Chemiluminescence (alsochemoluminescence) is the emission of light (luminescence) as the result of achemical reaction, i.e. a chemical reaction results in a flash or glow of light. A standard example of chemiluminescence in the laboratory setting is theluminol test. Here,blood is indicated byluminescence upon contact withiron inhemoglobin. When chemiluminescence takes place in living organisms, the phenomenon is calledbioluminescence. Alight stick emits light by chemiluminescence.
As in many chemical reactions, chemiluminescence starts with the combining of two compounds, say A and B, to give a product C. Unlike most chemical reactions, the product C converts to a further product, which is produced in an electronicallyexcited state often indicated with an asterisk:
A + B → C
C → D*
D* then emits a photon (hν), to give theground state of D:[1] I
D* → D +hν
In theory, onephoton of light should be given off for each molecule ofreactant. In practice, the yield ("quantum efficiency") is often low owing to side reactions.
Chemiluminescence differs fromfluorescence orphosphorescence in that the electronic excited state is the product of a chemical reaction rather than of theabsorption of a photon. It is the antithesis of aphotochemical reaction, in which light is used to drive an endothermic chemical reaction. Here, light isgenerated from a chemically exothermic reaction. The chemiluminescence might be also induced by an electrochemical stimulus, in this case is calledelectrochemiluminescence.
Chemiluminescence was first observed withlophine (triphenylimidazole).[2] When in basic solution, this compound converts to the imidazolate, which reacts with oxygen to eventually give adioxetane. Fragmentation of the dioxetane gives the excited state of an anionic diamide.[3]
Steps leading up to chemiluminescence of lophine.
Chemiluminescence in aqueous system is mainly caused by redox reactions.[4]
Luminol in analkaline solution withhydrogen peroxide in the presence of iron or copper,[5] or an auxiliaryoxidant,[6] produces 3-aminophtalate in an excited state, which exhibits chemiluminescence. The luminol reaction is
One of the oldest known chemiluminescent reactions is that of elementalwhite phosphorus oxidizing in moist air, producing a green glow. This is a gas-phase reaction of phosphorus vapor, above the solid, with oxygen producing excited states of(PO)2 and HPO.[7]
Another gas phase reaction is the basis ofnitric oxide detection in commercial analytic instruments applied to environmental air-quality testing.Ozone (O3) is combined withnitric oxide (NO) to formnitrogen dioxide (NO2) in an activated state [◊]:
The activatedNO2[◊] luminesces broadband visible to infrared light as it reverts to a lower energy state. Aphotomultiplier and associated electronics counts the photons that are proportional to the amount of NO present. To determine the amount ofnitrogen dioxide,NO2, in a sample (containing no NO) it must first be converted to nitric oxide, NO, by passing the sample through a converter before the above ozone activation reaction is applied. The ozone reaction produces a photon count proportional to NO that is proportional toNO2 before it was converted to NO. In the case of a mixed sample that contains both NO andNO2, the above reaction yields the amount of NO andNO2 combined in the air sample, assuming that the sample is passed through the converter. If the mixed sample is not passed through the converter, the ozone reaction produces activatedNO2[◊] only in proportion to the NO in the sample. TheNO2 in the sample is not activated by the ozone reaction. Though unactivatedNO2 is present with the activatedNO2[◊], photons are emitted only by the activated species that is proportional to original NO. Final step: Subtract NO from (NO + NO2) to yieldNO2[8]
Inchemical kinetics,infrared chemiluminiscence (IRCL) refers to the emission of infrared photons from vibrationally excited product molecules immediately after their formation. The intensities of infrared emission lines from vibrationally excited molecules are used to measure the populations of vibrational states of product molecules.[9][10]
The observation of IRCL was developed as a kinetic technique byJohn Polanyi, who used it to study theattractive or repulsive nature of thepotential energy surface for gas-phase reactions. In general the IRCL is much more intense for reactions with an attractive surface, indicating that this type of surface leads to energy deposition in vibrational excitation. In contrast reactions with a repulsive potential energy surface lead to little IRCL, indicating that the energy is primarily deposited as translational energy.[11]
Enhanced chemiluminescence (ECL) is a common technique for a variety of detection assays in biology. Ahorseradish peroxidase enzyme (HRP) is tethered to an antibody that specifically recognizes the molecule of interest. 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 furtheroxidation byhydrogen 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,[12][13] well below the detection limit for most assay systems.
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. Highly specialised chemiluminescence detectors have been used recently to determine concentrations as well as fluxes ofNOx with detection limits as low as 5 ppt.[14][15][16]
Analysis of inorganic species in liquid phase
Analysis of organic species: useful withenzymes, where the substrate is not directly involved in the chemiluminescence reaction, but the product is
Detection and assay of biomolecules in systems such asELISA andWestern blots
Combustion analysis: Certainfree 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.[19]
Chemiluminescence has been applied byforensic scientists to solve crimes. In this case, they use luminol and hydrogen peroxide. The iron from the blood acts as a catalyst and reacts with the luminol and hydrogen peroxide to produce blue light for about 30 seconds. Because only a small amount of iron is required for chemiluminescence, trace amounts of blood are sufficient.
In biomedical research, the protein that givesfireflies their glow and its co-factor,luciferin, are used to produce red light through the consumption of ATP. This reaction is used in many applications, including the effectiveness of cancer drugs that choke off a tumor's blood supply.[20] This form ofbioluminescence imaging allows scientists to test drugs in the pre-clinical stages cheaply.Another protein,aequorin, found in certain jellyfish, produces blue light in the presence of calcium. It can be used in molecular biology to assess calcium levels in cells. What these biological reactions have in common is their use ofadenosine triphosphate (ATP) as an energy source. Though the structure of the molecules that produce luminescence is different for each species, they are given the generic name of luciferin. Firefly luciferin can be oxidized to produce an excited complex. Once it falls back down to a ground state a photon is released. It is very similar to the reaction with luminol.
Many organisms have evolved to produce light in a range of colors. At the molecular level, the difference in color arises from the degree of conjugation of the molecule, when an electron drops down from the excited state to the ground state. Deep sea organisms have evolved to produce light to lure and catch prey, as camouflage, or to attract others. Some bacteria even use bioluminescence to communicate. The common colors for the light emitted by these animals are blue and green because they have shorter wavelengths than red and can transmit more easily in water.
In April 2020, researchers reported havinggenetically engineered plants glow much brighter than previously possible by inserting genes of thebioluminescent mushroomNeonothopanus nambi. The glow is self-sustained, works by converting plants'caffeic acid into luciferin and, unlike for bacterial bioluminescence genes used earlier, has a relatively high light output that is visible to the naked eye.[21][22][23][24]
^Shah, Syed Niaz Ali; Lin, Jin-Ming (2017). "Recent advances in chemiluminescence based on carbonaceous dots".Advances in Colloid and Interface Science.241:24–36.doi:10.1016/j.cis.2017.01.003.PMID28139217.
^Rauhut, Michael M. (1985), Chemiluminescence. In Grayson, Martin (Ed) (1985).Kirk-Othmer Concise Encyclopedia of Chemical Technology (3rd ed), pp 247 John Wiley and Sons.ISBN0-471-51700-3
^Stella, P., Kortner, M., Ammann, C., Foken, T., Meixner, F. X., and Trebs, I.: Measurements of nitrogen oxides and ozone fluxes by eddy covariance at a meadow: evidence for an internal leaf resistance to NO2, Biogeosciences, 10, 5997-6017,doi:10.5194/bg-10-5997-2013, 2013.
^Tsokankunku, Anywhere: Fluxes of the NO-O3-NO2 triad above a spruce forest canopy in south-eastern Germany. Bayreuth, 2014 . - XII, 184 P. ( Doctoral thesis, 2014, University of Bayreuth, Faculty of Biology, Chemistry and Earth Sciences)[1]
^Kuntzleman, Thomas Scott; Rohrer, Kristen; Schultz, Emeric (2012-06-12). "The Chemistry of Lightsticks: Demonstrations To Illustrate Chemical Processes".Journal of Chemical Education.89 (7):910–916.Bibcode:2012JChEd..89..910K.doi:10.1021/ed200328d.ISSN0021-9584.
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