Chemistry Forums for Students > Analytical Chemistry Forum
Fluorescence intensity and fading
Mania28:
Hi,
i´m working with carboxylated Latex-colloids which emit fluorescence light at certain wavelenghts. Unfortunately, the intensity of the emitted light fades away after a few minutes (I´m working with a fluorescence microscope). What is the reason of this loss of intensity? Can somebody explain it to me exactly?
Another question is: what is fluorescence quenching?
P:
Fluorescence is the light emitted by electrons falling from high energy states to low. After absorbing energy from a light source, a fluorescent sample has a finite period of fluorescence before all of the higher energy electrons have falled back to their ground state and have given off their 'stored' energy as light. Thus fluorescence intensity drops fairly quickly with time and drops to 0 swiftly.
Fluorescence quenching is where this time period of fluorescence is severely shortened - usually due to the presence of (sometimes deliberately introduced) impurities which cause the electrons in the higher energy states to fall faster or more directly to the ground states.
In PHOSPHORESCENCE - the sample contains more complex electronic structure and the higher energy electrons take much longer to make their way back down to the ground state. This is because they get stuck in 'meta-stable' states (fairly stable electronic configurations at higher energies). If you want your polymer to glow for longer then look into phosphorescence.
Hope this helps - I'm not an expert, but this what I can remember from my lectures from last decade.
Dude:
Are you referring to reasonably long time intervals (i.e. 5-10 minutes with a constant intensity light source)? If so, the slow loss of fluorescence in solution is likely due to degradation of the molecule. Look up words such as photobleaching or photodegradation in SciFinder or Google.
Mania28:
thanks for your answers. I´m working with long time intervals with constant intensity light source (fluorescence microscope). The latex colloids are adsorbed to a mineral surface. Could somebody explain to me shortly how photobleaching/photodegradation works?
P:
Never heard of photo bleaching, but in short:
Photodegredation of a polymer happens when light/uv/radiation hits the sample and is absorbed - a radical may be formed and the chain may break (this is called chain scission) or the radical may cause a reaction with a neighbouring chain and join with it (this is called crosslinking). Photodegredation is where excitations along the chain occur buy the interaction of the radiation (light/uv/electron beam etc..) and chain scission is observed - thus the polymer chains get shorter - thus degradation.
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