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Thetunable nature of weak polyelectrolyte multilayers makes themideal candidates for drug loading and delivery, water filtration,and separations, yet the lateral transport of charged molecules inthese systems remains largely unexplored at the single molecule level.We report the direct measurement of the charge-dependent, pH-tunable,multimodal interaction of single charged molecules with a weak polyelectrolytemultilayer thin film, a 10 bilayer film of poly(acrylic acid) andpoly(allylamine hydrochloride) PAA/PAH. Using fluorescence microscopyand single-molecule tracking, two modes of interaction were detected:(1) adsorption, characterized by the molecule remaining immobilizedin a subresolution region and (2) diffusion trajectories characteristicof hopping (D∼ 10–9cm2/s). Radius of gyration evolution analysis and comparisonwith simulated trajectories confirmed the coexistence of the two transportmodes in the same single molecule trajectories. A mechanistic explanationfor the probe and condition mediated dynamics is proposed based ona combination of electrostatics and a reversible, pH-induced alterationof the nanoscopic structure of the film. Our results are in good agreementwith ensemble studies conducted on similar films, confirm a previously-unobservedhopping mechanism for charged molecules in polyelectrolyte multilayers,and demonstrate that single molecule spectroscopy can offer mechanisticinsight into the role of electrostatics and nanoscale tunability oftransport in weak polyelectrolyte multilayers. [ABSTRACT FROM AUTHOR]

Two maximum likelihood estimation (MLE) methods weredevelopedfor optimizing the analysis of single-molecule trajectories that includephenomena such as experimental noise, photoblinking, photobleaching,and translation or rotation out of the collection plane. In particular,short, single-molecule trajectories with photoblinking were studied,and our method was compared to existing analytical techniques appliedto simulated data. The optimal method for various experimental caseswas established, and the optimized MLE method was applied to a realexperimental system: single-molecule diffusion of fluorescent molecularmachines known as nanocars. [ABSTRACT FROM AUTHOR]