**2. Fundamentals of surface enhanced Raman scattering**

Chandrasekhara Venkata Raman (Nobel Prize in Physics, 1930) and his student Kariamanickam Srinivasa Krishnan reported in a scientific paper entitled "A new type of secondary radiation," the phenomenon that we now call the Raman effect [26]. This effect involves inelastic scattering of light when the photons strike a certain medium containing molecules. Although the number of photons resulting from the elastic dispersion (Rayleight scattering) of light coming from a laser source is greater than that of the inelastic dispersion, thus giving rise to a more intense signal, it is the spectral information collected from the inelastic collision that is of interest for Raman spectroscopy [27–30]. Indeed, the bands that are observed in a Raman spectrum (Stokes and anti-Stokes shifted in relation to the excitation line) are associated to certain vibrational modes existing in molecules and materials and can thus give information about their structure and other properties. The use of Raman spectroscopy in certain areas of application was, until recently, limited by two main factors. One of these factors can be considered extrinsic to the phenomenon and depends essentially on the development of more sensitive and affordable equipment. The other factor is intrinsic to the Raman effect, since it is a physical process that results in low-intensity spectral bands, because of the low scattering cross-section (10−29 cm2 molecule−1), limiting its use as a high sensitive analytical technique [30–32].

The SERS (surface enhanced Raman scattering) effect was discovered about 44 years ago during studies applied to a silver electrode and an aqueous solution of pyridine [9]. Unexpectedly, this experiment revealed an increase in the Raman signal of pyridine adsorbed on the metal surface. A few years later, other research groups advanced the currently accepted explanation for this observation [33, 34]. The interpretation given considered that the Raman signal intensification occurred due to adsorption of molecules on rough metallic surfaces. It is generally accepted today that the intensification occurring in SERS is due to two distinct mechanisms: the chemical interaction of the analyte with the metallic surface (signal intensification 10–10<sup>2</sup> ) and the enhancement of the local electric field at the junction of the metallic NPs (signal intensification can reach up to 10<sup>11</sup>) [32, 35, 36]. The chemical mechanism (CM) for the SERS signal enhancement is related to the adsorption of molecular species on the metal surface, namely the surface selection rules, the type of interaction between the metal and the molecule and the chemical nature of the adsorbed molecule itself [36–38]. The electromagnetic mechanism (EM) is the dominant contribution to SERS and does not depend necessarily on the establishment of a chemical interaction between the analyte and the metal surface, contrary to what happens in the chemical mechanism. According to the EM, the intensification of the local electromagnetic field is mainly due to the excitation of the surface plasmons of the metal by the incident light [39–41]. While the CM is a short-range effect, the EM is a long-range effect, in the sense that it does not require that the molecular species is in contact with the metal surface and can still be observed a few nanometers of distance from the metal surface. **Figure 1** presents a scheme of

and which are expected to expand in the near future in several applications and techniques. Among these techniques, Surface Enhanced Raman Scattering (SERS) has emerged in the past decades with great importance. Although the discovery of the SERS effect dates back to the 70's, [9] in the last decades, this technique has been improved due to the huge progress observed in Raman instrumentation and also in recent research specific to nanoscale materials. Indeed, the design of SERS active and highly sensitive analytical platforms has been a major goal in several fields, due to the impact in trace chemical analysis, environmental moni-

This chapter intends to present recent developments concerning polymer based nanocomposites containing metal nanofillers for SERS applications, in some situations based on studies of metal loaded polymers prepared in our laboratory. In order to contextualize the SERS application of these polymer based nanocomposites, the synthesis of the nanostructures and some fundamental aspects of SERS are briefly introduced [17–25]. Polymer based composites for SERS, containing metallic fillers, have been largely documented for the vestigial detection of several bio-analytes. Illustrative examples on the use of polymer based composites as new platforms for SERS are described in more detail, reviewing their use for chemical analysis. The importance of Raman imaging in SERS studies is also explored because is a recent devel-

Chandrasekhara Venkata Raman (Nobel Prize in Physics, 1930) and his student Kariamanickam Srinivasa Krishnan reported in a scientific paper entitled "A new type of secondary radiation," the phenomenon that we now call the Raman effect [26]. This effect involves inelastic scattering of light when the photons strike a certain medium containing molecules. Although the number of photons resulting from the elastic dispersion (Rayleight scattering) of light coming from a laser source is greater than that of the inelastic dispersion, thus giving rise to a more intense signal, it is the spectral information collected from the inelastic collision that is of interest for Raman spectroscopy [27–30]. Indeed, the bands that are observed in a Raman spectrum (Stokes and anti-Stokes shifted in relation to the excitation line) are associated to certain vibrational modes existing in molecules and materials and can thus give information about their structure and other properties. The use of Raman spectroscopy in certain areas of application was, until recently, limited by two main factors. One of these factors can be considered extrinsic to the phenomenon and depends essentially on the development of more sensitive and affordable equipment. The other factor is intrinsic to the Raman effect, since it is a physical process that results in low-intensity spectral bands, because of the low scattering

The SERS (surface enhanced Raman scattering) effect was discovered about 44 years ago during studies applied to a silver electrode and an aqueous solution of pyridine [9]. Unexpectedly, this experiment revealed an increase in the Raman signal of pyridine adsorbed on the metal

molecule−1), limiting its use as a high sensitive analytical technique

toring, medical applications and food safety [10–16].

92 Raman Spectroscopy

cross-section (10−29 cm2

[30–32].

opment with a great potential for the research on new SERS substrates.

**2. Fundamentals of surface enhanced Raman scattering**

**Figure 1.** (A) Scheme illustrating the chemical mechanism (CM) that occurs in SERS. (a) Ground state chemical enhancement; (b) resonance Raman enhancement; (c) charge-transfer resonance enhancement, where Ef represents the local electromagnetic field; HOMO represents the highest occupied molecular orbital and LUMO represents the lowest unoccupied molecular orbital); (B) Scheme illustrating the electromagnetic mechanism (EF), in which the I represents the intensity of Raman; E<sup>i</sup> represents the incident electric field; Ei,s represents the field which is intensified by the metal and (E<sup>i</sup> + Ei,s) represents the incident enhanced field; E<sup>r</sup> represents the scattered Raman field, which can be intensified by the metal creating Er,s; (E<sup>r</sup> + Er,s) represents the scattered enhanced field. ([10] - Reproduced by permission of The Royal Society of Chemistry.).

the different types of contributions to the chemical enhancement mechanism and the electromagnetic enhancement mechanism in SERS.

reduction methods in the presence of a polymer; in the ex situ approach, the metallic NPs are previously synthesized and then mixed with a polymer matrix, forming homogeneous blends composites (blending method) or, after surface modification procedures applied at the filler's surfaces [10, 58, 59]. These preparative methods will be briefly described in the next sections. **Table 1** lists some methods for preparing these composites based on polymers

In this methodology, metallic nanofillers are produced by chemical reduction of a metal precursor using reducing agents such as sodium citrate or sodium borohydride, in the presence of a polymer. This strategy generates nanocomposites whose morphology can vary, such as in a polymeric shell and a metal core [60, 61, 77, 96, 102], a polymeric core and a metal shell [95] or a polymeric matrix having dispersed metallic fillers [68, 69, 97]. In particular cases, the polymer can act as reducing agent due to specific functional groups, avoiding the use of an external reducing agent [61, 73]. The advantages of this one-step approach relies on its

Blending or "grafting to" approach Ag [66, 67, 81–87]

UV light reduction of the metal ions Ag [66, 94]

Blending or "grafting to" approach Ag [104, 105]

In situ polymerization or "grafting from" approach Ag [113–118]

Electrospinning method Ag [126, 127]

Au [73–76] Cu [77–80]

SERS Research Applied to Polymer Based Nanocomposites

http://dx.doi.org/10.5772/intechopen.72680

95

Au [88–92] Cu [79] Au@Ag [93]

Au [94] Au@Ag [94]

Cu@Ag [103]

Au [106–112]

Au@Ag [125]

Fe3 O4 @Ag [48]

Au [126] Au@Ag [128]

Au [58, 108, 116, 119–124]

**Polymer matrix Preparative method Metallic NPs** Natural polymers Chemical reduction Ag [60–72]

Synthetic polymers Chemical reduction Ag [95–102]

**Table 1.** Preparative methods of polymer/metal nanocomposites.

of natural and synthetic origin.

**3.1. Chemical reduction (in situ method)**

The contribution of nanotechnology, and in particular of the Chemistry of Nanomaterials, to the resurgence of studies and applications of Raman spectroscopy in materials characterization and the improvement of analytical techniques cannot be surprising. Indeed, the SERS effect allows the range of applications of Raman spectroscopy to be expanded but it is still very dependent on the quality of the substrates used to obtain signal intensification. The substrates most used for these purposes are based on Ag and Au nanostructures, although the SERS effect using other types of surfaces have been reported [42–45]. A variety of studies have demonstrated that the enhancement of the localized electromagnetic field occurs in close vicinity of metal NPs, metal nanotips or in metal surfaces with specific nanopatterns. The positions in which the strongest enhancement of the local electromagnetic field is observed correspond to the so called hotspots, due to the strongly enhanced Raman signals observed for certain molecular adsorbates [46–52]. Chemistry provides the synthetic tools to control the morphology and size of metal nanoparticles as well as other nanostructured materials and therefore has been widely used in the development of highly sensitive and reproducible SERS substrates. It should be noted that the sensitivity of some of the SERS substrates reported in the literature allows the analyte detection at the single-molecule level [53–55]. The fact that Raman spectroscopy is a non-destructive technique and can also be used for materials imaging, are additional factors that make this technique increasingly relevant in chemical analysis and materials characterization. In particular, the SERS technique is a valuable tool for the surface characterization of materials. For example, it is possible to obtain information about the surface of a metal using molecules that once adsorbed will function as molecular probes [56]. In addition, it is possible to obtain information about the orientation of these molecules, applying surface selection rules that were established in earlier studies [11, 39, 57].
