Ion-selective permeability of an ultrathin nanoporous silicon membrane as probed by scanning electrochemical microscopy using micropipet-supported ITIES tips

Abstract

We report on the application of scanning electrochemical microscopy (SECM) to the measurement of the ion-selective permeability of porous nanocrystalline silicon membrane as a new type of nanoporous material with potential applications in analytical, biomedical, and biotechnology device development. The reliable measurement of high permeability in the molecularly thin nanoporous membrane to various ions is important for greater understanding of its structure-permeability relationship and also for its successful applications. In this work, this challenging measurement is enabled by introducing two novel features into amperometric SECM tips based on the micropipet-supported interface between two immiscible electrolyte solutions (ITIES) to reveal the important ion-transport properties of the ultrathin nanopore membrane. The tip of a conventional heat-pulled micropipet is milled using the focused ion beam (FIB) technique to be smoother, better aligned, and subsequently, approach closer to the membrane surface, which allows for more precise and accurate permeability measurement. The high membrane permeability to small monovalent ions is determined using FIB-milled micropipet tips to establish a theoretical formula for the membrane permeability that is controlled by free ion diffusion across water-filled nanopores. Moreover, the ITIES tips are rendered selective for larger polyions with biomedical importance, i.e., polyanionic pentasaccharide Arixtra and polycationic peptide protamine, to yield the membrane permeability that is lower than the corresponding diffusion-limited permeability. The hindered transport of the respective polyions is unequivocally ascribed to electrostatic and steric repulsions from the wall of the nanopores, i.e., the charge and size effects.

Figure 1

(a) Scheme of ion transport across ultrathin nanopore silicon membrane as induced by a micropipet-supported liquid/liquid interface as an SECM probe. (b) TEM image of the membrane with pores (bright circles) and diffracting nanocrystals (dark spots).

Figure 2

A micropipet (a, b) before and (c, d) after FIB milling as observed by (a, c) FIB and (b, d) SEM imaging. Scale bars, 1 μm.

Figure 3

(a) Model of SECM-induced ion transfer across a nanopore membrane (red dotted line). The black solid lines represent liquid/liquid interfaces with h/a = 0, 0.38, and 0.81 for monovalent ions, Arixtra, and protamine, respectively. The transfer of the target ions across the pipet-supported interface is diffusion-limited. The blue and green lines represent boundaries with zero normal flux and simulation space limits, respectively. The simulation space (light blue) is limited to the external solution surrounded by these boundaries while ion diffusion in the internal solution is not relevant to the tip current. (b) SECM approach curves at a SiO₂ substrate for TBA⁺, Arixtra, and protamine in 0.1 M PBS (solid lines). The probe scan rates are 0.3, 0.23, and 0.23 μm/s, respectively. The circles for the respective ions represent approach curves simulated with impermeable membranes for a = 1.9, 1.8, and 2.0 μm.

Figure 4

SECM approach curve to the nanopore membrane for TBA⁺ in 0.1 M PBS (solid line). Probe scan rate, 0.3 μm/s. The circles represent the approach curve simulated with the k value that gives the best fit with the experimental curve. The dashed and dotted lines were simulated for freely permeable (k_d = 4.2 × 10⁻² cm/s from eq 2 with l = 16 nm) and impermeable membranes, respectively. All simulated curves were obtained using (a, h/a, r_g/a) = (1.7 μm, 0, 1.2).

Figure 5

Plot of the membrane permeability versus the diffusion coefficients of transported ions. The corresponding diffusion coefficients are listed in Table 1. The permeability to Arixtra and protamine was measured with 0.10, 0.03, and 0.01 M PBS (see Table 2) while 0.10 M PBS was employed for monovalent ions.

Figure 6

SECM approach curves to the nanopore membrane for Arixtra in 0.10, 0.03, and 0.01 M PBS (solid lines). Probe scan rate, 0.23 μm/s. The circles represent approach curves simulated with the corresponding k values in Table 2. The dashed and dotted lines were simulated for freely permeable (k_d = 1.1 × 10⁻² cm/s from eq 2 with l = 16 nm) and impermeable membranes, respectively. All simulated curves were obtained using (a, h/a, r_g/a) = (1.8 μm, 0.81, 1.2).

Figure 7

SECM approach curves to the nanopore membrane for protamine in 0.10 and 0.03 M PBS (solid lines). Probe scan rate, 0.23 μm/s. The circles represent simulated approach curves with the corresponding k values in Table 2. The dashed and dotted lines were simulated for freely permeable (k_d = 2.8 × 10⁻³ cm/s from eq 2 with l = 16 nm) and impermeable membranes, respectively. All simulated curves were obtained using (a, h/a, r_g/a) = (2.0 μm, 0.38, 1.2).