FCS Classroom
What Physical Processes Cause Fluorescence Fluctuations?
Physical processes are typically in a state of dynamic equilibrium. For example, the dynamic process of diffusion causes a component's local concentration in a small section of a large sample solution to fluctuate about the average concentration.
FCS measures fluctuations in fluorescence intensity in a small section of the total sample volume (See What is the Confocal Volume?). These fluctuations can arise from volume-dependent processes, such as local concentration fluctuations of fluorescent particles. They can also arise from volume-independent processes that act on the fluorescent dyes.
Fluctuations Detected by FCS
Fluctuations in fluorescence intensity can result from the following volume-dependent and volume-independent processes:
- Random diffusion
- Directed flow (hydrodynamic and electrophoretic)
- Chemical Equilibrium
- Intersystem crossing between singlet and triplet states
- Nonradiative fluorescence resonance energy transfer (FRET)
For a list of FCS equations that describe these processes, please see our List of FCS Equations for Various Physical Processes.