TheInfoList

Fluorescence is the emission of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

by a substance that has absorbed light or other
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

. It is a form of
luminescence Luminescence is spontaneous emission of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived by the human eye. Visible light is usually defined ...

. In most cases, the emitted light has a longer
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

, and therefore a lower
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

energy, than the absorbed radiation. A perceptible example of fluorescence occurs when the absorbed radiation is in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

region of the
spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum Continuum may refer to: * Continuum (measurement) Continuum theories or models expla ...

(invisible to the human eye), while the emitted light is in the visible region; this gives the fluorescent substance a distinct color that can only be seen when exposed to
UV light Ultraviolet (UV) is a form of with from 10 (with a corresponding frequency around 30 ) to 400 nm (750 ), shorter than that of , but longer than s. UV radiation is present in , and constitutes about 10% of the total electromagnetic radi ...
. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike
phosphorescent Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike flu ...

materials, which continue to emit light for some time after. Fluorescence has many practical applications, including
mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific st ...

,
gemology Gemology or gemmology is the science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), organizes knowledge in the form of Testability, testable ex ...
,
medicine Medicine is the science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity, awareness, or understanding of someone or something, such as facts ( descriptive knowledge), skills (proced ...

, chemical sensors (
fluorescence spectroscopy Fluorescence spectroscopy (also known as fluorimetry or spectrofluorometry) is a type of electromagnetic spectroscopy that analyzes fluorescence light. Fluorescence is the emission of light Light or visible light is electromagnetic rad ...
),
fluorescent labelling In molecular biology and biotechnology, a fluorescent tag, also known as a fluorescent label or fluorescent probe, is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of ...
,
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
s, biological detectors, cosmic-ray detection,
vacuum fluorescent display A vacuum fluorescent display (VFD) is a display device A display device is an output device An output device is any piece of computer hardware Computer hardware includes the physical parts of a computer A computer is a machine th ...
s, and
cathode-ray tube File:CRT monochrome.png, 250px, Cutaway rendering of a monochrome CRT: 1. Deflection coils2. Electron beam3. Focusing coil4. Phosphor layer on the inner side of the screen; emits light when struck by the electron beam5.&nbs ...
s. Its most common everyday application is in ( gas-discharge)
fluorescent lamp A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor that uses to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a coating ...

s and
LED lamps An LED lamp or LED light bulb is an electric light An electric light is a device that produces visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum The electroma ...

, in which fluorescent coatings convert UV or blue light into longer-wavelengths resulting in white light which can even appear indistinguishable from that of the traditional but highly inefficient
incandescent lamp image of the tungsten filament of an incandescent light bulb An incandescent light bulb, incandescent lamp or incandescent light globe is an electric light with a wire #Filament, filament heated until it glows. The filament is enclosed in a gla ...

. Fluorescence also occurs frequently in nature in some minerals and in many biological forms across all kingdoms of life. The latter may be referred to as ''biofluorescence'', indicating that the
fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms fro ...

is part of or is extracted from a living organism (rather than an inorganic
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
or
stain A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Accidental staining may make materials ...
). But since fluorescence is due to a specific chemical, which can also be synthesized artificially in most cases, it is sufficient to describe the substance itself as ''fluorescent''.

# History

An early observation of fluorescence was described in 1560 by
Bernardino de Sahagún Bernardino de Sahagún (; c. 1499 – 5 February 1590) was a Franciscan friar, missionary priest and pioneering ethnographer who participated in the Catholic evangelization Image:Jakob Jordaens 002.jpg, The Four Evangelists In Christianity, eva ...

and in 1565 by
Nicolás Monardes Image:Portrait-of-Monardes-1569.png, 230px, Nicolás Monardes Nicolás Bautista Monardes (1493 – 10 October 1588) was a Spain, Spanish physician and Botany, botanist. Monardes published several books of varying importance. In ''Diálogo llam ...
in the
infusion Infusion is the process of extracting chemical compounds or flavors from plant material in a solvent such as water, oil An oil is any nonpolar chemical substance A chemical substance is a form of matter In classical physics and ...

known as ''
lignum nephriticum ''Lignum nephriticum'' (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of th ...
'' (
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

for "kidney wood"). It was derived from the wood of two tree species, ''
Pterocarpus indicus ''Pterocarpus indicus'' (commonly known as Amboyna wood, Malay padauk, Papua New Guinea rosewood, Philippine mahogany, Andaman redwood, Burmese rosewood, narra and asana in the Philippines, angsana, or Pashu padauk) is a species of ''Pterocarpus' ...
'' and ''
Eysenhardtia polystachya ''Eysenhardtia polystachya'' or kidneywood tree is a tree from Mexico, growing along forest edges and water courses at elevations of 150–3000 m. Previously it was used as a source of lignum nephriticum. References

Eysenhardti ...

''. The chemical compound responsible for this fluorescence is matlaline, which is the oxidation product of one of the
flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenol Polyphenols () are a large family of naturally occurring s characterized by multiples of units. They are abunda ...
s found in this wood. In 1819, and in 1822
René Just Haüy René Just Haüy () FRS MWS FRSE (28 February 1743 – 3 June 1822) was a French priest and mineralogist Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical ...

described fluorescence in
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride Calcium fluoride is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural scien ...

s,
Sir David Brewster Sir David Brewster KH PRSE FRS FSA Scot FSSA MICE A mouse, plural mice, is a small rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European lang ...

described the phenomenon for
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally ...

in 1833 and
Sir John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ) is a classical language belonging to th ...
did the same for
quinine Quinine is a medication used to treat malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced sympto ...

in 1845. In his 1852 paper on the "Refrangibility" (
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

change) of light,
George Gabriel Stokes Sir George Gabriel Stokes, 1st Baronet, (; 13 August 1819 – 1 February 1903) was an Anglo-Irish physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in a ...

described the ability of
fluorspar Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in cubic crystal system, isometric crystal habit, cubic habit, although octahedral and more complex isometric forms ...

and
uranium glass Uranium glass is glass Glass is a non- crystalline, often transparency and translucency, transparent amorphous solid, that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glas ...

to change invisible light beyond the violet end of the visible spectrum into blue light. He named this phenomenon ''fluorescence'' : "I am almost inclined to coin a word, and call the appearance ''fluorescence'', from fluor-spar .e., fluorite as the analogous term ''opalescence'' is derived from the name of a mineral." The name was derived from the mineral
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride Calcium fluoride is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural scien ...

(calcium difluoride), some examples of which contain traces of divalent
europium Europium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

, which serves as the fluorescent activator to emit blue light. In a key experiment he used a prism to isolate ultraviolet radiation from sunlight and observed blue light emitted by an ethanol solution of quinine exposed by it.

# Physical principles

## Mechanism

Fluorescence occurs when an excited molecule, atom, or
nanostructure 250px, The DNA structure at left (schematic shown) will self-assemble into the structure visualized by atomic force microscopy at right. Image from Strong. A nanostructure is a structure A structure is an arrangement and organization of interre ...

, relaxes to a lower energy state (usually the
ground state The ground state of a quantum-mechanical system is its lowest-energy In , energy is the that must be to a or to perform on the body, or to it. Energy is a ; the law of states that energy can be in form, but not created or destro ...
) through emission of a
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

without a change in
electron spin The electron is a subatomic particle, symbol or , whose electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of electric charge: ...
. When the initial and final states have different multiplicity (spin), the phenomenon is termed
phosphorescence Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike flu ...

. The ground state of most molecules is a
singlet state In quantum mechanics Quantum mechanics is a fundamental theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is ...
, denoted as S0. A notable exception is
molecular oxygenThere are several known allotropes Allotropy or allotropism () is the property of some chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an elemen ...

, which has a ground state. Absorption of a photon of energy $h \nu_$ results in an excited state of the same multiplicity (spin) of the ground state, usually a singlet (Sn with n > 0). In solution, states with n > 1 relax rapidly to the lowest vibrational level of the first excited state (S1) by transferring energy to the solvent molecules through non-radiative processes, including internal conversion followed by vibrational relaxation, in which the energy is dissipated as
heat In thermodynamics Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these ...

. Therefore, most commonly, fluorescence occurs from the first singlet excited state, S1. Fluorescence is the emission of a photon accompanying the relaxation of the excited state to the ground state. Fluorescence photons are lower in energy ($h \nu_$) compared to the energy of the photons used to generate the excited state ($h \nu_$) * Excitation: $\mathrm_0 + h \nu_ \to \mathrm_1$ * Fluorescence (emission): $\mathrm_1 \to \mathrm_0 + h \nu_$ In each case the photon energy $E$ is proportional to its
frequency Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...

$\nu$ according to $E=h\nu$, where $h$ is . The excited state S1 can relax by other mechanisms that do not involve the emission of light. These processes, called non-radiative processes, compete with fluorescence emission and decrease its efficiency. Examples include
internal conversion Internal conversion is a non-radioactive decay process wherein an excited atomic nucleus, nucleus interacts electromagnetism, electromagnetically with one of the Atomic orbital, orbital electrons of the atom. This causes the electron to be emitted ...
,
intersystem crossingIntersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different states spin multiplicity. Singlet and triplet states When an electron in a molecule with a singlet ground ...
to the triplet state, and energy transfer to another molecule. An example of energy transfer is Förster resonance energy transfer. Relaxation from an excited state can also occur through collisional
quenching In materials science The interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic discipline An academic discipline or academic field is a subdivision of knowledge that is Edu ...
, a process where a molecule (the quencher) collides with the fluorescent molecule during its excited state lifetime. Molecular
oxygen Oxygen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same ...

(O2) is an extremely efficient quencher of fluorescence just because of its unusual triplet ground state.

## Quantum yield

The fluorescence
quantum yield The quantum yield (Φ) of a radiation upThe international symbol for types and levels of ionizing radiation (radioactivity) that are unsafe for unshielded humans. Radiation, in general, exists throughout nature, such as in light and sound. ...
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.Valeur, Bernard, Berberan-Santos, Mario (2012). ''Molecular Fluorescence: Principles and Applications''. Wiley-VCH. . p. 64 : $\Phi = \frac$ The maximum possible fluorescence quantum yield is 1.0 (100%); each
photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamental particle is a that is not composed of other particles. Particles currently thought to be elementary include the fundamental s ...

absorbed results in a photon emitted. Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence is by the rate of excited state decay: : $\Phi = \frac$ where $_$ is the rate constant of
spontaneous emission Spontaneous emission is the process in which a quantum mechanical system (such as a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five carbon rings. ...
of radiation and :$\sum__$ is the sum of all rates of excited state decay. Other rates of excited state decay are caused by mechanisms other than photon emission and are, therefore, often called "non-radiative rates", which can include: dynamic collisional quenching, near-field dipole-dipole interaction (or
resonance energy transfer Resonance describes the phenomenon of increased amplitude that occurs when the frequency of a periodically applied force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of ...
), internal conversion, and
intersystem crossingIntersystem crossing (ISC) is an isoenergetic radiationless process involving a transition between the two electronic states with different states spin multiplicity. Singlet and triplet states When an electron in a molecule with a singlet ground ...
. Thus, if the rate of any pathway changes, both the excited state lifetime and the fluorescence quantum yield will be affected. Fluorescence quantum yields are measured by comparison to a standard. The
quinine Quinine is a medication used to treat malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced sympto ...

salt ''quinine sulfate'' in a
sulfuric acid Sulfuric acid (American spelling Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthogra ...

solution was regarded as the most common fluorescence standard, however, a recent study revealed that the fluorescence quantum yield of this solution is strongly affected by the temperature, and should no longer be used as the standard solution. The quinine in 0.1M perchloric acid (Φ=0.60) shows no temperature dependence up to 45°C, therefore it can be considered as a reliable standard solution.

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 The rate law or rate equation for a chemical reaction is an equation that links the initial or forward reaction rate with the concentrations or pressures of the reactants and constant parameters (normally rate coefficients and partial reaction order ...
: : where is the decay rate or the inverse of the fluorescence lifetime. This is an instance of
exponential decay A quantity is subject to exponential decay if it decreases at a rate proportional Proportionality, proportion or proportional may refer to: Mathematics * Proportionality (mathematics), the property of two variables being in a multiplicative rela ...

. Various radiative and non-radiative processes can de-populate the excited state. In such case the total decay rate is the sum over all rates: :$\Gamma_=\Gamma_ + \Gamma_$ where $\Gamma_$ is the total decay rate, $\Gamma_$ the radiative decay rate and $\Gamma_$ the non-radiative decay rate. It is similar to a first-order chemical reaction in which the first-order rate constant is the sum of all of the rates (a parallel kinetic model). If the rate of spontaneous emission, or any of the other rates are fast, the lifetime is short. For commonly used fluorescent compounds, typical excited state decay times for photon emissions with energies from the to
near infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natur ...
are within the range of 0.5 to 20
nanoseconds A nanosecond (ns) is an SI unit of time equal to one billionth of a second The second (symbol: s, abbreviation: sec) is the SI base unit, base unit of time in the International System of Units (SI) (French: Système International d’unités) ...
. The fluorescence lifetime is an important parameter for practical applications of fluorescence such as and
fluorescence-lifetime imaging microscopy Fluorescence-lifetime imaging microscopy or FLIM is an imaging technique based on the differences in the exponential decay rate of the fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound ...
.

## Jablonski diagram

The
Jablonski diagram In molecular spectroscopy Spectroscopy is the study of the interaction between matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be t ...

describes most of the relaxation mechanisms for excited state molecules. The diagram alongside shows how fluorescence occurs due to the relaxation of certain excited electrons of a molecule."Animation for the Principle of Fluorescence and UV-Visible Absorbance"
. ''PharmaXChange.info''.

## Fluorescence anisotropy

Fluorophores are more likely to be excited by photons if the transition moment of the fluorophore is parallel to the electric vector of the photon. The polarization of the emitted light will also depend on the transition moment. The transition moment is dependent on the physical orientation of the fluorophore molecule. For fluorophores in solution this means that the intensity and polarization of the emitted light is dependent on rotational diffusion. Therefore, anisotropy measurements can be used to investigate how freely a fluorescent molecule moves in a particular environment. Fluorescence anisotropy can be defined quantitatively as :$r =$ where $I_\parallel$ is the emitted intensity parallel to polarization of the excitation light and $I_\perp$ is the emitted intensity perpendicular to the polarization of the excitation light.

## Fluorence

Strongly fluorescent pigments often have an unusual appearance which is often described colloquially as a "neon color" (originally "day-glo" in the late 1960s, early 1970s). This phenomenon was termed "Farbenglut" by
Hermann von Helmholtz Hermann Ludwig Ferdinand von Helmholtz (31 August 1821 – 8 September 1894) was a German physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific research to advance knowledge in an Branch ...

and "fluorence" by Ralph M. Evans. It is generally thought to be related to the high brightness of the color relative to what it would be as a component of white. Fluorescence shifts energy in the incident illumination from shorter wavelengths to longer (such as blue to yellow) and thus can make the fluorescent color appear brighter (more saturated) than it could possibly be by reflection alone.

# Rules

There are several general
rules Rule or ruling may refer to: Human activity * The exercise of political Politics (from , ) is the set of activities that are associated with Decision-making, making decisions in Social group, groups, or other forms of Power (social and ...
that deal with fluorescence. Each of the following rules has exceptions but they are useful guidelines for understanding fluorescence (these rules do not necessarily apply to
two-photon absorption Two-photon absorption (TPA or 2PA) or two-photon excitation or non-linear absorption is the simultaneous absorption of two photon The photon ( el, φῶς, phōs, light) is a type of elementary particle In , an elementary particle or fundamen ...
).

## Kasha's rule

Kasha's rule Kasha's rule is a principle in the photochemistry of electronically excited molecules. The rule states that photon emission (fluorescence light. Fluorescence is the emission of light Light or visible light is electromagnetic radiation ...
dictates that the quantum yield of luminescence is independent of the wavelength of exciting radiation. This occurs because excited molecules usually decay to the lowest vibrational level of the excited state before fluorescence emission takes place. The Kasha–Vavilov rule does not always apply and is violated severely in many simple molecules. A somewhat more reliable statement, although still with exceptions, would be that the fluorescence spectrum shows very little dependence on the wavelength of exciting radiation.

## Mirror image rule

For many fluorophores the absorption spectrum is a mirror image of the emission spectrum. This is known as the mirror image rule and is related to the
Franck–Condon principles are shown favoring transitions between ''v'' = 0 and ''v'' = 2. The James Franck, Franck– Condon principle is a rule in spectroscopy Spectroscopy is the study of the interaction between matter In classical physics and general chemistry ...
which states that electronic transitions are vertical, that is energy changes without distance changing as can be represented with a vertical line in Jablonski diagram. This means the nucleus does not move and the vibration levels of the excited state resemble the vibration levels of the ground state.

## Stokes shift

In general, emitted fluorescence light has a longer wavelength and lower energy than the absorbed light. This phenomenon, known as
Stokes shift __NOTOC__ Stokes shift is the difference (in energy In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies m ...

, is due to energy loss between the time a photon is absorbed and when a new one is emitted. The causes and magnitude of Stokes shift can be complex and are dependent on the fluorophore and its environment. However, there are some common causes. It is frequently due to non-radiative decay to the lowest vibrational energy level of the excited state. Another factor is that the emission of fluorescence frequently leaves a fluorophore in a higher vibrational level of the ground state.

# In nature

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

## Compared to bioluminescence and biophosphorescence

### Fluorescence

Fluorescence is the temporary absorption of
electromagnetic Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

wavelengths from the
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
spectrum by fluorescent molecules, and the subsequent emission of light at a lower energy level. When it occurs in a living organism, it is sometimes called biofluorescence. This causes the light that is emitted to be a different color than the light that is absorbed. Stimulating light excites an
electron The electron is a subatomic particle (denoted by the symbol or ) whose electric charge is negative one elementary charge. Electrons belong to the first generation (particle physics), generation of the lepton particle family, and are general ...

, raising energy to an unstable level. This instability is unfavorable, so the energized electron is returned to a stable state almost as immediately as it becomes unstable. This return to stability corresponds with the release of excess energy in the form of fluorescence light. This emission of light is only observable when the stimulant light is still providing light to the organism/object and is typically yellow, pink, orange, red, green, or purple. Fluorescence is often confused with the following forms of biotic light, bioluminescence and biophosphorescence. Pumpkin toadlets that live in the Brazilian Atlantic forest are fluorescent.

### Bioluminescence

Bioluminescence Bioluminescence is the production and emission of light by a living organism. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some Fungus , fungi, microorganisms includ ...
differs from fluorescence in that it is the natural production of light by chemical reactions within an organism, whereas fluorescence is the absorption and reemission of light from the environment.
Fireflies The Lampyridae are a family (biology), family of insects in the beetle order (biology), order Coleoptera with more than 2,000 described species. They are soft-bodied beetles that are commonly called fireflies, glowworms, or lightning bugs for t ...

and
anglerfish The anglerfish are fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of ...

are two examples of bioluminescent organisms. To add to the potential confusion, some organisms are both bioluminescent and fluorescent, like the sea pansy Renilla reniformis, where bioluminescence serves as the light source for fluorescence.

### Phosphorescence

Phosphorescence Phosphorescence is a type of photoluminescence Photoluminescence (abbreviated as PL) is light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, percei ...

is similar to fluorescence in its requirement of light wavelengths as a provider of excitation energy. The difference here lies in the relative stability of the energized electron. Unlike with fluorescence, in phosphorescence the electron retains stability, emitting light that continues to "glow-in-the-dark" even after the stimulating light source has been removed. For example, stickers are phosphorescent, but there are no truly ''biophosphorescent'' animals known.

## Mechanisms

### Epidermal chromatophores

Pigment cells that exhibit fluorescence are called fluorescent chromatophores, and function somatically similar to regular
chromatophore Chromatophores are cells that produce color, of which many types are pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Genera ...
s. These cells are dendritic, and contain pigments called fluorosomes. These pigments contain fluorescent proteins which are activated by K+ (potassium) ions, and it is their movement, aggregation, and dispersion within the fluorescent chromatophore that cause directed fluorescence patterning. Fluorescent cells are innervated the same as other chromatophores, like melanophores, pigment cells that contain
melanin Melanin (; from el, μέλας ''melas'', "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the ami ...

. Short term fluorescent patterning and signaling is controlled by the nervous system. Fluorescent chromatophores can be found in the skin (e.g. in fish) just below the epidermis, amongst other chromatophores. Epidermal fluorescent cells in fish also respond to hormonal stimuli by the α–MSH and MCH hormones much the same as melanophores. This suggests that fluorescent cells may have color changes throughout the day that coincide with their
circadian rhythm A circadian rhythm (), or circadian cycle, is a natural, internal process that regulates the sleep–wake cycle and repeats roughly every 24 hours. It can refer to any process that originates within an organism (i.e., endogenous Endogenous subst ...

. Fish may also be sensitive to
cortisol Cortisol is a steroid hormone, in the glucocorticoid class of hormones. When used as a medication, it is known as hydrocortisone. It is produced in many animals, mainly by the ''zona fasciculata'' of the adrenal cortex in the adrenal gland. ...

induced
stress response The fight-or-flight response (also called hyperarousal or the acute stress response) is a physiological reaction that occurs in response to a perceived Psychological trauma, harmful event, Trauma (medicine), attack, or threat to survival. It was f ...
s to environmental stimuli, such as interaction with a predator or engaging in a mating ritual.

## Phylogenetics

### Evolutionary origins

The incidence of fluorescence across the
tree of life #REDIRECT Tree of life#REDIRECT Tree of life The tree of life is a fundamental widespread mytheme or archetype in many of the world's mythology, mythologies, religion, religious and philosophy, philosophical traditions. It is closely related ...

is widespread, and has been studied most extensively in cnidarians and fish. The phenomenon appears to have evolved multiple times in multiple
tax A tax is a compulsory financial charge or some other type of levy imposed on a taxpayer (an individual or legal entity In law Law is a system A system is a group of Interaction, interacting or interrelated elements that act accord ...
a such as in the anguilliformes (eels), gobioidei (gobies and cardinalfishes), and tetradontiformes (triggerfishes), along with the other taxa discussed later in the article. Fluorescence is highly genotypically and phenotypically variable even within ecosystems, in regards to the wavelengths emitted, the patterns displayed, and the intensity of the fluorescence. Generally, the species relying upon camouflage exhibit the greatest diversity in fluorescence, likely because camouflage may be one of the uses of fluorescence. It is suspected by some scientists that GFPs and GFP-like proteins began as electron donors activated by light. These electrons were then used for reactions requiring light energy. Functions of fluorescent proteins, such as protection from the sun, conversion of light into different wavelengths, or for signaling are thought to have evolved secondarily.

Currently, relatively little is known about the functional significance of fluorescence and fluorescent proteins. However, it is suspected that fluorescence may serve important functions in signaling and communication,
mating In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

, lures,
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard The leopard (''Pan ...

, UV protection and antioxidation, photoacclimation,
dinoflagellate The dinoflagellates (Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 m ...
regulation, and in coral health.

## Aquatic

Water absorbs light of long wavelengths, so less light from these wavelengths reflects back to reach the eye. Therefore, warm colors from the visual light spectrum appear less vibrant at increasing depths. Water scatters light of shorter wavelengths above violet, meaning cooler colors dominate the visual field in the
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
. Light intensity decreases 10 fold with every 75 m of depth, so at depths of 75 m, light is 10% as intense as it is on the surface, and is only 1% as intense at 150 m as it is on the surface. Because the water filters out the wavelengths and intensity of water reaching certain depths, different proteins, because of the wavelengths and intensities of light they are capable of absorbing, are better suited to different depths. Theoretically, some fish eyes can detect light as deep as 1000 m. At these depths of the aphotic zone, the only sources of light are organisms themselves, giving off light through chemical reactions in a process called bioluminescence. Fluorescence is simply defined as the absorption of electromagnetic radiation at one
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

and its reemission at another, lower energy wavelength. Thus any type of fluorescence depends on the presence of external sources of light. Biologically functional fluorescence is found in the photic zone, where there is not only enough light to cause fluorescence, but enough light for other organisms to detect it. The visual field in the photic zone is naturally blue, so colors of fluorescence can be detected as bright reds, oranges, yellows, and greens. Green is the most commonly found color in the marine spectrum, yellow the second most, orange the third, and red is the rarest. Fluorescence can occur in organisms in the aphotic zone as a byproduct of that same organism's bioluminescence. Some fluorescence in the aphotic zone is merely a byproduct of the organism's tissue biochemistry and does not have a functional purpose. However, some cases of functional and adaptive significance of fluorescence in the aphotic zone of the deep ocean is an active area of research.

### =Fish

= Bony fishes living in shallow water generally have good color vision due to their living in a colorful environment. Thus, in shallow-water fishes, red, orange, and green fluorescence most likely serves as a means of communication with
conspecifics Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organis ...
, especially given the great phenotypic variance of the phenomenon. Many fish that exhibit fluorescence, such as
sharks Sharks are a group of elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including shark Sharks are a group of elasmobranch fish characterized by a Chondrichthyes#Skeleton, cartilaginous skeleton, ...
,
lizardfish The Synodontidae or lizardfishes(or typical lizardfish to distinguish them from the Bathysauridae and Pseudotrichonotidae) are Benthic fish, benthic (bottom-dwelling) marine and estuarine bony fishes that belong to the Aulopiformes, aulopiform ord ...

,
scorpionfish The Scorpaenidae (also known as scorpionfish) are a family (biology), family of mostly ocean, marine fish that includes many of the world's most venomous species. As their name suggests, scorpionfish have a type of "sting" in the form of sharp spi ...

,
wrasses The wrasses are a family, Labridae, of marine fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a sister group to the tunicates, together forming ...
, and flatfishes, also possess yellow intraocular filters. Yellow intraocular filters in the
lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simple ...
and
cornea The cornea is the transparent Transparency, transparence or transparent most often refer to transparency and translucency, the physical property of allowing the transmission of light through a material. They may also refer to: Literal uses * ...

of certain fishes function as long-pass filters. These filters enable the species that to visualize and potentially exploit fluorescence, in order to enhance visual contrast and patterns that are unseen to other fishes and predators that lack this visual specialization. Fish that possess the necessary yellow intraocular filters for visualizing fluorescence potentially exploit a light signal from members of it. Fluorescent patterning was especially prominent in cryptically patterned fishes possessing complex camouflage. Many of these lineages also possess yellow long-pass intraocular filters that could enable visualization of such patterns. Another adaptive use of fluorescence is to generate orange and red light from the ambient blue light of the
photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...
to aid vision. Red light can only be seen across short distances due to attenuation of red light wavelengths by water. Many fish species that fluoresce are small, group-living, or benthic/aphotic, and have conspicuous patterning. This patterning is caused by fluorescent tissue and is visible to other members of the species, however the patterning is invisible at other visual spectra. These intraspecific fluorescent patterns also coincide with intra-species signaling. The patterns present in ocular rings to indicate directionality of an individual's gaze, and along fins to indicate directionality of an individual's movement. Current research suspects that this red fluorescence is used for private communication between members of the same species. Due to the prominence of blue light at ocean depths, red light and light of longer wavelengths are muddled, and many predatory reef fish have little to no sensitivity for light at these wavelengths. Fish such as the fairy wrasse that have developed visual sensitivity to longer wavelengths are able to display red fluorescent signals that give a high contrast to the blue environment and are conspicuous to conspecifics in short ranges, yet are relatively invisible to other common fish that have reduced sensitivities to long wavelengths. Thus, fluorescence can be used as adaptive signaling and intra-species communication in reef fish. Additionally, it is suggested that fluorescent tissues that surround an organism's eyes are used to convert blue light from the photic zone or green bioluminescence in the aphotic zone into red light to aid vision.

### =Sharks

= A new
fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms fro ...

was described in two species of sharks, wherein it was due to an undescribed group of brominated tryptophane-kynurenine small molecule metabolites.

### =Coral

= Fluorescence serves a wide variety of functions in coral. Fluorescent proteins in corals may contribute to photosynthesis by converting otherwise unusable wavelengths of light into ones for which the coral's symbiotic algae are able to conduct
photosynthesis Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Conversion (Doctor Who audio), "Conversion" (''Doctor Who'' audio), an episode of the audio drama ' ...

. Also, the proteins may fluctuate in number as more or less light becomes available as a means of photoacclimation. Similarly, these fluorescent proteins may possess antioxidant capacities to eliminate oxygen radicals produced by photosynthesis. Finally, through modulating photosynthesis, the fluorescent proteins may also serve as a means of regulating the activity of the coral's photosynthetic algal symbionts.

### =Cephalopods

= ''Alloteuthis subulata'' and ''Loligo vulgaris'', two types of nearly transparent squid, have fluorescent spots above their eyes. These spots reflect incident light, which may serve as a means of camouflage, but also for signaling to other squids for schooling purposes.

### =Jellyfish

= Another, well-studied example of fluorescence in the ocean is the
hydrozoan Hydrozoa (hydrozoans, from ancient Greek ὕδωρ, ''hydōr'', "water" and ζῷον, ''zōion'', "animal") are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most living in salt water. The col ...
''
Aequorea victoria ''Aequorea victoria'', also sometimes called the crystal jelly, is a bioluminescent hydrozoan jellyfish, or hydromedusa, that is found off the west coast of North America. The species is best known as the source of two proteins involved in biol ...

''. This jellyfish lives in the photic zone off the west coast of North America and was identified as a carrier of
green fluorescent protein The green fluorescent protein (GFP) is a protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within orga ...
(GFP) by
Osamu Shimomura was a and , and at (MBL) in and . He was awarded the in 2008 for the discovery and development of (GFP) with two American scientists: of Columbia University and of the University of California-San Diego. Biography Born in in 1928, S ...
. The gene for these green fluorescent proteins has been isolated and is scientifically significant because it is widely used in genetic studies to indicate the expression of other genes.

### =Mantis shrimp

= Several species of
mantis shrimp Mantis shrimp, or stomatopods, are carnivorous marine crustaceans of the order Stomatopoda, branching from other members of the class Malacostraca around 340 million years ago. Mantis shrimps typically grow to around in length, while a few c ...

, which are stomatopod
crustaceans Crustaceans (Crustacea ) form a large, diverse arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Eua ...
, including ''Lysiosquillina glabriuscula'', have yellow fluorescent markings along their antennal scales and
carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...
(shell) that males present during threat displays to predators and other males. The display involves raising the head and thorax, spreading the striking appendages and other maxillipeds, and extending the prominent, oval antennal scales laterally, which makes the animal appear larger and accentuates its yellow fluorescent markings. Furthermore, as depth increases, mantis shrimp fluorescence accounts for a greater part of the visible light available. During mating rituals, mantis shrimp actively fluoresce, and the wavelength of this fluorescence matches the wavelengths detected by their eye pigments.

### =Siphonophores

= ''
Siphonophorae Siphonophorae (from Greek ''siphōn'' 'tube' + ''pherein'' 'to bear') is an order Order or ORDER or Orders may refer to: * Orderliness Orderliness is associated with other qualities such as cleanliness Cleanliness is both the abstract state o ...
'' is an order of marine animals from the phylum
Hydrozoa Hydrozoa (hydrozoans; ) are a taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification sch ...

that consist of a specialized and polyp
zooid A zooid or zoöid is a single animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Over 1.5 million animal have ...

. Some siphonophores, including the genus Erenna that live in the aphotic zone between depths of 1600 m and 2300 m, exhibit yellow to red fluorescence in the
photophores A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. The ...

of their tentacle-like
tentilla with 2 tentacles and 8 arms In zoology, a tentacle is a flexible, mobile, elongated organ (anatomy), organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomical ...
. This fluorescence occurs as a by-product of bioluminescence from these same photophores. The siphonophores exhibit the fluorescence in a flicking pattern that is used as a lure to attract prey.

### =Dragonfish

= The predatory deep-sea dragonfish ''Malacosteus niger'', the closely related genus ''
Aristostomias ''Aristostomias'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer ...
'' and the species ''Pachystomias microdon'' use fluorescent red accessory pigments to convert the blue light emitted from their own bioluminescence to red light from suborbital
photophores A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. The ...

. This red luminescence is invisible to other animals, which allows these dragonfish extra light at dark ocean depths without attracting or signaling predators.

## Terrestrial

### Amphibians

Fluorescence is widespread among amphibians and has been documented in several families of frogs, salamanders and caecilians, but the extent of it varies greatly. The polka-dot tree frog (''Hypsiboas punctatus''), widely found in South America, was unintentionally discovered to be the first fluorescent amphibian in 2017. The fluorescence was traced to a new compound found in the lymph and skin glands. The main fluorescent compound is Hyloin-L1 and it gives a blue-green glow when exposed to violet or ultraviolet light. The scientists behind the discovery suggested that the fluorescence can be used for communication. They speculated that fluorescence possibly is relatively widespread among frogs. Only a few months later, fluorescence was discovered in the closely related ''Hypsiboas atlanticus''. Because it is linked to secretions from skin glands, they can also leave fluorescent markings on surfaces where they have been. In 2019, two other frogs, the tiny pumpkin toadlet (''Brachycephalus ephippium'') and red pumpkin toadlet (''B. pitanga'') of southeastern Brazil, were found to be have naturally fluorescent skeletons, which is visible through their skin when exposed to ultraviolet light. It was initially speculated that the fluorescence supplemented their already aposematic colours (they are toxic) or that it was related to mate choice (species recognition or determining fitness of a potential partner), but later studies indicate that the former explanation is unlikely, as predation attempts on the toadlets appear to be unaffected by the presence/absence of fluorescence. In 2020 it was confirmed that green or yellow fluorescence is widespread not only in adult frogs that are exposed to blue or ultraviolet light, but also among tadpoles, salamanders and caecilians. The extent varies greatly depending on species; in some it is highly distinct and in others it is barely noticeable. It can be based on their skin pigmentation, their mucous or their bones.

### Butterflies

swallowtail butterfly, Swallowtail (''Papilio'') butterflies have complex systems for emitting fluorescent light. Their wings contain pigment-infused crystals that provide directed fluorescent light. These crystals function to produce fluorescent light best when they absorb radiance from sky-blue light (wavelength about 420 nm). The wavelengths of light that the butterflies see the best correspond to the absorbance of the crystals in the butterfly's wings. This likely functions to enhance the capacity for signaling.

### Parrots

Parrots have fluorescent plumage that may be used in mate signaling. A study using mate-choice experiments on budgerigars (''Melopsittacus undulates'') found compelling support for fluorescent sexual signaling, with both males and females significantly preferring birds with the fluorescent experimental stimulus. This study suggests that the fluorescent plumage of parrots is not simply a by-product of pigmentation, but instead an adapted sexual signal. Considering the intricacies of the pathways that produce fluorescent pigments, there may be significant costs involved. Therefore, individuals exhibiting strong fluorescence may be honest indicators of high individual quality, since they can deal with the associated costs.

### Arachnids

Spiders fluoresce under UV light and possess a huge diversity of fluorophores. Remarkably, spiders are the only known group in which fluorescence is "taxonomically widespread, variably expressed, evolutionarily labile, and probably under selection and potentially of ecological importance for intraspecific and interspecific signaling". A study by Andrews et al. (2007) reveals that fluorescence has evolved multiple times across spider taxa, with novel fluorophores evolving during spider diversification. In some spiders, ultraviolet cues are important for predator-prey interactions, intraspecific communication, and camouflaging with matching fluorescent flowers. Differing ecological contexts could favor inhibition or enhancement of fluorescence expression, depending upon whether fluorescence helps spiders be cryptic or makes them more conspicuous to predators. Therefore, natural selection could be acting on expression of fluorescence across spider species. Scorpions are also fluorescent due to the presence of beta carboline in their cuticles.

### Platypus

In 2020 fluorescence was reported for several specimen of platypus.

### Plants

Many plants are fluorescent due to the presence of
chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally ...

, which is probably the most widely-distributed fluorescent molecule, producing red emission under a range of excitation wavelengths. This attribute of chlorophyll is commonly used by ecologists to measure photosynthetic efficiency. The ''Mirabilis jalapa'' flower contains violet, fluorescent betacyanins and yellow, fluorescent betaxanthins. Under white light, parts of the flower containing only betaxanthins appear yellow, but in areas where both betaxanthins and betacyanins are present, the visible fluorescence of the flower is faded due to internal light-filtering mechanisms. Fluorescence was previously suggested to play a role in pollinator attraction, however, it was later found that the visual signal by fluorescence is negligible compared to the visual signal of light reflected by the flower.

## Abiotic

### Gemology, mineralogy and geology

Gemstones, minerals, may have a distinctive fluorescence or may fluoresce differently under short-wave ultraviolet, long-wave ultraviolet, visible light, or X-rays. Many types of calcite and amber will fluoresce under shortwave UV, longwave UV and visible light. Ruby, Rubies, emeralds, and diamonds exhibit red fluorescence under long-wave UV, blue and sometimes green light; diamonds also emit light under X-ray radiation. Fluorescence in minerals is caused by a wide range of activators. In some cases, the concentration of the activator must be restricted to below a certain level, to prevent quenching of the fluorescent emission. Furthermore, the mineral must be free of impurities such as iron or copper, to prevent quenching of possible fluorescence. Divalent manganese, in concentrations of up to several percent, is responsible for the red or orange fluorescence of calcite, the green fluorescence of willemite, the yellow fluorescence of esperite, and the orange fluorescence of wollastonite and clinohedrite. Hexavalent uranium, in the form of the uranyl cation (), fluoresces at all concentrations in a yellow green, and is the cause of fluorescence of minerals such as autunite or andersonite, and, at low concentration, is the cause of the fluorescence of such materials as some samples of hyalite opal. Trivalent chromium at low concentration is the source of the red fluorescence of ruby. Divalent
europium Europium is a chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numb ...

is the source of the blue fluorescence, when seen in the mineral
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride Calcium fluoride is the inorganic compound In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural scien ...

. Trivalent lanthanides such as terbium and dysprosium are the principal activators of the creamy yellow fluorescence exhibited by the yttrofluorite variety of the mineral fluorite, and contribute to the orange fluorescence of zircon. Powellite (calcium molybdate) and scheelite (calcium tungstate) fluoresce intrinsically in yellow and blue, respectively. When present together in solid solution, energy is transferred from the higher-energy tungsten to the lower-energy molybdenum, such that fairly low levels of molybdenum are sufficient to cause a yellow emission for scheelite, instead of blue. Low-iron sphalerite (zinc sulfide), fluoresces and phosphoresces in a range of colors, influenced by the presence of various trace impurities. 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. Humic acids and fulvic acids produced by the degradation of organic matter in soils (humus) may also fluoresce because of the presence of aromatic cycles in their complex molecular structures. Humic substances dissolved in groundwater can be detected and characterized by spectrofluorimetry.

### Organic liquids

Organic solutions such anthracene or stilbene, dissolved in benzene or toluene, fluoresce with
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

or gamma ray irradiation. The decay times of this fluorescence are on 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. Scintillation (physics), Scintillation is defined a flash of light produced in a transparent material by the passage of a particle (an electron, an alpha particle, an ion, or a high-energy photon). Stilbene and derivatives are used in scintillation counters to detect such particles. Stilbene is also one of the gain mediums used in dye lasers.

### Atmosphere

Fluorescence is observed in the atmosphere when the air is under energetic electron bombardment. In cases such as the natural aurora, high-altitude nuclear explosions, and rocket-borne electron gun experiments, the molecules and ions formed have a fluorescent response to light.

### Common materials that fluoresce

* Vitamin B2 fluoresces yellow. * Tonic water fluoresces blue due to the presence of
quinine Quinine is a medication used to treat malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms Signs and symptoms are the observed or detectable signs, and experienced sympto ...

. * Highlighter ink is often fluorescent due to the presence of pyranine. * Banknotes, postage stamps and credit cards often have fluorescent security features.

# In novel technology

In August 2020 researchers reported the creation of the brightest fluorescent solid optical materials so far by enabling the transfer of properties of highly fluorescent
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
s via spatial and electronic isolation of the dyes by mixing cationic dyes with anion-binding cyanostar macrocycles. According to a co-author these materials may have applications in areas such as solar energy harvesting, bioimaging, and lasers.

# Applications

## Lighting

The common
fluorescent lamp A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor that uses to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a coating ...

relies on fluorescence. Inside the glass tube is a partial vacuum and a small amount of mercury (element), mercury. An electric discharge in the tube causes the mercury atoms to emit mostly ultraviolet light. The tube is lined with a coating of a fluorescent material, called the ''phosphor'', which absorbs ultraviolet light and re-emits visible light. Fluorescent lighting is more energy-efficient than incandescent lighting elements. However, the uneven spectrum of traditional fluorescent lamps may cause certain colors to appear different from when illuminated by incandescent light or daylight. The mercury vapor emission spectrum is dominated by a short-wave UV line at 254 nm (which provides most of the energy to the phosphors), accompanied by visible light emission at 436 nm (blue), 546 nm (green) and 579 nm (yellow-orange). These three lines can be observed superimposed on the white continuum using a hand spectroscope, for light emitted by the usual white fluorescent tubes. These same visible lines, accompanied by the emission lines of trivalent europium and trivalent terbium, and further accompanied by the emission continuum of divalent europium in the blue region, comprise the more discontinuous light emission of the modern trichromatic phosphor systems used in many compact fluorescent lamp and traditional lamps where better color rendition is a goal. Fluorescent lights were first available to the public at the 1939 New York World's Fair. Improvements since then have largely been better phosphors, longer life, and more consistent internal discharge, and easier-to-use shapes (such as compact fluorescent lamps). Some High-intensity discharge lamp, high-intensity discharge (HID) lamps couple their even-greater electrical efficiency with phosphor enhancement for better color rendition. White light-emitting diodes (LEDs) became available in the mid-1990s as LED lamps, in which blue light emitted from the semiconductor strikes phosphors deposited on the tiny chip. The combination of the blue light that continues through the phosphor and the green to red fluorescence from the phosphors produces a net emission of white light. Glow sticks sometimes utilize fluorescent materials to absorb light from the chemiluminescence, chemiluminescent reaction and emit light of a different color.

## Analytical chemistry

Many analytical procedures involve the use of a fluorometer, usually with a single exciting wavelength and single detection wavelength. Because of the sensitivity that the method affords, fluorescent molecule concentrations as low as 1 part per trillion can be measured. Fluorescence in several wavelengths can be detected by an Chromatography detector, array detector, to detect compounds from High-performance liquid chromatography, HPLC flow. Also, Thin layer chromatography, 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 excitement and release of photons by lower-energy atoms, making analysis more efficient.

## Spectroscopy

Usually the setup of a fluorescence assay involves a light source, which may emit many different wavelengths of light. In general, a single wavelength is required for proper analysis, so, in order to selectively filter the light, it is passed through an excitation monochromator, and then that chosen wavelength is passed through the sample cell. After absorption and re-emission of the energy, many wavelengths may emerge due to
Stokes shift __NOTOC__ Stokes shift is the difference (in energy In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies m ...

and various electron transitions. To separate and analyze them, the fluorescent radiation is passed through an emission monochromator, and observed selectively by a detector.

## Biochemistry and medicine

Fluorescence in the life sciences is used generally as a non-destructive way of tracking or analysis of biological molecules by means of the fluorescent emission at a specific frequency where 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 an extrinsic
fluorophore A fluorophore (or fluorochrome, similarly to a chromophore) is a fluorescent chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms fro ...

, a fluorescent
dye A dye is a color Color (American English) or colour (Commonwealth English) is the visual perception, visual perceptual Physical property, property corresponding in humans to the categories called ''blue'', ''green'', ''red'', etc. Colo ...
that 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:

### Microscopy

* When scanning the fluorescence intensity across a plane one has fluorescence microscope, 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. Labelling multiple antibodies with different fluorophores allows visualization of multiple targets within a single image (multiple channels). DNA microarrays are a variant of this. * Immunology: An antibody is first prepared by having 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. * FLIM (Fluorescence Lifetime Imaging Microscopy) can be used to detect certain bio-molecular interactions that manifest themselves by influencing fluorescence lifetimes. * Cell and molecular biology: detection of colocalization using fluorescence-labelled antibodies for selective detection of the antigens of interest using specialized software such as ImageJ.

### Other techniques

* FRET ( Förster resonance energy transfer, also known as ) is used to study protein interactions, detect specific nucleic acid sequences and used as biosensors, while fluorescence lifetime (FLIM) can give an additional layer of information. * Biotechnology: biosensors using fluorescence are being studied as possible Fluorescent glucose biosensors. * 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. * FACS (fluorescence-activated cell sorting). One of several important cell sorting techniques used in the separation of different cell lines (especially those isolated from animal tissues). * DNA detection: the compound ethidium bromide, in aqueous solution, has very little fluorescence, as it is quenched by water. Ethidium bromide's fluorescence is greatly enhanced after it binds to DNA, so this compound is very useful in visualising the location of DNA fragments in agarose gel electrophoresis. Intercalated ethidium is in a hydrophobic environment when it is between the base pairs of the DNA, protected from quenching by water which is excluded from the local environment of the intercalated ethidium. Ethidium bromide may be carcinogenic – an arguably safer alternative is the dye SYBR Green. * FIGS (Fluorescence image-guided surgery) is a medical imaging technique that uses fluorescence to detect properly labeled structures during surgery. * Intravascular fluorescence is a catheter-based medical imaging technique that uses fluorescence to detect high-risk features of atherosclerosis and unhealed vascular stent devices. Plaque autofluorescence has been used in a first-in-man study in coronary arteries in combination with optical coherence tomography. Molecular agents has been also used to detect specific features, such as stent fibrin accumulation and enzymatic activity related to artery inflammation. * SAFI (species altered fluorescence imaging) an imaging technique in electrokinetic phenomena, electrokinetics and microfluidics. It uses non-electromigrating dyes whose fluorescence is easily quenched by migrating chemical species of interest. The dye(s) are usually seeded everywhere in the flow and differential quenching of their fluorescence by analytes is directly observed. * Fluorescence-based assays for screening Toxicity, toxic chemicals. The optical assays consist of a mixture of environmental-sensitive fluorescent dyes and human skin cells that generate fluorescence spectra patterns. This approach can reduce the need for Animal testing, laboratory animals in biomedical research and pharmaceutical industry. *Bone-margin detection: Alizarin, Alizarin-stained specimens and certain fossils can be lit by fluorescent lights to view anatomical structures, including bone margins.

## Forensics

Fingerprints can be visualized with fluorescent compounds such as ninhydrin or DFO (1,8-Diazafluoren-9-one). Blood and other substances are sometimes detected by fluorescent reagents, like fluorescein. Fibers, and other materials that may be encountered in Forensic science, forensics or with a relationship to various collectibles, are sometimes fluorescent.

## Non-destructive testing

Fluorescent penetrant inspection is used to find cracks and other defects on the surface of a part. Dye tracing, using fluorescent dyes, is used to find leaks in liquid and gas plumbing systems.

## Signage

Fluorescent colors are frequently used in signage, particularly road signs. Fluorescent colors are generally recognizable at longer ranges than their non-fluorescent counterparts, with fluorescent orange being particularly noticeable.Hawkins, H. Gene; Carlson, Paul John and Elmquist, Michael (2000
"Evaluation of fluorescent orange signs"
, Texas Transportation Institute Report 2962-S.
This property has led to its frequent use in safety signs and labels.

## Optical brighteners

Fluorescent compounds are often used to enhance the appearance of fabric and paper, causing a "whitening" effect. A white surface treated with an optical brightener can emit more visible light than that which shines on it, making it appear brighter. The blue light emitted by the brightener compensates for the diminishing blue of the treated material and changes the hue away from yellow or brown and toward white. Optical brighteners are used in laundry detergents, high brightness paper, cosmetics, high-visibility clothing and more.

* Absorption-re-emission atomic line filters use the phenomenon of fluorescence to filter light extremely effectively. * Black light * Blacklight paint * Fiber photometry * Fluorescence-activating and absorption-shifting tag * Fluorescence correlation spectroscopy * Fluorescence image-guided surgery * Fluorescence in plants * Fluorescence spectroscopy * Fluorescent lamp * Fluorescent Multilayer Disc * Fluorometer * High-visibility clothing * Integrated fluorometer * Laser-induced fluorescence * List of light sources * Microbial art, using fluorescent bacteria * Mössbauer effect, resonant fluorescence of gamma rays * Organic light-emitting diodes can be fluorescent *
Phosphorescence Phosphorescence is a type of photoluminescence Photoluminescence (abbreviated as PL) is light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, percei ...

* Phosphor thermometry, the use of phosphorescence to measure temperature. * Spectroscopy * Two-photon absorption * Vibronic spectroscopy * X-ray fluorescence

# Bibliography

*

*

Fluorophores.org
the database of fluorescent dyes

Basic Concepts in Fluorescence
"A nano-history of fluorescence" lecture by David Jameson

Database of fluorescent minerals with pictures, activators and spectra (fluomin.org)

"Biofluorescent Night Dive – Dahab/Red Sea (Egypt), Masbat Bay/Mashraba, "Roman Rock""
YouTube. 9 October 2012. * Steffen O. Beyer
"FluoPedia.org: Publications"
fluopedia.org. * Steffen O. Beyer
"FluoMedia.org: Science"
fluomedia.org. {{Authority control Fluorescence, Dyes Molecular biology Radiochemistry