Fluorescence Correlation Lifetime Spectroscopy Study of Xanthene Derivatives in Water-In-Oil Micelles

Abstract

Fluorescence Lifetime Correlation Spectroscopy (FLCS) allows to extract autocorrelation functions from different emitters by using Time-Correlated Single Photon Counting information, extending the applicability of conventional Fluorescence Correlation Spectroscopy (FCS) [1]. Here, we have used a xanthene derivative, so-called Tokyo Green II (TG-II) [2,3], to study the size, diffusional properties, and dynamics of reverse water-in-oil micelles of Aerosol-OT (AOT) in isooctane (iC8). The dye undergoes a partition between the inner micellar aqueous core, in which the TG-II anion is the main form, and the AOT lipophilic side chains, where the TG-II neutral species is predominant. By applying FLCS we obtained the lifetime filtered autocorrelation (AC) curves of the anionic TG-II, which shows a characteristic lifetime of 3.70 ns, under different conditions. Fitting the TG-II anion AC curves to the appropriate model provided the diffusion coefficient and hydrodynamic radius of micelles at different ratios [water] / [AOT]. Interestingly, the AC functions also showed a strong anticorrelation time in the microsecond time range. This time represents micelle dynamics that changes the distribution of the TG-II neutral and anionic forms in the hydrophobic interface and the water core. [1] Kapusta et al., J. Fluoresc.17 (2007), 43. [2] Paredes et al., Phys. Chem. Chem. Phys.11 (2009), 5400. [3] Paredes et al., Phys. Chem. Chem. Phys.12 (2010), 323.