Increasing resonance energy transfer upon dilution: a counterintuitive observation in CTAB micelles

Abstract

We present a comprehensive study of two indocarbocyanines, DiI (1,1′-dioctadecyl-3,3,3′3′-tetramethylindocarbocyanine perchlorate) and DiD (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate), in water suspensions in the presence of a CTAB surfactant both above and below the critical micellar concentration. The very good affinity of the two dyes with CTAB allows them to be brought into aqueous solutions minimizing aggregation phenomena. When the dyes are loaded inside the micelles, stable fluorescent nanostructures are formed that can be exploited for fundamental studies and for imaging applications. Of special interest are micelles loaded with both dyes: indeed, the large local dye concentration inside the micelles allows observing resonance energy transfer in systems where the global dye concentration is maintained at a low level, so that detailed and robust spectroscopic characterization is possible. Quite impressively, the efficiency of resonance energy transfer is boosted when diluting the sample below the critical micellar concentration. This counterintuitive result is explained in terms of the very large affinity between the dyes and CTAB which favors the dynamical formation of small molecular clusters containing both dyes. Fluorescent micelles are widely used in bioimaging and pharmacokinetic applications. More specifically, the observation of resonant energy transfer in micelles or more generally in nanostructures loaded with two dyes is routinely exploited as a way to assess the nanostructure integrity. In this context, our results demonstrate the importance of robust spectroscopic characterization of the relevant systems in different environments in order to safely assess the viability of the integrity test of nanoparticles based on resonant energy transfer.