Renewable resources, such as plants and crops, are inexhaustible and clean sources of materials that, when used in industrial processes, often produce by-products. Strategic utilization of such industrial by-products (i.e., biomonomers) as starting materials for generating value-added products and building blocks in chemistry will have broad impact in industrial economy as well as in sustainable development (Lichtenthaler 2002, Corma 2007, Goldemberg 2007). The efficient utilization of renewable resources is possible for developing novel monomers, polymers, chemicals, and soft nanomaterials (John, Soft Matter 2006, Vemula, J. Am. Chem. Soc. 2006, Vemula 2007, John 2001, John 2002, John 2004, John, Angew. Chem. Int Ed. 2006, Rostrup-Nielsen 2005, Pagliaro 2007, Biermann 2000). Polymers are among the most important products of the chemical industry and are used for versatile applications in everyday life. Employing agricultural/industrial by-products in polymer applications (for instance, the production of packaging, textiles and other functional materials) will be highly advantageous due to their properties of being renewable and biodegradable. Additionally, these biomonomers may be converted into valuable polymers or novel amphiphiles to produce soft nanomaterials.
A primary by-product of cashew nuts is cashew nut shell liquid (CNSL), which is extracted from the by-product shells of the cashew nut. One can synthesize free radically polymerizable monomers from cardanol (a compound derived from CNSL) by simple modifications and then polymerize them for use in coating applications (John 1992, John 1993).
In addition to CNSL oil, another polymerizable oil is vegetable oil. 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 (Daniel 1964, Metzger 2006, Bieleman 2000), which undergo free radical-mediated autoxidation during the curing/drying process (Black 1978, Reich 1969) (FIGS. 1a-c). The use of naturally generated free radicals enables one to generate valuable oil-based products.
Coatings can be used to decorate or protect surfaces of interest (Bohannon 2005, Crisp 2003, Klaus 1999). 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 (Black 1978, Reich 1969) (FIG. 1c). In addition, literature reports suggest that free radicals are known to reduce metal salts to their uncharged MNPs (Zhang 2006, Okitsu 1997). Free radical-induced MNP synthesis is well studied (Zhang 2006, Okitsu 1997).
Several methods have been reported for the preparation of organic-inorganic hybrid materials; and most of the techniques used to incorporate metals into polymeric matrices involve either chemical reactions such as reduction (Aymonier 2002), mixing preformed metal nanoparticles with polymers (Liu 2005), or complicated physical techniques (Heilmann 2002), such as sputtering (Dowling 2003), plasma deposition (Jiang 2004), and layer-by-layer deposition (Dai 2002). All of these techniques add time, cost, multistep synthesis, and complexity to the overall process of fabricating metal-particle-doped materials.
Metal nanoparticles have attracted a great deal of attention because of their unusual optical and electronic properties (Colvin 1994) with potential application in the area of catalysis, (Hoffman 1992) electron microscopy markers, (Baschong 1990) gene therapy (Elghanian 1997) and sensors. (Shipway 1999) Recent interests focused towards developing new applications of nanoparticles having antifungal, antibacterial properties and can be used as coating materials or packaging materials. Attempts have been made to design such materials by embedding a antimicrobial agent in existing well known coating or packaging materials. Silver nanoparticles are known for its antibacterial properties have been used in fabrics, polymer for various applications. (Prashant 2005, Wang 1994, Chou 2005) Prashant et al. attached the silver nanoparticles on the surface of polyurethane foam and used it for water filter to avoid the bacterial contamination of surface water. (Prashant 2005) Wang et al prepared the antibacterial utltrathin film of titanium phosphate containing silver nanoparticles. (Wang 1994) Antibacterial cellulose acetate has also been made by incorporating silver nanoparticles in cellulose acetate based membrane. (Chou 2005)
Silver nanoparticles have also been used to incorporate coating materials to make antibacterial paints. In most of the approaches, either nanoparticles were synthesized separately and attached to different support, or silver ions were reduced in the presence of support using external reducing agent. Perhaps the same process can be used for the synthesis of nanoparticles and integrating them in coating materials for different application. Recently Willner and co-workers formed super lattice of citrate stabilized gold nanoparticles and cyclobis (paraquat p-phenylene) on the ammonium-functionalized indium tinoxide (ITO) surface using electrostatic interaction. (Shipway 1999) The Au nanoparticles in the super lattice provide a rough conductive array for the electrochemical sensing of the π-donor aromatic compound. Mirkin and co-workers used the optical properties of gold nanoparticles for the detection of DNA down to a concentration of 50 fM. (Taton 2000) Gold show catalytic activity for the oxidation of carbon monoxide at nanoscale at higher temperature (Haruta 1988). This catalytic activity is due to high surface free energy of nanoparticles, which makes them useful for protective gas masks and household room air fresheners etc. Gold particles have also been recognized as good catalyst for water gas shift reaction, propylene epoxidation, and benzene oxidation etc. (Bond 1999).
Much of the recent research focused on developing metal nanoparticles-based flat panel displays, radio frequency identification tags, sensors and other disposable electronics. Future technology demands the organic substrate based devices which can be fabricated entirely by printing to reduce the costs associated with lithography, vacuum processing and ultra clean room conditions. The main challenge is to use the low temperature conductor suitable for printing and inkjet printing technology compatible to fabricate at low temperature on low cost plastics. Metal nanoparticles have also been investigated for the electronic applications because of possibility of their use in printing circuits on plastic. (Huang 2003) The low resistance circuits were fabricated on plastic using alkanethiol protected metal nanoparticles dispersion as an ink at lower temperature.
Other than electronic application, the nanoparticles have also been used as a pigment in paints due to surface plasmon resonance in the visible region. More precisely, gold and silver nanoparticles have been known as an artistic ruby and yellow colorant for stained glass and fine glassware, due to their inherent surface plasmon absorption. The ruby red or yellow color of the stained glass is stable for hundreds of years. In contrary, the red color of organic dyes in traditional paints often fades away within several years due to the short lifetime of typical organic compounds. Nippon Paint has recently developed the technology for the use of paints for cars, based on a polymer-stabilized gold colloid. This paint appears black in shaded areas and red in illuminated areas, giving a dynamic effect as the vehicle is in motion due to varying light conditions. (Iwakoshi 2003) Use of this type of dynamic color effect could be envisaged for use in security devices, such as ‘watermarking’ of valuable or confidential documents, and biomedical testing kits. Titanium dioxide nanoparticles have been used in paints as a whitener as well as photo-active catalyst for hygiene or self cleaning application.
Another advantage of using metal nanoparticles in paints is their high reflectivity of infrared radiation. The heat-loss occurs in three ways: convection, conduction and radiation. Insulation is quite effective to reduce the heat loss due to convection and conduction however it have very little effect on heat loss due to radiation. Metal nanoparticles (Ag, Au, Al, Cu, Rh) have reflectivity of 98-99% in the infrared (IR) portion of the spectrum so paints containing metal nanoparticles will increase the reflection of radiant heat. Therefore use of such kind of paints inside of exterior will reduce heat loss by radiation. Current paints manufactured by ChemRex are claimed to reflect 30% of the radiant incident heat. The radiation of a room at 70° F. will peak at a wavelength of 10 microns, according to the black-body equation. Calculation, based on refractive indices of the particles and the paint, and a wavelength of 10 μm and particles size, shows that the reflectivity (scattering) increases linearly with the particle number density but not with the particle size.
Optical behavior of nanoparticles can be tuned by tailoring the shape of the particles. For example the optical absorption of gold nanoparticles can be tuned from visible region to near-infrared (NIR) region of electromagnetic spectrum by tailoring the shape of the particles from spherical to rod or triangular shape. The NIR absorption of the gold nanotriangles is expected to be having applications in hyperthermia of cancer cells and in IR-absorbing optical coatings. Sastry and co-workers showed that triangular gold nanoparticles coated glass films are highly efficient in absorbing IR radiation for potential architectural applications where the temperature in a compartment need to control due to expose of an infrared radiation. (Shankar 2005)
Most of the methods demand the synthesis of metal nanoparticles at large scale. Therefore it is most important to have a protocol for the synthesis of metal nanoparticles dispersion at large scale with precise control over the particles size and high metal concentration, yet most importantly keeping low production cost. Preparation of monolayer protected gold nanoparticles was achieved using the method previously documented by Brust et al. (Brust 1994) The key requirement for the scale-up of the nanoparticles was to reduce solvent levels used during the preparation stages. For example, it was reported that to prepare ca 0.25 g of the thiol-stabilized nanoparticle according to the Brust method would require ca 80 mL of toluene and 800 mL of ethanol for precipitation and purification. By extrapolation, 3 kg of product was reported to require 960 L of toluene and 10,000 L of ethanol. This was considered impractical for commercial production. With the described modifications, 0.5-1 kg quantities of gold chloride could be used to produce nanoparticles in 20 liter reaction vessels that were consistent in gold assay of the final product and also analytically similar for each batch. (Bishop 2002) The success of this technology is thought to be due to the low mobility of these nanoparticles during the early stages of heat treatment (50-150° C.) and also to their tendency to self-assemble and form ‘loose’ gold films before thermal decomposition of the stabilizing thiol ligand occurs.
Drying oils/alkyd resins are known as one of the oldest and the cheapest coatings materials and have attracted renewed interest because they are from renewable resources, like plant oils and independent of limited supply of petroleum-based products. Alkyd emulsions and high solid alkyd resins have shown a lot of success fulfilling the environmental demands. Moreover, life-cycle analysis of alkyd emulsion paints showed less effect on the environment than those based on acrylic dispersions. The possibility to obtain versatile, low cost, renewable, and low VOC emission products makes alkyd paints very attractive materials.
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 (Sambhy 2006, Lansdown 2002, Kenawy 2007). Silver-based antimicrobial agents receive much attention because of the low toxicity of the active Ag ion to human cells (Williams 1989, Berger 1976), 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 (Alt 2004), and their use for biological applications has been documented as well (Podsiadlo 2005). AgNPs are known to exhibit antibacterial properties and various research groups have investigated the mechanism of AgNP-mediated antibacterial activity (Morenes 2005, Gogoi 2006). 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 (Morenes 2005, Gogoi 2006).
One important aspect to consider is that although efficient antibacterial agents have been developed (Haldar 2006, Lewis 2005), they often fail to reach commercial needs owing to their complex, multi-step preparation methods and the high cost of production (Bohannon 2005). If the aim is to develop a general, simple (for example, single-step) procedure to make a solid surface bactericidal, then covalent attachment of polymers is probably not a viable option given the paucity of derivatization-amenable functional groups on most common surfaces.
Typically, nanoparticle synthesis involves external reducing agents and toxic organic solvents, which pose potential environmental and biological risks. Except for a few reports (Naik 2002, Raveendran 2003), it is difficult to find fully environmentally friendly methods for MNP synthesis.
Polymer-stabilized MNP composites (Morones 2007, Abyaneh 2007) are known to exhibit enhanced physicochemical stability, electrical and optoelectronic properties (Daniel 2004, Shan 2005). These composites are prepared either by simple entrapment of gold and silver nanoparticles (AuNPs and AgNPs, respectively) in a pre-synthesized polymer. Typically, the polymers have a thiol or a thiolate end group and are allowed to self-assemble on the MNPs' surface. The self-assembly occurs as a result of the specific interaction of the sulfur end-group with the surface (Hotchkiss 2007, Liu 2007, Fustin 2006). Another approach to creation of MNPs involves the reduction of gold salts with sodium borohydride in the presence of thio (Zheng 2004, Shimmin 2004) or dithioester (Shan 2003) end functionalized polymers. The reaction yields hybrids with AuNPs within the polymer shell. Physical entrapment of MNPs, however, has obstacles. For example, physical entrapment often produces heterogeneous hybrid materials. Importantly, it requires separate synthesis and purification of NPs and external doping into polymers (a multi-step process).
Oxidative drying of polyunsaturated oils is well known. 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.
The three main steps in the preparation of MNPs involve the choice of the solvent medium used for the synthesis, the selection of an environmentally benign reducing agent, and the selection of a non-toxic material for the stabilization of the MNPs (see Anastas 1998). Although there are several known reducing agents, the majority of processes reported so far use reducing agents such as sodium borohydride (NaBH4) and hydrazine (NH2—NH2). All of these are highly reactive chemicals and raise potential environmental and biological risks. Another and perhaps the most important issue is the choice of a capping agent to protect and passivate the nanoparticle surface, for better dispersion of MNPs.
Previously, novel organic-inorganic hybrid nanomaterials were prepared using self-assembled hydro/organogels (Vemula 2007, Vemula, Chem. Commun. 2006) and LCs as media for in situ synthesis of various MNPs (Zhang 2006, Okitsu 1997, Okitsu 1996).
In a prior art process, silver nanoparticles have been dispersed/incorporated in silicon rubber to achieve an antimicrobial effect, but in an amount less than cytotoxic silver concentration (U.S. Pat. No. 6,822,034). Silicon rubber is used in applications which include, for example, pan grips, camera eye caps, handles of bicycles, slipping preventative for spectacles, various rubber sheets and rubber coated cloth such as sheets and curtains that are used, for example in hospitals.
In another process, silver nanoparticles in organic matrix have also been used for antimicrobial activity for body care products (U.S. Pat. No. 6,720,006). A suspension containing silver nanoparticles with an individual size range of 5 to 50 nm was produced through thermal evaporation of silver into a liquid silicone oil base. Polypropylene granules are then co-extruded with this silicone oil using a Werner & Pfleiderer equipment to produce polypropylene granules containing up to 5% of the silver containing silicone oil. This master material was made into top sheets for diapers containing approximately 1000 ppm silver. The ELISA measurements demonstrated antibacterial efficacy.
Synthesis of nanoparticles (U.S. Pat. No. 6,974,493 and U.S. Pat. No. 6,929,675) in nonpolar medium is available. Harutyunyan, et al. (U.S. Pat. No. 6,974,493) synthesized the metal nanoparticles by heating or refluxing a mixture of two or more metal salts, such as metal acetates, and a passivating solvent, such as glycol ether, at a temperature above the melting point of the metal salts for an effective amount of time. Bunge, et al. (U.S. Pat. No. 6,929,675) followed different strategy which involves the thermal decomposition of organomettalic complexes of metal in organic phase. In this method, a solution of (CU(C6H2(CH3)3)5, (Ag(C6H2(CH3)3)4, or (Au(C6H2(CH3)3)5 is dissolved in a coordinating solvent, such as a primary, secondary, or tertiary amine; primary, secondary, or tertiary phosphine, or alkyl thiol, to produce a mesityl precursor solution. This solution was decomposed by injecting it into an organic solvent heated to a temperature of approximately 100° C.
In yet another process, organically functionalized metal nanoparticles have been synthesized by mixing a metal precursor with an organic surface passivant and reacting the resulting mixture with a reducing agent to generate a free metal while binding the passivant to the surface of the free metal to produce organically functionalized metal particles (U.S. Pat. No. 6,103,868).
There is a need for a simpler, environmentally friendly process of preparing MNP-embedded materials. Accordingly, an objective of the present invention is the preparation of potent antibacterial coatings in a single step at ambient conditions without using external reagents or excessive energy for practical applications. Capitalizing on the versatility and reliability of oils (such as oil-based paints), the present invention uses an oxidative drying mechanism (lipid autoxidation) in the presence of metal salts to generate and stabilize MNPs (e.g., AgNPs) in oil, which competes, e.g., with previously implemented AgNP-based bactericidal agents (Morones 2005, Gogoi 2006). The process of the present invention thus provides an environmentally friendly method for making antimicrobial coatings containing metal nanoparticles.