Fundamental and applied studies of polyion -sensitive membrane electrodes.
Esson, Joan Marie
1999
Abstract
The potentiometric responses of tridodecylmethylammonium (TDMA)-based polymer membrane electrodes toward sub-micromolar concentrations of a variety of biologically important polyanions was explored. Both the equilibrium and non-equilibrium EMF changes toward polyphosphates, nucleic acids, and negatively charged peptides were found to depend on the structural characteristics of the polyanion, including the molecular weight, lipophilicity, and charge density. A novel method to evaluate the binding between TDMA and these polyanions, as well as between the ion-exchanger dinonylnaphthalene sulfonate and polycations, was developed. The titrations of various polyions with surfactants that are structurally analogous to these ion-exchangers were monitored in aqueous solutions via surfactant-selective electrodes. The cooperative binding affinities determined from binding analysis of the titration data were found to correlate strongly with the equilibrium EMF changes of polyion-sensitive membrane electrodes toward the respective polyions. This further supports the complexation mechanism of cooperative ion-pairing within the membrane phase of these devices. Additionally, the cooperative binding affinities between the surfactants and polyions was influenced by the structure of the polyion, as well as of the surfactant. The dependence of the non-equilibrium EMF response on molecular weight was exploited to develop enzymatic assays employing polyanion-sensitive membrane electrode detection. Assays for acid phosphatase and RNase A were developed utilizing polyphosphate and RNA homopolymers as substrates with detection ranges within the clinically relevant regimes. The linear range of the assays could be modulated by changing the substrate size. Additionally, these enzymatic assays were highly specific for the target enzyme. Alternative assay formats were also investigated and their analytical utility was found to be strongly dependent on the specific structure of the synthetic polyionic substrate. Another bioanalytical application of polyanion-sensitive membrane electrodes was explored. Using a flow-through sample format, pH-cross-sensitive polyanion-sensitive electrodes were utilized for the pseudo-continuous measurements of heparin levels in samples as complex as whole blood. It was found that the initial kinetic responses, rather than the discrete EMF responses, could be utilized to determine heparin concentrations more reproducibly with such a system. Lastly, the response characteristics of membrane electrodes incorporating lipophilic guanidinium-based ion-exchangers were found to have an altered polyanion selectivity pattern compared to TDMA-based membrane electrodes.Subjects
Applied Fundamental Membrane Electrodes Polyion-sensitive Studies Tridodecylmethylammonium
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