Chapter 1: In vivo applications of fluorescence correlation spectroscopy.

Fluorescence correlation spectroscopy provides a sensitive optical probe of the molecular dynamics of life in vivo and in vitro. The kinetics of chemical binding, transport, and changes in molecular conformations are detected by measurement of fluctuations of fluorescence emission by sensitive marker fluorophores. The fluorophores within a defined volume are illuminated by laser light that excites their fluorescence. While conventional confocal illumination by short-wavelength laser light is sufficient for two-dimensional targets, multiphoton fluorescence excitation by simultaneous quantum absorption of two or more long-wavelength photons of approximately 100 fs laser pulses provides the more precise submicron three-dimensional spatial resolution required in cells and tissues. Chemical kinetics, molecular aggregation, molecular diffusion, fluid flows, photophysical interactions, conformational fluctuations, concentration fluctuations, and other dynamics of biological processes can be measured and monitored in volumes approximately 1 mum(3) at timescales from <1 mus and upward for many orders of magnitude. Theory, motivations, methods, in vivo applications, and future directions for improvement and new applications for fluorescence correlation spectroscopy are summarized in this chapter.