Single-molecule tracking
was used to characterize the mobility
of poly(ethylene glycol) chains at a solid–liquid interface
over a wide range of surface coverage. Trajectories exhibited intermittent
motion consistent with a generalized continuous time random walk (CTRW)
model, where strongly confined “waiting times” alternated
with rapid flights. The presence of three characteristic regimes emerged
as a function of surface coverage, based on an analysis of effective
short-time diffusion coefficients, mean-squared displacement, and
CTRW distributions. The dilute “site-blocking” regime
exhibited increasing short-time diffusion, less confined behavior,
and shorter waiting times with higher surface coverage, as anomalously
strong adsorption sites were increasingly passivated. At intermediate
values of surface coverage, the “crowding” regime was
distinguished by the exact opposite trends (slower, more confined
mobility), presumably due to increasing intermolecular interactions.
The trends reversed yet again in the “brush” regime,
where adsorbing molecules interacted weakly with a layer of extended
overlapping chains.
The surface diffusion of poly-L-lysine (PLL) in a planar nanoslit was studied using convex lens-induced confinement (CLiC) single-molecule tracking microscopy. Three surface chemistries were employed to understand the interplay of electrostatic and short-range interactions: an amine-functionalized silica surface, an oligo(ethylene oxide) (OEG)-modified surface, and a 1:1 mixture of the two ligands. Effective surface diffusion coefficients increased rapidly with slit height until saturating for slit heights <30 nm. While diffusion at a semi-infinite interface was significantly faster for OEG surfaces, the diffusion coefficient increased most rapidly with slit height for aminefunctionalized surfaces, resulting in surface diffusion within very thin slits being nearly independent of surface chemistry. Intermittent random walks were simulated within a planar slit geometry, using experimentally measured parameters obtained from diffusion at a single interface to account for the characteristic short-range interactions between PLL and each surface chemistry, and were in good agreement with experimental measurements.
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