Biomimetic potassium-selective nanopores

Abstract

Reproducing the exquisite ion selectivity displayed by biological ion channels in artificial nanopore systems has proven to be one of the most challenging tasks undertaken by the nanopore community, yet a successful achievement of this goal offers immense technological potential. Here, we show a strategy to design solid-state nanopores that selectively transport potassium ions and show negligible conductance for sodium ions. The nanopores contain walls decorated with 4'-aminobenzo-18-crown-6 ether and single-stranded DNA (ssDNA) molecules located at one pore entrance. The ionic selectivity stems from facilitated transport of potassium ions in the pore region containing crown ether, while the highly charged ssDNA plays the role of a cation filter. Achieving potassium selectivity in solid-state nanopores opens new avenues toward advanced separation processes, more efficient biosensing technologies, and novel biomimetic nanopore systems.

Fig. 1. Designing potassium-selective solid-state nanopores.

(A) Single nanopores with a tunable opening diameter were created in 30-nm-thick silicon nitride films by the process of dielectric breakdown. The first modification step led to the attachment of carboxyl groups. The second modification involved either symmetric attachment of 4′-aminobenzo-18-crown-6 ether (B) or asymmetric modification with the crown ether and ssDNA (C). (B) I-V curves in 1 M KCl and 1 M NaCl recorded for a 1-nm-diameter pore whose walls were decorated with crown ether, as shown in the scheme. The graph on the right summarizes ratios of currents in KCl and NaCl at 1 V before and after each modification step for the same nanopore. Ratios of currents for the nanopore before and after carboxylation are calculated on the basis of the recordings in 100 mM of the salts. (C) I-V curves in 1 M KCl and 1 M NaCl for a 0.6-nm-wide nanopore modified with crown ether and ssDNA. Selectivity of the nanopore is shown as ratios of ionic currents in KCl and NaCl solutions measured under the same conditions as in (B).

Fig. 2. Selectivity of nanopores toward potassium.

(A) Experimental ratios of ion currents in KCl and NaCl solutions for six independently prepared nanopores subjected to chemical modification with crown ether (CE) and ssDNA. Data for three different bulk concentrations of the salts are shown. The model fit is shown as dashed lines. (B) Experimental data of potassium selectivity for three nanopores modified only with crown ether. SDs of currents for individual voltages are shown in I-V curves in Fig. 1 and fig. S5.

Fig. 3. Phenomenological model of the potassium selectivity of solid-state nanopores.

(A) Scheme of the modeled system with geometrical parameters used in the model. (B) Diameter dependence of the selectivity sensitivity (m_S) to voltage. m_S is defined here as the slope of a linear fit of log(I_K/I_Na) versus voltage for 1 M KCl and NaCl solution concentrations. (C) I-V curves at three different bulk KCl concentrations for the same pore shown in Fig. 1C. (D) I-V curves at three different bulk NaCl concentrations for the same pore as (C). Symbols are for experimental data, while dashed lines represent model predictions using the parameters listed in Table 1. SDs of currents for individual voltages are shown in I-V curves in fig. S5.

Fig. 4. Ion current measurements in mixtures of KCl and NaCl for a 1-nm-diameter nanopore modified with crown ether and DNA.

(A) I-V curves in all conditions examined. (B) Ratio of the ionic currents in mixed salt solutions and in 1 M NaCl solution as a function of KCl concentration at +1 V and a constant total salt concentration of 1 M. The inset shows the magnitude of the ion current as a function of KCl concentration under the same conditions. Dashed lines show model predictions using the parameters given in Table 1 for a 1-nm-diameter pore.