Development of Electrochemical Sensors through Molecular Design and Sensor Modifications

Abstract

This dissertation focuses on three main research topics. One is the design and improvement of aqueous reference electrodes for long-term stability (Ch. 2-4). The second is the design of novel fluorous phase ion-transfer voltametric sensors (Ch. 5-6) and the third is the study and synthesis of a suitable probe for aldehydes to be self-assembled onto graphene-based electrochemical sensors (Ch. 7-8). Recently, the development of stable working electrodes for long-term electrochemical analysis has taken great strides. However, the improvement of references electrodes has not kept up. Commonly used, commercially available reference electrodes such as double-junction electrodes are not suitable for long term measurements as they must be refilled often. While glass-frit based reference electrodes have lower flow, they suffer from irregularities in their potential due to ionic strength changes, stirring, and contamination. To alleviate all these problems associated with these aforementioned aqueous reference electrodes, an Ag-AgCl capillary-based reference electrode was developed. Utilizing a capillary-based salt bridge provides a stable interface for electrochemical measurements, while also decreasing the volumetric flow of these electrodes to about 100 nL/h. The larger diameter of the capillary in comparison to glass frit pores eliminates the issues associated with charge screening as well. These characteristics not only reduces greatly the need to refill capillary-based reference electrodes, making them ideal for long-term measurement, but they also are ideal for use in low-volume samples where previously only quasi-reference electrodes could be used. The exploration of ion-transfer voltammetry using a fluorous phase is of great interest as a possible technique to detect and measure the presence of perfluorinated contaminants in many types of aqueous samples, including both environmental and biological. Two strategies for their detection through ion-transfer voltammetry with a fluorous phase are discussed, including thin-film ion transfer voltammetry and three phase electrodes. Thin-film electrodes typically rely on a conductive polymer to act as an ion-to-electron transducer which is then coated in a thin polymer membrane. While these electrodes have only been fabricated with organic membranes, here the goal is to modify these with thin fluorous membrane. Synthesis of a suitable highly fluorophilic monomer, 3-[tris-[2-(perfluorohexyl)ethyl]silyl]thiophene, which will be compatible with a fluorous membrane is described. Alternatively, three-phase electrodes could provide a strategy for the detection of perfluorinated contaminants. These electrodes rely on a three-phase boundary between two immiscible solutions and the working electrode to measure the transfer of ions from at the liquid-liquid interface. Again, these electrodes have only been reported using an aqueous/organic interface. Here the analysis of different three phase electrode set ups for use with a fluorous phase is presented, including a paper-based set up, a pencil graphite set up, and a gelled organic platinum mesh set up. Three phase electrodes require a redox active species in the hydrophobic phase. So, a suitable, more fluorophilic compound, (3-perfluorooctylpropanoyl)ferrocene, was synthesized and used in fluorous three phase electrodes to demonstrate the transfer of perfluorinated ions at the aqueous/fluorous interface. Lastly, the development of a probe for the detection of aldehydes with graphene-based electrochemical sensors is described. A direct comparison of the kinetics via NMR spectroscopy of two commonly used aldehyde derivatizing agents, phenyl hydrazine and O-phenylhydroxylamine, is presented. This work shows that phenylhydrazine reacts, on average, 71 times faster with aldehydes than does O-phenylhydroxylamine, indicating that development of a hydrazine derivative for aldehyde sensing purposes is preferred. The synthesis of 4-hexadecylphenylhydrazine is described as well as its successful self-assembly onto graphene from a solution of tetrahydrofuran resulting in a good fit with Langmuir adsorption theory (LogK=3.93) and indicating a concentration of 1.05 mM is needed for a 90% coverage monolayer.