Single Ion Diffusive Transport within a Poly(styrene sulfonate) Polymer Brush Matrix Probed by Fluorescence Correlation Spectroscopy

Abstract

Diffusive transport within complex environments is a critical piece of the chemistry occurring in such diverse membrane systems as proton exchange and bilayer lipid membranes. In the present study, fluorescence correlation spectroscopy was used to evaluate diffusive charge transport within a strong polyelectrolyte polymer brush. The fluorescent cation rhodamine-6G was used as a counterion probe molecule, and the strong polyelectrolyte poly(styrene sulfonate) was the polymer brush. Such strong polyelectrolyte brushes show promise for charge storage applications, and thus it is important to understand and tune their transport efficiencies. The polymer brush demonstrated preferential solvation of the probe counterion as compared to solvation by the aqueous solvent phase. Additionally, diffusion within the polymer brush was strongly inhibited, as evidenced by a decrease in diffusion constant of 4 orders of magnitude. It also proved possible to tune the transport characteristics by controlling the solvent pH, and thus the ionic strength of the solvent. The diffusion characteristics within the charged brush system depend on the brush density as well as the effective interaction potential between the probe ions and the brush. In response to changes in ionic strength of the solution, it was found that these two properties act in opposition to each other within this strong polyelectrolyte polymer brush environment. A stochastic random walk model was developed to simulate interaction of a diffusing charged particle with a periodic potential, to show the response of characteristic diffusion times to electrostatic field strengths. The combined results of the experiments and simulations demonstrate that responsive diffusion characteristics in this brush system are dominated by changes in Coulombic interactions rather than changes in brush density. More generally, these results support the use of FCS to evaluate local charge transport properties within polyelectrolyte brush systems, and demonstrate that the technique shows promise in the development of novel polyelectrolyte films for charge storage/transport materials.

Charge-Dependent Transport Switching of Single Molecular Ions in a Weak Polyelectrolyte Multilayer

“…Because of the excitation geometry and integration time of 30 ms, probes diffusing freely in solution ( D ≈ 10 8 nm 2 /s) 38 are visible only as a constant background signal and are therefore not counted during particle identification. 48 Probes adsorbed in the film bulk will bleach a short time after the sample is illuminated.…”

Transient Three-Dimensional Orientation of Molecular Ions in an Ordered Polyelectrolyte Membrane

“…13 Fluorescence correlation spectroscopy (FCS) yields translational diffusion constants for cationic R6G (with dimensions of < 1.5 nm) in these brushes at neutral pH of ~10 −14 m 2 /s. 12–13 FCS studies for this strongly charged dye:brush system reveal significantly slowed translational diffusion indicative of dye diffusion in the brush, without a significant component of fast translational diffusion indicative of diffusion in bulk water or near surfaces. 12–13 Single molecule blip analysis of the translational diffusion occurring in the brush furthermore reveals periods of very slow rotation/reorientation, as well as periods of faster molecular rotation that occurs on a time scale faster than the data collection.…”

“…By observing transport of positively charged R6G fluorescent molecules as they transit a sub-femtoliter focal volume, and employing a novel 3-angle polarization resolved detection scheme, we are able to provide a 3-dimensional description of transport within a model system of well-characterized poly(styrene sulfonate) (PSS) brushes. 12–13 …”

“…However, as discussed in previous work and in more detail below, because of the strong interaction between the brush and dye, observed translational diffusion in this system consists almost exclusively of brush:dye interactions. 12 Here, for a molecule with an absorption dipole oriented along z (the observation axis of the microscope), Θ is equal to 0. The use of high NA optics allows determination of the spherical coordinates associated with the molecular transition dipole.…”

Transport in Supported Polyelectrolyte Brushes

“…Note that in this image, the dimension of the thin film normal to the surface (along the z axis) is exaggerated; the thin films we study have dry thicknesses from 10 to 50 nm, while the focal volume dimension along the z axis is ∼ 2 μm. For details as to data acquisition and analysis, the reader is directed towards the published original work and associated Supplementary Information 39 . The overall results are described briefly below.…”