Three-Dimensional Tracking of Interfacial Hopping Diffusion

June 11th, 2019 by Double Helix Optics

The three-dimensional movement of molecules in liquids is well described by Brownian motion, however, the movement of molecules near interfaces is not as well understood, especially in the case of how molecules move across the surface of the cell. The Schwartz Lab at the University of Colorado, Boulder, using Double Helix’s SPINDLE® imaging system, was able for the first time to directly test and quantitatively characterize the 3D nature of surface diffusion.

The dynamics of macromolecules at solid-liquid interfaces underlie many applications including chemical sensing, catalysis, lubrication, and adhesion. In principle, this kind of movement is two-dimensional, but some theoretical models predict that the movement of the molecule consists of a series of hops, i.e. returns to the surface, by which a molecule stochastically re-adsorbs after a given hop according to a hypothetical parameter known as the “sticking coefficient.” While this parameter is theoretically the primary determinant of the flight length, to date there has been no actual support for the presence of multiple hops per flight.

To directly test and quantitatively characterize the supposed 3D nature of surface diffusion, The Schwartz Lab used a Double Helix SPINDLE module with a high-efficiency double helix engineered-PSF (DH-PSF) phase mask, enabling 3D tracking of molecular movement within that volume.

Using the multi-micron 3D tracking capability enabled by the DH-PSF, researchers verified that surface diffusion is, in fact, dominated by 3D flights. Moreover, the direct observation made it possible to determine the statistics of the desorption-mediated hops in detail. This included the waiting times between hops, the duration of hops, the 3D spatial extent of hops, and the probability of reabsorption after a hop. 

The knowledge from this research demonstrated that molecules search for reabsorption points more efficiently than predicted by conventional theory. This information has many applications including drug discovery, pharmaceutical purification, and wastewater remediation.

Read the brief and download the manuscript, Three-Dimensional Tracking of Interfacial Hopping Diffusion, published by the American Physical Society.

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Key Words:  molecular tracking, PSF engineering, 3D super resolution, 3D tracking