Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

Response of fluorescent molecular rotors in ternary macromolecular mixtures

Mingshan Chi, Anh-Thy Bui, Pierre Lidon, Yaocihuatl Medina-Gonzalez

TL;DR

This work interrogates how fluorescent molecular rotors (FMRs) respond to microenvironmental changes in complex PEG/water mixtures to address calibration challenges for quantitative microviscosity. By measuring density, viscosity, and rotor fluorescence lifetimes in binary and ternary PEG solutions, the authors test Förster-Hoffmann scaling and explore free volume theory as a framework for interpreting rotor dynamics. They find that FH holds with solvent- and MW-dependent exponents in binary solutions, and that, in ternaries, both specific volume and viscosity evolve nearly linearly with the heavier PEG proportion, supporting a linear mixing description with two limit environments. The study demonstrates that a local free volume perspective provides a robust interpretation, enabling more reliable calibration of FMRs in heterogeneous media and informing potential microviscosity mapping in complex systems.

Abstract

For a few decades, Fluorescent Molecular Rotors have been commonly employed as local probes of microviscosity in complex materials. However, without proper calibration, relating microviscosity to a physical parameter is unclear, which strongly limits their quantitative use in biological media for instance. In this study, the response of a molecular rotor in binary and ternary macromolecular aqueous solutions of polyethylene glycol (PEG) of different molecular weights is investigated in order to better rationalize the sensitivity of rotors to their cybotactic environment. More precisely, for the investigated composition range of ternary mixtures, it is shown that a linear mixing rule applies for fluorescence lifetime with the proportion of the two PEG, and with an increasing ratio of heavy PEG leading to larger lifetimes. These results allow to test more precisely the free volume theory, which has been proposed in the context of probing glass transition. Analysis show that while this theory semi-quantitatively captures the observation, its precise use raises some questions.

Response of fluorescent molecular rotors in ternary macromolecular mixtures

TL;DR

This work interrogates how fluorescent molecular rotors (FMRs) respond to microenvironmental changes in complex PEG/water mixtures to address calibration challenges for quantitative microviscosity. By measuring density, viscosity, and rotor fluorescence lifetimes in binary and ternary PEG solutions, the authors test Förster-Hoffmann scaling and explore free volume theory as a framework for interpreting rotor dynamics. They find that FH holds with solvent- and MW-dependent exponents in binary solutions, and that, in ternaries, both specific volume and viscosity evolve nearly linearly with the heavier PEG proportion, supporting a linear mixing description with two limit environments. The study demonstrates that a local free volume perspective provides a robust interpretation, enabling more reliable calibration of FMRs in heterogeneous media and informing potential microviscosity mapping in complex systems.

Abstract

For a few decades, Fluorescent Molecular Rotors have been commonly employed as local probes of microviscosity in complex materials. However, without proper calibration, relating microviscosity to a physical parameter is unclear, which strongly limits their quantitative use in biological media for instance. In this study, the response of a molecular rotor in binary and ternary macromolecular aqueous solutions of polyethylene glycol (PEG) of different molecular weights is investigated in order to better rationalize the sensitivity of rotors to their cybotactic environment. More precisely, for the investigated composition range of ternary mixtures, it is shown that a linear mixing rule applies for fluorescence lifetime with the proportion of the two PEG, and with an increasing ratio of heavy PEG leading to larger lifetimes. These results allow to test more precisely the free volume theory, which has been proposed in the context of probing glass transition. Analysis show that while this theory semi-quantitatively captures the observation, its precise use raises some questions.
Paper Structure (21 sections, 21 equations, 19 figures, 7 tables)

This paper contains 21 sections, 21 equations, 19 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Materials and methods
  3. Rotor synthesis
  4. Mixture formulation and characterization
  5. Fluorescence characterization
  6. Results
  7. Characterization of rotor fluorescence
  8. Binary mixtures
  9. Ternary mixtures
  10. Discussion
  11. Reminder of free volume theory for molecular rotors
  12. Direct use of free volume
  13. Local approach of free volume
  14. Conclusion
  15. List of solutions
  16. ...and 6 more sections

Figures (19)

  • Figure 1: Structure of the molecular rotor used in this study.
  • Figure 2: Fluorescence properties of rotor in ethylene glycol/water mixtures. (a) Absorption and emission spectra in pure ethylene glycol. Dashed line : absorption spectrum, normalized by maximum value and corrected for baseline, maximum of absorption is at $\lambda_\text{abs} = 532nm$; Continuous line : emission spectrum with excitation at maximum of absorption, normalized by maximum value, maximum of emission is at $\lambda_\text{em} = 556nm$. (b) Evolution of rotor normalized fluorescence intensity (black) and lifetime (red) in ethylene glycol/water mixtures (ethylene glycol weight fraction $w$ from $0.6$ to $1$). Continuous line are fit with Förster-Hoffmann equation \ref{['eq:FH_I_tau']} with $x=0.76$ for intensity and $x=0.65$ for lifetime.
  • Figure 3: Evolution of properties of binary PEG/water mixture as a function of PEG mass fraction $w$, for different PEG molar weight ($^\bullet$ PEG-62; $\bullet$ PEG-400; $^\blacklozenge$ PEG-2000; $\blacksquare$ PEG-6000; $\blacktriangledown$ PEG-20000). Arrows indicate increasing molecular weight $M$. (a) Specific volume $v_\text{m} = 1/\rho$. Data for $M=2e4g\per mol$ were not measured. (b) Viscosity $\eta$.
  • Figure 4: Evolution of rotor fluorescence lifetime $\tau$ in binary PEG/water mixtures, for different PEG molar weight ($^\bullet$ PEG-62; $\bullet$ PEG-400; $^\blacklozenge$ PEG-2000; $\blacksquare$ PEG-6000; $\blacktriangledown$ PEG-20000). Arrows indicate increasing molecular weight $M$. (a) Förster-Hoffmann representation of evolution of lifetime $\tau$ with viscosity $\eta$. Solid lines are fit with Equation \ref{['eq:FH_I_tau']} and inset represents evolution of exponent with PEG molar weight $M$. (b,c) Evolution of fluorescence lifetime $\tau$ of the FMR with PEG mass fraction $w$ in regular and semilogarithmic coordinates.
  • Figure 5: Properties of ternary mixtures of water, PEG-400 and PEG-2000 as a function of the proportion of PEG-2000, $W$. The color gradient represents the evolution of the total PEG mass fraction $w$, varying between $w=0.4$ and $0.6$ by step of $0.05$. The open symbols on axis $W=0$ and $W=1$ represent data obtained in binary PEG/water mixtures, displayed on Fig. \ref{['fig:carac_binary']}. Continuous lines are linear fit along Eq. \ref{['eq:ideal_ternary']}. Arrows indicate increasing PEG mass fraction $w$. (a) Evolution of viscosity $\eta$ in semilogarithmic coordinates. b) Evolution of specific volume $v_\text{m}$. (c) Evolution of the fluorescence lifetime $\tau$ of the FMR.
  • ...and 14 more figures