The behaviour of water in nanopores is very different from that of bulk water. Close to hydrophobic surfaces, the water density has been found to be lower than in the bulk, and if confined in a sufficiently narrow hydrophobic nanopore, water can spontaneously evaporate. Molecular dynamics simulations have suggested that a nanopore can be switched between dry and wet states by applying an electric potential across the nanopore membrane. Nanopores with hydrophobic walls could therefore create a gate system for water, and also for ionic and neutral species. Here, we show that single hydrophobic nanopores can undergo reversible wetting and dewetting due to condensation and evaporation of water inside the pores. The reversible process is observed as fluctuations between conducting and non-conducting ionic states and can be regulated by a transmembrane electric potential.

We observe single nanoparticle translocation events via resistive pulse sensing using silicon nitride pores described by a range of lengths and diameters. Pores are prepared by focused ion beam milling in 50 nm-, 100 nm-, and 500 nm-thick silicon nitride membranes with diameters fabricated to accommodate spherical silica nanoparticles with sizes chosen to mimic that of virus particles. In this manner, we are able to characterize the role of pore geometry in three key components of the detection scheme, namely, event magnitude, event duration, and event frequency. We find that the electric field created by the applied voltage and the pore's geometry is a critical factor. We develop approximations to describe this field, which are verified with computer simulations, and interactions between particles and this field. In so doing, we formulate what we believe to be the first approximation for the magnitude of ionic current blockage that explicitly addresses the invariance of access resistance of solid-state pores during particle translocation. These approximations also provide a suitable foundation for estimating the zeta potential of the particles and/or pore surface when studied in conjunction with event durations. We also verify that translocation achieved by electro-osmostic transport is an effective means of slowing translocation velocities of highly charged particles without compromising particle capture rate as compared to more traditional approaches based on electrophoretic transport.

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Room temperature ionic liquids (RTILs) are substances composed entirely of ions and are liquids at or below 100 degrees C. Ionic conductivity of RTIL is one of the most important physical properties of these unique substances that determine their potential applications as a new medium for capacitors, fuel and solar cells as well as in separation systems. The quality of performance of these devices relies on the understanding of ionic transport of RTIL on a nanoscale. In this letter, we use ionic current carried by RTILs in single nanopores as a probe for their nanoscale transport properties. We show that the conductivity of RTILs through nanopores is significantly less than corresponding bulk values. Our experiments allowed us to address the nature of the interaction of these confined RTILs with charged surfaces. Electrostatic interactions of RTILs with nanopores are the basis for the formation of ionic diodes rectifying transport of the constituent ions.

Ion currents through three types of rectifying nanoporous structures are studied and compared for the first time: conically shaped polymer nanopores, glass nanopipettes, and silicon nitride nanopores. Time signals of ion currents are analyzed by power spectrum. We focus on the low-frequency range where the power spectrum magnitude scales with frequency, f, as 1/f. Glass nanopipettes and polymer nanopores exhibit non-equilibrium 1/f noise, thus the normalized power spectrum depends on the voltage polarity and magnitude. In contrast, 1/f noise in rectifying silicon nitride nanopores is of equilibrium character. Various mechanisms underlying the voltagedependent 1/f noise are explored and discussed, including intrinsic pore wall dynamics, and formation of vortices and non-linear flow patterns in the pore. Experimental data are supported by modeling of ion currents based on the coupled Poisson-Nernst-Planck and Navier Stokes equations. We conclude that the voltage-dependent 1/f noise observed in polymer and glass asymmetric nanopores might result from high and asymmetric electric fields inducing secondary effects in the pore such as enhanced water dissociation.2

Hydrophobic interactions and local dewetting of hydrophobic cavities have been identified as a key mechanism for ionic gating in biological voltage-gated channels in a cell membrane. Hydrophobic interactions are responsible for rectification of the channels, i.e. the ability to transport ions more efficiently in one direction compared to the other. We designed single polymer nanopores with a hydrophobic gate on one side in the form of a single layer of C10 or C18 thiols. This nanoporous system behaves like an ionic diode whose direction of rectification is regulated by the pH of the electrolyte. In addition, reversible dewetting of the hydrophobic region of the pore was observed as voltage-dependent ion current fluctuations in time between conducting and non-conducting states. The observations are in accordance with earlier molecular dynamics simulations, which predicted the possibility of spontaneous and reversible dewetting of hydrophobic pores.

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