The development of nanostructured advanced materials based on the incorporation of metal nanoparticles has attracted the attention of the researchers [1–5]. The optical spectra of the metal nanostructures show an attractive plasmon resonance band, known as localized surface plasmon resonance (LSPR), which occurs when the conductive electrons in metal nanostructures collectively oscillate as a result of their interaction with the incident electromagnetic radiation [6, 7]. Such nanoplasmonic properties of the metal nanostructures are being investigated because of their unique or improved antibacterial, catalytic, electronic, or photonics properties [8–15]. In addition, their excellent optical properties make them ideal to use in optical fiber sensors in detecting physical or chemical parameters [16, 17].
A wide variety of methodologies are focused on the synthesis of metal nanoparticles with a fine control of the resultant morphology [18–24]. Of all them, chemical reduction methods from metal salts (i.e., AgNO3 or HAuCl4) are one of the most studied using adequate protective and reducing agents due to their simplicity [25–29]. Very recently, the high versatility of the poly(acrylic acid, sodium salt) (PAA) has been demonstrated as a protective agent of the silver nanoparticles because of the possibility of obtaining multicolor silver nanoparticles with a high stability in time by controlling the variable molar ratio concentration between protective and reducing agents . This weak polyelectrolyte (PAA) presents carboxylate and carboxylic acid groups at a suitable pH, being of great interest for the synthesis of metal nanoparticles. Specifically, the carboxylate groups of the PAA can bind silver cations, forming positively charged complexes, and a further reduction of the complexes to silver nanoparticles takes place [31–33].
One approach for incorporating metallic nanoparticles into thin films is based on in situ chemical reduction of silver cations to zero valent nanoparticles into a previously fabricated host matrices used as a template. This in situ synthesis process of metallic nanoparticles can be applied to several well-known deposition techniques such as sol-gel process , electrospinning , or layer-by-layer (LbL) assembly . Among of all them, LbL assembly shows a higher versatility for tailoring nanoparticles due to the use of polyelectrolytes with specific functional groups . Furthermore, a thermal post-treatment of the films makes possible the fabrication of chemically stable hydrogels  because a covalent cross-link via amide bonds between the polymeric chains of the polyelectrolytes has been induced [38–40] with a considerable improvement of their mechanical stability.
In this work, two weak polyelectrolytes, poly(allylamine hydrochloride) (PAH) as a cationic polyelectrolyte and PAA as an anionic polyelectrolyte, have been chosen to build the multilayer structure. The pH-dependent behavior of the PAA makes possible to control the proportion of carboxylate and carboxylic acid groups [41–44]. The carboxylate groups are responsible of the electrostatic attraction with the positive groups of the PAH, forming ion pairs to build sequentially adsorbed multilayers in the LbL assembly. In addition, the carboxylic acid groups are known as nanoreactor host sites which are available for a subsequent metal ion exchange with the proton of the acid groups. More specifically, the carboxylic acid groups are responsible of binding silver cations via metal ion exchange (loading solution). Once silver ions have been immobilized in the films, a chemical reduction of the silver ions to silver nanoparticles (AgNPs) takes place when the films are immersed in the reducing solution. Several approaches have been presented in the bibliography using different loading and reduction agents as well as weak or strong polyelectrolytes [45–49]. Nevertheless, weak polyelectrolyte LbL templates (such as PAH and PAA) offer the additional advantage of an adjustable pH-dependent degree of ionization, which is a key parameter when in situ synthesis process (ISS) approach is used.
Alternatively, AgNPs-loaded LbL films can be built up using polyelectrolyte-capped metal nanoparticles. The use of PAA as a protective agent of the silver nanoparticles (PAA-AgNPs) plays a key role for a further incorporation into LbL films . The carboxylate groups at a specific pH value are used to build the sequentially adsorbed multilayer structure with a cationic polyelectrolyte, preserving their aggregation of the AgNPs into the LbL films . Henceforward, this approach of a successive incorporation of AgNPs of a specific morphology into LbL films will be referred as layer-by-layer embedding (LbL-E) deposition technique.
In this work, a comparative study about the synthesis and incorporation of AgNPs into thin films obtained by layer-by-layer assembly is presented using two alternative chemical methods. The first methodology is the ISS which is based on a first step of thin film fabrication, and then a second step where the synthesis of silver nanoparticles into the films is performed. The second methodology is the LbL-E deposition technique which follows a different order because firstly silver nanoparticles of a specific shape are synthesized, and then their incorporation into thin films using the LbL assembly is performed. Although both processes use the same reagents, remarkable differences related to the size, distribution, or maximal wavelength position of the LSPR band have been observed. Additionally, a thermal post-treatment was performed to fabricate stable hydrogel films with a better chemical stability via cross-link of the polymeric chains. This comparative study can be useful to the further design of advanced hybrid coatings based on metallic nanoparticles and polymeric materials.