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Causes of Fluorescence Intensity Fluctuations

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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.