On-paper Synthesis of Silver Nanoparticles for Antibacterial Applications

Recent years have seen remarkable progress in research and development of metal nanoparticles (NPs) that takes advantage of their unique optical, magnetic, electronic, catalytic and other physicochemical properties, in a wide range of practical and potential applications such as energy, environmental, biomedical and chemical engineering (Feldheim & Foss, 2002; Ma et al., 2006; Jena & Raj, 2008; Zotti et al., 2008). For example, gold (Au) is a chemically inert metallic element in bulk form; however, AuNPs possess colorful plasmon resonances useful for bio-sensing (Shukla et al., 2005; Yokota et al., 2008), and can catalyze chemical reactions (Bond et al., 2006; Ishida & Haruta, 2007), due to their quantum size effects. NPs of inexpensive base metals, such as copper (CuNPs) have recently attracted much attention as innovative nanomaterials for applications such as highly-active gas reforming catalysts for hydrogen production (Gadhe & Gupta, 2007) and effective marine antifouling coatings (Anyaogu et al., 2008). In general, however, practical utilization of nanosized materials involves considerable difficulties since metal NPs are hard to handle directly, and easily aggregate to minimize their surface area. The inevitable aggregation of metal NPs often nullifies their unique functionalities, and eventually yields ordinary bulk metals. For that reason, an area of ongoing research has focused on effective immobilization of metal NPs on easily handled supports such as porous membranes (Dotzauer et al., 2006) and nanostructured inorganic sheets (Wang et al., 2008). Of the various metal NPs, silver NPs (AgNPs) are of increasing interest because of their high conductivity (Li et al., 2005) and tunable optical responsiveness (McFarland & Van Duyne, 2003). Moreover, it is well known that Ag exhibits potent antibacterial properties with low toxicity for humans and animals by comparison with other heavy metals (Alt et al., 2004; Shah et al., 2008). Ag and Ag-compounded materials are effective for both Gram-negative and Gram-positive bacteria, whereas the efficacy of conventional antibiotics varies with the species of bacteria (Shah et al., 2008). Many researchers have recently reported that AgNPs demonstrate excellent antibacterial activity (Sondi & Salopek-Sondi, 2004; Gogoi et al., 2006; Pal et al., 2007; Navarro et al., 2008). However, effective methods for immobilization of AgNPs for practical use are insufficiently advanced, and incorporation of AgNPs into various matrices is being actively investigated. For instance, it was reported that AgNPs were synthesized on poly(ethylene glycol)-polyurethane-titanium dioxide (TiO2) films via 14