Chemiluminescence

  Chemiluminescence (sometimes "chemoluminescence") is the emission of light (luminescence) without emission of heat as the result of a chemical reaction. Given reactants A and B, with an excited intermediate ◊, 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

where:

• where 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% Q_C, quantum efficiency.

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 light by chemiluminescence.

Additional recommended knowledge

What is the Sensitivity of my Balance?

Essential Laboratory Skills Guide

Daily Sensitivity Test

Liquid-phase reactions

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

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

The quantum efficiency, Q_C is 1%. For the laboratory experiment see references ^[1],[2].

• Cyalume^®, as used in a lightstick, emits light by chemiluminescence of a fluorescent dye (also called sensitiser) activated by cyalume reacting with hydrogen peroxide in the most efficient non-enzymatic reaction known.^[3]

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.^[4].

• Oxalyl chloride (C₂O₂Cl₂) produces light when oxidized - but only in the presence of a sensitiser, similar to the above examples. If oxalyl chloride is treated with H₂O₂ in non-aqueous media (e.g. CH₂Cl₂) in the presence of a 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.

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) an orange light is emitted.

• TMAE (tetrakis(dimethylamino)ethylene) emits clear blue-green light upon oxidation by air.
• Pyrogallol (1,2,3-trihydroxibenzene) is also capable of light emission. 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 hypochlorite (NaClO) are mixed, red light is emitted. It is barely visible, though - for this reason a sensitiser is often included to boost light emission in terms of brightness and intensity. Again, both colour and intensity of light depend on the 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. ^[5].
• Solutions containing ions of manganese (VII, IV, III) show Chemiluminescence (690 nm) when reduced by sodium borohydride at low pH to Mn(II) ^[6]

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.^[3]
• 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 through 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 activated 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

Bioluminescence

Main article: Bioluminescence

Chemiluminescence 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 chemiluminescence 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 chemiluminescent compound.

ECL

Enhanced chemiluminescence (ECL) 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 ECL 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 (ECL review), well below the detection limit for most assay systems.

The mechanism of action for a typical ECL reagent:

Applications

• 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
• detection and quantitation of biomolecules in assay systems such as ELISA and Western blots
• DNA sequencing using pyrosequencing

References

Horiba