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PhD Defence Zinaida Kostiuchenko

mass transport in electrochemical nanofluidic detectors 

Zinaida Kostiuchenko is a PhD Student from the research group Bio Electronics. Her supervisor is professor Serge Lemay from the faculty of Science and Technology (TNW)

The ability to perform analysis of substances on a small scale is an important aspect of modern technology. Electrochemical detectors, suitable for miniaturization and integration with electronic components, are excellent candidates for this task as they can provide information on both composition and quantity of analyte present in a sample. At the highest level of miniaturization, nanogap devices significantly amplify the signal by redox cycling of analyte molecules between two parallel, closely spaced electrodes, making it possible to operate with tiny sample volumes (order femtoliter) and simultaneously detect low concentrations down to single molecules. Integration of these detectors in a fluidic system is essential for practical purposes. However, the presence of convective flow introduces an additional layer of complexity into the analysis of experimental results. In this thesis we consider mass transport in the nanochannels and its coupling with redox cycling processes taking place in nanogap devices.

The first chapter has a tutorial character, it briefly introduces the main concepts and terminology of both electrochemistry and nanofluidics that we later employ.

The second chapter is devoted to introducing in more detail ionic transport in nanofluidic devices. Particular emphasis is placed on the aspect of concentration polarization. We first discuss concentration polarization in the classic case of an inert electrolyte confined in a nanochannel whose height is comparable to the Debye length. We then introduce a thought experiment in which two reservoirs in which opposite redox reactions take place are connected. We propose that here a different form of concentration polarization occurs due to the difference in the diffusion coefficients between reduced and oxidized molecules.

In the third chapter we study mass transport in a nanofluidic channel under the condition of pressure driven flow using a two-electrode generator-collector configuration. The low height-to-length ratio in these channels suggests that mass transport can be described satisfactorily using only the 1D Nernst-Planck equation. We validate this assumption by demonstrating agreement between experimental results and analytical calculations. The transition between the diffusion-dominated and convection-dominated transport is defined by the longitudinal Peclet number and depends on the spacing between electrodes.

In the fourth chapter we examine the operation of nanogap devices under the conditions of pressure driven flow. We show that with the decrease of the flow rate the redox current magnitude goes down and the cycling voltammograms change their shape from reversible to quasi-reversible. Nanogap devices residing in the same nanochannel demonstrate cross-talk at low flow velocities: the redox current magnitude of one pair changes depending on the potential applied to electrodes at another pair. We consider several possible sources for these local and non-local changes of the value of the redox current and analyze the impact of each of them.

In the fifth chapter we focus our attention on the streaming effects developed in the fluidic system associated with nanogap devices when the concentration of the supporting electrolyte goes down. We utilize nanogap devices located in the ends of the nanochannel to measure the magnitude of the effect upstream and downstream with respect to the channels region and observe a clear presence of the streaming potentials at concentrations of 100µM and lower. The experimental results qualitatively follow estimated trends, but the precise values are off, which we attribute to uncertainty in the parameters governing the calculations.

Finally, in the sixth chapter we explore a different electrochemical system consisting of a bipolar electrode imbedded in a membrane. Both numerically and analytically, we quantify fluctuations in the number of electrons and the potential due to reduction and oxidation processes occurring at the electrode ends. We argue that for the smallest nanosized electrodes it is possible to achieve one-to-one coupling between the oxidation and the reduction of single molecules at the opposite sides.