Browsing by Subject "potentiometry"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    Application of Fluorous Polymer Matrixes in Ion-Selective Elcetrodes
    (2016-08) Lugert-Thom, Elizabeth
    Polyperfluoro(4-vinyloxy-1-butene), which is also known as Cytop, and poly[4,5-difluoro-2,2,-bis (trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) copolymers with dioxole monomer contents of 65% or 87% (known as Teflon AF1600 and Teflon AF2400, respectively) were plasticized with four fluorous compounds. While plasticization of all polymers with perfluoroperhydrophenanthrene, perfluoro(1-methyldecalin), a perfluorotetraether with three trifluoromethyl side groups and one hydrogen atom, and a linear perfluorooligoether with an average of 14.3 ether groups per molecule was successful, these four plasticizers affected the twelve blends very differently. A threshold of plasticization beyond which further increases in the plasticizer volume fraction did not further affect the glass transition temperature, Tg, was observed for some blends. Also, the limit of miscibility ranged from as low as 20% plasticizer content to complete miscibility at all volume fractions. The blends of Teflon AF2400 or Teflon AF1600 with high contents of the linear perfluorooligoether provided Tg values as low as –114 ºC, lower than for any other fully miscible blend. The occurrence of two glass transitions in an intermediate range of plasticizer volume ratios for these two types of blends can be explained by distinct local environments rather than macroscopic phase separation, as anticipated by the Lodge-McLeish model. In spite of the widespread use of perfluorinated solvents with amino and ether groups in a variety of application fields, the coordinative properties of these compounds are poorly known. It is generally assumed that the electron withdrawing perfluorinated moieties render these functional groups rather inert, but little is known quantitatively about the extent of their inertness. This chapter reports on the interactions between inorganic monocations and perfluorotripentylamine and 2H-perfluoro-5,8,11-trimethyl-3,6,9,12-tetraoxapentadecane, as determined with fluorous liquid-membrane cation-selective electrodes doped with tetrakis[3,5-bis(perfluorohexyl)phenyl]borate salts. The amine does not undergo measurable association with any ion tested, and its formal pKa is shown to be smaller than –0.5. This is consistent with the nearly planar structure of the amine at its nitrogen center, as obtained with density functional theory calculations. The 2HPFTE interacts very weakly with Na+ and Li+. Assuming 1:1 stoichiometry, formal association constants were determined to be 2.3 and 1.5 M-1, respectively. This disproves an earlier proposition that the Lewis base character in such compounds may be non-existent. Due to the extremely low polarity of fluorous solvents and the resulting high extent of ion pair formation, a fluorophilic electrolyte salt with perfluoroalkyl substituents on both the cation and the anion had to be developed for these experiments. In its pure form, this first fluorophilic electrolyte salt is an ionic liquid with a glass transition temperature, Tg, of -18.5 ºC. Interestingly, the molar conductivity of solutions of this salt increases very steeply in the high concentration range, making it a particularly effective electrolyte salt. Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2,-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (known as Teflon AF) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis(3,5-bis(perfluorohexyl)phenyl)borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+-ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability and maintained their selectivity over at least four weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 103.5, 101.8, 106.8 and 104.4 M–1, respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride C(꞊O)F groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(꞊O)F groups can be eliminated chemically. In this work we demonstrate the remarkable stability of fluorous-based ion-selective electrode (ISE) membranes by exposing them to a cleaning-in-place treatment, CIP, as it is used in many industrial processes. The sensing membranes were made up of a linear perfluoropolyether as membrane matrix, 0.5 mmol/kg ionic sites (tetrakis[3,5-bis(perfluorohexyl)phenyl]borate), 2 mmol/kg ionophore (tris[(perfluorooctyl)propyl]amine or tris[(perfluorooctyl)pentyl]amine), and Teflon AF2400. To mimic a typical CIP treatment, the electrodes were repeatedly exposed for 30 min to 3.0% NaOH solution at 90 ºC (pH ≈12.7). After ten exposures and a total of 5 h at 90 ºC, the fluorous sensing membranes doped with the more selective ionophore still showed the ability to respond with a theoretical (Nernstian) slope without loss in selectivity. Addition of a fluorophilic electrolyte salt reduced the membrane resistance by an order of magnitude.
  • Thumbnail Image
    Item
    Development and Optimization of Planar Potentiometric Sensors for Point-of-Care Use
    (2023-10) Herrero, Eliza
    The monitoring of electrolytes and charged biomolecules in body fluids is a crucial step in both the diagnosis and management of many diseases, including chronic kidney disease and cardiovascular disease. Ion-selective electrodes (ISEs) are considered the gold standard in analyzing these analytes in clinical settings due to their high selectivity, near instant response time, and linear response. These ISEs are generally incorporated into a mainframe clinical blood analyzer which, due to the high cost, fragility, and need for trained staff to operate, are in centralized hospitals or laboratories. As a result, patients living in remote, or resource-limited areas often do not have access to such clinical diagnostics. There is, therefore, a need for point-of-care based ISEs, characterized by low-cost, high ease of use, and portability. Despite recent interest in the development of point-of-care based ISEs, there remain fundamental issues in the design and performance of these sensors, which I address in my research. This thesis presents work that supports the advancement of point-of-care based ISEs in several key areas.While paper has been proposed as a substrate for point-of-care sensors, it has impurities from manufacturing and being natural in origin. Moreover, its structure and surface composition are highly heterogeneous, which are disadvantageous when designing sensors for high reproducibility. In this work, I propose the use of a novel synthetic textile with higher purity and a more controlled structure to serve as a supporting substrate for miniaturized, membrane-free ISEs. To expand the versatility of these devices, I embedded both ion-sensing and reference membranes into the polyester fabric and successfully measured the activity of chloride (a highly relevant clinical biomarker) in aqueous solutions and 100% blood serum. This is the first example of an ISE that both embeds membranes into a fabric and uses the fabric to wick samples into contact with those membranes. I also determined the effect of pore structure on device performance, a finding applicable not only to textile-based ISEs, but also to other porous materials such as paper. I showed that devices fabricated on the textile had an order of magnitude improvement in the lower limit of detection (LOD) of chloride as compared to analogous paper-based devices. I also further the understanding of the sources of non-ideal performance in paper based ISEs through a systematic study of both sensor materials and interactions between materials and aqueous samples. While it has been suggested by many that these limitations are due to interactions of paper with sample or sensing components, to date this has not been thoroughly investigated. To this end, I studied interactions of target ions with paper by using a range of analytical techniques. My data shows two main reasons that explain the sub-optimal performance of paper-based devices for chloride sensing, which I explain and propose novel fabrication techniques to overcome. A key performance parameter in ISEs designed to be used outside of central laboratories is that of reproducibility, with the goal of calibration-free devices. Our group has previously improved sensor-to-sensor reproducibility with the use of redox buffers, which buffer redox-active impurities in the system. I propose the use of a novel cobalt(III/II)bis(terpyridine) as a hydrophilic redox buffer to be incorporated into the inner filling solution of ISEs for anion sensing. Conventional ISEs with a plasticized poly(vinyl)chloride ion-exchange membrane for Cl– and the redox buffer incorporated into the inner filling solution resulted in a E0 SD of 0.3 mV–one of the lowest reported SD thus far in the literature. The redox buffer was also found to be compatible with reference membranes as well as textile-based sensing setups. As the purpose for these devices is to be used in clinical diagnostics, it is also crucial to increase the number of analytes measured to include clinically relevant ions such as K+, Ca2+, and pH. I therefore also show the use of textile-based devices with ionophorecontaining membranes that selectively complex target ions. While a textile-based Ca2+ ISE was fabricated and successfully detected Ca2+ in aqueous samples, performance limitations arose in the detection of K+, H+, Ag+, and CO32-. A study of the effects of textile coating techniques and considerations of material-membrane interactions seek to address these shortcomings.
  • Thumbnail Image
    Item
    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.