Chemoluminescence

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Image:Lightsticks 3.jpg Chemoluminescence (sometimes "chemiluminescence") is the emission of light (luminescence) as the result of a chemical reaction. Most simply, given reactants A and B, with an excited intermediate ◊, we have,

[A] + [B] → [◊] → [Products] + light

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% Q_C, quantum efficiency.

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

Where 3-APA is 3-aminophthalate. And 3-APA[◊] is the excited state florescing as it decays to a lower energy level.

A standard example of chemoluminescence 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 chemoluminescence takes place in living organisms, the phenomenon is called bioluminescence.

A lightstick emits light by chemoluminescence.

1 Liquid-phase reactions
2 Gas-phase reactions
3 Bioluminescence
4 ECL
5 Applications of chemoluminescence
6 See also
7 External links and references

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Liquid-phase reactions

Image:Luminol.jpg Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper<ref name=luminol1>Template:Cite web</ref>, or an auxiliary oxidant<ref name=luminol2>Template:Cite web</ref>, produces chemoluminescence. The luminol reaction is

luminol + H₂O₂ → 3-APA[◊] → 3-APA + light

The quantum efficiency, Q_C is 1%. For the laboratory experiment see references <ref name=luminol1/>^,<ref name=luminol2/>.

Cyalume, as used in a lightstick, emits light by chemoluminescence of a fluorescent dye activated by cyalume reacting with hydrogen peroxide in the most efficient non-enzymatic reaction known.<ref name=Rauhut>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. ISBN 0-471-51700-3</ref>

cyalume + H₂O₂ + dye → phenol + 2CO₂ + dye[◊]

When the activated fluorescent dye decays to a lower energy level, light is given off. The color depends upon the dye. For a list of dyes see reference <ref>Helmenstine, Anne Marie (Aug 10, 2004). Light stick chemistry, retrieved Sept. 22, 2004.</ref>.

There are a number of other chemiluminescence reactions. Some of them are briefly described here.

Ru(bipy)₃²⁺ is a ruthenium(II) complex which undergoes oxidation to ruthenium(III) if certain oxidizing agents are introduced. If ruthenium(III) complex is then reduced in alkaline medium, emission of light occurs.

First, there is a reaction:

2Ru(bipy)₃²⁺ + PbO₂ + 4H⁺ → 2Ru(bipy)₃³⁺ + Pb²⁺ + 2H₂O

Here, Ru(III) is obtained. Further reaction includes use of solution of sodium tetrahydroborate(III), NaBH₄ in alkaline medium. When the solution is added, Ru(III) is reduced to Ru(II) and we get orange light.

TMAE (chemically tetrakis(dimethylamino) ethylene) can be oxidized by air. When that happens, emission of clear blue green light occurs.

Oxalyl chloride (C₂O₂Cl₂) produces light when oxidized - but only in the presence of a sensitiser. That can be any compound with rigid system of aromatic rings or a compound, which includes enough double or triple bonds in its structure to store electrochemical energy through means of vibration. The excited molecule (in this case, oxalyl chloride) transfers its electrochemical energy in certain phase of chemical reaction to sensitiser, thus making the sensitiser excited and capable of light emission. Good examples are rodamine 6 G, fluorescein, violanthrone and similar compounds. Anyway, if oxalyl chloride is treated with H₂O₂ in non-aqueous media (e.g. CH₂Cl₂) in the presence of sensitiser, emission of light is obtained. The colour, intensity and duration of light emission depend on the sensitiser used. Rodamin 6 G gives bright orange light with moderate duration of emission.

Pyrogallol (chemically 1,2,3-trihydroxibenzene) is also capable of light emission under right circumstances. If an aqueous solution of pyrogallol, NaOH and K₂CO₃ is mixed with formaldehyde, short-lived red emission occurs.

Pure oxygen (O₂) can also emit light. If solutions of 30% hydrogen peroxide and 5% alkaline sodium chlorate(I) (NaClO) are mixed, red light is emitted. It is barely visible, though - for this reason, in such experiment sensitiser is often included to boost light emission in terms of brightness and intensity. Again, both colour and intensity of light depend on sensitiser used.

Lucigenin oxidation is also very well known among chemiluminescence reactions. If an aqueous lucigenin solution is mixed with highly alkaline aqueous solution containing ethanol or acetone and hydrogen peroxide, very bright green emission is produced that decays to greenish blue and finally blue emission. The duration of the emission can be up to a couple of minutes under the right circumstances.

For more information about how to perform experiments mentioned, see the reference <ref>Bassam Z. Shakhashiri: Chemical Demonstrations, Volume 1, University of Wisconsin 1983.</ref>.

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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 excited states (PO)₂ and HPO.<ref name=Rauhut />

Another gas phase reaction is the basis of nitric oxide detection in commercial analytic instruments applied to environmental air quality testing. Ozone is combined with nitric oxide to form nitrogen dioxide in an activated state.

NO+O₃ → NO₂[◊]+ O₂

The activated NO₂[◊] luminesces broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier and associated electronics counts the photons which are proportional to the amount of NO present.

To determine the amount of nitrogen 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 which is proportional to NO₂ before it was converted to NO.

In the case of a mixed sample containing both NO and NO₂, the above reaction yields the amount of NO and NO₂ combined in the air sample, assuming that the sample is passed though the converter.

If the mixed sample is not passed through the converter, the ozone reaction produces activated NO₂[◊] only in proportion to the NO in the sample. The NO₂ in the sample is not activated by the ozone reaction. Though unactivated NO₂ is present with the acitvated NO₂[◊], photons are only emitted by the activated species which is proportional to original NO.

Final step, subtract NO from (NO + NO₂) to yield NO₂

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Bioluminescence

Main article: Bioluminescence

Chemoluminescence takes place in numerous living organisms, the American firefly being a widely studied case of bioluminescence.

The firefly reaction has the highest known quantum efficiency, Q_C of 88%, for chemoluminescence reactions. ATP (adenosine tri-phosphate), the ubiquitous biological energy source, reacts with luciferin with the aid of the enzyme luciferase to yield an intermediate complex. This complex combines with oxygen to produce a highly fluorescent compound.

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ECL

Enzymatic Chemiluminescence (ECL) is a common technique for a variety of detection assays in biology. a horseradish peroxidase molecule (HRP) is tethered to the molecule of interest (usually by immunoglobulin staining). This then locally catalyzes the conversion of the ECL reagent into a sensitized reagent, which on further oxidation by hydrogen peroxide, produces a triplet (excited) carbonyl which emits light when it decays to the singlet carbonyl.

The mechanism of action for a typical ECL reagent:

Image:ECL.png

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Applications of chemoluminescence

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). 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

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