Chemiluminescence is the generation of electromagnetic radiation as light, without heat, by the release of energy from a chemical reaction. While the light can, in principle, be emitted in the ultraviolet, visible or infrared region, those emitting visible light are the most common. They are also the most interesting and useful.
Bioluminescence is when light is produced by a chemical reaction which originates in an living organism.
Chemiluminescence (sometimes "chemoluminescence") is the emission of light with limited emission of heat (luminescence), as the result of a chemical reaction. Given reactants (substance that is added to a system in order to bring about a chemical reaction) A and B, with an excited intermediate (formed from the reactants and reacts further to give the directly observed products of a chemical reaction.) we have:
[A] + [B] → [◊] → [Products] + light
For example, if [A] is luminol and [B] is hydrogen peroxide in the presence of a suitable catalyst we have:
luminol + H2O2 → 3-APA[◊] → 3-APA + light
- 3-APA is 3-aminophthalate
- 3-APA[◊] is the excited state fluorescing as it decays to a lower energy level.
The decay of the excited state[◊] to a lower energy level is responsible for the emission of light. In theory, one photon of light should be given off for each molecule of reactant, or Avogadro's number of photons per mole. In actual practice, non-enzymatic reactions seldom exceed 1% QC, quantum efficiency.
In a chemical reaction, reactants collide to form a transition state, the enthalpic maximum in a reaction coordinate diagram, which proceeds to the product. Normally under reacting conditions, reactants form products of lesser chemical energy. The difference in energy between reactants and products is turned into heat, physically realized as excitations in the vibrational state of the normal modes of the product. Since vibrational energy is generally much greater than the thermal agitation, it is rapidly dispersed into the solvent through solvent molecules' rotation and translation. This underlies how exothermic reactions make their solutions hotter. In a chemiluminescent reaction, the direct product of a reaction is delivered in an excited electronic state, which then decays into an electronic ground state through either fluorescence or phosphorescence, depending on the spin state of the electronic excited state formed. The reason why this is possible is that chemical bond formation can occur on a timescale faster than electronic transitions, and therefore can result in discrete products in excited electronic states.
Chemiluminescence differs from fluorescence in that the electronic excited state is derived from the product of a chemical reaction rather than the more typical way of creating electronic excited states, namely absorption. It is the antithesis of a photochemical reaction, in which light is used to drive an endothermic chemical reaction. Here, light is generated from a chemically exothermic reaction.
A standard example of chemiluminescence in the laboratory setting is found in the luminol test, where evidence of blood is taken when the sample glows upon contact with iron. When chemiluminescence takes place in living organisms, the phenomenon is called bioluminescence. A lightstick emits a form of light by chemiluminescence.
Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper, or an auxiliary oxidant, produces chemiluminescence.
Cyalume, as used in a lightstick, emits light by chemiluminescence of a fluorescent dye (also called fluorescor) activated by cyalume reacting with hydrogen peroxide in the presence of a catalyst,such as sodium salicylate. It is the most efficient chemiluminescent reaction known.
Gas-phase reactions: One of the oldest known chemoluminescent reactions is that of elemental white phosphorus oxidizing in moist air, producing a green glow. This is actually a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing the excited states (PO)2 and HPO.
Topics of Interest
- gas analysis: for determining small amounts of impurities or poisons in air. Other compounds can also be determined by this method (ozone, N-oxides, S-compounds). A typical example is NO determination with detection limits down to 1 ppb
- analysis of inorganic species in liquid phase
- analysis of organic species: useful with enzymes, where the substrate isn't directly involved in chemiluminescence reaction, but the product is
- detection and quantitation of biomolecules in assay systems such as ELISA and Western blots
- DNA sequencing using pyrosequencing
- Lighting objects. Chemiluminescence kites, emergency lighting, glow sticks (party decorations).
- Combustion analysis: certain radical species (such as CH* and OH*) give off radiation at specific wavelengths. The heat release rate is calculated by measuring the amount of light radiated from a flame at those wavelengths.
- children's toys
Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios for "living" and the Latin lumen "light". Bioluminescence is a naturally occurring form of chemiluminescence where energy is released by a chemical reaction in the form of light emission. Adenosine triphosphate (ATP) is involved in most instances. The chemical reaction can occur either inside or outside the cell. In bacteria, the expression of genes related to bioluminescence is controlled by an operon called the Lux operon. Bioluminescence has appeared independently several times (up to 30 or more) throughout evolution. Bioluminescence occurs in marine vertebrates and invertebrates, as well as microorganisms and terrestrial animals. Symbiotic organisms carried within larger organisms are also known to bioluminesce.
Enhanced chemiluminescence is a common technique for a variety of detection assays in biology. A horseradish peroxidase enzyme (HRP) is tethered to the molecule of interest (usually through labeling an immunoglobulin that specifically recognizes the molecule). This enzyme complex, then catalyzes the conversion of the enhanced chemiluminescent substrate into a sensitized reagent in the vicinity of the molecule of interest, which on further oxidation by hydrogen peroxide, produces a triplet (excited) carbonyl which emits light when it decays to the singlet carbonyl. Enhanced chemiluminescence allows detection of minute quantities of a biomolecule. Proteins can be detected down to femtomole quantities (Enhanced CL review), well below the detection limit for most assay systems.
Electrochemiluminescence or electrogenerated chemiluminescence (ECL) is a kind of luminescence produced during electrochemical reactions in solutions. In electrogenerated chemiluminescence, electrochemically generated intermediates undergo a highly exergonic reaction to produce an electronically excited state that then emits light. ECL excitation is caused by energetic electron transfer (redox) reactions of electrogenerated species. Such luminescence excitation is a form of chemiluminescence where one/all reactants are produced electrochemically on the electrodes. ECL is usually observed during application of potential (several volts) to electrodes of electrochemical cell that contains solution of luminescent species (polycyclic aromatic hydrocarbons, metal complexes) in aprotic organic solvent (ECL composition).
Lyoluminescence refers to the emission of light while dissolving a solid into a liquid solvent. It is actually a form of chemoluminescence. The most common lyoluminescent effect is seen when solid samples which have been heavily irradiated by ionizing radiation are dissolved in water. The total amount of light emitted by the material increases proportionally with the total radiation dose received by the material up to a certain level called the saturation value.
Luminol (C8H7N3O2) is a versatile chemical that exhibits chemiluminescence, with a striking blue glow, when mixed with an appropriate oxidizing agent. It is a white to slightly yellow crystalline solid that is soluble in most polar organic solvents, but insoluble in water.
Peroxyoxalate chemiluminescence was first reported by Rauhut in 1967 in the reaction of diphenyl oxalate. The emission is generated by the reaction of an oxalate ester with hydrogen peroxide in the presence of a suitably fluorescent energy acceptor. This reaction is used in glow sticks.
Phosphorescence is a specific type of photoluminescence related to fluorescence. Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs. The slower time scales of the re-emission are associated with "forbidden" energy state transitions in quantum mechanics. As these transitions occur less often in certain materials, absorbed radiation may be re-emitted at a lower intensity for up to several hours. The study of phosphorescent materials led to the discovery of radioactivity in 1896.
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