Conceptual basis and instrumentation

The fluctuations SI(t) of the fluorescence around the mean value f , defined as 

The FCS autocorrelation functions4 contain two types of information (Thompson, 1991)  

4) In several papers, only the time-dependent part of the autocorrelation function is considered, and the definition is then 

The decay rate represents the average duration of the fluctuation signal. 

Two-photon FCS has been successfully developed it offers the advantage of 3-D resolution (see Section 11.2.1.2) (Chen et al., 2001). 

G(0) can yield information on molecular weights (e.g. labeled proteins and nucleic acids) and molecular aggregation. In these applications, the laser beam is focused through a microscope objective to a small spot on the specimen the latter is laterally translated through the beam by a computer-controlled microstepping stage, at a speed higher than the rate of translational diffusion of the species under study. This technique is called scannir -FCS. 

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