MMT-supported Ag nanoparticles for chitosan nanocomposites: Structural properties and antibacterial activity
Introduction
Chitosan, a crystalline polysaccharide obtained from crustaceans, is the deacetylation form of chitin, the second most abundant natural polymer. Because of the great advantage provided by the use of polymers obtained from renewable sources, chitosan has been one of the most attractive biopolymers due to its biocompatibility, biological activity and biodegradability. It has been used to design and fabricate new materials with functional properties applied in various fields, ranging from waste management and medicine to food processing and packaging (Wang, Chen, & Tong, 2006).
The enhancement of barrier and mechanical properties of chitosan-based films as well as the improvement of their dimensional stability has been achieved through the development of nanocomposites obtained by adding plasticizer and various types of layered silicates such as montmorillonite to the polymer matrix (Gunster et al., 2007, Kasirga et al., 2012, Lavorgna et al., 2010, Paluszkiewicz et al., 2011, Petrova et al., 2012, Tang et al., 2009, Wang et al., 2005, Xu et al., 2006). In order to further impart new functionalities to biopolymers for broadening their application fields, the use of suitable nanoparticles is highly demanded. In particular, silver nanoparticles, alone or supported on inorganic platelets (Patakfalvi et al., 2003, Sotiriou et al., 2012), can be used as filler into polymeric structures with the aim to endow antibacterial and antimicrobial properties to the obtained nanocomposites, thus making them suitable for a variety of target applications such as textile, biomedical and food packaging materials (Sotiriou et al., 2012). As for the latter field of application, several studies have demonstrated the effectiveness against microbial growth in foods of silver nanoparticles loaded in biopolymers such as sodium alginate, polyvinylpyrrolidone, cellulose based absorbent pads and hydroxypropyl ethylcellulose (de Azeredo, 2013).
Chitosan has already been investigated as matrix to incorporate silver compounds or silver nanoparticles (Zhang, Luo, & Wang, 2010). In particular, different silver compounds were incorporated into various forms of chitosan matrix, including solution (Bin Ahmad et al., 2012, Hsu et al., 2011), gel (An et al., 2011, Krishna Rao et al., 2012) and film (Lopez-Carballo et al., 2013, Pinto et al., 2012, Regiel et al., 2013). All these papers mainly deal with the structural characterization of the obtained nanocomposites and with the study of their antimicrobial and antibacterial activity. However, the knowledge about the silver ions release and the capability to control their release kinetic is essential in order to assess the applicability of silver nanocomposites in sectors such as medicine and food packaging. Wang, Liu, Ji, Ren, and Ji (2012) reported that the release of silver ions from chitosan-Ag/PVP nanocomposite films shows a burst release process in the first day whereas Martınez-Abad, Lagaron, and Ocio (2012) showed that the release of Ag+ from a silver-based EVOH film in water takes place within 30 min, all samples reaching the equilibrium before the first hour. However, it is possible to design silver engineered nanoparticles, i.e. inorganic porous hosts with silver partially immobilized, able to release silver ions with a fine control and high effectiveness over time. In fact the use of bamboo charcoal supporting silver compounds delayed the sustained release of silver ions in aqueous environment over 70 h for diffusion effects (Yang, Wu, Liu, Lin, & Hu, 2009), whereas Ag and Ag2O nanoparticles supported on nanostructured SiO2 were able to release Ag+ ions by controlled dissolution of the oxide layer and further oxidation of metal silver (Sotiriou et al., 2012).
Moreover, in the last years, an innovative class of inorganic engineered nanoparticles based on layered silicates such as montmorillonite supporting silver ions or silver metal have been proposed (Incoronato et al., 2010, Praus et al., 2008). In these nanoparticles the release of silver ions is controlled by both diffusion effects and oxidation reactions of silver to silver ions which depend on the size of silver nanoparticles and on the water wettability. Incoronato et al. (2010) embedded the obtained silver-montmorillonite particles into agar, zein and polycaprolactone matrices and showed that the water uptake of the polymeric matrix is the key parameter associated with the antimicrobial effectiveness of these active systems.
However, to the best of our knowledge, there is no paper reporting on the design of a chitosan-based nanocomposite films in which tailored functionalities were introduced using silver-montmorillonite nanoparticles. In this work we propose novel active nanocomposite films consisting of chitosan filled with silver-montmorillonite nanoparticles obtained by ion exchange reaction. In the novel multifunctional bionanocomposite, the inorganic carrier has been used as filler exhibiting a twofold advantage. It serves as silver supporting material for a slow and sustained release of silver ions in an aqueous medium and to confer better performance to the obtained films such as dimensional stability and mechanical properties. The resulting films were widely characterized in order to have a deeper understanding of the correlations between their structure and properties; their in vitro antimicrobial activity was also assessed in order to verify their effectiveness as active systems.
Section snippets
Materials
Chitosan (CS) powder (molecular weight in the range 310–375 kDa and deacetylation degree >75%), silver nitrate (AgNO3) and glacial acetic acid (HAc) were purchased from Sigma–Aldrich (Milan, Italy). The unmodified pristine clay (Na+-montmorillonite) was purchased from Southern Clay Products, Inc., TX. Glycerol by Fluka (Italy) was used as plasticizer.
Preparation of silver/montmorillonite nanoparticles
Silver-montmorillonite (Ag-MMT) nanoparticles were prepared by silver ions exchange reaction (Incoronato et al., 2010). Five grams of Na-MMT were
Results and discussion
A preliminary characterization of chemical and physical properties of Ag-MMT particles has been previously reported by some of the authors (Incoronato et al., 2010). It has been proved that the total content of silver in the Ag-MMT particles is equal to 0.037 g/g. The presence of the characteristic UV silver surface plasmon bands at 290, 350, 450 nm shows that residual Ag+ ions or silver nanoparticles with average size of 2 nm, 10 nm and 40 nm are simultaneously present.
In order to have a deeper
Conclusion
Silver-montmorillonite (Ag-MMT) active nanoparticles were obtained by allowing silver ions from nitrate solutions to replace the Na+ ions of natural montmorillonite and to be reduced by thermal treatment. It has been proved that both Ag metallic and AgO/Ag2O nanoparticles are mainly located on the surface of MMT platelets with a preferential location on the edges.
The obtained nanoparticles were embedded into chitosan matrix and successfully used to prepare bionanocomposites film exhibiting
Acknowledgements
Financial support from the program PON Ricerca e Competitività 2007–2013, co-financed by the European Regional Development Fund (ERDF), within the Research Project PON02_00186_3417392 “Innovative packaging solutions to extend shelf life of food products” is gratefully acknowledged. The authors also thank Mrs. A. Aldi for technical support and Mrs. C. Del Barone for TEM analysis.
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