how is the mirror made

This article is about wave reflectors (mainly, specular reflection of visible light). For other uses, see Mirror (disambiguation

A mirror, reflecting a vase

A mirror or looking glass is an object with at least one polished and therefore specularly reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or diminished images or focus light or simply distort the reflected image

Mirrors are commonly used for personal grooming (in which case the old-fashioned term "looking-glass" can be used), decoration, and architecture. Mirrors are also used in scientific apparatus such as telescopes and lasers, cameras, and industrial machinery. Most mirrors are designed for visible light; however, mirrors designed for other types of waves or other wavelengths of electromagnetic radiation are also used, especially in non-optical instruments

History

A sculpture of a lady looking into a mirror, India

The first mirrors used by people were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass. Examples of obsidian mirrors found in Anatolia (modern-day Turkey) have been dated to around 6000 BC. Polished stone mirrors from central and south America date from around 2000 BC onwards.[1] Mirrors of polished copper were crafted in Mesopotamia from 4000 BC,[1] and in ancient Egypt from around 3000 BC.[2] In China, bronze mirrors were manufactured from around 2000 BC,[3] some of the earliest bronze and copper examples being produced by the Qijia culture

Metal-coated glass mirrors are said to have been invented in Sidon (modern-day Lebanon) in the first century AD,[4] and glass mirrors backed with gold leaf are mentioned by the Roman author Pliny in his Natural History, written in about 77 AD.[5] The Romans also developed a technique for creating crude mirrors by coating blown glass with molten lead[6

Reflecting parabolic mirrors were first described by the Arabian physicist, Ibn Sahl, in the 10th century[7]. Ibn al-Haytham discussed concave and convex mirrors in both cylindrical and spherical geometries,[8] carried out a number of experiments with mirrors, and solved the problem of finding the point on a convex mirror at which a ray coming from one point is reflected to another point.[9] By the 11th century, clear glass mirrors were being produced in Moorish Spain.[10

Some time during the early Renaissance, European manufacturers perfected a superior method of coating glass with a tin-mercury amalgam. The exact date and location of the discovery is unknown, but in the 16th century, Venice, a city famed for its glass-making expertise, became a centre of mirror production using this new technique. Glass mirrors from this period were extremely expensive luxuries.[11] The Saint-Gobain factory, founded by royal initiative in France, was an important manufacturer, and Bohemian and German glass, often rather cheaper, was also important

The invention of the silvered-glass mirror is credited to German chemist Justus von Liebig in 1835.[12] His process involved the deposition of a thin layer of metallic silver onto glass through the chemical reduction of silver nitrate. This silvering process was adapted for mass manufacturing and led to the greater availability of affordable mirrors. Nowadays, mirrors are often produced by the vacuum deposition of aluminium (or sometimes silver) directly onto the glass substrate

Manufacturing

Mirrors are manufactured by applying a reflective coating to a suitable substrate. The most common such substrate is glass, due to its transparency, ease of fabrication, rigidity, and ability to take a smooth finish. The reflective coating ("silver") is typically applied to the back surface of the glass, so that it is protected from corrosion and accidental damage. (Glass is much more scratch-resistant than most substrates

In classical antiquity mirrors were made of solid metal (bronze, later silver) and were too expensive for widespread use, as well as being prone to corrosion. Due to the low reflectivity of polished metal these mirrors also gave a darker image than modern ones, making them unsuitable for indoor use with the artificial lighting of the time (candles or lanterns

The method of making mirrors out of plate glass was invented by 16th-century Venetian glassmakers on the island of Murano, who covered the back of the glass with mercury, obtaining near-perfect and undistorted reflection. For over one hundred years Venetian mirrors installed in richly decorated frames served as luxury decoration for palaces throughout Europe, but the secret of the mercury process eventually arrived to London and Paris during the 17th century, due to industrial espionage. French workshops succeeded in large scale industrialization of the process, eventually making mirrors affordable to the masses, although mercury's toxicity remained a problem[citation needed

In modern times the mirror substrate is shaped, polished and cleaned, and is then coated. Glass mirrors are most often coated with non-toxic silver or aluminium, implemented by a series of coatings

tin
silver
chemical activator
copper
paint

The tin is applied because silver will not bond with the glass. The activator causes the tin/silver to harden. Copper is added for long-term durability.[13] The paint protects the coating on the back of the mirror from scratches and other accidental damage

In some applications, generally those that are cost-sensitive or that require great durability, mirrors are made from a single, bulk material such as polished metal

For technical applications such as laser mirrors, the reflective coating is typically applied by vacuum deposition on the front surface of the substrate. This eliminates double reflections (a weak reflection from the surface of the glass, and a stronger one from the reflecting metal) and reduces absorption of light by the mirror. Technical mirrors may use a silver, aluminium, or gold coating (the latter typically for infrared mirrors), and achieve reflectivities of 90–95% when new. A protective transparent overcoat may be applied to prevent oxidation of the reflective layer. Applications requiring higher reflectivity or greater durability where wide bandwidth is not essential use dielectric coatings, which can achieve reflectivities as high as 99.999% over a narrow range of wavelengths

Effects

See also: Mirror image and Specular reflection

In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image). There are also concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. Lastly, there are convex mirrors, where a parallel beam becomes divergent, with the rays appearing to diverge from a common intersection "behind" the mirror. Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a commonly-used approximation. Parabolic reflectors resolve this, allowing incoming parallel rays (for example, light from a distant star) to be focused to a small spot; almost an ideal point. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel

A beam of light reflects off a mirror at an angle of reflection that is equal to its angle of incidenc " if the size of a mirror is much larger than the wavelength of light". That is, if the beam of light is shining on a mirror's surface at a 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical in the opposite direction

This law mathematically follows from the interference of a plane wave on a flat boundary"of much larger size than the wavelength

Applications

Reflections in a spherical convex mirror. The photographer is seen at top right

Safety and easier viewing

Rear-view mirrors are widely used in and on vehicles (such as automobiles, or bicycles), to allow drivers to see other vehicles coming up behind them. Some motorcycle helmets have a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective surfaces inside the helmet that together function as a rear-view mirror.[1] There exist rear view sunglasses, of which the left end of the left glass and the right end of the right glass work as mirrors

Convex mirrors are used to provide a wider field of view than a flat mirror, and are often used on vehicles, especially large trucks, to minimise blind spots. They are sometimes placed at road junctions, and corners of places such as parking lots to allow people to see around corners to avoid crashing into other vehicles or shopping carts. They are also sometimes used as part of security systems, so that a single video camera can show more than one angle at a time

Mouth mirrors or "dental mirrors" are used by dentists to allow indirect vision and lighting within the mouth. Their reflective surfaces may be either flat or curved. Mouth mirrors are also commonly used by engineers to allow vision in tight spaces and around corners in equipment

Two-way mirrors

Main article: Two-way mirror

A two-way mirror (two-way as viewers on both sides use it), sometimes called a one-way mirror or one-way glass (because it is possible to see through it in one direction only), reflects some percentage of the light incident on it and transmits the remainder to the other side. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, transmitting some light and reflecting the remainder (from both sides

It is typically used as an apparently normal mirror in a brightly lit room, with a much darker room on the other side. People on the brightly lit side see their own reflection—it looks like a normal mirror. People on the dark side see through it—it looks like a transparent window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly

The reality television program Big Brother makes extensive use of two-way mirrors throughout its set to allow cameramen in special black hallways to use movable cameras to film contestants without being seen

The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is quite different: to split a beam of light so that part, usually about half, passes straight through, while the other part is reflected. In a typical scientific application the two resulting beams are made to interfere after traversing different paths. An unusual single-lens reflex camera used a half-silvered mirror to create an image of the scene both in the film plane and in the viewfinder

One-way mirrors work by overwhelming dim transmitted light with bright reflected light. A true one-way mirror that actually allows light to be transmitted in one direction only without requiring external energy is not possible as it violates the second law of thermodynamics: if we place a cold object on the transmitting side and a hot one on the blocked side, radiant energy would be transferred from the cold to the hot object

One-way windows can be made to work with polarized light in the laboratory without violating the second law. This is an apparent paradox that stumped some great physicists, although it does not allow a practical one-way mirror for use in the real world. [14][15] Optical isolators are one-way devices that are commonly used with lasers

Signalling

Main article: Heliograph

With the sun as light source, a mirror can be used to signal by variations in the orientation of the mirror. The signal can be used over long distances, possibly up to 60 kilometres on a clear day. This technique was used by Native American tribes and numerous militaries to transmit information between distant outposts

Mirrors can also be used for rescue, especially to attract the attention of search and rescue helicopters. Specialised signalling mirrors are available and are often included in military survival kits

Technology

Televisions and projectors

Microscopic mirrors are a core element of many of the largest high-definition televisions and video projectors. A common technology of this type is Texas Instruments' DLP. A DLP chip is a postage stamp-sized microchip whose surface is comprised of an array of millions of microscopic mirrors. The picture is created as the individual mirrors move to either reflect light toward the projection surface (pixel on), or toward a light absorbing surface (pixel off

Other projection technologies involving mirrors include LCoS. Like a DLP chip, LCoS is a microchip of similar size, but rather than millions of individual mirrors, there is a single mirror that is actively shielded by a liquid crystal matrix with up to millions of pixels. The picture is formed as light is either reflected toward the projection surface (pixel on), or absorbed by the activated LCD pixels (pixel off). LCoS-based televisions and projectors often use 3 chips, one for each primary color

Large mirrors are used in rear projection televisions. Light (for example from a DLP as mentioned above) is "folded" by one or more mirrors so that the television set is compact

Instruments

Telescopes and other precision instruments use front silvered or first surface mirrors, where the reflecting surface is placed on the front (or first) surface of the glass (this eliminates reflection from glass surface ordinary back mirrors have). Some of them use silver, but most are aluminum, which is more reflective at short wavelengths than silver. All of these coatings are easily damaged and require special handling. They reflect 90% to 95% of the incident light when new. The coatings are typically applied by vacuum deposition. A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in the air. Front silvered mirrors have to be resurfaced occasionally to keep their quality

The reflectivity of the mirror coating can be measured using a reflectometer and for a particular metal it will be different for different wavelengths of light. This is exploited in some optical work to make cold mirrors and hot mirrors. A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmissive to infrared light. A hot mirror is the opposite, the coating preferentially reflects infrared. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. For instance, aluminum mirrors are commonly coated with silicon dioxide or magnesium fluoride. The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied

A dielectric coated mirror used in a dye laser. The mirror is over 99% reflective at 550 nanometers, (yellow), but will allow most other colors to pass through

For scientific optical work, dielectric mirrors are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. Such mirrors are often used in lasers

In astronomy, adaptive optics is a technique to measure variable image distortions and adapt a deformable mirror accordingly on a timescale of milliseconds, to compensate for the distortions

Although the most of mirrors are designed to reflect visible light, surfaces reflecting other forms of electromagnetic radiation are also called "mirrors". The mirrors for other ranges of electromagnetic waves are used in optics and astronomy. Mirrors for radio waves are important elements of radio telescopes

A Mangin mirror is a combination lens and concave mirror and is widely used in optical instruments and even sometimes in cameras.[2] [3][4
Face-to-face mirrors

A dielectric mirror used in lasers

Two or more mirrors placed exactly face to face give the appearance of an infinite regress. Some devices use this to generate multiple reflections

Fabry-Pérot interferometer
Laser (which contains an optical cavity
some types of catoptric cistula
momentum-enhanced solar sail
and the mirror momentum ((solar sail))to in the mirror
the mirror cause by the enternet mirror and the mirror in the house and the room

Military applications

It has been said that Archimedes used a large array of mirrors to burn Roman ships during an attack on Syracuse. This has never been proven or disproved; however, it has been put to the test. Recently, on a popular Discovery Channel show, MythBusters, a team from MIT tried to recreate the famous "Archimedes Death Ray". They were successful at starting a fire on a ship at 75 feet away; however, previous attempts to light the boat on fire using only the bronze mirrors available in Archimedes' time were unsuccessful, and the time taken to ignite the craft would have made its use impractical, resulting in the MythBusters team deeming the myth "busted". (See solar power tower for a practical use of this technique)

seasonal lighting

A multi-facet mirror in the Kibble Palace conservatory, Glasgow, Scotland

Due to its location in a steep-sided valley, the Italian town of Viganella gets no direct sunlight for seven weeks each winter. In 2006 a €100,000 computer-controlled mirror, 8×5 m, was installed to reflect sunlight into the town's piazza. In early 2007 the similarly situated village of Bondo, Switzerland, was considering applying this solution as well.[16][17] Mirrors can be used to produce enhanced lighting effects in greenhouses or conservatories

Leisure

Decoration

Mirrors, typically large and unframed, are frequently used in interior decoration to create an illusion of space, and amplify the apparent size of a room

Mirrors are used also in some schools of feng shui, an ancient Chinese practice of placement and arrangement of space to achieve harmony with the environment

The softness of old mirrors is sometimes replicated by contemporary artisans for use in interior design. These reproduction antiqued mirrors are works of art and can bring color and texture to an otherwise hard, cold reflective surface. It is an artistic process that has been attempted by many and perfected by few

A decorative reflecting sphere of thin metal-coated glass, working as a reducing wide-angle mirror, is sold as a Christmas ornament called a bauble

Entertainment

The hall of mirrors, commonly found in amusement parks, is an attraction in which a number of distorted mirrors are used to produce unusual reflections of the visitor. Mirror mazes, also found in amusement parks, contain large numbers of mirrors and sheets of glass. The idea is to navigate the disorientating array without bumping into the walls

Mirrors are often used in magic to create an illusion. One effect is called Pepper's ghost. Illuminated rotating disco balls covered with small mirrors are used to cast moving spots of light around a dance floor. Mirrors are employed in kaleidoscopes, personal entertainment devices invented in Scotland by sir David Brewster

Art

Filippo Brunelleschi discovered linear perspective with the help of the mirror, Leonardo da Vinci called the mirror the "master of painters". He recommended, "When you wish to see whether your whole picture accords with what you have portrayed from nature take a mirror and reflect the actual object in it. Compare what is reflected with your painting and carefully consider whether both likenesses of the subject correspond, particularly in regard to the mirror." The mirror is the central device in some of the greatest of European paintings: Jan Van Eyck's Arnolfini Portrait, Diego Velazquez's Las Meninas and Edouard Manet’s A Bar at the Folies-Bergère. Without a mirror, the great self portraits by Dürer, Rembrandt, Van Gogh and Frida Kahlo could not have been painted. M. C. Escher used special shapes of mirrors in order to have a much more complete view of the surroundings than by direct observation (Hand with Reflecting Sphere). István Orosz’s anamorphic works are images distorted such way that they only become clearly visible when reflected in a suitably-shaped and positioned mirror. Some other contemporary artists use mirrors as the material of art, like in mirror-sculptures and paintings on mirror surfaces. Some artists build special mirror installations as the neon mirror cubes by Jeppe Hein

Painters depicting someone gazing into a mirror often also show the person's reflection. This is a kind of abstraction—in most cases the angle of view is such that the person's reflection should not be visible. Similarly, in movies and still photography an actor or actress is often shown ostensibly looking at him or herself in the mirror, and yet the reflection faces the camera. In reality, the actor or actress sees only the camera and its operator in this case, not their own reflection

Literature

Mirrors play a powerful role in cultural literature, from the self-loving Narcissus of Greek Mythology to the Biblical reference to Through a Glass Darkly. The evil queen in the European fairy-tale Snow White asked, "Mirror, mirror, on the wall... who's the fairest of them all?" Some of the best-loved uses of mirrors in literature include Lewis Carroll's Through the Looking Glass and the Mirror of Erised in the Harry Potter series. Horror movies about mirrors include Candyman and Mirrors

Mirrors and superstition

There are many legends and superstitions surrounding mirrors. Mirrors are said to be a reflection of the soul, and they were often used in traditional witchcraft as tools for scrying or performing other spells. It is also said that mirrors cannot lie. They can show only the truth, so it is a very bad omen indeed to see something in a mirror which should not be there. Also there is a legend that a newborn child should not see a mirror until its first birthday as its soul is still developing. If the child sees its reflection it is said that it will die

It is a common superstition that someone who breaks a mirror will receive seven years of bad luck.[18] One of the many reasons for this belief is that the mirror is believed to reflect part of the soul, therefore, breaking the mirror will break part of the soul. However, the soul is said to regenerate every seven years, thus coming back unbroken. To counter this one of many rituals has to be performed, the easiest of which is to stop the mirror from reflecting the broken soul by grinding it to dust.[19] The belief might also simply originate from the high cost of mirrors in times gone past. It is also said that tapping the broken mirror on a gravestone seven times will allow the soul to heal. Another option is to bury the mirror, also preventing the mirror from reflecting the broken soul. However, if the mirror is both touched to the gravestone and buried, the bad luck will remain. If you are in this position, the only course of action is to dig up the mirror and grind it to dust. Finally, this dust must be sprinkled around the same gravestone on which the mirror was initially tapped.[citation needed

In days past it was customary in the southern United States to cover the mirrors in a house where the wake of a deceased person was being held. It was believed that the person's soul would become trapped in a mirror left uncovered. This practice is still followed in other countries (Greece), extending to everything that could reflect the deceased person's face (like TV appliances); another explanation given is that the devil will appear in the reflection of the dead. Mirrors falling from walls or otherwise breaking or cracking mysteriously were said to be haunted

According to legend, a vampire has no reflection in mirrors because it is an undead creature and has already lost its soul

Spectrophobia is the fear of mirrors

Another superstition claims it is bad luck to have two mirrors facing each other.[20

A staple of childhood slumber parties is the game Bloody Mary, which involves chanting "Bloody Mary" three times in a darkened room while staring into a mirror. There are many versions of the game, but the general idea is that "Mary" will appear in the mirror and attempt to harm or kill the person who has summoned her. Thanks to a series of popular horror movies based on a supernatural killer who haunted mirrors, the phrase "Candy Man" may be substituted for Mary

Mirrors and animals

The Asian elephant can recognise its own reflection in a mirror

Experiments have shown that only large-brained social animals are able to recognise that a mirror shows a reflection of themselves.[21

Animals that have shown they are able to use a mirror to study themselves

Asian elephants
Bonobos
Common chimpanzees
Dolphins
Magpies
Orangutans

Unusual types of mirror

Other types of reflecting device are also called "mirrors". For example metallic reflectors are used to reflect infrared light (such as in space heaters), or microwaves

4.5metre high acoustic mirror near Kilnsea Grange, East Yorkshire, UK

An acoustic mirror is a passive device used to reflect and perhaps to focus sound waves. Acoustic mirrors were used for selective detection of sound waves, especially during World War II. They were used for detection of enemy aircraft prior to the development of radar. Acoustic mirrors are used for remote probing of the atmosphere; they can be used to form a narrow diffraction-limited beam.[22] They can also be used for underwater "imaging

Active mirrors are mirrors that amplify the light they reflect. They are used to make disk lasers.[23] The amplification is typically over a narrow range of wavelengths, and requires an external source of power

An atomic mirror is a device which reflects matter waves. Usually, atomic mirrors work at grazing incidence. Such a mirror can be used for atomic interferometry and atomic holography. It has been proposed that they can be used for non-destructive imaging systems with nanometer resolution.[24

Cold mirrors are dielectric mirrors that reflect the entire visible light spectrum while efficiently transmitting infrared wavelengths. Conversely, hot mirrors reflect infrared light while allowing visible light to pass. These can be used to separate useful light from unneeded infrared to reduce heating of components in an optical device. They can also be used as dichroic beamsplitters

Corner reflectors use three flat mirrors to reflect light back towards its source. They are used for emergency location, and even laser ranging to the Moon

X-ray mirrors produce specular reflection of X-rays. All known types work only at angles near grazing incidence, and only a small fraction of the rays are reflected.[25

A non-reversing mirror is a mirror that provides a non-reversed image of its subject

Looking deeply into polymer solar cells (9/14/2009

Researchers from the Eindhoven University of Technology and the University of Ulm have made the first high-resolution 3D images of the inside of a polymer solar cell. This gives them important new insights in the nanoscale structure of polymer solar cells and its effect on the performance. The findings were published online in Nature Materials on Sunday 13 September

The investigations shed new light on the operational principles of polymer solar cells

Cost-effective, flexible and lightweight

These solar cells do not have the high efficiencies of their silicon counterparts yet. Polymer cells, however, can be printed in roll-to-roll processes, at very high speeds, which makes the technology potentially very cost-effective. Added to that, polymer cells are flexible and lightweight, and therefore suitable to be used on vehicles or clothing or to be incorporated in the design of objects

Hybrid polymer solar cells

In these hybrid solar cells, a mixture of two different materials, a polymer and a metal oxide are used to create charges at their interface when the mixture is illuminated by the sun. The degree of mixing of the two materials is essential for its efficiency. Intimate mixing enhances the area of the interface where charges are formed but at the same time obstructs charge transport because it leads to long and winding roads for the charges to travel. Larger domains do exactly the opposite. The vastly different chemical nature of polymers and metal oxides generally makes it very difficult to control the nanoscale structure. The Eindhoven researchers have been able to largely circumvent this problem by using a precursor compound that mixes with the polymer and is only converted into the metal oxide after it is incorporated in the photoactive layer. This allows better mixing and enables extracting up to 50% of the absorbed photons as charges in an external circuit

Nanoscale mixing

The importance of the degree of mixing was clearly demonstrated by visualization of the structure of these blends in three dimensions. Traditionally such visualization has been extremely challenging, but by using 3D electron tomography, the team has been able to resolve the mixing with unprecedented detail on a nanoscale. From these images the researchers at the Institute of Stochastics in Ulm have been able to extract typical distances between the two components, relating to the efficiency of charge generation, and analyze the percolation pathways, that is, how much of each component is connected to the electrode. These quantitative analyses of the structure matched perfectly with the observed performance of the solar cells in sunlight

Future

Even though these hybrid polymer solar cells are among the most efficient reported to date for this class, their power conversion efficiency of 2% in sunlight must be enhanced to make them really useful. This will be realized by improving the control over the morphology of the photoactive blend, for example by creating polymers that can interact with the metal oxide and by developing polymers or molecules that absorb a larger part of the solar spectrum. At such point, the intrinsic advantages of hybrid polymer solar cells in terms of low cost and thermal stability of the nanoscale structure could be fully exploited

Note: This story has been adapted from a news release issued by the Eindhoven University of Technology

Opto-electronic nose sniffs out toxic gases (9/15/2009

Imagine a polka-dotted postage stamp that can sniff out poisonous gases or deadly toxins simply by changing colors

Kenneth Suslick, the Schmidt Professor of Chemistry at the University of Illinois at Urbana-Champaign, and his team developed a sensor that uses reactive pigments to detect toxic industrial chemicals. - Photo courtesy of Kenneth Suslick

As reported in the Sept. 13 issue of the journal Nature Chemistry, Kenneth Suslick and his team at the University of Illinois have developed an artificial nose for the general detection of toxic industrial chemicals (TICs) that is simple, fast and inexpensive - and works by visualizing odors. This sensor array could be useful in detecting high exposures to chemicals that pose serious health risks in the workplace or through accidental exposure

"Our device is simply a digital multidimensional extension of litmus paper. We have a six by six array of different nanoporous pigments whose colors change depending on their chemical environment," said Suslick, the Schmidt Professor of Chemistry at the U. of I. "The pattern of the color change is a unique molecular fingerprint for any toxic gas and also tells us its concentration. By comparing that pattern to a library of color fingerprints, we can identify and quantify the TICs in a matter of seconds

To create the sensor array, the researchers print a series of tiny colored dots - each a different pigment - on an inert backing such as paper, plastic or glass. The array is then digitally imaged with an ordinary flatbed scanner or an inexpensive electronic camera before and after exposure to an odor-producing substance. And, unlike other electronic-nose technologies that have been tried in the past, these colorimetric sensors are not affected by changes in relative humidity

While physicists have radiation badges to protect them in the workplace, chemists and workers who handle chemicals have no good equivalent to monitor their exposure to potentially toxic chemicals

This project, which was funded by the National Institute of Environmental Health Sciences at the National Institutes of Health, exemplifies the types of sensors that are being developed as part of the NIH Genes, Environment and Health Initiative

"This research is an essential component of the GEI Exposure Biology Program that NIEHS has the lead on, which is to develop technologies to monitor and better understand how environmental exposures affect disease risk," said NIEHS director Linda Birnbaum. "This paper brings us one step closer to having a small wearable sensor that can detect multiple airborne toxins

To test the application of their color sensor array, the researchers chose 19 representative examples of toxic industrial chemicals. Chemicals such as ammonia, chlorine, nitric acid and sulfur dioxide at concentrations known to be immediately dangerous to life or health were included

The laboratory studies used inexpensive flatbed scanners for imaging. The researchers have developed a fully functional prototype handheld device that uses inexpensive white LED illumination and an ordinary camera, which will make the whole process of scanning more sensitive, smaller, faster, and even less expensive. It will be similar to a card-scanning device. The device is now being commercialized by iSense, located in Palo Alto, Calif., and Champaign

The researchers say older methods relied on sensors whose response originates from weak and highly non-specific chemical interactions, whereas this new technology is based on stronger dye-analyte interactions that are responsive to a diverse set of chemicals. The power of this sensor to identify so many volatile toxins stems from the increased range of interactions that are used to discriminate the response of the array

"One of the nice things about this technology is that it uses components that are readily available and relatively inexpensive," said David Balshaw, Ph.D. program administrator at NIEHS. "Given the broad range of chemicals that can be detected and the high sensitivity of the array to those compounds, it appears that this device will be particularly useful in occupational settings

Note: This story has been adapted from a news release issued by the University of Illinois at Urbana-Champaign

Green tea component may help preserve stored platelets, tissues (9/16/2009

In two separate studies, a major component in green tea, epigallocatechin-3-O-gallate (EGCG), has been found to help prolong the preservation of both stored blood platelets and cryopreserved skin tissues. Published in the current double issue of Cell Transplantation (18:5/6), now freely available on-line at http://www.ingentaconnect.com/content/cog/ct, devoted to organ preservation and transplantation studies from Japan, the two complimentary studies have shown that EGCG, known to have strong anti-oxidative activity, can prolong platelet cell "shelf life" via anti-apoptosis (programmed cell death) properties and preserve skin tissues by controlling cell division

Dr. Suong-Hyn Hyon, lead author on both studies and associate professor in the Institute for Frontier Medical Sciences in Kyoto, Japan, says that EGCG, a green tea polyphenol, is a known anti-oxidation and anti-proliferation agent, yet the exact mechanism by which EGCG works is not yet known. However, some of the activity of EGCG is likely to be related to its surface binding ability

Enhanced platelet preservation

Using standard blood banking procedures, the storage duration for platelet cells (PCs) is limited to five days internationally or three days in Japan. During storage, PCs undergo biochemical, structural and functional changes, and PCs may lose membrane integrity and haemostatic functions, such as aggregability and affinity for surface receptors. Thus, PC shortages often occur. When EGCG was added to blood platelet concentrates, aggregation and coagulation functions were better-maintained after six days, perhaps due to EGCG's anti-oxidative ability. Researchers suggested that EGCG inhibited the activation of platelet functions and protected the surface proteins and lipids from oxidation

"Functions were restored by the maintained surface molecules with the detachment of ECGC by washing," noted Dr. Hyon. "EGCG may lead to an inhibition of platelet apoptosis and lower rates of cell death, offering a potentially novel and useful method to prolong platelet storage period

EGCG enhances life of cryopreserved skin grafts
Another team of Japanese researchers studied the effects of using EGCG on frozen, stored skin tissues. As with platelet storage, the storage of skin tissue for grafting presents problems of availability and limitations on the duration of storage

"To provide best outcomes, skin grafts must be processed and stored in a manner that maintains their viability and structural integrity until they are needed for transplantation," explained Dr. Hyon. "Transplant dysfunction often occurs as the result of oxidation. A better storage solution could prevent this

It is known that polyphenols in green tea promote the preservation of tissues, such as blood vessels, cornea, islet cells, articular cartilage and myocardium at room temperature. Also, it is known that ECGC has stronger anti-oxidant activities than vitamin C because of its sterochemical structure and is reported to play an important role in preventing cancer and cardiovascular diseases

This study examined how EGCG might help extend the preservation duration of frozen rat skin tissues and found that skin grafts could be protected from freeze-thaw injuries when EGCG was absorbed into various membrane lipids and proteins. Results of the study showed that EGCG enhanced the viability and stored duration of skin grafts up to seven weeks at 4 degrees C

"The storage time of skin grafts was extended to 24 weeks by cryopreservation using EGCG and the survival rate was almost 100 percent," noted Dr. Hyon

"These studies highlight the benefits of using natural compounds such as ECGC to enhance the preservation of stored tissues, possibly due to their anti-oxidative properties" said Dr. Naoya Kobayashi, guest editor of this double issue of Cell Transplantation

Note: This story has been adapted from a news release issued by the Cell Transplantation Center of Excellence for Aging and Brain Repair

New X-ray technique illuminates reactivity of environmental contaminants (9/17/2009

Thanks to a new analytical method employed by researchers at the University of Delaware, scientists can now pinpoint, at the millisecond level, what happens as harmful environmental contaminants such as arsenic begin to react with soil and water under various conditions

Quantifying the initial rates of such reactions is essential for modeling how contaminants are transported in the environment and predicting risks

The research method, which uses an analytical technique known as quick-scanning X-ray absorption spectroscopy (Q-XAS), was developed by a research team led by Donald Sparks, S. Hallock du Pont Chair of Plant and Soil Sciences and director of the Delaware Environmental Institute at UD. The work is reported in the Sept. 10 Early Edition of the Proceedings of the National Academy of Sciences and will be in the Sept. 22 print issue

Postdoctoral researcher Matthew Ginder-Vogel is the first author of the study, which also involved Ph.D. student Gautier Landrot and Jason Fischel, an undergraduate student at Juniata College who has interned in Sparks's lab during the past three summers

The research method was developed using beamline X18B at the National Synchrotron Light Source at Brookhaven National Laboratory in Upton, N.Y. The facility is operated by the U.S. Department of Energy

"This method is a significant advance in elucidating mechanisms of important geochemical processes, and is the first application, at millisecond time scales, to determine in real-time, the molecular scale reactions at the mineral/water interface. It has tremendous applications to many important environmental processes including sorption, redox, and precipitation," Sparks said

"My group and I have been conducting kinetics studies on soils and soil minerals for 30 years," Sparks added. "Since the beginning I have been hopeful that someday we could follow extremely rapid reaction processes and simultaneously collect mechanistic information

X-ray spectroscopy was invented years ago to illuminate structures and materials at the atomic level. The technique has been commonly used by physicists, chemists, materials scientists, and engineers, but only recently by environmental scientists

"In studying soil kinetics, we want to know how fast a contaminant begins to stick to a mineral," Ginder-Vogel says. "In general, these reactions are very rapid -- 90 percent of the reaction is over in the first 10 seconds. Now we can measure the first few seconds of these reactions that couldn't be measured before. We can now look at things as they happen versus attempting to freeze time after the fact," he notes

For their study, the UD researchers made millisecond measurements of the oxidation rate of arsenic by hydrous manganese oxide, which is a mineral that absorbs heavy metals and nutrients

Contamination of drinking water supplies by arsenic is a serious health concern in the United States and abroad. The poisonous element occurs naturally in rocks and minerals and is also used in a wide range of products, from wood preservatives and insecticides, to poultry feed

The toxicity and availability of arsenic to living organisms depends on its oxidation state -- in other words, the number of electrons lost or gained by an atom when it reacts with minerals and microbes. For example, arsenite [As(III)] is more mobile and toxic than its oxidized counterpart, arsenate [As(V

"Our technique is important for looking at groundwater flowing through minerals," Ginder-Vogel notes. "We look at it as a very early tool that can be incorporated into predictive modeling for the environment

Note: This story has been adapted from a news release issued by the University of Delaware

Graphene and gallium arsenide: 2 perfect partners find each other (9/18/2009

It is the marriage of two top candidates for the electronics of the future, both excentric and extremely interesting: Graphene, one of the partners, is an extremely thin fellow and besides, very young. Not until 2004 was it possible to specifically produce and investigate the single layer of carbon atoms. Its electronic properties are remarkable, because, among other things, its electrons can move so tremendously fast. It is a perfect partner for gallium arsenide, the semiconductor that allows tailoring of its electrical properties and which is the starting material of the fastest electrical and opto-electronic components. Besides, it is possible to produce gallium arsenide with an atomic-layer-smooth surface; this should suit well as a support for graphene

Scientists of the Physikalisch-Technische Bundesanstalt (PTB) have now, with the aid of a special design, succeeded in making graphene visible on gallium arsenide. Previously it has only been possible on silicon oxide. Now that they are able to view with a light optical microscope the graphene layer, which is thinner than one thousandth of a light wavelength, the researchers want to measure the electrical properties of their new material combination. As experts for precision measurements, the PTB physicists are thus especially well equipped to do this

They use the principle of the anti-reflective layer: If on a material one superimposes a very thin, nearly transparent layer of another material, then the reflectivity of the lower layer changes clearly visibly. In order to make their lower layer of gallium arsenide (plus graphene atomic layer) visible, the PTB physicists chose aluminium arsenide (AlAs). However, it is so similar to gallium arsenide (GaAs) in its optical properties that they had to employ a few tricks: They vapour-coated not only one, but rather several wafer-thin layers. "Thus, even with optically similar materials it is possible, in a manner of speaking, to 'grow' interference effects", Dr. Franz-Josef Ahlers, the responsible department head at PTB, explained. "This principle is known from optical interference filters. We have adapted it for our purposes

First of all, he and his colleagues calculated the optical properties of different GaAs and AlAs layers and optimized the layer sequence such that they could expect a sufficiently good detectability of graphene. Following this recipe, they got down to action and with the molecular beam epitaxial facility of PTB accurately produced a corresponding GaAs/AlAs crystal atom layer. Then in the same procedure as with silicon oxide, it was overlaid with graphite fragments. "Different from silicon but as predicted by the calculation, although single carbon layers are no longer visible at all wavelengths of visible light, it is, however, possible, e.g. with a simple green filter, to limit the wavelength range such that the graphene is easily visible", explained Ahlers.In the photo, all lighter areas of the dark GaAs are covered with graphene. From the degree of lightening it is possible to conclude the number of individual layers of atoms. The marked areas are 'real', that is, single-layer graphene. But next to them, there are also two, three and multiple layers of carbon atoms, which also have interesting properties. This arrangement was confirmed again with another method, Raman spectroscopy

Following such a simple identification with a normal light optical microscope, the further steps in the manufacture of electrical components from graphene surfaces are now possible without any difficulty. Thus the PTB scientists can now begin to accurately measure the electrical properties of the new material combination

Note: This story has been adapted from a news release issued by the Physikalisch-Technische Bundesanstalt (PTB

Smaller isn't always better (Catalyst simulations could lower fuel cell cost (9/19/2009

Imagine a car that runs on hydrogen from solar power and produces water instead of carbon emissions. While vehicles like this won't be on the market anytime soon, University of Wisconsin-Madison researchers are making incremental but important strides in the fuel cell technology that could make clean cars a reality

Materials science and engineering assistant professor Dane Morgan and Ph.D. student Edward (Ted) Holby have developed a computational model that could optimize an important component of fuel cells, making it possible for the technology to have a more widespread use. Essentially, they investigate how particle size relates to the overall stability of a material, and their model has shown that increasing the particle size of a fuel cell catalyst decreases degradation and therefore increases the useful lifetime of a fuel cell

Fuel cells are electrochemical devices that facilitate a reaction between hydrogen and oxygen, producing electrical power and forming water. In the type of fuel cells Morgan is researching, called proton exchange membrane fuel cells, or PEMFCs, hydrogen is split into a proton and electron at one side of the fuel cell (the anode). The proton moves through the device while the electron is forced to travel in an external circuit, where it can perform useful work. At the other side of the fuel cell (the cathode), the protons, electrons and oxygen combine to form water, which is the only waste product

Though the premise sounds straightforward, there are multiple hurdles to producing efficient fuel cells for widespread use. One of these hurdles is the catalyst added to aid the reaction between protons, electrons and oxygen at the cathode. Current fuel cells use platinum and platinum alloys as a catalyst. While platinum can withstand the corrosive fuel cell environment, it is expensive and not very abundant

To maximize platinum use, researchers use catalysts made with platinum particles as small as two nanometers, which are approximately 10 atoms across. These tiny structures have a large surface area on which the fuel cell reaction occurs. However, platinum catalysts this small degrade very quickly

"The stability of bulk versus nanoparticle materials can be understood intuitively by thinking of cheese," says Morgan. "When you leave a large chunk of cheese out and the edges get crusty, the surface is destroyed, but you can cut that off and there is still a lot of cheese inside that is good

"But if you crumble the cheese into tiny pieces and leave it out, you destroy all of your cheese because a larger fraction of the cheese is at the surface

Rapid catalyst degradation means the fuel cell doesn't last long, and the U.S. Department of Energy estimates fuel cells must function for 5,000 hours, or approximately seven months of continuous use, to be practical for automotive energy solutions

Morgan and Holby, who are working in collaboration with Professor Yang Shao-Horn from the Massachusetts Institute of Technology, have found a possible solution to the rapid degradation problem: When it comes to catalyst particle size, sometimes smaller isn't better

Their modeling work, which is funded by 3M and the U.S. Department of Energy, shows that if the particle size of a platinum catalyst is increased to four or five nanometers, which is approximately 20 atoms across, the level of degradation significantly decreases. This means the catalyst and the fuel cell as a whole can continue to function for much longer than if the particle size was only two or three nanometers

The research into the fundamental physics of particle size will be useful as scientists extend their platinum studies to exploring platinum alloys, which can reduce platinum consumption when used as fuel cell catalysts. Morgan is beginning to research models to study size effects on the stability of platinum alloys, such as copper-platinum and cobalt-platinum catalysts

"Fuel cells are just one of many energy technologies - solar, battery, etc. - with enormous potential to reduce our dependence on oil and our carbon emissions," says Morgan. "Computer simulation offers a powerful tool to understand and develop new materials at the heart of these energy technologies

Note: This story has been adapted from a news release issued by the University of Wisconsin-Madison

fluorescence

Fluorescence is the emission of visible light by a substance that has absorbed light of a differing, usually invisible, wavelength. Absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength. A shorter wavelength emission is sometimes observed from multiple photon absorption. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the characteristics of the particluar fluorescent substance

Fluorescent minerals

The term 'fluorescence' was coined by George Gabriel Stokes in his 1852 paper[1]; the name was given as a description of the essence of the mineral fluorite, composed of calcium fluoride, which gave a visible emission when illuminated with "invisible radiation" (UV radiation

Photochemistry
Fluorescence occurs when a molecule, atom or nanostructure relaxes to its ground state after being electrically excited
Excitation: so+hvex→ s1
Fluorescence (emission): s1→ so + hvem here hν is a generic term for photon energy with h = Planck's constant and ν = frequency of light. (The specific frequencies of exciting and emitted light are dependent on the particular system.)
State S0 is called the ground state of the fluorophore (fluorescent molecule) and S1 is its first electronically) excited state

A molecule in its excited state, S1, can relax by various competing pathways. It can undergo 'non-radiative relaxation' in which the excitation energy is dissipated as heat (vibrations) to the solvent. Excited organic molecules can also relax via conversion to a triplet state which may subsequently relax via phosphorescence or by a secondary non-radiative relaxation step
Relaxation of an S1 state can also occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen (O2) is an extremely efficient quencher of fluorescence because of its unusual triplet ground state
Molecules that are excited through light absorption or via a different process (e.g. as the product of a reaction) can transfer energy to a second 'sensitized' molecule, which is converted to its excited state and can then fluoresce. This process is used in lightsticks to produce different colors

Quantum yield

The fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed

Lifetime

The fluorescence lifetime refers to the average time the molecule stays in its excited state before emitting a photon. Fluorescence typically follows first-order kinetics

Applications

There are many natural and synthetic compounds that exhibit fluorescence, and they have a number of applications. Some deep-sea animals, such as the Greeneye, use fluorescence

Lighting

The common fluorescent tube relies on fluorescence. Inside the glass tube is a partial vacuum and a small amount of mercury. An electric discharge in the tube causes the mercury atoms to emit ultraviolet light. The tube is lined with a coating of a fluorescent material, called the phosphor, which absorbs the ultraviolet and re-emits visible light. Fluorescent lighting is very energy-efficient compared to incandescent technology, but the spectra produced may cause certain colours to appear unnatural. A compact fluorescent lamp can replace incandescent lighting
In the mid 1990s, white light-emitting diodes (LEDs) became available, which work through a similar process. Typically, the actual light-emitting semiconductor produces light in the blue part of the spectrum, which strikes a phosphor compound deposited on the chip; the phosphor fluoresces from the green to red part of the spectrum. The combination of the blue light that goes through the phosphor and the light emitted by the phosphor produce a net emission of white light
Glow sticks sometimes utilize fluorescent materials to absorb light from the chemiluminescent reaction and emit light of a different color

Fluorescent paint and plastic lit by UV tubes

Analytical chemistry
Fluorescence in several wavelengths can be detected by an array detector, to detect compounds from HPLC flow. Also, TLC plates can be visualized if the compounds or a coloring reagent is fluorescent. Fluorescence is most effective when there is a larger ratio of atoms at lower energy levels in a Boltzmann distribution. There is then a higher probability of lower energy atoms being excited and releasing photons, making analysis more efficient
Fingerprints can be visualized with fluorescent compounds such as ninhydrin

Biochemistry and medicine
Main article: Fluorescence in the life sciences

Endothelial cells under the microscope with three separate channels marking specific cellular components

Fluorescence in the life sciences is used generally as a non-destructive way of tracking or analysis biological molecules by means of the fluorescent emission at a specific frequency were there is no background from the excitation light, as relatively few cellular components are naturally fluorescent (called intrinsic or autofluorescence). In fact, a protein or other component can be "labelled" with a extrinsic fluorophore, a fluorescent dye which can be a small molecule, protein or quantum dot, finding a large use in many biological applications. The quantification of a dye is done with a Spectrofluorometer and finds additional applications in:the following
when scanning the fluorescent intensity across a plane one has Fluorescence microscopy of tissues, cells or subcellular structures which is accomplished by labeling an antibody with a fluorophore and allowing the antibody to find its target antigen within the sample. Labeling multiple antibodies with different fluorophores allows visualization of multiple targets within a single image (multiple channels). DNA microarrays are a variant of this
Automated sequencing of DNA by the chain termination method; each of four different chain terminating bases has its own specific fluorescent tag. As the labeled DNA molecules are separated, the fluorescent label is excited by a UV source, and the identity of the base terminating the molecule is identified by the wavelength of the emitted light
. Ethidium bromide fluoresces orange when intercalating DNA and when exposed to UV light
FACS) fluorescent-activated cell sorting)
DNA detection: the compound ethidium bromide, when free to change its conformation in solution, has very little fluorescence. Ethidium bromide's fluorescence is greatly enhanced when it binds to DNA, so this compound is very useful in visualising the location of DNA fragments in agarose gel electrophoresis. Ethidium bromide can be toxic - a safer alternative is the dye SYBR Green
Immunology: An antibody has a fluorescent chemical group attached, and the sites (e.g., on a microscopic specimen) where the antibody has bound can be seen, and even quantified, by the fluorescence
Additionally Fluorescence resonance energy transfer used to study protein interactions, detect specific nucleic acid sequences and used as biosensors, while fluorescent lifetime can give an additional layer of information

Gemology, mineralogy, geology, and forensics
Gemstones, minerals, fibers, and many other materials which may be encountered in forensics or with a relationship to various collectibles may have a distinctive fluorescence or may fluoresce differently under short-wave ultraviolet, long-wave ultra violet, or X-rays
Many types of calcite and amber will fluoresce under shortwave UV. Rubies, emeralds, and the Hope Diamond exhibit red fluorescence under short-wave UV light; diamonds also emit light under X ray radiation
Crude oil (petroleum) fluoresces in a range of colors, from dull brown for heavy oils and tars through to bright yellowish and bluish white for very light oils and condensates. This phenomenon is used in oil exploration drilling to identify very small amounts of oil in drill cuttings and core samples
Organic liquids
Organic liquids such as mixtures of anthracene in benzene, toluene, or stilbene in the same solvents, fluoresce with ultraviolet or gamma ray irradiation. The decay times of this fluorescence is of the order of nanoseconds since the duration of the light depends on the lifetime of the excited states of the fluorescent material, in this case anthracene or stilbene