Lifetime-Weighted FCS and 2D FLCS: Advanced Application of Time-Tagged TCSPC

Time-tagged TCSPC (time-correlated single photon counting) is a special acquisition mode of TCSPC with which one determines not only the excitation-emission delay time of detected photons but also their arrival times measured from the start of the experiment. Time-tagged TCSPC enables us to examine slow fluctuation of fluorescence lifetimes, which is particularly important in the study of heterogeneous or fluctuating systems at the single-molecule level. In this chapter, we describe recent development of new methods using time-tagged TCSPC, aiming at showing their high potential in studying dynamics of complex systems. We depict two closely related methods based on fluorescence correlation spectroscopy (FCS), i.e., lifetime-weighted FCS and two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS). These methods enable us to quantify fluorescence lifetime fluctuations on the microsecond timescale. Showing examples including the study of a biological macromolecule, we demonstrate the usefulness of these two methods in real applications. In addition, we present another application of time-tagged TCSPC, which analyzes photon interval time for characterizing timing instability of photon detectors.