A look at a nanomaterial recently demonised by environmental campaigners despite its safe use for decades. However, this is only part of the story.
The rapid development of nanosilver products is based on this material’s unique physical properties. Nanosilver is the nanoform of silver characterized by being spherical particles of a size ranging from 1-250 nanometres (nm). It is commercialized as powder, flakes, grains, ingots, etc., and is sold in suspension (in water, alcohol or surfactant) and as a dry powder. Nanosilver is furthermore available in preparations (e.g. as a coating agent, in alloys, etc.) and in articles (electrodomestic appliances, in textiles, in food packages, etc.).
Environmental and public pressure organizations have targeted nanosilver as a key area in their campaign against the use of new nanomaterials. Nanosilver is in fact, despite its scale, not a new material and has been regulated and used safely for decades. Nanoscale silver has long been registered under biocide regulations, with over 50% of all EPA registered silver products based on nanoscale silver.
Elemental silver has been used for thousands of years as a powerful antibacterial, antifungal and antiviral agent, but in the past few years, the use of nanosilver as a biocide has experienced a revival. Due to the unique properties of silver at the nanoscale it is widely used in a number of consumer and medical products, mainly taking advantage of its high anti-microbial activity. Applications currently on the market include wound dressings, personal care products, powdered colours, varnish, textile, paper, interior and exterior paints, printing colours, water and air purification, dietary supplements, polymer-based products and foils for antibacterial protection such as washing machines and medical device coatings (implants and catheters). There are several hundred consumer and industrial products currently available that include nanoscale silver. Products on the markets include Acticoat Wound Care with Nanocrystalline Silver and I-Flow SilverSoaker Nanosilver Catheters. However these materials are currently viewed as representing one of the largest potential sources of public exposure to nanomaterials and the potential detrimental effects thereof.
PROPERTIES OF NANOSILVER
ANTI-BACTERIAL
Nano silver is a powerful and natural antimicrobial agent that has been proven highly effective in fighting against a broad spectrum of Gram-negative and Gram-positive bacteria. Nano silver is presumed to exert its antimicrobial effect through the dual mechanisms of denaturation and oxidation. The use of elemental silver as an anti-bacterial agent goes back thousands of years. The ancient Egyptians, Romans and Chinese all used silver in for its anti-bacterial properties. Acting as a catalyst, it reportedly disables the enzyme that one-celled bacteria, viruses, and fungi need for their oxygen intake without causing corresponding harm to human enzymes or other parts of the human body chemistry. The result is the destruction of disease-causing organisms without any detrimental effects on the surrounding human tissue. Silver is known to kill over 650 types of bacteria.
ANTI-FUNGAL
Nanosilver is an effective, fast-acting fungicide against a broad spectrum of common fungi. Although the exact mechanism against fungi is unclear, it is believed they are similar to those associated with the anti-bacterial action.
ANTI-VIRAL
iSilver has long been known for its antimicrobial properties, but its medical applications declined with the development of antibiotics. However, nanosilver is a very effective anti-viral agent, inhibiting viruses from binding to cells. It has been proven to exert antiviral activity against HIV-1 at non-cytotoxic concentrations. Silver’s mode of action is presumed to be dependent on Ag+ ions, which strongly inhibit bacterial growth through suppression of respiratory enzymes and electron transport components and through interference with DNA functions.
ANTI-INFLAMMATORY
Nanocrystalline silver dressings with anti-inflammatory activity have been commercially available for more than a decade. They improve wound healing via potent anti-inflammatory activity. Nanosilver modulates cytokines involved in wound healing.
SURFACE PLASMON RESONANCE
Silver nanoparticles can be used in liquid radiation detectors due to the irradiation-induced surface plasmon resonance (SPR) phenomena they exhibit. Decreasing nanoparticle size (diameter ≤ 10 nm) is associated with a red-shift and broadening of the plasmon-related absorption peak.
PLASMONIC HEATING
Silver nanoparticles have been remotely activated using laser irradiation, causing not only absorption of photons but also heat transfer from the nanoparticles to the surrounding polymer matrix for drug delivery applications. The local heating disrupts the polymer matrix and allows the encapsulated material/drug to be released.
METAL-ENHANCED FLUORESCENCE
Silver nanomaterials have been utilized for metal enhanced fluorescence applications. The intrinsic spectral properties of fluorophores can be altered by metallic nanostructures. The proximity of metallic nanosilver results in an increase in the intensity of low quantum yield Applications include immunoassays and DNA/RNA detection.
APPLICATIONS OF NANOSILVER
CATALYSIS
Nanosilver is attractive for catalytic applications as their high surface area to volume ratio of the material provides high surface energy, which promotes surface reactivity such as adsorption and catalysis. Nanosilver has been to catalyse many reactions in industrial processes such as CO oxidation, benzene oxidation to phenol, photodegradation of gaseous acetaldehyde and the reduction of the p-nitrophenol to p-aminophenol.
PACKAGING
Nanoscale silver is being developed as an anti-microbial and bacteriostatic agent for food and non-food packaging. They have been incorporated into biodegradable films in food packaging. The use as anti-microbials in food packaging has been highlighted as potentially releasing nanosilver into the environment as well as increasing bacterial resistance.
WATER TREATMENT
Twenty per cent of the world’s population is currently living without access to safe water for drinking, personal hygiene, and domestic use and 80% of human disease is due to contaminated drinking water. The anti-bacterial properties of nanosilver make it attractive for application in water purification and treatment. The United Nations Environment Program (2004) has listed silver-treated ceramic water filters as an appropriate technology for water purifications in developing countries and regions affected by natural disaster. Nanosilver coated water filters use two mechanisms to disinfect water. The first is by filtration; any harmful microorganisms or particles larger than 1 μ m are removed from the water. The second mechanism is by colloidal silver induced antibacterial action. Nanosilver is also used to treat water in swimming pools. One product on the market is Home Water Purification Floatron, a small solar-powered ionization product, which uses silver and copper ions to purify and soften water in swimming pools.
TEXTILES
The market for antimicrobial textiles has grown greatly in the last few years, driven by the increased need of consumers for fresh, clean and hygienic clothing. Nanosilver is used widely in anti-microbial textiles finishes. Products on the market include nanosilver coated socks that kill the bacteria associated with foot odour. Textile applications are one of the main areas that are most open to regulatory scrutiny. Several studies have shown that widespread use of silver nanoparticles in consumer products, especially textiles, likely results in the distribution of nanoparticles in lakes and streams via leaching into wastewater during washing. A 2008 study showed that socks lose nearly all of their silver content within a few washings. In January 2012, The Natural Resources Defense Council filed a lawsuit in the US federal court against the U.S. Environmental Protection Agency (EPA) in a bid to limit public exposure to antimicrobial nanosilver used in clothing and other textiles such as blankets, pillow cases and interior textiles.
MEDICINE
Nanosilver has been incorporated in numerous medical applications including diagnostics, wound care, drug delivery, medical devices and coatings. Nanosilver coated polymers have been incorporated into plastic catheters, for their antibacterial and disinfecting effect. Nanosilver is also used as an antibacterial additive for poly (methyl methacrylate) (PMMA), used in bone implants. Nanoparticle silver is also used in wound dressings, diabetic socks, scaffolds, sterilization materials in hospitals, medical textiles, and contraceptive devices. Nanocrystalline silver wound dressing inhibit the growth of bacteria allowing wounds to heal more quickly. Nanosilver is also used in dentistry for making artificial teeth and in eye care for coating contact lenses. More advanced applications include imaging of cell cancers; inhibition of the HIV-1 virus from binding to the host cells through the use of silver nanoparticles and also in drug delivery through plasmonic photoactivation of hollow polyelectrolyte-multilayer capsules incorporating silver nanoparticles and containing drug molecules.
ELECTRONICS
Nanosilver materials are under development for replacing conventional silver powders and flakes in inks, pastes, and adhesives in conductive circuitry or electrodes. Their usage is a result of their high electrical and thermal conductivity and enhanced optical properties. Applications include the preparation of active waveguides in optical devices, inks for printed circuit boards, optoelectronics, nanoelectronics (such as single-electron transistors, and electrical connectors), subwavelength optics, data storage devices, nonlinear optics, high density recording devices, intercalation materials for batteries, making micro-interconnects in integrated circuits (IC) and integral capacitors. Silver inks are used to replace wires and act as flat wires in printed circuit boards. In addition, silver inks are also used in solder for circuit connections.
PAPER
Nanosilver is being utilized in the paper industry. DocuGuard uses silver-based paper to protect hospital case notes and medical files against the proliferation of bacteria. The company is also exploring future applications including business stationery, envelopes, brochures and book-binding materials.
NANOSILVER INKS
Nanosilver is being used in conductive inks with low processing temperatures for printing electronic devices on plastic or paper. Companies are developing nanosilver inks for RFID antennas. Methode Electronics produces a nano silver ink that can be printed on several types of media including various coated polymer films.
TRANSPARENT CONDUCTORS
Silver has the highest electrical conductivity of all metals, high thermal conductivity and the lowest contact resistance. Nanosilver is being developed as an alternative to sputter-deposited ITO transparent conductors. Companies involved in product development in this market include Blue Nano, Cambrios Agfa, Blue Nano, Carestream Advanced Materials, Cima Nanotech, Dow Chemical PolyIC, Ferro, Saint-Gobain, Sigma Technologies, Suzhou NanoGrid Technology and Sumitomo Metals and Mining.
