Heterogeneous Translational Dynamics of Rhodamine B in Polyelectrolyte Multilayer Thin Films Observed by Single Molecule Microscopy

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Abstract
The lateral diffusion dynamics of rhodamine B(RB) in polyelectrolyte multilayer (PEM) thin films has been studied with single-molecule confocal fluorescence microscopy. The Films were made with sodium poly(sodium 4-styrenesulfonate) (PSS) and poly(diallydimethlyammonium chloride) (PDDA). Analysis of the real-time emission intensity traces reveals three diverse components of translational motion: (1) fast diffusion of RB through the confocal detection volume; (2) reversible tracer adsorption processes; and (3) nanoconfined diffusion, These processes cover a wide range of time scales. Analysis via fluorescence correlation spectroscopy (FCS) involves multicomponent fitting of the autocorrelated emission data. The model includes it free Brownian diffusion parameter, D, and two rate constants of desorption, k(-1) and k(-2). For RB in a PSS/PDDA thin film made with 0.01 M NaCl in the polyelectrolyte buildup solutions, D = 1.7x10(-7) cm(2)/s,k(-1)=30 s(-1), and k(-2) = 0.1 s(-1). FCS was also performed on RB/PEM samples made with NaCl concentrations of the buildup solutions ranging from 0.01 to 0.7 M. A weak dependence of D and k-1 on NaCl concentration was observed while k(-2) increased linearly with [NaCl].

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