Browsing by Subject "reference electrode"

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    Development of Electrochemical Sensors for Analytical and Biomedical Applications
    (2019-08) Chen, Xin
    The focus of this dissertation is on two main topics: the development of chemical sensors with reduced biofouling for applications in biological samples (Chapter I–II), and the development of chemical sensors with improved biocompatibility (Chapter III–V). Conventional polymeric membrane-based ion-selective electrodes (ISEs) rely on plasticized poly(vinyl chloride) (PVC) as sensor membranes. The plasticizers that solubilize PVC backbone—a prerequisite for PVC-phase ISEs—leach out gradually, resulting in a limited sensor lifetime. Polar groups in the plasticizer may also lower the sensor selectivity. To improve selectivity and expand working ranges, fluorous-phase ISEs relying on nonpolar perfluorinated compounds as sensing membrane were developed. A novel fluorophilic ionophore was synthesized and used to make ionophore-doped fluorous-phase ISEs with Nernstian responses and an optimal working range centered around neutral pH—suitable for most biological samples. The reproducibility of fluorous-phase ISEs was enhanced by a new electrode body design. Importantly, fluorous-phase ISEs maintained their excellent selectivity after prolonged exposure in serum whereas PVC-phase ISEs lost selectivity considerably. Insights were also obtained on the optimal ionophore-to-ionic site ratio. To improve biocompatibility, silicone-based reference and ion-selective electrodes were developed to eliminate plasticizers. Reference electrodes doped with several ionic liquids showed sample-independent and long-term stable potentials in artificial blood electrolytes and serum samples. Potassium-selective silicone-based ISEs developed with two ionophores and two silicones showed Nernstian responses and good selectivities. In an attempt to prevent leaching of ionophores from ISE membrane into samples, a well-known potassium ionophore was covalently attached to silicone membranes. Miniaturized microelectrodes suitable for implantable devices were also developed based on this platform. In a similar effort, plasticizer-free polymethacrylate-based ISEs exhibited Nernstian responses to pH and selectivities comparable to PVC-phase ISEs. To further improve biocompatibility for applications in the pharmaceutical and food industries, either an ionophore or ionic site or both were covalently attached to sensor membranes. Sensors with either ionophore or ionic site attached provided similar good characteristics whereas when both were attached, Nernstian responses were not found consistently. Furthermore, heating experiments showed that sensors exposed to 90 ˚C heating maintained good selectivity.
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    Miniaturizable Ion-Selective Electrode System: Solid Contact Electrode and Liquid Junction Free Reference Electrode
    (2014-05) Zou, Xu
    Ion-selective electrodes (ISEs) are widely used as important, analytical tools to determine the concentration of a broad range of ions for clinical analysis. As a potentiometric sensor, an ISE is comprised of two major parts: an ion-selective electrode and a reference electrode, both of which are vital in guaranteeing accurate and reliable measurement results. Conventional ISEs have an interior solution that forms a liquid contact with a selective membrane. However, complications due to evaporation and freezing in certain circumstances lead to irreproducibility, instability of the signals, and shortened life expectancy. In addition, miniaturization is an obstacle for this type of electrode. A universal method for a solid contact electrode that is calibration-free, durable, reproducible, and inexpensive to fabricate. A plasticized poly(vinylchloride) doped with tris(1,10-phenanthroline) cobalt(2+) tetrakispentafluorophenylborate and tris(1,10-phenanthroline) cobalt(3+) tetrakispentafluorophenylborate has been developed as a transducer layer. This layer has shown very reproducible potential in potassium chloride solutions with a potentil of standard deviation of 0.5 mV and response slope of 61mV / decade to varying concentrations. A redox buffer platform, based on the more lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 4,4'-dinonyl-2,2'-bipyridyl, was developed and applied to K+, Na+, Ca2+, H+, and CO32- with emf values of an electrode-to-electrode standard deviation as low as 0.7 mV (2% error in concentration) The reference electrode must maintain a constant potential over long periods of continuous measurements. Used in real life samples, salt bridges for conventional reference electrodes clog with proteins and lipids, suffer from contamination by sample components. A new method was developed that involves the application of a current pulse to a hydrophobic ion-doped membrane, thereby controlling transmembrane ion fluxes to obtain a sample independent reference electrode potential. The concentration of the ions released into the sample depended on the current amplitude and length and can be explained quantitatively by diffusion theory. The observable stability of the potential exhibited in this study by reference electrodes exposed to serum was particularly promising in view of biological and medical applications that require long term monitoring.