Fluorophore - Fluorescence

Fluorescence

The fluorophore absorbs light energy of a specific wavelength and re-emits light at a longer wavelength. The absorbed wavelengths, energy transfer efficiency, and time before emission depend on both the fluorophore structure and its chemical environment, as the molecule in its excited state interacts with surrounding molecules. Maximum wavelengths of absorption (≈ excitation) and emission (for example, Absorption/Emission = 485 nm/517 nm) are the typical terms used to refer to a given fluorophore, but the whole spectrum may be important to consider. The excitation wavelength spectrum may be a very narrow or broader band, or it may be all beyond a cutoff level. The emission spectrum is usually sharper than the excitation spectrum, and it is of a longer wavelength and correspondingly lower energy. Excitation energies range from ultraviolet through the visible spectrum, and emission energies may continue from visible light into the near infrared region.

Main characteristics of fluorophores are :

• Maximum excitation and emission wavelength (expressed in nanometers (nm)): corresponds to the peak in the excitation and emission spectra (usually one peak each),
• Extinction Coefficient (or molar absorption, in Mol−1cm−1) : links the quantity of absorbed light, at a given wavelength, to the concentration of fluorophore in solution.
• Quantum yield : efficiency of the energy transferred from incident light to emitted fluorescence (= number of emitted photons per absorbed photons)
• Lifetime (in picoseconds): duration of the excited state of a fluorophore before returning to its ground state. It refers to the time taken for a population of excited fluorophores to decay to 1/e (≈0.368) of the original amount.
• Stokes shift: difference between the max excitation and max emission wavelengths.

These characteristics drive other properties, including the photobleaching or photoresistance (loss of fluorescence upon continuous light excitation). Other parameters should be considered, as the polarity of the fluorophore molecule, the fluorophore size and shape (i.e. for polarization fluorescence pattern), and other factors can change the behavior of fluorophores.

Fluorophores can also be used to quench the fluorescence of other fluorescent dyes (see article Quenching (fluorescence)) or to relay their fluorescence at even longer wavelength (see article FRET)

See more on fluorescence principle.