© 2008 Nature Publishing Group ARTICLES Silver-nanoparticle-embedded antimicrobial paints based on vegetable oil ASHAVANI KUMAR 1 * † , PRAVEEN KUMAR VEMULA 2 † , PULICKEL M. AJAYAN 1 * AND GEORGE JOHN 2‡ 1 Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA 2 Department of Chemistry, The City College of New York, and The Graduate School and University Center of The City University of New York, New York, New York 10031, USA *Present address: Department of Mechanical Engineering and Material Science, Rice University, Houston, Texas 77005, USA † These authors contributed equally to this work ‡ e-mail: john@sci.ccny.cuny.edu Published online: 20 January 2008; doi:10.1038/nmat2099 Developing bactericidal coatings using simple green chemical methods could be a promising route to potential environmentally friendly applications. Here, we describe an environmentally friendly chemistry approach to synthesize metal-nanoparticle (MNP)- embedded paint, in a single step, from common household paint. The naturally occurring oxidative drying process in oils, involving free-radical exchange, was used as the fundamental mechanism for reducing metal salts and dispersing MNPs in the oil media, without the use of any external reducing or stabilizing agents. These well-dispersed MNP-in-oil dispersions can be used directly, akin to commercially available paints, on nearly all kinds of surface such as wood, glass, steel and different polymers. The surfaces coated with silver-nanoparticle paint showed excellent antimicrobial properties by killing both Gram-positive human pathogens (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli). The process we have developed here is quite general and can be applied in the synthesis of a variety of MNP-in-oil systems. Common household oil paint, the oldest form of modern paints, uses a binder that is derived from vegetable oils, obtained from linseed or soya bean. Alkyd paints are based on alkyd resins (vegetable-derived drying oils), which contain a variety of polyunsaturated fatty-acid chains, commonly linoleic and linolenic acid and their triglycerides 1–3 , which undergo free-radical-mediated autoxidation during the curing/drying process 4,5 (Fig. 1a–c). The use of naturally generated free radicals would enable us to generate paint-based value-added products. Coatings on surfaces of interest decorate or protect the surfaces 6–8 . In general, several natural oils, drying oils in particular, are excellent coating materials, and when exposed to air, they form a tough scratch-free film as a result of the oxidative drying (lipid autoxidation) process that occurs through a widely accepted ‘free-radical’ mechanism in the presence of atmospheric oxygen 4,5 (Fig. 1c). In addition, literature reports suggest that free radicals are known to reduce metal salts to their uncharged metal nanoparticles 9,10 (MNPs). Hence, we took advantage of the free radicals that are generated during the natural drying process of drying oils/alkyd paints for the preparation of silver- and gold-nanoparticle- (AgNP and AuNP) embedded paints (in situ); AgNP-embedded paints are of particular interest owing to their potential bactericidal activity. Several methods have been reported for the preparation of organic–inorganic hybrid materials; most of the techniques used to incorporate metals into polymeric matrices involve either chemical reactions such as reduction 11 , mixing preformed metal nanoparticles with polymers 12 or complicated physical techniques 13 , such as sputtering 14 , plasma deposition 15 and layer- by-layer deposition 16 . All of these techniques add time, cost, multi- step synthesis and complexity to the overall process of fabricating metal-particle-doped materials. Hence, the preparation of MNPs without using external reagents in a single step (in situ) by excluding extra purification processes or transfer protocols will have advantages over the present methods. To overcome the above- mentioned hurdles, we have been working on developing efficient supramolecular organic soft materials as hosts for the synthesis and stabilization of inorganic MNPs 17–19 . Here, we have used the naturally occurring autoxidation/drying process in vegetable-based drying oils as a tool to prepare MNPs in situ. Silver and silver-based compounds are highly antimicrobial by virtue of their antiseptic properties to several kinds of bacterium, including Escherichia coli and Staphylococcus aureus 20–22 . Silver- based antimicrobial agents receive much attention, because of the low toxicity of the active Ag ion to human cells 23,24 , as well as it being a long-lasting biocide with high thermal stability and low volatility. However, although previous studies on silver and AgNPs have revealed some insights into the application of silver in several areas, little is known about the toxicity of AgNPs, where the size and surface area are recognized as important determinants for toxicity. AgNPs have been shown to possess good biocompatibility with mouse fibroblasts and human osteoblasts 25 , and their use for biological applications has been widely documented 26 . AgNPs are known to exhibit antibacterial properties and various research groups have investigated the mechanism of AgNP-mediated antibacterial activity 27,28 . As the size of the silver particles decreases down to the nanoscale regime, their antibacterial efficacy increases because of their larger total surface area per unit volume 27,28 . One important aspect to consider is that although efficient antibacterial agents have been developed 29,30 , they often fail to reach commercial needs owing to their complex, multi-step preparation methods and the high cost of production 6 . If the aim is to develop a general, simple (for example, single-step) procedure to make a solid surface nature materials ADVANCE ONLINE PUBLICATION www.nature.com/naturematerials 1