Measurements of the size and correlations between ions using an electrolytic point contact

Abstract

The size of an ion affects everything from the structure of water to life itself. In this report, to gauge their size, ions dissolved in water are forced electrically through a sub-nanometer-diameter pore spanning a thin membrane and the current is measured. The measurements reveal an ion-selective conductance that vanishes in pores <0.24 nm in diameter—the size of a water molecule—indicating that permeating ions have a grossly distorted hydration shell. Analysis of the current noise power spectral density exposes a threshold, below which the noise is independent of current, and beyond which it increases quadratically. This dependence proves that the spectral density, which is uncorrelated below threshold, becomes correlated above it. The onset of correlations for Li + , Mg 2+ , Na + and K + -ions extrapolates to pore diameters of 0.13 ± 0.11 nm, 0.16 ± 0.11 nm, 0.22 ± 0.11 nm and 0.25 ± 0.11 nm, respectively—consonant with diameters at which the conductance vanishes and consistent with ions moving through the sub-nanopore with distorted hydration shells in a correlated way.

Beyond mass spectrometry, the next step in proteomics

“…The problem with sensitivity hobbling MS can be resolved using a tool of a different stripe, i.e., a nanometer-diameter pore or nanopore through a thin membrane (74)(75)(76)(77)(78). A nanopore is the ultimate analytical tool with extreme single-molecule sensitivity derived from its minuscule volume.…”

“…It is now technologically within our grasp to fix these problems by creating a subnanopore spanning a thin amorphous inorganic membrane. A subnanopore can be created by sputtering with a tightly focused, high-energy electron beam in a STEM through an amorphous silicon nitride membrane nominally 10 nm thick (75)(76)(77)(78)147). Pores made this way generally have a negatively charged surface and can be so small that only single dehydrated cations permeate across the membrane through them (Fig.…”

“…Pores made this way generally have a negatively charged surface and can be so small that only single dehydrated cations permeate across the membrane through them (Fig. 7A, i to iv) (78). High-angle annular dark-field (HAADF) images acquired with an aberration-corrected STEM (Fig.…”

“…7A) have exposed the main structural features of these pores. The HAADF-STEM images acquired under different tilt conditions relative to the axis of the electron beam reveal pores with a biconical topography and an irregular waist >0.25 nm in diameter (78). The biconical subnanometer topography is important because it focuses the electric field to a nanometer-scale extent near the waist, and the current density there is proportional to the electric field (Fig.…”

“…Still, if the thin membrane is small enough in area, it can be mechanically robust and chemically resilient so that it can resist denaturants like SDS, -mercaptoethanol (BME), acids (HCl), high electric field, and high temperature. Accordingly, proteins denatured by heat, SDS, and BME can be analyzed intact with a subnanopore (75)(76)(77), and a pore that is fouled can be rehabilitated with denaturing agents, although the subnanopore topography through a silicon nitride membrane might etch a bit in HCl (78,112). Another side benefit is that the SDS used to maintain denaturation of a protein is supposed to form a uniform, negatively charged shell along the protein backbone (155) that results in a rod-like molecular configuration ( Fig.…”