A comparative study of two different approaches for the incorporation of silver nanoparticles into layer-by-layer films
© Rivero et al.; licensee Springer. 2014
Received: 15 April 2014
Accepted: 1 June 2014
Published: 13 June 2014
In this work, a comparative study about the incorporation of silver nanoparticles (AgNPs) into thin films is presented using two alternative methods, the in situ synthesis process and the layer-by-layer embedding deposition technique. The influence of several parameters such as color of the films, thickness evolution, thermal post-treatment, or distribution of the AgNPs along the coatings has been studied. Thermal post-treatment was used to induce the formation of hydrogel-like AgNPs-loaded thin films. Cross-sectional transmission electron microscopy micrographs, atomic force microscopy images, and UV-vis spectra reveal significant differences in the size and distribution of the AgNPs into the films as well as the maximal absorbance and wavelength position of the localized surface plasmon resonance absorption bands before and after thermal post-treatment. This work contributes for a better understanding of these two approaches for the incorporation of AgNPs into thin films using wet chemistry.
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.
Poly(allylamine hydrochloride) (Mw 56,000), poly(acrylic acid, sodium salt) 35 wt.% solution in water (PAA) (Mw 15,000), silver nitrate solution (> 99% titration, 0.1 N AgNO3), and dimethylamine borane complex (DMAB) were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used without any further purification. Aqueous solutions of 0.01 M of both PAH and PAA were prepared using ultrapure deionized water (18.2 MΩ) and adjusted to pH values 7.0 and 9.0 by the addition of a few drops of HCl or NaOH 1 M.
Fabrication of the thin films
All the thin films have been fabricated using a 3-axis Cartesian robot from Nadetech Innovations SL (Sarriguren, Spain). The LbL assembly was performed by sequentially exposing the glass slides to cationic and anionic polyelectrolytes with an immersion time of 2 min. A rinsing step in deionized water was performed between the two polyelectrolyte baths. The combination of a cationic monolayer with an anionic monolayer is called bilayer. More details of the LbL assembly can be found elsewhere .
In situ synthesis of the silver nanoparticles
This process starts with a first step of a multilayer coating fabrication using the LbL assembly of cationic (PAH) and anionic (PAA) polyelectrolytes. A second step is where the ISS of the AgNPs into the polymeric coating was carried out.
The polymeric thin films are firstly immersed in an aqueous solution of silver nitrate (AgNO3 0.01 N) at room temperature for 5 min, removed, and rinsed with ultrapure water. Then, once the silver ions have been incorporated into films via ion exchange, a further in situ chemical reduction of the silver cations (Ag+) to silver nanoparticles (Ag0) was performed at room temperature. The films are immersed in an aqueous solution of dimethylamine borane complex (DMAB 0.01 N) for 5 min, removed, and rinsed with ultrapure water.
Layer-by-layer embedding deposition technique
This synthesis process is based on a first step of synthesis of silver nanoparticles with a desired shape, and then a second step where a further incorporation of the synthesized silver nanoparticles into a thin film using the LbL-E deposition technique is performed.
Silver nanoparticles have been synthesized at room temperature via chemical reduction process of an aqueous solution of silver precursor (AgNO3) with an aqueous solution of reducing agent (DMAB). More details of the synthesis can be found elsewhere . In LbL-E, the PAA functionalized AgNPs were used as polyanion (PAA-AgNPs) in the LbL protocol, as it was described in ‘Fabrication of the thin films’ section.
A thermal post-treatment was carried out in the resultant LbL films using temperatures from 50°C to 200°C in a furnace for a period of time of 2 h. The heat-treated cross-linked films have enhanced durability when immersed in aggressive conditions for several hours (buffer solution pH 10) and no delamination of the films was observed, while untreated films were severely damaged.
Characterization of the thin films
UV-vis spectroscopy (UV-vis) was used to characterize the optical properties of the silver nanoparticles incorporated into the thin films. Measurements were carried out with a Jasco V-630 spectrophotometer (Jasco Inc., Easton, MD, USA).
Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize both the distribution of the nanoparticles and the morphology of the resultant thin films. The samples were scanned using a Veeco Innova AFM (Veeco Instruments, Inc., Plainview, NY, USA), in tapping mode and a Carl Zeiss UltraPlus FESEM (Carl Zeiss AG, Oberkochen, Germany).
Transmission electron microscopy (TEM) was used to characterize the cross section of the thin films. The coatings were performed onto polystyrene coverslips which were cut off and embedded in an epoxy resin. Then, ultrathin cross sections were obtained and immediately mounted onto 200 mesh copper grids. Measurements were performed using transmission electron microscope Carl Zeiss Libra 120 at 80 kV.
Results and discussion
In situ synthesis process of the silver nanoparticles
The weak polyelectrolyte nature of the PAH/PAA matrix makes the pH of the polyelectrolyte dipping solutions determine the number of free carboxylic acid present in the multilayer thin film. The PAA polyanion presents carboxylate and carboxylic acid groups at a suitable pH where the carboxylate groups are responsible for the electrostatic attraction with the cationic groups of the polycation (PAH), forming ion pairs to build sequentially adsorbed multilayers in the LbL assembly. However, the carboxylic acid groups are available for a subsequent ionic exchange for the introduction of inorganic ions such as silver (loading AgNO3 solution) and a further in situ chemical reduction of the silver cations (Ag+) to AgNPs using a reducing agent (reduction DMAB solution). This loading/reduction (L/R) cycles have been repeated up to four times.
Thickness evolution of the thin films obtained by ISS process
LSPR (λmax; Amax)
288 ± 5
[PAH(9.0)/PAA(9.0)]40 + 1 L/R cycle
291 ± 4
421.3 nm; 0.04
[PAH(9.0)/PAA(9.0)]40 + 2 L/R cycles
289 ± 16
422.1 nm; 0.09
[PAH(9.0)/PAA(9.0)]40 + 3 L/R cycles
296 ± 8
422.8 nm; 0.79
[PAH(9.0)/PAA(9.0)]40 + 4 L/R cycles
294 ± 8
424.6 nm; 1.07
A study about the thickness evolution of the LbL films before and after the ISS process as well as the maximum wavelength position and absorbance related to the LSPR absorption band is performed, as it can be observed in Table 1. An important consideration is that the resultant thickness after the L/R cycles (from 1 to 4 cycles) is very similar to that of only polymeric LbL coating. As a conclusion, when the number of L/R cycles is increased during the fabrication process, a higher amount of AgNPs are synthesized while the overall thickness of the film remains almost unaltered.
Thickness evolution of the thin films obtained by ISS process after thermal treatment
LSPR (λmax; Amax)
[PAH(9.0)/PAA(9.0)]40+ 4 L/R cycle
294 ± 8
424.6 nm; 1.07
[PAH(9.0)/PAA(9.0)]40+ 4 L/R cycles
277 ± 9
424.6 nm; 1.10
[PAH(9.0)/PAA(9.0)]40+ 4 L/R cycles
256 ± 7
424.6 nm; 1.16
[PAH(9.0)/PAA(9.0)]40+ 4 L/R cycles
212 ± 7
436.8 nm; 1.63
[PAH(9.0)/PAA(9.0)]40+ 4 L/R cycles
194 ± 7
477.1 nm; 1.57
Layer-by-layer embedding deposition technique
As it was previously commented in the ‘Methods’ section, AgNPs with a specific protective agent (PAA-AgNPs) were firstly synthesized prior to the LbL assembly of the coating .
Thickness evolution of the thin films obtained LbL-E deposition technique
LSPR (λmax; Amax)
63 ± 5
421.3 nm; 0.017
165 ± 4
432.1 nm; 0.13
507 ± 16
432.3 nm; 0.77
642 ± 12
432.6 nm; 1.18
Thickness evolution of the thin films obtained LbL-E deposition technique after thermal treatment
LSPR (λmax; Amax)
642 ± 12
432.6 nm; 1.18
611 ± 16
432.6 nm; 1.20
600 ± 12
432.6 nm; 1.26
552 ± 9
432.6 nm; 1.68
452 ± 10
446.9 nm; 1.66
A comparative study between ISS process and LbL-E deposition technique
In this section, a comparative study about both processes will be shown for a better understanding of the incorporation of AgNPs into thin films using wet chemistry reactions. In order to establish any significant differences, the evolution of the thin films will be studied for the higher number of bilayers and L/R cycles at room temperature (ambient) and after thermal post-treatment of 200°C. In addition, a study about the distribution of the AgNPs into the thin films will be necessary to understand the shift of the LSPR absorption bands.
As a conclusion of both processes, the use of PAA as a protective agent of the AgNPs in the LbL-E deposition technique is of vital importance because it can prevent cluster formation along the coating, although it is possible to appreciate nanoparticles of higher size along the coating thickness. To sum up and according to the results, LbL-E deposition technique allows the incorporation of AgNPs of higher size along the film, whereas cluster formation mixed with AgNPs of small size is only observed for the ISS process.
This work is based on the synthesis and incorporation of silver nanoparticles into thin films using two alternative techniques with remarkable differences, the ISS process and the LbL-E deposition technique. Firstly, both processes are separately analyzed as a function of several parameters such as the pH value of the dipping polyelectrolyte solutions, thickness evolution, or temperature effect. Secondly, a comparative study between both processes has been performed in order to establish the difference in the size and distribution of the nanoparticles into the LbL films.
In both methodologies, the presence of a weak polyelectrolyte such as poly(acrylic acid, sodium salt) is the key for synthesizing metallic silver nanoparticles due to its pH-dependent behavior, making possible to obtain carboxylate and carboxylic acid groups as a function of the pH value. For the ISS process, the presence of free carboxylic acid groups is the key for the introduction of silver ions which are further reduced to silver nanoparticles. However, in the case of the LbL-E deposition technique, PAA is acting as an encapsulating agent of the nanoparticles and these AgNPs are incorporated into thin films by the electrostatic attraction between the polycation (PAH), and the carboxylate groups of the PAA capped the nanoparticles (PAA-AgNPs).
The location of the LSPR absorption bands varies from 424.6 nm for the ISS process to 432.6 nm for the LbL-E deposition technique. However, a post-thermal treatment produces a wavelength shift of the LSPR absorption bands, being more significant for the ISS process because the LSPR maximum wavelength position is displaced at 46 nm in comparison with only 13 nm in the LbL-E deposition technique. In addition, the full width at half maximum is higher for the ISS film (224 nm) in comparison with the LbL-E film (108 nm).
A morphological characterization (SEM, TEM, or AFM) is performed in order to clarify the size and distribution of the nanoparticles in the LbL films. SEM images indicate that a higher amount of AgNPs with less size is synthesized for the ISS process. Cross-sectional TEM micrographs and AFM phase images indicate the cluster formation of AgNPs in the topographic distribution of the ISS process which is not observed in the LbL-E films. These remarkable differences between both processes related to the distribution, size, and partial aggregation have a considerable influence in the final location of the LSPR absorption bands. In addition, the great importance of using a protective agent such as PAA-AgNPs in the LbL-E deposition technique is to prevent the aggregation of the AgNPs during the fabrication process and after thermal post-treatment. To our knowledge, this is the first time that a comparative study of the synthesis and incorporation of AgNPs into thin films is presented in the bibliography using two alternative methods with the same chemical reagents based on wet chemistry.
This work was supported by the Spanish Ministry of Economy and Competitiveness through TEC2010-17805 Research Project, Innocampus Program and Public University of Navarra (UPNA) research grants. Special thanks to CEMITEC for the utilization of the SEM.
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