Chemiluminescence

(physical chemistry) Emission of light as a result of a chemical reaction without an apparent change in temperature.

The type of luminescence wherein a chemical reaction supplies the energy responsible for the emission of light (ultraviolet, visible, or infrared) in excess of that of a blackbody (thermal radiation) at the same temperature and within the same spectral range. Below 900°F (500°C), the emission of any light during a chemical reaction is a chemiluminescence. The blue inner cone of a bunsen burner or the Coleman gas lamp are examples.

Many chemical reactions generate energy. Usually this exothermicity appears as heat, that is, translational, rotational, and vibrational energy of the product molecules; whereas, for a visible chemiluminescence to occur, one of the reaction products must be generated in an excited electronic state (designated by an asterisk) from which it can undergo deactivation by emission of a photon. Hence a chemiluminescent reaction, as shown in reactions (1) and (2), can be regarded as the reverse of a photochemical reaction.
1

2

The energy of the light quantum hv (where h is Planck's constant, and v is the light frequency) depends on the separation between the ground and the first excited electronic state of C; and the spectrum of the chemiluminescence usually matches the fluorescence spectrum of the emitter. Occasionally, the reaction involves an additional step, the transfer of electronic energy from C* to another molecule, not necessarily otherwise involved in the reaction. Sometimes no discrete excited state can be specified, in which case the chemiluminescence spectrum is a structureless continuum associated with the formation of a molecule, as in the so-called air afterglow: NO + O → NO₂ + hv (green light).See also Photochemistry.

Only very exothermic, or “exergonic,” chemical processes can be expected to be chemiluminescent. Partly for this reason, most familiar examples of chemiluminescence involve oxygen and oxidation processes; the most efficient examples of these are the enzyme-mediated bioluminescences.

Electrogenerated chemiluminescence, also known as electrochemiluminescence, is a luminescent chemical reaction in which the reactants are formed electrochemically. Electrochemical reactions are electron-transfer reactions occurring in an electrochemical cell. In such a reaction, light emission may occur as with chemiluminescence; however, the excitation is from the application of a voltage to an electrode. In chemiluminescence, the luminophore is excited to a higher energetic state by means of a chemical reaction initiated by mixing of the reagents. In electrogenerated chemiluminescence, the emitting luminophore is excited to a higher energy state by reactions of species that are generated at an electrode surface by the passage of current through the working electrode. Upon decay to the electronic ground state, light emitted by the luminophore (fluorescent or phosphorescent) can be detected. The luminophore is typically a polycyclic hydrocarbon, an aromatic heterocycle, or certain transition-metal chelates. See also Electron-transfer reaction.

Chief among the developments since the early research and discovery of electrogenerated chemiluminescence was the construction of instrumentation for detection of electrogenerated chemiluminescence. These instruments made it possible for the methodology to be used by practitioners other than electrochemists.

Measurement of the light intensity of electrogenerated chemiluminescence is very sensitive and is proportional to the luminophore concentration. Trace amounts of luminophore as low as 10⁻¹³ mol/liter can be detected, making electrogenerated chemiluminescence very useful in analytical and diagnostic applications.

A commercial application of the phenomenon forms the basis of a highly sensitive technique for detection of biological analytes such as deoxyribonucleic acid (DNA), ribonucleic acid (RNA), proteins, antibodies, haptens, and therapeutic drugs in the clinical laboratory. The technique combines a binding assay method and a system for detecting electrogenerated chemiluminescence.