Noise Properties of Ion Current in Rectifying Nanopores

Abstract

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.