Synthesis and characterization of chitosan and silver loaded chitosan nanoparticles for bioactive polyester

Abstract

The paper focuses on the synthesis of chitosan nanoparticles (CSN) by ionic gelation with sodium tripolyphosphate and subsequently its loading with silver ions to produce silver loaded chitosan nanoparticles (Ag-CSN). The aim was to enhance the antibacterial property of chitosan in the nanoparticle form and thus improve its bactericidal efficacy when applied on polyester fabrics. The average particle size of CSN and Ag-CSN was 115 nm and 165 nm, respectively. The structure of CSN and Ag-CSN was studied by XRD, FTIR, DSC, TGA and TEM analysis. The minimum inhibitory concentration of both the CSN and Ag-CSN against Staphylococcus aureus bacteria was found to be 50 and 500 times less, respectively, as compared to bulk chitosan. Silver loading on the synthesized CSN showed synergistic antimicrobial effect against S. aureus bacteria. The release of Ag⁺ from Ag-CSN finished polyester fabric is substantiated by antibacterial testing which shows a clear zone of inhibition.

Introduction

The textile materials have generated a considerable interest in the medical field where these wide range of materials in the form of monofilament, multifilament, woven or nonwoven structures are being used as sutures, bandages, scaffolds, wound dressing, masks, surgical gowns and hospital linen, etc. A lot of commercial medical textile products are available with antimicrobial property wherein the growth of micro-organisms is controlled by treatment with antimicrobial agents. Antimicrobial agents can also be integrated in textiles substrates to make these rot proof, mildew stain proof and to prevent perspiration odor resulting from microbial growth on textiles.

The major classes of synthetic antimicrobial agents for textiles include triclosan, metal and their salts, organometallics, phenols, quaternary ammonium compounds and organosilicons, etc. (Purwar & Joshi, 2004). But these chemicals would often yield highly toxic or otherwise undesirable by-products. In view of these environmental and ecological concerns and the lack of alternatives that meet all of the aforementioned effectiveness, durability and safety, researchers have been exploring the use of natural substances in treating and producing textile products with antimicrobial properties. The antimicrobial activity of natural dyes (Gupta et al., 2004, Joshi et al., 2009) on textiles has been discussed in literature. The effect of various plant extracts on the bacteria has been studied by a number of researchers in the past (Da and ErdoUrul, 2003, ErdoUrul, 2002, Reddy et al., 2001). The antibacterial properties of neem oil in combination with other herbal oils such as clove, tulsi and karanga have been used to impart antimicrobial finish on cotton textiles (Joshi et al., 2009, Thilagavathi et al., 2005). Work on antimicrobial finishing of 100% cotton and PET/cotton blend fabric using neem seed extract has been recently reported by our group (Joshi et al., 2007, Purwar et al., 2008). Chitosan, a biopolymer derived from a component found in crustacean shells called chitin, has long been known to possess antimicrobial attributes. Antimicrobial textiles using chitosan is also extensively reported in the literature (Lee et al., 1999, Seong et al., 1999, Shin et al., 2001). Antimicrobial fibers obtained from chitosan are readily available in the market for potential use (Kumar, 1999, Nam et al., 2001).

More recently researchers working in the area of textiles and polymers are investigating the possible applications of nanotechnology for producing more appealing and highly functional value-added textile/polymeric substrates. Coating the surface of textiles and clothing or incorporating the fibres with nanoparticles is the latest approach for the production of highly active textile surfaces with a range of functionality such as UV blocking, antimicrobial, flame retardant, water repellant and self-cleaning properties, etc. While antimicrobial properties are exerted by nano silver, several metal oxide nanoparticles such as nano TiO₂, ZnO, SiO₂, etc. as coatings impart self-cleaning and flame-retardant properties.

On the other hand, nano-biotechnology researchers are actively focused towards new technique of drug delivery through nanoparticles. The use of chitosan nanoparticle in protein and drug delivery system is being actively researched and reported in the literature (Kevin et al., 2001, Xu and Du, 2003).

Although there is a lot of literature on use of chitosan based antimicrobial textiles where bulk chitosan has been used as coating or finishing (Joshi et al., 2009), but there is no systematic research carried out on characterization and the application of chitosan nanoparticles and silver loaded chitosan nanoparticles on polyester substrates to have enhanced antibacterial activity. Recently, Yang, Wang, Huang, and Hon (2010) studied the application of nanochitosan for wool fabric finishing where they showed the effect of molecular weight on the chitosan nanoparticle size and zeta potential and the effect of nanochitosan concentration on shrink-proof and antibacterial properties of the wool fabric. Moreover, most of the studies reported on chitosan nanoparticles for drug delivery or gene therapy application (Jayakumar et al., 2010, Kong et al., 2010).

This study aims to investigate the use of chitosan nanoparticles for antimicrobial textile applications as chitosan in nano form is highly active because of very high surface area to volume ratio and expected to have desirable bioactivity even at very low concentrations. Initially, a detailed physical, chemical and thermal properties of synthesized nanoparticles vis-à-vis bulk chitosan has been investigated. A comparative study has also been carried out to investigate the enhancement of bioactivity of nanochitosan treated polyester over the bulk chitosan finished polyester at the same concentration.

It is well-known that a single antimicrobial agent cannot work effectively against both the Gram-positive and Gram-negative bacteria and also a broad range of microbes. Chitosan shows antimicrobial activity on textile substrates at relatively high concentrations and does not show release property from the treated textiles for its medical application as sutures, bandages, etc. Silver is a known antimicrobial agent with a wide spectrum of activity. But, incorporation of silver nanoparticles on textile substrates has been challenge because of its low charge density. Therefore, for instance, silver nanoparticles are often encapped with poly(methacrylic acid) (PMA) to generate negative charge on the surface for attachment on nylon and silk fibres (Dubas, Kumlangdudsana, & Potiyaraj, 2006) which can reduce the antimicrobial efficacy of the silver particle as well. In the present case, chitosan acts as a stabilizing agent and prevent the oxidation of silver which forms black color on the textile substrates by transforming silver to silver oxides.

Therefore, the synthesis of chitosan nanoparticles loaded with silver ion can also be investigated so as to be effective against a wide range of microbes with sustained release of silver ion over time. Also silver loaded in chitosan nanoparticle could be easily incorporated in the textile structure along with the chitosan nanoparticles without the need of the capping agents. Thus, the present work would be an attempt to synthesize chitosan nanoparticles and silver loaded chitosan nanoparticles and to integrate them on polyester fabric for imparting enhanced antibacterial property for a wide range of medical textile applications.