Two-dimensional fluorescence correlation spectroscopy with modulated excitation

Overlap of multiple states or multiple species in a chemical system often creates a congested fluorescence spectrum that is difficult to interpret. The resolution of component spectra is essential for the understanding of the structure and dynamics of such multicomponent systems. In this paper, two-dimensional fluorescence correlation spectroscopy (2D FCS) is presented for the dissection of component spectra using the time correlation function. In 2D FCS, the time response of fluorescence intensity is collected at various wavelengths upon an external perturbation. The time correlation function is evaluated between wavelengths, A two-dimensional fluorescence correlation spectrum, or a plot of the correlation intensity as a function of two wavelength axes, resolves the overall spectrum into component spectra. The characteristics of the two-dimensional time correlation function are demonstrated in the frequency domain fluorescence spectroscopy in which the sinusoidally modulated excitation provides the external perturbation. Using 2D FCS, fine vibronic structures of the component fluorescence emission spectra were completely resolved from a strongly overlapped one-dimensional mixture spectrum. The existence of multiple microenvironments of a probe molecule in a biological system is evidenced by nonzero asynchronous correlation intensities. The corresponding spectra are retrieved from correlation analysis. Unlike traditional resolution methods in fluorescence spectroscopy based on statistical fitting of fluorescence decays, 2D FCS can resolve species whose fluorescence decays are linked by the rate constants in chemical reactions and species displaying multiexponential decay kinetics.