Probing Polyelectrolyte Multilayers using NSOM

NSOM topography (a) and fluorescence images from the spectral region of the acceptor (b) and donor (c). Image b corresponds to emission >570 nm and image c corresponds to emission <550 nm. Arrows indicate areas of strong emission in both the donor and acceptor spectral regions

Polyelectrolyte layer-by-layer (LbL) assembly has shown promise as a route for fabricating thin films with controlled thickness and designed functionality. This technique begins with a substrate with an excess surface charge on which alternating layers of polycations and polyanions are deposited by dipping the substrate into a dilute solution of the appropriate polyelectolyte salt. Thin films incorporating a variety of functionalities have been demonstrated using this technique, due in large part to the fact that the construction of the film is predominantly controlled by electrostatics (hydrophobic interactions may play a role as well). This method has several advantages over Langmuir-Blodgett deposition and self-assembly methods based on coordination chemistry. These include: Deposition from aqueous solution, extreme flexibility (i.e. no specific functional group is required), no limitation on substrate shape or size, and unlike other self-assembly methods, LbL films do not propagate voids in the film structure.
Current methods of analysis of polyelectrolyte self-assembled films including such spatially-averaged techniques as ellipsometry, GAXRD, neutron diffraction, and UV-Vis spectrophotometry indicate well-ordered layer-by-layer growth. While these techniques are essential for probing physical properties of the overall film, little is known regarding the mesoscale morphology and its influence on the materials properties. Since the polyelectrolyte requires that the properties defined on a nanoscale in an individual layer extend to the macroscale.

Near-field scanning optical microscopy data from a sample with two spacer bilayers (n = 2) between donor and acceptor layers. Local spectra (10 sec. integration) were collected in the areas marked in the fluorescence image. The spectra (1,2) are shown with a combined fit to the donor and acceptor contribution (red line), and the residual to the fit. Calculated energy transfer efficiencies (c) for spectra 1 and 2 were 0.13 and 0.18 respectively

Laser Scanning Confocal Micrograph of spatially isolated Oregon Green 488 molecules that have been bound to PAH and incorporated in an LbL film. Circled spots indicate molecules exhibiting fluorescence intermittency.