Selective Ion Transport Control and Single Molecule Dynamics in Nanopore Electrode Arrays

Advances in nanofabrication techniques have enabled construction of sophisticated nanoscale architectures, which allows electrochemical measurements in low dimensional nanostructures that have the capability to illuminate electrochemical phenomena in a unique way, especially compared to conventional macroscopic electrochemistry which relies on bulk diffusion. In particular, nanopore electrode arrays (NEAs), which are composed of a set of ring/disk nanoelectrodes in cylindrical or conical nanopores, is a useful architecture to investigate a number of interesting nanoelectrochemical characteristics in confined volumes, typ. 10^-18 L, such as ion/molecular transport behavior and electrochemical kinetics. Also, this small confined volume makes it possible to explore the dynamics of chemical reactions involving a single or a few molecules. The work in this dissertation addresses three main topics: the development of massively parallel NEAs for selective ion/molecular transport control, sensitive electrochemical detection, and spectroelectrochemical single enzyme molecule studies in nanoconfined geometries.

The first topic focuses on the demonstration of biomimetic ion gating in block copolymer (BCP)-coated NEAs in response to external stimuli, such as pH and ionic charge state. The structures consist of a pH-responsive, charge-selective dual-gating BCP membrane, capable of self-organizing into highly ordered nanocylindrical domains. Introducing the BCP membrane onto NEAs allows the BCP to serve as a pH-gate, controlling ion transfer into the nanopores, because it exhibits pH-dependent structural transitions, i.e., being positively-charged, hydrophilic, and swollen at pH values below pK_a of the BCP, but charge-neutral, hydrophobic, and collapsed structures at pH > pK_a. These structural properties support on-off transport switching at pH values near the pK_a with excellent anion permselectivity membrane at pH < pK_a. Redox species are charge-selectively transported through the BCP membrane into NEAs where they can then be sensitively detected by electrochemical signal amplification by redox cycling at 100 nm gap dual-ring nanoelectrodes.

Extending this concept, the second topic addresses the active control of mass transport based on electrowetting in the BCP membrane when formed at an NEA surface. Potential-induced wetting and dewetting behavior is characterized in pH-responsive BCP membranes fabricated in hierarchically-organized structures on NEAs. In these structures the BCP blocks mass transport across the hydrophobic BCP at pH > pK_a, while it acts as an anion exchange membrane at pH < pK_a. Interestingly, however, mass transport across the hydrophobic BCP nanochannels at pH > pK_a, is switched on by sufficiently negative potentials applied across the BCP membrane, resulting in the electrolyte solution being introduced into, and then isolated in, the nanopores. The potential-induced wetting and dewetting transitions and their effect on voltammetric response in the BCP and NEA structures are characterized as a function of potential, pH, and ionic strength. In addition, chronoamperometry and redox cycling are used to further characterize the potential response.

In the third topic, NEAs are used in a bifunctional zero mode waveguide (ZMW) configuration that combines electrochemistry with spectroscopy for single enzyme molecule studies. The potential-dependent fluorescence dynamics of single glutathione reductase (GR) molecules are investigated in an electrochemical ZMW, where a single Au ring electrode embedded in each nanopore simultaneously serves to control electrochemical potential and to confine electromagnetic radiation in a zeptoliter-scale observation volume within the nanopores. The redox state of GR and its response to the enzyme substrate is monitored at single molecule level by correlating electrochemical and spectroscopic signals under external potential control in the presence of a redox mediator.