Two nanopores are fabricated with diameters of around 7 nm and about 20 nm as shown in the right inset of Figure 1b. The chips with nanopore fabricated on are cleaned in piranha solution and treated in oxygen plasma for 30 s on both sides prior to use. As shown in Figure 1b, the chip is assembled into a polymethylmethacrylate flow cell and sealed by means
of silicone elastomer gaskets . Two Ag/AgCl electrodes are immersed in two electrolyte compartments separated by the chip for setting up a transmembrane potential and detecting the transmembrane ionic currents through the nanopore. The ionic current is measured at 100 kHz with low-pass filtering at 10 kHz using a resistive feedback amplifier (EPC10, HEKA Elektronik, Rheinland-Pfalz, Germany). All salt solutions are degassed, filtered, and adjusted to pH 8.0 using 10 mM Tris–HCl and 1 mM Selleck LEE011 EDTA at pH 8.0 at room temperature. The λ-DNA (48.5 kbp, about 16.2-μm long) we used is purchased from Takara Bio, Inc. (Otsu, Japan) and put in the cis chamber (chamber with cathode). A voltage of 600 mV is applied on the trans side. All measurements are taken inside a dark Faraday cage. Talazoparib Figure 1 The setup of measuring the ionic currents through a nanopore. (a) Schematic illustrations of the nanopore fabrication
process and (b) the microfluidic setup. FIB, focused ion beams; PMMA, polymethylmethacrylate; Ⓐ, electrometer. Results and discussion Figure 2 shows the current–voltage curves for nanopores with diameters of 7 and 20 nm in various salt solutions. There are four set data representing the open pore ionic conductance, which include three set data for the 20-nm diameter nanopore in 1 M KCl, 0.5 M MgCl2 + 0.5 M KCl, triclocarban and 1 M MgCl2 solutions and one set data for the 7-nm diameter nanopore in 1 M MgCl2 solution. The open pore ionic conductance of a cylindrical nanopore in high ionic strength solutions with diameter d open and thickness h can be expressed as [30, 31] (1) where σ is the bulk electrolytic conductivity. In this paper, it
is set as σ KCI = 9.83 Sm −1, at 18°C for 1 M KCl and 1 M MgCl2 according to reference . Given the bulk electrolytic conductivity, the open pore conductance for a nanopore can also be estimated from formula (1). Based on formula (1), it is estimated that the open pore conductance for the 20-nm diameter nanopore in the three type solutions of 1 M KCl, 0.5 M MgCl2 + 0.5 M KCl, and 1 M MgCl2 should depend directly on the bulk electrolytic conductivity and the salt concentration. The predicted ratio for the open pore conductance in the above three solutions is 1:1.13:1.25, which agrees well with the measured value of 1:1.19:1.37 extracted from Figure 2. The open pore conductance for the 7-nm diameter nanopore can also be calculated. The predicted result is 18.56 nS, which is consistent with the experimental results, too. Figure 2 I – V curves for different nanopores in different solutions.