Fluorescence induced by exposure to ultraviolet light in vials containing various sized cadmium selenide (CdSe) quantum dots.

Fluorescence is a luminescence, i.e. optical phenomenon in cold bodies, in which a molecule absorbs a high-energy photon, and re-emits it as a lower-energy (longer-wavelength) photon. The energy difference between the absorbed and emitted photons ends up as molecular vibrations (heat). Usually the absorbed photon is in the ultraviolet, and the emitted light (luminescence) is in the visible range, but this depends on the absorbance curve and Stokes shift of the particular fluorophore. Fluorescence is named after the mineral fluorite (calcium fluoride), which exhibits this phenomenon Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... General Name, Symbol, Number Cadmium, Cd, 48 Chemical series Transition metals Group, Period, Block 12, 5, d Density, Hardness 8650 kg/m3, 2 Appearance Silvery gray metallic Atomic properties Atomic weight 112. ... General Name, Symbol, Number selenium, Se, 34 Series nonmetals Group, Period, Block 16 (VIA), 4, p Density, Hardness 4790 kg/m3(300K), 2 Appearance grey, metallic lustre Atomic properties Atomic weight 78. ... A quantum dot is a potential well that confines electrons in three dimensions to a region of the order of the electrons de Broglie wavelength in size, a few nanometers in a semiconductor. ... Luminescence is any emission of electromagnetic radiation. ... An optical phenomenon is any observable event which results from the interaction of light and matter. ... In physics, the photon (from Greek φοτος, meaning light) is a quantum of excitation of the quantised electromagnetic field and is one of the elementary particles studied by quantum electrodynamics (QED) which is the oldest part of the Standard Model of particle physics. ... Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... Stokes shift is the difference (in wavelength or frequency units) between positions of the band maxima of the absorption and luminescence spectra of the same electronic transition. ... A fluorophore is a fluorescent molecule. ... Minerals are natural compounds formed through geological processes. ... Octahedral fluorite crystals from New Mexico, USA Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF2. ...

Equation

This means that the system starts in state S₁, and after the fluorescent emission of a photon with energy hν, it is in state S₂ where:

h = Planck's constant and Plancks constant, denoted h, is a physical constant that is used to describe the sizes of quanta. ...

ν = frequency of the fluorescing light Sine waves of various frequencies; the lower waves have higher frequencies than those above. ...

Rules

Kasha–Vavilov rule The quantum yield of luminescence is independent of the wavelength of exciting radiation.

Jablonski diagram Describes most of the relaxation mechanism for excited state molecules.

Applications

There are many natural and synthetic compounds that exhibit fluorescence, and they have a number of applications:

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 light. The emitted light is in the ultraviolet range and is invisible, and also harmful to living organisms, so the tube is lined with a coating of a fluorescent material, called the phosphor, which absorbs the UV and re-emits visible light. Architect lamps Lighting refers to the devices or techniques used for illumination, usually referring to artificial light sources such as lamps or flashlights. ... A compact fluorescent lamp A fluorescent lamp is a type of electric lamp that excites argon and mercury vapor to create luminescence. ... Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... A phosphor is a substance that can exhibit the phenomenon of fluorescence (glowing during absorption of radiation of another kind) or phosphorescence (sustained glowing without further stimulus). ...

Recently, "white LEDs" (light-emitting diodes) have become 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 a reflector; the phosphor fluoresces in the orange part of the spectrum, the combination of the two colors producing a net effect of apparently white light. Various light-emitting diodes (5 mm reds, 3 mm greens and yellows) A light-emitting diode (LED) is a semiconductor device that emits incoherent monochromatic light when electrically biased in the forward direction. ... A semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. ...

Biochemistry and medicine

There is a wide range of applications for fluorescence in this field. Large biological molecules can have a fluorescent chemical group attached by a chemical reaction, and the fluorescence of the attached tag enables very sensitive detection of the molecule. Examples:

• automated sequencing of DNA by the chain termination method; each of four different chain terminating bases has its own specific fluorescent tag. As the labelled 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.
• 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
• The DNA microarray
• 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 quantitated, by the fluorescence.
• FACS (fluorescent-activated cell sorting)
• Fluorescence has been used to study the structure and conformations of DNA and proteins. This is especially important in complexes of multiple biomolecules.
• Aequorin, from the jellyfish Aequorea victoria, produces a blue glow in the presence of Ca²⁺ ions (by a chemical reaction). It has been used to image calcium flow in cells in real time. The success with aequorin spurred further investigation of A. victoria and led to the discovery of Green Fluorescent Protein (GFP), which has become an extemely important research tool. GFP and related proteins are used as reporters for any number of biological events including such things as sub-cellular localization. Levels of gene expression are sometimes measured by linking a gene for GFP production to another gene.

Also, many biological molecules have an intrinsic fluorescence that can sometimes be used without the need to attach a chemical tag. Sometimes this intrinsic fluorescence changes when the molecule is in a specific environment, so the distribution or binding of the molecule can be measured. Bilirubin, for instance, is highly fluorescent when bound to a specific site on serum albumin. Zinc protoporphyrin, formed in developing red blood cells instead of hemoglobin when iron is unavailable or lead is present, has a bright fluorescence and can be used to detect these problems. Space-filling model of a section of DNA molecule Deoxyribose nucleic acid (DNA) is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life (and many viruses). ... The chain termination or Sanger or dideoxy method is a process used to sequence (read the bases) of DNA. It is named after Frederick Sanger who developed the process in 1975. ... Ethidium bromide (EtBr) is an intercalating agent commonly used as a nucleic acid stain in molecular biology laboratories for techniques such as agarose gel electrophoresis. ... Digital printout of an agarose gel electrophoresis of cat-insert plasmid DNA Agarose gel electrophoresis is a method used in molecular biology to separate DNA strands by size, and to determine the size of the separated strands by comparison to strands of known length. ... A DNA microarray, the different colours indicate relative expression of different genes. ... Fluorescent-activated cell sorting is a type of flow cytometry, a method for sorting a suspension of biologic cells into two or more containers, one cell at a time, based upon specific light scattering and fluorescent characteristics of each cell. ... Binomial name Aequorea victoria (Murbach and Shearer, 1902) Aequorea victoria is a luminescent jellyfish found off the west coast of America. ... The green fluorescent protein (GFP) is a protein from the jellyfish Aequorea victoria and the sea pansy that fluoresces green when exposed to blue light. ...

Gemology, mineralogy 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. A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut or faceted and polished is collectible or can be used in jewellery. ... Minerals are natural compounds formed through geological processes. ... For the meaning of fiber in nutrition, see dietary fiber. ... Forensics or forensic science is the application of science to questions which are of interest to the legal system as well as social sciences such as archaeology. ... A collectible (or collectable) is a manufactured item designed for people to collect. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...

Rubies and the Hope Diamond exhibit red fluorescence under short-wave UV light; diamonds also emit light under X ray radiation. Ruby is a red gemstone, a variety of the mineral corundum in which the color is caused mainly by chromium. ... Hope Diamond in museum The Hope Diamond is a large deep blue diamond currently housed in the Smithsonian Institution. ... An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength in the range of 10 nanometers to 100 picometers (corresponding to frequencies in the range 30 PHz to 60 EHz). ...

See also

• phosphorescence

Phosphorescence is a radiative transition involving a change in the spin multiplicity of a molecule. ...

External links

• Jablonski diagram (http://www.shsu.edu/~chemistry/chemiluminescence/JABLONSKI.html)
• Fluorescence on Scienceworld (http://scienceworld.wolfram.com/physics/Fluorescence.html)
• Scorpion detection using UV LEDs (http://johnbokma.com/pet/scorpion/detection-using-uv-leds.html)
• Handbook on fluorescent probes used in biology (http://www.probes.com/handbook/) from the company Molecular Probes