Fluorescence Quenching

Fluorescence Quenching

Fluorescence Quenching

Kinetic formulation for determining the rate of fluorescence Quenching

When a molecule absorbs light, electrons in its constituent atoms gets excited and promoted to a higher energy level. When electrons in this excited state lose energy and return to the ground state, they release this energy in the form of heat or radiation. Light emitted during this process is known as fluorescence and the molecules that show this activity are called fluorophores.
An electronically excited molecule may emit a quantum of energy by fluorescence or may transfer its excited energy by collision with other molecules called quenching.
Fluorescence quenching is a physicochemical process that lowers the intensity of emitted light from fluorescent molecules.

The quenching of fluorescence provides a method of determining the rate constant for the reaction between the excited singlet state molecules and the quencher.
X + hν → X*
X* ---Kf---> X + hν'     (Fluorescence)
X* + Q ---Kq---> X + Q + K.E.     (Quenching)
The total rate of deactivation of X* is-
−[dX]/dt = Kf[X*] + Kf[X*][Q]
If the intensity of incident light is Io and the fluorescent light is If, The fraction of excited molecules that fluorscence is Yf i.e.-
Yf = If / Io
or, Yf = Kf[X*] / (Kf[X*] + Kq[X*][Q])
or, Yf = 1 / (1 + (Kq / Kf)[Q])
or, Io / If = 1 + (Kq / Kf)[Q]
This equation is known as Stern-Volmer Equation
A plot of Io / If vs [Q] is linear with slope equal to Kq / Kf.
Fluorescence Quenching