Bioanalytical Applications of Chemically Modified Surfaces

Driscoll, Peter F

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Bioanalytical Applications of Chemically Modified Surfaces

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The design and development of chemically modified surfaces for bioanalytical applications is presented. Chemical surface modification is demonstrated to be a method to control surface properties on the molecular level by selecting the appropriate substrate, linking chemistry, and terminal group functionality. These systems utilize spontaneous interactions between individual molecules that allow them to self-assemble into larger, supramolecular constructs with a predictable structure and a high degree of order. Applications investigated in this thesis include: surface patterning, switchable surface wettability, and biological sensor devices that combine surface based molecular recognition, electrochemical detection methods, and microfluidics. A multilayered approach to complex surface patterning is described that combines self-assembly, photolabile protecting groups, and multilayered films. A photolabile protecting group has been incorporated into molecular level films that when cleaved leaves a reactive surface site that can be further functionalized. Surface patterns are created by using a photomask and then further functionalizing the irradiated area through covalent coupling. Fluorophores were attached to the deprotected regions, providing visual evidence of surface patterning. This approach is universal to bind moieties containing free amine groups at defined regions across a surface, allowing for the development of films with complex chemical and physico-chemical properties. Systems with photoswitchable wettability were developed by fabricating multilayered films that include a photoisomerizable moiety, cis-/trans- dicarboxystilbene. When this functionality was incorporated into a multilayered film using non-covalent interactions, irradiation with light of the appropriate wavelength resulted in a conformational change that consequently changed the hydrophobicity of the substrate. Methods were investigated to increase the reversibility of the photoswitching process by creating surface space between the stilbene ligands. Utilizing mixed monolayers for spacing resulted in complete isomerization for one cycle, while the use of SAMs with photolabile groups produced surfaces that underwent isomerization for three complete cycles. A microfluidic device platform for ion sensing applications has been developed. The platform contains components to deliver small volumes of analyte to a surface based microelectrode array and measure changes in analyte concentration electrochemically in an analogous method to that used in conventional electrochemical cells. Crown ether derivatives that bind alkali metal ions have been synthesized and tested as ionophores for a multi-analyte device of this type, and the sensing platform was demonstrated to measure physiological relevant concentrations of potassium ions. Advantages of this design include: high sensitivity (uM to mM), small sample volumes (less than 0.1 mL), multi-analyte capabilities (multiple working electrodes), continuous monitoring (a flow through system), and the ability to be calibrated (the system is reusable). The self-assembled systems described here are platform technologies that can be combined and used in molecular level devices. Current and future work includes: photopatterning of gold and glass substrates for directed cell adhesion and growth, the design and synthesis of selective ion sensors for biological samples, multi-analyte detection in microfluidic devices, and incorporating optical as well as electrochemical transduction methods into sensor devices to allow for greater sensitivity and self-calibration.

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