Direct Evidence of Ionic Fluxes Across IonSelective Membranes: A Scanning Electrochemical Microscopic and Potentiometric Study (original) (raw)
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2010
Recognition and transport of important species at the membrane of a biological cell are critical for regulation of intracellular communication, metabolic pathways, vital internal conditions, and pharmaceutical drug uptake. Both processes are mediated by membrane-bound proteins functioning as pores, channels, and transporters that recognize and facilitate the transport of ions, nucleic acids and sugars. This whole process can be driven actively by membrane potential against the concentration gradient of transported species. In my PhD work, I fundamentally characterized dynamics of active ion transport, both in the presence and absence of recognition events, at liquid/liquid interfaces to understand electrochemically-controlled interfacial ion recognition and transfer. A deeper understanding of the kinetic and thermodynamic properties is achieved to realize applications in biomedical and environmental science, sensor technology and nanotechnology. The interface between two immiscible solutions served as an artificial model of a cell membrane. By manipulation of the interfacial potential, the active transport of ionic species was mimicked, which was monitored by an ionic current. Micrometer and nanometer sized interfaces were formed experimentally at the orifice of micropipets and nanopipets to probe ion-transfer reactions. Micropipet/nanopipet voltammetry was advanced to accurately obtain quantitative kinetic and thermodynamic parameters through numerical simulations of ion transfer and diffusion. Ion transfer rates for reversible and
Analytical Chemistry, 1999
The processes determining the lower detection limit of carrier-based ion-selective electrodes (ISEs) are described by a steady-state ion flux model under zero-current conditions. Ion-exchange and coextraction equilibria on both sides of the membrane induce concentration gradients within the organic phase and, through the resulting ion fluxes, influence the lower detection limit. The latter is shown to improve considerably when very small gradients of decreasing primary ion concentration toward the inner electrolyte solution are created. By merely altering the concentration of the inner electrolyte, detection limits may vary by more than 5 orders of magnitude. Very large gradients, however, are predicted to lead to significant depletion of analyte ions in the outer membrane surface layer and thus to apparent super-Nernstian response. The currently recommended IUPAC definition of the lower detection limit leads to nonrealistic values in such cases. Small changes in the concentration profiles within the membrane may have large effects on the response of the ISE at submicromolar levels and enhance its sensitivity to interferences during trace determinations. The model studies presented here demonstrate that trace level measurements with ISEs are feasible but often require higher membrane selectivities than expected from the Nicolskii equation.
Analytica Chimica Acta, 1998
A¯ow cell with a poly(vinyl chloride) (PVC) neutral-ionophore liquid-membrane ion-selective electrode has been developed for¯ow injection potentiometry (FIP). The¯ow system was optimised and ®ve substituted azacrown ethers: 7,16-dithenoyl-1,4,10,13-tetroxa-7,16-diazacyclooctadecane (DTODC), 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC), 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC), 1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TC) and 4,7,13,16-tetrathenoyl-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TTOTC) synthesised and investigated as the ionophores in Pb 2 (DTAODC, TTOTC) and Hg 2 (DTDC, DTODC, TC, TTOTC) selective electrodes. The short contact times between analyte and ionophore in FIP allow the measurement of a strongly complexing ion such as Hg 2 that saturates the electrodes in batch analysis, or in continuous¯ow analysis that comes to a steady state. For the mercury-selective electrodes with ionophores with amide functional groups (TTOTC and DTODC) a carrier of 10 mM potassium nitrate was found to increase the speed of response and recovery to baseline. The linear calibration range for a DTAODC lead-selective electrode was pPb2.0 to 5.0 with a slope of 32.5 mV decade À1 and for a TTOTC mercury-selective electrode, pHg3.0 to 5.5 with a slope of 28.4 mV decade À1 . Highly reproducible measurements were obtained (RSD <2%) at a¯ow rate of 3.0 ml min À1 giving a typical throughput of 40 samples h À1 for Pb 2 and 30 samples h À1 for Hg 2 . # 1998 Elsevier Science B.V. All rights reserved.
Validation criteria for developing ion-selective membrane electrodes for analysis of pharmaceuticals
Accreditation and Quality Assurance, 1998
The problem of validation criteria for developing ion-selective membrane electrodes for the analysis of pharmaceuticals arises from the connection between the reliability of ion-selective membrane electrodes construction and the reliability of the analytical information. Liquid membrane selective electrodes are more suitable for validation than the solid variety. The influence of the stability of ion pair complexes from the membrane on various parameters (e.g. response, limit of detection, and selectivity) is discussed. Validation criteria are proposed.
Membranes
The use of external electronic enforcement in ion-sensor measurements is described. The objective is to improve the open-circuit (potentiometric) sensitivity of ion sensors. The sensitivity determines the precision of analyte determination and has been of interest since the beginning of ion-sensor technology. Owing to the theoretical interpretation founded by W.E. Nernst, the sensitivity is characterized by the slope and numerically predicted. It is empirically determined and validated during calibration by measuring an electromotive force between the ion sensor and the reference electrode. In practice, this measurement is made with commercial potentiometers that function as unaltered “black boxes”. This report demonstrates that by gaining access to a meter’s electrical systems and allowing for versatile signal summations, the empirical slope can be increased favorably. To prove the validity of the approach presented, flow-through ion-sensor blocks used in routine measurements of bl...
Electroanalysis, 2000
A normal pulse voltammetric detection mode for amperometric solvent polymeric membrane ion sensors is described. These sensors function on the basis of ion transfer voltammetry into an organic membrane phase of high viscosity. To avoid sensor drift, it is required that sample ions extracted within a measurement period are quantitatively stripped off the sensing membrane before the next measurement step. The time required for complete back extraction of previously extracted ions must be substantially longer than for the uptake process. Indeed, more than 40% of extracted ions are predicted to remain in the membrane phase if the stripping time equals the uptake time. This suggests that cyclic voltammetry is generally an inadequate method for a reliable application/characterization of these sensors. The pulsed method imposes discrete potential pulses onto the membrane that are incrementally changing with time to cover the total desired potential range. Between each uptake pulse a suf®ciently long stripping pulse around 0 V is applied. Optimization of uptake and stripping times are performed, and comparative data with cyclic voltammetry are shown. Normal pulse voltammetric detection scans show strictly the current response for the ion uptake process, and are free of superimposed stripping waves. This characteristic aids in elucidating the nature of each observed wave and can therefore also be used for qualitative purposes. The scans also show higher sensitivity than in classical cyclic voltammetry. Experiments are here limited to ionophore-free membranes as model systems.
The Journal of Physical Chemistry B, 1999
A theoretical description of the steady-state potential response of ionophore-based ion-selective electrodes is presented that, so far, is the most general formalism available. The treatment considers membrane systems with any number of ionophores, differently charged cations, anions, and fixed or stationary ionic sites. The theory accounts for thermodynamically controlled selectivity characteristics, as well as for various diffusioninduced effects resulting from transmembrane ion fluxes at zero current. The phenomena discussed in detail include apparent super-and sub-Nernstian responses and detection limits. An extension of the treatment for time-dependent phenomena is also given. The present approach can be applied for optimizing the selectivity coefficients and improving the detection limits of ionophore-based ion-selective electrodes.
Polymer Membrane Ion-Selective Electrodes-What are the Limits?
Electroanalysis, 1999
This article reviews recent advances in the ®eld of potentiometric solvent polymeric membrane electrodes. These sensors have found widespread applications in a variety of ®elds, especially in the area of clinical diagnostics. Emphasis is given on the discussion of the theoretical and practical limits of ionophore-based ion-selective electrodes, with a special focus on electrode sensitivities, characterization of selectivities and dramatic improvements in detection limits. Advances in ionophore design and in the underlying model assumptions are also discussed. It is shown that a multitude of exciting new research possibilities have recently emerged in this well±established ®eld.
Analytical Chemistry, 2006
The ion transfer across the water-solvent polymeric membrane interface is investigated by using a new device based on a modification of a commercial ion-selective electrode body that permits the accommodation of a platinum counter electrode inside the inner filling solution compartment and, therefore, use of a four-electrode potentiostat with ohmic drop compensation. This device is used here to apply two different double potential pulse techniquessdifferential pulse voltammetry and additive differential pulse voltammetryswhich are more advantageous than other voltammetric techniques, such as normal pulse voltammetry or cyclic voltammetry, for the determination of the characteristic electrochemical parameters of the system. This is due to the concurrence of two factors in these double potential pulse techniques, the peak-shaped response together with a considerable reduction of undesirable current contributions.