Nanotechnology is the development and application of nanoparticles (nanomaterials), which are pieces of matter smaller than 100 nanometers. The width of a human hair is about 100,000 nanometers, so a nanoparticle is at least 1,000 times smaller than the width of a hair. A nanoparticle cannot be seen with the human eye or with most ordinary microscopes. Electron microscopes are used to view nanoparticles. These microscopes use beams of electrons to create highly magnified images.
In nanoparticle form, many ordinary compounds such as gold or aluminum have altered chemical, physical, optical, and electronic characteristics, and the technology needed to easily and efficiently produce and apply nanoparticles is still being developed. Carbon nanoparticles, for example, can be hundreds of times stronger than steel, by some measurements. Applications that exploit the strength of carbon nanoparticles are still mainly in development, but they have been proposed as material for sports equipment and other items that require a combination of low weight and high strength.
Nanoparticles have unique properties because their surface area is large relative to their volume. They can be coated with a relatively large amount of substances, such as drugs that can be carried to target cells, or enzymes that will catalyze, or facilitate, a chemical reaction. Many nanoparticles have properties that make them suitable for use as semi-conductors, which are materials with electrical-carrying properties suitable for computers and other electronic technologies. Other proposed applications are in imaging, for example, to distinguish cancerous from normal tissue. Nanoparticles may also used in electronics, household appliances, automobiles, and cosmetics.
This wide range of uses suggests to some that an explosive growth in their applications may occur. The worldwide market for nanotechnology has been predicted to be $1 trillion by 2015, according to one estimate.
In solid form, pure carbon atoms can form graphite or diamond. In 1985, scientists at the University of Sussex in England and Rice University in the United States found that carbon can also form nanoparticles in the shape of regular, symmetrical spheres, tubes, or sheets. They named these molecules fullerenes, after Richard Buckminster Fuller, an architect whose geodesic dome buildings, made of interlocking polygons, had the same form as some of the nanoparticles. The 1996 Nobel Prize in Chemistry was awarded to the discoverers of fullerenes.
The sources of nanoparticles can be natural or human-generated. Natural sources in the environment include particular airborne matter generated by the combustion of forest fires and volcanoes. Because of their small size, viruses may also be considered nanoparticles. As any matter degrades or is destroyed by burning, minute particles of the matter may become airborne. Some of the particles may fall into the category of nanoparticles. Heating may accelerate degradation of any matter, so any surface can produce nanoparticles because airborne particles are released as the matter is heated. Therefore, incinerators, electric motors, and internal combustion engines, as well as everyday cooking, may produce nanoparticles.
The term "natural" is often used to describe nanomaterials that are biological, such as viruses, or that are produced during the natural aging or weathering of substances. The term 'ultrafine particles' (UFP) is often applied to nanoparticles that are incidental such as the unintentional airborne by-products of volcanoes or industrial or automotive processes. The terms nanoparticle, or manufactured, may be used for nanomaterials that are deliberately produced for consumer applications, although the terms are often used interchangeably. Nanotoxicology is the study of the potential hazards of nanoparticles to humans, animals, and the environment.
Air pollution is contamination of air by smoke and harmful gases, mainly oxides of carbon, sulfur, and nitrogen, produced from automobile exhausts, industrial emissions, or burning rubbish. Airborne pollutants can have serious negative effects on both indoor and outdoor air quality. Particulate air pollution is a specific type of particulate airborne matter. It contains a heterogeneous, non-uniform mixture of incidental nanoparticles that includes metals such as lead, arsenic, zinc, gold and silver; sulfate and nitrate ions; and organic materials such as the residue from wood or fossil fuels degraded by combustion. The complexity of the material being burned or consumed determines the composition of the particular airborne matter. Most of the nanoparticles in particulate air pollution are generated as unintended byproducts of industrial or manufacturing processes and vehicle exhaust.
Manufacturing: Manufacturers of nanoparticles are found globally and include large manufacturing companies like DuPont, as well as smaller, more specialized companies. Several techniques are used to generate nanoparticles, including high-tech versions of ordinary mills that grind materials using fine beads. Materials may also be ground using techniques that do not generate uniform batches of nanoparticles, and the particles of desired size removed by filtration after grinding. Nanoparticles may also be generated by precipitating or spraying particles with a solution of the desired material.
Each technique has limitations. For example, techniques that generate particles of diverse sizes may produce waste, or nanopollution. Precipitation techniques may involve chemicals that must be removed from the nanoparticles before use.
Personal and health applications: Carbon nanoparticles that can neutralize skin-damaging oxidants are used in cosmetics. Nanoparticles of titanium dioxide that can form thin, nearly transparent films are used in sunscreens. Nanoparticle silver, which may have antibacterial properties, is used in anti-odor socks because bacterial growth in sweaty socks can contribute to foot odor. Nanoparticles have been proposed as drug delivery systems, because they may often access cells that cannot be reached by other, larger particles, and some nanoparticles may be internalized into cells, enabling delivery of therapeutics directly into cells.
Applications in materials: Nanoparticles are used as coatings for some stain-repellent fabrics and scratchproof surfaces, such as eyeglasses. Films of ceramic nanoparticles on solar cells may increase their efficiency. Carbon black is a type of nanoparticle that is composed of carbon, like fullerenes, which are uniformly spherical carbon nanoparticles. In carbon black, however, the nanoparticles do not have the same symmetrical form. Carbon black molecules form grape-like clusters that can be used in rubber as a reinforcing agent found in tires, and as a pigment, for example, in printer inks. (Carbon black is not the same as other carbon products with similar names, such as black carbon or activated carbon).
Mechanism of action: When in nanoparticle form, meaning less than 100 nanometers in diameter, many ordinary compounds such as gold or aluminum have altered chemical, physical, optical, and electronic properties. Because of their small size, nanoparticles can be made into very fine, lightweight films, for example, to coat surfaces such as stain-repellent fabrics or the surface of solar cells. Because of their small size, nanoparticles may enter areas of the body that particles of larger size cannot access. Therefore, nanoparticles are being explored as delivery systems for drugs.
Pure carbon nanoparticles may form spheres, tubes, or sheets. Many have properties of great strength and superconductivity that are being explored for practical uses such as in electronics and sports equipments. Pure carbon nanoparticles may act as antioxidants that neutralize oxidants that can damage and age tissue, so they have been used in cosmetics. Current uses of pure carbon nanoparticles and nanotubes, which are nanoparticles in tube form, are somewhat specialized. For example, they are used in certain solar cells and limited electronic applications. Carbon black, a nanomaterial that is used in rubber products (e.g., tires) and as a pigment in inks, is produced by burning oil in a high-temperature furnace designed to produce carbon black.
Nanoparticles of zinc oxide or titanium dioxide (also called micronized titanium), are found in some sunscreens, where they absorb ultraviolet (UV) light. These compounds are white, so they can be seen on the skin, but as nanoparticles, they are small enough to be transparent so the nanoparticle form is used in sunscreens for cosmetic reasons. Nanoparticles used in previously available sunscreens could potentially be damaging to the skin because after absorbing UV light, they emitted the energy as photoelectrons that could speed up the production of reactive oxygen species (ROS), which are highly reactive forms of oxygen that can cause damage to cellular components like DNA. Newer versions of sunscreen nanoparticles may be modified to prevent ROS production, but product labels might not specify what types of nanoparticles are used, or even if any nanoparticles are present.
Impact: Humans have always been exposed to nanoparticles, but interest in their effect on human health is increasing because technological advances have increased their use. Manufactured nanoparticles are used in paints, automobiles, cosmetics, and toothpaste, among other applications.
The impact on human health of ingesting or absorbing nanoparticles is still being investigated and definitive data are lacking. Skin absorption appears to be limited unless the skin is injured or undergoes mechanical stress, like stretching. Once inside the body, however, the high surface area-to-volume ratio appears to make nanoparticles more difficult to clear from the body than larger particles. Some physiological mechanisms for clearing small particles involve physically sweeping them away, so a relatively high surface area may make this task more difficult. The surface-to-volume ratio may also change the properties of diffusion, meaning the tendency to migrate according to concentration, of materials. This may also affect the clearance of nanoparticles from the body.
Inhaled nanoparticles: Carbon nanoparticles in the form of nanotubes are being investigated for adverse effects when inhaled because their shape is similar to asbestos, although they are composed of a different material. When deliberately introduced into the lungs of rats or mice, they produce damage to the lungs that in some ways resembles damage caused by asbestos. The effect on humans is unclear, however, because carbon nanoparticles and nanotubes come in many different forms. Therefore, it is difficult to compare studies that investigate different forms. In addition, it is unclear what proportion of nanotubes becomes airborne under manufacture and normal use, and of that, what fraction of nanotubes humans would actually inhale.
The International Agency for Research on Cancer (IARC) classifies carbon black, a nanomaterial used in rubber and printer toners and inks, as possibly carcinogenic, or cancer-causing, to humans, based on animal studies. Short-term exposure is not known to be life-threatening.
As particulate air pollution, small particles, including nanoparticles, may cause lung inflammation when inhaled. They may be especially harmful to individuals with conditions such as asthma or chronic obstructive pulmonary disease (COPD). Pollutant particles may also have cardiovascular effects. Deliberate introduction of diesel exhaust particles, for example, in small animal studies, may cause thrombosis, or the generation of blood clots within blood vessels. Epidemiologic studies have found a link between particulate air pollution and an increase in cardiovascular disease, although the mechanism by which inhaled particles increase disease risk is still under investigation. Metal-based ultrafine particles that are a component of air pollution may induce the production of damaging reactive oxygen species (ROS).
Sunscreen: Nanoparticles of zinc oxide or titanium dioxide can be found in some sunscreens. Studies on the degree to which titanium dioxide or other nanoparticles are absorbed through the skin have come to differing conclusions.
In 2007, a Consumer Reports test found no difference in the effectiveness between sunscreens with and without nanoparticles. Of 19 sunscreens tested, eight listed zinc oxide or titanium dioxide as ingredients. Only one of these stated that it contained nano-zinc, which is nanoparticles of zinc, although all contained nanoparticles, according to the Consumer Report tests.
Nanoparticles used in previously available sunscreens could potentially be damaging to skin, since after absorbing UV light, they emitted the energy as photoelectrons that could speed up the production of reactive oxygen species (ROS), which are highly reactive forms of oxygen that can cause damage to cellular components like DNA. Newer versions of sunscreen nanoparticles may be modified to prevent ROS production, but product labels might not specify what types of nanoparticles are used, or even if any nanoparticles are present.
Manufacturing: The large-scale manufacture of nanomaterials poses a unique situation. Much of the research on the health and occupational hazards of nanoparticles has been on heterogeneous (non-uniform) mixtures of airborne particles. The unintended particles generated as part of air pollution are often referred to as ultrafine particles (UFP). The more uniformly-sized products of deliberately engineered and manufactured nanoparticles may or may not have the same effects and may or may not carry the same risks. Manufactured nanoparticles are generated in solid or liquid, rather than gaseous or airborne phases. Nonetheless, the National Institute for Occupational Safety and Health (NIOSH) recommends implementing basic control systems, such as controlled air supply systems, for workplaces where nanoparticles may be generated, while risks assessment studies continue.
FUTURE RESEARCH OR APPLICATIONS
Potential future health applications: Nanoparticles are small enough that, if inhaled or ingested, they might enter the bloodstream or lymph system and invade individual cells. For this reason, they are being explored as a possible delivery system for drugs, like anticancer therapies. Their high surface-to-volume ratio means that they can be coated with a relatively large amount of a therapeutic compound. Their ability to be taken up by cells means they may be used to deliver a drug to a target within cells, or to cells that are difficult to reach, such as those in the brain.
Gene therapy: Gene therapy is the technique of delivering human genes into a patient in order to treat or prevent an illness. For example, in many cancer cells, a mutant p53 gene contributes to the cancerous characteristics of the cell. Many inherited diseases could potentially be alleviated, or cured, by delivery of a non-mutant version of a gene to the cells that are most affected by a particular mutation.
Most gene therapy techniques are still in the experimental stage, and most recently researched gene therapies rely on viruses as delivery systems. In such cases, scientists change the genetic makeup of a viral vector so it carries normal human DNA instead of viral DNA. In other words, the virus' disease-causing genes are removed, and normal human genes are inserted. Although the viruses are engineered to be as harmless as possible, there is still a potential risk of adverse inflammation and immune reactions. Silica nanoparticles are being explored as alternative gene delivery systems, for example, delivering non-mutant p53 genes to cancer cells.
Tissue ablation: Energy-absorbing metal nanoparticles are being explored as potential anticancer therapies. Nanoparticles delivered to tumor tissues by means of antibodies, which are proteins made by the immune system that bind specific proteins, for example, tumor-specific surface molecules may coat the tumor. Delivery of energy absorbed by the nanoparticles, for example, as waves of energy called "near-infrared" energy waves may thermally ablate, or heat the tissue to a temperature that would kill the tumor cells.
Imaging: Another application based on the ability of nanoparticles to enter the body and be dispersed to various organs, is delivery systems for probes used to image, or make pictures of, specific tissues. Preliminary tests have used probes, or detectors, made of nanoparticles to visualize tumors as a potential way to diagnose cancer. Nanoparticle-delivered probes appear to produce brighter, less blurry pictures than those currently used, possibly because their small size produces a more uniform coating of the tissue.
Fullerenes: Fullerenes are pure-carbon nanoparticles of spherical shape that have numerous potential applications. Theoretically, their possible uses range from bulletproof clothing and sports equipment to gene delivery systems for gene therapy. At this time, fullerenes are not widely in use, but many applications are being explored.
Environmental impact: Research on the environmental effects of nanoparticles is being funded by the U.S. Environmental Protection Agency (EPA), among other agencies. Ultrafine particles, (UFP), such as those produced by combustion engines, volcanoes, or forest fires, may remain airborne for days and may be transported over thousands of miles.
Different types of nanomaterials have different abilities to aggregate or dissolve in water, affecting their environmental impact. Some properties of nanoparticles may be exploited for human benefit, such as the ability of titanium dioxide nanoparticles to generate microbe-killing reactive oxygen species (ROS), which are highly reactive oxygen atoms that could be used for water purification. However, the same properties may have adverse effects. For example, they may unintentionally affect environmentally important microbial populations.
This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).
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