Current fluctuations in ionic nanopores

Thesis

From electrical current to ecological and biological systems, fluctuations are characterised based on the frequency dependence of the power spectral density. Surprisingly - given the diversity of the systems considered - the power spectra of many systems reveal an inverse power- law dependence on the frequency f in certain regimes. This ubiquitous phenomenon famously known as “1/f noise” has triggered an abundance of investigations aimed at understanding its mechanism. Recently, “1/f noise” has also been observed in the ionic electric current through biological ion channels, nanometre-scale membrane pores and solid-state nanopores. Identifying and understanding the source of 1/f noise in nanopores emerges as a crucial prerequisite to design nanopore systems for their use in technological devices, such as nanopore- based DNA sequencers. At the same time, the current power spectrum in nanopores contains a wealth of information which, if extracted, would help towards a detailed characterisation of the nanopore’s microscopic properties.

To analyse the mechanism behind the occurrence of 1/f in nanopores, we study nanopores using Langevin-dynamics simulations and analytical methods. Ions move through the pore driven by an applied electric field, and the induced current is recorded. The power spectral density calculated from this data indicates the existence of a power law frequency dependence in an extended frequency regime.

To verify a series of conclusions drawn from experimental investigations, we systematically vary all parameters characterising the system, including the geometry, the electric field, the ion density and the flexibility of the pore wall. Furthermore, we derive an analytical expression for the current power spectral density and compare it to the results from the Langevin-dynamics simulations. Finally, we evaluate the role of hydrodynamic interactions and find that it does not induce any major changes in the current power spectral density. Our studies allow us to uncover the mechanism that leads to the power law behaviour: interactions among the ionic components.

Authors: Zorkot, M

• DOI: 10.5287/ora-vq71ann6e
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford