Correlation Spectroscopy Now

Fluorescence Correlation Spectroscopy (FCS) was conceived in 1972 (Magde, D., E. Elson, and W.W. Webb, Physical Review Letters 29(11), 705-708, 1972) to add chemical kinetics to the challenges in chemical physics that had been successfully understood by optical measurements of the fluctuations of density and concentration using quasi-elastic light scattering. Correlation spectroscopy of the kinetics of fluorescence fluctuations associated with fluorophore concentration fluctuations analyze chemical kinetics and molecular diffusion. The recent status of FCS and its progeny are summarized nicely in a recent book edited by R. Rigler and E. S. Elson. But here we consider what we need now from FCS for research on the dynamics of life processes, and how we might meet these challenges. For example: typical enzyme kinetics requires FCS at micromolar substrate concentrations, cell signaling requires nanomolar spatial resolution of dynamic spatial correlations, multi-component interactions require fluorescence cross-correlation of many markers, detection of the dynamics of weakly fluorescent fluorophores and harmonic correlation signals is needed, protein structural fluctuation dynamics and folding measurements are needed, detection and identification of sparse molecules at femtomolar concentrations challenges FCS.