Photobleaching

In optics, photobleaching (sometimes termed fading) is the photochemical alteration of a dye or a fluorophore molecule such that it permanently is unable to fluoresce. This is caused by cleaving of covalent bonds or non-specific reactions between the fluorophore and surrounding molecules.^[1] Such irreversible modifications in covalent bonds is caused by transition from a singlet state to the triplet state of the fluorophores. The number of excitation cycles vary to achieve full bleaching. In microscopy, photobleaching may complicate the observation of fluorescent molecules, since they will eventually be destroyed by the light exposure necessary to stimulate them into fluorescing. This is especially problematic in time-lapse microscopy.

However, photobleaching may also be used prior to applying the (primarily antibody-linked) fluorescent molecules, in an attempt to quench autofluorescence. This can help to improve signal-to-noise ratio.

Photobleaching may also be exploited to study the motion and/or diffusion of molecules, for example via the FRAP, in which movement of cellular components can be confirmed by observing a recovery of fluorescence at the site of photobleaching, or FLIP techniques, in which multiple rounds of photobleaching is done so that the spread of fluorescence loss can be observed in cell.

Loss of activity caused by photobleaching can be controlled by reducing the intensity or time-span of light exposure, by increasing the concentration of fluorophores, by reducing the frequency and thus the photon energy of the input light, or by employing more robust fluorophores that are less prone to bleaching (e.g. Alexa Fluors or DyLight Fluors). To a reasonable approximation, a given molecule will be destroyed after a constant exposure (intensity of emission X emission time X number of cycles) because, in a constant environment, each absorption-emission cycle has an equal probability of causing photobleaching.

Lifetime

Depending on their specific chemistry, molecules can photobleach after absorbing just a few photons, while more robust molecules can undergo many absorption/emission cycles before destruction:

• Green fluorescent protein: 10⁴–10⁵ photons; 0.1–1.0 second lifetime.
• Typical organic dye: 10⁵–10⁶ photons; 1–10 second lifetime.
• CdSe/ZnS quantum dot: 10⁸ photons; > 1,000 seconds lifetime.

This use of the term "lifetime" is not to be confused with the "lifetime" measured by fluorescence lifetime imaging.

See also

• Ozone depletion

References

External links

• Introduction to Optical Microscopy an article about photobleaching
• Viegas MS; Martins TC; Seco F; do Carmo A (2007). "An improved and cost-effective methodology for the reduction of autofluorescence in direct immunofluorescence studies on formalin-fixed paraffin-embedded tissues". Eur J Histochem. 51 (1): 59–66. PMID 17548270.