Role of Brownian Force for Molecular Translocation in Nanopores

Based on the previous experiments, detecting fast nanopore translocating analytes requires a high-frequency measurement system that guarantees a time resolution of better than 1 μs. This limitation may still make it uncertain whether all translocation events are unambiguously captured, as the challenge effectively shifts from increasing the sampling bandwidth to dealing with the rapidly increasing noise at frequencies typically above 10 kHz. In this work, we introduce a numerical simulation model as an alternative to discern translocation events under different experimental settings: pore size, bias, analyte charge state, salt concentration, and electrolyte viscosity. The model allows simultaneous analysis of the forces acting on a large analyte cohort along their individual trajectories, which are responsible for the analyte movement that ultimately leads to nanopore translocation. By following the analyte trajectory, it is found that the Brownian force dominates the analyte movement in the electrolyte until the last moment when the electro-osmotic force determines the final translocation act. The mean dwell time of analytes mimicking streptavidin decreases from ~6 to ~1 ms with increasing the bias voltage from ±100 to ±500 mV. The simulated translocation events are in qualitative agreement with our experimental data using streptavidin. The simulation model is also useful for the design of new solid-state nanopore sensors.