Quenching refers to any process which decreases the

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

intensity of a given substance. A variety of processes can result in quenching, such as excited state reactions, energy transfer, complex-formation and collisional quenching. As a consequence, quenching is often heavily dependent on

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

and

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

. Molecular

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

iodide An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine de ...

ions and

acrylamide Acrylamide (or acrylic amide) is an organic compound with the chemical formula CH2=CHC(O)NH2. It is a white odorless solid, soluble in water and several organic solvents. From the chemistry perspective, acrylamide is a vinyl-substituted primary ...

are common chemical quenchers. The chloride ion is a well known quencher for quinine fluorescence. Quenching poses a problem for non-instant spectroscopic methods, such as

laser-induced fluorescence Laser-induced fluorescence (LIF) or laser-stimulated fluorescence (LSF) is a spectroscopic method in which an atom or molecule is excited to a higher energy level by the absorption of laser light followed by spontaneous emission of light. It was f ...

. Quenching is made use of in

optode An optode or optrode is an optical sensor device that optically measures a specific substance usually with the aid of a chemical transducer. Construction An optode requires three components to function: a chemical that responds to an analyte, a p ...

sensors; for instance the quenching effect of oxygen on certain

ruthenium Ruthenium is a chemical element with the symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to most other chemical ...

complexes allows the measurement of

oxygen saturation Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...

in solution. Quenching is the basis for

Förster resonance energy transfer Förster resonance energy transfer (FRET), fluorescence resonance energy transfer, resonance energy transfer (RET) or electronic energy transfer (EET) is a mechanism describing energy transfer between two light-sensitive molecules (chromophores). ...

(FRET) assays. Quenching and dequenching upon interaction with a specific molecular biological target is the basis for activatable optical contrast agents for

molecular imaging Molecular imaging is a field of medical imaging that focuses on imaging molecules of medical interest within living patients. This is in contrast to conventional methods for obtaining molecular information from preserved tissue samples, such as h ...

. Many dyes undergo self-quenching, which can decrease the brightness of protein-dye conjugates for

fluorescence microscopy A fluorescence microscope is an optical microscope that uses fluorescence instead of, or in addition to, scattering, reflection, and attenuation or absorption, to study the properties of organic or inorganic substances. "Fluorescence microscop ...

, or can be harnessed in sensors of proteolysis.

Mechanisms

Förster resonance energy transfer

There are a few distinct mechanisms by which energy can be transferred non-radiatively (without absorption or emission of photons) between two dyes, a donor and an acceptor.

(FRET or FET) is a dynamic quenching mechanism because energy transfer occurs while the donor is in the excited state. FRET is based on classical dipole-dipole interactions between the transition

dipole In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...

s of the donor and acceptor and is extremely dependent on the donor-acceptor distance, ''R'', falling off at a rate of 1/''R''⁶. FRET also depends on the donor-acceptor spectral overlap (see figure) and the relative orientation of the donor and acceptor transition dipole moments. FRET can typically occur over distances up to 100 Å.

Dexter electron transfer

Dexter (also known as Dexter exchange or collisional energy transfer, colloquially known as Dexter Energy Transfer) is another dynamic quenching mechanism. Dexter electron transfer is a short-range phenomenon that falls off exponentially with distance (proportional to e^−''kR'' where k is a constant that depends on the inverse of the van der Waals radius of the atom) and depends on spatial overlap of donor and quencher molecular orbitals. In most donor-fluorophore–quencher-acceptor situations, the Förster mechanism is more important than the Dexter mechanism. With both Förster and Dexter energy transfer, the shapes of the absorption and fluorescence spectra of the dyes are unchanged. Dexter electron transfer can be significant between the dye and the solvent especially when hydrogen bonds are formed between them.

Exciplex

Exciplex An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has a valence shell completely filled with electrons (for example, noble gases). In this case, form ...

(excited state complex) formation is a third dynamic quenching mechanism.

Static quenching

The remaining energy transfer mechanism is static quenching (also referred to as contact quenching). Static quenching can be a dominant mechanism for some reporter-quencher probes. Unlike dynamic quenching, static quenching occurs when the molecules form a complex in the ground state, i.e. before excitation occurs. The complex has its own unique properties, such as being nonfluorescent and having a unique absorption

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. The word was first used scientifically in optics to describe the rainbow of colors i ...

. Dye aggregation is often due to

hydrophobic In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water. Hydrophobic molecules tend to be nonpolar and, t ...

effects—the dye molecules stack together to minimize contact with water. Planar aromatic dyes that are matched for association through hydrophobic forces can enhance static quenching. High temperatures and addition of surfactants tend to disrupt ground state complex formation.

Collisional quenching

Collisional quenching occurs when the excited fluorophore experiences contact with an atom or molecule that can facilitate non-radiative transitions to the ground state. ... Excited-state molecule collides with quencher molecule and returns to ground state non-radiatively.

References

{{DEFAULTSORT:Quenching (Fluorescence) Fluorescence Reaction mechanisms