Power Spectral Analysis to Understand Noise and Fluctuations in Nanoscale Systems

Apparently random fluctuations, or 'noise', are a feature of experimental systems at all length scales. This can be particularly problematic for nanoscale systems, where small measurable quantities and short length- and time- scales lead to low signal-to-noise ratios. Previous computational and theoretical work has established some mechanistic links between microscopic fluctuating quantities and physical properties, to obtain system properties from noise. Experimentally, determining similar links has proven more difficult; real systems are complex and contain many interconnected processes happening in concert - for example, diffusion, adsorption, inter-particle interactions, and drift. As such, developing new methods to interpret noise in experiments could unlock a valuable new source of information, aiding the development of nanotechnological devices including functionalized nanopores for sensing and filtration. The aim of the project is to identify robust mechanistic links between microscopic fluctuations and physico-chemical processes in nanoscale systems where adsorption plays a key role. To quantify noise, we use power spectral density (PSD) analysis - an effective tool which decomposes fluctuations in time into their different frequency components. PSDs reveal information about the system by their characteristic shape, with specific corner frequencies and scalings as 1/falpha at high frequencies (fractional noise). While commonly used to characterize nanopore data, PSDs are less popular in other nano- and micro-scopic systems (for example, colloidal microfluidics) where alternative correlation functions are favoured. The project will combine theoretical and experimental approaches, to interpret fluctuation phenomena across a range of length scales relevant to soft matter systems. Over the last three decades, nanopores have developed as a molecular sensing technology, primarily for single-molecule sensing of DNA. The first nanopore sensors were biological pores, but the field has since expanded with the advent of solid-state pores like glass nanopipettes. Nanopores are still most commonly used for sensing or sequencing of proteins and DNA, but the central technique - resistive pulse sensing - depends on the detection of large drops in the ionic current through the pore. Smaller analytes like short polymers do not cause a measurable drop in the current; however, it has recently been demonstrated that their presence can be detected by a change to the PSD of the current. It is hypothesized that this change results from their adsorption to the pore walls, but the precise adsorption mechanism is unknown even in bulk solution. Using noise in this way to reveal interactions inside pores is a novel method of exploring particle dynamics inside such systems. Furthermore, by comparing these results to larger systems involving colloids, it will be possible to assess the extent to which scaling laws in power spectra linked to diffusion and adsorption persist across length and time scales. This project falls within the EPSRC Biophysics and Soft Matter Physics research area. It involves collaboration with Sophie Marbach at Sorbonne University.