Effect of Electrolyte Concentration and Pore Size on Ion Current Rectification Inversion

Abstract

A thorough understanding of nanoscale transport properties is vital for the development and optimization of nanopore sensors. The thickness of the electrical double layers (EDLs) at the internal walls of a nanopore, as well as the dimensions of the nanopore itself, plays a crucial role in determining transport properties. Herein, we demonstrate the effect of the electrolyte concentration, which is inversely proportional to the EDL thickness, and the effect of pore size, which controls the extent of the electrical double layer overlap, on the ion current rectification phenomenon observed for conical nanopores. Experimental and numerical results showed that as the electrolyte concentration is decreased, the rectification ratio reaches a maximum, then decreases, and eventually inverts below unity. We also show that as the pore size is decreased, the rectification maximum and the inversion take place at higher electrolyte concentrations. Numerical investigations revealed that both phenomena occur due to the shifting of ion enrichment distributions within the nanopore as the electrolyte concentration or the pore size is varied.

Figure 1

Thickening of the EDL as concentration is decreased and the rectified current–voltage curve, as well as the associated ion concentrations relative to the bulk concentration at the different potentials.

Figure 2

Rectification ratio is a function of both electrolyte concentration and pore size. The change in the rectification ratio as a function of electrolyte concentration for four different nanopore sizes obtained (A) experimentally and (B) numerically. Error bars are calculated as the standard error of measurements from a minimum of six nanopores. Representative current–voltage curves for each pore size and electrolyte concentration are shown in Figure S8.

Figure 3

Normalized ion enrichment (C_av/C_bulk) values at (A) 1 mM, (B) 0.05 mM, and (C) 0.005 mM concentrations, corresponding to the prerectification maximum, postrectification maximum, and postrectification inversion regions, respectively. The normalized ion enrichment values were extracted from the central axisymmetric axis of the 109 nm pore. The gray reference line corresponds to the narrowest end of the conical region. The normalized cation and anion enrichment traces are shown in Figure S5.

Figure 4

Normalized ion enrichment (C_av/C_bulk) peak shifts inside or outside the nanopore as a function of electrolyte concentration postrectification maximum. The normalized ion enrichment values were extracted from the central axisymmetric axis of the 109 nm nanopipette at different electrolyte concentrations. The point markers indicate the shifting location of the enrichment maximum. Note that a value <1 indicates depletion. The normalized cation and anion enrichment traces are shown in Figure S6.

Figure 5

Normalized ion enrichment (C_av/C_bulk) peak shifts as the pore size is varied postrectification inversion (RR <1). The normalized ion enrichment values were extracted from the central axis symmetric axis at a 0.001 mM electrolyte concentration for the different pore sizes. The end of the conical nanopipette region is located at 0 nm. The point markers indicate the shifting location of the enrichment maximum. The normalized cation and anion enrichment traces are shown in Figure S7.