Calcium ions play important roles in many physiological processes, yet their concentration is much lower than the concentrations of potassium and sodium ions. The selectivity of calcium channels is often probed in mixtures of calcium and a monovalent salt, e.g., KCl or NaCl, prepared such that the concentration of cations is kept constant with the mole fraction of calcium varying from 0 and 1. In biological channels, even sub-mM concentration of calcium can modulate the channels' transport characteristics; this effect is often explained via the existence of high affinity Ca 2+ binding sites on the channel walls. Inspired by properties of biological calcium-selective channels, we prepared a set of nanopores with tunable opening diameters that exhibited a similar response to the presence of calcium ions as biochannels. Nanopores in 15 nm thick silicon nitride films were drilled using focused ion beam and e-beam in a transmission electron microscope and subsequently rendered negatively charged through silanization. We found that nanopores with diameters smaller than 20 nm were blocked by calcium ions such that the ion currents in mixtures of KCl and CaCl 2 and in CaCl 2 were even ten times smaller than the ion currents in KCl solution. The ion current blockage was explained by the effect of local charge inversion where accumulated calcium ions switch the effective surface charge from negative to positive. The modulation of surface charge with calcium leads to concentration and voltage dependent local charge density and ion current. The combined experimental and modeling results provide a link between calcium ion-induced changes in surface charge properties and resulting ionic transport.

Understanding ion transport in nanoporous materials is critical to a wide variety of energy and environmental technologies, ranging from ion-selective membranes, drug delivery, and biosensing, to ion batteries and supercapacitors. While nanoscale transport is often described by continuum models that rely on a point charge description for ions and a homogeneous dielectric medium for the solvent, here, we show that transport of aqueous solutions at a hydrophobic interface can be highly dependent on the size and hydration strength of the solvated ions. Specifically, measurements of ion current through single silicon nitride nanopores that contain a hydrophobic–hydrophilic junction show that transport properties are dependent not only on applied voltage but also on the type of anion. We find that in Cl–-containing solutions the nanopores only conducted ionic current above a negative voltage threshold. On the other hand, introduction of large polarizable anions, such as Br– and I–, facilitated the pore wetting, making the pore conductive at all examined voltages. Molecular dynamics simulations revealed that the large anions, Br– and I–, have a weaker solvation shell compared to that of Cl– and consequently were prone to migrate from the aqueous solution to the hydrophobic surface, leading to the anion accumulation responsible for pore wetting. The results are essential for designing nanoporous systems that are selective to ions of the same charge, for realization of ion-induced wetting in hydrophobic pores, as well as for a fundamental understanding on the role of ion hydration shell on the properties of solid/liquid interfaces.

Surface charge plays a very important role in biological processes including ionic and molecular transport across a cell membrane. Placement of charges and charge patterns on walls of polymer and solid-state nanopores allowed preparation of ion-selective systems as well as ionic diodes and transistors to be applied in building biological sensors and ionic circuits. In this article, we show that the surface charge of a 10 nm diameter silicon nitride nanopore placed in contact with a salt gradient is not a constant value, but rather it depends on applied voltage and magnitude of the salt gradient. We found that even when a nanopore was in contact with solutions of pH equivalent to the isoelectric point of the pore surface, the pore walls became charged with voltage-dependent charge density. Implications of the charge gating for detection of proteins passing through a nanopore were considered, as well. Experiments performed with single 30 nm long silicon nitride nanopores were described by continuum modeling, which took into account the surface reactions on the nanopore walls and local modulation of the solution pH in the pore and at the pore entrances. The results revealed that manipulation of surface charge can occur without changing pH of the background electrolyte, which is especially important for applications where maintaining pH at a constant and physiological level is necessary. The system presented also offers a possibility to modulate polarity and magnitude of surface charges in a two-electrode setup, which previously was accomplished in more complex multielectrode systems.

The electrical-double layer (EDL) model is fundamental to our understanding of interactions in ionic solutions, and is widely used in chemical, biological, and technological contexts, particularly in the description of aqueous electrolyte solutions. However, recent experiments have raised questions regarding the validity of this model in polar, aprotic solvents; some observations, such as a surface potential that changes sign with increasing salt concentration, are not consistent with the EDL picture. We demonstrate in a model system, acetonitrile at a silica interface, that solvent organization dictates the ionic distributions. Ion-transport measurements in nanopores, surface-selective spectroscopy, and molecular dynamics simulations reveal that the distribution of ions in acetonitrile at a silica interface is determined by the lipid-bilayer-like organization that the interface imposes upon the liquid, which accounts for the change in sign of the potential. Our findings emphasize the importance of including solvent molecules and ions explicitly in descriptions of solid/liquid interfaces.

Solid-state nanopores offer a novel platform for single molecule analysis and characterization. In particular, they have attracted significant attention as tools for high-throughput, robust, and low-error DNA sequencing. The successful implementation of solid-state nanopores in emerging third-generation DNA sequencing applications is contingent upon developing methods for scalable fabrication, high-accuracy output, and integration with low-noise electronic architectures. In this presentation, I will cover recent progress our group has made in the production, characterization, and integration of these devices. New high-resolution methods for fabricating nanopores down to the singleatom level (0.5 nm) -ion beam irradiation, photo-oxidation, and controlled chemical etching -will first be discussed. I will then describe recent successes in incorporating solid-state nanopores with high-frequency (10 MHz), lownoise amplifiers and subsequent advances in signal processing. Lastly, a brief perspective on the outlook of solid-state nanopores as well as ongoing work with ion-channel-like pores in two-dimensional (2D) materials will be provided.