CoreâShell Nanoparticle-Enhanced Raman Spectroscopy - American

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Core−Shell Nanoparticle-Enhanced Raman Spectroscopy Jian-Feng Li,*,†,‡ Yue-Jiao Zhang,† Song-Yuan Ding,† Rajapandiyan Panneerselvam,† and Zhong-Qun Tian*,† †

State Key Laboratory for Physical Chemistry of Solid Surfaces, MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, College of Chemistry and Chemical Engineering, iChEM, Xiamen University, Xiamen 361005, China ‡ Department of Physics, Xiamen University, Xiamen 361005, China ABSTRACT: Core−shell nanoparticles are at the leading edge of the hot research topics and offer a wide range of applications in optics, biomedicine, environmental science, materials, catalysis, energy, and so forth, due to their excellent properties such as versatility, tunability, and stability. They have attracted enormous interest attributed to their dramatically tunable physicochemical features. Plasmonic core−shell nanomaterials are extensively used in surface-enhanced vibrational spectroscopies, in particular, surfaceenhanced Raman spectroscopy (SERS), due to the unique localized surface plasmon resonance (LSPR) property. This review provides a comprehensive overview of core−shell nanoparticles in the context of fundamental and application aspects of SERS and discusses numerous classes of core−shell nanoparticles with their unique strategies and functions. Further, herein we also introduce the concept of shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) in detail because it overcomes the long-standing limitations of material and morphology generality encountered in traditional SERS. We then explain the SERS-enhancement mechanism with core−shell nanoparticles, as well as three generations of SERS hotspots for surface analysis of materials. To provide a clear view for readers, we summarize various approaches for the synthesis of core−shell nanoparticles and their applications in SERS, such as electrochemistry, bioanalysis, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, we exemplify about the future developments in new core−shell nanomaterials with different functionalities for SERS and other surfaceenhanced spectroscopies.

CONTENTS 1. Introduction 1.1. Properties of Core−Shell Nanoparticles 1.2. Pure Metal Nanostructures in SERS 1.3. History of Core−Shell Nanoparticles in SERS 1.4. Scope of this Review 2. Classification and Techniques 2.1. Classes of Core−Shell Nanoparticles in SERS 2.2. Ultrathin Transition Metal Shells 2.2.1. “Borrowing SERS” Strategy 2.2.2. SERS Activity 2.2.3. Effect of Shell Thickness and Core Size 2.2.4. Multimetal Shells 2.3. Ultrathin Nonmetal Shells (SHINERS) 2.3.1. Shell-Isolated Mode 2.3.2. Advantages of Shell-Isolated Mode over Contact Mode 2.3.3. 3D-FDTD Simulations of Electric Field Distribution 2.3.4. Various Types of SHINs 2.4. Thick Shells 2.4.1. Metal−Metal Core−Shell Nanoparticles with Tunable SPR 2.4.2. Raman Marker (Tag)

2.4.3. Dielectric-Metal Core−Shell Nanoparticles (Nanoshell) 2.4.4. Metal Film Over Nanospheres (FON) 2.4.5. Magnetic-Metal Core−Shell Nanoparticles 2.4.6. Hybrid Core−Shell Nanostructures 3. Theory 3.1. General Consideration on Enhancement Strategy in Raman Scattering 3.2. Surface Plasmon and Surface-Enhanced Raman Scattering 3.3. Concepts of SERS Hotspots 3.3.1. First-Generation Hotspots 3.3.2. Second-Generation Hotspots 3.3.3. Third-Generation Hotspots 4. Synthesis and Characterization of Core-Shell Nanoparticles 4.1. Different Shapes of Plasmonic Cores 4.1.1. Nanospheres 4.1.2. Nanocubes 4.1.3. Nanorods

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Special Issue: Vibrational Nanoscopy Received: August 30, 2016 Published: March 8, 2017

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DOI: 10.1021/acs.chemrev.6b00596 Chem. Rev. 2017, 117, 5002−5069

Chemical Reviews 4.1.4. Nanowires 4.1.5. Multibranched Nanoparticles 4.2. Core−Shell Nanoparticles with Ultrathin Shell 4.2.1. Metal−Metal Core−Shell Nanoparticles with Ultrathin Shell 4.2.2. Metal-Nonmetal Core−Shell Nanoparticles with Ultrathin Shell 4.2.2.2. Alumina Shell 4.2.2.3. Amorphous Carbon Shell 4.3. Core−Shell Nanoparticles with Thick Shell 4.3.1. Metal−Metal Core−Shell Nanoparticles with Thick Shell 4.3.2. Nonmetal−Metal Core−Shell Nanoparticles with Thick Shell 4.3.2.2. Magnetic-Metal Core−Shell Nanoparticles 4.3.3. Metal-Nonmetal Core−Shell Nanoparticles with Thick Shell 4.3.4. Hybrid Core−Shell Nanostructures with Thick Shell 5. Applications 5.1. Electrochemistry 5.1.1. Surface Adsorption 5.1.2. Surface Reaction 5.2. Bioanalysis 5.2.1. DNA Detection 5.2.2. Immunoassays 5.2.3. Bacteria Detection 5.2.4. Bioimaging 5.2.5. Disease Diagnosis and Therapy 5.3. Sensing 5.3.1. pH Sensing 5.3.2. Temperature Sensing 5.3.3. Biosensing 5.4. Food Safety 5.4.1. Pesticide Residues 5.4.2. Dairy Product Detection 5.4.3. Alcoholic Beverages 5.4.4. Tobacco Products 5.5. Environment Safety 5.5.1. Heavy Metal Ions Detection 5.5.2. Explosive Detection 5.5.3. Biowarfare Agent Detection 5.5.4. Toxic Chemicals 5.6. Cultural Heritage Objects 5.7. Materials 5.7.1. Monitoring the Synthesis of Nanomaterials 5.7.2. Corrosion Inhibition 5.7.3. Semiconductor Materials 5.7.4. Biomaterials 5.7.5. Polymer Materials 5.8. Catalysis 5.8.1. In Situ Monitoring of Catalytic Process 5.8.2. In Situ Study of Catalysis at High Temperature 5.8.3. Tuning the Properties and Performances of Multifunctional Catalysts 5.8.3.2. Influence of the Composition and Structure of Shell Material on Catalysis 5.9. Energy Storage and Conversion 5.9.1. Photocatalysis and Solar Cell

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5.9.2. Lithium Battery 5.9.3. Solid Oxide Fuel Cell 6. Summary and Outlook 6.1. Summary 6.2. New Functional Core−Shell Nanoparticles for SERS 6.3. New Core−Shell Nanoparticles for Other Surface-Enhanced Spectroscopies Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION The developments and innovations in nanoscience and nanotechnology have opened numerous opportunities in various fields,1−4 such as optics,5,6 catalysis,7−9 manufacturing technology, microelectronics, computer technology, biomedicine,10−12 environmental science,13−15 and energy,16,17 due to the unique properties of nanomaterials compared to those of bulk materials. In the early stages, nanomaterials with a single component were studied extensively, and numerous superior properties were observed, including higher ratio of surface atoms, tunable optical properties, ease of processing, and excellent catalytic performance. With the rapid development of synthesis and characterization techniques, researchers found that much better properties could be obtained for multicomponent nanomaterials compared to single-composition nanomaterials, and the diversities in their composition and structure could significantly enrich their applications in various fields. Therefore, core−shell nanomaterials have emerged as one of the hot research topics in recent years.18,19 In this review, first we discussed about the outstanding properties of core−shell nanoparticles and then explained the unique strategies and applications in SERS.

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1.1. Properties of Core−Shell Nanoparticles

Among all multicomponent nanomaterials, core−shell nanoparticles have attracted increasing research attention due to their outstanding properties as follows.20,21 (1) Versatility: a core− shell nanoparticle consists of an inner core and an outershell made of different material; therefore, the combination of different properties of different materials leads to several novel properties of core−shell materials, thus expanding their applications in electronics, optics, magnetism, and catalysis.22 (2) Inexpensive: in general, the shell layer requires a low weight ratio of noble or transition metals in contrast to pure metal nanoparticles because only the surface atoms contribute to the processes such as catalytic reactions. For example, the use of noble metals can be significantly reduced by coating its thin layer on an inexpensive carrier.23 (3) Tunability: the properties of core−shell nanoparticles can be easily and dramatically tuned by changing the size, shape, morphology, and components of core, as well as thickness, shape, and components of shell materials.24,25 (4) Stability and dispersibility: the coated shell can protect the nanoparticles from aggregation, sintering, or the effect of other reagents.26 (5) Biocompatibility: the biocompatibility is one of the important issues from the perspective of practical bioapplication, and the biocompatibility of core−shell nanoparticles can be improved by coating with silica (SiO2),

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Figure 1. Illustration of the development of core−shell nanoparticles for SERS in order to expand the materials and surface morphology generalities over the past four decades. (a) The first discovery of SERS effect on roughened Ag electrode. (b) SERS of Ag or Au colloids. “Borrowing” strategy of (c) silver nanoislands on n-GaAs electrode and (d) deposited transition metal on roughened Au electrode. (e) Single molecule SERS. (f) Silica core-Au shell nanoshell. (g) UPD and redox replacement transition metal on Au nanoparticles. (h) SERS Tag. (i) Au core-transition metal shell nanoparticles synthesized by wet-chemical method. (j) Alumina layer on Ag film over nanosphere (AgFON) substrates. (k) Silica shell isolated nanoparticles. (l) Graphene shell isolated nanoparticles.

the electronic structure, due to the distinct change in dielectric constant and the electrons shared across the boundary. With the unique LSPR property, plasmonic nanomaterials are extensively used in Raman spectroscopy to enhance the Raman signals of probe molecules with exquisite sensitivity, which is commonly called “surface-enhanced Raman spectroscopy” (SERS).

polymer, etc. The toxicity can be decreased, and the shell can be easily modified with biomolecules.27 (6) Controllability: the release or leaching of the core can be controlled by changing the environmental pH, ionic strength, or temperature, which is an important requirement in drug delivery.28 Owing to the unique chemical and physical properties, core− shell nanoparticles have been widely applied in many fields, including drug delivery and release, bioimaging, catalysis, and optical spectroscopy such as photoluminescence and vibrational spectroscopy. In particular, one of the most significant physical properties of noble metal nanostructures is its plasmonic property, which is a product of the resonant interaction between an electromagnetic radiation and the free electron like metal nanomaterials, such as gold, silver, and copper nanoparticles.12,29−31 If the frequency of light matches the resonant frequency of collective oscillating electrons, the incident light could resonantly interact with the noble metal nanostructures, and the electromagnetic field around the nanoparticles could be remarkably reshaped, leading to the effective concentration of the incident light at the spatially narrow region around the nanostructures. The resonant behavior in coinage metal nanoparticles could be termed as localized surface plasmon resonance (LSPR).32 The core−shell nanomaterials containing noble metals are plasmonic core−shell nanomaterials. Recently, plasmonic core−shell nanomaterials have been employed for a miscellany of applications such as solar cell,33−35 photocatalysis,36−39 sensor,40−42 biomedical diagnosis,43−45 and imaging,46,47 due to their distinct advantages in boosting the performance of corresponding devices or processes and fascinating optical properties. For example, in energy conversion, well-designed plasmonic nanostructures can enhance the absorption of light in solar cells or drive photocatalysis. Moreover, they can also be used for sensors because the LSPR property is sensitive to the surrounding environment. In biomedical diagnosis, plasmonic nanoparticles are extensively used for photothermal therapy as the LSPR can be tuned to nearinfrared (NIR) region which is suitable for the biological window. These nanoparticles can also be used for imaging by labeling with reporter molecules. Compared to individual metal nanoparticles, core−shell nanoparticles have demonstrated some special properties which provide great significance for understanding the fundamental phenomena of the plasmonic field. Coating a shell on a core can significantly modify the optical properties and

1.2. Pure Metal Nanostructures in SERS

Raman spectroscopy is a powerful vibrational optical spectroscopic technique based on the inelastic scattering of light by the molecule of interest. It can provide specific fingerprint information about a wide range of target molecules.48 Raman effect was first experimentally observed by the Indian physicist C. V. Raman in 1928,49 who subsequently earned the Noble Prize in 1930 “for his work on the scattering of light and for the discovery of the effect named after him”. However, the detection sensitivity of Raman spectroscopy was found to be intrinsically low in contrast to that of infrared absorption or fluorescence emission spectroscopy.50 Therefore, Raman spectroscopy was typically not used in the fields of trace analysis and surface science because of the extremely low amount of probe molecules. In 1977, Van Duyne et al. discovered the SERS effect on roughened Ag electrodes (Figure 1a), which was an important breakthrough in Raman spectroscopy.51−53 Initially, in 1974, Fleischmann et al. observed enormously enhanced Raman signals of pyridine molecules adsorbed on a silver electrode roughened by an electrochemical method. However, the authors attributed the enormous Raman signals to the increase in the number of adsorbed molecules on the corrugated surface of the electrodes with the enlarged surface area.51 Nonetheless, after careful calculations, Van Duyne recognized that the Raman signal of adsorbed molecules was 105−106 times higher than that of the bulk pyridine. They concluded that the anomalously intense Raman signal could not be explained merely by the increased amount of adsorbed molecules due to the enlarged surface area, and instead, should be attributed to the true enhancement in Raman scattering efficiency itself. Jeanmaire et al. and Albrecht et al. showed that an enhancement in the Raman signal was due to a localized electromagnetic field around the metallic nanostructures.52,53 This effect was known as surface-enhanced Raman scattering (SERS). The discovery of SERS immediately triggered a significant interest in surface-enhanced Raman studies because the low detection sensitivity was no longer a fatal flaw for surface Raman 5004


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In 1999, Xu demonstrated the detection of molecular vibrations in single hemoglobin (Hb) protein molecules attached to isolated or immobilized silver nanoparticles by SERS.62 Combined with scanning electron microscope (SEM)/atomic force microscope (AFM) analysis, they found that no Raman signal was detected on single Ag nanoparticle; however, the majority of the dimers/trimers were active in Raman. Moreover, among the “hot” dimers, those with their dimer axis oriented parallel to the incident electric field consistently produced the highest SERS intensity. Significantly, stronger electromagnetic field could be generated in the nanoparticle−nanoparticle junctions, which are well-known as the “hot spots”,63 due to the coupling effect between nanoparticles. The Raman signal of molecules in the hot spots areas can be considerably enhanced. The high quality single molecular SERS spectra have solved the limitation of molecule generality and made SERS become one of the most promising tools for trace analysis in life and medical sciences as well as security and environmental protection with single-molecule sensitivity.64 On the other hand, expansion of material generality is another fundamental issue of SERS, as strong enhancement could only be obtained on Au, Ag, and Cu surface in the early stages. Researchers had paid extensive attention to explore the enhancement effect on other metals, such as Pt, Pd, Rh, Ni, Ti, or Co.65−70 However, the enhancement was significantly low and the probe molecules on transition metals were barely detected. This limitation greatly restricted the applications of SERS in other fields, resulting in a low tide of SERS from 1980’s to the mid-1990’s. An important step forward was taken from 1990’s, when Tian’s group developed various surface roughening procedures to improve the enhancement of transition metals.71 They obtained the enhancement of 1−3 orders of magnitude from bare Pd, Pt, Rh, Ru, Fe, Co, and Ni electrodes.72−77 However, the SERS signal was quite weak, due to the optical damping nature of these metals and the nonuniform roughened surface (please refer to the detailed discussion on optical properties of transition metals in section 3.2). In this case, the advantage of controllable shape and size of nanoparticle makes the nanoparticles the promising candidates for more sensitive and uniform SERS substrate.78−81 For example, Xia’s group successfully synthesized Pt and Pd nanocubes as SERS substrates, which exhibited 3−10 times higher enhancement than roughened electrodes.82

spectroscopy. Furthermore, SERS can be used for in situ studies even in electrochemical/aqueous environments with ultrahigh sensitivity and excellent spectral resolution in a nondestructive manner. Noteworthy, signals from the species located on the surface of metal nanostructures can only be enhanced greatly, while that from solution still remains unchanged. Thus, the interference from the unadsorbed molecules in solution can be effectively eliminated or reduced, which is an outstanding advantage of SERS. Thus, SERS can offer abundant information about target molecules on physical, chemical, and biological surfaces or interfaces, including the surface bonding, orientation, and conformation of the adsorbed molecules. Besides, SERS can also be applied for in situ monitoring of reactions/biological processes at surfaces or interfaces.54−56 SERS is an exceptional technique for the characterization of a small number of molecules bound to or near plasmonic surfaces. Moskovits interpreted the origin of SERS to plasmon resonances of metal bumps on the roughened surface.57 It was found that only a few free-electron-like metals, mainly metals of Group 1B (Cu, Ag, Au) and alkalis could generate optical conductive resonance in the visible spectral range. More importantly, Moskovits further predicted that similar enhancement could be obtained for Ag, Au, and Cu colloids. Further in 1979, Creighton et al. reported the enhancement of Raman scattering by using chemically synthesized monodisperse Ag and Au colloids (Figure 1b).58 Sodium borohydride was used as a reducing agent to reduce silver nitrate or chloroauric acid to obtain yellow or purple sol, respectively. The extinction maximum was at 400 nm for Ag sol and 525 nm for Au sol, indicating that the particle size was smaller than the wavelength of light. Addition of pyridine molecules into the sols led to the enhancement in the enhanced Raman signals. The enhancement was dependent on the matching of the extinction maximum with the excitation wavelength, indicating that surface plasmon oscillations contributed to the enhancement of Raman scattering. Importantly, Moskovits’ prediction and Creighton’s experimental verification indicated that SERS substrates were well-extended from roughened coinage metals films or electrodes to coinage colloids which are now named fancily as nanoparticles. The nanoparticle-based SERS significantly expand the scope of SERS application from electrochemistry to material science and life science. Benefitted from the rapid development of nanoscience in the 1990s, SERS has again attracted a widespread interest. In 1995, Natan et al. fabricated monodisperse gold and silver nanoparticles on a substrate modified by monolayers of polymer with functional groups such as thiol, amine, or cyanide, which can strongly interact with gold or silver nanoparticles. For the first time, nanoparticles were immobilized on the substrate to obtain strong SERS enhancement.59 After these pioneering studies, much great progress has been made in the field of highly sensitive nanoparticle-based SERS. One of the most important breakthroughs was the development of single-molecule SERS (SMSERS), which was reported by Kneipp’s group60 and Nie’s group61 independently (Figure 1e). Nie et al. reported SM-SERS of rhodamine 6G (R6G) molecules on immobilized citrate reduced Ag nanoparticles with nanometer-sized gaps (nanogaps). They estimated that each nanoparticle carried an average of one R6G molecule at a concentration of 2 × 10−10 M. The additional SERS enhancement was over 10 orders of magnitude relative to the normal Raman, which was comparable to singlemolecule fluorescence.

1.3. History of Core−Shell Nanoparticles in SERS

Though the limitation of substrate generality in SERS has been circumvented partially by devising and utilizing various transition metal nanostructures such as roughened electrodes or nanoparticles with shape corner or spikes, the SERS enhancement on the surface of the nanostructures was found to be very weak compared to that on the surface of SERS-active metals including Ag, Au, and Cu. Molecules with weak Raman cross section were not observed in many systems or conditions. In order to solve this problem, a strategy of “borrowing SERS activity” has been developed and utilized. The approach of “borrowing SERS activity” involved the “borrowing” of the SERS activity from Au or Ag nanostructures coated with ultrathin shells of various transition metals to improve the Raman signals of molecules adsorbed on transition metal surface at the early stage. This strategy was developed based on the well-known electromagnetic enhancement mechanism of SERS (please refer to section 3.2), which indicates that electromagnetic fields can reach several nanometers away from the Au or Ag nanoparticle surface, 5005


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boosted by the long-range effect of electromagnetic field. In other words, the molecules without direct contact with the nanostructure surface can also experience the electromagnetic field, and their Raman signals can be enhanced accordingly. The first study demonstrating the strategy of “borrowing” was proposed and reported by Van Duyne et al. in 1983.83,84 They deposited discontinuous and high-SERS-active silver nanoislands on a non-SERS-active material such as n-GaAs electrode. Surprisingly, Raman signal of molecules adsorbed on n-GaAs electrode was clearly observed. Figure 1c exhibits that the very strong electromagnetic field generated on Ag nanoislands results in an enhancement in the signal of adjacent molecules on n-GaAs surface due to the long-range effect. However, this method suffers from a limitation that the species should be selectively adsorbed on non-SERS-active substrate but not on high-SERSactive Ag nanostructures. This is because the signals from molecules adsorbed on Ag nanostructures are much stronger than that from semiconductor n-GaAs, which may provide misleading information about the probe molecules. However, most of the molecules tend to get adsorbed on high-SERS-active materials. Therefore, it is difficult to determine which measured Raman signals come from the probe molecules adsorbed on the surface of the semiconductor in interest. In order to overcome the above-mentioned disadvantage, Fleischmann’s group and Weaver’s group independently developed an alternative strategy in 1987 involving deposition of an ultrathin layer of weak or non-SERS-active materials on Ag and Au nanostructure substrates.85−89 Ag or Au nanostructures can generate enormous enhancement in the electromagnetic field near their surfaces, thus Raman signals of target molecules can be enhanced even if the molecules are not in direct contact with the SERS active substrate.90−92 By employing the “borrowing” method, high quality SERS spectra of molecules adsorbed on transition metal shells can be successfully obtained. However, the enhancement decreases exponentially with the increase in the distance from the SERS-active core. Thus, the transition metal shell should be ultrathin, usually a few atomic layers. At the same time, the ultrathin metal film is also required to be pinhole-free, in order to block the direct adsorption of molecules on the SERS-active core. Such types of ultrathin and pinhole-free metal layers were not successfully prepared until the late 1990’s. To overcome this issue, Weaver’s group fabricated a perfect metal film on gold electrodes through optimizing electrodeposition parameters.93−96 However, the expansion of this method was difficult because different materials required different deposition conditions; therefore, it was successfully applied to only a few metal surfaces. To overcome this limitation, they further invented a more general procedure to deposit Pt group metal films on roughened gold electrodes by underpotential-deposition (UPD) of copper followed by spontaneous redox replacement with Pt group metal cations. As demonstrated by carbon monoxide (CO) and ethylene adsorption behavior, the as-prepared thin films were highly uniform and pinhole-free, and their SERS enhancement factors were 3−4 times greater than that from the directly deposited Pt films.97 In 2002, the Weaver group further expanded the UPD and redox replacement procedure to the coating of Au nanoparticles with a transition metal layer, in order to prepare more versatile nanoparticle-based SERS substrates.98 Similarly, gold nanoparticles were first assembled on a clean functionalized indium tin oxide (ITO) surface by a coupling agent via the interaction with amine groups. Then a monolayer of copper film was coated on them by UPD method. The as-prepared copper film was then

replaced by Pt or Pd transition metal. Thus, a pinhole-free and single-atomic-layer thick transition metal layer was obtained on the surface of gold nanoparticles. The electrochemical cyclic voltammetry test and the SERS spectra of adsorbed CO and ethylene demonstrated that a strong SERS enhancement could be obtained and also the surface properties of Pt could be preserved. Although it exhibited strong SERS enhancement, this procedure was very complex, involving surface functionalization, electrochemical UPD procedure, and chemical redox replacement. Furthermore, it was difficult to obtain thick transition metal layers. Thus, a simple, direct, and accurate method was definitely required to improve the SERS substrate generality. In 2004, Tian’s group developed a more versatile and straightforward wet-chemical synthesis method to fabricate the Au core transition metal shell (Au@TM, TM = Pt, Pd, Rh, Ru, Co, and Ni) nanoparticles, in order to expand SERS to transition metal surfaces.99−102 This transition metal shell contained only a few atomic layers, and the inner Au nanoparticle acted as a plasmonic core to enhance the Raman signals of molecules adsorbed on the transition metal shell. By this method, the enhancement was strong enough (about 4−5 orders of magnitude) to fulfill the molecular-level investigation. Furthermore, the properties of nanoparticles could directly represent the properties of the transition metal because the nanoparticle surface was completely covered by the transition metal shell. Thus, the detection of molecules on the transition metal surface can be realized succinctly. Another superiority of chemically synthesized Au@TM is that the shell thickness could be tuned by changing the ratio of Au nanoparticles to transition metal precursors, which is very simple and convenient, and this procedure can be easily expanded to the preparation of various transition metal shells. The as-synthesized nanoparticles can be directly assembled on the electrode surface without the coupling agent interference, leading to more accurate control of potential. Therefore, the generality of materials has been partially solved by the “borrowing SERS activity” strategy. However, for other materials, such as metal oxides, polymeric membranes, insulators, or biological membranes, it is unmanageable if not impossible to coat them as uniform ultrathin shells on gold or silver nanoparticles. On the other side, surface generality is another stumbling block in applying SERS. For example, single-crystal is a great model system because of its well-defined surface, and it can be efficiently used to study the mechanism of electrochemistry, catalytic reactions, and other important surface physicochemical processes. However, single-crystal surfaces exhibit the existence of no or an extremely weak SERS effect. Therefore, one way to study the single-crystal by SERS is to excite surface plasmon polaritons (SPP) on the single-crystal surface by using an attenuated-total-reflection (ATR) Raman cell. For example, Otto et al. utilized the ATR Raman cell with Otto configuration to obtain an enhancement of 1−2 orders of magnitude for pyridine molecules adsorbed on Cu single-crystal surfaces.103,104 However, it was very difficult to adjust the ATR setup with Otto configuration. Another type of ATR Raman cell with the Kretschmann configuration was also used to obtain SERS from smooth metal surfaces.105 However, this optical configuration requires deposition of an ultrathin metal film on a quartz substrate, which is not a well-defined single-crystal surface. Overall, Raman enhancements by these two delicate ATR setups were still too weak to achieve the detection of trace amounts of species on the surface, and they could only be used for Au, Ag, or Cu metals, as SPP can only be excited on these materials. 5006


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Figure 2. Classes of core−shell nanoparticles in SERS.

thousands of tips, thus the obtained Raman signal is very strong. Besides, the ultrathin and pinhole-free shell is used to avoid the direct contact of molecules with gold or silver NPs, thus the obtained signal is just from the interested species adsorbed on the substrate. In principle, this method can be used for the surface with any material and morphology, thus high quality Raman spectrum can be obtained from single-crystals of platinum, gold, or silicon. Furthermore, yeast cells, pesticide residues on fruits, and even biological tissues can also be studied or detected. These applications attest that SHINERS has very wide applicability. Simultaneously, SHINERS is also becoming a promising and powerful tool in various fields, such as surface science, material science, biological analysis, pharmaceutical analysis, food safety, and environmental protection.115−118 Furthermore, based on the shell-isolated mode, researchers have also developed a variety of shells such as alumina (Al2O3),119,120 manganese oxide,121 titanium oxide (TiO2),122 carbon,123 and graphene124−126 for SHINERS, which further broadens the scope of its applications.

Therefore, practical application of ATR-Raman set up in various systems has still been a great challenge. The surface generality problem of SERS was then subtly solved by the discovery of tip-enhanced Raman spectroscopy (TERS), which was conceptualized in 1985 by Wessel et al.106 and experimentally realized in 2000 independently by several groups.107−110 Anderson utilized a gold-coated AFM tip to selectively produce SERS for localized Raman spectroscopy.107 The combination of AFM with SERS provided increased sensitivity, selectivity, and spatial resolution over a conventional Raman microprobe. At the same time, Kawata et al. coated the AFM tip with a silver layer and observed the amplification of the near-field signal.108 Moreover, Zenobi et al. observed brilliant cresyl blue with a silver-coated AFM probe.110 Furthermore, Pettinger et al. combined SERS and scanning tunnel microscope (STM) on the Ag(111) sample.109 These pioneer studies introduced the concept of TERS and opened up a new field for Raman spectroscopy. During TERS analysis, a Au or Ag tip end can generate an enhanced localized electromagnetic field under the excitation of a suitable laser, thus the Raman signal of molecules near the tip can be enhanced because the sample borrows the enhancement from the tip itself. Therefore, the material and morphology of the substrate are not limited, leading to significant expansion in the surface and substrate generalities and its excellent applications in surface science. TERS technique has extremely high spatial resolution down to several nanometers and can be applied to any substrate and material. Molecular and topographic information can be obtained simultaneously, which can be directly correlated with the spectroscopic data. These advantages significantly promote the applications of TERS in various fields, including single molecules detection, monitoring reactions, etc. and in investigating processes at metal/vacuum, metal/gas, and metal/ electrolyte interfaces, as well as in biorelated systems.111−113 However, the obtained TERS signals are not strong because there is only one tip working as a signal amplifier. Moreover, if the measurements are under aqueous environments, the species in solution lead to easy contamination of the tip, resulting in distinct interference of Raman signals from the substrate being tested. Therefore, Tian’s group developed a new generation of Raman technique in 2010, known as “shell-isolated nanoparticleenhanced Raman spectroscopy” (SHINERS) to overcome the limitations of materials and morphology generalities in SERS.114 In general, gold or silver nanoparticles are employed with a layer of ultrathin and pinhole-free inert silica shell. Each Au core-inert shell nanoparticle can be considered as a TERS tip. Under a laser spot, the shell-isolated nanoparticles monolayer is equal to

1.4. Scope of this Review

The main objective of this review is to focus on the importance of various core−shell nanoparticles and their applications in SERS. On the basis of the origin of SERS signals, core−shell nanoparticles can be classified into two following categories: ultrathin shell and thick shell (section 2). Nanoparticles with an ultrathin shell are developed based on “borrowing SERS activity” strategy to overcome the materials and morphology generality limitations of SERS. With an ultrathin transition metal shell, reactions catalyzed by transition metals can be detected and monitored by the in situ SERS method. Ultrathin dielectric or inert shell leads to the isolation of the inner plasmonic core from probe molecules and environment, thus the target molecules can be characterized and identified on any surface. On the other hand, core−shell nanoparticles with a thick shell such as glass shell, polymer shell, and molecular shells, including DNA, protein, or other biomolecules have also been developed and utilized for various applications. In section 3, we introduce the theory of enhancement mechanism of various surface-enhanced spectroscopic techniques with core−shell nanoparticles, highlighting mainly the physical enhancement followed by three generations of SERS hotspots. Various procedures for synthesis of core−shell nanoparticle synthesis are discussed in section 4 as well as various applications of core−shell nanoparticles in SERS are also elaborated in section 5. To provide a clear elucidation, applications of core− shell nanoparticles in SERS have been summarized in several fields, including electrochemistry, bioanalysis, sensor, food safety, environmental safety, cultural heritage, materials, catalysis, and energy storage and conversion. Finally, some developments 5007


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molecules increases, thus, the shell must be ultrathin to obtain strong enhancement. 2.2.1. “Borrowing SERS” Strategy. On the basis of Weaver’s electrochemical deposition method, Tian’s group developed a versatile and more straightforward chemical method to fabricate Au core transition metal shell (Au@TM) nanoparticles to expand SERS to transition metals.99−102,128,129 The inner core is a highly SERS-active metal nanoparticle which can generate strong electromagnetic field to enhance the Raman signal of molecules adsorbed on coated transition metals. Because the nanoparticle surface is completely covered by a transition metal, the surface properties of the Au@TM nanoparticles are similar to bulk transition metal properties. The advantage of the chemical method is that the shell thickness can be easily controlled by the concentration of transition metal salt solution. This chemical synthesis approach is simple, convenient, fast, and the experimental steps were reduced largely. Moreover, the synthesized nanoparticles can be directly assembled on an electrode surface without any coupling agent, thus, the electrode potential can be controlled accurately. The feasibility of Au@TM nanoparticles in SERS can be demonstrated by the 3-dimensional finite-difference timedomain (3D-FDTD) simulations. As shown in Figure 3, the

of new functional/materials core−shell nanoparticles for SERS and for other surface-enhanced spectroscopies such as shellisolated tip-enhanced Raman, infrared, fluorescence, nonlinear, and SPR spectroscopies are discussed in section 6.

2. CLASSIFICATION AND TECHNIQUES 2.1. Classes of Core−Shell Nanoparticles in SERS

On the basis of the origin of SERS signals and their enhancement mechanism, core−shell nanoparticles in SERS can be broadly categorized into two following types: core−shell nanoparticles with ultrathin shell and thick shell, respectively. The nanoparticles with ultrathin shells mainly “borrow” the strong electromagnetic field of SERS-active core to enhance the Raman signals of molecules on or near the shell surface; however, the nanoparticles with thick shells are usually accompanied by multifunctions to broaden SERS applications. Figure 2 clearly illustrates that the ultrathin shell can be classified into two categories: metal shell and nonmetal shell. The former refers to the transition metal shell, such as Pt, Pd, and Ru. The latter includes silica shell, Al2O3 shell, graphene shell, TiO2 shell, carbon (C) shell, and other oxide shells. Metal shell nanoparticles are mainly developed based on the “borrowing SERS activity” strategy to overcome the material limitation of SERS, which allows SERS studies on other metals, in particular, transition metals. Thus, the reactions catalyzed by transition metals can be easily detected and monitored by the in situ SERS method. Furthermore, nonmetal ultrathin shell nanostructures such as silica shell-isolated nanoparticles are considered as the isolated mode to protect the inner plasmonic core (Raman signal amplifier) and avoid direct contact with probe molecules and environment. With this isolated mode, the long-standing limitations of material and morphology generality have been overcome to a large extent. Overall, SHINERS is an appropriate approach to detect, characterize, and identify molecules on various materials and substrates, as well as single-crystals. Thick shell nanoparticles consist of metal shell nanoparticles and nonmetal shell nanoparticles with various multifunctions. The combination of Au−Ag or Ag−Au core−shell colloids possesses the properties of both metals, thus broadening the interval of available excitations. The combination of magnetic material with Au or Ag shell makes it suitable for bioseparation and detection purposes. Moreover, nanoparticles with silica core and Au/Ag shell are suitable materials in NIR region. Besides, nanoparticles with thick nonmetal shells such as thick silica shells, polymer shells or molecular shells including DNA and protein are usually used as SERS tags for bioanalysis and bioimaging.

Figure 3. FDTD stimulation of the electromagnetic field distribution of Au@Pt nanoparticles (left). The relationship of SERS enhancement with the distance from Au core (right). Reproduced with permission from ref 102. Copyright 2007 the Royal Society of Chemistry.

electromagnetic field is still strong around the Pt shell surface, if the transition metal shell is ultrathin. Therefore, Raman signals of molecules adsorbed on the Pt or Pd shell can be enhanced by the strong electromagnetic field. On the basis of 3D-FDTD simulation in Figure 3, we found that when the shell thickness increases, the distance between molecules and Au core increases and the electromagnetic field enhancement decreases exponentially. Eventually, the SERS signals decrease exponentially. Thus, in order to obtain strong SERS signals, the coated transition metal shell thickness must be controlled in a few nanometers, and the probe molecules should be located in close proximity to the metal core. 2.2.2. SERS Activity. Au@TM nanoparticles were coated with ultrathin and pinhole-free transition metal shells (several atomic layers) by a seed-mediated growth method. To demonstrate the SERS activity of the Au@TM nanoparticles, 55 nm [email protected] nm Pt nanoparticles were used as an example to compare with electrochemically roughened Pt electrodes and Pt nanocubes, by using CO as probe molecules.102 As shown in Figure 4, it can be easily found that the SERS signal of Au@Pt nanoparticles is about 200 times stronger than roughened Pt electrodes and about 40 times higher than 12 nm Pt nanocubes.

2.2. Ultrathin Transition Metal Shells

SERS enhancement is determined by the nature and morphology of metal nanomaterials. For example, only a few free-electron-like metals like Au, Ag, or Cu can generate strong electromagnetic enhancement, thus the probe molecules should be located near the noble metal surface.54,57,120,127 Many efforts have been made to break the limitation of material generality and expand SERS applications to other metals, especially the transition metals which are widely used in catalysis. For example, roughened transition metal electrodes or Pd/Pt nanoparticles were developed as SERS substrates. However, these substrates still cannot support strong enhancement on transition metals. Weaver’s group developed a “borrowing” strategy by coating a transition metal layer on a roughened Au electrode to overcome this problem.98 However, there will be an exponential decrease of enhancement effect if the distance between metal core and probe 5008


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and FDTD simulations. The experiment data were in good agreement with the FDTD calculations. The enhancement of Raman signals will also be affected by the size of metallic cores.79 Because the dielectric constant of nanoparticles depends on the size of the nanoparticles, the SERS effect of nanoparticles depends on two competing effect: surface scattering and radiation damping. The surface scattering can affect the imaginary part of the dielectric constant, which leads to a dramatic change in surface plasmons, hence the enhancement of SERS is strongly influenced. Generally, larger nanoparticles may induce stronger enhancement, when excited by longer wavelength lasers, such as 633 or 785 nm. On the other hand, radiation damping will become severe when the particle size increases. This core size effect has been studied by changing the size of Au cores with 0.7 nm Pd shell (Figure 5c). It was found that the maximum enhancement was obtained by Au@Pd nanoparticles with around 120−130 nm core size. The experimental data was in good agreement with the simulation results.130 In the case of core−shell nanoparticles, we found an interesting phenomenon that when the Au nanoparticles were coated with 1−2 layers of Pt or Pd, the frequency of CO adsorbed on Pt surface shifts to a higher wavenumber and to a lower wavenumber for Pd compared with bulk metals (similar to thick shells).131 The observed phenomena with shell-dependent Raman shift frequency of adsorbed CO molecules could be used to reveal the different work functions of these three metals and various electronic structures of the surface metals. Furthermore, Au@TM nanoparticle-based SERS can be used for efficiently predicting minor changes of the chemisorption or electro-chemisorption structures in a target system. For example, we used Au@Pd nanoparticles with different shell thicknesses for CO electrooxidation. As shown in Figure 6c, the peak potential of CO oxidation was 0.57 V for the Au@Pd nanoparticles with a

Figure 4. SERS spectra of CO adsorbed on (a) 55 nm [email protected] nm Pt nanoparticles, (b) 12 nm Pt nanocubes, and (c) a roughened Pt electrode. Reproduced with permission from ref 102. Copyright 2007 the Royal Society of Chemistry.

2.2.3. Effect of Shell Thickness and Core Size. The SERS activity of core−shell nanoparticles depends on two elements, the thickness of shell and the size of metallic core. Figure 5a

Figure 5. (a) SERS spectra of CO adsorbed on Au@Pt NPs with different Pt shell thicknesses in a solution of 0.1 M HClO4 saturated by CO at 0.0 V. (b) Dependence of the normalized intensity of CO adsorbed on Au@Pt nanoparticles on the Pt shell thickness (black line) and the corresponding FDTD calculations (red line). Inset is FDTD simulation of the electric field distribution on the surface of 55 nm Au@ 1.5 nm Pt dimer. Reproduced from ref 101. Copyright 2006 American Chemical Society. (c) SERS spectra of p-aminothiophenol (PATP) adsorbed on Au@Pd nanoparticles with different core sizes. (d) The normalized SERS intensity of PATP to Au core size. Reproduced with permission from ref 130. Copyright 2008 John Wiley & Sons, Ltd. Figure 6. SERS spectra of CO adsorbed on (a) Au@Pd and (b) Au@Pt nanoparticles with different shell thicknesses (a−e) (0.35, 0.7, 1.4, 2.8, and 7.0 nm) in a CO-saturated solution of 0.1 M HClO4 at 0.0 V. Linear voltammograms of CO electrooxidation of (c) Au@Pd/GC electrode and (d) Au@Pt/GC electrode with different shell thicknesses (0.35, 1.4, and 7.0 nm) in a CO-saturated solution of 0.1 M HClO4. Scan rate: 0.1 V/s. Reproduced from ref 131. Copyright 2016 American Chemical Society.

shows the SERS spectra of CO adsorbed on 55 nm Au@Pt nanoparticles with different shell thicknesses.101 As the Pt shell thickness increases, the SERS signal of CO decreases rapidly. Figure 5b shows the effect of Pt shell thickness to the normalized integrated intensity of C−O stretching mode which belongs to on-top adsorbed CO and the comparison between experiments 5009


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Figure 7. Schematic of three different working modes. Contact mode: (a) bare nanoparticles and (b) Au core transition metal shell nanoparticles. (c) Noncontact mode: TERS. (d) Shell-isolated mode: Au core inert silica shell nanoparticles. Reproduced with permission from ref 114. Copyright 2010 Nature Publishing Group.

were used to examine probe molecules adsorbed on the metal shell surface. In this case, the molecules are in direct contact with nanoparticles, which can be called as contact mode SERS (Figure 7).114 With contact working mode, enhanced Raman signals can be obtained, however, the amplifier and carrier must be integrated in the same nanostructure. Thus, to obtain the information of molecules on a non-SERS-active material, the non-SERS-active material must be coated on a SERS-active material like Au or Ag, which is usually very challenging and complex. For many other materials, such as oxides, insulators, polymers, or biological membranes, it is very difficult, if not impossible to coat them as uniform ultrathin shells onto Au/Ag nanoparticles. Moreover, atomically flat surfaces of various single-crystals, which are commonly used in surface science, electrochemistry, and semiconductor technology, were almost inaccessible by SERS method. Later, TERS was invented in 2000, with a noncontact working mode (Figure 7c).110,145,146 In this mode, probe molecules are adsorbed on any substrate and a Au or Ag tip can act as a Raman signal amplifier to enhance the Raman signal of molecules without any direct contact. Hence, the information on molecules on any substrate can be detected easily with a high spatial resolution.113 By this way, the substrate generality problem can be solved. While for TERS, the total Raman signal is rather weak because only one tip is acting as an amplifier. In 2010, Tian’s group invented “shell-isolated nanoparticleenhanced Raman spectroscopy” (SHINERS)114,117,147 with a shell-isolated working mode which combines borrowing strategy and TERS (Figure 7d). In this case, every gold nanoparticle core acts as a tip, thus it is equal to hundreds or thousands of tips working at the same time in the laser spot. Therefore, the SHINERS signal is 3 orders of magnitude greater than a single TERS tip. 2.3.2. Advantages of Shell-Isolated Mode over Contact Mode. SHINERS method employs the concept of shell-isolated mode which is a combination of direct contact (SERS) mode and noncontact (TERS) mode. This shell-isolated mode overcomes the long-term limitation of SERS because it allows SERS measurements on various target systems that were inaccessible

monolayer of Pd, while it is 0.7 V for bulk Pd (7.0 nm Pd shell). It indicated that Au@Pd nanoparticles with a monolayer of Pd exhibited better performance rather than bulk Pd. On the contrast, it exhibits contrary phenomenon in Au@Pt system. 2.2.4. Multimetal Shells. Compared to bimetallic nanostructures, trimetallic nanostructures usually exhibit more outstanding catalytic properties because of the synergistic effect of multi metals.132,133 At the same time, the Au core may affect the catalytic performance of transition metal shells, because of the synergistic effect of multimetals.134,135 The synergistic effect of trimetallic nanostructures has been investigated by many groups. The surface electronic structure and the high utilization of Pt or Pd will contribute to the enhanced mass activity.136−140 Sun’s group has synthesized various core−shell nanoparticles, such as Au@FePt3 with Au core and FePt3 alloy shell as a highly durable electrocatalyst to be applied in the oxygen reduction reaction (ORR), which is important for proton-exchange membrane fuel cells.141 Pt-on-(Au@Pd) trimetallic structure was also prepared by Xu’s group for catalytic ethanol electrooxidation.142 Trimetallic nanostructures presented high reactivity and fair stability for electrocatalytic reactions. Furthermore, due to the plasmon-enhancement from the Au/Ag cores, the core−shell nanoparticles can also be applied for detecting and monitoring the catalytic reaction processes by SERS. On the basis of the excellent properties of trimetallic catalysts, Tian’s group synthesized Au@Pd@Pt trimetallic nanoparticles and in situ monitored the catalytic process of formic acid and CO oxidation.143 Schlücker’s group also fabricated Au/Pt/Au core− shell nanoraspberries for in situ quantitative monitoring of Ptcatalyzed reactions by SERS method.144 The in situ quantitative SERS spectra provide the chemical identity of the involved molecular species and also quantify their relative contributions, which is a primary requirement for establishing a reaction mechanism and testing kinetic models. 2.3. Ultrathin Nonmetal Shells (SHINERS)

2.3.1. Shell-Isolated Mode. With the “borrowing” strategy for transition metals, the material limitation of SERS has been solved to a certain extent. The nanostructures mentioned above 5010


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previously because of the material and surface morphology issues. Moreover, SHINERS can be used in many complicated environments such as in aqueous solutions and biological systems. Generally, researchers use bare Au or Ag NPs for surfaceenhanced Raman studies because they present a strong SERS effect. Since the first experimental reports on SERS, the contact mode measurements have been employed by many researchers in the field of SERS. Thus, we believe it is indispensable to make a detailed and systematic discussion and comparison about the superiority of SHINERS method compared with the traditional SERS techniques. Shell-isolated nanoparticles (SHINs) possess significant advantages over bare nanoparticles,147 for instance, the inert silica shell protects the nanoparticles from agglomeration, oxidation, and also prevents the direct contact between SERSactive metal cores and probe molecules/surface. Though the bare silver or gold nanoparticles exhibit strong Raman signals which have been utilized by many researchers for various applications, there are several problems associated with the contact mode SERS measurements using bare nanoparticles. First, bare nanoparticles will have contact with chemical environments (Figure 8a), especially in liquid environments,

Second, in contact mode measurements, charge transfer can possibly occur between the metal nanoparticles and other metallic substrates (Figure 8b) because their Fermi levels are different. This may remarkably affect the electronic structure of the target system.149,150 Especially in a catalytic reaction with a transition metal (e.g., Pt or Pd), the electronic structures of the catalyst may affect the catalytic performance. For instance, Figure 8e shows the SERS and SHINERS spectra of carbon monoxide adsorbed on a Pt single-crystal surface. Two visible Raman bands at 2060 and 2125 cm−1 were observed. The former is the stretching vibration mode of CO adsorbed on Pt, and the latter band at 2125 cm−1 is the CO adsorbed on Au. Actually, the band of 2125 cm−1 is unwanted because it may lead to false interpretation of the spectrum. It can be seen that the frequency of CO vibration on Pt is different in the SHINERS and SERS spectra. In SHINERS spectra, the band is at 2072 cm−1 and it shifts to 2060 cm−1 in SERS spectra. The shift is because of the charge transfer from bare Au nanoparticles to the Pt single-crystal surface due to their different work functions (the function of Au is 5.1 eV and Pt is 5.65 eV).151 The charge transfer effect may increase the electron density at the Pt surface, leading to the obvious shift of CO stretching frequency. The true CO stretching frequency on Pt surface at 2072 cm−1 was obtained by SHINERS. Thus, SHINERS can provide original and important vibrational information on surface-adsorbed species. Third, target molecules may interact with bare nanoparticles, thus, the electron density of target molecules and its adsorption configuration may be changed. As shown in Figure 8c, molecules may possibly adopt a double-end adsorption configuration but not a single-end adsorption configuration.152 Hence, the direct contact with probe molecules will lead to obvious changes in the SERS spectral features and also lead to misinterpretation of spectra.153 In addition, some probe molecules may undergo photocatalytic reactions under laser excitation due to a direct contact of its functional group with the metal surface (yellow molecules in Figure 8c).153−157 Finally, SHINs exhibit better chemical stability and long-term performance than bare nanoparticles due to the inert dielectric coating. Thus, SHINs are widely used for several practical applications. As shown in Figure 8 (panels f and g), when pyridine was added into Au@SiO2 SHINs sol, the SHINs sol remains in stable condition for as long as 240 h.158 However, without the protection of the silica shell, the color of bare Au nanoparticles sol changed in only 3 min. After 15 min, the nanoparticles were aggregated completely. By this way, SHINERS can overcome the above-mentioned problems because of its unique isolated mode. With an ultrathin and inert layer coated on the Au core, the Au particles are isolated with probe molecules. Therefore, the method is feasible in both the liquid and gas phase. Importantly, Raman signals will be originated from the target surface/molecules and will not be affected by other factors. The inert silica shell prevents the direct contact of Au nanoparticles with the substrate surface, thus no charge transfer occurs between the nanoparticle and the substrate. Therefore, the molecular orientation/electronic structure will not be changed, and photocatalysis reaction will occur. 2.3.3. 3D-FDTD Simulations of Electric Field Distribution. Generally, 3D-FDTD method158,159 is used to simulate the electric field distribution, to calculate the Raman enhancement of SHINs and to explore the mechanism of SHINERS/SERS. Figure 9 illustrates the electric-field distribution of a 2 × 2 array of SHINs on a smooth surface of gold.114 As the shell becomes

Figure 8. Schematic illustrations of several disadvantages in contact mode measurements: (a) contact with a chemical environment. (b) Charge transfer between a nanoparticle and a substrate. (c) Contact with probe molecules. (d) Au core is isolated by an inert shell. (e) Comparison of SERS and SHINERS spectra of CO adsorption on a Pt(111) electrode in 0.1 M HClO4 solution saturated with CO gas. The photographs display the stability comparison between (f) shell-isolated nanoparticles and (g) bare nanoparticles. Reproduced with permission from ref 147. Copyright 2015 Royal Society of Chemistry.

such as in electrochemical and biological systems, or in gas environments, for heterogeneous catalysis. Many matrix species in these target systems may adsorb on the nanoparticles surface, eventually the Raman signal of the adsorbed species on bare nanoparticles will be observed with the Raman signal of target species adsorbed on the target surface. Even if there is no matrix species in the gaseous or liquid environment, probe molecules may diffuse from the target surface to the bare nanoparticles surface.148 5011


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stronger SERS enhancement effect and their SPR property can be tuned in a wide range of wavelength.165,166 Besides different shapes and sizes of plasmonic cores, different shell materials such as Al2O3, carbon shell, TiO2, MnO2, Ag2S, and graphene shell have also been prepared for various applications. Due to the acidic and basic resistance of MnO2, plasmonic core can be coated with an ultrathin MnO2 shell to greatly enhance their long-term stability in strong acid or base media as well as thermal stability.121 To further expand the application of SHINERS, semiconductor TiO2 shell can be coated on metal cores for photocatalysis and solar cells.166 Another promising shell in SHINERS would be the graphene shell.125,126 Graphene is a perfect 2D atomic crystal with a single atom layer of carbon, and it exhibits much unique superiority for SERS, such as unique electron structure, chemical inertness, atomic uniformity, single atom layer thickness, pinhole-free, and excellent biocompatibility. Therefore, it is a promising material for SERS studies, for instance, graphene-isolated nanoparticles combine the unique properties of both SERS-active metal nanostructure and graphene. Moreover, the SERS enhancement may come not only from the electromagnetic field enhancement of SERS-active metal core but also from the chemical enhancement of graphene, which is called graphene-enhanced Raman scattering (GERS).125 Though great progress has been achieved recently, there are still several limitations which may restrict the applications of SHINERS. (1) The enhancements of traditional SHINs should be further improved, so that ultratrace amounts or even single molecules of surface species with short lifetime can be detected by SHINERS. (2) The shell must be ultrathin to obtain high enhancement. However, it is still a great challenge to coat a pinhole-free shell whose thickness is less than 1 nm, especially on highly SERS active silver nanoparticles. (3) The stability of SHINs should be improved, so that it can be applied in some harsh conditions, such as strong acid or base solutions, strong oxidative environments, and high temperature. (4) The functionality of SiO2 shell is not very good, which greatly limits its applications. The shell material can be extended to other materials for wider applications, such as Al2O3, TiO2, MnO2, or 2D materials (like graphene or MoS2). (5) SHINERS lacks spatial resolution. Shell-isolated tip-enhanced spectroscopy can be developed to overcome this limitation.

Figure 9. 3D-FDTD simulated electric field distribution for a 2 × 2 array of SHINs on a smooth gold surface under 633 nm laser. (a) Side view and (b) top view. (c) SHINERS spectra of pyridine adsorbed on a smooth Au surface placed with Au@SiO2 NPs with different silica shell thicknesses. (d) The shell thickness dependence of the integrated SHINERS intensity of pyridine (Py) (black ■) and the corresponding 3D-FDTD calculation result (red ▲). Reproduced with permission from ref 114. Copyright 2010 Nature Publishing Group.

thinner, the distance between the Au core and the smooth Au surface decreases. Thus, LSPR can be excited in the nanogap between Au core and Au plate to create a strong electromagnetic field enhancement, which will lead to a huge enhancement of Raman signals of probe molecules located in the nanogap. By this way, a monolayer of probe molecules adsorbed on a substrate can be detected effectively. The 3D-FDTD calculations prove that the electric field is enhanced about 85 times for the SHINs of 4 nm shell and about 142 times for the SHINs of 2 nm shell. The highest enhancement of Raman signal is 5 × 107 and 4 × 108 for the SHINs of 4 nm shell and 2 nm shell, respectively. The strongest enhancement is located at the nanogap between the particle and the substrate, which are called hot spots. Since the Raman enhancement is dependent on the distance between the Au nanoparticle and the smooth gold surface, 3DFDTD was used to simulate with a 2 × 2 array of SHINs with different shell thicknesses. For Raman measurements, pyridine was used as the probe molecule and the results were consistent with the simulation data. As expected, in both experimental and theoretical results, when the shell thickness was increased, the Raman intensity decreased exponentially. 2.3.4. Various Types of SHINs. After SHINERS discovery, various types of SHINs were developed with an ultrathin inert shell and different metallic cores. In general, there are two kinds of preparation methods: chemical method and physical method. To meet different requirements or applications, SHINs can be prepared in different sizes, shapes, and materials. For example, in order to obtain a higher enhancement, 120 nm Au core SHINs were used because they exhibit strongest electromagnetic field enhancement at 633 nm excitation.160 Furthermore, nanocubes and nanorods SHINs were also synthesized because of their tunable surface plasmon resonance (SPR).161−164 By adjusting the aspect ratio of nanorods, the SPR absorption can be tuned to the near-infrared region, which can be useful for biological systems. To further improve the SERS enhancement factor, Ag SHINs were prepared because Ag nanoparticles exhibit much

2.4. Thick Shells

Core−shell nanoparticles with thick shells are widely used in SERS. On the basis of their functions and applications, these nanoparticles can be classified into several categories. (1) Metalmetal core−shell nanoparticles usually refer to the Au/Ag core− shell nanoparticles with tunable SPR properties, outstanding enhancement, and stability. Because of their tunable SPR properties, the preparation of Ag/Au core−shell nanoparticles allows the combination of the SERS properties of both metals and allows a wide range of excitation wavelengths. (2) SERS markers or tags, namely core−shell nanoparticles with Raman reporter molecules between the plasmonic core and the inert shell. The shell is usually made up of silica, polymer, or large molecules including DNA, protein, or other biomolecules and these nanostructures are suitable for bioanalysis and bioimaging. (3) Dielectric-metal core−shell nanoparticles, namely nanoshell, invented by Halas et al. are suitable materials for NIR region. (4) Metal film over nanospheres (FON). In this nanostructure, silica or polystyrene spheres can be self-assembled orderly on solid surfaces then followed by deposition of Au or Ag film. This FON 5012


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instance, Au−Ag bimetallic core−shell multilayer nanostructures were synthesized by using ascorbic acid (AA) and cetyltrimethylammonium bromide (CTAB), as reducing and capping agents, respectively. These materials showed different optical properties, as proved by the UV−vis spectroscopy data. As shown in Figure 10b, bare Au nanoparticles exhibit a small extinction peak at about 520 nm. After coating with Ag shell, it blue shifts to much lower values (at about 440 nm), which is around the typical SPR band of Ag. This peak would shift back to about 600 nm after the formation of Au@Ag@Au nanocomposites, which is similar to that of bare nanoparticles but with a much stronger intensity.168 More interestingly, both SPR properties of Au and Ag can be preserved by further coating Ag shell on Au@Ag@Au. These obvious changes directly demonstrate the possibility of tuning SPR properties by core− shell nanostructures. 2.4.2. Raman Marker (Tag). SERS is an ultrasensitive analytical technique, which has been widely used in many systems, especially in biological systems. One of the most applied concepts is “SERS tags” which combines plasmonic nanomaterials and Raman markers and protected by a thick shell.176,177 This design provides extremely strong Raman signals for Raman markers because the marker molecules are directly adsorbed on SERS-active materials like Au or Ag. Moreover, the protective shell not only protects nanoparticles from aggregation but also has good biocompatibility. Therefore, it is suitable for bioanalysis.178,179 In this type of studies, silica, polymer, or other biomolecules are usually used as the shell materials. Silica has many inherent advantages, such as chemical inertness, biocompatible property, and being optically transparent. These properties make silica a promising candidate in optics, biotechnology, and other fields. Besides silica, some nontoxic and biocompatible polymers have attracted more attention recently. As compared with silica, polymers possess several advantages such as nontoxic, flexible shell thickness, and weak adsorption, and the abilities of biodistribution and pharmacokinetic properties are superior.180−183 Therefore, some researchers pay more attention on polymer shells for biological applications. 2.4.2.1. Silica Shell. The first work of silica shell SERS tag was demonstrated in 2003 by Natan’s group. They prepared Au or Ag nanoparticles with Raman active molecules adsorbed on their surface then coated silica layer on them to provide mechanical and chemical stability.184 They designed and synthesized the kind of “glass-coated, analyte-tagged nanoparticles (GANs)” which included the Raman tag molecules onto the surface of bare Au or Ag colloids and then was protected by the silica layer. Notably, SERS has various valuable implications compared to other approaches such as fluorescence, electrochemical detection, chemiluminescence, etc. For instance, the width of Raman peaks is much narrower than fluorescence bands, and there is no photobleaching in Raman spectroscopy (when laser power is not high). The excitation of Raman can be shifted to the red or nearinfrared region which can minimize the effect of background fluorescence. Therefore, SERS is a suitable technique for the immunoassay. Generally, SERS immunoassay involves three steps as shown in Figure 11. First, antibodies should be immobilized on the substrate surface, and then the immobilized antibodies can capture antigens in solution. Next, these bonded antigens can capture the molecule-labeled nanoparticles, on which other antibodies assembled. By this way, a sandwich substrate can be developed and applied in the immunoassay. Because of the specific recognition of the antigen−antibody, nanoparticles modified with antibodies will only be interacted

structure provides good reproducibility in SERS studies and has promising applications in quantitative SERS. (5) Magnetic-metal core−shell nanoparticles combine magnetic materials with Au or Ag because it is a suitable candidate for bioseparation and detection purposes. Particularly, gold shell has good biocompatibility, thus the multifunctional magnetic-Au core−shell nanoparticles are suitable for bioseparation, biological analysis, and subsequent immunoassay. Additionally, there are some other hybrid core−shell nanostructures which can provide extremely strong SERS enhancement with single-molecule detection sensitivity. 2.4.1. Metal−Metal Core−Shell Nanoparticles with Tunable SPR. Plasmonic materials, such as Au or Ag, can generate enhanced electric field around nanoparticles during the interaction with an electromagnetic radiation, leading to an enormous enhancement of Raman signals of molecules on the nanoparticle surface. The LSPR effect is generated only when the frequency of incident light matches with the surface electrons oscillation, therefore, it is very sensitive to the electronic structure of nanomaterials.167−170 Thus, the LSPR or the SERS enhancement is strongly affected by the size, shape, and material of nanoparticles, which urges the researchers to tune the SPR properties of nanomaterials for specific applications. However, it still remains a great challenge to prepare more active plasmonic nanomaterials, such as Ag with complex structures, though interesting SPR properties of them have been predicted. An alternative strategy to solve this problem is employing a seed-mediated method to synthesize such nanomaterials with desired shapes or compositions by using gold nanoparticles as a template, whose size and shape can be easily manipulated due to its chemically inert property.171,172 For example, it is well-known that the SPR peak of Ag nanoparticles will have a red-shift remarkably on larger particles. However, the direct synthesis of homogeneous Ag nanoparticles with a size bigger than 100 nm is very difficult, which greatly limits the application of Ag nanoparticles in the near-IR region. Li’s group prepared Ag nanoparticles with a size range from 50 to 300 nm by a seed-mediated method.173 As shown in Figure 10a, the visible absorption peaks red shift greatly with the increasing size, indicating the tunable SPR properties of Ag nanoparticles. On the other hand, the optical properties can be tuned in a much wider range for core−shell nanoparticles, as it will be affected by both core and shell material. Thus, broader SPR tunability can be achieved by independently manipulating the structure and composition of core or shell material.174,175 For

Figure 10. UV−vis absorption spectra of (a) different sizes and (b) structures of Au@Ag nanoparticles. Reproduced with permission from ref 173. Copyright 2016 John Wiley & Sons, Ltd. Reproduced with permission from ref 168. Copyright 2005 Royal Society of Chemistry. 5013


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Figure 11. (a) Schematic diagram of the GANs synthesis and (b) the use of GANs to label an immunoassay. Reproduced from ref 184. Copyright 2003 American Chemical Society.

Hg2+ and it exhibits high sensitivity and selectivity.187 Chen’s group prepared Au nanoparticles coated with polystyrene-blockpoly(acrylic acid) (PS154-b-PAA60), which is an amphiphilic diblock copolymer. The PS layer is hydrophobic, thus the contact between Au nanoparticles and ionic oxidants can be avoided. The PAA is hydrophilic, and can not only help to prevent aggregation but also act as an anchor. Kim’s group prepared Ag powders coated with cationic poly(allylamine hydrochloride) (PAH) and anionic poly(acrylic acid) (PAA) polyelectrolytes.188,189 The PAA layer could be derivatized with biotin-derivatized poly(Llysine). It can be used in molecular sensing and recognition by SERS. Duan’s group developed a new strategy to synthesize core− shell metal nanoparticles with an interior, Raman tag-encoded nanogap by taking advantage of nanoparticle templated selfassembly of amphiphilic block copolymers and localized metal precursor reduction by redox-active polymer brushes.190,191 The nanogap size can be tailored flexibly in the range of sub-2 nm, leading to the highest SERS enhancement. And the surface functionalization of the nanogapped Au nanoparticles with aptamer targeting ligands allows for specific recognition and ultrasensitive detection of cancer cells. 2.4.3. Dielectric-Metal Core−Shell Nanoparticles (Nanoshell). Another important type of core−shell nanoparticles is coating a metal shell onto a dielectric nanoparticle core, which is called “nanoshell” and has been systematically studied by Halas et al. In this strategy, the dielectric material serves as a core and coated with a layer of nobel metal.25,192 It can provide 100 times stronger enhancement than a single nanoparticle. The nanoshell structures show tunable LSPR properties and can generate strong optical absorption and scattering. By tuning the ratio of core and shell, the extinction range can be tuned from the visible region to the near-infrared region. Figure 13a shows the theoretical optical resonances of nanoshell with different shell thicknesses, which were calculated by discrete dipole approximation (DDA) method.42 The core diameter was 120 nm, and the shell thickness varied from 20 to 5 nm, leading to the change of their SPR from ∼700 nm to ∼1000 nm. Figure 13b shows the growth of Au shell on 120 nm diameter silica nanoparticles. The lower five spectral curves follow the evolution of the optical absorption as coalescence of the gold layer progresses. This behavior is responsible for the initial red shift in the peak absorbance from 550 to 800 nm. And it can be seen that with the shell growing thicker, the SPR is shifted to lower wavelengths. From right to left, the shell thickness was 20, 26, 29, and 33 ± 4 nm. This kind of nanomaterial is essential for biological and medical applications. Particularly, in the nearinfrared region, the optical transmission through tissue will be good and the surface of the gold shell can be modified easily to make it easier to be combined with biological systems.5,46,47,193

with the corresponding antigen. Finally, the antigen can be identified by the Raman signals of molecule markers. The antigen or antibody will not affect the Raman signals because the silica shell thickness is more than 5 nm, the outer antibody molecules are too far from the inner Au core, thus the Raman signal of antibodies will not be enhanced. Nie’s group also reported a similar structure by using dye molecules embedded in nanoparticles with core−shell structure.185 2.4.2.2. Polymer Shell. One of the widely used polymers in SERS tag is PEG (polyethylene glycol). Nie et al. utilized a mixture of thiol-modified PEG and heterofunctional PEG (SHPEG-COOH) to coat on Au nanoparticles which already bonded with a Raman marker (Figure 12).180 The biocompatible and

Figure 12. Schematic diagram of cancer cell targeting by using antibodyconjugated SERS nanoparticles. Preparation of targeted SERS nanoparticles by using a mixture of SH-PEG and a heterofunctional PEG (SH-PEG-COOH). Covalent conjugation of an EGFR-antibody fragment occurs at the exposed terminal of the heterofunctional PEG. Reproduced with permission from ref 180. Copyright 2007 Nature Publishing Group.

nontoxic nanoparticles were applied for in vivo tumor targeting and SERS detection. The PEG shell ensured that the gold gel can safely biodistribute in vivo, and the SH-PEG-COOH could have covalent conjugation with an epidermal growth factor receptor (EGFR)-antibody fragment, to detect EGFR-positive and EGFR-positive cancer cells. The SERS spectra show the difference from two kinds of cancer cells in control experiments and also in in vivo target detection. Additionally, there are many other polymers that have been used for SERS studies. For instance, Gao et al. fabricated Ag@ polyaniline (PANI) nanofibers by a thermodynamically controlled process.186 Nanofibers with several micrometers length can be obtained by this way, and the polymer shell can be easily removed by N,N-dimethylformamide to generate 1D SERS substrate. Liu used an electrochemical method to prepare Au@polypyrrole (Au@PPy) core−shell nanocomplexes in aqueous solutions.183 Based on the responsive Raman intensity of PANI to Hg2+, Wang et al. prepared Ag@polyaniline to detect 5014


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electromagnetic field of the nanoshell surface is stronger than the nanosphere and its field intensity of individual nanoshells seems similar to that of the dimer structure which is in agreement with the experimental results. 2.4.4. Metal Film Over Nanospheres (FON). To obtain high reproducibility in SERS measurements, the uniformity of SERS substrates and the controllability of the metallic nanostructures morphology both play crucial roles in SERS measurements. A commonly used method called nanosphere lithography (NSL), which is used to fabricate ordered arrays for reliable SERS measurements.195,196 Generally, “film over nanospheres” (FON) are formed by depositing a metal layer on selfassembled dielectric nanospheres (Figure 15).196−198 The

Figure 13. (a) Theoretically calculated optical resonances of silica core gold nanoshells over a range of core radius shell thickness ratios. Inset is the visual demonstration of the tunability of metal nanoshells. (b) Growth of gold shell on 120 nm diameter silica nanoparticle. Reproduced with permission from ref 42. Copyright 2004 SAGE Publishing Group.

The nanoshell structure can provide a strong plasmon resonance at the surface.19 By optimizing the core−shell ratio, the surface plasmon resonance can be tuned to match with the excitation laser to enhance the localized electromagnetic field. Halas’s group utilized the nanoshell structure to investigate its Raman enhancement.194 They compared four kinds of structures, including individual gold nanoparticle, individual nanoshell, gold nanoparticles dimer, and nanoshell dimer. Pmercaptobenzoic acid (4-MBA), a nonresonant molecule, was used as a probe molecule. It was found that for Au nanoparticles, only dimers can produce strong SERS signal, and no signal was obtained from a single nanoparticle. While for the nanoshell, both individual and dimer structures can generate SERS signals with similar enhancement. They also used the FDTD method to stimulate the local electromagnetic field of these structures to interpret the contributions of every nanostructure to the SERS intensity observed in the experiment. As shown in Figure 14, the

Figure 15. Schematic illustration of the fabrication of Au or Ag metal film over nanospheres (FON) substrate. (a) Closed packed array of nanospheres. (b) View along a row of nanospheres. (c) View along a column of nanospheres. Reproduced from ref 197. Copyright 2002 American Chemical Society.

dielectric cores are usually polystyrene (PS) nanospheres or silica nanospheres, and they can be easily self-assembled on the substrate and placed in an orderly arrangement. Then Au or Ag metal layer can be deposited on the surface by physical vapor deposition.32 Besides, if the nanospheres are removed after Au or Ag deposition, Au or Ag triangles or spots array can be obtained (top-down method).199 With the same template, sphere voids structure can be obtained by electrochemical deposition on the substrate (bottom-up method) and followed by removal of the spheres.200 Further, these kinds of metal arrays could be applied in optoelectronic and photobiological devices. Van Duyne’s group used this method to get well-ordered Ag nanostructures for SERS because the NSL method is simple, inexpensive, and has great reproducibility and uniformity. For FON structures, the LSPR can be easily controlled by the core size and the thickness of the deposited metal to match with the excitation wavelength.201 Particularly, AgFON and AuFON have extremely high SERS enhancement and reproducibility. 2.4.5. Magnetic-Metal Core−Shell Nanoparticles. Recently, magnetic nanoparticles receive great attention due to its magnetic effects and stability. Importantly, magnetic properties of magnetic nanoparticles can be used for separation, bioseparation, or preconcentration processes.202−204 Moreover, magnetic nanoparticles can be combined with plasmonic materials to form bifunctional nanoparticles with plasmonic activities and magnetic properties. Recently, magnetic nano-

Figure 14. Electromagnetic near field enhancement at the excitation laser’s wavelength (633 nm) for (a) an isolated Au nanosphere, (b) an isolated Au nanoshell, (c) a roughened Au nanoshell, (d) an adjacent nanosphere pair with an axis perpendicular to the incident polarization, (e) an adjacent nanoshell pair with the interparticle axis perpendicular to the incident polarization, (f) an adjacent nanosphere pair with the interparticle axis parallel to the incident polarization, and (g) a nanoshell dimer with the interparticle axis parallel to the incident polarization. Reproduced from ref 194. Copyright 2005 American Chemical Society.

electromagnetic field distributions of individual nanosphere, nanoshell, and their dimers were calculated at the excitation laser of 633 nm. The dimers with an axis both perpendicular and parallel to the incident polarization were stimulated. It can be seen that when the axis is parallel to the incident polarization, the strongest electromagnetic field appeared at the junction of two particles, which is called a hotspot. For individual particles, the 5015


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Figure 16. Schematic illustration of the separation of magnetic-Au core−shell nanoparticles. Reproduced with permission from ref 205. Copyright 2016 Elsevier B.V.

realized the importance of the gap and also obtained hot spot SERS. Importantly, this SERS-active GSNDs method could apply to other dyes or biosystems. Another hybrid structure is metal core@molecule@metal shell.217−219 Briefly speaking, the authors prepared Au shell on a dye marked and DNA modified Au sphere, to obtain a well-defined ∼1 nm gap between the Au core and the Au shell (Figure 17b). This work showed good reproducibility, and the enhancement ability of the Au- NNPs was up to 108. Thus, this kind of nanostructures could achieve single-molecule level detection with a narrow signal intensity distribution.

particles have developed rapidly because of the manipulability in external magnetic fields (Figure 16).205 The plasmonic materials, such as Au or Ag, can generate LSPR to enhance the local electric field around nanoparticles to enhance the molecules adsorbed on the particle surface. Moreover, gold shell has good biocompatibility and can be applied in various biological systems. Hence, the multifunctional magnetic-Au core−shell nanoparticles are suitable for bioseparation and subsequent immunoassay. Graham’s group mainly researches a range of functionalized metallic nanoparticles which can be used for a variety of different purposes including disease diagnosis, treatment, etc.206−211 Recently, they reported a “SERS off” competitive binding type assay utilizing a plasmonic silver (AgNP) and silver-coated magnetic nanoparticle (Ag@MNP) for the detection of the toxin bisphenol A (BPA).212 2.4.6. Hybrid Core−Shell Nanostructures. Among various SERS substrates, Au nanostructures are easy to prepare with good stability, while Ag nanostructures can exhibit higher enhancement; however, it is difficult to control the morphology and shape. Therefore, many hybrid core−shell structures are designed and fabricated to obtain strong SERS enhancement by combining their advantages. DNA is a commonly used molecule to assemble organized structures such as dimer or multimer nanostructures.213−215 These structures are well-defined and have good reproducibility which can act as promising SERS substrates. For example, Suh and co-workers fabricated gaptunable gold−silver core−shell nanodumbells (GSNDs),216 in which the gap was obtained by means of a single-target-DNA hybridization to form Au dimer and then coated with different shell thicknesses of silver to form a tunable nanogap(Figure 17a). When the dye molecule was placed at the junction of the GSNDs, the enhancement factor could be greater than 1010. This work

3. THEORY SERS combines vibrational Raman scattering from molecules with plasmonics of metal nanostructures. The entire theory of SERS, including physical and chemical enhancement mechanisms has been examined by several generations of scientists since the discovery of SERS in the mid-1970’s. The primary mechanism of SERS is electromagnetic (EM) enhancement. In this chapter, we first introduce the fundamental principle of enhancement strategy in Raman scattering, followed by the concept of SPR, especially LSPR which is important to understand the EM enhancement mechanism of SERS. Finally, we present the key concept in SERS, the SERS hotspot, and discuss three generations of SERS hotspots in detail. 3.1. General Consideration on Enhancement Strategy in Raman Scattering

The linear Raman intensity for a vibrational mode of a molecule following the Placzek’s polarizability theory with regard to the instrumental and surface factors could be evaluated by,220 k = Imn

27 π 5 I0(ν0 − νmn)4 32 c 4

∑ |(αρσ )mn |2 NA ΩQTmT0 × G ρσ

(1)

where I0 is the incident intensity, ν0 and νmn are the incident frequency and vibrational frequency (cm−1) of the kth normal mode, respectively, N is the number density of the adsorbates (molecules cm−2), A is the surface area illuminated by the laser beam (cm2), Ω is the solid angle of the collection optics (sr), QTmT0 is the product of the detector efficiency, the throughput of the dispersion system, and the transmittance of the collection optics, respectively. For more intense Raman intensities, the instrumental and detection factors such as QTmT0, Ω, N, and A could be optimized. Furthermore, the incident laser with much shorter wavelength such as ultraviolet laser could be used as an excitation source for much larger contribution from the term (ν0 − νmn)4 to total Raman intensities. Besides, the shorter wavelength could turn out to the enhanced polarizability derivatives (αρσ)mn due to possible resonance or preresonance Raman processes,221 in which the incident frequency is equal to or near the frequency of

Figure 17. Schematic illustration of two hybrid structures: (a) Au@Ag nanodumbells and (b) DNA-anchored nanobridged nanogap particles. Reproduced with permission from ref 216. Copyright 2010 Macmillan Publishers Limited. Reproduced with permission from ref 217. Copyright 2011 Macmillan Publishers Limited. 5016


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Figure 18. Schematics of Raman enhancement in a molecule−antenna system. Reproduced with permission from ref 222. Copyright 2011 Nature Publishing Group.

a molecular chromophore. However, a strong fluorescence background will be accompanied by the resonant Raman signals, which significantly decrease the contrast of the Raman spectrum of the analyte. Therefore, a high-quality linear Raman spectrum of monolayer sample could hardly be obtained without any large enhancement factor, G. The linear Raman intensity of a free molecule depends on the induced dipole p(ωR) at the Raman scattering frequency (ωR), which could be approximately expressed with the inner product of two independent quantities (eq 2). One is the electric field strength for the excitation of the measured molecules, the other is the Raman polarizability derivatives which reflects the modulation efficiency of the local incident photons to the inelastic Raman photons. For stronger electric field strength, we could either increase the incident electric field strength E0(ω0) or increase the local electric field strength (Eloc in eq 3) by means of some optical resonance processes supporting giant local EM enhancement, such as optical antenna (Figure 18a).222 However, the high-power incident laser typically limits the ultimate sensitivity by the surface damage threshold. The strategy to locally enhance electric field strength, g1(ω0, rm) in eq 3, is just to spatially modulate and focus incident field to the vicinity of the optical receiving antenna. This strategy could significantly increase local interaction power between photon and the molecules close to optical antenna but with less laser power. In this way, the molecules can experience the excess electric field strength at incident frequency ω0 and produce stronger Raman scattered photon at the Raman scattering frequency ωR. pm(ωR , rm) = αmI(ωR , ω0)E loc(ω0 , rm)

(2)

E loc(ω0 , rm) = g1(ω0 , rm)E0(ω0)

(3)

pA(ωR , rA) = αA(ωR ) ·E loc(ωR , rA) = C2αA(ωR ) ·pm(ωR , rm)

The total signals that detected by the detector at far field come from the additive local source p(ωR) = pm(ωR, rm) + pA(ωR, rA),223,224 p(ωR ) = pm(ωR , rm)(I + C2αA(ωR )) = pm(ωR , rm)g2(ωR , rA)

(6)

where g2(ωR, rA) is 1 + C2αA. Substitute eqs 2 and 3 to eq 6, p(ωR ) = [g1(ω0 , rm)] × [g2(ωR , rA)αmI(ωR , ω0) ·E0(ω0)] (7)

The total Raman intensity I(ωR) is proportional to | p(ωR) | , 2

I(ωR ) ∝ |g1(ω0 , rm)|2 × |g2(ωR , rA)αmI(ωR , ω0) ·E0(ω0)|2 (8)

As we can understand that the total Raman intensity I(ωR) depends on two factors, one is the local field enhancement g1(ω0, rm) in the first term of eq 8 and the other is the apparent Raman polarizability derivatives αIm(ωR, ω0)g2(ωR, rA) which is typically proportional to the volume of the molecule−antenna systems and should be much larger than αIm(ωR, ω0) of free molecules.225 The second enhancement could be considered as the enhanced modulation efficiency of local incident photons to Raman photons due to mutual excitation between the molecules and the antenna. Therefore, a molecule−transmitting-antenna system should be designed to transmit Raman signals from the reactive near-field region where molecules locate to the far field region where the detector locates (Figure 18b). To concentrate the two factors for the giant enhancement of Raman signals into one question, is there any optical antenna which could simultaneously act as both a receiving antenna and a transmitting antenna in optical frequency with low intrinsic loss, strong local EM field, and high directionality for giant enhancement of Raman signals? The solution is that the optical antenna is typically composed of gold or silver nanostructures, which can support surface plasmon resonance (SPR). Researchers such as Moskovits,54,57 Creighton,58 Chen,226 Chang,227,228 Pettinger,229 Gersten,230,231 Gersten & Nitzan, 2 31 ,2 32 McCall, 2 33 Kerker, 2 23 ,2 34 ,2 35 Metiu, 2 36 , 23 7 Schatz,238−240 etc. majorly contributed to the revealing of the SPR mechanism in SERS from the late of 1970’s to mid-1980’s. Please refer to the detailed notes and reviews on the discovery of SPR mechanism of SERS in refs 241−245.

where Eloc and E0, the local and incident electric field strength at the position rm where the molecules are located in the presence and absence of optical antenna at the incident frequency ω0, respectively, g1(ω0, rm) is the enhancement factor of incident electric field strength, αIm(ωR, ω0) is the Raman polarizability derivatives at Raman scattering frequency ωR under illumination of incident laser at incident frequency ω0. Furthermore, the antenna at rA would also be locally excited by the point dipolar source pm (ωR, rm) nearby, and then the local electric field strength at rA is E loc(ωR , rA) = C2·pm(ωR , rm)

(5)

(4)

where C2 depends on the relative position of molecules and antenna. In the induced dipole approximation, the induced dipolar term of the antenna pA(ωR, rA) could be expressed as the inner product elastic polarizability tensor of antenna αA and Eloc(ωR, rA), 5017


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3.2. Surface Plasmon and Surface-Enhanced Raman Scattering

E loc(ω0 , rm) = E0(ω0)z ̂ − E0(ω0)

The conduction electrons in metal or metal-like nanomaterials can be coherently excited by the incident light or fast electrons and are collectively oscillating at the metal−dielectric interfaces. The collective oscillating mode of electrons is referred to as surface plasmons, and the nanomaterials that support surface plasmons are named as plasmonic materials.246,247 A plasmonic material should have nanocurved surfaces and/or nanocrevices to support surface plasmon resonance (SPR) effect. There are two types of surface plasmons: (i) localized surface plasmons (LSP), in which coherent electrons oscillate around the NP surfaces (as shown in Figure 19a) or nanoscale crevices (as

εM − εd ⎡ z ̂ 3z ⎤ ⎢⎣ 3 − 5 rm⎥⎦ εM + 2εd r r (9)

The corresponding frequency-dependent extinction spectrum ILSP (i.e., LSP resonance (LSPR) spectrum) could further derived as,32,120 ILSP(ω0) = 12πNa3εd3/2ω0 ⎡ ⎤ Im(εM (ω0)) ⎥ /c⎢ 2 2 ⎣ (Re(εM (ω0)) + 2εd) + (Im(εM (ω0))) ⎦ (10)

As we can see from eq 9 and 10 both Eloc and ILSP will approach their maximum when (εM + 2εd) approaches ZERO [i.e., Re(εM) is approximately equal to −2εd and Im(εM) is close to ZERO]. In other words, only materials with moderately negative Re(εM) and with positive and close-to-zero Im(εM) could support strong LSPR and support giant local field enhancement for giant SERS enhancement.252 It is evident from Figure 20b, only gold, silver, and copper in transition metals could meet these rigid criteria to support surface plasmon with low intrinsic losses. Typically, nanostructures supporting LSP instead of SPP are commonly employed as SERS-active substrates because of the much stronger local EM enhancement at LSPR.120 There are several methods to excite LSP from plasmonic nanomaterials. LSP can be excited by incident light source, which can focus the light to a nanoscale edge, tip, or crevice, thereby enhancing the local EM field intensity by 2−5 orders of magnitude.225 LSP could also be excited by local oscillating sources (such as molecular oscillating electric dipoles, quadrupoles, etc.) nearby the plasmonic nanomaterials, which could directionally reradiate the local EM field into the far-field region in several wavelengths distance. Nanomaterials with desired plasmonic characteristics have supported a vast array of applications in surface-enhanced spectroscopy, SPR sensors, subwavelength waveguides, nanolasers, metamaterials, plasmonic circuits, surface science, photocatalysis, and solar cells, which form the components of a new field called plasmonics.254−259 Actually, SERS and SPR sensors are the two subdivisions of plasmonics and also fueling the field to a next level. Here, we clearly explain the working principle of SERS. The giant enhancement in SERS could be primarily attributed to the enhancement of the local EM field in the vicinity of the nanostructures and attributed to the enhanced efficiency of irradiation of the nanostructures mainly due to LSPR.225 In addition, five types of chemical effects may also contribute to the change of the relative and total Raman intensity,245 (1) ground

Figure 19. Schematic diagrams illustrate the enhanced Raman scattering effect due to the excitation of (a) localized surface plasmon at a Au nanosphere/air interface or by (b) propagating surface plasmon (surface plasmon polariton) at Au thin film/air interface excited by incident laser beam at Au thin film/prism interface in the Kretschmann-Raether attenuation total reflection (ATR) mode.

shown in Figure 25) and (ii) propagating surface plasmons [i.e., surface plasmon polaritons (SPPs)], in which the coherent electrons oscillate as a longitudinal wave at extended metal surfaces (as shown in Figure 19b).32 All metallic materials with a negative real part of dielectric function Re(ε) (Figure 20a) and with a positive imaginary part of dielectric function Im(εM) (Figure 20b) could be considered as plasmonic materials in certain wavelength region.248−250 Physically, the smaller Im(εM) is, the less intrinsic absorption loss is in light scattering processes.251 To understand the LSP of a metal nanosphere dispersed in bulk dielectric medium (with dielectric constant εd), we consider the Eloc outside a single nanosphere under electrostatic approximation,252,253

Figure 20. (a) Real part and (b) imaginary part of experimental dielectric function of typical transition metals (data on Au, Ag, and Cu from ref 250; Ti, V, Cr, Mn, Fe, Co, Ni, and Pd from ref 251; and Pt from ref 253 in visible region). 5018


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Figure 21. Schematics of electromagnetic enhancement mechanism in SERS. Reproduced with permission from ref 225. Copyright 2016 Nature Publishing Group.

this step could approximately be expressed with the second power of the local EM field strength at ωR.223 For low-frequency vibrational modes of adsorbed molecules, the incident and Raman scattered frequency, and thus the EFs of the first and second steps, G1(ω0) and G2(ωR), are usually comparable. Therefore, the SERS EF is approximately proportional to the fourth power of the enhancement of the local EM field strength,223,280−283 It should be noted that SERS enhancement of molecules located at the interparticle nanogap of gold or silver NP dimers and oligomers could be extremely large, even supporting singlemolecule detection.60−62,284 Moreover, EM and SERS EFs in the nanogap region of Ag or Au oligomers strongly depend on the size of the nanogap. As shown in Figure 22, when the gap size of a

states interaction, such as surface-complexation or chemisorption-induced increase of Raman polarizability, in the condition of far-away-from resonance;260−263 (2) resonant or resonance-like Raman process, such as resonant Raman of free molecules, chemisorption-induced molecular resonance, or preresonance Raman processes;264−267 (3) photon-driven electron-transfer from molecules to metal or the opposite;268−272 (4) transient electron-enriched states due to extremely negatively applied potential in electrochemistry273,274 or due to the donation of generated electrons to analytes;275 and (5) bias-voltageenhanced Raman in molecular junctions.276−278 It should be noted that chemical effect in SERS depends on the detailed electronic interaction in molecule−substrate systems. The general principles and the enhancement magnitudes should be carefully demonstrated and detected by rationally designed experiments. Here we concentrate the discussion on the EM enhancement mechanism of SERS by taking SERS on a gold nanosphere dimer as an example. EM enhancement in SERS is a two-step process.223,225,279 First, local EM field enhancement mainly at the gap and the accompanied induced dipole (the long and thick pink arrows in Figure 21) of the gold nanosphere dimer at ω0 are generated by exciting LSP of the nanostructure. In this step, plasmonic nanomaterials act as receiving optical antennae to transform the incident laser source to the near field, which could excite the molecular induced dipole pm (ωR, rm) at position rm and at ωR (eq 2). In the second step, the enhancement arises from the apparent Raman polarizability derivatives of the molecule−nanomaterial system [αIm(ωR, ω0)g2(ωR, rA)], which are typically 1−2 orders of magnitude larger than Raman polarizability derivatives of the free molecules αIm(ωR, ω0). This enhancement results from strong mutual excitation between the induced dipole of molecules [pm(ωR, rm), short red arrows in Figure 21) and the induced dipole (and even multipoles) of nanostructures pA(ωR, rA), long red arrows in the Figure 21]. Here, the plasmonic nanomaterials act as transmitting optical antennae to transmit the local Raman signals in the reactive near field to the far field at ωR. The EF in

Figure 22. Effect of gap size on SERS activity of a model molecule located at the midpoint of the gap of a Au nanosphere dimer. Reproduced with permission from ref 225. Copyright 2016 Nature Publishing Group.

Au nanosphere dimer is reduced from 10 to 2 nm, the SERS EF increases from 105 to 109.282,285 However, the local EM field and SERS EF cannot be unlimitedly increased by decreasing the gap size down to 0.4 nm, due to the quantum tunneling effects of electrons between the coupled Au NPs.286−289 5019


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Figure 23. LSPR and the average SERS EF vs (a and b) the incident wavelength of a single gold-dielectric core−shell nanosphere and (c and d) that of a single gold-transition metal core−shell nanosphere (TM: Ag, Pt, and Pd). The core diameter is 60 nm, shell thickness is 2 nm.

3.3. Concepts of SERS Hotspots

and a large amount of electromagnetic energy is directly absorbed by those materials instead of stored in the near field. Therefore, the average SERS EF is much smaller than that on a single pure Au nanosphere (Figure 23d). For much higher SERS activity of core−shell nanoparticles, we could learn from bare Au or Ag NPs with higher SERS-activity. For instance, a single Au@SiO2 or Au@Pt NP with sharp corners such as the nanocube (Figure 24, panels c and d) and/or with

As discussed above, the SERS EF is approximately proportional to the fourth power of the local electric field strength. Importantly, the EM field around plasmonic materials is not uniformly distributed but highly localized in the spatially narrow regions (“SERS hotspots”), such as nanotips, interparticle nanogaps, or particle−substrate nanogaps.63,92,113,177,236,285,290−293 Considering the different nature of EM coupling in single plasmonic nanostructures, coupled plasmonic nanostructures, and hybrid structures with plasmonic structure and the probe materials, we have classified three generations of SERS hotspots in SERS-active nanostructures.225 3.3.1. First-Generation Hotspots. The first generation hotspots are usually generated from single nanostructures such as a single nanosphere, nanocube, or nanorod freely suspended in a homogeneous medium. These hotspots exhibit moderate SERSactivity.225 For a single core−shell nanoparticle, the electric field distribution and the SERS activity depend on the refractive index of shell materials. For gold-dielectric core−shell nanoparticles Au@Dielectric, the resonant wavelength of LSP slightly red shifts and the SERS activity becomes much larger as the refractive index of shell materials increases (Figure 23, panels a and b).252 The slightly red-shifted resonant wavelength as the increased refractive index of shell materials could be understood by considering the resonance condition, [Re(εM(ω0)) + 2εd → 0]. The effective dielectric constant εd will increase, when dielectric constant of εL of the shell material increases (if εL > εd). Then Re(εM) should be more negatively changed to meet the resonance condition, which indicates that the incident frequency ω0 should be smaller (i.e., the resonance wavelength should be red-shifted). The enlarged SERS activity on the surface of dielectric shell with higher refractive index could be understood by the fact that light has the tendency to be “bended” into a higher refractive index medium, which results in much higher local enhancement of the EM field on the surface of the dielectric shell. For gold transition metal core−shell nanoparticles (Au@TM), the SERS activity on the surfaces of a TM shell depends on the dielectric function of TM.102 Typically, the TMs beyond coinage metals suffered from large optical loss in visible spectral range,

Figure 24. First-generation SERS hotspots. Distribution of SERS enhancement factors (EFs) of (a) Au@SiO2 nanosphere, (b) Au@Pt nanosphere, (c) Au@SiO2 nanocube, and (d) Au@Pt nanocube. The average/maximum EFs for them are 314/1414, 158/719, 3066/68704, and 591/12923, and the excitation lines for the simulation are at 537, 540, 587, and 600 nm, respectively. The core diameter or side length is 60 nm, shell thickness is 2 nm.

intraparticle gaps, such as Au and Ag nanostars, nanoflowers, and mesocages could be rationally designed for much higher SERSactivity in contrast to a Au@SiO2 or Au@Pt nanosphere with much less SERS-activity (Figure 24, panels a and b). In addition, the aspect ratio of nanorod core could be well-tuned to make the resonance wavelength of LSP agree well with the output frequency of incident lasers of the Raman measurement available in Lab.294 Then the practical problem is how to coat the ultrathin dielectric or TM layer on the nanostructured plasmonic cores. 3.3.2. Second-Generation Hotspots. The second generation SERS hotspots are generated from coupled nanostructures with controllable interparticle nanogaps or interunit nanogaps in nanopatterned surfaces (Figure 25).225 Typically 5020


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Figure 25. Second-generation SERS hotspots for trace-molecule detection. (a−d) FEM simulations of SERS-enhancement distribution in typical Au nanoparticles. The excitation lines for the simulation are shown in the figures, which are the resonance wavelength at the localized SPR peak. Particle size is 60 nm, and (a and b) shell thickness or (c and d) gap size is 2 nm. The average/maximum EFs for (a), (b), (c), and (d) are 1.01 × 105/ 1.21 × 107, 2.35 × 105/ 8.74 × 107, 1.47 × 106/4.11 × 109, and 2.41 × 105/1.24 × 108. (e) Au nanoparticles aggregate (left) and oligomer (right). (f) Au core−satellite nanostructure (left) and vertical self-assembly of Au nanorods on a support (right). (g) Nanobump (left) or nanovoid arrays (right) were prepared by depositing Ag on the preassembled SiO2 or polystyrene spheres. (h) Nanoheptamer (left) and nanocone quadrumer (right). (c−h) are adapted with permission from ref 225. Copyright 2016 Nature Publishing Group.

ceramics practically cannot be squeezed into the extremely tiny and narrow regions of the hotspots formed by coupled nanostructures, as shown in Figure 25, panels a−h.225 More importantly, the LSPR and local EM field distribution in the plasmonic nanostructures will be modified in the presence of probe materials nearby, compared to the same plasmonic nanostructures dispersed in a homogeneous medium. Therefore, it is essential to design plasmonic nanostructures that can produce hotspots right on the surface of the materials to be probed by taking into account of light scattering from a hybrid structures consisting of plasmonic nanostructures and the probe materials. The hotspot generated from the hybrid structures can be considered as third-generation hotspots. Representative examples of such hybrid structures are shown in Figure 26. It is evident from Figure 26a-ii,225 that the average SERS EF for a single Au nanosphere on a Pt surface is about 1 order of magnitude larger than that of the nanosphere on a Si surface (Figure 26a-i) due to the plasmonic coupling between the Au NP and Pt surfaces. Interestingly, a single Au nanocube, or nanobar with flat facet instead of a Au nanosphere could be used for much higher SERS activity probably due to cavity waveguide-like plasmonic mode in the tiny region of nanoparticle−substrate gap (Figure 26b). More interestingly, plasmonic NP clusters such as gold dimer instead of single NPs on the surface of a probe material can be used to obtain higher sensitivity (Figure 26c). SHINs can be used to create third-generation hotspots on material surfaces. Because SHINs composed of plasmonic Au or Ag cores with ultrathin (1−5 nm) chemically and electrically inert shells (for example, of SiO2 or Al2O3).147 The advantages of

nanostructures supporting second-generation hotspots could be NP dimers as shown in Figure 25, panels a−d. There are different types of coupled nanostructures such as NP aggregates, oligomers (Figure 25e), core−satellite nanostructures, NP arrays, a core−satellite nanostructured assembly with small Au NPs assembled on a larger Au NP, vertical self-assembly of Au nanorods on supporters (Figure 25f), nanostructured surfaces with nanobump and nanovoid arrays (Figure 25g), an individual nanoheptamer and a nanocone quadrumer (Figure 25h). The second-generation hotspots exhibit excellent SERS activity. The average SERS intensities from coupled plasmonic nanostructures are typically 2−4 orders of magnitude greater than that from single nanostructures, and they are more commonly used for trace-molecule detection. In the case of Au@Dielectric nanoparticle dimer, the EM field in the interparticle gap could be well-controlled by controlling the thickness of dielectric shell in addition to controlling the nanogap (Figure 25a). However, EM field in the gap of a Au@ TM dimer is extremely sensitive to the size of the nanogap (Figure 25b), which is similar to a bare Au nanoparticle dimer (Figure 25c). In the case of second-generation SERS hotspots in core−shell NPs, the probe molecules are attracted to and then “squeezed” into the hotspots through diffusion, specific adsorption or target-binding, and/or optical forces to optimize the SERS enhancement. 3.3.3. Third-Generation Hotspots. Typically, first- and second-generation hotspots are not well-preferable for surface analysis of many materials. Because most commonly used probe materials such as silicon wafer, carbon materials, thin film, or 5021


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SHINERS eliminate several weaknesses of contact-mode SERS; however, both interparticle plasmonic coupling and NP−substrate coupling in SHINs-on-substrate systems are moderate when compared to bare NP-on-substrate systems. Thus, the average SHINERS EF is about 6 and 5 times weaker than that from a bare Au NP dimer on Si and Pt surfaces, respectively (Figure 26, panels c and d).225

4. SYNTHESIS AND CHARACTERIZATION OF CORE-SHELL NANOPARTICLES Core−shell nanoparticles are usually synthesized by a seedmediated method. Thus, the shape of core normally determines the final morphology of core−shell nanoparticles. Herein, we start with introducing the preparation procedures of frequently used nanoparticles (cores), followed by the preparation of different types of core−shell nanoparticles. 4.1. Different Shapes of Plasmonic Cores

Generally, plasmonic Au or Ag nanoparticles are used as a core material in core−shell nanostructures. Researchers have synthesized various shapes of Au or Ag nanoparticles, such as nanospheres, nanorods, nanowires, nanocubes, nanostars, etc. (Figure 27). In the past several decades, the above-mentioned nanostructures exhibited various novel properties such as SPR tunability and excellent catalysis properties. There are abundant articles describing the method and mechanism of synthesis of various plasmonic nanoparticles.29,31,164,295−307 Here we select some simple and commonly used nanostructures as examples. 4.1.1. Nanospheres. Spherical nanoparticles are the most commonly used nanoparticles because of the simple conventional preparation method. Au nanospheres are usually prepared by Frens’ method.308 Briefly, a particular amount of HAuCl4 solution was placed in a round-bottom flask and heated to its boiling. Then sodium citrate was added quickly, and, after 2−3 min, the solution became brownish red. Next, the mixture was refluxed for 40 min and then cooled down to room temperature. Notably, the size of the nanospheres can be tuned by the amount of sodium citrate because high concentration of sodium citrate leads to smaller nanospheres. By this way, small nanosphere with uniform size and shape can be obtained, usually smaller than 50 nm. To get larger nanospheres with uniform size and shape, a seed-mediated growth method is needed. The small and uniform nanoparticles act as seeds and more HAuCl4 solution was added, at the same time ascorbic acid can act as a reducing agent and sodium citrate can act as a stabilizing agent to reduce HAuCl4 to Au atoms to grow larger nanoparticles. Apparently, sodium citrate is not only acting as a reducing agent but also acting as a stabilizing agent to make a uniform and monodispersed Au particles. It is noteworthy to mention that the rapid injection of sodium citrate in the already boiling solution results in much more nucleation and subsequently a lower

Figure 26. Third-generation hotspots for surfaces analysis. FEM simulations of SERS-enhancement distribution for “hybrid” nanostructures of SERS-active nanoparticles on Pt or Si probe materials. The particle size is 60 nm, and the shell thickness (d) is 2 nm. The sizes of the particle−substrate gap and interparticle gap for (c and d) are 1 and 2 nm, respectively. The average/maximum EFs for (a-i), (b-i), (c-i), and (d-i) are 94/2.03 × 103, 1.48 × 106/1.42 × 107, 1.04 × 105/6.85 × 106 (6.76 × 107), and 1.74 × 104/8.37 × 105 (6.56 × 106), respectively. The average/ maximum EFs for (a-ii), (b-ii), (c-ii), and (d-ii) are 1.30 × 103/4.32 × 104, 7.31 × 105/5.15 × 106, 1.02 × 106/9.01 × 107 (7.19 × 107), and 2.07 × 105/1.16 × 107 (1.39 × 107), respectively. Reproduced with permission from ref 225. Copyright 2016 Nature Publishing Group.

SHINERS are as follows:225 (1) the ultrathin yet pinhole-free shell separate the core from the material surface (and environment), thus ensuring that there is almost no electron interaction between probe material surfaces and SERS-active metal core; (2) the chemically inert shell prevents interparticle fusion and particle−metal substrate fusion, which significantly improves the reliability and stability of Raman signals from the target surface; (3) the shell thickness can be used to tune intercore gap, and core−probe-materials gap (the former is typically larger than the latter), which consequently improves the particle−probe-material EM coupling for selectively exciting probe materials; and (4) the Au or Ag cores create an enhanced local EM field on the surface of probe materials without distorting its structure (Figure 26d).

Figure 27. SEM and TEM images of different shapes of nanoparticles. (a) gold nanospheres, (b) silver nanocubes, (c) gold nanorods, (d) silver nanowires, and (e) gold multibranched nanoparticles. Reproduced from ref 305. Copyright 2011 American Chemical Society. Reproduced with permission from ref 297. Copyright 2008 Wiley-VCH. Reproduced with permission from ref 306. Copyright 2014 The Royal Society of Chemistry. Reproduced from ref 303. Copyright 2007 American Chemical Society. Reproduced from ref 307. Copyright 2014 American Chemical Society. 5022


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amount of AgNO3 was adjusted to obtain nanorods with different aspect ratios. In the presented experiments, ascorbic acid (AA) served as a mild reducing agent and could not make Au3+ reduction without the presence of Au seeds in the growth process. Especially the addition of Au seeds to growth solution can increase the overall growth rate. Also, the ratio of Au3+ to reducing agent can make a significant influence on the rod size. Further the Ag+ ions can regulate the aspect ratio which also depends on the ratio of Au seed to Au salt ratio.312 With respect to Ag+, AA is not able to reduce Ag+, but the formation of AgBr can inhibit the growth due to the selective adsorption on the Au particle surface. The surfactant CTAB is stable and can prohibit the aggregation of nanoparticles due to its preferential adsorption on the long axis of growing Au nanoparticles. Furthermore, silver nanorods were typically synthesized with gold nanorods as seeds to grow silver shells. We discuss about this nanostructure in the section of Au−Ag core−shell nanoparticles. 4.1.4. Nanowires. A typical synthetic method for silver nanowires is the polyol method.303 First, ethylene glycol was heated at 160 °C for 1 h. At the same time, AgNO3 and PVP were dissolved in ethylene glycol separately, while the PVP ethylene glycol solution contained little amount of NaCl and Fe(acac)3. Then these two ethylene glycol solutions were injected into the hot solution simultaneously to generate the silver nanowires. Au NWs were synthesized by reducing HAuCl4 by KBH4 with Triton X-114 (TX-114) as a capping agent.313 TX-114 was used as a thermosensitive nonionic surfactant, and it was used as a structure-director to form the NWs and protective agent to keep NWs from aggregating. For Ag nanowires, ethylene glycol can be served as solvent and reducing agent in 160 °C. The capping agent PVP can regulate the relative growth rate on various crystal surfaces. Also, controlling the molar ratio of the repeating unit of PVP to Ag+ can change the morphologies and aspect ratio of nanowires. Simply, the decreasing ratio can make higher aspect ratios. On the other hand, the molecular weight of PVP also makes an influence on the yield and quality of nanowires. Further the coordination reagent, selectively adsorbing and desorbing on the surface, can kinetically control the growth rates of various faces. At the initial stage of reduction reaction, the seeds formation can be controlled by the injection rate via a syringe pump. 4.1.5. Multibranched Nanoparticles. As Cui et al. reported, Au multibranched nanoparticles, also called as nanostars, were synthesized by reducing HAuCl4 by AA with the presence of microscale AgNO3. Or it can be synthesized by a seed-mediated growth method.307 2-[4-(2-Hydroxyethly)-1piperazinlyl] ethane-sulfonic acid (HEPES) was used as a shape-directing agent, and hydroxylamine (NH2OH) acted as a reducing agent. Many other methods have been reported based on the reduction of HAuCl4 in the presence of PVP or CTAB. Recently, Liz-Marzán et al. reported a method to synthesize gold nanostars by seed-mediated growth method in N,N-dimethylformamide (DMF).314,315 They found that DMF was a suitable reductant for HAuCl4 with PVP as a capping/stabilizing reagent. With high concentration of PVP, HAuCl4 can be reduced without an external energy source in the absence of seeds. Further, they found that with different aged times of growth solutions (DMF solution containing PVP and HAuCl4), gold nanostars with different optical spectra can be obtained. Moreover, the molar ratio of HAuCl4 and seed can significantly affect the shape and optical spectra of the nanostructures.

growth process. The explanation is that reducing agent can consume more Au3+, leading to a slow growth process. The color change of colloid also showed that the nucleation and growth was completed rapidly. It is well-known that changing the mole ratio of reactant can affect the rate of nucleation and growth process, and the size is determined by the number of nuclei. Silver nanoparticles can also be synthesized by the sodium citrate reducing method, but the particle size is not uniform and the shape cannot be controlled effectively. To get shapecontrolled silver nanoparticles, the polyol synthesis method is a common method.303 In this method, ethylene glycol acts as both solvent and reducing agent. Ethylene glycol is thermally oxidized to glycoaldehyde, which can reduce Ag+ to Ag and spherical silver nanoparticles can be obtained by this method. Typically, ethylene glycol was heated to 153 °C for 1 h. Then two ethylene glycol solutions contained silver nitrate, and PVP with NaCl were simultaneously injected into the hot ethylene glycol and finally the obtained silver nanospheres were single crystals. 4.1.2. Nanocubes. Gold nanocubes were synthesized by a procedure reported by Sau and Murphy.309 First, a certain amount of HAuCl4 was reduced by NaBH4 to prepare seeds. Then the seeds were kept for 1 h before use. Next the mixture of CTAB, HAuCl4, and ascorbic acid were made as growth solution, and the CTAB-stabilized seeds were added into the growth solution to get gold nanocubes. Ag nanocubes were synthesized by reducing silver nitrate by ethylene glycol at 160 °C with poly(vinylpyrrolidone) (PVP) as a capping reagent.303,310 PVP is a shape-control agent and ethylene glycol plays two roles: reductant and solvent. The shape of the product is strongly dependent on the reaction conditions. If the molar ratio of PVP and AgNO3 is controlled at 1.5, silver nanocubes can be obtained. Typically, silver nitrate and PVP were dissolved in ethylene glycol separately and then simultaneously injected into hot ethylene glycol by a two-channel syringe pump with a slow rate. In the first minute of the reaction, silver ions were reduced to silver atoms then the atoms immediately form nuclei. Ethylene glycol can act as both mild reductant and organic solvent. It holds a more mild reduction environment than that in aqueous solution. PVP, a capping reagent, can selectively adsorb on the surface, which would depress the growth along the (001) direction and promote the growth on (111) plane. Actually, the adsorption of PVP on the Ag surface is achieved by coordination bonding with the O and N atoms of pyrollidone ring. Further, the morphology of Ag cubes can also be affected by some reaction conditions such as reaction temperature, the concentration of Ag+, growth time, and the molar ratio of the repeating unit of PVP to Ag+. 4.1.3. Nanorods. Au NRs were prepared by a seed-mediated growth method, which was first reported by Murphy’s group.304,311 In their work, they used citrate-capped Au nanoparticle as seeds. Afterward, El-Sayed et al. found that CTAB-capped Au seeds were single-crystalline, which can be used to obtain gold nanorods with unprecedented high yields.12,164 Typically, HAuCl4 and CTAB mixture was reduced by ice-cold NaBH4 at 25 °C to form 3.5 nm gold seed solution. The resulting solution was brownish yellow color, and it was stored at 25 °C for 2 h. Later, CTAB and AgNO3 were mixed with HAuCl4 solution. After ascorbic acid was added, the color of growth solution was changed from dark yellow to colorless. Then seed solution was added to the growth solution, and the solution should be stirred for some seconds. Then the mixture was kept at 27−30 °C. After 10−20 min, the colorless solution gradually changed to brownish red and the solution was kept for 6 h. The 5023


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Figure 28. Schematic illustration of the synthesis of core−shell nanoparticles with an ultrathin shell.

stirred for 30 min for a complete reduction of H2PdCl4 to obtain Au@Pd NPs (Figure 29a).131 Other transition metal shells such

Usually the direct reduction of metal salt to metal holds a wide size range, but there is a narrow distribution using seed-mediated growth. Importantly, the higher concentration of PVP in DMF is propitious to produce uniform and monodisperse nanostars. The reduction ability of PVP in DMF also plays a primary role to control the reduction kinetics of AuCl4− on the seeds surface. In addition, silver nanostars were also synthesized with gold nanostars as seed, which are classified as core−shell nanoparticles. Additionally, there are many other nanostructures such as nanoprism, triangular,316,317 hexagonal,318 octahedron319 nanoparticles and nanodisks,320,321 nanobelts,322 and so on. Besides the methods described above, currently, the environmentally benign nanoparticle synthesis processes that do not use toxic chemicals in the synthesis protocol have been developed. Biological method is a kind of green synthesis method which uses bacteria and fungi for the synthesis of metal nanoparticles. Shankar et al. synthesized Au, Ag, and Au@Ag nanoparticles using Neem (Azadirachta indica) leaf broth.323 And they also synthesized triangular gold nanoprisms with a biological method.324

Figure 29. (a) SEM images of Au@Pd nanoparticles. The inset is TEM image of a Au@Pd nanoparticle. Reproduced from ref 131. Copyright 2016 American Chemical Society. (b) SEM image of Au@Pt nanoparticles. The inset is TEM image of a Au@Pt nanoparticle. Reproduced from ref 131. Copyright 2016 American Chemical Society. (c) TEM image of an Au@Pt nanorod. The inset is crystal-lattice mapping of the area enclosed by the yellow circle. Reproduced with permission from ref 306. Copyright 2014 The Royal Society of Chemistry. (d) TEM image of Au@Pd−Ag nanocubes. Reproduced from ref 326. Copyright 2015 American Chemical Society. (e) TEM image of Au@Pt nanowires. Reproduced from ref 327. Copyright 2014 American Chemical Society. (f) TEM image of Au@Pt multibranched nanoparticles. Reproduced from ref 307. Copyright 2014 American Chemical Society.

4.2. Core−Shell Nanoparticles with Ultrathin Shell

Core−shell nanoparticles are usually prepared based on the template core to grow shell by chemical synthesis method such as seed-mediated growth method or physical method such as ALD or metal deposition. Finally, the prepared core−shell nanoparticles can be assembled on a substrate for Raman measurements (Figure 28). 4.2.1. Metal−Metal Core−Shell Nanoparticles with Ultrathin Shell. 4.2.1.1. Metal Shell. Au core transition metal nanoparticles were coated with ultrathin transition metal layers (several atomic layers) by a seed-mediated growth method.100,101,131,325 Au nanoparticles served as the seeds, and the transition metal ions were reduced by ascorbic acid to form the shell. Here, we use Au@Pd nanoparticles as an example to explain the preparation procedure. First, Au NPs were synthesized by Frens’ method308 as mentioned above. In the next step, the transition metal layer was coated on the Au NPs surface. Au sol was placed in a round-bottom flask, and a certain amount of H2PdCl4 was added according to the shell thickness. The solution was cooled down to 4 °C, and then the ascorbic acid solution was dropped through a syringe. The dropping speed was controlled by a step motor, and after dropping, the solution was

as Pt, Ru, Rh, Ni, or Co can also be prepared by a similar method.99,129 After being centrifuged for 2 times, the nanoparticles were assembled on electrodes or other substrates for SERS measurements. Significantly, nanoparticles with different shapes exhibit different properties. For example, Bao et al. synthesized Au nanorods then coated with a Pt shell (Figure 29c).306 After Au NRs were obtained, a certain amount of H2PtCl6 was reduced by ascorbic acid to coat Au NRs. Nanocube is another commonly used nanostructure in nanoscience. For example, Qin’s group synthesized bifunctional Ag@Pd−Ag nanocubes with codeposi5024


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for Pt, because it grows on Pd as clusters, the amount of H2PtCl6 can be calculated as H2PdCl4. The author used monolayerequivalents (θPt) to describe the amount of H2PtCl6 added to each nanoparticle. The synthesis of 55 nm Au@Pd@Pt with 2 monolayers of Pd and θPt ≈ 0.5 nanoparticles was used as an example. Au nanoparticles of 55 nm were prepared with Frens’ method, and H2PdCl4 were added into the gold sol at 4 °C in an ice bath. And then ascorbic acid was slowly added to the solution to reduce the H2PdCl4. After 30 min, the mixture was heated to 80 °C and H2PtCl6 were added. Once again, ascorbic acid was dropped in to reduce H2PtCl6. Finally, Au@Pd@Pt nanoparticles were obtained as shown in Figure 30b. 4.2.2. Metal-Nonmetal Core−Shell Nanoparticles with Ultrathin Shell. 4.2.2.1. Silica Shell. After SHINERS discovery, various types of SHINs were developed with an ultrathin inert shell and different metallic cores. In general, there are two kinds of preparation methods, chemical method and physical method. The widely used SHINs, Au@SiO2 NPs, are prepared by a chemical method because the chemical method is cost-effective and flexible, and it is simple with high reproducibility.114,160 First, Au or Ag NPs were synthesized by the chemical reduction method, and then the metal core surface was functionalized by silane coupling agent. Usually, the gold nanoparticles with a diameter of 55 nm are employed in most Raman measurements because those nanoparticles can be easily prepared, meanwhile they exhibit strong Raman enhancement for most of the applications. The 55 nm Au NPs were prepared by the classical sodium citrate reduction method.308 In the next step, 30 mL of the Au NPs solution was placed in a round-bottom flask under stirring. Then 0.4 mL of 1 mM APTMS solution was added. After stirring for 15 min, 3.2 mL of 0.54% sodium silicate solution with a pH ∼ 10.3 was added into the flask. After stirring 3 min, the reaction mixture was transferred to a 95 °C bath. After a certain time, the reaction can be stopped by removing and placing it an ice water bath. The silica shell thickness can be tuned by controlling the reaction time. For example, the SHINs with ∼2 nm thickness of shell need to react for 20 min, and 4−5 nm shell thickness needs 60 min. Moreover, the shell thickness is also influenced by the pH, temperature, and the concentration of Au nanoparticles. To obtain huge enhancement, SHINs can be prepared in different sizes. Similarly, different shapes of nanoparticles can be synthesized for a variety of applications (Figure 31). In order to get a higher enhancement, 120 nm Au cores were used because it exhibits strongest electromagnetic field enhancement at 633 nm excitation.160 Furthermore, nanocubes and nanorods were coated with inert shells, because of their tunable SPR property.12,161,301 By adjusting the aspect ratio of nanorods, the SPR absorption can be tuned to the near-infrared region, which can be useful for biological systems. To further enhance the SERS enhancement factor, Ag SHINs were prepared because AgNPs exhibit a strong SERS enhancement effect and the SPR of Ag nanoparticles can be tuned at a wide range of wavelength.173,328 4.2.2.2. Alumina Shell. Besides chemical methods, the physical method can also be used to fabricate SHINs. For example, atomic layer deposition (ALD) method is a widely used technique to prepare metal NPs under discontinuous gas flow.329,330 The ALD method makes use of self-limiting surface reactions, which leads to layer-by-layer growth to control the interfacial thickness with high uniformity. The Al2O3 shell is usually fabricated by this method, which can get ultrathin and uniform shell without pinholes. Van Duyne reported a method to

tion of Pd and Ag atoms on the Ag nanocube surface (Figure 29d).326 Na2PdCl4 and AgNO3 with PVP as a capping agent and AA as a reductant were added into Ag nanocubes solution. With the presence of Ag+ and AA, the galvanic reaction between Ag and Pd2+ can be effectively suppressed, thus the Pd and Ag atoms were codeposited on the nanocube surface in a site-by-site manner. Liu et al. prepared submonolayer Pt-coated Au nanowires (NWs), and they found that TX-114 played another important role to cover Pt on Au NWs (Figure 29e).327 Because of the free d orbital of transition metals, TX-114 can form a strong bond with Pt atom, thus the interaction among Pt atoms is weakened. Therefore, the newly deposited Pt atom tends to be located on Au surface rather than form a Pt assembly. Moreover, the multibranched core−shell nanoparticles were prepared by depositing transition metals on multibranched cores (Figure 29f).307 4.2.1.2. Multimetal Shell. Multilayer or more complex nanostructures were prepared for SERS application in catalysis. Schlücker’s group synthesized Au/Pt/Au core−shell nanoraspberries by depositing Ag on Au nanoparticles, and then the Ag shell was replaced by a Pt shell by galvanic replacement.144 Finally, another Ag shell was deposited on the Pt shell and followed by galvanic replacement of Au precursor. Briefly, Au nanoparticles were also prepared by the sodium citrate reducing method as described above. Then silver nitrate solution was dropped into the heating solution to grow a silver layer. After 1 h, H2PtCl6 solution was added and then a dark mauve solution was obtained. Here the Ag layer was replaced by Pt layer, and then the product was centrifuged and washed two times. The product was resuspended in water and heated to boiling. Later, another silver nitrate solution was added followed by sodium citrate solution with boiling for 1 h. The last step was growing Au protuberance. HAuCl4 solution and citrate solution were added simultaneously to the gold−platinum−silver multilayer nanoparticle solution for a 20 min reaction time then the target nanostructures were obtained as shown in Figure 30a.

Figure 30. (a) TEM images of Au/Pt/Au core−shell nanoraspberries. The inset is a TEM of a single nanoraspberry. Reproduced from ref 144. Copyright 2011 American Chemical Society. (b) SEM image of Au@ Pd@Pt nanoparticles. The inset is TEM of a single nanoparticle. Reproduced with permission from ref 143. Copyright 2011 Royal Society of Chemistry.

Tian’s group synthesized trimetallic Au@Pd@Pt nanoparticles by a seed-mediated growth method.143 The Au core surface was totally covered with Pd atoms as a consecutive monolayer, while the Pt grew on the Pd shell as a set of clusters. The advantage of this method is that the Pd shell thickness and Pt cluster coverage can be controlled by adjusting the molar ratios of H2PdCl4 and H2PtCl6 to Au seeds. The amount of H2PdCl4 was determined by the total surface of Au nanoparticles. And an extra 25% H2PdCl4 was added to ensure the complete coverage of Pd layer. However, 5025


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Figure 31. HR-TEM images of different types of SHINs with silica shell. (a) 55 nm Au@SiO2 nanoparticle, (b) 120 nm Au@SiO2 nanoparticle, (c) Au@ SiO2 nanocube, (d) Au@SiO2 nanorod, and (e) Ag@SiO2 nanoparticle. Reproduced with permission from ref 147. Copyright 2015 The Royal Society of Chemistry.

deposit an ultrathin alumina layer on Ag film over nanosphere (AgFON) substrates by ALD method.32,201,291,331 They demonstrated that the Al2O3-modified AgFON substrates have brilliant SERS effect and longer temporal stability. Ma et al. also used the ALD technique to fabricate a subnanometer Al2O3 shell with a few pinholes on Ag nanorods (Figure 32a).119

ZnO. This can protect the Ag nanoparticles from being corroded, and Ag@C core−shell nanocomposites can be successfully prepared. Pan et al. also reported the synthesis of Ag@C nanocomposites by a modified hydrothermal strategy and obtained thinner carbon shell by changing the reaction temperature and time.333 The thicknesses of carbon shell for samples prepared at 140 °C for 360 min, 100 °C for 80 min, and 60 °C for 60 min were about 2.5, 1.5, and 1 nm, respectively. Another approach for coating plasmonic cores with a carbon shell was based on the carbonization of long chain molecules adsorbed on their surface.123,336 In this strategy, long chain molecules, such as CTAB, were first assembled on the surface of Au/Ag nanoparticles, and then carbonated under hydrothermal conditions or in concentrated sulfuric acid to form an ultrathin carbon shell. CTAB-functionalized Ag nanoparticles were first prepared by using ascorbic acid as a reducing agent and then hydrothermally treated at about 160 °C for 2 h to generate Ag@ C core−shell nanoparticles. The carbon shell synthesized by this method is pinhole free and only about 2 nm thick, which was used to probe 4-mercaptobenzoic acid (4-MBA). 4.2.2.4. Graphene Shell. Graphene is a promising twodimensional material with a thickness of single atom layer and shows various advantages, such as near-infrared absorption and good biocompatibility. Thus, the graphene-isolated gold nanoparticles (GIAN) have a wide range of applications in biomedical research including photothermal therapy and bioimaging. The most common method for the preparation of the nanomaterials with plasmonic-core graphene-shell is chemicalvapor-deposition method (CVD).337 Synthesis of Au@graphene by CVD method was first reported by Knecht et al.,338 and then improved by Tan et al.126 and Zhang et al.125 In this method, Au/ Ag nanoparticles were assembled onto a substrate, and then the graphene shell was grown on the Au nanoparticles by pyrolysis of carbon sources, such as xylene or methane, under a controlled atmosphere and high temperature. As shown in Figure 33, an ultrathin graphene-like shell was coated on the surface of Au nanoparticles, and its shell thickness can be well-controlled by manipulating the CVD growth time. Besides the CVD method, there are also a few studies reported for the coating of graphene via another strategy. Zhang et al.339 reported the preparation of graphene-veiled gold film by a two-step procedure, in which the 8 nm gold film was first prepared by vacuum thermal evaporation and graphene was then transferred onto its surface. By this method, the SERS activity can almost remain unchanged even after being veiled by graphene. Zhao et al. even developed a wetchemical method for the synthesis of [email protected] The precursor of gold was reduced by NaHB4 in the presence of preprepared graphene oxide, which was synthesized by a standard Hummer’s method. After reaction for 24 h, Au@GO NPs were obtained and had a core−shell-like structure as indicated by TEM analysis.

Figure 32. (a) SEM image of Ag@Al2O3 nanorods with pinhole. The inset is HRTEM image of an Ag@Al2O3 nanorod. Reproduced from ref 119. Copyright 2016 American Chemical Society. (b) Schematic diagram of ALD method to prepare Ag@Al2O3 nanoparticles. (c) HRTEM of Ag@Al2O3 nanoparticle. Reproduced with permission from ref 332. Copyright 2015 John Wiley & Sons, Ltd.

Additionally, Zhang et al. coated the ultrathin Al2O3 shell on metal nanoparticles by ALD method for SERS applications (Figure 32, panels b and c).332 The unique advantage of this method is that the metal nanoparticles were floated in a chamber by gas flow, hence the nanoparticles can be coated with a uniform shell. 4.2.2.3. Amorphous Carbon Shell. Amorphous carbon materials exhibit various advantages, such as acid and base resistance, excellent electroconductibility and good adsorption properties. Importantly, coating plasmonic cores with an ultrathin carbon shell can greatly improve the resistance to acid, base, and other solvents. Pan et al., Zhang et al., and Chen et al. developed a hydrothermal route for the synthesis of Ag@C nanocomposite.333−335 In this strategy, ultrapure water, glucose, ZnO powder, and preprepared Ag nanoparticles or AgNO3 were added into an autoclave with a Teflon inner wall, which was then transferred to an oven at 150 °C. After a certain reaction time, it was cooled down to room temperature and the products were separated and washed for several times. The obtained solids were then characterized by SEM and TEM, and it was found that an ultrathin carbon shell was coated on the surface of Ag nanoparticles. The shell thickness can be tuned by the reaction time. The average thicknesses of the carbon layer were about 4.5, 7.3, and 10.5 nm for reaction times at 3, 4, and 5 h, respectively. The authors also found that ZnO powder was essential for the synthesis process. ZnO can readily neutralize the acid generated during the hydrothermal process, as it can easily be corroded under acid conditions (pH < 5). Thus, the pH value during reaction can be maintained at about 6−6.5 in the presence of 5026


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amount of KMnO4 and K2C2O4. They also found that this material showed better SERS activity as well as high stability under strong alkaline media. Thus, they can be directly applied to in situ SHINERS study under these conditions. 4.3. Core−Shell Nanoparticles with Thick Shell

4.3.1. Metal−Metal Core−Shell Nanoparticles with Thick Shell. 4.3.1.1. Au−Ag Core−Shell Nanoparticles. Since the direct synthesis of large Ag nanoparticles is a challenging task, researchers developed an alternative pathway to achieve this goal by using Au NPs as seeds for the synthesis process, leading to the formation of Au@Ag nanoparticles.172 Zhang et al.341 first reported the synthesis of Au@Ag core−shell like structure by a seed-mediated growth method. Typically, gold seeds with a diameter of about 12 nm were first prepared by the standard Frens’ method.308 Then, Ag shell was coated on the as-prepared Au seeds by reducing AgNO3 with ascorbic acid. The shell thickness can be tuned by changing the reaction conditions, such as amount of AgNO3, reaction temperature, and pH value. The shape of Au@Ag nanoparticles can also be manipulated by changing the seed or synthesis procedure and conditions. For instance, Au@Ag nanorods can be synthesized by using gold nanorod as seeds (Figure 35b),167,342 and Au@Ag nanocubes can also be obtained by using CTAC as a capping agent.343

Figure 33. (a) Schematic diagram of preparation process of metal@ graphene nanoparticles. SEM images of (b) Cu@graphene, (c) Ag@ graphene, and (d) Au@graphene nanoparticles. Insets are TEM images of graphene shell. Reproduced from ref 124. Copyright 2014 American Chemical Society.

Due to the SERS enhancement from the gold cores and the characteristic Raman signals of GO shell, this material was then successfully utilized for intracellular Raman imaging in HeLa cells. 4.2.2.5. Other Materials Shell. In the above section, we have discussed various preparation procedures for various materials. In addition, some other materials have been used to coat metal nanoparticles, such as TiO2, MnO2, and Ag2S shell, and have been prepared for various applications (Figure 34). For instance, Mao’s group synthesized Ag@TiO2 by TOAA (titanium oxide acethylacetonate) and MPA (mercaptoundecanoic acid).122 Briefly, 14 mL of freshly prepared Ag NPs solution was centrifuged and then dispersed into 12 mL of ethanol. After that, 40 μL of 0.25 M MPA solution with pH of 9.7−10 was added, and 4 mL water was added subsequently. After 30 min stirring, 1.5 mL TOAA (1 mg TOAA dissolved in 2 mL ethanol) was added dropwise, and then kept stirring for 12 h to complete the hydrolysis reaction. Additionally, in order to overcome the limitation of traditional SiO2- or Al2O3-coated Au nanoparticles in the application in alkaline media, ultrathin, pinhole-free MnO shell-coated Au nanoparticles were successfully synthesized via the reaction between KMnO4 and K2C2O4 in Tian’s group.121 In a typical procedure, Au nanoparticles were added into an aqueous solution of KOH with a pH value about 9.5. Then, a certain amount of KMnO4 and K2C2O4 were added into the mixtures, and the temperature was raised to 60 °C for 2 h to complete the reaction. As demonstrated by TEM analysis, the thickness of MnO2 shell could be easily tuned by changing the

Figure 35. (a) SEM image of Au@Ag nanoparticles. Inset, the corresponding TEM image. Reproduced from ref 172. Copyright 2012 American Chemical Society. (b) TEM image of Au@Ag nanorods. Reproduced with permission from ref 167. Copyright 2012 WIley-VCH. (c) TEM image of Ag@Au nanocubes. Inset, a Ag@Au nanocube. Reproduced from ref 343. Copyright 2014 American Chemical Society.

4.3.1.2. Ag−Au Core−Shell Nanoparticles or Other Metals. Sun et al.344 have established a facile seed-mediated strategy for the synthesis of Ag@Au nanoparticles. In their strategy, Ag seeds with a diameter of about 13 nm were prepared in oleylamine at 180 °C. Then, Ag@Au was obtained via the growth of Au shell on the seed under octadecene by using oleylamine as both reducing agent and capping agent. They further found that the shell thickness can be tuned by different reaction times. Ag@Au nanoparticles can also be prepared by a replacement reaction due to the different reduction potentials between Ag and Au.345 In this method, citrate-stabilized Ag nanoparticles were first prepared via NaBH4 reduction of AgNO3. The as-prepared Ag

Figure 34. HRTEM images of SHINs with different shell materials. (a) Ag@TiO2, (b) Ag@Al2O3, (c) Au@MnO2, and (d) Au@Ag2S. Reproduced with permission from ref 160. Copyright 2012 Nature America, Inc. 5027


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and co-workers used a monolayer and double-layer PS spheres template to form Ag triangles and Ag spots array (top-down) (Figure 37).331,348 This kind of metal arrays could be further

nanoparticles were transferred to toluene by ligand exchange with dodecylamine. HAuCl4 was then added to the above mixture to start the replacement reaction. The author also found that the solvent and temperature would significantly affect the final structure and composition of the nanocomposites. Furthermore, Ag@Au core−shell nanocubes can also be prepared by depositing Au atoms on the surface of Ag nanocubes via galvanic replacement reaction.343 4.3.2. Nonmetal−Metal Core−Shell Nanoparticles with Thick Shell. 4.3.2.1. Dielectric-Metal Core−Shell Nanoparticles. Halas and co-workers synthesized the nanoshell nanostructures by a seed growth method.25,346 First, they prepared silica cores by Stöber’s method,347 and then according to the diameter of silica particles, a certain amount of freshly distilled tetraethoxysilane was mixed with ammonium hydroxide, water, and ethanol. After 1 day, the silica nanoparticles were obtained and then functionalized with aminopropyltriethoxysilane (APTES). APTES served as a coupling agent that the silane group interacted strongly with the SiO2, and the amine groups were connected with small Au NPs which was reduced by tetrakis(hydroxymethyl) phosphonium chloride (THPC) with the diameter of 2 nm. Then a K-gold solution was prepared by mixing K2CO3 solution and HAuCl4 solution. The K-gold solution was mixed with silica-gold seed solution and reduced by NaBH4 or formaldehyde or CO to form the outer Au shell, by varying the ratio of the seed solution and K-gold solution, the shell thickness can be controlled (Figure 36), thus the SPR of the nanoshell can be tuned conveniently.

Figure 37. Schematic diagram of nanosphere lithography for the fabrication of ordered nanostructures. Reproduced with permission from ref 348. Copyright 2008 The Royal Society of Chemistry.

applied in optoelectronic and photobiological devices. The same group continuously used NLS method to get well-order Ag nanostructures, and several experiments were carried out to obtain the relationship between the LSPR peak position of Ag nanoparticles and the size or dielectric environment of the nanoparticles. Bartlett and co-workers200 prepared 700 nm diameter polystyrene spheres on a substrate, and then they used electrochemical deposition to deposit platinum (bottom-up method), gold, and silver with various thicknesses on a PS template, followed by dissolution in tetrahydrofuran to remove the PS sphere, and finally obtained a hexagonal closely packed monolayer of metallic nanostructures. By studying the reflection spectra, the authors found that when the thickness of film is smaller than the PS spheres, the metallic structures showed a plasmon gratinglike behavior, but when the thickness is bigger than one-half of the PS sphere, the roughness of the nanostructures became obvious, and the grating effect was not observed. 4.3.2.2. Magnetic-Metal Core−Shell Nanoparticles. Several kinds of magnetic nanoparticles have been synthesized, including iron oxides such as Fe3O4 and γ-Fe2O3 or pure metals such as Fe and Co or alloys such as CoPt, FePt, etc.; here, we introduce the commonly used Fe3O4 and γ-Fe2O3 (see Figure 38). There are several popular methods to synthesize magnetic nanoparticles.302 The most simple and convenient way to get iron oxides is the recipitation method.349 The Fe2+ and Fe3+ salt solutions are mixed, and NaOH is added as the reductant. To get monodisperse magnetite nanoparticles, many kinds of organic additives were used as a stabilizing agent such as polyvinlyalcohol (PVA) or trisodium salt of citric acid, oleic acid, etc. Lyon et al. used tetramethylammonium hydroxide (TMAOH) as a stabilizing agent because when pH = 12, the interactions between N(CH3)4+ counterions and adsorbed OH− anions can stabilize the solution and prevent particle aggregation.350 To prepare γ-Fe2O3, freshly prepared Fe3O4 particles were dissolved in HNO3 and heated to 90−100 °C for 30 min to oxidize the particles to γ-Fe2O3. And then Au shell can be formed by reducing Au3+ on the iron oxide surface by iterative hydroxylamine seeding procedure.202,203,351 HAuCl4 and NH2OH·HCl

Figure 36. TEM images of growth of Au nanoshell on 120 nm silica core. (a) Initial gold colloid-decorated silica nanoparticles. (b−e) Gradual growth and coalescence of gold colloid on silica nanoparticle surface. (f) Complete Au nanoshell. Reproduced with permission from ref 25. Copyright 1998 Elsevier Science B.V.

To obtain high reproducibility in SERS spectra, both the uniformity of the SERS substrate and the controllability of the metallic nanostructures morphology199 play crucial roles in this procedure. One solution is to use template techniques. First, a well-defined substrate should be fabricated using the template materials (often silica or polystyrene). Then, metallic film should be deposited on the substrate (usually electrochemical deposition (bottom-up method) and vapor deposition (topdown method). Finally, the template materials should be removed by dissolution or other ways. By this way, we can obtain a highly ordered morphology of metal nanostructures. In the case of FON structures, a well-defined substrate should be fabricated using the template materials (often silica or polystyrene), and then the metallic film should be deposited on the substrate.197 By this way, we can obtain a highly ordered morphology of metal nanostructures. For example, Van Duyne 5028


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Figure 39. (a) Schematic diagram of the synthesis of Au@SiO2 with Raman reporter inside nanoparticles. (b) TEM image of Au@SiO2 nanoparticles. Reproduced from ref 185. Copyright 2003 American Chemical Society. Figure 38. Magnetic concentration of Fe3O4@Au nanoparticles. Photographs of a drop of the nanoparticle solution on a silica chip (a) before and (b) after the concentration. SEM images of the (c) overview and (d) the center area of Fe3O4@Au nanoparticles after concentration. Reproduced with permission from ref 363. Copyright 2012 Elsevier B.V.

displace the marker molecules, and it will stabilize the Raman signals instead, even in a harsh environment. Besides, the PEG shell could create hot spots to improve the enhancement of the nanoparticles and also prevent other nonspecific interactions with metal nanoparticles. Rotello and co-workers provided an efficient sensor for protein detection. The sensor was prepared through the following procedures: Au nanoparticles were first functionalized with molecules with thiolated ligands and then connected with fluorescent poly(p-phenyleneethynylene) (PPE) polymers by electrostatic interaction.181 Because of the FRET process, the fluorescence of fluorescent polymers could be quenched. When protein analytes were added, they would strongly interact with the cationic Au nanoparticles, the fluorescent polymers would be displaced and released from the Au surface, and then the fluorescence would be restored. And changing the cationic terminal, the bonding ability of the fluorescent polymers or the protein, the kind of “chemical nose/tongue” approach, has been successfully used to identify several proteins in a rapid and efficient way. Other polymers also show well biodistribution and pharmacokinetic properties. Cui’s group352 utilized R6G-tagged Ag aggregates to obtain high SERS effects, coated with PVP as a stabilizer, due to the biocompatibility of the PVP shell; this kind of colloidal solution could be used to study living cells. Choo et al.353 fabricated Au nanorods with mercaptopyridine (MPY) as a Raman reporter, coated poly(sodium 4-styrene-sulfonate) (PSS) to change the charge of Au@MPY to negative, and then immobilized specific antibodies such as anti-HER2 (human epidermalgrowth factor receptor-2) antibodies on the Au nanorod surface through electrostatic interactions. Finally, these antibody-conjugated gold nanorods were used for the highly sensitive targeting and imaging of HER2 markers, which were expressed on the surface membrane of cancer cells and they were clinically molecular markers of breast cancer. 4.3.3.3. Metal-Molecule Core−Shell Nanoparticles. Besides inert silica shell or polymer shell, many kinds of biomolecules are also used as shells which are suitable for biological applications. Among them, DNA is commonly used because of its complementary property. In general, thiol-functionalized DNA is attached on the nanoparticle surface with strong Au−S or Ag− S bond. At the same time, Raman markers with strong Raman signal are also assembled on the nanoparticle surface. Thus, the modified nanoparticle can be used to capture the target DNA. If strong Raman signals can be obtained, it means that the target DNA is captured.

were repeatedly added several times, and it can be found that the clear solution became purple at the addition of Au3+ and gradually changed to deep pink during the successive iterations. There are many other methods to grow Au shell on the magnetic core. For example, the cores can be modified with a coupling agent to attract gold ions or atoms. Several polymers with amine groups were used as a coupling agent, such as poly-L-histidine (PLH) or polyethylenimine (PEI) because the amine groups can attach with gold ions. Finally, the gold ions can be reduced by ascorbic acid to grow the Au shell. 4.3.3. Metal-Nonmetal Core−Shell Nanoparticles with Thick Shell. 4.3.3.1. Metal-Silica Core−Shell Nanoparticles. Some researchers synthesized Au@Raman reporter@SiO2 nanoparticles for SERS applications.185 Briefly, the authors synthesized a certain size of Au colloid by reducing a certain concentration of HAuCl4 solution with sodium citrate,308 stirred it with Raman markers or added a coupling agent before the marker molecules, and then coated the metal NPs with a thinner silica shell. The Raman marker should be added after the coupling agent because Raman markers can strongly interact with Ag/Au nanoparticles, so the previously linked coupling molecules would be displaced by the Raman markers. Then, a thin silica shell was coated to protect the molecules attached on Au nanoparticles and also to prevent NPs aggregation. After that, a second thicker silica shell was coated by Stober’s method (Figure 39).185,347 The second silica shell was used to further modify with other tags or antibody. Several researchers also attempted to modify the shell with different biomolecules and also changed different materials and shapes of the metal core to design different experiments to conduct detection, immunoassay, and imaging. 4.3.3.2. Metal−Polymer Core−Shell Nanoparticles. Polymers possess several advantages such as biocompatibility, biodistribution, and modifiability, and because of these advantages they are widely used in various fields like biology, surface science, and medicinal chemistry. One of the star molecules is polyethylene glycol (PEG),180 a routine experiment always carried out as follows: first, Au colloids should be modified with Raman markers, and then it will be attached with SHfunctionalized PEG. The thiol-functionalized PEG would not 5029


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modified with Raman dyes-tagged DNA strands. To obtain enhanced Raman signals, a second metal layer on the DNA (or any other Raman reporter) modified Au nanoparticles should be reduced to form the metal-molecule-metal core−shell nanostructures (Figure 40, panels c and d).

To get higher sensitivity and enhancement, another metal shell is formed to couple with core particle to provide strong electromagnetic field. For example, Mirkin’s group fabricated Au nanoparticles covered by DNA and labeled with Raman-active dyes and then coated a layer of Ag nanoparticles.354 The covered DNA were used to link with target DNA, the dyes were used to provide strong Raman signal, and the Ag nanoparticles were used to enhance the Raman signals. They used different dyes to label different DNA, and because of the complementary property, the multiplexed detection of DNA would be realized. And other biomolecules such as bovine serum albumin (BSA) is also commonly used as a protective shell. Liposomes are also promising shells for nanoparticles because of their great biocompatibility and self-assembly into organized structures. Various molecular shells can expand the application of core− shell nanoparticles in biological systems. 4.3.4. Hybrid Core−Shell Nanostructures with Thick Shell. This category can be divided into three configurations. The first configuration is nanoparticles linked with DNA and labeled with Raman markers to fabricate hot spots in the gap of two nanoparticles to obtain strong Raman enhancement. For example, the gold−silver core−shell nanodumbbells (GSNDs) structure was reported by Suh et al.216 In detail, first, Au nanoparticles were modified with two different DNA, one iwas the protected DNA and another was the target-capture DNA. Two kinds of Au nanoparticles were used in this design. One of them was additionally modified with Raman active Cy3 molecule, and they would be separated by magnetic nanoparticles modified with the DNA complements. After that, probe A was modified with a target-capture DNA, and probe B was modified with B target-capture DNA with Cy3 mixed together with target DNA. Then the DNA-tethered Au dimer can be obtained and the Cy3 located at the gap. As shown in Figure 40 (panels a and b). The second one uses Raman markers and DNA-modified Au nanoparticles and then coated with another metal shell.217 DNA was used to fabricate a bridge to facilitate the formation of the nanobridged nanogap and Raman markers located in the gap to provide strong SERS signals. In detail, Au nanoparticles were

5. APPLICATIONS SERS technique is a fingerprint vibrational spectroscopy with ultrahigh sensitivity. Core−shell nanoparticles with unique properties are suitable for application in various fields if combined with SERS technique, such as electrochemistry, bioanalysis, sensing, food safety, environment safety, cultural heritage object, material science, catalysis, energy storage and conversion, and so forth. In order to help readers find the proper strategy for their interested systems easily and rapidly, detailed information about the applications of SERS using core−shell nanoparticles as substrates is summarized in Table 1. 5.1. Electrochemistry

5.1.1. Surface Adsorption. 5.1.1.1. Surface Water Structure on Transition Metals. Raman spectroscopy can provide rich fingerprint information about molecular structure, bonding, and orientation of chemical and biological molecules. Raman peak shape, frequency, and half peak width are sensitive to the molecular bonding and its environment. Particularly, water is the most widely used and important solvent in electrochemical studies. It has strong absorption in infrared region, while in the visible region there is no absorption. Thus, the Raman scattering of bulk water is very weak, and it does not interfere with Raman spectra. Moreover, SERS spectra can be acquired in a wide range from 50 to 4000 cm−1 (Figure 41). Overall, SERS is an appropriate spectroscopic technique for electrochemical systems when compared with infrared spectroscopy (IR) and sum frequency generation (SFG). Tian’s group investigated the interfacial water structures on metal film electrodes, including Au, Au@Pd, and Au@Pt nanoparticles film electrodes. The Au@Pd and Au@Pt nanoparticles were used to explore the interfacial water structure on the Pd or Pt surface.128 By utilizing the core−shell nanoparticles, the molecules with weak signals adsorbed on weak-SERS-active metal shell could be observed. By this borrowing activity strategy, the Raman spectra of surface water on Pt-group metals were obtained. The Stark tuning rates of the O−H stretching vibration for Au, Pd, and Pt were 64, 76, and 14 cm−1 V−1, respectively. It indicates that the interaction betweenf water and nanoparticles surface plays an important role, and it is different for these three metals. For Pt, a full monolayer of hydrogen was adsorbed on the top site as the first layer, and water molecules were located as the second outer layer. For Pd, the hydrogen atoms may be adsorbed inside bulk Pd, so no Pd−H vibration can be observed and water molecules adhered onto the bare Pd surface directly. For Au, the interaction of Au with hydrogen atoms is very feeble so water molecules directly adsorbed on the Au surface. 5.1.1.2. Adsorption at Single-Crystal Surfaces. Single-crystal surface is highly preferred in surface science, catalysis, and electrochemistry due to its well-defined and atomically flat surface structure. With these properties, surface related studies at single-crystal surfaces can be correlated with theoretical calculations. Unfortunately, it is not possible to directly perform SERS measurements at single-crystal surfaces which do not support SPR. Shell-isolated nanoparticles can be used as signal amplifiers to create a strong electromagnetic field at single-crystal surfaces without any interference compared with bare nano-

Figure 40. TEM images of two types of hybrid core−shell nanostructures. (a and b) TEM images of gold−silver core−shell nanodumbbells structure. Reproduced with permission from ref 216. Copyright 2010 Nature Publishing Group. (c and d) TEM images of gold nanobridged nanogap particles. Nanobridges within the Au-NNP are indicated by red arrows in (d). Reproduced with permission from ref 217. Copyright 2011 Nature Publishing Group. 5030


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Table 1. Various SERS Applications Using Core-Shell Nanoparticles

concentration of pyridine, and the applied potential to the adsorption behavior. Figure 42a shows the cyclic voltammograms (CVs) of three low-index Au(hkl) surfaces in 1 mM pyridine. It shows great distinction at different single-crystal facets, which indicates that the surface structure of the electrode has a significant impact on the adsorption behavior of pyridine. The adsorption potential at which pyridine formed a full

particles. In this way, the structural information on molecules adsorbed at the single-crystal surface can be clearly examined and explained. Li’s group employed electrochemical SHINERS to in situ probe the adsorption behavior of pyridine at gold single-crystal electrode surfaces in an electrochemical environment.118 They carefully examined the influence of crystallographic orientation, 5031


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Figure 41. Potential dependent SERS spectra and corresponding models of water adsorbed on Pt, Pd, and Au surface. Reproduced with permission from ref 128. Copyright 2007 Royal Society of Chemistry.

Figure 42. (a) CVs of three gold single-crystal electrodes in 0.1 M NaClO4 solution containing 1 mM Py. (b) Potential-dependent SHINERS spectra of Py adsorbed on Au(111). (c) The trend of Raman frequency shift with potential in different crystal facets. (d) The trend of Raman intensity with potential in three different crystal facets. Reproduced with permission from ref 118. Copyright 2015 American Chemical Society.

Au(111) electrode. The potential range increases from −0.8 to 0.4 V with 0.1 V increments. The strong peaks at 1010 and 1035 cm−1 are the characteristic peaks of pyridine. The peak of 1011 cm−1 is attributed to the ν1 ring breathing mode, and another peak is assigned to the ν12 symmetric triangular mode. Figure 42 (panels c and d) shows the Raman frequency and normalized intensity of the peak of 1010 cm−1 in three Au(hkl)

monolayer on Au surface follows the order: Au(111) at 0.24 V > Au(100) at −0.04 V > Au(110) at −0.27 V. This trend is consistent with the sequence of potential of zero charge Epzc of these three Au single-crystals without pyridine. Thus, it is reasonable to conclude that the electrode charge plays a dominant role in the adsorption process. Figure 42b shows the SHINERS spectra of pyridine in different potentials on the 5032


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Figure 43. (a) SHINERS spectra of hydrogen adsorbed on a Pt(111) surface at different potentials. Reproduced from ref 114. Copyright 2010 Nature Publishing Group. (b) SHINERS spectra of hydrogen adsorbed at a Ru(111) surface at different potentials. Reproduced with permission from ref 387. Copyright 2011 The Royal Society of Chemistry. (c) The schematic diagram of viologen molecule with V2+ and V+• mode at the Au single-crystal surface. Reproduced from ref 357. Copyright 2011 American Chemical Society. (d) The schematic diagram and SHINERS spectra of 2,2′-bipyridine adsorbed at Au(100). Reproduced from ref 356. Copyright 2012 American Chemical Society.

Figure 44. (a) Potential-dependent SHINERS spectra of BMIBF6 at Au (111) electrode. (b) Proposed interfacial structure of Au single-crystal electrode/imidazolium-based ionic liquid at different potential regions. Reproduced with permission from ref 359. Copyright 2014 Royal Society of Chemistry.

faces plotted against potentials for the ν1 mode. As shown in Figure 42c, the Raman frequency of the ν1 mode have little increment in the negative potential for all crystal facets. In higher potential, however, a sharp increase of the frequency of ν1 mode occurred and the Stark tuning rate is large. The obvious change indicates that the pyridine adsorption behavior or orientation is different in different potentials. In negative potential region, the frequency of ν1 mode is lower, which suggests the flat orientation of pyridine. In this adsorption, pyridine molecules interact with metal by π-orbitals and this binding interaction is weak. Oppositely, the frequency of pyridine increases rapidly in higher potential. This might arise from a vertical orientation of pyridine because of the strong interaction of nitrogen atom lone-pair electrons with the metal. The sharp increase of frequency occurred in different potentials at three Au(hkl) facets, and the potential order is Au(111) < Au(100) < Au(110). The order is also the same as the trend of Epzc, and this phenomenon means that the interaction between pyridine and single-crystal surface is influenced by surface charge; after the monolayer adsorption is complete, the interaction becomes stronger. Figure 42d shows the intensity of the ν1 mode increases as the order of Au(111)
Au(110) ≫ Au(100). This order is in contrast to the activities of three single-crystal facets in oxygen reduction reaction. This may be because the

5.2. Bioanalysis

5.2.1. DNA Detection. When the analytes were placed in the junction of the core−shell, the SERS enhancement can be enhanced dramatically. Therefore, several researchers pay attention to the multilayer metal core−shell nanostructures. Mirkin et al. designed a model in which Au nanoparticles were modified with Raman dye-labeled oligonucleotides and then 5036


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Figure 49. (a) Scheme diagram of the sandwich-assay for DNA detection. (b) Left is the detection results of the different DNA sequences (every sequence has individual Raman dyes); right is different colors represent different Raman dyes’ spectrum. Reproduced with permission from ref 354. Copyright 2002 the American Association for the Advancement of Science.

utilized the functionalized nanoparticles to detect the labeling substrate-bound oligonucleotides (Figure 49) because the diameter of the Au nanoparticles is too small to produce the electromagnetic-field to enhance the signal.354 Thus, they prepared Ag particles around the Cy3-labeled nanoparticles, which can produce a large Raman scattering enhancement (100 fold) comparing with conventional fluorescent detection of DNA. The nanoshell structure has been served as a robust and stable substrate to probe biological species such as DNA. DNA is the primary and important biomolecule because it carries the genetic information on the organism. Thus, the detection of DNA is significant for biology. Along this direction, the SERS method can be used to detect the DNA molecules due to its high sensitivity. Even single molecule like adenine can be detected by SERS. Halas and co-workers developed a simple protocol to bind DNA with Au nanoshell by thermal pretreatment prior to improve the SERS spectra quality and reproducibility.193 They also extended this protocol to examine the orientation of dsDNA. DNA can bind with metal shell surface via thiol group. First, they relaxed the ssDNA to an extended conformation by thermal pretreatment. Thus, the DNAs were at a stretched conformation and then the complementary DNA sequences were added with a 1:1 molar ratio with DNA solution in the buffer of TE/50 mM NaCl. The target DNA was deposited onto a nanoshell substrate. After overnight, the excess DNA was washed by TE or TE/50 mM buffer. It can be seen in Figure 50 that the SERS spectra of DNA treated with this method has good reproducibility compared with untreated DNA. This indicates that the good reproducibility is not related with the DNA sequence. The different reproducibility between thermally pretreated and untreated DNA may be due to the uncoiling of ssDNA induced by thermal pretreatment. This treatment led to extended ssDNA chains, so the ssDNA tended to bind with the nanoshell surface regularly. The ordered and monolayer adsorption on the nanoparticle surface can enhance the reproducibility and uniformity of the SERS signals. Therefore, the DNA molecules were thermally pretreated to bind to nanoshell and to obtain good quality SERS spectra. 5.2.2. Immunoassays. Antigens can be detected by a sandwich structure due to the interaction of antigen−antibody. A sandwich model was designed by depositing Au on oxidized γFe2O3 NPs.349 The γ-Fe2O3 NPs were synthesized by completely oxidizing Fe3O4 NPs. And Au shell was deposited by Lyon’s method, which used a hydroxylamine as a reductive agent and HAuCl4 was added dropwise. This process was repeated three

Figure 50. (a) Schematic diagram for the SERS detection of DNA via metal nanoshell core−shell nanoparticles. (b) SERS spectra of untreated ssDNA (curve a) and pretreated DNA (curve b). Reproduced from ref 193. Copyright 2008 American Chemical Society.

times to obtain Fe2O3@Au NPs. In this study, goat antimouse IgG and goat antihuman IgG were used as antibody and human IgG and mouse IgG were antigen. The Fe2O3@Au NPs were assembled with goat antihuman IgG, and bovine serum albumin (BSA) was added to block active sites between antibodies. Then the immuno-Fe2O3@Au NPs were labled with MBA molecules to provide strong Raman signals. Figure 51 shows the procedure of separating two different antigens by Fe2O3@Au NPs and then

Figure 51. Schematic diagram for separation process of antigens with Fe2O3/Au magnetic nanoparticles and the determination of the separation efficiency. Reproduced from ref 349. Copyright 2009 American Chemical Society. 5037


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react with amine groups with the MUC4, and the 4-NTP acted as the Raman markers. The capture substrates were modified with the antigens, and then used the Au nanoprobes to capture the antigens. Once the antibody−antigens reaction occurred, the Raman spectra of 4-NTP was obtained, and the aim to detect the MUC4 was realized with a high detection sensitivity. 5.2.3. Bacteria Detection. Zhang et al. prepared magnetic Fe3O4/Au core−shell nanoparticles for fast concentration and sensitive detection of bacteria simultaneously.363 The concentration was realized by using an external magnetic field, and the bacteria were detected by SERS. They dropped 10 μL solution mixed with 9 μL 2 × 105 cfu mL−1 bacterial E. coli K12 and 1 uL 30 μg mL−1 Fe3O4@Au NPs solution to a clean silicon chip. By applying an external magnetic field, the bacterial cells were concentrated in a dot. After that, the nanoparticles were pulled toward the center by the magnetic field, and the bacterial cells were pushed by the nanoparticles in the meantime. By this way, the bacterial cells were concentrated and the concentration efficiency was calculated by the ratio of bacterial cells number density with and without concentration. In the magnetic concentrated dot area, the number density of bacterial cells was 60 times higher than that without concentration. It can be seen in the SEM images that Fe3O4@Au NPs directly attached to the bacterial cell wall, thus, the Raman signals of bacterial cells were enhanced. They examined three bacteria strains by this method and found that different SERS spectra could be obtained, especially the bands around 800 and 1000 cm−1. The authors also used principle component analysis (PCA) method to analyze the SERS spectra to distinguish these three bacteria. The shift and intensity of Raman peaks were applied in PCA, as shown in Figure 53; the spectra of the same bacteria clustered together suggest that the spectra have good reproducibility, and different bacteria can be distinguished clearly. Magnetic Fe2O3−Au core−shell nanparticles were ultilized by Gu and co-workers in biological separation and immunoassay by SERS method. The bioseparation was realized by appling an external magnetic field. And immunoassay, which is a common method for biochemical analysis, can be carried out by SERS. SERS-active plasmonic nanomaterials have become promising substrates for in vivo imaging and multiplex detection.391−393 Register et al. investigated the use of SERS-encoded gold nanostars for in vivo detection and demonstrated in vivo SERS detection of gold nanostars using small animal (rat) as well as large animal (pig) models.394 Stuart et al. detected glucose in vivo in a rat model with the aid of a surgically implanted optical

detected by SERS. The nanoparticles were modified with goat antihuman IgG, so only human IgG in solution could be captured by nanoparticles because of the strong interation between antibodies with their corresponding antigens. Then the immunoFe2O3@Au NPs were separated by using an external magnetic field. The remaining antigens in solution were reacted with Au NPs fixed with goat antihuman IgG and labeled molecules. No Raman signal was obtained, which indicated that nearly no human IgG exsit in solution. Almost all of the antigens were separated with high separation efficiency. Therefore, magnetic Fe2O3@Au NPs have potential applications in bioseparation. Sandwich configurations are widely used in immunoassays. Wang et al. utilized Raman markers tagged and antibodymodified Au nanoparticles to conduct the detection of pancreatic cancer (PC), a lethal cancer (Figure 52).368 Nowadays the

Figure 52. Schematic description of SERS-based immunoassay assay. (a) The process of gold substrates extract antigens from the solution. (b) Synthesis of Raman makers and antibodies modified Au nanoparticles as the nanoprobes. (c) Formation of sandwich immunoassay for SERS readout. Reproduced from ref 368. Copyright 2011 American Chemical Society.

routine detection for PC is to detect the MUC4 in an enzymelinked way. The MUC4 is a suitable biomarker for PC. So the author synthesized certain sizes of Au nanoparticles as the substrates for the antibody and then functionalized with DSP and 4-nitrothiophenol mixture. The succinimidyl ester of DSP could

Figure 53. (a) Schematic of SERS analysis of bacterial cell wall with Au-MNPs. (b) Principle component analysis of three different bacteria. Reproduced with permission from ref 363. Copyright 2012 Elsevier B.V. 5038


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Figure 54. (a) Mapping images and (b) corresponding SERS spectra of 10 different nanoparticles in vivo. (c) SERS mapping results of deep tissue after the injection of five unique SERS nanoparticle batches. Reproduced from ref 176. Copyright 2013 American Chemical Society.

Figure 55. (a) Raman spectrum of GIANs at 632 nm excitation, which shows the G and D bands of graphene. (b) Bright field images and Raman imaging of MCF-7 cells without and with GIANs in D mode and G mode. (c) Bright field images and TPL imaging of HeLa cells and 95-C cells incubated with GIAN and GIAN-Sgc8. Reproduced with permission from ref 126. Copyright 2014 Nature Publishing Group.

window.395 Their works demeonstrate that the SERS technique can be a powerful tool for detection of various cancers and diseases. 5.2.4. Bioimaging. In vivo SERS imaging has been used to image tumors and guide surgical removal of tumors (particularly for elucidating tumor margins).391,396−398 It has also been used for multiplexing capabilities to simultaneously detect several markers (analytes or Raman reporters), which is a great advantage of SERS, especially in the context of in vivo biosensing. Zavaleta et al. reported multiplexed imaging in a live mice by SERS tags.365,399 They prepared ten SERS tags with unique fingerprint for each molecule marker. The SERS tag consists of a 60 nm Au core, a kind of unique Raman active molecules adsorbed on Au nanoparticle surface, and then coated by a silica

layer to get a 120 nm core−shell nanoparticle. The Au core was used as a Raman signal amplifier to increase the detection sensitivity. The silica shell was used to increase the stability of nanoparticles and avoid direct contact with the external environment. To examine the in vivo multiplexed imaging ability of SERS tags, first, they injected 10 SERS tags separately by subcutaneous (s.c.) injection in superficial (skin) of a nude mouse. Then the injected area was mapped by a Raman microscope. After being analyzed by their postprocessing software, it can be found that all 10 s.c. injections have been separated correctly (Figure 54a). This experiment demonstrated that the 10 SERS tags can be applied in multiplexed imaging. Further, they examined the feasibility of the multiplexed imaging by SERS tags in deep tissues like liver. The SERS tags 5039


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Figure 56. Anti-EGFR/Au nanorods incubate with malignant and nonmalignant cancer cells. At 80 mW (10 W/cm2), the malignant cells show obvious injury by the laser spots (the black circles in the pictures), meanwhile the nonmalignant cells keep no injury until the laser power up to 120 mW. Reproduced from ref 367. Copyright 2006 American Chemical Society.

strong bands at 1325 and 1596 cm−1 were attributed to the D band and G band of graphene, respectively. Additionally, the graphene shell can be modified with various functional groups, such as PEG molecule, thus the hydrophobic shell becomes a hydrophilic surface, improving its biocompatibility. The graphene isolated gold nanoparticles (GIANs) have a wide range of applications in biomedical research. To do a series of studies in cell imaging and photothermal enhanced chemotherapy, Tan’s group has utilized GIAN (Figure 55).126 Compared with organic molecules, graphene has a larger Raman cross section and more stable structures. Because of the strong electromagnetic field of Au core and chemical enhancement of graphene shell, the Raman signals can be enhanced enormously. Hence, graphene is an appropriate candidate for cell imaging. Figure 55b shows the Raman imaging of MCF-7 breast cancer cell loaded with GIANs. Either G band or D band can be used for imaging cell. It can be seen that the Raman signal was mainly distributed in the cytoplasm while not in the nucleus. Compared to fluorescence imaging, the Raman signal of MCF-7 has higher resolution due to its narrower bandwidth. Moreover, fluorescent molecules are prone to self-quenching, while the Raman signals are more stable. In addition to silica and polymer shells, many researchers are interested on the molecular shells. A molecular shell can be formed with DNA molecules to form a special configuration,

were injected into the tail vein, and then the particles tend to be captured by the Kupffer cells of the reticuloendothelial system, thus, the particles were accumulated in the liver and the imaging of liver can be realized. Because the particles were mixed together to be injected, the spectra may overlap, leading to certain mismatches. The author chose five SERS tags with the least overlapped spectra and inject them to a nude mouse via the tail vein. After 24 h, a multiplexed imaging was obtained by Raman mapping (Figure 54c). It represents that all five SERS tags accumulated in liver simultaneously, and the five tags can be correctly identified by a software. But their distribution was not homogeneous throughout the liver, leading to problems for quantification. They solved the quantification problem by a mixture of various concentrations of four SERS tags and then correlated SERS concentration with Raman intensity. They found that each SERS channel revealed a linear correlation. Therefore, the Raman tags can be successfully applied in multiplexed imaging in both superficial and deep tissue after s.c. and intravenous (i.v.) injections. As discussed earlier, many other materials can be used as a shell to protect SERS-active cores. In particular, graphene is a promising two-dimensional material with single atom layer. Graphene has the property of near-infrared absorption which is suitable for photothermal therapy, and the property of strong Raman signal makes it a great Raman tag for bioimaging. The 5040


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would scatter red light in the dark field, thus we can clearly distinguish the malignant cell from the normal one. The authors practiced the nanorods in photothermal therapy of cancer cells: HSC and HOC malignant cells and HaCaT nonmalignant cells. It is certified that these antibodies conjugated Au nanorods not only could differentiate the malignant and nonmalignant cells but also could destruct the malignant cells by using the half energy of the laser power which could kill the nonmalignant ones. Chemotherapy is another common method for the treatment of cancer, and combining photothermal therapy with chemotherapy can significantly increase the therapeutic effect. In this regard, GIANs are used to enhance the chemotherapy.126 For example, doxorubicin (Dox) is a kind of commonly used anticancer drugs, and GIAN can connect with Dox through π−π stacking (Figure 57). It was found that under laser radiation, the

such as the dimer mode, forming the gap between or within nanoparticles, and by using dye-tagged DNA sequences, we could directly detect the target on the solution or the substrate. Besides DNA, amino acids, peptides, and other kinds of molecules can be used for molecular shells because they have a wide range of applications. Lee et al. reported encapsulated tags of RhB@Au nanoflowers, RhB as the Raman reporter, following the package with denatured BSA molecules to gain a compact molecular shell. The shell of denatured BSA molecules presents a negative charge because of carboxylic acid groups which could be further modified with other ligands to detect proteins. In terms of multimodal and multifunctional structural design, several excellent research has been reported recently. For example, Gao et al. mainly used multicolor quantum dots for multiplexed optical coding of biomolecules, multiplexed biological detection, and imaging.400−403 They developed a multicolor multicycle in situ imaging technology, which is capable of creating detailed quantitative molecular profiles for individual cells at the resolution of optical imaging.404 5.2.5. Disease Diagnosis and Therapy. Nanomaterials are also applied in biological systems to combine multiple functionalities into a single system. Generally, nanomaterials such as gold nanoparticles, graphene, quantum dots, and carbon nanotubes have been used for various applications, such as biological imaging, biomedicine, diagnosis, etc. Gold nanoparticles like nanocages and nanorods are appropriate biological materials due to their strong near-infrared absorption, thus they can effectively convert near-infrared light to heat through photothermal processes. On the other hand, gold nanostructure is widely used in biological imaging because it is a great twophoton luminescence (TPL) material. Further, Au NPs are commonly used as a SERS substrate. However, molecules in the biological systems may have direct contact with bare nanoparticles, which may cause interference to the Raman signals. Moreover, the long-time irradiation of laser may lead to carbonization of molecules adsorbed on the bare nanoparticle surface. Therefore, to eliminate the above-mentioned problems, an inert shell is required to prevent the interaction between nanoparticles and biomolecules. Consequently, shell isolated nanoparticles are highly preferred and widely applied in this field. Silica-coated nanoparticles have been applied in biological systems, for instance, Xu’s group utilized SHINERS technique to obtain fingerprint information to investigate the type II microcalcifications in breast tissues.366,405 They obtained a series of normal Raman and SHINERS spectra. The distinction between different tissues by PCA, which can distinguish the type II microcalcifications, was also analyzed. This is the first time that SHINERS was used in the diagnosis of breast cancer cells. This study indicates that SHINERS is a promising technique in early diagnosis of breast cancer. Au nanorod was also a promising candidate to be applied in therapeutic human diseases. With a suitable aspect ratio, Au nanorod could absorb and scatter light in NIR region, which makes it suitable for human issue transmission wave band. ElSayed and co-workers367 utilized antiepidermal growth factor receptor (anti-EGFR) antibodies conjugated on Au nanorods (Figure 56). The antibodies tagged Au nanorods were incubated with the malignant cells (oral epithelial cell, HOC 313 clone8, and HSC 3) and the nonmalignant cells (epithelial cell, HaCat). Through antigen−antibody interaction with the overexpressed EGPR, Au tagged nanorods could specifically bond with malignant cells. In lab microscopy, the bonded Au nanorods

Figure 57. (a) Bright field images of cells after NIR photothermal therapy with varying concentration of GIAN solution or varying irradiation times. (b) The cell viability in varying concentration of GIAN solution (c) The cell viability at varying irradiation times. Reproduced with permission from ref 126. Copyright 2014 Nature Publishing Group.

effect of chemotherapy of Dox modified with GIAN was better than Dox-only samples, indicating that GIANs exhibited photothermal enhancement to the chemotherapeutic effect. Additionally, Gao et al. synthesized multilayered gold nanoshells for enhanced photothermal therapy.406 In order to create an ever higher plasmonic tunability, a gold-silica-gold multilayered nanoshells structure, known as “nanomatryoshka”, has been created by encapsulating gold cores into the gold nanoshells. The intensified coupling between the inner core and outer shell plasmons makes it a much more effective NIR photothermal transducer than conventional gold nanoshells. Consequently, such MLGSs can kill cancer cells efficiently under the exposure to NIR 808 nm laser. 5.3. Sensing

Plasmonic materials have been widely designed and prepared for various applications, ranging from near-field optics, spectroscopies, solar cell, chemical- and biosensors. Using plasmonic core−shell nanostructures as a sensing probe is an attractive topic with numerous applications. The metal core provides the strong electromagnetic field to enhance the spectrum signals, and the functionalized shell can realize the stimuli-responsive to the external environment. Therefore, the core−shell nanostructures 5041


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can combine the sensing and SERS techniques. Here we briefly introduce some applications in pH sensing, temperature sensing, and biosensing. 5.3.1. pH Sensing. Nie et al. used plasmonic nanostructures core and polymers shell to sense the ambient pH.407 In Figure 58,

Figure 59. Thermoresponsive N-isopropylacrylamide (NIPAM)-coated Au nanoparticles trap the Raman markers from 4 to 60 °C to 4 °C. Thermoresponsive N-isopropylacrylamide (NIPAM)-coated Au nanoparticles trap the fluorescent molecules from 4 to 60 °C to 4 °C. Reproduced with permission from ref 408. Copyright 2009 Wiley-VCH. Figure 58. (a) The different configurations of the pH-responsive group in different pH environments. (b) Functionalization of Au nanoparticles with Raman markers and pH-responsive group. (c) Different configurations of the nanoprobes make significant differences in the Raman spectrum. Reproduced from ref 407. Copyright 2009 American Chemical Society.

because the molecules were located too close to the metal core, and at the stretched mode of the shell, the fluorescence restored. 5.3.3. Biosensing. Nowadays, multitudinous patients suffer from diabetes mellitus because their bodies lose the produce or respond function of the insulin, which regulates the glucose in an abnormal level. Now the routine detection method is taking a small sample of the patients’ blood, which is painful and unconventional. Therefore, the future aim is making progress toward in vivo detection, minimally damage sensing. The wellorder FON substrates can exhibit a strong electromagnetic enhancement and could be synthesized in microscale and nanoscale which can be exploited to detect glucose in vivo. To detect glucose molecules, the Van Duyne group modified the AgFON surface with alkanethiol molecules to form a partition layer (Figure 60).198 However, this approach can be further improved by changing various factors, such as the optimum alkanethiol molecules, the wavelength and power of the laser, and the suitable acquisition time.

the pH plasmonic sensor consisted of Raman markers tagged Au nanoparticles, and a pH-responsive polymer polymethacrylic acid (PMAA) coated the functionalized Au nanoparticles. First, Au nanoparticles were modified with Raman markers, and then PMAA linked PEG was used to encapsulate the modified Au nanoparticles. When pH > 4, the pH sensor showed no SERS signal because the negative carboxylic acids of the PMAA swelled, which make the polymer far away from each other by the electrostatic repulsions and lead to a strong steric effect between the SERS NPs. At pH < 3, PMAA exhibited a condensed configuration, meanwhile it exhibited a strong SERS signal because of the shorter distance between the plasmonic nanoparticles which enhanced the SERS enhancement. 5.3.2. Temperature Sensing. Alvarez-Puebla and coworkers combined thermal-stimuli responsive materials and SERS method for a temperature sensor.408 In this work, the authors used the porous properties of polymer shells to trap molecules, which have no functional groups. As shown in Figure 59, the plasmonic NPs were coated with a polystyrene(PS) shell then linked with the thermoresponsive N-isopropylacrylamide (NIPAM) to form the Au@pNIPAM nanocomposites. Later, thermoresponsive nanoprobes were used to detect the noninteracting molecules. At 4 °C, no SERS signal was observed because of the large spatial separation between the molecules and the nanoprobes. But at 60 °C, the NIPAM shell collapsed and the distance was reduced, so huge SERS signals were obtained. Besides the Raman molecules, the fluorescent molecules could be trapped in the porous polymer shells, and at the condensed configuration of the polymer shell, the fluorescence quenched

Figure 60. Schematic description of the AgFON structure for glucose detection. Reproduced from ref 198. Copyright 2003 American Chemical Society. 5042


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The detection limit was 5.0 × 10−8 M in the melamine-Hg2+ system with the Ag@C/Ag NPs structure. 5.4.3. Alcoholic Beverages. In the alcoholic beverages industry, ethyl carbamate (EC) is a common byproduct of the fermentation process, which is the ester of carbamic acid. EC can be found in various food products and alcoholic beverages such as wine, beer, rice wine cheese, vinegar, bread, and soy sauce. However, EC is potentially toxic to humans because it has a carcinogenic and mutagenic effect; when EC is metabolized into viny carbamate and then followed by epoxidation, the product will result in DNA adduction and mutation which will damage the lungs, liver, and mammary glands. Thus, the detection of EC in beverages and food products is significant. Yang et al. prepared Au@Ag core−shell nanoparticles to quantitatively detect ethyl carbamate in alcoholic beverages.371 Using the SERS technique, they detected ethyl carbamate in three different alcoholic beverages, including vodka, obstler, and white rum. The band at 1003 cm−1 was the characteristic band for the quantitative evaluation of ethyl carbamate. The as-prepared substrates reached the limit of detection of 9.0 × 10−9 M, 1.3 × 10−7 M, and 7.8 × 10−8 M for vodka, obstler, and white rum, respectively. 5.4.4. Tobacco Products. Nicotine is one of the main psychoactive ingredients in tobacco which leads to the addiction of tobacco smoking, and it will cause health damage in lungs and can even lead to premature death. Therefore, the detection of nicotine in tobacco products is very important. To achieve this goal, Bell et al. developed an approach of rapid and quantitative SERS analysis of nicotine by polymer-coated Ag nanoparticles.372 In this work, d5-pyridine was used as an internal standard for quantitative analysis of nicotine. Pyridine is an appropriate internal standard because its chemical structure is similar to nicotine. Both the target molecules and standard molecules attached to the surface through pyridyl ring, hence the perturbation to the nanoparticles will affect them in a similar way. 1030 cm−1 was assigned to nicotine, and 974 cm−1 was assigned to the internal standard. The intensity ratio of these two bands versus concentration was highly linear with R2 ∼ 0.998 in a concentration range of 0−10 ppm. With this method, they obtained the limit of detection of 0.1 ppm.

5.4. Food Safety

5.4.1. Pesticide Residues. The accuracy and convenience of SHINERS make it widely used for food safety, drug analysis, and environmental analysis. Interestingly, SHINs can be simply spread on the target surface for Raman measurements. For example, this method can be used to detect pesticide residues on a fruit, food, or vegetable.114,147 Figure 61 shows normal Raman

Figure 61. (a) Ordinary Raman spectra of orange contaminated by parathion before (I) and after (II). SHINERS spectra of parathion contaminated orange (III) and cabbage (IV). SHINERS spectra of fenthion contaminated cabbage (V). SHINERS spectra of cabbage contaminated by two pesticides (VI). (b) In situ inspection of pesticide residues on food/fruit by a portable Raman spectrometer. Reproduced with permission from ref 147. Copyright 2015 The Royal Society of Chemistry.

spectra of clean pericarps of orange and contaminated pericarps by parathion. There were only two bands at 1165 and 1526 cm−1 in both spectra, and the bands were assigned to carotenoid molecules. When the SHINs spread on the contaminated surface, two bands at 1109 and 1339 cm−1 due to methyl parathion can be observed clearly. Furthermore, the multiple detection of pesticide on vegetables can also be realized by SHINERS. This study suggests that with SHINs, one can accurately and quickly detect the parathion residues or pesticide residues on fruits/ vegetables. 5.4.2. Dairy Product Detection. In the dairy industry, melamine is usually added to improve the measured protein content because it contains many nitrogen atoms, but melamine is a highly toxic chemical to human beings. Chen et al. have synthesized Ag@C core−shell nanoparticles by a hydrothermal method and then they modified the Ag@C nanoparticles with small silver nanoparticles in the outer shell.370 The as-prepared substrate exhibited a 1 × 107 enhancement factor, and the substrates were used to detect melamine with a detection limit of 5 × 10−8 M. They also found that metal ions can affect the detecion of melamine. It means that the melamine-metal ion system can be used to quantitatively detect the content of heavy metal ions. They examined a series of concentrations of mercury ion to the SERS intensity of melamine. And they found that the SERS intensity of melamine would reduce with the concentration of mercury ion decreasing. When the concentration was below to 5.0 × 10−8 M, the band at 685 cm−1 was disappeared.

5.5. Environment Safety

5.5.1. Heavy Metal Ions Detection. Heavy metal ions are most common bioaccumulative pollutants and will lead to serious damage to human health. Among them, mercury ion (Hg2+) is one of the most toxic solvated ions. Even a low concentration will result in high toxicity, thus the mercury contamination has been a global problem. Therefore, it is necessary to develop a highly sensitive and precise detection method for Hg2+ ions. Toward this direction, Ma et al. designed SiO2-coated Au−Ag core−shell nanoparticles to detect Hg2+ at picomolar concentrations.373 They prepared the Rhodamine 6Gderived Schiff base to bond to the amino group modified on the silica shell surface. When Hg2+ was added, the SERS signal was decreased due to the detachments of −R groups induced by Hg2+. This tunable ability of the chemical bonding of the substrate can improve the selectivity and sensitivity of the SERS sensor dramatically. The limit of detection of Hg2+ was 0.33 pmol L−1, and this structure can also be used as a pH sensor. It is wellknown that R6G-derivative exists as isomers with different chemical structures and spectroscopic properties, such as spirocyclic and open-cycle forms. In an alkaline environment, most of the −R groups of NR-Rs exist in spirocyclic form, which 5043


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makes less −R groups fall to the “hot spot” of the NRs and the substrates exhibit weak SERS enhancement. 5.5.2. Explosive Detection. Environment safety is also a significant matter because it is closely related to people’s life. Trinitrotoluene (TNT) is a kind of explosive and environmentally deleterious substance. Therefore, the trace detection of TNT has attracted considerable interest among researchers. Yang’s group used the SHINERS technique with shell isolated nanoparticles to detect trinitrotoluene (TNT) (Figure 62).374

example, the Ray group developed a sensitive nanostructure to detect TNT in a highly sensitive way. In their work, cysteinetagged Au nanoparticles were used for the detection. Because cysteine could react with TNT to form the Meisenheimer complex then the Meisenheimer complex can form a bond with the cysteine-modified Au nanoparticles through the electrostatic interaction. This electrostatic interaction can result in aggregation of Au NPs and eventually produce several “hot spots” which can enhance the enhancement factor up to 109. By this way, a significant detection of TNT can be achieved with the molecular shells. 5.5.3. Biowarfare Agent Detection. Bacillus anthracis is a kind of biowarfare agent, and dicarboxylic pyridine acid (DPA) can be used as a biomarker for the detection of this bacterial spores. Van Duyne’s group utilized Al2O3-modified AgFON to detect DPA by adjusting the structure of Ag FON to match with 750 nm laser using a portable Raman spectrometer.409 The detection range of spore concentration was 10−14−10−12 M. Importantly, the limit of detection was 2600 spores, which is below the anthrax infection dose (10000 spores). Additionally, they coated a 1 nm alumina layer on the Ag FON surface by atomic layer deposition (ALD) to enhance the stability of the substrates and the binding affinity with analytes. Because the carboxylate groups of DPA have strong interaction with alumina, this structure is suitable for bacillus spores’ detection. With the modification of alumina on Ag FON, the detection of spores was further improved to 1400 spores. Moreover, the stability of the substrate was increased from 40 days to 9 months. 5.5.4. Toxic Chemicals. In some cases, pinhole is not a defect if the Raman signal of molecules adsorbed on the core material because analytes can directly contact the SERS-activemetal core, and they also have high stability because of the protection of the inert shell. For example, to detect cyanide molecules, Ma et al. prepared Ag nanorods coated with Al2O3 as a SERS substrate (Figure 63).119 They prepared Ag nanorods by a physical method and deposited a thin layer of alumina by the

Figure 62. Schematic illustration for the synthesis of Au@PAT and its application in the detection of TNT. Reproduced with permission from ref 374. Copyright 2012 Royal Society of Chemistry.

The authors successfully prepared ultrathin and compact Au@ poly(2-aminothiophenol) (PAT) nanoparticles which can selectively identify TNT molecules because of the formation of Meisenheimer complexes between TNT and amino groups. Moreover, this nanostructure has good stability in strong acid or alkaline with pH from 2.02 to 12.95. Along this direction, some other core−shell nanoparticles were also used to detect TNT; for

Figure 63. (a) The schematic diagram of Ag NRs@Al2O3 with and without pinholes. (b) SERS spectra of DPA on Ag NRs@Al2O3-1 substrate with different concentrations. (c) SERS signals of NaCN at different concentrations on Ag NRs@Al2O3 substrate. (d) Actual cyanide concentrations versus their predicted values in the range from 1 to 100 ppb with the PLS model. Reproduced from ref 119. Copyright 2016 American Chemical Society. 5044


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alizarin, purpurin, carminic acid, laccaic acid, and brazilein, can be easily discriminated by using this method. However, in some cases, the direct contact between the dyes and the metal surface may cause charge transfer effect, which may result in some unknown changes. Moreover, this direct interaction may also change the orientation of the dye molecules and the spectrum may be different. Therefore, it is desirable to prevent the direct interaction between metal nanoparticles and dye molecules. To solve this issue, Tian’s group utilized Au core silica shell nanoparticles to investigate the thiazine dye thionine, which has been used in leather, textiles and papers.147 This molecule can interact with metal nanoparticles in various ways such as through the lone pair of electrons of N or S heteroatom, or amine group, or through the conjugated π ring of this molecule. Thus, it is difficult to obtain the specific spectrum of this molecule with SERS. With an inert shell, SHINERS can be used to obtain Raman spectra without any interference. This behavior indicates that the direct interaction of molecule and metal nanoparticles can affect the spectrum. However, with shell isolated mode this influence can be prevented and high quality SERS spectra can be obtained.

ALD method. The alumina shell was in subnanometer thickness with pinholes. It exhibits excellent SERS sensitivity because of the ultrathin alumina shell and has a long lifetime due to the protection of Al2O3. The substrate is very uniform, so it is suitable for quantitative SERS analysis. They detected a trace amount of cyanide molecules with this substrate because cyanide molecules are highly toxic. For example, it may cause fatal damage when exposed to 300 ppm of cyanide. Thus, a rapid and accurate testing method is required to detect trace amounts of cyanide. With the Ag NRs@Al2O3 substrate, quantitative detection of cyanide was realized and the detection limit can reach 1 ppb. 5.6. Cultural Heritage Objects

Due to the rapid and nondestructive properties, Raman spectroscopy is a commonly used technique for cultural heritage. SERS has developed rapidly over the past decades for the detection and identification of natural or manmade organic molecules such as pigments and dyes. Generally, colorants molecules exhibit strong fluorescent effects in traditional Raman spectroscopy, which can be eliminated in the SERS method. In the SERS study, colorant molecules are adsorbed on metal nanoparticles surface thus quenching the fluorescence, due to the energy transfer to the metal directly. Therefore, Raman signals will be greatly enhanced as a result of both the chemical and electromagnetic effects of nanoparticles. Van Duyne has made great efforts in detection of pigments and dyes, which has been widely used in the oil painting, by using Ag film over nanospheres (AgFONs) as the SERS substrate. The AgFONs were prepared by depositing a Ag film on the hexagonal-shaped array consisted of polystyrene or silica spheres. An important property of AgFONs is that the LSPR of this structure can be tuned easily by changing the sphere size. The authors utilized AgFONs substrates to identify and characterize many dyes and pigments such as purpurin, lac, cochineal, red dyestuffs alizarin, etc (Figure 64).376,410−412 Further, the authors

5.7. Materials

5.7.1. Monitoring the Synthesis of Nanomaterials. As discussed in the introduction section, nanomaterials have been widely used in many important fields due to their unique properties. However, controllable synthesis of nanomaterials is still very challenging because several fundamental issues during the synthesis process are not clear yet. For example, how do the factors, such as reduction potential, temperature, precursor concentration, and diffusivity control the size, shape, and morphology of nanoparticles? All these factors still needed to be understood to efficiently prepare the next generation nanostructures. Thus, developing the in situ method to monitor the synthesis process is becoming one of the hottest research topics in nanotechnology.413,414 SERS can be easily applied in various conditions, such as in aqueous solution, gas phase, or media at variable temperature and pressure. Furthermore, influence of molecules in the solvent on the results can be excluded, due to the selective enhancement of the Raman signals in SERS measurements. These advantages make SERS become a versatile and promising method for monitoring the synthesis process of nanomaterials. Recently, Kalyanaraman et al. demonstrated that SERS could be used to study the deposition of CdS on Ag nanoparticles via liquid phase chemical bath deposition (CBD), which is a very important method for the synthesis of core−shell nanostructure materials.377 Figure 65a shows the Raman spectra of CdS-coated Ag film as a function of deposition times. No Raman peak for silver sulfide (Ag2S) could be found in the spectra, indicating that the deposition would not lead to the formation of Ag2S. The Raman peak at about 466 cm−1 corresponds to quartz, and Raman band at 305 and 600 cm−1 can be attributed to CdS, which would increase with the deposition time. As shown in Figure 65b, the intensity of the peak at 305 cm−1 increased then reached a stable value after about 10 min. At the same time, it can also be observed that bare CdS directly deposited on pure quartz showed much weaker Raman signals. These results indicate that CdS is selectively deposited on the surface of Ag nanoparticles, thus the Raman signals from CdS can be significantly enhanced by the electromagnetic field generated on Ag. This can also be proved by the decreased trend of SERS enhancement factors with deposition time, as shown in Figure 65c. They further used

Figure 64. (a) SERS spectra of red pigment obtained by FON substrate. (b) SEM of a FON substrate. Reproduced from ref 376. Copyright 2010 American Chemical Society.

optimized the LSPR of AgFONs to match with the excitation wavelength, thus resonant Raman signal of dyes could be obtained. They also optimized AgFON for the NIR and IR region to identify eosin Y, which is an early modern synthetic dye and had been extensively used by Vincent van Gogh. The NIR or IR laser is more suitable for analyzing cultural objects because it minimizes photodegradation compared with visible laser. Additionally, they found that AgFONs exhibit greater enhancement by 1064 nm excitation than using 785 nm. Recently, AgFONs were also employed to discriminate some dyes with similar color, which can hardly be distinguished by visual inspection. For example, a series of natural red dyes, such as 5045


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trimethylaluminum (TMA) as precursor. During the ALD process, they found that the Raman signals for Al-CH3 and C− H of Al-(CH3)x surface species would increase or decrease along with the dosing of TMA or H2O, respectively. These results directly demonstrated that the surface reaction happened during each half cycle of the ALD process. Furthermore, they also studied the influence of molecule monolayer adsorbed on the surface, such as toluenethiol and 4-mercaptobenzoic acid, on the ALD process. Al−C stretching and no thiol vibrational frequency shifts were observed on toluenethiol-functionalized AgFON, while opposite results were observed on 4-mercaptobenzoic acid functionalized sample. These findings indicate that the mechanisms for coating Al2O3 on AgFON functionalized with toluenethiol or 4-mercaptobenzoic acid are very different. As shown in Figure 66, for toluenethiol-functionalized AgFON, TMA can penetrate through SAMs and react directly with Ag. However, for 4-mercaptobenzoic acid functionalized sample, TMA will react with the terminal COOH. These findings not only provide detailed information for the ALD process but also can guide the development of a more efficient ALD process. 5.7.2. Corrosion Inhibition. Gewirth’s group utilized SHINERS to investigate the formation of the benzotriazole (BTAH) film on copper single crystals and copper polycrystalline surface.378 The corrosion of Cu is a serious problem in oxidizing environments. Benzotriazole is an effective corrosion inhibitor that has usually been utilized in polishing slurries or plating baths to prevent the corrosion of Cu and related alloys. BTAH can form a coordination polymer film which prevents oxidation of copper. Therefore, it is essential to understand the formation of BTAH film to inhibit the corrosion process. Figure 67 shows the SHINERS spectra of Cu(111), Cu(100), and Cu(poly) at different potentials. The peak of 1140 cm−1 was attributed to the in-plane bending of NH, 1160 cm−1 was assigned to the combination of the bending modes of NH and the asymmetric stretching of triazole, and 1190 cm−1 was associated with the triazole ring breathing C−C−C in plane bending of BTA−. Thus, the peak at 1140 cm−1 is just related with the surface-attached BTAH, and the peak at 1190 cm−1 is due to the formation of the BTA Cu film in the process of BTAH oxidation on Cu electrodes. It is found that the peak intensities of 1020 and 1190 cm−1 increase in comparison to the other peaks when the potential is

Figure 65. (a) SERS spectra of CdS-coated Ag nanoparticles, (b) corresponding intensities of the peak at 305 cm−1, and (c) SERS enhancement factors as a function of deposition time. (d) Schematic diagram for the deposition process. Reproduced from ref 377. Copyright 2015 American Chemical Society.

LSPR spectroscopy to in situ monitor the mechanism for the growth of CdS on Ag. Combined with the simulation results, it can be concluded that the deposition process follows a 3D growth mechanism. As shown in Figure 65d, small CdS islands were first formed on the surface of Ag nanoparticles, then more CdS were preferentially deposited onto these islands, leading to the formation of CdS-coated Ag core−shell structure. Synthesis of core−shell type nanomaterials under gas phase can also be monitored by SERS. Van Duyne et al. studied the coating process of Al2O3 on Ag film-on-nanoparticles (AgFON) by atomic layer deposition (ALD) with operando SERS, in order to better understand the fundamental mechanism of this process. 415 ALD coating of Al 2O3 proceeded by using

Figure 66. Proposed mechanism for the deposition of Al2O3 on AgFON functionalized with different SAMs by ALD. Reproduced from ref 415. Copyright 2016 American Chemical Society. 5046


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then remained constant when further scanning in the cathodic direction on Cu(100) surface. And on Cu(111) surface, the ratio increased continually until −0.3 V, as the potential swept to the cathodic direction. While on the polycrystalline surface, the ratio began to increase until −0.3 V as the potential swept to the positive direction, and in scanning to the cathodic direction, the ratio began to decrease at −0.4 V and finally to the initial ratio. The film growth process in three electrode surfaces is different. The film formation on Cu single-crystal is an irreversible process, while the polycrystalline Cu surface exhibits reversible film growth. The reason for the difference is due to the presence of grain boundaries which may forbid the formation of more and larger irreversible oligomers. As discussed earlier, SHINERS can be used on single-crystal surfaces. Additionally, SHINERS can be used for quantitative analysis, especially to solve practical problems. For example, in the process of gold extraction from ores, the formation of a passive layer will suppress the gold-thiosulfate leaching, thus the full dissolution of gold is prevented. Lipkowski’s group utilized SHINERS to record the formation of a passive layer in the gold leaching process from thiosulfate solutions at the gold surface.379 First, (3-aminopropyl)triethoxysilane (APTES) was used to functionalize the gold core, and then sodium silicate solution was added to grow a protective SiO2 layer coating the gold core. The silica layer prevents the direct interaction between the gold core and the thiosulfate electrolyte to improve the stability of nanoparticles. The Raman signals of APTES can be seen in the SHINERS spectra, thus it can act as an internal standard to offset the fluctuations in different SHINERS spectra recorded over a long period of time. To investigate the effects of the thiosulfate electrolyte during gold leaching, a Au electrode was modified with SHINs and then immersed into the thiosulfate electrolyte. The Raman spectrum of the electrode surface was collected from 300 to 600 min. It was found that several additional bands appeared during the long immersion time, while these bands were not observed in the Raman spectra at shorter immersion time. The rate of gold leaching can be implied from the quantitative changes of the intensity of the Raman peaks. The electrochemical experiment shows that the rate of gold leaching decreases rapidly in the first 50 min and then decreases slowly at an almost linear relation with immersion time. Figure 68b shows the normalized Raman intensity of three bands with a period of immersion time. The band at 382 cm−1

Figure 67. Potential-dependent SHINERS spectra for Cu(100), Cu(111), polycrystalline Cu, and roughened polycrystalline Cu sweep in (a) positive and (b) negative directions in 0.75 mM BTA and 0.1 M H2SO4. (c) The corresponding potential-dependent ratio of peak intensities for 1190 cm−1 /1140 cm−1 bands. Reproduced with permission from ref 378. Copyright 2012 John Wiley & Sons, Ltd.

swept in the positive direction. As the potential is swept in the negative direction, the intensity remains for the Cu single-crystal faces while it disappears in the polycrystalline Cu. This indicates that the process on Cu single-crystal is irreversible and reversible on polycrystalline Cu. Figure 67b shows the potential-dependent relative intensity of 1190 cm−1 versus 1140 cm−1 because the peak at 1190 cm−1 is related to BTAH and the one at 1140 cm−1 is related to BTA−. Hence, the increase of 1190/1140 ratio is assigned to the growth of the BTA−Cu film. As shown in Figure 67, that in single-crystal surfaces, the ratio of 1190/1140 cm−1 increases with the potential scanning from −0.7 V to the anodic direction. This behavior is in agreement with the formation of BTA−Cu film. The increase of the ratio stopped at −0.2 V, and it

Figure 68. (a) SERS spectra of the Au electrode modified with SHINs immersed in a 0.1 M Na2S2O3 electrolyte with pH = 10.0 for a long time. (b) The relationship of intensity with immersion time of the [Au(S2O3)2]3− complex (red), adsorbed sulfide (blue), and polymeric sulfur (green). Reproduced from ref 379. Copyright 2015 American Chemical Society. 5047


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was attributed to the [Au(S2O3)2]3− complex at the surface, 316 cm−1 was assigned to the adsorbed sulfide, and the peak at 460 cm−1 was assigned to the polymeric sulfur species. The band of [Au(S2O3)2]3− complex was correlated well with the gold leaching process. The intensity of 382 cm−1 decreased quickly at an early time and then decreased slowly at a longer time. In contrast, the intensities of adsorbed sulfides (316 cm−1) and polymeric sulfur species (460 cm−1) bands increased with the leaching time. These results quantitatively indicate that the gold leaching in the thiosulfate solution is prevented by the formation of sulfur species. The adsorbed sulfur prevents the interactions between the gold surface and thiosulfate. Thus the formation of [Au(S2O3)2]3− complex is inhibited at the gold−solution interface, and the full dissolution of gold is limited. 5.7.3. Semiconductor Materials. Semiconductor materials are widely used in photocatalysis because of their wide band gaps. There are different kinds of semiconductors such as SiC, AlN, GaN, InN, BN, etc., and many metal oxides such as TiO2, ZnO, and CuOx. SERS can be used to investigate their properties to understand their special role in photocatalysis and other applications. Si is the most commonly used semiconductor, thus, it is necessary to understand the surface property of Si. By utilizing silica-coated Au nanoparticles and appropriate surface treatment by HF solution, Si−H band at 2149 cm−1 can be observed.114 When the silicon wafer was further cleaned by O2 plasma, the Si−H band disappeared. This result indicates that core−shell nanoparticles enhanced Raman spectroscopy has a promising application in semiconductor industrial processes. Raman scattering is one of the most effective methods to study the vibrational information on bulk semiconductors, thin films, and nanostructures, due to the strong laser field coupled with crystal lattice. Raman spectroscopy can be used to study the vibration of optical phonons in semiconductor nanostructures and discover new physical effects, including super lattice vibration and confinement of optical phonons. However, Raman studies on nanostructures are limited by their low Raman cross-section, inhomogeneous size, and shape. Thus, it is a great challenge to observe the confinement of optical phonon modes. SERS is a suitable technique to study optical phonons in semiconductor nanostructures.380 As Figure 69 shows, the Raman spectra of the CdS without Ag nanoclusters appeared as only a weak feature peak near 300 cm−1 associated with longitudinal optical phonons localized in CdS nanocrystal.

However, after depositing Ag nanoclusters on the surface of nanocrystal layers, the Raman signals were enhanced. And the Raman peak for different order of longitudinal optical phonons also appeared. These results showed that SERS technique can be used to determine the lattice vibration as well as high quality structural information. 5.7.4. Biomaterials. It has been known that vital activities of microorganisms can produce extracellular polymeric substance (EPS) if microorganisms adhere to the surface. The EPS biofilm contains genetic material, polysaccharides, lipids, proteins, and humic-like substances. Thus, biomaterials are considered as a complex system, and it is difficult to investigate the growth process of biofilms. To understand the biofilm composition and the changes during the growth process, Efeoglu et al. utilized silver core chitosan shell nanoparticles to in situ monitor the formation of biofilm of two model bacteria.381 The chitosan shell was used to isolate Ag nanoparticles from bacteria to decrease the impact of Ag nanoparticles to the biological media secreted by bacteria. Moreover, chitosan is a natural polymer with great biocompatibility and biodegradability. In addition, because of the −NH2 groups on the surface, it can interact with DNA and RNA through electrostatic interaction. The SERS spectra of S. cohnii in different incubation time indicated the generation of secondary metabolite production and then transformed to the stationary phase. The band at 678 cm−1 gave the information on the DNARNA concentration, and 802 cm−1 was assigned to uracil of RNA and increased at the biofilm formation process. The disappearance of the band is consistent with the transition to the stationary phase. Therefore, core−shell nanoparticles enhanced Raman spectroscopy is a promising method to investigate biomaterials. The cell membrane of yeast cell plays important roles in biological functions. Silica shell nanoparticles can also be used to characterize the structure of the living cell membrane. Tian’s group chose yeast cells, which are widely used as model eukaryotic organisms in genetics and biology as the research target. First, the yeast cells were incubated with SHINs for 3 h and then placed on a quartz window for Raman measurements. The SHINERS spectra obtained from different points of the yeast cell wall after incubating with Au@SiO2 NPs were very different from the normal Raman of yeast cells, and the main bands at 1166, 1414, 1488, and 1587 cm−1 were similar to the SERS spectra of mannoproteins. This is because mannoproteins are the main components of the yeast cell wall. This report demonstrates that SHINERS can be used for cell wall proteins detection, and it is a convenient and safe technique for exploring the dynamic process of biological systems. 5.7.5. Polymer Materials. Zhang et al. investigated the interface structure of a-PMMA thin film by dip-coating it on Ag nanoparticles.416 It can be observed that the bands for the R-CH3 symmetric bend and CH2 rocking vibration, which were perpendicular to axis of the molecules, were greatly enhanced in the SERS spectrum. However, the band for CH2 symmetric bend vibration, which was mostly parallel to the molecular axis, decreased significantly in the SERS spectrum, as compared with the normal Raman result. These results indicate that a-PMMA is aligned parallel to the substrate at the interface. Additional information about the structure of the molecule was also revealed by SERS. For example, they found that the C=O and C−O−C stretching modes in the ester group of a-PMMA were parallel and perpendicular to the metal surface, respectively. Furthermore, they found that conformation of the ester group would change after thermal annealing at a high temperature (100 or 120 °C), and the C−O−C stretching modes would become parallel to the

Figure 69. (a) SHINERS study of hydrogen adsorption on Si(111) wafer. Curve I, Curve II, and Curve III show the SHINERS spectra on Si(111) wafer treated with 98% sulfuric acid, 30% HF solution, and O2 plasma, respectively. Reproduced with permission from ref 114. Copyright 2010 Nature Publishing Group. (b) Raman spectra of CdS nanocrystals without and with Ag nanoclusters (curves 1 and 2, respectively), excited wavelength is 476.5 nm. Reproduced with permission from ref 380. Copyright 2016 Science Direct. 5048


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Figure 70. (a) In situ SERS spectra during the reduction of pNTP to pATP. (b) SERS spectra of pATP on bare 80 nm Au NPs (curve I), bare 80 nm Au NPs (curve II), or Au SHINs (curve III) modified with 5 nm Au NPs, which was covered with 4-ATP. (c) Schematic diagram of 4,4′-DMAB and pATP on the Au surface. Reproduced from ref 382. Copyright 2013 American Chemical Society.

Figure 71. (a) SERS spectra of the reduction of pNTP to pATP with Ag SHIN core-Ag NPs satellites nanostructures in different reaction conditions. (b) Schematic illustration of six-electron photoinduced reaction. Reproduced with permission from ref 383. Copyright 2015 Nature Publishing Group.

surface. These findings demonstrate that SERS may be a powerful tool for the analysis of the molecular structure and conformation of polymer materials.

were not observed when the Au core was protected with the silica shell. This result indicated that 4,4′-DMAB resulted from the direct adsorption of pATP on Au cores. This is because Au core has strong plasmonic activity, and it will induce the photoreaction of the molecules adsorbed on the metal surface. Under the laser illumination, the “hot” electrons will be excited on the Au core surface, then transfer to the reactants, leading to the conversation of pATP to DMAB. Oppositely, with SHINs, the photoinduced coupling reaction was avoided because the charge transfer was blocked. To explore the mechanism of pNTP to pATP conversion, recently, Schlücker’s group developed a photocatalytic strategy, in which hot electrons were involved, for the reduction of pNTP to pATP by using similar 3D metal core−satellite nanostructures.383 As shown in Figure 71, 100 nm Ag SHINs were modified with a submonolayer of ∼25 nm Ag NPs on the silica shell surface. Because small Ag NPs are incapable of catalyzing the hydride reduction reaction, the reduction of pNTP to pATP is due to the photocatalytic activity of the core−satellite superstructures. It can be clearly seen in Figure 71 that only in the presence of both H+ and Cl−, the band at 1590 cm−1 can be observed, indicating that the photocatalytic reduction of pNTP needs both H+ and Cl−. On the basis of the in situ SERS study, the authors proposed a reaction mechanism involving sixelectron photocatalytic reaction (Figure 71). Under the participation of chloride ions, silver atoms undergo photo-

5.8. Catalysis

5.8.1. In Situ Monitoring of Catalytic Process. As mentioned earlier, SHINERS technique can also be used to monitor the catalytic reactions. Schlücker’s group fabricated 3D Au nanostructure constituted of Au core, silica shell, and Au nanoparticles satellites.382 This superstructure was applied to in situ monitor Au-catalyzed reactions such as the reduction of 4nitrothiophenol (pNTP) to 4-aminothiophenol (pATP). With the protection of silica shell, direct contact of reaction species with plasmonic Au core could be excluded, thus photocatalytic side reactions were avoided. At the same time, the SERS enhancement was still strong enough to probe the reaction species, as the silica shell was only 1−2 nm. Figure 70 shows the SERS spectra of the species in this reaction at a different reaction time. The bands at 1569 and 1591 cm−1 could be assigned to pNTP and pATP, respectively. It can be clearly seen that the pNTP signal was gradually reduced, while that of pATP signal was increased. Two additional control experiments were performed to explore the role of the thin silica shell (Figure 70). Without the silica shell protection, the authors observed the bands at 1145, 1391, and 1437 cm−1, which were attributed to 4,4′dimercaptoazobenzene (4,4′-DMAB). However, these signals 5049


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Figure 72. (a) Schematic illustration for probing the local chemical reactions by LSPR. The plasmonic resonance spectra (b) of single SHIN on a 15 nm Pd film and concentration of H2. Reproduced from ref 417. Copyright 2013 American Chemical Society.

induced oxidation and become Ag+, namely hot holes, and then combined with the chloride ions to form the Ag halide. Then pNTP was reduced by the hot electrons, which were generated by the excitation of LSPR. At last, the Ag halide was decomposed to the Ag atom and X− via photodissociation. This finding demonstrates that the in situ SHINERS technique allows researchers to directly identify the reaction species. This kind of online and label-free method is promising to examine the mechanism of catalytic reactions. Shell-isolated nanoparticles were also used to probe local chemical reactions in real time. Liu’s group utilized a single 120 nm Au@SiO2 NP as a strong light concentrator to generate strong LSPR to probe the hydrogen uptake in Pd.417 The hydrogen uptake in Pd can lead to a phase transition because hydrogen molecules will dissociate to the atomic hydrogen and then diffuse into Pd to form palladium hydride (PdH). PdH is more semiconductor-like than metal-like, leading to its dielectric function change. Thus, LSPR will change after hydrogen introduction into the reaction chamber even with a low concentration. As shown in Figure 72, the plasmonic resonance manifested a clear change in intensity and frequency when hydrogen was added. These properties make it become an excellent sensor for the detection of H2. Besides the above materials, there are also a few other oxide shell coated Au/Ag materials which have been reported in the literature for various applications. For example, in order to overcome the limitation of traditional SiO2- or Al2O3-coated Au nanoparticles in the application in alkaline media, ultrathin and pinhole-free MnO2 shell coated Au nanoparticles were successfully synthesized in Tian’s group.121 It showed very good SERS activity and was very stable under strong alkaline media. Thus, this Au@MnO2 material can be directly applied to the in situ SHINERS study under alkaline conditions, such as alkaline fuel cell. 5.8.2. In Situ Study of Catalysis at High Temperature. Most heterogeneous catalytic reactions are operated at very high temperatures. Thus, increasing the thermal stability of SHINs is of significant importance for the pratical applications of SHINERS in catalysis. Recently, a few studies also demonstrated that the thermal stability of SHINs could also be improved by coating with Al2O3 shell. Van Duyne et al.418 first proved that the LSPR properties of Ag triangular nanoparticles covered by Al2O3 shell could be maintained even after being treated under 500 °C. As shown in Figure 73a, a remarkable change in the LSPR can be observed for bare Ag nanoparticles treated at 100 °C. With increased temperature, the change became more and more distinct. On the basis of the corresponding SEM study, it can be attributed to the evolution of the morphology of nanoparticles.

Figure 73. LSPR spectroscopy of bare (a) Ag NPs and (b) Ag SHINs with Al2O3 shell treated under N2 at different temperatures. Reproduced from ref 418. Copyright 2007 American Chemical Society. (c) Scheme of the interfacial sturcture of phosphotungstic acid (PTA) or vanadium on Al2O3-coated SERS substrate. Reproduced from ref 422. Copyright 2011 American Chemical Society.

However, as for the Ag SHINs with 1 nm Al2O3 shell, almost no change for the LSPR spectra can be observed except for that of the sample treated at 500 °C, which shows a decreased intensity of the LSPR peak. The influence of the Al2O3 shell on the LSPR properties of these materials can be more easily distinguished in Figure 73c. The LSPR peaks of bare Ag nanoparticles rapidly shift to lower values with increased temperature, while almost no change can be observed for that of Au@Al2O3. As the LSPR properties directly determine the SERS behaviors, it is reasonable to apply the strategy to increase the SERS stability of Ag nanoparticles. Dai et al.419 studied the influence of the Al2O3 shell on the stability of SERS activity of Ag nanowire by using R6G as a probe molecule. They found almost no enhancement of Raman signals can be observed for bare nanowire treated at 400 °C in air, while the enhancement factor was about 4.4 × 104 for fresh bare nanowire. On the contrary, the Raman enhancement factors were about 3.0 × 104 and 1.1 × 104 for fresh and treated Ag@ Al2O3, respectively. This result indicates that coating Ag with an ultrathin Al2O3 shell can greatly improve the stability of SERS active NPs. Though thermal stability of SERS substrates can be improved via Al2O3 coating, it will decrease the SERS enhancements slightly. Van Duyne et al. have systematically studied the effect of 5050


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Figure 74. In-situ SERS monitoring of the selective hydrogenation of pNTP into pATP over (a) Au@Pt multibranched nanoparticles. Reproduced from ref 307. Copyright 2014 American Chemical Society. (b) Ag@Pd−Ag nanocubes. Reproduced from ref 326. Copyright 2015 American Chemical Society. (c) Au nanowires@submonolayer-Pt. Reproduced from ref 327. Copyright 2014 American Chemical Society. (d) Au nanorods@Au−Pd alloy horns. Reproduced from ref 423. Copyright 2013 American Chemical Society.

Figure 75. Schematic illustration for the (a) synthesis procedure and the SEM image of Au@Pt@Au. The inset in (b) shows photographs of the products during different synthesis steps. (c) In situ SERS spectra of the hydrogenation of pNTP to pATP with different volumes of NaBH4 solution (the added volumes increase from bottom curve to top curve). Reproduced from ref 144. Copyright 2011 American Chemical Society.

the thickness of Al2O3 shell on the SERS intensity.420 They found that SERS enhancement decreased rapidly with the increase of shell thickness, which was well-consistent with 3D-FDTD calculations. A similar trend was also reported by Zhang et al. for Al2O3-coated Ag nanorods, though pinhole ratio instead of shell thickness was used to correlate SERS intensity with the shell thickness in their study.421 They also found Ag@Al2O3 with less pinholes exhibited better thermal stability but sacrificed SERS efficiency. Thus, it is an important topic to balance these two influences to obtain better substrates with higher thermal stability as well as SERS sensitivity. Recently, these thermally stable Ag@Al2O3 SHINs were applied to the in situ study of the structure changes of phosphotungstic acid (PTA) and V2O5 under high temperatures.422 In their study, Al2O3 shell not only enhanced the thermal stability but also could work as a support for the catalysts. As the SERS intensity decreased rapidly with the increase of the distance to the Ag substrate, the highest Raman enhancement factors can be obtained at the interfacial region between the PTA or V2O5 and the alumina surface. Thus, the interfacial structure and properties could be studied, and it was found they were quite different for PTA and V2O5 as indicated by the relative Raman

intensity of different chemical bonds. As shown in Figure 73c, both the W=O and W−O−W bonds were located around the surface of Al2O3 for PTA. However, as for V2O5 on Al2O3, only the V−O−Al was located near the interface, while the V=O bond was away from this area. These findings demonstrate that thermally stable SHINs may be a promising material for the in situ SHINERS study of heterogeneous catalysis. 5.8.3. Tuning the Properties and Performances of Multifunctional Catalysts. 5.8.3.1. Different Core Shapes in Catalysis. On the other hand, one of the most important properties of transition metals is their catalytic behavior. Au core transition metal shell NPs combine the catalytic property of transition metal shell and the enhanced SERS activity of Au core, thus they can be used for in situ detection or monitoring of various catalytic reactions. Many nanostructures with different morphologies were fabricated to achieve this goal. For example, Cui et al. developed a multibranched nanoparticle substrate of Au@Pt core−shell for in situ SERS study of catalytic reactions (Figure 74a).307 This structure can provide stronger enhancement in SERS and better catalytic performance because of dense hot spots and larger surface area. Bao et al. prepared sub-5 nm Pt nanoparticles decorated Au@Pt nanorods nanostructures.129 5051


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lungs and stomach. Figure 76 shows the SERS spectra of 2,4-D molecules with different concentrations. The nanocomposites

Because nanorods have a great property of tunable LSPR wavelength, it can be tuned to get a stronger enhancement. Meanwhile, Li et al. also synthesized Ag@Pd−Ag nanocubes with both SERS sensitivity and catalytic activity toward the hydrogenation of 4-nitrothiophenol (pNTP), and they found their activity could be greatly improved by manipulating the Pd/ Ag ratio (Figure 74b).326 Liu et al. reported a novel procedure for the synthesis of submonolyer-Pt coated Au nanowires (Figure 74c).327 These materials not only showed very high SERS sensitivity but also improved catalytic performance, which made them promising platforms for in situ SERS study of catalytic process. As shown in Figure 74d, Huang et al. synthesized Aunanorods@Au−Pd alloy horns nanocomposites (HIF-AuNR@ AuPd) through seed-mediated growth strategy, in order to introduce Au−Pd alloy horns as catalytic sites on the surface of gold nanorods.423 On the basis of the high-resolution electron microscopy analysis, the fine structure of the nanocomposites was clearly characterized, and it was found that Au−Pd alloy horns were bound to the gold nanorod with high-index facets. Therefore, it showed extremely high activity toward the reduction of pNTP due to the exposed high-index facets of Au−Pd horns. 5.8.3.2. Influence of the Composition and Structure of Shell Material on Catalysis. Schlucker’s group fabricated Au/ Pt/Au core−shell nanoraspberries for in situ quantitative monitoring Pt-catalyzed reactions by SERS method (Figure 75).144 This nanostructure possesses both high SERS activity and a large surface area. Thus, in situ SERS can be used to monitor a Pt-catalyzed reaction, and the reaction can be directly applied in colloidal suspension, which can be applied in the actual system. By in situ SERS, the authors examined the reduction of 4-NTP by NaBH4 and the unknown species can be identified efficiently. In situ quantitative SERS spectra provide the chemical identity of the involved molecular species and also quantify their relative contributions, which is a primary requirement for establishing a reaction mechanism and testing kinetic models.

Figure 76. (a) Normal Raman spectrum of solid 2,4-D and its SERS spectra with concentrations of 10−4 and 10−6 adsorbed on the Ag-TiO2 core−shell nanocomposites. (b) SERS signal of 2,4-D before and after UV irradiation. Reproduced with permission from ref 428. Copyright 2014 Elsevier B.V.

provided strong Raman signals of 2,4-D molecules, and even at a concentration of 10−6 M, the bands of 2,4-D are clearly observed. Thus, it can be used to detect the residual of 2,4-D in plants and crops. After irradiation by UV light, the SERS signal was declined and this behavior indicates that the 2,4-D molecules adsorbed on the surface of TiO2 are photocatalytically degraded to CO2, HCl, and H2O. Because of the self-cleaning function, the substrate is recyclable. Similarly, Alessandri et al. also found that Au-coated ZnO nanorods showed the bifunction of both SERS detection and photocatalysis.429 Gold shell was deposited on the ZnO nanorods, which were preprepared via a seed-mediated hydrothermal method, by sputtering at room temperature, leading to the formation of ZnO nanorods core-gold shell-like structure. As ZnO was an excellent photocatalyst, while gold can work as a SERS substrate, this ZnO@Au nanocomposite can be used in SERS analysis of the photocatalytic process. The Raman signals of methyl blue (MB) can be clearly seen before UV irritation. However, they will rapidly decrease under UV irradiation and completely vanish after 30 min, indicating that photocatalytic degradation of MB can perform easily on ZnO@Au. It can also be observed that the photocatalytic activity of this core−shell-like material is very stable, and almost no activity decrease happens even after 10 cycles. Interestingly, the authors also found that ZnO@Au showed better catalytic performance as compared with bare ZnO, which may arise from the higher separation efficiency of charges as the formation of Au−semiconductor interface. These findings demonstrate that the ZnO@Au core−shell-like nanocomposites may be a promising and reusable SERS platform for the detection of dyes. Additionally, dye-sensitized solar cells (DSSCs) are widely studied to improve the efficiency of solar cells. Understanding the adsorption behavior and configuration of molecules at the semiconductor−molecule interface is essential for the development of more efficient DSSCs, as it will greatly affect the charge transfer efficiency. Recently, Mao’s group studied the TiO2− N719 interface by using Ag2@TiO2 dimeric nanoparticles as SERS substrates (Figure 77).166 This nanostructure consists of two Ag nanoparticles connected by DNAs then coated with TiO2. By this nanostructure, a 10-fold higher enhancement than Ag@TiO2 was achieved in lower energy 785 nm laser, thus the investigation of TiO2−N719 could be achieved. With electrochemical SERS (EC-SERS), they found that N719 was first adsorbed at the interface via the SCN group. However, with the

5.9. Energy Storage and Conversion

Due to the limited reserves of fossil fuel and increased energy demand and environmental issues, great attention has been paid to obtain efficient and clean pathways for energy storage and conversion. Among these efforts, development of lithium batteries, direct fuel cells, and exploitation of solar energy are becoming the most important part of advanced technology for energy utilization. 5.9.1. Photocatalysis and Solar Cell. One of the most important pathways for the utilization of solar energy is converting it to chemical energy by photocatalysis, such as hydrogen production through water splitting,424−426 capture and conversion of CO2, and synthesis of high value-added chemicals.427 Coating photocatalysts, such as TiO2, CuOx, and CdS, on plasmonic cores, or the other way round, can not only enhance the catalytic performance but also achieve real-time analysis of the reaction mechanism or process by SERS. Titanium oxide is a commonly used material in photocatalysis. Bao et al. synthesized Ag-TiO2 core−shell nanocomposites to decompose and detect organic pollutants in the environment.428 Due to the electron transfer in the interface of Ag and TiO2, the Ag-TiO2 nanocomposites can exhibit better photocatalytic performance compared to bulk TiO2. In this study, (2,4dichlorophenoxy)-aceticacid (2,4-D) molecules were detected. Generally, this molecule is widely used as an herbicide, and it can enrich in animal organisms and lead to harmful influences to 5052


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the target material analysis, are blocked. Moreover, the SiO2 shell prevents the nanoparticles aggregation, which will reduce signal enhancement. From the SHINERS spectra results, they directly observed the formation of Li2O on lithium-rich cathode during the charging/discharging process and discussed the mechanism of this formation. SHINERS method was used to study the formation of solid electrolyte interphase (SEI) species, which were formed on the surface of electrode materials because of the reduction of nonaqueous electrolyte in Li-ion batteries.430 With Au@SiO2 NPs, clear bands can be seen from the SHINERS spectra of electrochemically discharged Cu foil, which correspond to LiOH·H2O and Li2CO3. While without Au@SiO2 NPs, no peak can be observed as shown in Figure 78b. 5.9.3. Solid Oxide Fuel Cell. In this regard, Liu et al. simply mixed SHINs with a bulk catalyst material to study solid oxide fuel cell (SOFC) anodes.385 The surface structure and composition of catalyst material affect the activity and stability of SOFC electrodes. A trace quantity of modified catalysts can remarkably improve the reaction rate. Gadolinium doped ceria (GDC) is a commonly used electrolyte material. However, the deposition of inactive phase, like coking, may quickly degrade electrode performance. Therefore, the detection of such surface species is significant to reveal the mechanism of electrode processes. They fabricated Ag@SiO2 nanoparticles with 60 nm core and 10 nm shell for real-time probing SOFC anodes at high temperature of 500 °C. Figure 79a demonstrated the high temperature SERS effect of SHINs. The spectra of GDC thin film on a silicon wafer has no obvious peak at 460 cm−1 which is attributed to the F2g mode of doped ceria. While with the Ag@ SiO2 nanoparticles applied to the surface, a clear peak of the F2g mode can be seen. Even after annealing in 10% H2 at 400 °C for 30 min, the peak remains obvious and it indicates that the thermal stability of SHINs is significant enough for catalytic reactions at a high temperature. Figure 79d shows the in situ SHINER spectra of the carbon deposition process at a polished nickel surface at 450 °C. The carbon deposition is the main reason for degradation of nickelbased anodes because of the decrease of active sites and the damage of surface structures. Initially, the deposition was quickly slowed down over time, and it can be seen that the D-band at 1350 cm−1 increased over time. This study demonstrates that SHINERS is a sensitive and appropriate technique to investigate the surface species in catalytic/chemical reactions.

Figure 77. (a) Structure of N719 and Ag2@TiO2 dimeric core−shell nanocomposite. (b) Raman spectra of N719 by using Ag@TiO2 or Ag2@TiO2 dimer. Reproduced from ref 166. Copyright 2015 American Chemical Society.

decrease of potential, the molecules were then bonded to the interface through carboxyl groups, leading to the orientation change of its bipyridine ring. These results demonstrated that in situ SHINERS study could be a promising method to reveal the fundamental insights of the physicochemical process in DSSCs and guide the design of more effective DSSCs. 5.9.2. Lithium Battery. SHINERS can also be used to study energy devices like lithium ion batteries (LIBs). With the rapid development of electric vehicles and energy storage for grid-scale applications, there is a great demand for high-energy density lithium-ion batteries. Lithium-rich cathode materials such as Li[NixLi(1−2x)/3Mn(2−x)/3]O2 (0 ≤ x ≤ 0.5) (LLNMO) have developed rapidly in the past decade because of their high reversible capacities (>270 mAh g−1). However, the mechanism of the charging/discharging process is still unclear, and some mechanisms of modification are also uncertain. Exploring the influence factors of redox reactions during the charge/ discharging progress is significant for improving electrochemical performance of lithium ion batteries. To in situ monitor redox reactions on lithium-rich cathodes, Hwang’s group has utilized SHINERS technique with Au@SiO2 NPs (Figure 78a).384 The inert silica shell assures the isolation between gold core and the probe material, thus the Raman signal of adsorbate on the gold nanoparticle surface, which will interfere

6. SUMMARY AND OUTLOOK In this section, we first summarize the various types of core−shell nanoparticles used in SERS with unique structures and wide

Figure 78. (a) In situ SHINERS study of solid electrolyte interphase formation on the surface of electrode in Li-ion battery. (b) Raman spectra of Cu foil, discharged Cu foil with and without nanoparticles. Reproduced with permission from ref 430. Copyright 2014 Elsevier B.V. 5053


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with unique functions, is a promising direction for new core− shell nanoparticle-enhanced Raman spectroscopy. The newly functionalized core−shell nanoparticles for SERS can also be applied in various fields, such as photocatalysis, solar cell, device, and life science. Finally, we discuss about the concept of core− shell nanoparticle enhancement for other surface-enhanced spectroscopies, such as surface-enhanced IR, fluorescence, and nonlinear spectroscopies. 6.1. Summary

Various types of core−shell nanoparticles have been developed and extensively used in SERS. The core−shell nanoparticles with ultrathin transition shells are mainly developed based on the “borrowing SERS strategy” concept to overcome the material limitation of SERS. Thus, reactions catalyzed by transition metals have been probed by the in situ SERS method. Moreover, the core−shell nanoparticles with ultrathin nonmetal shells, such as SiO2, Al2O3, graphene, and TiO2 shells, have been developed to protect the plasmonic core and avoid the direct contact with probe molecules and environment. By changing the working mode from direct contact (SERS) or noncontact (TERS) to the shell-isolated mode, SHINERS has overcome the long-standing limitations of material and morphology generalities. It is a suitable approach to detect, characterize, and identify molecules on various materials and substrates, in particular, on single-crystal surfaces. In the past few years, this technique has been used to probe surface molecular adsorption and in situ monitor surface

Figure 79. (a) Raman spectra of (1) Ag@SiO2 NPs on silica wafer, (2) GDC film deposited on a silica wafer, (3) Ag@SiO2 nanoparticle on GDC film before annealing, (4) after annealing, and (5) Raman spectra of a GDC pellet for reference. (b) Carbon deposition on Ni foil. (1) Ag@SiO2 on silica wafer, (2) blank Ni foil, and (3) Ag@SiO2 NPs loaded Ni foil. Time-resolved normal (c) Raman and (d) SERS analysis of coking and carbon removal on the nickel surface. Reproduced with permission from ref 385. Copyright 2014 Royal Society of Chemistry.

applications. Development of an ultrathin pinhole-free shell material, especially for 2D materials such as graphene and MoS2

Table 2. Comparison of Different Types of Core-Shell Nanoparticles

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semiconductor material in photocatalysis, as well as one of the most promising electrocatalysts for electron-driven hydrogenevolution reaction (HER). Comparing with traditional Pt catalysts, MoS2 nanosheet is a significantly important nonprecious candidate because of its low cost, high chemical stability, and good catalytic performance toward HER. Therefore, if the MoS2 monolayer can be coated onto SERS-active metal nanoparticle, it is promising to in situ probe the (photo-) catalytic process and to reveal the mechanism of the reaction. More importantly, attention should also be paid to the possible novel surface and interfacial properties of 2D material shells due to electronic interactions between plasmonic core and 2D materials shell in the core−shell nanoparticles. In other words, the surface properties of the 2D materials could be modulated not only by employing various types of 2D materials shell themselves but also by the specific plasmonic core. For example, several ultraviolet or infrared plasmonic materials (as mentioned above), or other visible plasmonic materials beyond Au, Ag, Cu, and their alloys, such as metal nitrides (TiN, ZrN), silicides, borides, and germanides, could also be developed as new plasmonic core materials, capability of tuning the electronic structure and surface properties of the 2D material shells. We believe that a new family of 2D-material plasmonic core−shell nanoparticles could be developed with ultrahigh SERS activity to solve several other still unknown but worth-expecting issues. The new core−shell nanoparticle could provide enormous and versatile choice properties for hosting and/or probing other molecules or other materials in both fundamental and application research. Owing to the increasing fuel prices and uncertain supply, increasing attention has been paid to the development of efficient materials for harvesting and conversion of solar energy via the photocatalytic process. Among these materials, plasmonic nanostructures-semiconductors nanocomposites with core− shell structures are one of the most promising and outstanding candidates for enhancing the photocatalytic efficiency.35,38,39,449,450 This is attributed to the maximum interfacial interactions between them as well as their tunable SPR band ranging from UV to near-IR region by manipulating their composition, size, shape, and structure. Though great progress has been made in this field, there is a critical demand for the development of more efficient plasmonic photocatalysts for the utilization of solar energy. Furthermore, these nanostructures can be used to explore the mechanism and fundamental details about the conversion process. Core−shell nanostructures can efficaciously combine the plasmonic material and photocatalysis material in solar cells to improve the efficiency of solar utilization. They can also be applied in photoanode of solar cells to inhibit the recombination of interfacial charge in the photocatalysis process. Thus, core−shell nanostructures act as promising candidates for the fabrication of solar cells.

catalytic reactions at single-crystal surfaces which is helpful to understanding the reaction mechanism. Furthermore, a wide range of practical applications can be realized with SHINERS, such as bioanalysis, corrosion inhibition, lithium battery, solid oxide fuel cells, and food and environmental safety, because of its high sensitivity, stability, and simplicity. The core−shell nanoparticles with thick metal shells, for instance Ag/Au mixed colloids, combine the advantages of both metals and expand a wider spectral range of excitation wavelengths in SERS. Similarly, the combination of magnetic materials as the cores with Au or Ag shells makes it a suitable candidate for bioseparation and detection. Moreover, SiO2/PS sphere core Au/Ag shell nanoparticles are good for SERS in nearinfrared region and surface-enhanced infrared spectroscopy in mid-infrared region. Besides, Au/Ag core silica (or polymer) shell nanoparticles are also well-known as SERS tags for bioanalysis and bioimaging. Furthermore, various shell materials exhibit different functions. For example, SiO2 and graphene exhibit great biocompatibility, TiO2 is the most popular photocatalytic material, and Al2O3 can be used as the support layer for other nanocatalysts. Thus, there is a huge space to develop new functionalized core− shell nanoparticles for SERS in various fundamental and practical research. The comparison of different types of core−shell nanoparticles has been stated in Table 2. 6.2. New Functional Core−Shell Nanoparticles for SERS

In recent years, new plasmonic materials have been explored that might enable SERS to be carried out in ultraviolet, visible, and near-infrared wavelength regions. It will be promising to develop shell-isolated nanoparticles with core materials such as Al, Ga, In, Sn, Tl, Pb, and Bi431 nanoparticle for ultraviolet SHINERS. Similarly, shell-isolated nanoparticles with core materials such as transparent conductive oxides including ITO, Al:ZnO, and Ga:ZnO; n-type semiconductors, including n-GaAs, n-InP, and n-Si247,432−434 could also be developed for near-infrared SHINERS. With regard to the shell materials, silica shell can be dissolved in an alkaline media, which restricts the applications of silica shell-isolated nanoparticles. Moreover, massive preparation of silica shell with a shell thickness of less than 1 nm without pinholes is difficult because SiO2 is typically porous. Therefore, undeniably a lot more systematic explorations are demanded for the development of other more stable shell materials. Among various shell materials, graphene is the thinnest 2D material, which has been recently used as a shell material in core− shell nanoparticles.435−438 The sensitivity of SERS can be further enhanced up to one more order of magnitude by using the Augraphene core−shell nanoparticles. Besides the atomic or molecular thickness of 2D materials, notably, the 2D materials themselves are functional materials which are highly preferable for various applications such as photovoltaics,439−442 semiconductors,443−446 electronic fields,438,447,448 etc. Graphene as the most popular 2D material has been studied in almost all fields, and it has also been combined with SERS to fabricate core−shell nanostructures for bioimaging, disease diagnosis, and therapy as well.124−126 Besides graphene, other 2D materials such as hexagonal boron nitride (h-BN) and MoS2 in principle could also be used as shell materials to obtain functional core−shell nanoparticles as well. The development of core−shell nanostructure with these 2D materials and their heterostructures may lead to unique properties for catalytic research. 2D MoS2 is a commonly used

6.3. New Core−Shell Nanoparticles for Other Surface-Enhanced Spectroscopies

The concept of shell-isolated nanoparticle-enhancement can be expanded to the other surface-enhanced spectroscopies, such as shell-isolated tip-enhanced spectroscopy, shell-isolated nanoparticle-enhanced fluorescence (SHINEF), and shell-isolated nanoparticle-enhanced second-harmonic generation (SHINESHG), etc. Recently, inspired by SHINERS, the SITERS technique was developed by Zenobi and Li’s group.451 They utilized a chemical method to coat an ultrathin and pinhole-free silica shell on gold 5055


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Surface-enhanced fluorescence (SEF) is another popular technique which uses core−shell nanoparticles for various purposes. In SEF, on one side, the fluorophore should be very close to a gold or silver nanoparticle for giant enhancement. On the other side, the fluorophore should not directly or almost contact with the metal nanoparticle surfaces in order to avoid fluoresncence quenching due to the energy transfer or charge transfer. Therefore, an appropriate distance between fluorescent molecules and nanoparticles (typically, 2−20 nm) is required to obtain strong fluorescence intensities. The shell-isolated strategy by using ultrathin dielectric shelled nanopartices such as Au@ SiO2 could be naturally applied for shell-isolated nanoparticleenhanced fluorescence (SHINEF).452−458 Aroca et al. demonstrated the first example of SHINEF by spreading the Au@SiO2 NPs on the surface of the fluorophore molecules, self-assembled monolayer of octadecyl rhodamine B (R18) with 94-fold enhancement in fluorescence (Figure 81a).455 Ag@SiO2 nanoparticles exhibit stronger electromagnetic enhancement and can be used in a broader spectral region from 300 to 1000 nm. Li et al. have reported very recently that 1000-fold enhancment could be obtained by using well-optmized Ag@SiO2 core−shell nanoparticles on different types of fluorescent molecules (Figure 81b).458 This is attributed to the fact that not only the excitation rate of fluorescent molecules can be accelerated due to the local enhanced electromagnetic field but also the radiation rate can be increased, leading to a decrease in the lifetime. Therefore, the final fluorescence intensity can be increased. This ultrahigh sensitive spectroscopy is highly suitable for trace analysis and surface science. However, a precisely controlled shell thickness is required for obtaining the most enhanced fluorescence. With SHINs, enormous enhancement can be obtained in fluoroscence spectroscopy as well as Raman spectroscopy simultaneosuly.456 Figure 81c demonstrates that crystal violet (CV) is excited by 532 nm laser, and the Raman signal (∼550 nm) can be clearly distinguished from the fluoresence signal (∼670 nm). Therefore, the shell-isolated mode can be used to enhance Raman and fluorescence signals, and the fingerprint information on target molecules and an ultrahigh sensitivity can thus be obtained simultaneously. Besides, Li’s group and Yang’s group cooperated together to develop plasmon-enhanced SHG using SHINs.459 The uniformly assembled Au@SiO2 NPs on a smooth Au surface with different shell thicknesses were utilized to verify the exponential dependence of SHG intensity on the shell thickness, indicating plasmon coupling strength between the NPs and the Au substrates (Figure 82). They synthesized 55 nm Au@SiO2

or silver tips for TERS (Figure 80). By this method the ultrathin shell thickness could be well controlled with the pinhole-free

Figure 80. (a) Scheme of a TERS tip protected by silica shell. HRTEM of sicila shell-isolated (b) Au tip and (c) Ag tip and (d) the comparison of stability of a bare Ag tip (squares) and a silica shell isolated Ag tip (triangles). Reproduced from ref 451. Copyright 2016 American Chemical Society.

property which was proved by CV and TERS measurements. Similar to SHINERS, the ultrathin inert shell can not only avoid the tip contamination but also maintain the high SERS enhancement from the TERS tip. Moreover, the shell-isolated tips outperform bare tips with long time stability, especially for silver tips which get oxidized in the air in several days (Figure 80d). The stability is largely increased, and it is beneficial for commercialization. Thus, this method paves a way to more widespread applications of TERS in electrochemistry and biological systems. The shell-isolated strategy could in principle be extended to other surface-enhanced vibrational spectroscopies such as surface-enhanced infrared spectroscopy (SEIRS) by coating IR transparent materials such as AgBr, Si, or ZnSe onto the surface of a nanoshell (dielectric-gold core−shell nanoparticle). Furthermore, dielectric-graphene core−shell nanoparticles or nanowires have also been proposed as a SEIRS substrate with much higher activity than the nanoshell counterpart. Futher experimental synthesis of the designed core−shell nanoparticles should be carried out to verify the theoretical prediction before practical applications.

Figure 81. SHINEF spectra of (a) a monolayer R18 film with and without Au@SiO2 nanoparticles. Reproduced with permission from ref 455. Copyright 2011 John Wiley & Sons, Ltd. (b) A monolayer rhodamine isothiocyanate (RITC) film with and without Ag@SiO2 nanoparticles. Reproduced from ref 458. Copyright 2015 American Chemical Society. (c) CV molecules modified with and without Ag@SiO2 nanoparticles. Reproduced with permission from ref 456. Copyright 2012 John Wiley & Sons, Ltd. 5056


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materials for LSPR and SERS sensing. The LSPR of the smart core−shell nanoparticles could be very sensitive to their local environments, such as temperature, humidity, pH, and electric or magnetic field which may induce the expansion or shrinkage of polymer. For example, Tenhu et al. coated Au nanoparticles with a stimuli-sensitive diblock copolymer to fabricate a pH and temperature sensor (Figure 83, panels c and d).463,464 The polymer shell was poly(methacrylic acid)-block-poly(N-isopropylacrylamide) (PMMA-b-PNIPAM), with the PNIPAM bound to the particle and PMAA as stimuli-sensitive aggregates. The properties of the aggregates could be modified by temperature or pH, leading to a change in LSPR peak.

Figure 82. Schematic diagram of shell-isolated nanoparticles for plasmon-enhanced SHG. Reproduced from ref 459. Copyright 2015 American Chemical Society.

AUTHOR INFORMATION Corresponding Authors

nanoparticles with the shell thicknesses of 1, 2, 3, 4, and 6 nm, respectively, and assembled uniform submonolayer of SHINs on the smooth Au surface. Thus, the gap size was dependent on silica shell thickness. The SHINs with 1 nm shell on the Au film showed the highest SHG intensity, which might be attributed to the avoidance of exchange of charges between nanoparticles and substrates. With the increase in the shell thickness, a monotonic exponential attenuation trend of SHG intensities could be obtained. The quantitative relationship reveals the promising possibility to use PESHG as an ultrasensitive optical method to measure nanoscale distances. Core−shell nanoparticles could also be applied for LSPR spectroscopy even with real-time single molecular sensitivity. The LSPR spectroscopy for nanosensing utilizes the quantitative relationship between resonance wavelength of LSPR and the refractive index of adsorbed analytes. In general, bare plasmonic nanoparticles are less functionalized; however, core−shell nanoparticles can be functionalized easily and employed to improve the sensitivity of SPR sensors.460,461 Along this direction, Long’s group developed core−shell nanoparticlesbased LSPR sensor.462 They found that the LSPR spectra of Au@ Cu nanoparticles were linearly red-shifted with the linear increase in the concentration of nicotinamide adenine dinucleotide (NADH) (Figure 83, panels a and b). Besides, plasmonic-smart polymers or stimuli-response polymers core− shell nanoparticles could be developed for new types of smart

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jian-Feng Li: 0000-0003-1598-6856 Zhong-Qun Tian: 0000-0002-9775-8189 Notes

The authors declare no competing financial interest. Biographies Jian-Feng Li is a full Professor of Chemistry at Xiamen University. He received his BSc degree in Chemistry from Zhejiang University in 2003 and his Ph.D. degree in Chemistry under the supervision of Prof. ZhongQun Tian at Xiamen University in 2010. He is a principal inventor of SHINERS (shell-isolated nanoparticle-enhanced Raman spectroscopy). His research interests include SERS, core−shell plasmonic nanostructures, (single-crystal) electrochemistry, and surface catalysis. Yue-Jiao Zhang received her BSc degree from Department of Chemistry at Zhejiang University in 2013. She is currently a Ph.D. candidate under the supervision of Prof. Jian-Feng Li at the department of Chemistry at Xiamen University. Her research is focused on surface-enhanced Raman and fluorescence spectroscopy and plasmonic nanoparticles. Song-Yuan Ding is a Research Fellow in Collaboration Innovation Center of Chemistry for Energy Materials (iChEM) at Xiamen

Figure 83. (a) Schematic diagram of the LSPR sensor of Au@Cu nanoparticles for NADH. (b) The plasmon resonance Rayleigh scattering spectra of Au@Cu nanoparticles in different concentrations of NADH. Reproduced with permission from ref 462. Copyright 2011 Wiley-VCH. (c) The synthesis of a Au nanoparticle coated with (PMMA-b-PNIPAM). (d) The relationship of LSPR with temperature under different pH. Reproduced from ref 464. Copyright 2007 American Chemical Society. 5057


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University. He received his BSc degree in Chemistry at University of Science & Technology of China in 2005 and a Ph.D. in Chemistry under the supervision of Prof. Zhong-Qun Tian at Xiamen University in 2012. His research mainly focuses on developing theory and modeling of plasmon-enhanced spectroscopies (SERS, SEIRS, SHINERS, and TERS), interfacial electrochemistry, and molecular assembly. Rajapandiyan Panneerselvam graduated from University of Madras (India) in 2009. He obtained his Ph.D. degree from National Chung Hsing University (Taiwan) in 2014. Later, he worked as a postdoctoral fellow under the direction of Prof. Zhong-Qun Tian. His current research interests include SERS, SHINERS, spectroelectro-chemistry, bio-SERS, nanoparticle synthesis, and assembly. Zhong-Qun Tian is a full Professor of Chemistry at Xiamen University, China. He obtained his BSc at Xiamen University in 1982 and his Ph.D. under the supervision of Prof. Martin Fleischmann at the University of Southampton in 1987. He is a Member of the Chinese Academy of Sciences and an advisory board member for over ten international journals. Currently, his main research interests include SERS, spectroelectrochemistry, nanochemistry, plasmonics, and catalytical molecule assembly.

ACKNOWLEDGMENTS The authors thank Z. Wang, R. Zare, Z. L. Yang, T. Wandlowski, J. Lipkowski, B. Ren, D. Y. Wu, B. W. Mao, A. Gewirth, S. Schlucker, and R.F. Aroca for helpful discussions, and. C. Y. Li, Y. F. Huang, J. C. Dong, S. B. Li., H. Zhang, M. Meng, J. Yi, E. M. You, and J. Yang for their experimental assistance. J. F. Li and Z. Q. Tian acknowledge support from the NSFC (Grants 21522508, 21427813, 21521004, and 21533006), MOST of China (Grants 2011YQ030124 and 2015CB932301), “111” Project (B16029), the Fundamental Research Funds for the Central Universities (20720150039), and the Thousand Youth Talents Plan of China. REFERENCES (1) Spanhel, L.; Weller, H.; Henglein, A. Photochemistry of Semiconductor Colloids. 22. Electron Injection from Illuminated Cadmium Sulfide into Attached Titanium and Zinc Oxide Particles. J. Am. Chem. Soc. 1987, 109, 6632−6635. (2) Youn, H. C.; Baral, S.; Fendler, J. H. Dihexadecyl Phosphate, Vesicle-Stabilized and In Situ Generated Mixed Cadmium Sulfide and Zinc Sulfide Semiconductor Particles: Preparation and Utilization for Photosensitized Charge Separation and Hydrogen Generation. J. Phys. Chem. 1988, 92, 6320−6327. (3) Daniel, M. C.; Astruc, D. Gold Nanoparticles: Assembly, Supramolecular Chemistry, Quantum-Size-Related Properties, and Applications toward Biology, Catalysis, and Nanotechnology. Chem. Rev. 2004, 104, 293−346. (4) Aragay, G.; Pino, F.; Merkoci, A. Nanomaterials for Sensing and Destroying Pesticides. Chem. Rev. 2012, 112, 5317−5338. (5) West, J. L.; Halas, N. J. Engineered Nanomaterials for Biophotonics Applications: Improving Sensing, Imaging, and Therapeutics. Annu. Rev. Biomed. Eng. 2003, 5, 285−292. (6) Stockman, M. I. Nanofocusing of Optical Energy in Tapered Plasmonic Waveguides. Phys. Rev. Lett. 2004, 93, 137404. (7) Astruc, D.; Lu, F.; Aranzaes, J. R. Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis. Angew. Chem., Int. Ed. 2005, 44, 7852−7872. (8) Crooks, R. M.; Zhao, M. Q.; Sun, L.; Chechik, V.; Yeung, L. K. Dendrimer-Encapsulated Metal Nanoparticles: Synthesis, Characterization, and Applications to Catalysis. Acc. Chem. Res. 2001, 34, 181− 190. 5058


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