**3. Polymer based nanocomposites**

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 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 of natural and synthetic origin.
