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

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 zeta(pore) and the protein zeta potential zeta(protein) 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, zeta(protein) - zeta(pore). 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.