Surface-enhanced Raman spectroscopy (SERS) is a popular and potential area of investigation in many applications because of its high sensitivity even at mono-molecular level. SERS substrates that typically composed of metal nanostructures can enhance the Raman signal level up to several orders of magnitude, making it a powerful analytical tool in chemical and biomedical applications. The present book chapter is aimed to provide insight about design and latest developments on metal nanoparticles and their application in the field of SERS. The chapter starts with the discussion of basic concept and theories of Raman scattering and SERS based on examples from recent research. It then primarily reviews various potential metallic nanostructures and their geometry as SERS substrates, followed by recent reports and theories on application of bimetallic nanostructures for the purpose. Toward the end, we briefly discuss the research progress in designing hybrid SERS substrates using emerging materials like photonic crystals and graphene.
- Raman spectroscopy
- plasmonic materials
- vibrational spectroscopy
- metal nanostructures
- electromagnetic enhancement
Raman spectroscopy is a type of vibrational spectroscopy which is based on inelastic scattering of photons from a chemical or biological molecule, causing changes in the scattered photon frequency that is proportional to the difference in the vibrational energy levels in the molecule. It can therefore give us various information about the target molecule, like chemical composition and structure of the molecule, its surface information including surface or interfacial reactions, impurity identification and so on [1, 2]. The phenomenon of Raman scattering was discovered in 1928 by Sir Chandrasekhara Venkata Raman, for which he received Nobel prize in physics in 1930. In his discovery, he observed that certain molecules scatter a small portion of incident light at a different frequency, where the scattering is therefore inelastic type and the energy difference between the incident and scattered light is proportional to the vibrational frequency of the molecule . Although he used sunlight to explain his discovery, modern day Raman spectroscopy uses high energy monochromatic laser light to excite samples.
Figure 1 illustrates the Raman effect in terms of an incident laser light and the vibrational energy levels of a sample. Once the incident light hit the sample, the energy can be absorbed, transmitted or scattered by the sample. The major portion of the scattered light from the sample carry same frequency (
Since only a small fraction of incident photons show inelastic scattering, the Raman signal is therefore a very weak signal which sometimes can be very difficult to detect even by using latest detector technologies available till date. Therefore, a new technique was developed in 1973 by Martin Fleischmann, Patrick J. Hendra and A. James McQuillan at the University of Southampton, UK to enhance the Raman signal of a molecule by adsorbing it onto a rough metallic surface. Since the technique is surface sensitive, it was termed as ‘
At the beginning, this chapter will briefly discuss the SERS mechanism based on both the proposed routes, followed by discussions on some recent developments in applications of noble metal nanostructures in terms of SERS signal enhancement. Toward the end, the chapter will also briefly touch upon some emerging nonmetallic materials which have potential SERS applications.
2. Principle of SERS
The mechanism behind SERS enhancement is always an area of interest for several researchers to understand and explain the phenomena. There are two commonly accepted theories which were proposed and demonstrated by various groups [4, 5]. The first one is the electromagnetic (EM) enhancement and the second one is chemical enhancement. In any cases, samples are typically placed on or at a proximity of a nanostructured metallic substrate, known as SERS substrate, where the enhancement occurs as a result of the interaction among the incoming light, the target sample molecule and the metallic surface. Nanostructures of noble metals, such as silver and gold, are common examples for SERS substrates since they do not have any Raman active modes for themselves.
The EM enhancement occurs when excitation of localized surface plasmon resonance (LSPR) modes of the metallic nanostructures occur at the resonance frequency of the incoming light, which can be considered as an EM wave . At this condition, localized dipoles are created resulting in an enhancement in the localized electric field around the metal nanostructure (Figure 2a). The magnitude of such induced dipoles is dependent on the incident electric field strength (
The chemical enhancement, on the other hand, involves enhancement through charge transfer mechanism between the substrate and the target molecule (Figure 2b) [8, 9]. Upon chemisorption of target molecules on the metallic substrate, new electronic states are formed having possible intermediate energies resonating with the resonance frequency of the metal. Under resonance, exchange of charges occurs between the substrate and the molecule resulting an enhancement in the Raman signal. However, chemical enhancement mechanism is molecule specific and typically shows lower enhancement factor (in the order of 103–105) compared to the EM enhancement, where the enhancement can be in the order of 108–1012 .
The SERS enhancement factor (
In Eq. (1),
3. Preparation of SERS substrates
It is now clear that SERS enhancement is dependent on the interaction between the target molecule and SERS substrate. Therefore, preparation of the SERS substrate is crucial for the success of the SERS enhancement. The SERS enhancement is mainly contributed by EM enhancement, which is highly dependent on the size, shape and structure of the material used to prepare the substrate. Typically, nanostructured plasmonic materials, like gold (Au), silver (Ag) and copper (Cu), are used for the preparation of SERS substrates. However, application of Cu is limited since the material show oxidation problem in air. Gold and silver are, on the other hand, most widely used due to their high stability compared to copper and most importantly they have LSPR frequencies in the visible to NIR range where most of the Raman scattering occurs. In this part, we will therefore briefly describe the application of some of the promising plasmonic noble metals for SERS substrates.
3.1. Nanoparticles of plasmonic metals
Nanoparticles of noble metals are most commonly used materials for the preparation of SERS substrates, because of their LSPR properties. These properties are useful only when the materials are in nanoscale, that is, less than 100 nm in dimensions. Several papers are now available reporting the synthesis and development of noble metal nanostructures focusing on their tailored shapes and sizes for SERS applications. Colloidal monodisperse metal nanoparticles show high SERS enhancement due to the controlled size and shape of the nanoparticles. A systematic study on colloidal Ag nanostructures with sizes ranging from 60 to 100 nm showed dependency of the SERS enhancement on the shape and size of the nanoparticles . It was found that
The size of the metal nanoparticles also is another crucial factor contributing to the SERS enhancement. It is well known that for LSPR effect, the size of the metal nanoparticles should be comparable to or smaller than the wavelength of the incident light. However, at very small size the nanoparticles start to show poor polarization resulting in loss of LSPR properties, making them inoperable for SERS applications . A recent study on the size effect of Ag nanoparticles on SERS showed reduction (in the order of 102) in the signal
3.2. Bimetallic nanoparticles
As the name suggests, the bimetallic nanoparticles are composed of two types of metals atoms in various configurations. These bimetallic nanoparticles are not only interesting for SERS point of view, but also for their other tunable properties like catalytic, optical, and magnetic properties. There are two common configurations of bimetallic nanoparticles used for SERS applications, namely alloyed and core-shell nanoparticles. In the alloyed configuration, the two types of metals are mixed homogeneously at their atomic level and hence the surface of the nanoparticle contains both the metal atoms. The
The core-shell bimetallic nanoparticles, on the other hand, have inhomogeneous distribution of the two metals, where one metal termed as
3.3. Anisotropic plasmonic nanostructures
As described in Section 3.1, anisotropic metal nanostructures enhance the localized electric field resulting in improved SERS signal enhancement. Especially, nanorods or nanowire-shaped metal nanostructures are mostly preferred since they also exhibit surface plasmon polariton (SPP) propagation along the metal-surrounding interface. Template assisted bundled Ag nanowires were reported for SERS detection by Lee et al.  and demonstrated ~50-fold increase in the signal intensity for benzenethiol (BT) used as a Raman probe. The enhancement was attributed to the hot spots created at the tips of the Ag nanowires, where most of the probe molecules were adsorbed. Another strategy for preparing monolayers of Ag nanowires using Langmuir-Blodgett (L-B) technique was reported by Tao et al. . In both the cases, the goal was to obtain closely packed Ag nanowires and form hot spots in the region of close contact. The template-assisted method resulted in almost vertically standing Ag nanowires, while L-B method, which is a relatively less complex method, provided a bed of horizontally aligned monolayer of Ag nanowires. The
Other types of anisotropic nanostructures, such as nanoprism, nanostar, nanosheets of plasmonic metals have also been investigated as SERS substrate by several researchers [42, 43, 44, 45]. Due to the presence of corners, edges or branches, such nanostructures always exhibit strong EM enhancement. Highly branched concave nanostructures of bimetallic Au-Pd were reported for their improved catalytic and SERS properties . SERS application of Ag nanoprism and factors affecting its enhancement factors were investigated by Ciou et al. . They presented three possibilities that contribute to the total SERS enhancement in case of the Ag nanorpism. They were namely higher target molecule adsorption, improved SPR resonance in the nanoprism with the excitation laser compared to the regular spherical Ag nanoparticles and presence of Ag nanoclusters along with the nanoprisms.
3.4. Patterned SERS substrates
While a significant development is taking place in the areas of solution processed SERS substrate preparations, reports on preparation of SERS substrates using non-solution processed techniques to directly transfer patterns on a surface are also surfacing. For example, lithographic techniques are widely used these days to produce highly sensitive, stable and reproducible SERS substrates [48, 49, 50]. One of the major advantages for lithographic techniques is the precise control over the geometry and arrangement of the nanostructured substrates, which is crucial for enhanced SERS signal. In this regard, electron beam (E-beam) lithography was employed to produce Au nanostructures showing the highly dependency of the SERS enhancement factors on the geometry and arrangement of the nanostructures . Van Duyne and coworkers [52, 53, 54] have invented a novel nanosphere lithography (NSL) technique to produce metallic nano-triangle patterns (Figure 7). In their technique, a self-assembled monolayer of similar sized metal nanospheres on a substrate was used a mask and then a thin layer of metal film (such as Au or Ag) was deposited in the gaps using evaporation or vapor phase techniques. The mask layer was then removed by simply applying ultrasonic vibration resulting in regularly spaced triangular shaped metallic patterns which showed
Periodic metallic structures and plasmonic gratings are also reported to exhibit SERS enhancement. A two-dimensional finite array of Au nanopatches was investigated by Grande et al.  reporting
4. Nontraditional SERS substrates
When traditional plasmonic materials are widely used for SERS substrates, few researchers also started to explore other possible nontraditional materials to prepare SERS substrates. Photonic crystal fibers with embedded metal nanoparticles as a potential SERS substrate were reported [60, 61]. The coverage density of the plasmonic metal nanoparticle in these fibers is the key for the successful SERS applications of such configurations. Zhao et al.  has reported significant increasing of the electromagnetic field in amorphous Si photonic crystals embedded in multiple metallo-dielectric units. Sensitivity level of such systems is found to be up to picomolar concentration levels of probe molecules. Another photonic crystal SERS substrate composed of plasmonic SiO2 nanotubes deposited on silicon nitride substrates was demonstrated by Xu et al. , exhibiting almost 10 times higher Raman signal intensities (see Figure 8).
Single sheet of carbon atoms, known as graphene was also explored as a potential SERS substrate [64, 65]. For graphene no evidence of EM enhancement is reported, and SERS enhancement is therefore mainly contributed by the chemical enhancement route exhibiting
Chemically enhanced SERS using semiconductor TiO2 nanoparticles was also explored by Musumeci et al.  reporting an
Without any doubt, SERS is a very powerful technique in various fields such as biomedical, material science, pharmaceutical, and even sensor application. With the emergence of nanotechnology as an enabling technology, applications of noble metal nanostructures with remarkable optoelectrical properties for SERS applications becomes a major area of R&D, which is not only limited to the synthesis of plasmonic nanostructures for SERS substrates, but also includes patterning of surfaces at nanoscale to enhance the effect. Challenges are there in terms of producing highly sensitive, large area robust and reusable SERS substrates with low-cost fabrication techniques. This chapter highlighted some of the numerous developments happening in this area, exhibiting wide range of SERS substrates available to date.