Chemiluminescence (also chemoluminescence) is the emission of light (luminescence), as
the result of a chemical reaction. There may also be limited emission of heat. Given reactants
A and B, with an excited intermediate ◊,
[A] + [B] → [◊] → [Products] + lightFor example, if [A] is luminol and [B] is hydrogen
peroxide in the presence of a suitable catalyst we have:
C
8
H
7
N
3
O
2
luminol
+
H
2
O
2
hydrogen
peroxide
⟶ 3
−
APA
[ ◊
]
⟶
3
−
APA
+ light
{\displaystyle {\ce {{\underset {luminol}{C8H7N3O2}}+{\underset {hydrogen\ peroxide}{H2O2}}->3-APA[\lozenge
]->{3-APA}+light}}} where:
3-APA is 3-aminophthalate 3-APA[◊] is the vibronic excited state fluorescing
as it decays to a lower energy level.
== General description == The decay of this excited state[◊] to a
lower energy level causes light emission. In theory, one photon of light should be given
off for each molecule of reactant. This is equivalent to Avogadro's number of photons
per mole of reactant. 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, reactants form products of lesser chemical energy. The
difference in energy between reactants and products, represented as
Δ
H
r x
n
{\displaystyle \Delta H_{rxn}} , 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 rapidly disperses in
the solvent through molecular rotation. This is how exothermic reactions make their solutions
hotter. In a chemiluminescent reaction, the direct product of the reaction is an excited
electronic state. This state then decays into an electronic ground state and emits light
through either an allowed transition (analogous to fluorescence) or a forbidden transition
(analogous to phosphorescence), depending partly on the spin state of the electronic
excited state formed. Chemiluminescence differs from fluorescence
or phosphorescence in that the electronic excited state is the product of a chemical
reaction rather than of the absorption of a photon. 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. The chemiluminescence might be also
induced by an electrochemical stimulus, in this case is called electrochemiluminescence.
A standard example of chemiluminescence in the laboratory setting is the luminol test.
Here, blood is indicated by luminescence upon contact with iron in hemoglobin. When chemiluminescence
takes place in living organisms, the phenomenon is called bioluminescence. A light stick emits
light by chemiluminescence.
== Liquid-phase reactions == Chemiluminescence in aqueous system is mainly
caused by redox reactions.
Luminol in an alkaline solution with hydrogen peroxide in the presence of iron or copper,
or an auxiliary oxidant, produces chemiluminescence. The luminol reaction is
C
8
H
7
N
3
O
2
luminol
+
H
2
O
2
hydrogen
peroxide
⟶ 3
−
APA
[ ◊
]
⟶
3
−
APA
+ light
{\displaystyle {\ce {{\underset {luminol}{C8H7N3O2}}+{\underset {hydrogen\ peroxide}{H2O2}}->3-APA[\lozenge
]->{3-APA}+light}}}
== Gas-phase reactions ==
One of the oldest known chemiluminescent reactions is that of elemental white phosphorus oxidizing
in moist air, producing a green glow. This is a gas-phase reaction of phosphorus vapor,
above the solid, with oxygen producing the excited states (PO)2 and HPO.
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[◊]+ O2The activated NO2[◊] luminesces
broadband visible to infrared light as it reverts to a lower energy state. A photomultiplier
and associated electronics counts the photons that 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 that is proportional to NO2 before it was converted to NO. In the
case of a mixed sample that contains 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 emitted only by the activated species that is proportional to
original NO. Final step: Subtract NO from (NO + NO2) to yield NO2
== Infrared chemiluminescence == In chemical 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.The observation of IRCL was developed as a kinetic technique
by John Polanyi, who used it to study the attractive or repulsive nature of the potential
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.
== Enhanced chemiluminescence == Enhanced chemiluminescence is a common technique
for a variety of detection assays in biology. A horseradish 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 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, well below the detection limit
for most assay systems.
== 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). 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 of NOx with detection limits as low as 5 ppt.
Analysis of inorganic species in liquid phase Analysis of organic species: useful with enzymes,
where the substrate is not directly involved in the chemiluminescence reaction, but the
product is Detection and assay of biomolecules in 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.
Glow sticks.
== Biological applications == Chemiluminescence has been applied by forensic
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 gives fireflies 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. This form of bioluminescence 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 of adenosine 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.
Luciferin
+
O
2
+ ATP
→
Luciferase
Oxyluciferin
+
CO
2
+ AMP
+ PPi
+ light
{\displaystyle {\ce {Luciferin{}+O2{}+ATP->[{\text{Luciferase}}]Oxyluciferin{}+CO2{}+AMP{}+PPi{}+light}}} 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
wavelength than red and can transmit more easily in water.
Chemiluminescence is different from fluorescence. Hence the application of fluorescent proteins
such as Green fluorescent protein is not a biological application of chemiluminescence.
== See also == Eclox
Electrochemiluminescence List of light sources
Lyoluminescence Will-o'-the-wisp