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

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 a 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 = 1.7 x 10(-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].