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Fluorescence of Dyes

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Fluorescence

In recent years fluorescence spectroscopy has become a very important tool in biology, pharmacy and medicine. Numerous bio-molecules can be observed, identified and traced by labeling with a fluorescent dye. For a long time it has been common to make cells and organelles visible by staining with dyes. Nowadays, making use of fluorescence, even the motion of single dye-labeled molecules can be observed in living cells.

Characterization of Fluorescence

On absorption of a photon a dye molecule is excited to a meta-stable state. In most dyes the absorbed energy is dissipated very fast and converted into heat. Only in relatively few dyes these non-radiative transitions are sufficiently slow so that the radiative transition from excited to ground state can successfully compete: the dye is deactivated by emitting a photon (it fluoresces).
A fluorescent dye can be characterized by the spectral properties (excitation and emission spectrum), the fluorescence quantum yield (h_Fl), and the fluorescence decay time (t_Fl). The fluorescence of a dye is independent of the wavelength of excitation.

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