DiPyMe in SDS Micelles: Artifacts and Their Implications in the Interpretation of Micellar Properties
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Date
2015-11-10
Authors
Fowler, Michael
Hisko, Victoria
Henderson, Jason
Casier, Remi
Li, Lu
Thoma, Janine L.
Duhamel, Jean
Journal Title
Journal ISSN
Volume Title
Publisher
American Chemical Society
Abstract
This study provides experimental evidence that di(1-pyrenylmethyl) ether or DiPyMe, a well-known fluorescent probe employed to determine the microviscosity of surfactant or polymeric micelles, is being hydrolyzed in the presence of water upon UV irradiation. This effect was established from a careful analysis of the fluorescence spectra and decays acquired with aqueous solutions of DiPyMe dissolved in micelles of sodium dodecyl sulfate (SDS). The size of the SDS micelles could be adjusted from an aggregation number (N-agg) of 70 to 172 by increasing the ionic strength of the aqueous solution from 0.0 to 0.5 M NaCl. The hydrolysis of DiPyMe was much reduced in the larger SDS micelles. While the degradation of DiPyMe in aqueous solutions of SDS micelles affected the analysis of the fluorescence spectra, model-free analysis (MFA) of the fluorescence decays of DiPyMe could reliably retrieve the rate constant < k > of excimer formation for DiPyMe. After calibration with mixtures of organic solvents of known macroscopic viscosity, the < k > values obtained for DiPyMe yielded the microviscosity (mu eta) of the SDS micelles as a function of salt concentration. The mu eta was found to increase from 4.0 to 8.8 mPa.s as the salt concentration increased from 0.0 to 0.5 M. This study demonstrated that, regardless of the problem of its hydrolysis that jeopardizes its use in steady-state fluorescence experiments, DiPyMe remains an extremely valuable probe for describing the microviscosity of hydrophobic domains in aqueous solution as long as its decays are analyzed with a model that accounts for the presence of degradation products as the MFA does.
Description
This document is the Accepted Manuscript version of a Published Work that appeared in final form in Langmuir, copyright © American Chemical Society after peer review and technical editing by publisher. To access the final edited and published work see http://dx.doi.org/10.1021/acs.langmuir.5b02770
Keywords
Intramolecular Excimer Formation, Sodium Dodecyl-Sulfate, Hydrophobically-Modified Poly(N-Isopropylacrylamides), Intra-Molecular Excimer, Fluorescence Spectroscopy, Forming Probe, Labeled Macromolecules, Pyrene Fluorescence, Solvent Polarities, Phase-Transitions