Dye encapsulation and exchange in block copolymer micelles

Abstract

Amphiphilic block copolymers, consisting of covalently attached hydrophobic and hydrophilic polymer blocks, can self-assemble into spherical micelles in aqueous solution. Block copolymer micelles can solubilize molecules for use as nanocarriers in applications of drug delivery, catalysis, and self-healing materials. Guest exchange dynamics are known to affect the stability, performance and lifetime of the nanocarriers, so thorough understanding of this process is critical. This work first explores the encapsulation of cyanine3 (Cy3) alkyne and BHQ-2 amidite dyes by poly(1,4-butadiene-b-ethylene oxide) (PB-PEO) micelles in aqueous solution using dynamic light scattering, UV-Vis spectroscopy, and fluorescence spectroscopy. The highly hydrophobic BHQ-2 amidite molecules were stably encapsulated within the micelles. Cyanine3 alkyne molecules were found to be less hydrophobic and could migrate out of the PB-PEO micelles into the aqueous solution. Additionally, this work investigates the guest exchange mechanism with fluorescence resonance energy transfer. Apparent guest exchange occurred within one minute under quiescent and magnetically stirred conditions. Multiple hypotheses were examined but no conclusive theory could be presented as to why this effect occurred. Vortex mixing induced significant guest exchange compared to quiescent and magnetically stirred conditions. Within 2 to 5 min, complete guest exchange had occurred. This time frame was found to be in concurrence with theoretical estimates of a guest exchange process mediated through the air-water interface.