Spatiotemporal electrochemical detection in nanofluidic device
The main focus of this thesis is to explore mass-transport processes for redox-active analytes in concentrated supporting electrolytes when they are driven by external pressure through nanofluidic channels with embedded electrodes. The principal devices employed in these experiments are so-called nanogap electrodes, which consist of two electrodes located in the floor and roof of the nanochannel. Electrochemical reactions taking place at the electrodes can profoundly disturb the concentration distribution of analytes along the axial direction of the nanochannel in several manners. First, changes in the redox state of molecules at different electrodes can cause significant variations in the distribution of analyte concentrations under steady-state conditions. Second, time-dependent changes in electrode potential can cause local, transient fluctuations in the analyte concentration. These represent a new class of phenomena in nanofluidics next to the more established electrokinetic and depletion-enrichment effects.
Jin Cui is a PhD student in the research group NanoIonics, directed by Serge Lemay.
The large surface-to-volume ratio that exists in nanofluidic devices means that any interactions between analyte molecules and surfaces manifest themselves more strongly than in conventional systems. After a general introduction in Chapter 1, Chapters 2 and 3 of this thesis investigate how transient concentration disturbances can be created by the near-instantaneous switching on and off of an electrochemical reaction. A pressure-driven convective flow can then transport these transient concentration profiles downstream. Throughout this process, further interaction with the channel walls can cause different species to become separated, resulting in a new form of electrochemical chromatography. After demonstrating the basic effect, we explore how it could be further optimized by varying the geometry and controlling the potential of the channel walls via additional ‘gate’ electrodes spanning the entire length of the channel.
An originally unforeseen consequence of introducing multiple nanogap transducers and/or gate electrodes in a single channel is that it can lead to significant coupling between different regions of the system. Local changes in the redox state imposed by electrodes at different biases means that large gradients can develop between different regions. Chapter 4 addresses how, in some cases, this can result in a rearrangement of the analyte concentration distribution along the entire channel, leading to an apparent non-local coupling between electrodes.
In addition to these three core chapters, we explored further potential applications of electrochemistry in bioanalytical research. Chapter 5 describes exploratory attempts to detect single conducting polymer molecules as they form a conducting pathway between two closely spaced electrodes. This approach can be further developed to become a transduction mechanism in single-molecule detection assays suitable for detection of short DNA oligomers. Chapter 6 instead shows how electrochemical detection at microelectrodes can yield the same enzyme kinetic parameters as conventional photo-spectroscopy techniques, a preparatory work prior to studying the kinetics of single enzyme in nanogap devices.
Starting-time: 12.30h in Building Waaier - Prof.dr. G. Berkhoff-zaal
