Fluorescence

Fluorescence is a luminescence which is mostly found as an 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.

Equation:

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

h = Planck's constant and
v = frequency of the fluorescing light

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.

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

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 ultraviolet and re-emits visible light.

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.

The modern mercury vapor streetlight is said to have been evolved from the fluorescent lamp.

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 an ultraviolet 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 with techniques such as fluorescence resonance energy transfer. 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 extremely 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.

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 will flouresce under shortwave ultraviolet.

Rubies, emeralds, and the Hope Diamond exhibit red fluorescence under short-wave ultraviolet light; diamonds also emit light under X ray radiation.