Electrically Facilitated Translocations of Proteins through Silicon Nitride Nanopores: Conjoint and Competitive Action of Diffusion, Electrophoresis, and Electroosmosis

TLDR

Solid‑state nanopores hold promise for probing single proteins, yet protein passage through them is complex, showing unexpected direction, rate, and duration behavior. The study investigates how electrokinetic effects influence the translocation of avidin through silicon nitride nanopores. The authors measured the zeta potentials of the nanopore and the protein as a function of solution pH to quantify electrokinetic contributions. Electroosmotic flow can dominate or reverse electrophoretic transport, and when the two cancel, diffusion drives translocation; the direction is predictable from the difference in zeta potentials.

Abstract

Solid-state nanopores bear great potential to be used to probe single proteins; however, the passage of proteins through nanopores was found to be complex, and unexpected translocation behavior with respect to the passage direction, rate, and duration was observed. Here we study the translocation of a model protein (avidin) through silicon nitride nanopores focusing on the electrokinetic effects that facilitate protein transport across the pore. The nanopore zeta potential ζpore and the protein zeta potential ζprotein are measured independently as a function of solution pH. Our results reveal that electroosmotic transport may enhance or dominate and reverse electrophoretic transport in nanopores. The translocation behavior is rationalized by accounting for the charging states of the protein and the pore, respectively; the resulting translocation direction can be predicted according to the difference in zeta potentials, ζprotein − ζpore. When electrophoresis and electroosmosis cancel each other out, diffusion becomes an effective (and bias-independent) mechanism which facilitates protein transport across the pore at a significant rate.