According to Ocean Optics, Inc., their fiber optic oxygen sensors use the fluorescence of a chemical complex in a sol-gel to measure the partial pressure of oxygen. The pulsed blue LED sends light, at ~475 nm, to an optical fiber. The optical fiber carries the light to the probe. The distal end of the probe tip consists of a thin layer of a hydrophobic sol-gel material. A sensor formulation is trapped in the sol-gel matrix, effectively immobilized and protected from water. The light from the LED excites the formulation complex at the probe tip. The excited complex fluoresces, emitting energy at ~600 nm. If the excited complex encounters an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence signal.
The degree of quenching correlates to the level of oxygen concentration or to oxygen partial pressure in the film, which is in dynamic equilibrium with oxygen in the sample. The energy is collected by the probe and carried through the optical fiber to the spectrometer.
Oxygen as a triplet molecule is able to quench efficiently the fluorescence and phosphorescence of certain luminophores. This effect (first described by Kautsky in 1939) is called "dynamic fluorescence quenching." Collision of an oxygen molecule with a fluorophore in its excited state leads to a non-radiative transfer of energy. The degree of fluorescence quenching relates to the frequency of collisions, and therefore to the concentration, pressure and temperature of the oxygen-containing media.
Source: Ocean Optics, Inc. http://www.oceanoptics.com/products/sensortheory.asp
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