic states become accessible, which serves as resonant intermediate states resulting in the increase of the intensity. Ultimately, the chemical enhancement mechanism is a short-range effect due to the required direct adsorbate-surface interaction, which generally limits to only the first layer of adsorbed molecules [3, 14].
One of the important parameters to characterize SERS substrates is the enhancement factor (EF), which is especially true for the practical application of SERS and the comparison with the theoretical calculation [10, 15]. In fact, the most widely used definition of the enhancement factor is according to the following equation [1, 15, 16], which describes the overall enhancement in Raman scattering In the above equation, while denotes the intensity of the special band for the spectra of probe molecules which absorb on the SERS substrate, denotes the intensity of the same band for the spectra of probe molecules in the Raman (non-SERS) measurement. and are the corresponding number of analyte molecules in the focal volume. Generally the magnitude of EF is in the range of 104–108; however, in the single molecule SERS detection, the EF can reach up to 1014. A comprehensive study of enhancement factor was reported according to .
In general, the EM and chemical enhancement exist simultaneously, but the proportion of contributions is different, in which the former can give 104–106 enhancement and the latter usually provides 10–100 enhancement. At present, researchers focus on the EM enhancement because the EM enhancement is stronger than the chemical enhancement and more helpful for the practical applications of SERS substrates.
3. Noble Metallic Nanostructures Used as SERS Substrates
According to the theoretical background mentioned before for SERS, the EM enhancement provides the most contribution to the total enhancement of Raman signal. However, the EM enhancement is mainly due to LSPR, which is dependent on the shape, size, and structure of the material . Hence, it is of great importance to design rational materials for achieving the strongest enhancement.
Noble metallic nanostructures have been widely used in SERS because of their LSPR properties . As a result, there have appeared many researches focusing on preparing ideal noble metallic nanostructures as SERS substrates . Taking Ag and Au nanostructures as examples, various shapes have emerged , such as nanocubes [18, 19], nanorods [20, 21], nanocaps , nanochains [22, 23], nanoplates , honeycomb and hexagonally structured patterns , and nanoclusters . Monodisperse nanoparticles show high SERS enhancement because of their special shapes and sizes. For example, a systematic study on Ag nanocubes with a size ranging from 60 to 200 nm, in which the intensity of SERS increased with the size and the particles with sharper curvature showed a high sensitivity in contrast to normal particles with the similar size . The size of nanoparticles must be smaller than the wavelength of the light to produce LSPR, but it should not be too small, otherwise it may result in poor polarization of nanoparticles and hence poor plasmon resonance . For the nanoclusters and nanopatterns, the shape of patterns and the size of gaps between particles may influence the interparticle coupling, which plays an important role in generating enhanced signals .
As a result, to achieve high sensitive SERS signals, fabrication of effective noble metallic nanostructures is crucial.
4. Fabrication of Metallic Nanostructures for SERS
With the development of nanotechnology, nanofabrication has improved for meeting simple, low-cost, large-scale, and green requirements. In this section, we will introduce several common methods, which have been used to generate noble metal nanostructures for SERS applications.
4.1. Solution-Phase Synthesis
4.1.1. Polyol Process
The polyol process is a simple and versatile method for preparing metal nanostructures with various shapes and sizes [11, 29–31], which show potential optical applications. The enhancement effect of SERS is dependent on the shape and size of materials; thus, the polyol process is a good choice for preparing SERS substrates.
In a typical polyol synthesis [32, 33], the salt precursor and polymeric capping agent are injected into a preheated polyol, especially ethylene glycol, which serves as both a solvent and a reducing agent. The nucleation and growth could be controlled through varying the reaction conditions, such as temperature, reagent concentration, and types of additive ions, thus controlling the final products.
The nucleation and growth mechanisms in the polyol process have been discussed in detail [11, 29]. In this section, we take the synthesis of Ag nanostructures as an example. Typically, to obtain Ag nanostructures, silver nitrate (AgNO3) is used as metal salt precursor, ethylene glycol (EG) as the reductant and solvent, poly(vinyl pyrrolidone) (PVP) as the stabilizer and crystal-habit modifier [11, 30]. During the reaction process, initially formed small Ag clusters grow larger and become more stable which usually called seeds. The seeds may show three different structures: single crystalline, single twinned, and multiply twinned. By introducing additive chemical agents or ions, the growth speed of different crystalline faces can be changed, resulting in the formation of nanoparticles with different shapes by the selective growth of dominate crystalline faces. There are many works using different additive ions leading to different shapes with SERS applications. For example, Wiley et al. reported shape-controlled synthesis of various Ag nanostructures with this method  as shown in Figure 1. According to their study multiply twinned seeds grew into Ag pentagonal nanowires with addition of chloride and Fe3+, in which chloride prevents seeds from aggregating and Fe3+ protects twinned seeds from etching by chloride and oxygen. Without adding Fe3+, Cl−/O2 pair will dissolve both multiply twinned and singly twinned seeds, resulting in the products dominated by Ag cuboctahedrons or cubes. By adding Br− into the solution, only single twinned seeds retained, ultimately resulting in the formation of Ag nanobars or right bipyramids. More recently, they produced large-scale Ag nanocubes by introducing argon into the NaHS-mediated synthesis . However, in contrast to Ag synthesis, there is a notable difference in the mechanism for growing Au nanostructures: PVP preferentially promote the growth of facets but not the facets for Ag nanostructures, thus resulting in different shapes [29, 34, 35]. When using tetrachloroaurate trihydrate (HAuCl4·3H2O) as precursor, diethylene glycol (DEG) or tetraethylene glycol (TEG) as reductant, PVP as capping agent, various polyhedral nanostructures have been prepared . Moreover, adding Ag+ into the solution, Au nanocubes with the size in the range of ~100 nm have been fabricated .
4.1.2. Seed-Mediated Growth
Another popular chemical synthesis method for preparing well-defined shapes and sizes is using preformed nanocrystals as seeds for further growth, which is generally called seed-mediated growth [11, 36]. As a major advantage over conventional methods, the introduction of preformed seeds into a synthesis process allows nucleation and growth separately, thus making it easier to obtain a desired morphology by only manipulating the growth process. In general, metal atoms could deposit on seeds with the same composition or different composition, which are respectively called homogeneous or heterogeneous growth. The difference in lattice constant between the seed and the deposited metal will influence the final products. Besides, there are other influence factors [29, 36]: (1) the internal structure of the seed, for example, twin defects, stacking faults, and grain boundaries, which show a higher reactivity; (2) the presence and types of capping agents, such as surfactants, polymer stabilizers, ionic species, as well as unknown contaminants, which could bond selectively to different crystalline facets; and (3) the reduction or growth kinetics, which may influence the forming and dissolve of the seeds. Therefore, we can receive products with different shapes and morphologies through using different seeds and under different experimental conditions.
By using preformed spherical or cubic seeds, Ag nanocubes with uniform edge lengths controllable in the range of 30–200 nm have been prepared, in which the size was adjusted by varying the amount of AgNO3 precursor added in the reaction solution . In their study, the calculated EFs of Ag nanocubes did not vary much with the increase of size because and both became larger with increasing size of the cube. However, the enhancement factors have a relationship with the laser polarization. Recently by adding ionic or covalent bromides into the typical seed-mediated system, Ag nanobars with different aspect ratios were achieved , in which EFs showed anisotropy and were found to be dependent on the aspect ratios of nanobars, its orientation is relative to laser polarization, as well as the wavelength of excitation, as shown in Figure 2. A characteristic feature of anisotropic structures such as nanorods, nanowires is that its plasmonic band splits into two components, which corresponds to the longitudinal and transverse components of the localized surface plasmon resonance . Faceted pentagonal Ag nanorods with tunable dimensions have also been reported by using photochemically synthesized decahedra seeds, in which the SERS enhancement dropped with the increase of the aspect ratio . The report also suggests that interparticle cavities play a dominant role in SERS enhancement.
In addition, special Au nanostructures could be readily achieved by using different preformed seeds. For example, recently Au nanohexapods were prepared with single-crystal Au octahedral seeds, in which the seeds were also produced by seeded growth . Core-shell nanostructure has also been prepared through an improved seed-mediated growth .
According to the two methods described above, they are usually used to prepare various monodisperse metal nanoparticles. While self-assembly is a typical process, in which metal nanoparticles spontaneously organize into special arrays or patterns with controllable size, structure, and composition. To date, different assembly approaches have been employed to assemble different dimensional metal nanoarchitectures based on corresponding metal nanoparticles [40, 41]. These typical assembly approaches include layer-by-layer (LBL) assembly [42, 43], solvent-induced evaporation , Langmuir-Blodget assembly , external field-driven assembly , ion or small molecule or polymer induced assembly [22, 23, 47], liquid-liquid interface assembly , and so forth.
Chainlike 1D nanostructure has been widely fabricated through assembling nanoparticles [49–51]. For example, Yang et al. reported the assembly of Ag nanoparticles into nanochains under the inducing of cetyltrimethylammonium bromide (CTAB) and the assistance of 11-mercaptoundecanoic acid (MUA)  as shown in Figure 3. In their process, CTAB linked together the facets of neighboring Ag nanoparticles and MUA prevented excess aggregations of nanoparticles. Their results show that the chain length could be easily controlled by adjusting the amount of CTAB and MUA. The maximum SERS enhancement factor around 2.6 × 108 was observed on the Ag monolayer mainly composing of four-particle nanochains, which was due to the enhancement of localized electromagnetic field. The localized surface plasmon coupling at the interstitial sites of Ag nanochains resulted in the enhanced localized electromagnetic field. More Recently gold nanoparticle linear aggregates were generated by using a siloxane surfactant, in which the enhancement factor reached to 2.4 × 106, that was about 5 times stronger than the isolated Au nanosphere . In 2011, acid-directed self-assembly of Ag nanoparticles into special nanostructures has been achieved without using any polymer surfactant or capping agent . In the process, ascorbic acid reduced the precursor solution, and with the assistance of citric acid perfect microspheres composing of nanosheets were produced. Helical nanochains were also prepared by self-assembly of spherical colloids in V-shaped grooves .
In addition, metal nanoparticles can be immobilized on solid substrates resulting in 2D structures through chemical attachment [53, 54], electrostatic interaction , capillary force driving , and direct transfer of pre-assembled nanoparticle film , focusing on which numerous works have been published . Recently under the guiding of polystyrene-b-poly(4-vinylpyridine) (P4VP-b-PS) block copolymer, citrate-stabilized gold nanospheres assembled into arrays on the Si substrate driving by electrostatic interaction between the positively charged pyridinium groups on the substrate and the negatively charged surface ligands of the nanoparticles . Their products showed a uniform response regardless of orientation and excellent reproducibility, which are very important for practical applications of SERS. Besides, nanoparticle cluster arrays were reported by using nanopatterns of poly(styrene-block-2-vinylpyridine) (PS-b-P2VP) .
Finally, the metal nanoparticle clusters have been obtained through controllable assembly. Different morphol-ogies can be generated under usage of molecular linkers , asymmetrically functionalized nanoparticles , and controlling the aggregation kinetics .
4.2. Template Technique
Template technique offers another way to prepare nanostructures with well-defined size and shape under the assistance of templates. By using different templates, different morphologies could be obtained. There are many approaches for preparing metal nanomaterial through hard templates, such as anodic alumina oxide (AAO) [62, 63], polycarbonate membranes (PCM) , polymethylmethacrylate (PMMA) , monolayer colloidal crystal , and so forth, as well as soft templates, including micelles , reverse micelles, DNA, and so forth [11, 68]. In this section, we will describe some popular templates used to generate special noble metal nanostructures.
Anodic alumina oxide (AAO), also known as porous anodic alumina (PAA), is a very popular substrate for producing various metal nanostructures, such as nanowire, nanorod, nanopore, and nanodot arrays. Silver nanopores with abundant sub-5 nm gaps as SERS substrates were reported by electron beam deposition Ag on the porous side through controlling the temperature and the deposited Ag thickness, of which the EF reached to 107 using rhodamine 6G (R6G) as the probe molecule . High ordered gold nanodots were also fabricated by using AAO template with the thickness of 750 nm , in which Raman signal of thin silicon membranes on polyethylene terephthalate (PET) substrates was enhanced and exhibited structure information.
Monolayer colloidal crystal (MCC) is another interested template for producing various metal nanopatterns, which is also widely used in fabricating SERS substrates. In general, there are two cases: metal materials depositing on the top of colloidal crystals and metal materials depositing on the interspaces of the colloidal crystal monolayer, which result in nanocaps, honeycomb, and hexagonal arrays. MCC could be prepared by self-assembly process . The target metal material can be deposited on the prepared MCC templates through plasma sputtering , magnetron sputtering , and so forth. In most cases, the templates finally need to be removed to produce ideal metal nanopatterns. For example, tunable Au nanoshell arrays were prepared by using pretreated uniform polystyrene (PS) monolayer colloidal crystals as template . By varying the plasma sputtering deposition time, different nanopatterns with different spacing were generated, which showed different SPR intensity, thus tuning the SERS intensity. An order Ag nanocaps were also fabricated through magnetron sputtering Ag on the two-dimensional PS monolayer colloidal crystal template, which showed a high SERS intensity with an enhance factor ~1012 using 4-mercaptopyridine (4-Mpy) as the probe molecule .
In addition, recently by sputtering deposition Ag and Au nanoneedles arrays were obtained, in which the firstly deposited carbon nanoneedles arrays serve as an template inducing the formation of final products [70, 71]. Rodríguez-Fernández fabricated Au semishells using Janus silica particles as templates .
4.3. Other Methods
Besides the methods mentioned before for fabricating SERS substrates, there have been other methods, such as combination of nanoparticles with special structure or material [73, 74], screen printing , and sonochemical synthesis , and winkle-confined drying of collides .
For example, Ag nanoparticles deposited on the porous silicon in the AgNO3 solution, in which Ag aggregates formed and the sizes could be controlled by varying the concentration of AgNO3 solution . The SERS performance was measured by detecting R6G and crystal violet (CV), which showed remarkable Raman signal enhancement. Besides, a facile and fast method was reported by chemically depositing Ag on polyaniline films treated by hydrazine in AgNO3 solution, as well as adding lactic acid, which ultimately forming flower-like Ag nanostructure in less than one minute . The prepared Ag nanostructure showed highly response to 4-mercaptobenzoic acid (4-MBA) up to ppb level. Large-scale fabrication was achieved through screen printing . The printing ink consisted of Ag nanoparticles and sodium carboxymethylcellulose (CMC), in which the nanoparticles were reduced by sodium citrate and CMC was used to adjust the viscosity of the ink. The ink was placed on the woven mesh and forced into the image areas of the woven mesh as hard squeegee moving across the mesh. 3D Ag microflowers have been prepared by a simple and sonochemical method, which showed high sensitivity of SERS for R6G and 4-mercaptobenzoic acid (MBA) molecule . In the procedure, AgNO3 and L-ascorbic acid (AA) were mixed and sonicated using an ultrasonic generator at room temperature. The Ag nanoparticles of different sizes were achieved through controlling the ultrasonic time and power, as well as the reactant molar ratio. Gold nanoparticle arrays were produced through confining gold nanoparticle collides by wrinkled polydimethylsiloxane (PDMS) during drying .
5. Practical Applications of SERS
Raman spectrum can inherently imply rich information of analyte molecules, and SERS with higher signal intensity make it possible to detect analyte in very low concentration, which improves its practical applications. There have been a large number of works focusing on applications of SERS in various fields, including trace chemical detection, such as dye molecules [70, 78, 79], illicitly sold narcotics , food additives , and residual pesticide trace detection [20, 80, 81], as well as bioanalysis [82, 83] and explosives detection [84, 85].
5.1. Applications of SERS in Trace Chemical Detection
Dye molecules such as R6G and MG are reported to be used as SERS probe molecules. For example, Ag nanoneedles array substrates prepared by a simple Ar+ ion sputtering technology were used to detect R6G at a very low concentration ~10−11 M . Besides, many bands are distinctly observed in the spectra even when the R6G concentration was down to 10−15 M, by using Ag nanosheets-assembled micro-hemispheres as SERS substrates . There was another report showing that the detection concentration for R6G on the Ag microflower substrates was as low as 10−17 M . Because of the low detection limit of R6G, it is useful for studies on single molecule SERS. In addition, dyes can produce vibrant colors, thus make it treasured since antiquity . The easy and facile detection of dye molecules may promote the historical and cultural research.
The detection of trace-level hazardous chemicals is also in high demand because of the increasing threat from harmful environments and unreliable food safety. For example, Yang et al. detected ketamine hydrochloride down to 27 ppb within 3 s, which offers significant applications in both biomedical diagnostics and forensics field  as shown in Figure 4.
Melamine, a chemical compound, has been widely used in milk, infant formula, and pet food as an additive to increase protein content because of its high nitrogen content (66% by mass). However, since 2007, melamine, with its contaminant cyanuric acid, has become prominent because of the milk scandal. As a facile and simple spectroscopy technique, SERS has been used to detect melamine content. For instance, Zhang et al. reported that melamine with a concentration of ~5 ppm could be readily detected by using the core-shell Ag nanostructure assembled by nanosheets . In addition, pesticide residues have also been detected through SERS and using metal nanostructure substrates. For example, gold nanorods and silver nanocubes were used to detect three different herbicides: the organochlorine compound 2,4-D, the organophosphorus compound trichlorfon, and the triazinic compound ametryn, which were fabricated in solution phase . Silver nanoparticles decorated silicon nanowires have been used to in situ detect pesticide residues on a cucumber surface with a high SERS intensity, which were prepared using CTAB as the soft template to induce nanoparticles absorbing on the nanowires . Moreover, Li et al. in situ detected parathion residues on fresh orange using Au-SiO2 core-shell nanostructure  as shown in Figure 5. And this provides an important potential to rapidly detect pesticide residue on the fruits and vegetables, which is crucial to diet safety.
5.2. Applications of SERS in Biosensing
SERS has been widely used for bioanalysis , including detecting biomolecules [87, 88], pathogens sensing [89, 90], cancer diagnosis [91, 92], urine components detection , and in vivo molecular probing in live cells , which play an important role in the life science for health care or therapeutic treatment.
Biomolecules, such as DNA, can be detected using SERS. There have been reports focusing on detecting the sequence of DNA molecule by SERS technique, which based on the mixed DNA-functionalized silver nanoparticles probe  or using gold nanoparticle aggregates . Bacteria, one of the pathogens, have been identified using SERS through internally or externally depositing Ag and Au colloids on bacteria . The applications of SERS on cells for detecting pathogens have also been studied, as well as the SERS-based immunoassay. Binding the antibody and the metal nanoparticles can be used to detect the special antigen, which serves as biomarker of corresponding cancer . Moreover, recently there has been a study focusing on the detection of ß-agonist in urine, in which the chemometric method was introduced for high sensitivity . Their work provided a potential use of SERS in drug test and clinic detection. Much effort to meet reliable, fast, and high specific detection requirements remains to be proceeded.
5.3. Applications of SERS in Explosives Detection
The detection and analysis of explosives are important to homeland security, environment cleaning, military issues, and land mine detection. For instance, Ni–Au nanocarpets (NCs) were prepared through a galvanic replacement reaction, starting from prefabricated Ni NCs, which showed high activity and reproducibility as SERS substrates for detecting explosives with a low concentration about 10−7 M . Silver-gold bimetallic nanostructures were achieved by sunlight inducing and DNA template assisting . By varying the molar ratio of silver to gold, the morphologies of products could be controlled. The prepared Ag-Au nanostructure showed a very low detection concentration of TNT down to 10−15 M through SERS technology. Stable and reproducible substrates for detecting explosives are still challenging.
In this paper, we mainly discuss the theoretical background of SERS, some synthesis methods of SERS substrates, and their applications in different fields. Although there have been many studies focusing on the theories, some issues remain unclear and unsure, such as the attribution of chemical mechanism. With the development of designing various nanostructures, it is possible to further understand the enhancement theories and the factors influencing the enhancement. Different dimensional nanostructures have been prepared through various methods, such as polyol process, seed-mediated synthesis, self-assembly and template technique. Using these noble metal nanostructures, SERS has been used in detecting various molecules and thus has been applied into different real fields. However, it is still a challenge to produce highly sensible, reproducible, and stale substrates using a simple, low-cost, large-scale, and rapid method. Further works should be continued to generate high-performance SERS substrates for wider applications.
Y. Yang thanks the Century Program (One-Hundred-Talent Program) of the Chinese Academy of Sciences for special funding support. This study was also supported in part by Funds from the National Natural Science Foundation of China (no. 51071167, 51102266), the Instrument Developing Project of the Chinese Academy of Sciences (no. YZ200939), the Shanghai Yangtze River Delta Science Project (no. 11495810100), and the Shanghai Pujiang Program (10PJ1410700). Y. Yang is also thankful for the support by Fund from Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences (no. 12CS01).
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