| Abstract
| - We employed negatively charged fluorescein (FL), positively charged rhodamine 6G (R6G), and neutralNile Red (NR) as molecular probes to investigate the influence of Coulombic interaction on their depositioninto and rotational mobility inside polyelectrolyte multilayer (PEM) films. The entrapment efficiency ofthe dyes reveals that while Coulombic repulsion has little effect on dye deposition, Coulombic attractioncan dramatically enhance the loading efficiency of dyes into a PEM film. By monitoring the emissionpolarization of single dye molecules in polyethylenimine (PEI) films, the percentages of mobile R6G, NR,and FL were determined to be 87 ± 4%, 76 ± 5%, and 68 ± 3%, respectively. These mobility distributionssuggest that cationic R6G enjoys the highest degree of rotational freedom, whereas anionic FL shows theleast mobility because of Coulombic attraction toward cationic PEI. Regardless of charges, this highpercentage of mobile molecules is in stark contrast to the 5−40% probe mobility reported from spun-castpolymer films, indicating that our PEI films contain more free volume and display richer polymer dynamics.These observations demonstrate the potential of using isolated fluorescent probes to interrogate the internalstructure of a PEM film at a microscopic level.
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