Modeling and simulation of stochastic phenomena in carbon nanotube-based single molecule sensors

Abstract

Shrinking sensors to the nanoscale introduces novel selectivity mechanisms and enables the ultimate sensitivity limit, single-molecule detection. Single-walled carbon nanotubes, with a bright fluorescence signal and no photobleaching, are a platform for implantable near-IR sensors capable of selectively detecting a range of small-molecules including the radical signalling molecule nitric oxide, the hormone estradiol, and sugars such as glucose. Selectivity is achieved by engineering an adsorbed phase of polymers, DNA, or surfactants at the nanotube/solution interface. Understanding these sensors requires a range of modeling and simulation tools and presents a unique opportunity to learn how these phases interact with small molecules. This thesis work discusses methods and limits to integrating data from many noisy stochastic sensors, show how these sensors can be used to monitor nitric oxide inside cells with unprecedented spatiotemporal resolution, and describes what is needed to engineer a selective adsorbed phase. In addition, another method of stochastic detection is described based on the stochastic ionic pore-blocking of transport inside individual single-walled carbon nanotubes. We discuss the current state-of-the-art for making and analysing devices with a single nanometer-scale pore, which necessarily leads to stochastic transport fluctuations. We also present work on the analysis on many devices with single characterized SWCNT pores. A maximum in transport rates inside SWCNTs with diameters of approximately 1.6 nm is shown and discussed, with implications for how we model transport at this scale and the design of new SWCNT membranes. Finally, we discuss how complex surfaces of interconnected nanoscale structures could lead to new materials with interesting mechanical properties. One example of such a structure is an interlocking sheet of graphene rings, analogous to macroscopic chainmail. Such a sheet would have interesting properties, as entropic out-of-plane fluctuations would lead to a negative Poisson's ratio, known as an auxetic material. We present simulations for what the properties of a sheet might look like. In addition, we present simulations for how these properties change as a membrane is strained and showing the conditions over which these surfaces have desirable properties. These results offer a path towards materials with tunable auxetic properties.