Silver Nanoparticles: Past, Production, and Potential
Silver metal. (Source:Mineral Information Institute)
Contents
History
Silver has a long history of anti-microbial use, dating back to the Phoenicians who used silver as a natural biocide to coat milk bottles, and more recently the metal is finding use in the form of silver nanoparticles. Silver’santimicrobial properties have also been exploited in both medicine and in the home. Burn patients sometimes use silver sulfadiazine creams to prevent infection at the burn site and at least one appliance company has incorporated silver into their washing machines.[i] Currently silver is used in the expanding field of nanotechnology and appears in many consumer products. Consumer products that use silver nanoparticles, or AgNPs, include; acne creams, baby pacifiers, computer keyboards, clothing that protects from emitting body odor (e.g. socks and athletic wear), and deodorizing sprays.
Function and Toxicity
The antimicrobial properties of silver are associated with silver ions. An ion is typically measured in pico-meters, where one pico-meter is equal to 1 × 10-12 meters. A nanoparticle is measured in nanometers, where one nanometer is equal to 1 × 10-9 meters. Nanoparticles have diameters ranging from 1-100nm. Although researchers and scientists are still exploring the specifics behind how the nano-sized silver (AgNP) releases silver ions, the nanoparticles, which are non-ionic, allow for these silver ions to be delivered more effectively and they provide a larger surface area for release. For example, the nanoparticle size allows for the antimicrobial properties of silver ions to coat products, surgical tools, etc. for sterilization purposes. However, as these products are being used, consumers may be exposed to increasing amounts of silver, or silver in a novel way, because the nanoparticle delivery allows for deeper penetration into more sensitive tissues. The size of these particles enables them to move easily through cell membranes including lung tissue and cells.[ii] The daily exposure limit for all forms of silver, as determined by the National Institute for Occupational Health and Safety, is 0.01 mg/m3. Silver (not necessarily silver nanoparticles) decreases the function of mitochondrial organelles in eukaryotic species.
Silver Nanoparticles in the Environment
In addition to biomedical concerns and toxicity associated with silver nanotechnology, as nanoparticle technology increases in coming years, it is important to also understand its behavior in a range of ecosystems. This includes: the cycling of AgNPs through the environment; the introduction of AgNPs into different systems; movement from one system into another (e.g. from the atmosphere into an aquatic system); the effects of AgNPs in those systems; and accumulation. Where and how AgNPs are released is heavily dependent on the method of production, though many of the studies on AgNPs and the environment have been based on industrial production and incineration. Additionally, the level of toxicity and environmental fate of AgNPs is dependent on characteristics of the particle, such as shape and size, which can change with method of production or synthesis.[iii] Nanoparticles can enter the environment as terrestrial, aquatic, and atmospheric contaminants.
A recent study on the aerosolization and release of silver nanoparticles indicated several different ways AgNPs may enter the atmosphere, including suspension as individual particles, chemical reaction with other molecules, and becoming coated with other compounds that subsequently condense. The study also summarized consumer products that claim to use nanotechnology with their potential for aerosolization (rated from “none” to “high”). Of these, disinfectant sprays, humidifiers, and hair dryers exhibited the highest potential for aerosolization of AgNPs, while food containers, mineral supplements, and hardware exhibited the lowest. [iv]
The main routes of introduction of AgNPs into aquatic environments are from waste streams produced during production, use, or disposal of products containing AgNPs and from atmospheric deposition.[v] Understanding the effects of AgNPs in aquatic environments may become increasingly important as synthesis and use of nanoparticle technology grows.
Studies are underway to determine the biomagnification and bioaccumulation of silver nanoparticles in the environment, and in particular aquatic ecosystems. As consumer products such as silver-nanoparticle coated socks and acne creams are washed (or washed off in the case of facial creams) the silver nanoparticles enter the wastewater system. Silver as an element is water-soluble and thus readily enters the wastewater system. Studies of sewage sludge have shown that when AgNPs enter the wastewater system they turn into silver sulfide nanoparticles, which are highly water insoluble.[vi] Studies on the toxicity of silver nanoparticles or silver sulfide nanoparticles are underway.
Trout and other aquatic species are currently being used as an indicator species to model the effects of silver nanoparticles. Although experimental settings use higher concentrations of AgNPs than would be found in natural settings, the results of such experiments in trout suggest that there is a correlation between silver nanoparticle treatments and cell viability which indicates preferential accumulation of particles in the liver of trout.[vii] Other studies have attempted to identify differences in toxicity associated with different routes of exposure in fish. One study suggests that ingesting agglomerated nanoparticle material (particles stuck together) may be more toxic than uptake via gills,[viii] while the mechanism of toxicity of silver nanoparticles seems to be associated with silver ions, rather than the particles.
Nanosilver Synthesis
Nanosilver is synthesized by either “controlled synthesis” or “green synthesis”. Controlled synthesis utilizes a two-step reduction process, while green synthesis entails a three-step process.
In controlled synthesis, a strong reducing agent, such as borohydride, is applied to the starting material (usually AgNO3) to form small silver particles in step one. A weaker reducing agent is employed to enlarge these small silver particles through further reduction in step two. This two-step process is used in place of a one-step reduction because it is easier to control synthesis for larger silver nanoparticles. Researchers often perform nanoparticle synthesis in the presence of stabilizers to keep nanoparticles from aggregating.
In green synthesis, a solvent is selected and applied in step one. A reducing agent is utilized in step two that is relatively non-toxic to the environment (e.g. water). A nontoxic stabilizer is applied in step three. Researchers choose solvents, reducing agents, and stabilizers based on green criteria-components that will have no adverse environmental effects. Green synthesis can be split into several subcategories, as described in the examples below. Note that these categories are not necessarily inclusive, but serve as examples.
Polysaccharides
Polysaccharides are long carbohydrate molecules that are soluble in water. Thus, water is used as a solvent in the polysaccharide synthesis method. They can be used both as a reducing agent and as a stabilizer in silver nanoparticle synthesis. This synthesis method produces highly stable silver nanoparticles.
Tollens
This method relies on a modified Tollens synthesis reaction. The synthesis reaction is shown in the equation below. Saccharides are carbohydrates that, in the presence of ammonia (the solvent), reduce silver ions to form silver nanoparticles. Two surfactants are added to stabilize the nanoparticles: sodium dodecyl sulfate (SDS) and polyoxyethylenesorbitane monooleate. This method is advantageous because it produces size-controllable silver nanoparticles in a single synthesis step.
Ag(NH3)2+ (aq) + RCHO (aq) --> Ag (s) + RCOOH (aq)
Irradiation
Irradiation synthesis does not require a reducing agent. Within this method, there are a number of techniques. One uses a laser to irradiate a silver salt and surfactant aqueous solution. The surfactant stabilizes the silver nanoparticles. This forms AgNPs of well-defined size and shape. A photosensitization technique also uses a laser to synthesize AgNPs using benzophenone. In photosensitization, low laser powers and short irradiation times yield larger AgNPs (~20 nm) and increased laser powers and longer irradiation times produce smaller AgNPs (~5 nm). Ionizing radiation reduces silver ions to form silver nanoparticles. In general, it is more difficult to control silver nanoparticle size and shape for this method than for controlled synthesis.
Biological
Microorganisms like fungi and bacteria can create metallic nanoparticles via their metabolic systems. An example is Pseudomonas strutzeri. This bacteria exists naturally in silver mine material. It reduces silver ions as it metabolizes them to form silver nanoparticles. Biological nanosilver synthesis is relatively quick, which makes it a desirable method. However, unlike other synthesis methods, less is known about the system mechanism that makes this possible.
POMs (polyoxometalates)
POMs are usually anions composed of transition metals. They dissolve in water, so water is an appropriate solvent for POM synthesis. Because they are anions, they can donate electrons to silver ions (Ag+) to reduce them. They can also stabilize the silver nanoparticle product. POMs are versatile in that the POM determines the size and shape of the synthesized nanoparticle. Thus, different POMs can be chosen to make silver nanoparticles of varying sizes and shapes.
The synthesis method choice is largely dependent on the desired shape and size of the AgNPs. In controlled synthesis, different reducing agents can be used to form rod-, sphere-, or cone-shaped Ag NPs. Smaller nanoparticles are more reactive due to increased surface area. This makes them more desirable than their larger counterparts for treating antibiotic resistant strains of Staphylococcus aureus, which plague hospitals, for example. In general, size and shape of silver nanoparticles determine their chemical properties, so synthesis is a crucial step in safely using these nanomaterials.
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