Fluorophore exchange kinetics in block copolymer micelles with varying solvent–fluorophore and solvent–polymer interactions

Abstract

Fluorescence spectroscopy was employed to characterize the kinetics of guest exchange in diblock copolymer micelles composed of poly(ethylene oxide-b-ε-caprolactone) (PEO-PCL) diblock copolymers in water/tetrahydrofuran (THF) mixtures which encapsulated fluorophores. The solvent composition (THF content) of the micelle solution was varied as a means of modulating the strength of interactions between the fluorophore and solvent as well as between the micelle core and solvent. A donor-acceptor fluorophore pair was employed consisting of 3,3′-dioctadecyloxacarbocyanine perchlorate (DiO, the donor) and 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI, the acceptor). Through the process of Förster resonance energy transfer (FRET), energy was transferred from the donor to acceptor when the fluorophores were in close proximity. A micelle solution containing DiO was mixed with a micelle solution containing DiI at t=0, and the emission spectra of the mixed solution were monitored over time (at an excitation wavelength optimized for the donor). In micelle solutions containing 5 and 10 vol% THF in the bulk solvent, an increase in the acceptor peak intensity maximum occurred over time in the post-mixed solution, accompanied by a decrease in the donor peak intensity maximum, indicating the presence of energy transfer from the donor to the acceptor. At long times, the FRET ratios (acceptor peak intensity divided by the sum of the acceptor and donor peak intensities) were indistinguishable from that determined from pre-mixed micelle solutions of the same THF content (in pre-mixed solutions, DiO and DiI were encapsulated within the same micelle cores). In the micelle solution containing 20 vol% THF, the fluorophore exchange process occurred too quickly to be observed (the FRET ratios measured from the solutions mixed at t=0 were commensurate to that measured from the pre-mixed solution). A time constant describing the guest exchange process was extracted from the time-dependence of the FRET ratio through fit of an exponential decay. An increase in the THF content in the micelle solution resulted in a decrease in the time constant, and the time constant varied over five orders of magnitude as the THF content was varied from 5–20 vol%.

Enhancing the Stability and Immunomodulatory Activity of Liposomal Spherical Nucleic Acids through Lipid‐Tail DNA Modifications

“…The fluorescence spectrum was then taken after incubation at 25 °C at 0, 0.25, and 24 h, with a plate reader (Synergy H4, BioTek Instruments, Inc., Winooski, VT, USA, 9.0 nm slit width, Excitation 480 nm). The FRET ratio was calculated using the formula [33]…”

Effect of vortex‐induced physical stress on fluorescent properties of dye‐containing poly(ethylene glycol)‐block‐poly(lactic acid) micelles

“…The DiO/PEG-b-PLA and DiI/PEG-b-PLA micelle samples showed a fluorescence emission peak at 507 nm (DiO emission peak) and 568 nm (DiI emission peak), respectively ( Figure 2 b), solid lines). On the DiO/DiI/PEG-b-PLA micelles containing two dyes, a small peak at 507 nm was observed but the dominant peak was found at 568 nm, indicating the occurrence of Förster resonance energy transfer (FRET) from DiO to DiI 24,31,32,38,[40][41][42][43] (Figure 2(c), solid line). As FRET occurs only when DiO (donor dye) and DiI (acceptor dye) are present within a few nanometers of each other, 43 the result confirmed the formation of block copolymer micelles and incorporation of these hydrophobic dyes into the micellar core.…”

“…Micelles were prepared by following the previously reported procedures [31][32][33] with minor modifications. One milliliter of the acetone solution containing PEG-b-PLA block copolymer (10.0 mg), DiO (0.015 mg), and DiI (0.075 mg) was prepared by mixing stock solution of each component (See Supporting Information for more detail on the procedure).…”

“…Hydrophobic dye-containing amphiphilic block copolymer micelles have been previously used in several studies on the dye transfer from micelles to biological membranes, 31 proteins, 33,40 and/or dye exchange between micelles. 32 In this study, we adapted DiO and/or DiI-containing PEG-b-PLA block copolymer micelles as a model system in order to investigate the effect of vortexing-induced mechanical stresses on the fluorescent properties of dye-containing block copolymer micelles ( Figure 1).…”

“…It has been previously reported that both DiO and DiI dyes are soluble in acetone but these dyes show little to no solubility in water. 32,44,45 Therefore, fluorescence properties of DiO and DiI dyes in the absence of block copolymer micelles were studied. Our control experiment confirmed that both DiO and DiI dyes were soluble in acetone and the obtained solution showed fluorescence emission peak at 507 and 568 nm, respectively.…”