Fluorescence correlation spectroscopy
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Fluorescence correlation spectroscopy (FCS) is a type of spectroscopy based on the measurement of fluorescence intensity and the analysis of it fluctuations, which can be due to the diffusion of the observed fluorophore in the excitation volume or to changes in the fluorescence quantum yield arising from chemical reactions. Measurements are usually made on only a few molecules at a time - of the order of 10 milliseconds - which is achieved by illuminating tiny volumes (around 1 femtoliter), available by employing two-photon excitation microscopy or confocal microscopy.
FCS is used to measure:
- translational diffusion coefficients of macromolecules
- the number of fluorescent molecules under observation
- the relative fluorescence yield of different molecules in a (inhomogeneous) solution, which gives the amount of each species
[edit] Formulas
The linear autocorrelation function:
with
- nj = the number of photons in bin j of width tj.
