Fluorescence quenching

Introduction
Quenching of fluorescence is a physiochemical process that describes the decreasing of the fluorescent intensity of fluorphores. There are two different ways of quenching: static and dynamic quenching. It has to be mentioned that there is a difference between quenching and the decrease of fluorescence because of an high state of molecular excitement or chemical changes of dye (like oxydation).

Static Quenching
Static quenching (or contact quenching) of fluorescence is, when a Fluorphor (F) and a Quencher (Q) are creating a complex (FQ) which is not fluorescent. The chemical equation therefore is: F + Q ⇌ FQ

The chemical equilibrium (Ks) between the Fluorphor, the Quencher and the complex FQ is formed by the law of mass action and equal to the Stern-Volmer-constant (Ksv). There [FQ] stands for the concentration of the complex FQ, [F] for the concentration of the loose Fluorfor and [Q] for the loose Quencher.

The Stern-Volmer-equation describes the dependence of the fluorescent intensity of a fluorescent dye on the concentration of an quenching material (Quencher). It was created by the two physical chemists Otto Stern and Max Volmer in 1919. There F0 is the fluorescent intensity of the fluorescent dye without the quencher, F is the fluorescent intensity of fluorescent dye with the quencher. Ks stands for the Stern-Volmer-constant and [Q] for the concentration of the quencher.

Applications
Quenching is a really good visible physical phenomenon, that's why it is used as an indicator for molecular processes.

1) Indicator for Potassium-ions The ends of short DNA fragments (telomer sequence), connected with a fluorescent dye and quencher through a covalent bond, are divided in a solution. The dye is producing light. If the there are any potassium-ions present, the DNA fragment is wrapping around the potassium-ion and the endings are touching each other. The fluorescence is quenched (lightning stops).

2) Indicator for DNA hybridisation If DNA strand is hybridising with an opposite strand it gets quite linear and stiff. Fluorphor and Quencher are attached to the ends. If the bases are paring correctly the Fluorphor and Quencher disconnect and the quenching stops.

3) Indicator of loose Oxygen On a Rubidium complex u can show the level of oxygen saturation depending on the level of quenching.