Intermittent Molecular Hopping at the Solid-Liquid Interface

Abstract

The mobility of molecules on a solid surface plays a key role in diverse phenomena such as friction and self-assembly and in surface-based technologies like heterogeneous catalysis and molecular targeting. To understand and control these surface processes, a universally applicable model of surface transport at solid-liquid interfaces is needed. However, unlike diffusion at a solid-gas interface, little is known about the mechanisms of diffusion at a solid-liquid interface. Using single-molecule tracking at a solid-liquid interface, we found that a diverse set of molecules underwent intermittent random walks with non-Gaussian displacements. This contrasts with the normal random walk and Gaussian statistics that are commonly assumed for molecular surface diffusion. The molecules became temporarily immobilized for random waiting times between surface displacements produced by excursions through the bulk fluid. A common power-law distribution of waiting times indicated a spectrum of binding energies. We propose that intermittent hopping is universal to molecular surface diffusion at a solid-liquid interface.

Interfacial Molecular Searching Using Forager Dynamics

“…Consistent with previous observations for a variety of molecules at the solid-liquid interface [29,32], the Step size distribution of DNA Molecules on a hydrophobic Surface (--). Gaussian distribution using the mean diffusion coefficient of DNA molecules on a hydrophobic Surface (--).…”

“…Interestingly, recent observations by our group and others suggest that the behavior of surfaceadsorbed molecules qualitatively mimics the motion favored for sparse prey searches [29][30][31][32]. In particular, proteins, polymers and small molecules all exhibit intermittent motion that corresponds to a CTRW with periods of slow or confined local diffusion alternating with flights comprising a heavy-tailed distribution.…”

Unified superresolution experiments and stochastic theory provide mechanistic insight into protein ion-exchange adsorptive separations

“…Superresolution techniques have yet to be applied to molecularscale investigations of separation methods, but recent efforts that identified heterogeneous interactions between proteins and biological membranes (44) demonstrate the potential. Pioneering diffraction-limited single-molecule studies of separations on relatively simple supports (45)(46)(47)(48)(49)(50)(51)(52) and adsorption at interfaces (53,54) provide incentive for applying higher-resolution spectroscopic analyses to a more realistic stationary phase. Because the ion-exchange process relies on maximizing the number of Significance Adsorption of proteins underlies the purification of biopharmaceuticals, as well as therapeutic apheresis, immunoassays, and biosensors.…”

3D Imaging of Hopping Molecules

“…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.…”

“…Thus, the surface diffusion was fast and nearly Brownian for surfaces from which the macromolecule experienced electrostatic repulsion and became dramatically slower and more subdiffusive for more attractive surfaces. These ensemble-averaged statistical trends can presumably be explained microscopically by the details of intermittent trajectories [18], e.g., waiting times, hop distances, and sticking coefficients, which can, in turn, be directly related to molecular-level interactions.…”