Abstract
The effective surface diffusion of biopolyelectrolytes within nanoscale confinement is integral to biosensing, biocatalysis, bioseparations (such as chromatography columns utilizing nanoporous media), as well as biophysical processes. Confinement influences the surface-mediated diffusion of biopolyelectrolyte chains due to the presence of electric double layers and the increased frequency of transient polymer/surface interactions. Here, the diffusion of poly-L-lysine (PLL) in a nanoslit was studied using Convex Lens-induced Confinement (CLiC) single-molecule tracking microscopy. Three surface chemistries were employed to understand and compare the effects of electrostatic and short-range interactions, respectively. In all cases, the effective surface diffusion coefficient increased rapidly with slit height until it saturated at its value for a semi-infinite interface for slit heights <30 nm. The evolution of the diffusion coefficient with slit height was faster for amine-functionalized surfaces with which PLL exhibited stronger short-range interactions. These hydrogen-bonding interactions influenced the intermittent random walk behavior by increasing the waiting times between flights and the re-adsorption probability (a.k.a. sticking coefficient) during flights. Intermittent random walks were simulated within a planar slit geometry, using the appropriate adsorption probabilities and waiting time distributions (obtained from diffusion at a single interface) to account for the characteristic short-range interactions between PLL and each type of surface chemistry. The simulated step-size distributions were in good agreement with experimental measurements, indicating that the fine details of diffusion in nanoscale confinement can be quantitatively described by a model that combines confined Brownian motion with transient surface adsorption and desorption, using parameters obtained only from the behavior at a single interface in a semi-infinite geometry. These results highlight the importance of short-range biopolyelectrolyte/surface interactions on diffusion within confined environments and moreover, demonstrate the complex relationship between electrostatics, confinement, and local polymer and surface chemistry on biopolyelectrolyte surface diffusion within a nanoslit.
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