Charge-Dependent Transport Switching of Single Molecular Ions in a Weak Polyelectrolyte Multilayer

Abstract

The tunable nature of weak polyelectrolyte multilayers makes them ideal candidates for drug loading and delivery, water filtration, and separations, yet the lateral transport of charged molecules in these 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 polyelectrolyte multilayer thin film, a 10 bilayer film of poly(acrylic acid) and poly(allylamine hydrochloride) PAA/PAH. Using fluorescence microscopy and single-molecule tracking, two modes of interaction were detected: (1) adsorption, characterized by the molecule remaining immobilized in a subresolution region and (2) diffusion trajectories characteristic of hopping (D ∼ 10–9 cm2/s). Radius of gyration evolution analysis and comparison with simulated trajectories confirmed the coexistence of the two transport modes in the same single molecule trajectories. A mechanistic explanation for the probe and condition mediated dynamics is proposed based on a combination of electrostatics and a reversible, pH-induced alteration of the nanoscopic structure of the film. Our results are in good agreement with ensemble studies conducted on similar films, confirm a previously-unobserved hopping mechanism for charged molecules in polyelectrolyte multilayers, and demonstrate that single molecule spectroscopy can offer mechanistic insight into the role of electrostatics and nanoscale tunability of transport in weak polyelectrolyte multilayers.

Characterization of Porous Materials by Fluorescence Correlation Spectroscopy Super-resolution Optical Fluctuation Imaging

“…For each step, the magnitude of the displacement is based on a user-defined diffusion coefficient and distribution width, the step size was sampled from a normal distribution and added to the particles previous location. 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 An in-house constructed wide-field TIRF microscope was used to measure samples after equilibration on the microscope stage for fifteen minutes. The beam of a solid state 532 nm laser (Coherent, Compass 315M-100SL) was focused at the edge of a 1.45 NA, 100x, oil-immersion objective (Carl-Zeiss, alpha Plan-Fluar) for through-the-objective TIRF microscopy.…”

Three-Dimensional Tracking of Interfacial Hopping Diffusion

“…While interfacial diffusion is nominally two-dimensional (2D) and conventionally described in terms of 2D Brownian motion, longstanding theoretical models [7][8][9][10][11][12][13][14][15][16] have predicted that interfacial mass transport could actually be dominated by "flights" through an adjacent liquid phase, which would dramatically alter the nature of interfacial molecular motion; an understanding of this process is necessary in order to rationally control mass transport at surfaces. Recent experimental results indirectly support these predictions, by measuring the 2D projection of trajectories for atoms, molecules, polymers, and nanoparticles, in thin films, at solid/liquid interface, and on lipid bilayers, which can be represented as an intermittent process, with periods of apparent immobility alternating with long "flights" comprising a heavy-tailed distribution [17][18][19][20][21][22][23][24][25][26]. However, the evidence for the presence of three-dimensional (3D) hops remains indirect, and critical aspects of the proposed "hopping" process remain a mystery.…”

Tuning the Flight Length of Molecules Diffusing on a Hydrophobic Surface

“…Because of this phenomenon, flights were observed on a polyelectrolyte surface only when there was a strong electrostatic attraction between the small-molecule probes and the surface. 23 The frequency of flights is not the only contributor to the overall mobility of the molecules, however. The distribution of flight lengths is also critically important.…”