www.acsprf.org

The project’s goal was to understand the relationship between charge/mass transport within a supported polyelectrolyte membrane, and the associated local polymer structure, with the overall objective of tuning this response.  The specific aims were 1) to investigate and understand diffusive ion transport within supported polyelectrolyte brushes 2)  to investigate and understand probe ion orientation within supported polyelectrolyte brushes.  The final year of this project saw significant progress in elucidating transport in supported polyelectrolyte brushes, and in developing advanced data analysis techniques that allow us to extract more useful data from complex systems. 

Project 1: Three-Dimensional Orientational Dynamics in Supported Polyelectrolyte Brushes

The properties of high numerical aperture confocal imaging optics were exploited to extract three-dimensional orientational dynamics of fluorescent counterion probes in supported polystyrene sulfonate (PSS) brushes.  For the first time experimentally, molecular dipole dynamics were followed in three dimensions in real time using confocal imaging techniques.   

The decoupling of rotational and translational diffusion is a hallmark of transport in charged and crowded environments.  In our pursuit of optimized storage/release conditions in polyelectrolyte brushes, we aimed to understand how the individual modes in the overall heterogeneous transport processes within PSS brushes were dependent on the brush structure.  By combining this new analytical technique and advanced simulations, it was possible to determine that there is a preferred 3-D orientational motion that is distinct and reproducible for each PSS brush.  It was also possible to identify orientational switching behavior that might correspond to the counterion probe switching between sulfonate coordination sites. 

The overall impact of this work lies in that it provides a mechanism by which we can determine if the hopping and switching behavior of counterion probes within the PSS brushes corresponds to motion along polymer strands, between polymer strands, or between lamemae that have formed due to the separation of hydrophobic/hydrophilic domains, which has been hypothesized to be a limiting factor in polyelectrolyte efficiency in fuel cell applications.  The results and data discussed above were presented in publications in Journal of Physical Chemistry Letters as noted in the publications list.

Project 2: Advanced Data Analysis Techniques

The resolution of heterogeneous interactions probed in single-molecule experiments is dominated by shot-noise.  Among the several techniques for processing single-molecule time series data, wavelet shrinkage is proven to reduce noise.  Standard wavelet algorithms can also introduce unwanted artifacts when acting on discontinuous signals, because they are not customized for the discrete nature of single-molecule data.  In this project, modifications to the basic method that are specific to smFRET were developed and tested on simulated systems.  It was found that, despite its simplicity compared to its more modern cousins, the Haar wavelet basis produces the most optimally denoised estimates.  Other efforts were spent assessing various thresholding methods, developing a time-localized noise estimator, and implementing a translation-invariant wavelet transformation to reduce artifacts associated with discontinuities and inadequate distinction of noise.  The time-local estimator enhances noise reduction by 5-20 %, and translation-invariant transformation nearly eliminates the aforementioned artifacts.  Kinetic parameters extracted from denoised estimates are accurate to within 5 % of the simulated values. 

Overall, the impact of this work is broad, in that it results in measurably improved resolution in single molecule measurements, which will allow us to extract more reliable information from heterogeneous systems.  Additional results of these efforts include compiled analysis algorithms that are made publicly available on our web site for others’ implementation and testing.  The methods developed and the interpretations resulted in a paper published in the Journal of Physical Chemistry B, as noted in the publications list. 

Project 3: Interactions of Probes with Soft, Tunable Surfaces

            This project continues our efforts to understand transport in crowded environments, and in particular to understand if polyethylene glycol (PEG) brushes, traditionally used to prevent bio-fouling at interfaces, serve as physical barriers or are penetrable.  In addition, a goal was to understand if novel PEG architectures could be used to selectively tune surface-protective properties.   Fluorescence correlation spectroscopy and single molecule event analysis were used to quantify the presence and extent of surface penetration of a variety of solutes, including proteins within PEG brushes with tunable surface properties.  The data support a sieve-like model in which size-exclusion principles determined the extent of probe-PEG interactions.  We have also observed what we hypothesize to be size exclusion effects when allowing the dyes to exchange their native counterion for a larger one. In these instances, the dyes with larger hydrodynamic radii are excluded from interaction with the linear PEG. We have varied the pore size of the polymer at the surface with PEG dendrons and observed that density of PEG is also an important parameter. Overall, we found that there is a strong relationship between the probe size, mobility, and density of PEG on the surface.

The significance of this works lies with the conclusions that PEG surfaces are in fact porous, and can serve as tunable sieves.  In the larger sense, these results suggest a pathway towards tunable surfaces.  This work resulted in a manuscript published in Colloids and Surfaces B: Biointerfaces.