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

the different types of contributions to the chemical enhancement mechanism and the electro-

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].

By definition a composite is formed by at least two distinct materials whose chemical identity is preserved in the final material. Typically, a composite contains a material (filler) that is dispersed in a larger amount of a distinct material that acts as the host matrix. In the case of having a polymer as the host, such material is referred as a polymer based composite. For the particular situation, in which the fillers have at least one dimension at the nanoscale, the material is called a nanocomposite. Hence, polymers, either synthetic or of natural origin, that contain inorganic nanoparticles form an important class of nanocomposites. In particular, polymer based nanocomposites containing metallic nanoparticles (NPs) as fillers, are the object of this chapter due to their role as hybrid substrates for SERS analysis. Several approaches have been reported in order to produce polymer nanocomposites containing metal NPs as fillers. Briefly, these preparative methods can be divided in two main categories, depending if the metal nanofillers were generated in situ or previously prepared and then used for the composite fabrication. These approaches will be designated here as i) in situ and ii) ex situ. In the in situ method, the metallic nanofillers are prepared by chemical

magnetic enhancement mechanism in SERS.

94 Raman Spectroscopy

**3. Polymer based nanocomposites**

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


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

simplicity, although the resulting nanocomposites may exhibit inhomogeneity in terms of morphology and fillers distribution in the polymer matrix [10, 129].

example, Brust and co⁻workers have described the preparation of Ag loaded agarose hydrogels, in which the metallic nanofillers are trapped inside the polymer network due to the agarose capability to dry and rehydrate. In this particular case, the use of agarose as polymeric matrix provides the formation of a recyclable SERS substrate, in which the 1-naphthalenethiol used as

Nanocomposites containing gelatin [61, 76, 77], natural rubber [73] and distinct synthetic polymers [98–101] have been also prepared using in situ methods. For instance, Wu and his team have reported the fabrication of Ag loaded poly(styrene) microspheres using the chemical reduction method for the SERS detection of organic molecules such as dyes. [98].

> )2 ] +

(PVP) and poly(dopamine) (PDA) as linkers to attach the Ag ions into the PS microspheres

Blending metallic nanofillers with a polymeric matrix is a simple methodology to fabricate nanocomposites with efficient SERS activity [84, 104, 106, 107, 124]. Besides its simplicity, this is a cost-effective and easily scalable method over a large area [90, 130]. In the literature, there are some reports on the use of synthetic polymers such as poly(methyl metacrylate) [105, 109], poly(*t*-butyl acrylate) [107], polystyrene-block-poly(acrylic acid) [110] and poly(vinyl alcohol) [128] as matrices for the incorporation of metallic NPs and subsequent use as SERS platforms. Although not so common in SERS research context, natural polymers can also be used as polymeric matrices for the preparation of polymer based composites, in which water compatible metal NPs are normally used. Marsch *et al.* have reported the development of a surface enhanced resonance Raman scattering (SERRS) substrate containing Ag NPs with a positive surface charge, due to a poly-L-lysine coating, which was then used in the analysis of organic anions [86]. In their work, these polymer based composites were successfully used as SERS substrates for the detection of bilirubin, a organic molecule of clinical interest formed as a metabolic waste product of heme breakdown. More recently, Chang *et al.* have reported flexible SERS substrates based on common filter paper loaded with gold nanorods, which exhibited more than two orders of magnitude SERS enhancement compared to silicon-based SERS substrates [92]. The authors have demonstrated that these platforms are excellent candidates for trace chemical and biological detection due to their efficient uptake, and transport of the ana-

Other examples of bionanocomposites containing metallic NPs have been reported including those based on chitosan [81], pullulan [83], cellulose [67, 79, 82], gelatin [85, 88] and carrageenan [84, 89]. **Figure 3** illustrates the use of biopolymers and Ag NPs in the fabrication of composites for antimicrobial and SERS applications. In certain situations, these applications can be complementary, such as in the case of producing a gel that due to the presence of Ag not only has the ability for SERS detection but also lasts longer periods of time due to the

In ex situ methods, the polymer can be used either as a continuous phase or as an aqueous emulsion. For example, Lee *et al.* have reported the preparation of several metallic nanofillers

as precursor and the poly(vinylpyrrolidone)

SERS Research Applied to Polymer Based Nanocomposites

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

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analyte can be washed out by dialysis and the composite can be reused again [60]

lytes from the dispersing liquid to the surface of metal nanostructures.

The authors have used the complex [Ag(NH3

surface by hydroxylic and aminic groups.

**3.2. Blending or "grafting to" approach**

antimicrobial characteristics.

In situ preparation is commonly used to prepare biopolymers based nanocomposites because natural polymers might be used to induce control over NPs size and aggregation state [63, 70], promoting the required biocompatibility for specific applications namely medical diagnostic and target detection in SERS [60, 64]. Polysaccharides such as chitosan [64, 65, 71, 72, 74], agarose [60, 63, 70], glucose [102], hyaluronic acid potassium salt [75] and cellulose [67–69] have been used as polymeric matrices for the one-step preparation of such polymer based nanocomposites. The preparation of composites containing polysaccharides in which in situ chemical or UV light reduction of metal ions occurred has been subject of great interest. Indeed, these nanocomposites can be applied in a variety of domains such as antimicrobial agents [66, 80], platforms for chemical detection [68, 69], textile dyeing monitoring process [67] and electronic paper [79]. For example, bacterial cellulose has been investigated as an alternative host matrix to vegetable cellulose in the preparation of electronic paper, SERS substrates and antimicrobial agents by using metal NPs as fillers. More recently, we have demonstrated that Raman imaging is a useful technique to characterize and monitor the textile dyeing process of antimicrobial fabrics [67]. **Figure 2** provides examples of cellulose composites for diverse applications.

The use of polysaccharides in the form of hydrogels have also been reported as matrices for dispersing metallic NPs, in certain cases showing capacity to collapse or be lyophilized upon drying and recover their structure by rehydration and subsequent use in SERS analysis [60, 63, 65]. For

**Figure 2.** Examples of applications for cellulose based nanocomposites loaded with metal NPs: a) Raman image obtained using the integrated intensity of the Raman band at 1620 cm−1 in the SERS spectra of methylene blue 10−4 M using a linen based composite containing Ag NPs as substrate (excitation at 633 nm); [67] Ag/bacterial cellulose nanocomposites for SERS biodetection of phenylalanine; [69] c) Nanocomposites containing copper based NPs and cellulose to be used as antibacterial agents against *Staphylococcus aureus* [80].

example, Brust and co⁻workers have described the preparation of Ag loaded agarose hydrogels, in which the metallic nanofillers are trapped inside the polymer network due to the agarose capability to dry and rehydrate. In this particular case, the use of agarose as polymeric matrix provides the formation of a recyclable SERS substrate, in which the 1-naphthalenethiol used as analyte can be washed out by dialysis and the composite can be reused again [60]

Nanocomposites containing gelatin [61, 76, 77], natural rubber [73] and distinct synthetic polymers [98–101] have been also prepared using in situ methods. For instance, Wu and his team have reported the fabrication of Ag loaded poly(styrene) microspheres using the chemical reduction method for the SERS detection of organic molecules such as dyes. [98]. The authors have used the complex [Ag(NH3 )2 ] + as precursor and the poly(vinylpyrrolidone) (PVP) and poly(dopamine) (PDA) as linkers to attach the Ag ions into the PS microspheres surface by hydroxylic and aminic groups.
