Coupling electrochemistry and microfluidics for biosensor development

Abstract

Biosensors are valuable analytical tools across both scientific and medical fields. Improving and miniaturizing biosensors is an area of great interest in academic, medical, and diagnostic settings. There is a constant need to improve these systems by increasing accessibility through lower costs, greater portability, and enhanced ease-of-use. Interfacing microfluidics and electrochemical methods shows great potential to address these needs. The work described in this thesis aims to address gaps in biosensor development, including increasing accessibility and improving sensing capabilities such as sensitivity, selectivity, and resolution, by combining electrochemistry and microfluidics to develop new tools for use in a range of biosensor systems. The primary focus of this research was to couple electrochemical detection methods with microfluidic devices for bioanalytical applications. Two main topics are reported. The first is a fluid transport mechanism employing the gas permeability of poly(dimethylsiloxane) (PDMS). Degassed PDMS pumps provide a simple, portable, inexpensive method to generate controlled fluid flow in a microfluidic device to transport a sample to electrodes for electrochemical detection. The second topic reported does not aim to address cost or portability of the system, but rather focuses on improving the capabilities of electrode arrays as chemical imaging platforms. In this work, a platinum microelectrode system was developed for biomarker detection, primarily nitric oxide and norepinephrine. Microfluidic devices interfaced directly with the electrodes provided precise control of fluid delivery to the sensors enabling both localized control of chemical concentrations as well as selective chemical stimulation of living tissue. The microelectrodes, when arranged in a high-density array, provided a platform capable of achieving electrochemical biomarker detection and imaging from live tissue slices with high spatiotemporal resolution. Both technologies described required the effective interfacing of microfluidic devices with electrochemical sensors to generate biomarker detection platforms. Custom microfluidic systems were developed to directly integrate biological samples into the platforms, including dried serum spots on a filter paper matrix and live ex vivo murine adrenal slices embedded in agarose. To achieve reproducible biomarker detection in complex biological matrices, electrochemical cleaning methods were developed and utilized for electrode maintenance. All of the tools described in this thesis were designed to address specific applications, but were also intended to be translatable to other systems. The degassed PDMS pump could be used as a fluid transport mechanism for other microfluidic devices, improving the simplicity and portability of systems that could otherwise be limited by external pumping equipment. Similarly, the strategies described for interfacing microfluidic devices with the reported electrode arrays and platinum microelectrodes could be applied to other silicon microchips to accomplish precise control of fluid delivery to the electrodes. The technology developed to generate an electrochemical imaging platform could be further pursued to achieve a high level of chemical selectivity by employing alternative waveforms, such as fast scan cyclic voltammetry, or electrode modifications to better elucidate the role of chemical gradients in biological systems. All of these tools, when applied to other bioanalytical platforms, could continue to advance the field of biomarker detection using microfluidic systems.