Probing Access Resistance of Solid-State Nanopores with a Scanning-Probe Microscope Tip

An apparatus that integrates solid-state nanopore ionic current measurement with a scanning-probe microscope is developed. When a micrometer-scale scanning-probe tip is near a voltage-biased nanometer-scale pore (10–100 nm), the tip partially blocks the flow of ions to the pore and increases the pore access resistance. The apparatus records the current blockage caused by the probe tip and the location of the tip simultaneously. By measuring the current blockage map near a nanopore as a function of the tip position in 3D space in salt solution, the relative pore resistance increases due to the tip and ΔR/R0 is estimated as a function of the tip location, nanopore geometry, and salt concentration. The amplitude of ΔR/R0 also depends on the ratio of the pore length to its radius as Ohm's law predicts. When the tip is very close to the pore surface, ≈10 nm, experiments show that ΔR/R0 depends on salt concentration as predicted by the Poisson and Nernst–Planck equations. Furthermore, the measurements show that ΔR/R0 goes to zero when the tip is about five times the pore diameter away from the center of the pore entrance. The results in this work not only demonstrate a way to probe the access resistance of nanopores experimentally; they also provide a way to locate the nanopore in salt solution, and open the door to future nanopore experiments for detecting single biomolecules attached to a probe tip.