Chemoluminescence

From Academic Kids

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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% QC, 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.


Contents

Liquid-phase reactions

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Luminol.jpg

Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper[1], or an auxiliary oxidant[2], produces chemoluminescence. The luminol reaction is

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

The quantum efficiency, QC is 1%. For the laboratory experiment see reference [1] or [2]

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.[4]

cyalume + H2O2 + dye → trichlorophenol + 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 [3].

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. [4]

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

Bioluminesence

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, QC 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.

ECL

Enzymatic Chemiluminescence (ECL) is a common technique for a variety of detection assays in biology. an 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:

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ECL.png
Image:ECL.png

External links and references

[1] Luminol chemistry laboratory demonstration [1] (http://www.3rd1000.com/labs/lumine.htm)

[2] Luminol chemistry laboratory demonstration #2 [2] (http://www.york.ac.uk/org/seg/salters/chemistry/ResourceSheets/luminol.PDF)

[3] Helmenstine, Anne Marie (Aug 10, 2004). Light stick chemistry, retrieved Sept. 22, 2004. [3] (http://chemistry.about.com/library/weekly/aa031703a.htm)

[4] 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

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