This dissertation presents novel integration of impedimetric-based sensing with microfluidic devices towards the development of biosensors. The applications for the devices discussed herein include an investigation of the electrical double layer at the electrode/electrolyte interface, an investigation of the electrical double layer when confined between two electrodes with a gap smaller than the Debye length, i.e. overlapping double layers, and impedimetric-based diagnosis of antibody-antigen binding inside a network of nanocavities. The importance of the electrode/elapectrolyte interface is addressed in many chapters, which is a crucial issue in biosensors. The electrical double layer is omnipresent, and this dissertation talks about how the double layer can be both good and bad for biosensing applications.
The first chapter offers a general introduction to impedance spectroscopy and microfluidic integration towards the development of biosensors. The importance of the electrical double layer found at the interface between an electrode and electrolyte and the use of equivalent circuit modeling is also discussed.
Chapter 2 offers in depth information into the impedimetric characteristics of the electrode/electrolyte interface with a mind towards biologically relevant solutions. Special attention is paid to the electrical double layer by using equivalent circuit modeling to isolate the interface with respect to the bulk. Different electrode surfaces are compared and discussed.
Chapter 3 builds upon the previous chapter as it investigates two relatively new electrode materials that have great potential for impedance spectroscopy of biological materials in microdevices, iridium oxide and PPy/PSS (a polyelectrolyte). Focus is placed on the ability of these electrode films to reduce the influence of electrical double layer capacitance on the impedance spectrum for biological applications.
Chapter 4 introduces the concept of a nanogap capacitor, which consists of two electrodes separated by a nanoscale gap. This chapter extensively reviews the existing literature related to nanogap capacitors, and compares the different types of nanogaps and their respective applications. Additionally, this chapter introduces and discusses the concept of overlapping double layers. A numerical solution to overlapping double layers inside a nanogap and how it relates to impedance spectroscopy is also presented.
Chapter 5 introduces an all-semiconductor nanogap device designed for biosensing applications. Characterization of the nanogap sensor is shown including an extensive equivalent circuit model. Applications of this nanogap, including protein sensing, DNA hybridization, and water behavior are demonstrated.
Chapter 6 demonstrates measurements using the nanogap capacitors to detect the conformational changes in a well known protein. A well-known alkaline conformational change of cytochrome c is induced by changing the ph of the solution. The change in shape is recorded in the impedance spectrum and demonstrates the ability of impedance-based nanogaps to act as biosensors.
Chapter 7 expands on the uses of nanogap capacitors by investigating the impedance spectrum of water as it freezes under different ionic conditions. The importance of water in everyday life is profound, and these nanogap devices could offer insight into the behavior of water and ions in the nanoscale.
Chapter 8 presents a different impedimetric device with diagnostic applications. A nanocavity network is formed in a facile manner inside of a microfluidic channel by packing beads into a column. The complex impedance through this beadpack is monitored over time as a means to sense the binding of proteins to the surface. Due to the electrical nature of the detection, this setup offers the potential for inexpensive and rapid point of care diagnostics. The relative merits of microfluidic devices that incorporate bead-based assays are also discussed.
The final chapter will briefly conclude the results obtained during my tenure as a graduate student as well as offer some discussion about the future direction for the microdevices discussed here.
Appendix A presents results from a feasibility study performed in Ecuador to assess the use of point of care microdevices for diagnostics. A direct result from the project discussed in Chapter 8, I traveled through Ecuador to specifically investigate using point of care devices for Dengue fever screening and monitoring. This chapter offers insight into using microdevices discussed in this dissertation for developing world diagnostics. (Abstract shortened by UMI.)