On the pH-Responsive, Charge-Selective, Polymer-Brush-Mediated Transport Probed by Traditional and Scanning Fluorescence Correlation Spectroscopy

Abstract

The complete and reversible charge-selective sequestration of fluorophores by a weak polyelectrolyte brush, poly(2-(dimethylamino) ethyl-methacrylate) (PDMAEMA) was demonstrated using Fluorescence Correlation Spectroscopy (FCS). The chemistry and thickness of the weak polyelectrolyte PDMAEMA was tuned reversibly between neutral and cationic polymer forms. Thus, by switching the pH successively, the brush architecture was tuned to selectively trap and release anionic dye probes, while continuously excluding cationic molecules. In addition, line-scan FCS was implemented and applied for the first time to a synthetic polymer system, and used to identify a new, slower diffusion time on the order of seconds for the sequestered anionic probe under acidic conditions. These results, which quantify the selective sequestration properties of the PDMAEMA brush, are important because they enable a better understanding of transport in polymers, and establish a spectroscopic means of evaluating materials with proposed applications in separations science, charge storage/release and environmental remediation.

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

“… 10 , 12 As a result, the ratio of positive to negative charge near the surface of a PEM film incorporating one or more weak polyelectrolytes can be tuned by adjusting the pH of the solution, and this charge ratio determines not only the electrostatic character of the film, but the nanoscale structure of the film itself. 8 , 10 In previous research we have shown that changing the degree of ionization of a weak polyelectrolyte brush allows for reversible and charge-selective sequestration of probe molecules, 13 which supports their use in drug release applications. 14 , 15 …”

Polymer Free Volume Effects on Protein Dynamics in Polystyrene Revealed by Single-Molecule Spectroscopy

“…28−30 For example, Schwartz et al showed how poly(ethylene glycol) (PEG) brush grafting decreased fibronectin adsorption, but longer PEG chains led to more unfolding and longer residence times for adsorbed fibronectin. 31 Other single-molecule studies have established relationships between the microstructures of polymer brushes 13,32,33 and self-assembled monolayers 34,35 with molecular probes, but a relationship between thin film polymer packing and protein dynamics is lacking. Using total internal reflectance geometry 36−38 achieves increased signal-tonoise by suppressing emission from molecules diffusing in the bulk.…”

“…Many common techniques suffer from ensemble averaging , or require ex situ sample conditions to measure. , Single-molecule fluorescence spectroscopy is a robust method for studying protein–polymer interactions allowing for many proteins to be tracked in real time below the diffraction limit of light. − For example, Schwartz et al showed how poly­(ethylene glycol) (PEG) brush grafting decreased fibronectin adsorption, but longer PEG chains led to more unfolding and longer residence times for adsorbed fibronectin . Other single-molecule studies have established relationships between the microstructures of polymer brushes ,, and self-assembled monolayers , with molecular probes, but a relationship between thin film polymer packing and protein dynamics is lacking. Using total internal reflectance geometry − achieves increased signal-to-noise by suppressing emission from molecules diffusing in the bulk.…”

Molecular Approaches to Chromatography Using Single Molecule Spectroscopy

“…When transport is too slow, the diffusion coefficient cannot be accurately extracted due to long acquisition time required; the maximum lag time must be on the order of 5 000 times the longest characteristic diffusion time before the observed diffusion constant converges to the expected value. 69 Movement of the focal volume to parallelize the acquisition over the spatial dimension and reduce time points between data has been performed to analyze slower diffusion in techniques such as raster scan image correlation spectroscopy, 70 − 72 line scan FCS, 73 , 74 and circular scanning FCS. 75 , 76 Alternatively, multiple focal volumes separated by a set distance have been used in dual focus FCS to quantify the diffusion time from one focal volume to the other, obtaining information on a longer time scale in addition to the typical diffusion within each individual focal volume.…”