Subdiffraction-Resolution Optical Measurements of Molecular Transport in Thin Polymer Films

Abstract

The measurement of molecular transport within polymer films yields information about the internal structural organization of the films and is useful in applications such as the design of polymeric capsules for drug delivery. Layer-by-layer assembly of polyelectrolyte multilayer films has been widely used in such applications where the multilayer structure often exhibits anisotropic transport resulting in different diffusivities in the lateral (parallel to the film) and transverse (normal to the film) directions. Although lateral transport can be probed using techniques such as fluorescence recovery after photobleaching (FRAP), it cannot be applied to probing transverse diffusivity in polymer films smaller than the diffraction limit of light. Here we present a technique to probe the transport of molecules tagged with fluorphores in polymer films thinner than the optical diffraction limit using the modulation of fluorescence emission depending on the distance of the tagged molecules from a metal surface. We have used this technique to probe the diffusion of proteins biotin and bovine serum albumin (BSA) in polyelectrolyte multilayer films. We also studied the interdiffusion of chains in multilayer films using this technique. We observed a 3 order of magnitude increase in interdiffusion as a function of the ionic strength of the medium. This technique, along with FRAP, will be useful in studying anisotropic transport in polymer films, even those thinner than the diffraction limit, because the signal in this technique arises only from transverse and not lateral transport. Finally, this technique is also applicable to studying the diffusion of chromophore-labeled species within a polymer film. We demonstrate this aspect by measuring the transverse diffusion of methylene blue in the PAH-PAA multilayer system.

Polyelectrolyte multilayers for bio‐applications: recent advancements

“…(iv) There is less understanding of molecular transport across the bulk of PEM matrix. Probing the molecules in PEM is quite an important parameter to consider for variety of practical applications ranging from bio-applications to other technological areas such as membrane filtration, which are mainly dependent on location and availability of molecules across the porous architecture of PEM [63].…”

Probing Bio-Molecules across Polyelectrolyte Multilayers

“…This technique offers the ability to study transport in ultrathin polymer films (thickness 100-200nm) [19]. This technique is based on the distance-dependent periodic modulation of fluorescence intensity emitted from the fluorescently labelled molecule.…”

Fluorescence Recovery after Photobleaching in Ultrathin Polymer Films

“…In other words, the FRAP experiments done with ultrathin PEM films cannot probe diffusion through the PEM matrix effectively. This is important because of the recent report where transverse diffusion was optically probed in ≈100 nm films . Previous works on PEM FRAP has employed the analysis procedure developed for only a single channel recovery process and ignored the factors discussed above which are important for accurate parameter estimation.…”

“…This is important because of the recent report where transverse diffusion was optically probed in ≈100 nm films. [29] Previous works on PEM FRAP has employed the analysis procedure developed for only a single…”