Monitoring the microenvironment inside polymeric micelles using the fluorescence probe 6-propionyl-2-dimethylaminonaphthalene (PRODAN)

Abstract

6-propionyl-2-dimethylaminonaphthalene (PRODAN) is a fluorescent molecule with sensitivity to environmental polarity and hydrogen bond interactions. Although it has been extensively used to study organized systems such as vesicles and reverse micelles, its potential application to characterize polymeric micelles remains largely unexplored. Molecular probes can provide valuable information about the physicochemical properties of these micelles, and thus about the environment they offer to solubilize cargos. The present study aimed to find out what new insights could be gained by using PRODAN for such a purpose. The model systems chosen were D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS) and polyvinyl caprolactam-polyvinyl acetate-poly-ethylene glycol (Soluplus®) micelles. Steady-state and time-resolved fluorescence emission spectroscopy revealed that PRODAN was wholly incorporated into the micelles when the polymer concentration exceeded 9.9x10-4 M (TGPS) and 2.1x10-5 M (Soluplus®). The probe sensed a less polar environment inside Soluplus® micelles, whose core was found to be more fluid than that of TGPS micelles through red edge excitation shift (REES) experiments. A calculation of the well-known polarity parameter ET(30) confirmed that the environment was less polar within Soluplus® micelles than within TGPS ones. These results correlate well with the known loading capacity of both systems to solubilize different drugs, and therefore demonstrate that PRODAN is a powerful probe to characterize polymeric micelles.