Fluorescence

Fluorescence is an optical phenomenon 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 ending up as molecular vibrations (heat). Usually the absorbed photon is in the ultraviolet, and the emitted light (luminescence) is in the visible range. Fluorescence is named after the mineral fluorspar (calcium fluoride), which exhibits this phenomenon.

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

Table of contents

1 Lighting 2 Biochemistry & Medicine 3 Gemmology, Mineralogy and Forensics

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.

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.

Biochemistry & 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 termninating 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 chip
• 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.
• something about that fluorescent compound that binds to Ca++ so that changes in intracellular calcium can be viewed in real time with a fluorescent microscope???
• FACS (Fluorescent activated cell sorting)
• Fluorescent proteins such as the Green Fluorescent Protein (GFP) as reporters for any number of biological events including such things as sub-cellular localization and level of expression.

Gemmology, Mineralogy and Forensics

Gemstones, Minerals, fibers and many other materials which may be encountered in forensics or with relationshp to various collectibles may have a distinctive fluorescence or may fluoresce differently under short wave ultraviolet, long wave ultra violet or X-rays.

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