Matthew R. Powell

Electric-field-induced wetting and dewetting in single hydrophobic nanopores

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.

Nanoprecipitation-assisted ion current oscillations

Nanoscale pores exhibit transport properties that are not seen in micrometre-scale pores, such as increased ionic concentrations inside the pore relative to the bulk solution, ionic selectivity and ionic rectification. These nanoscale effects are all caused by the presence of permanent surface charges on the walls of the pore. Here we report a new phenomenon in which the addition of small amounts of divalent cations to a buffered monovalent ionic solution results in an oscillating ionic current through a conical nanopore. This behaviour is caused by the transient formation and redissolution of nanoprecipitates, which temporarily block the ionic current through the pore. The frequency and character of ionic current instabilities are regulated by the potential across the membrane and the chemistry of the precipitate. We discuss how oscillating nanopores could be used as model systems for studying nonlinear electrochemical processes and the early stages of crystallization in sub-femtolitre volumes. Such nanopore systems might also form the basis for a stochastic sensor.

Noise Properties of Ion Current in Rectifying Nanopores

Studying noise properties of ion currents in nanopores can improve detection limits for nanopore sensors as well as give insight into behavior of transport at the nanoscale. We focused on the 1/f⊥alpha noise that is observed in the low frequency regime of the ion current power spectra with the exponent alpha∼1. We found that 1/f noise in single conically shaped nanopores in polymer films and glass nanopipettes exhibits asymmetric noise properties with respect to voltage polarity which are not observed for cylindrical and silicon nitride nanopores. The noise asymmetry is shown by the normalized power spectra, which present the noise amplitude at a given frequency, typically 1 Hz for these measurements, divided by the ion current squared. The conically shaped structures rectify the ion current and the currents for the forward bias exhibit noise that increases with voltage in an exponential manner, and are weakly KCl concentration dependent. The normalized noise of currents in the reverse bias is typically voltage-independent but increases with the increase of KCl concentration. The difference in noise properties of the currents is most pronounced when the pore diameter is comparable to the thickness of the electrical double-layer. We discuss two models, which could explain the observed effects: (i) presence of air bubbles, and (ii) crowding of ions at the pore entrance.