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Abstract:
The concept of cross-correlation spectroscopy actually follows as a logical consequence if one wants to extend the common established fluorescence autocorrelation analysis to achieve a larger detection specificity. Instead of mathematically comparing, i.e. convoluting, a single fluorescence time signal with itself at different times, two different signals of whatever origin are now compared with each other, again as a function of time. Consequently, while autocorrelation analysis of a recorded signal provides us with information about its self-similarity in time, thus in particular about characteristic repetitive processes of significant duration, cross-correlation of two different signals reveals underlying mechanisms that relate the measured quantities with each other. These two signals do not necessarily both have to be fluorescence traces; it is possible and potentially attractive to compare fluctuations in the emission behavior of fluorescent probes with any other parameter that can be measured sensitively enough to reveal fluctuations on a single molecule or single particle scale. In this respect, cross-correlation is a valuable concept if one wants to follow different parameters at once but also if long-distance or long-timescale effects have to be investigated. Although there exist manifold ways to apply the cross-correlation concept, in particular two quite simple modes have now been experimentally established: two-volume fluorescence cross-correlation, which is discussed in detail elsewhere in this book (Chap. 18), and dual-color fluorescence cross-correlation which we want to highlight here in particular.
