You are here:
State-Of-The-Science Review: Everything NanoSilver and More
VARNER, K. E., A. El-Badawy, D. Feldhake, AND R. Venkatapathy. State-Of-The-Science Review: Everything NanoSilver and More. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-10/084, 2010.
Silver has been valued throughout history for many of its properties that are useful to humans. It is used as a precious commodity in currencies, ornaments, jewelry, electrical contacts and photography, among others. One of the most beneficial uses of silver has been as a potent antibacterial agent that is toxic to fungi, viruses and algae. Silver has long been used as a disinfectant; for example, the metal has been used in treating wounds and burns because of its broad-spectrum toxicity to bacteria as well as because of its reputation of limited toxicity to humans. In nanotechnology, a nano particle is defined as a small object or particle that behaves as a whole unit in terms of its transport and properties. Nanotechnology takes advantage of the fact that when a solid material becomes very small, its specific surface area increases, which leads to an increase in the surface reactivity and quantum-related effects.
The physical and chemical properties of nanomaterials can become very different from those of the same material in larger bulk form. Nanomaterials (such as nanotubes and nanorods) and nanoparticles are particles that have at least one dimension in the range of 1 to 100 nm. Nanoparticles are classified solely based on their size, and may or may not exhibit size-related properties that differ significantly from those observed in bulk materials (ASTM, 2006; Buzea et al., 2007). Due to the properties of silver at the nanoscale, nanosilver is nowadays used in an increasing number of consumer and medical products. Nanomaterials are nanoparticles that have special physicochemical properties as a result of their small size (Buzea et al., 2007).
Silver has been known to be a potent antibacterial, antifungal and antiviral agent, but in recent years, the use of silver as a biocide in solution, suspension, and especially in nano-particulate form has experienced a dramatic revival. Due to the properties of silver at the nano level, nanosilver is currently used in an increasing number of consumer and medical products. The remarkably strong antimicrobial activity is a major reason for the recent increase in the development of products that contain nanosilver. Of the more than 1000 consumer products that claim to contain nanomaterials, more than a quarter of them contain nanosilver. Examples of consumer products that contain nanosilver include food packaging materials, food supplements, textiles, electronics, household appliances, cosmetics, medical devices, water disinfectants, and room sprays. While most of these nanosilver-containing products were in the past manufactured in North America, manufacture of nanosilver-containing products is shifting to the Far East, especially China, South Korea, Taiwan and Vietnam.
Currently, tracking products that contain nanosilver is getting to be difficult because the products are almost always packaged under numerous brand names, and current labeling regulations do not require that the nanomaterial be listed as an ingredient. Knowledge of silver nanomaterials synthesis methods is important from an environmental perspective. This information allows for the identification of characteristics and morphologies of the produced silver nanomaterials that are crucial for a more focused approach when evaluating their environmental fate, transport and toxicity. The main challenge in nanomaterials synthesis is the control of their physical properties such as obtaining uniform particle size distribution, identical shape, morphology, chemical composition and crystal structure.
There are an extensive number of synthesis methods of silver nanoparticles that are readily available in the literature. All reported methods can be classified and categorized since they all follow common approaches and the differences are limited to the specific reactants used and the reaction conditions. Categories such as top-down versus bottom-up, green versus non-green and conventional versus non-conventional have been reported. Physical methods such as milling or attrition, repeated quenching and photolithography are usually involved in the top-down strategies while bottom-up techniques start with silver salt precursor that is reduced in a chemical reaction. Synthesis methods can also be grouped under conventional and unconventional methods.
Conventional synthesis methods include the use of citrate, borohydride, two phase (water-organic) systems, organic reducers, and inverse micelles in the synthesis process. Unconventional methods include laser ablation, radiocatalysis, vacuum evaporation of metal, and the Svedberg method of electrocondensation.